Patent Publication Number: US-11040901-B2

Title: Removing nitrate from water

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of, and claims priority under 35 U.S.C. § 120 to, U.S. patent application Ser. No. 15/952,940, filed on Apr. 13, 2018, and entitled “Removing Nitrate From Water,” which is a continuation of, and claims priority under 35 U.S.C. § 120 to, U.S. patent application Ser. No. 15/284,720, filed on Oct. 4, 2016, and entitled “Removing Nitrate From Water,” now U.S. Pat. No. 9,994,466, the entire contents of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This document relates to systems and methods for removing nitrate from water and, more particularly, systems and methods for a denitrification system for aquaria or recirculating aquaculture systems, or both. 
     BACKGROUND 
     Water holding tanks, such as aquaria and other vessels that form animal habitats, often recycle and reuse the water (e.g., saltwater or freshwater) within the tank. In some cases, several treatment techniques may be used to clean and treat the recycled water. For example, unwanted particulates may be removed and a chemical composition of the water may be adjusted. In some cases, animals that live in the tank, as well as the food in which they consume, are creators of harmful substances in the water, e.g., through uneaten feed decomposition and animal waste. Nitrate, for example, may be a byproduct of such uneaten animal feed and animal waste that, when occurring in undesirable concentrations within the water, can affect animal health. 
     SUMMARY 
     In a general implementation, a sulphur denitrification system includes a liquid input fluidly coupled to a source of saltwater that includes nitrate; a liquid output fluidly coupled to the source of saltwater; a plurality of vertically-oriented tanks, at least one of the tanks including a liquid inlet that is fluidly coupled to the liquid input to receive a flow of the saltwater, a volume configured to enclose a plurality of sulphur particles that support denitrification bacteria that biologically transform the nitrate into at least one of nitrous oxide or nitrogen gas, and a liquid outlet fluidly coupled to the liquid output and the liquid inlets of the tanks; and a circulation system configured to circulate a portion of the saltwater though the liquid input to the liquid inlets of the plurality of tanks, through the plurality of tanks, and from the liquid outlets of the tanks to the liquid output and the liquid inlets of the tanks. 
     In an aspect combinable with the general implementation, the plurality of tanks are fluidly coupled in parallel between the liquid input and the liquid output. 
     In another aspect combinable with any of the previous aspects, at least one of the plurality of tanks includes a volume configured to enclose a plurality of calcium carbonate particles. 
     Another aspect combinable with any of the previous aspects further includes a carbon dioxide injection device fluidly coupled to the volume configured to enclose the plurality of calcium carbonate particles, the carbon dioxide injection device configured to circulate a flow of carbon dioxide into the volume. 
     Another aspect combinable with any of the previous aspects further includes a flow control device fluidly coupled between the liquid outlets of the tanks and the liquid inlets of the plurality of tanks. 
     In another aspect combinable with any of the previous aspects, the flow control device includes a modulating valve positioned in a bypass conduit coupled between the liquid outlets of the tanks and the liquid inlets of the plurality of tanks. 
     In another aspect combinable with any of the previous aspects, the denitrification bacteria includes  Thiobacillus denarificans.    
     In another aspect combinable with any of the previous aspects, at least one of the plurality of tanks further includes a gas release device positioned at or near a top portion of the tank, the gas release device configured to open and fluidly couple the volume of the tank to an ambient environment at or above a set pressure in the volume. 
     In another aspect combinable with any of the previous aspects, the liquid output includes an expansion valve and a gas release device positioned adjacent the expansion valve, the expansion valve configured to reduce a pressure of the saltwater in the circulation system to at or near ambient pressure. 
     In another aspect combinable with any of the previous aspects, the circulation system includes one or more pumps fluidly coupled to the liquid input, the liquid output, and the plurality of tanks with fluid conduits. 
     Another aspect combinable with any of the previous aspects further includes a temperature control system that includes at least one temperature sensor mounted in at least one of the liquid input, the liquid output, or the liquid outlet; and a heater mounted in at least one of the liquid input, the liquid output, or the liquid outlet, and configured to raise a temperature of the flow of the saltwater. 
     Another aspect combinable with any of the previous aspects further includes an ozone system that includes at least one oxidation reduction potential sensor mounted in at least one of the liquid input, the liquid output, or the liquid outlet; and one or more ozone contactors mounted in the liquid output and configured to inject ozone into the flow of the saltwater. 
     In another aspect combinable with any of the previous aspects, the source of saltwater includes an artificial animal habitat. 
     In another aspect combinable with any of the previous aspects, the artificial animal habitat includes at least one of an aquarium or a recirculating aquaculture system. 
     In another general implementation, a sulphur denitrification process includes circulating saltwater from a source of saltwater that includes nitrate through a liquid input to a plurality of vertically-oriented tanks; circulating at least a portion of saltwater through liquid inlets of at least one of the plurality of tanks and through a plurality of sulphur particles that support denitrification bacteria within the tank; transforming at least a portion of the nitrate, with the denitrification bacteria, to at least one of nitrous oxide or nitrogen gas; circulating the portion of saltwater from the volumes of the plurality of tanks through liquid outlets of the plurality of tanks; proportioning the portion of saltwater circulated from the liquid outlets into a first flow of saltwater and a second flow of saltwater; circulating the first flow of saltwater from the liquid outlets to the source of saltwater; and circulating the second flow of saltwater to the liquid inlets of the plurality of tanks. 
     An aspect combinable with the general implementation further includes circulating the portion of saltwater through the vertically-oriented tanks in parallel. 
     Another aspect combinable with any of the previous aspects further includes circulating another portion of the saltwater through a liquid inlet of another of the plurality of tanks and through calcium carbonate enclosed within the other tank. 
     Another aspect combinable with any of the previous aspects further includes injecting carbon dioxide into the other tank that encloses the calcium carbonate; and increasing a rate of dissolution of the calcium carbonate into the other portion of saltwater based on the injected carbon dioxide. 
     In another aspect combinable with any of the previous aspects, injecting carbon dioxide into the other tank that encloses the calcium carbonate includes comparing a pH level of the saltwater to a threshold value pH; based on the comparison, injecting the carbon dioxide into the other tank that encloses the calcium carbonate. 
     Another aspect combinable with any of the previous aspects further includes controlling a flow rate of the second flow of saltwater from the liquid outlets to the liquid inlets. 
     In another aspect combinable with any of the previous aspects, the denitrification bacteria includes  Thiobacillus denitrificans.    
     Another aspect combinable with any of the previous aspects further includes releasing nitrogen gas from the at least one tank to an ambient environment. 
     Another aspect combinable with any of the previous aspects further includes reducing a pressure of the first flow of saltwater circulating to the source of saltwater to at or near ambient pressure; and based on the reduction of pressure, releasing nitrogen gas from the first flow of saltwater. 
     Another aspect combinable with any of the previous aspects further includes reducing a pressure of the second flow of saltwater circulating to the liquid inlets of the plurality of tanks to at or near ambient pressure; and based on the reduction of pressure, releasing nitrogen gas from the second flow of saltwater. 
     In another aspect combinable with any of the previous aspects, the source of saltwater includes an artificial animal habitat. 
     In another general implementation, a system includes a water holding tank that includes a volume of water that includes nitrate; a denitrification system fluidly coupled to the water holding tank to receive a flow of water from the water holding tank, circulate the flow of water through a plurality of denitrification chambers, and transform at least a portion of the nitrate into nitrogen gas in the chambers; and a control system. The control system includes a sensor positioned in the flow of water; one or more hardware processors; and a data store coupled to the one or more hardware processors and storing instructions which, when executed by the one or more hardware processors, causes the one or more hardware processors to perform operations including receiving, from the sensor, a measurement associated with a nitrate concentration in the flow of water; comparing the measurement with a nitrate concentration setpoint; and adjusting a component of the denitrification system based on the measurement exceeding the nitrate concentration setpoint. 
     In an aspect combinable with the general implementation, adjusting a component of the denitrification system includes modulating at least one valve of the denitrification system to reduce a rate of flow of the water recirculated directly from outlets of the plurality of denitrification tanks to inlets of the denitrification tanks. 
     In another aspect combinable with any of the previous aspects, adjusting a component of the denitrification system includes adjusting a speed of a pump of the denitrification system to reduce a rate of flow of the water from the water holding tank to the denitrification system. 
     In another aspect combinable with any of the previous aspects, a component of the denitrification system includes injecting a carbon dioxide fluid into a pH balancing tank of the denitrification system to increase a rate of dissolution of calcium carbonate or aragonite into the flow of water. 
     In another aspect combinable with any of the previous aspects, adjusting a component of the denitrification system includes adjusting an expansion valve of the denitrification system to decrease a pressure of the flow of water circulated from the denitrification system back to the water holding tank. 
     In another general implementation, a method includes circulating saltwater from a saltwater holding tank to a plurality of tanks; circulating a first portion of the saltwater through sulphur and denitrification bacteria stored in at least one of the plurality of tanks to reduce a nitrate concentration in the saltwater at a rate of nitrate concentration reduction; circulating a second portion of the saltwater through calcium carbonate or aragonite stored in at least another of the plurality of tanks; mixing the first and second portions of the saltwater downstream of the plurality of tanks; injecting a carbon dioxide fluid into the another of the plurality of tanks; and increasing the rate of nitrate concentration reduction in the saltwater based on injecting the carbon dioxide fluid. 
     An aspect combinable with the general implementation further includes increasing, based on the injected carbon dioxide fluid, a concentration of calcium carbonate or aragonite dissolved into solution in the mixed first and second portions of the saltwater. 
     In another aspect combinable with any of the previous aspects, increasing the rate of nitrate concentration reduction in the saltwater based, at least in part, on injecting the carbon dioxide fluid includes increasing the rate of nitrate concentration reduction in the saltwater based on the increased concentration of calcium carbonate or aragonite dissolved into solution in the mixed first and second portions of the saltwater. 
     One, some, or all of the implementations of a denitrification system according to the present disclosure may include one or more of the following features. For example, the denitrification system of the present disclosure may more efficiently, e.g., pumping less water and using smaller sized equipment, implement a denitrification process to remove nitrate from an animal habitat relative to conventional denitrification systems. As another example, the denitrification system of the present disclosure may remove more nitrate on an absolute basis than conventional denitrification systems. As yet another example, the denitrification system of the present disclosure may be implemented seamlessly within a larger water treatment system for an animal habitat as compared to conventional denitrification systems. As another example, the denitrification system of this present disclosure may operate more reliably and offer less service or process interruption, thereby providing very stable operating conditions for a the living and breathing biological process of denitrification. Embodiments of the denitrification system may optimize these parameters for greater process longevity. As another example, the denitrification system of the present disclosure may offer greater integration capabilities as example embodiments may allow the denitrification system to be constructed as a skid unit to provide a “plug-and-play” system for a more widespread range of water sources and parameter types than anything that is currently offered. As another example, the denitrification system of the present disclosure may offer a lower maintenance or operator friendly version as compared to conventional denitrification. As another example, the denitrification system of the present disclosure may operate with a saltwater or freshwater water source. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a schematic diagram of an example implementation for a saltwater treatment system according to the present disclosure. 
         FIGS. 2A-2B  illustrate schematic diagrams of example implementations of a denitrification system for a saltwater treatment system according to the present disclosure. 
         FIGS. 3A-3C  illustrate an example implementation of a tank for a denitrification system according to the present disclosure. 
         FIG. 4  illustrates a schematic diagram of a control system for a denitrification system according to the present disclosure. 
         FIG. 5  illustrates a schematic diagram of a controller of a control system that may be applied to any of the computer-implemented methods and other techniques described in the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     This document discusses implementations of a denitrification system that operates to transform nitrate (e.g., soluble) in a flow of water to nitrite and then to, e.g., nitrous oxide (N x O x ) and then nitrogen gas (N 2 ), to lower a nitrate concentration in the flow of water. In some aspects, the flow of water originates in an animal habitat or other recirculating aquatic animal holding system that supports marine fish and other animals with a manufactured volume of saltwater (or with a volume of freshwater). The denitrification system described in the present disclosure, for example, utilizes a recirculation loop to increase hydraulic retention time of the flow of water within the denitrification system, thereby increasing an efficiency of the denitrification process. In some aspects, the denitrification process is an autotrophic sulphur denitrification process that also includes pH balancing of the water to increase the efficiency of the denitrification process. 
       FIG. 1  illustrates a schematic diagram of an example implementation for a saltwater treatment system  100 . Generally, the saltwater treatment system  100  may include or be fluidly coupled to a source of saltwater, such as saltwater source  102 , to receive a flow of saltwater  101 , filter the flow of saltwater  101 , and treat the flow of saltwater  101  (e.g., mechanically, chemically, biologically, or a combination thereof), before the flow of saltwater  101  is returned to the source. In some aspects, the source of saltwater may be an enclosed tank, such as an aquarium or recirculating aquaculture system. The enclosed tank, in some aspects, may be fluidly coupled to a source of natural seawater (e.g., ocean, sea, or other source). In alternative aspects, the enclosed tank may not be fluidly coupled to a source of natural seawater and, for instance, may be located in a geographic area that is not immediately adjacent a source of natural seawater (e.g., in a landlocked area). In some aspects, the enclosed tank, as an aquarium or recirculating aquaculture system, may support living animals, such as fish, mammals, crustaceans, and other animals. 
     In some aspects, aquaria systems and recirculating aquaculture systems may have different configurations based on the different purposes of such systems. For example, an aquarium may be comprised of a single animal habitat tank, or multiple independent animal habitat tanks that are fluidly coupled to share a water source but independent in that animals from one tank may not typically or ever be transferred to another tank. In some aspects, a particular animal or animals may spend its life or a majority of its life in a single tank of an aquarium system. Concerns for an aquarium system may include maintaining low animal populations within a particular tank so that animal density within the tank does not exceed a particular threshold (e.g., so that animal viewing is not impeded, animal life expectation is maximized, and otherwise). 
     In contrast, a recirculating aquaculture system (RAS) may include a system or series of systems of one or more freshwater, brackish, or saltwater (artificial or natural in type) animal holding or grow out tanks. These tanks are not necessarily habitats but may be utilized for the purposes of supporting animal life and rapid growth of animal life for the purpose of eventual harvest and preparation of the animal life (e.g., fish) for human consumption. An RAS may have a high animal stocking density (or densities) compared to aquaria systems, such as several hundred kilograms of fish mass per cubic meter of water. Such RAS animal density may be over 100 times greater than in a typical aquaria setting. 
     These animal stocking densities often require animal feed rates relative to aquaria systems that generate high levels of dissolved nitrogen that becomes nitrate, as well as large amounts of carbon dioxide due to animal respiration. Filtration components in RAS, like aquaria systems, may include mechanical, biological, and chemical filtration. In some aspects of RAS, a gas exchange system may be included (e.g., within system  100 ) due to high levels of dissolved carbon dioxide within the saltwater source  102 . Carbon dioxide may be mechanically stripped from the saltwater  101  utilizing rapid agitation in RAS and harvested in a negative pressure ducting system. 
     System  100  includes, as shown in the example implementation of  FIG. 1 , the saltwater source  102 , which is fluidly coupled, through conduits  107  (e.g., piping), to a number of treatment subsystems, such as a sand filter system  108 , a foam fractionation system  110 , a denitrification system  112 , an ozone system  114 , a deaeration system  120 , and a return sump  122 . The system  100  further includes, in this example, pumps  106   a ,  106   b ,  116 , and  118 , that are positioned to circulate a flow of saltwater from the saltwater source  102  to one or more of the components of system  100 . 
     The saltwater source  102 , in this example, is an enclosed tank (e.g., with a top open to an ambient environment) that includes an aquarium or recirculating aquaculture system that supports animal life, such as fish and other animals. In this example embodiment, the saltwater source  102  may hold or contain about 4 million gallons of saltwater  101 , while the system  100 , including conduits, piping, and the aforementioned subsystems, may hold or contain more than 6 million gallons of saltwater  101 . The saltwater  101  enclosed within the system  100  may be initially manufactured from freshwater, salt, and other additives (e.g., chemicals) or may be circulated into the system  100  from a natural source of seawater (if available). For simplicity, certain components of the system  100 , such as chemical additive systems, make-up saltwater systems, nutrient additive systems (for animal and plant feeding) are omitted from the schematic illustration of  FIG. 1 . 
     As shown, fluid conduits  103  and  105  couple the saltwater source  102  to the return sump  122 . The fluid conduit  103  couples a top surface of saltwater  101  in the saltwater source  102  to the return sump  122 , while the fluid conduit  105  couples a bottom portion (e.g., between 30 and 40 feet deeper than the top surface of the saltwater  101 ) of the saltwater  101  to the return sump  122 . In some implementations, the saltwater  101  may flow, e.g., by gravity, from the saltwater source  102 , through surface skimmers (in conduit  103 ) and bottom drains (in conduit  105 ) to the return sump  122 . The return sump  122 , in this example, is divided into two sections, chamber  124   a  and chamber  124   b , which are divided by a wall. Chamber  124   a  is fluidly coupled, by the conduits  103  and  105 , to the saltwater source  102 . Chamber  124   a  is also fluidly coupled, through the pump  106   a , to the foam fractionation system  110 . Chamber  124   b  is fluidly coupled to receive the flow of saltwater  101  from the foam fractionation system  110 , as well as other components of the system  100 . 
     The foam fractionation system  110  (i.e., a protein skimming system), generally, includes multiple tanks that comprise a recirculating process loop from chamber  124   a  back to chamber  124   b  of the return sump  122 . The foam fractionation system  110  targets the removal of dissolved organic or liquid waste by incorporating air/water to make foam, and, as shown, may include a pump  119  that recirculates at least a portion of the saltwater  101  through the tanks of the system  110 . The polarity of the water molecule and the surface tension of the foam create an affinity for these waste products that allow the dirty foam to be “fractioned” off the top of the tanks in the system  110 , thus removing the waste products from the process. In some aspects, a small amount of ozone gas can be dosed to the air/water stream in the foam fractionation system  110  to augment this process and also create micro flocculation which assists in other treatment of the saltwater  101  (e.g., in the sand filter system  108 ). 
     The sand filter system  108  is fluidly coupled to the chamber  124   b  of the return sump  122 , and includes multiple tanks for rapid particulate removal from the flow of saltwater  101 . The sand filter system  108 , once loaded with particulate material, may also be backwashed periodically to clean them. Aside from mechanically filtering particulates from the water, the sand filter system  108  also creates a location for aerobic heterotrophic bacteria, discussed in more detail later, to thrive. For example, due to high water flow and a large surface area within each tank of the sand filter system  108 , some biological filtration, as well as mechanical filtration, takes place during operation. For instance, bio-films within a tank of the sand filter system  108  may synthesize nutrients such as nitrogen, phosphorus and organic carbon. In some examples, the sand filter system  108  can remove particles in the saltwater flow of approximately 20 microns and larger. 
     As shown in this example, the denitrification system  112  and ozone system  114  are fluidly coupled to receive portions of the flow of saltwater  101  through system  100  from the sand filter system  108 . For instance, pump  118  is positioned to circulate a portion of the flow of saltwater  101  through the denitrification system  112 , while the pump  116  is positioned to circulate a portion of the flow of saltwater  101  through the ozone system  114 . 
     The denitrification system  112 , as shown, receives a portion of the flow of saltwater  101  that circulates through the system  100 . Generally, the denitrification system  112  biologically converts nitrate in the saltwater  101  to nitrite and then, nitrous oxide, and then (potentially) nitrogen gas. For example, in aquaria and recirculating aquaculture systems, changes in the water source, such as water changes to the saltwater  101  in the saltwater source  102 , may be necessary due to an accumulation of nitrogen-based products from food and animal waste that ultimately end up in the form of dissolved nitrate (NO 3 ). This nitrate may be formed as an end product of the nitrifying portion of the nitrogen cycle and can cause chronic health issues in animals within the aquaria, such as fish and invertebrates, at high concentrations. The nitrifying portion of the nitrogen cycle is aerobic and occurs readily in water and on substrates in closed aquarium or recirculating aquaculture systems due to the presence and abundance of aerobic bacteria and dissolved oxygen. Under such conditions, these environmental bacteria, called nitrifyers, readily oxidize ammonia, which is the dominant excreted animal waste product, to nitrite and then to nitrate. The denitrifying portion of the nitrogen cycle, which reduces the nitrate back to gaseous nitrogen, can be carried out by denitrifyers. Denitrifyers, in some aspects, can be facultatively anaerobic bacteria that utilize alternative oxygen sources such as the nitrate compound in low oxygen environments. This may be more difficult to achieve in closed system aquaria, e.g., due to the desirable or required anaerobic conditions being difficult to replicate and sustain in an otherwise aerobic environment. 
     Problems associated with high nitrate concentrations in saltwater (or freshwater) can be addressed by water changes in the proportion of 10-30% of a total volume per week in a saltwater (or freshwater) source. Water changes, for example, include circulating a portion of the saltwater out of a closed system while also circulating another portion of new saltwater (e.g., manufactured or from a natural source of saltwater) into the system. The new saltwater (or freshwater) helps dilute excessive nitrate concentrations and thus sustain a healthy water chemistry for the animals. Alternatively, a denitrification system, such as the denitrification system  112 , may be used to transform all or a portion of the nitrate in the saltwater to nitrogen gas, which can then be removed from the saltwater. 
     In example implementations, the denitrification system  112  may be a sulphur denitrification system  112 . For example, a sulfur denitrification (SDN) system may implement an autotrophic process that requires little or no external feeding or supplementation with a carbon energy source. By contrast, heterotrophic denitrification relies on externally supplied carbon, usually in the form of ethanol or methanol to be the energy source for the reaction. SDN processes cultivate sulfur-loving facultatively anaerobic bacteria that reduce nitrate (NO 3 ) to nitrous oxides (N x O x ) and ultimately nitrogen gas (N 2 ). Due to their anaerobic nature, applications of SDN may occur inside a closed reactor containing elemental sulfur at water flow rates determined to preserve low oxygen tensions. Under such conditions, bacteria such as  Thiobacillus denitrificans , and other similar sulfur-loving bacteria, thrive on elemental sulfur and reduce nitrates to nitrogen gas, as shown in Equation 1:
 
11S+10NO 3   − +4.1HCO 3   − +0.5CO 2 +1.7NH 4   + +2.5H 2 O—0.92C 5 H 7 NO 2 +11SO 4   −2 +5.4N 2 +9.62H + 
 
     Sulfur is the medium that the bacteria reside on and also acts as the electron donor or energy source for the bacteria. The denitrification process also consumes available carbon in the form of carbon dioxide, carbonate and bicarbonate as primary constituents of the reaction. Although readily available in natural seawater, carbonate can become depleted in a recirculated SDN process loop. In some aspects of saltwater treatment system  100 , a calcium carbonate source, such as aragonite, may be placed within the system  100  (or within another system fluidly coupled to the system  100 , such as the denitrification system  112 ) to supplement and optimize the SDN process. 
     The ozone system  114 , as shown, receives a portion of the flow of saltwater  101  that circulates through the system  100 . In some aspects, ozone gas may be generated onsite and injected into the saltwater in the system  100 . The ozone may be injected, in some examples, at both the foam fractionation system  110  and at ozone contactors as part of the ozone system  114  that is located in the system  100  downstream of the sand filter system  108 . Ozone contactors may act as a form of chemical filtration including color removal, organic removal and disinfection for the saltwater source  102 . The ozone&#39;s oxidizing effects are measured using oxidation reduction potential (ORP) sensors that control the flow of ozone gas to the ozone contactors in the ozone system  114 . Ozonated saltwater from the foam fractionation system  110  (through the sand filter system  108  and denitrification system  112 ) and ozone system  114  is circulated to the deaeration system  120 . 
     Prior to returning to the saltwater source  102 , the saltwater  101  in the system  100  is circulated to the deaeration system  120 . The deaeration system  120  includes one or more degassing towers that balance dissolved gasses in the saltwater  101 , such as oxygen and carbon dioxide, into equilibrium with the atmosphere. In some aspects, the deaeration system  120  may also be a beneficial bio-film growth location for biological filtration to occur in the saltwater. Treated saltwater  101  may be circulated from the deaeration system  120 , e.g., by gravity feed in conduit  109 , back to the saltwater source  102 . 
     One or more additional systems and subsystems, not shown in  FIG. 1 , may also be part of the saltwater treatment system  100 . For example, one or more heat exchangers (e.g., plate and frame, shell and tube, or other form of heat exchanger) may be thermally coupled within the system  100  and also to one or more heating sources (e.g., boilers) and one or more cooling sources (e.g., chillers). For example, in some aspects, first sides of the one or more heat exchangers may be fluidly coupled to the conduits  107  within the system  100 , while second sides of the one or more heat exchangers may be fluidly coupled to the heating sources, the cooling sources, or both. The first and second sides of the heat exchangers may be thermally coupled to exchange heat between the saltwater  101  and the heating or cooling sources. 
     The saltwater treatment system  100  may also include a backwash recovery system, which can capture sand filter backwash waste water in one or more basins. In some aspects, there may be at least one “clean” basin and at least one “dirty” basin. Using the dirty basin(s) and the clean basin(s), sand filter backwash water can be batch treated with a dedicated sand filter, foam fractionation and ozone contact for the recovery system (that operate as described for these features within the system  100 . Once batch filtered and recovered to the clean basin(s), the backwash water be reused and circulated to the saltwater source  102 . 
     The saltwater treatment system  100  may also include a flow control system. The flow control system can include the pumps  106   a,    106   b ,  116 ,  118 , and  119  for example, to pump the streams of saltwater  101  in the system, one or more flow pipes, such as conduits  103 ,  105 ,  107 , and  109  through which the streams are flowed and one or more valves to regulate the flow of the saltwater  101  through the pipes. In some implementations, a flow control system can be operated manually. For example, an operator can set a flow rate for each pump and set valve open or close positions to regulate the flow of the saltwater  101  through the pipes in the flow control system. Once the operator has set the flow rates and the valve open or close positions for all flow control systems distributed across the saltwater treatment system  100 , the flow control system can flow the saltwater  101  within the system  100  under constant flow conditions, for example, constant volumetric rate, or other flow conditions. To change the flow conditions, the operator can manually operate the flow control system, for example, by changing the pump flow rate or the valve open or closed position. 
     In some implementations, a flow control system can be operated automatically. For example, the flow control system can be connected to a computer system to operate the flow control system. The computer system can include a computer-readable medium storing instructions (such as flow control instructions and other instructions) executable by one or more processors to perform operations (such as flow control operations). An operator can set the flow rates and the valve open or close positions for all flow control systems distributed across the saltwater treatment system  100  using the computer system. In such implementations, the operator can manually change the flow conditions by providing inputs through the computer system. Also, in such implementations, the computer system can automatically (that is, without manual intervention) control one or more of the flow control systems, for example, using feedback systems implemented in the saltwater treatment system  100  and connected to the computer system. For example, a sensor (e.g., a pressure sensor, temperature sensor, flow sensor, nitrate sensor, or other sensor) can be connected to a pipe through which the saltwater  101  flows. The sensor(s) can monitor and provide a flow condition (e.g., a pressure, temperature, flow rate, nitrate concentration, or other flow condition) of the saltwater  101  to the computer system. In response to the flow condition exceeding a threshold (e.g., a threshold pressure value, a threshold temperature value, a threshold flow value, a threshold nitrate concentration value, or other threshold value), the computer system can automatically perform operations. 
     In an example operation of the saltwater treatment system  100 , saltwater  101  is circulated from the saltwater source  102  to chamber  124   a . In some aspects, the saltwater source  102  may contain about 4 million gallons of the saltwater  101  and a portion of the saltwater is continuously or periodically circulated to the chamber  124   a . The saltwater  101  can then be circulated to the foam fractionation system  110 , where, e.g., dissolved organic and liquid waste can be removed and ozone can be put into the saltwater  101 . In some aspects, the amount of saltwater  101  circulated to the foam fractionation system  110  at about 65,000 gallons per minute (gpm). 
     The saltwater  101  is circulated back to chamber  124   b  from the foam fractionation system  110  and then to the sand filter system  108 , where particulate matter is removed from the saltwater  101 . As with the foam fractionation system  110 , the sand filter system  108  may handle about 65,000 gpm of the saltwater  101 . 
     The saltwater  101  that is circulated from the sand filter system  108  is then circulated to the deaeration system  120 . Portions of the saltwater  101 , e.g., portions of the 65,000 gpm that flow through the foam fractionation system  110  and the sand filter system  108 , are circulated to the denitrification system  112  and then to the ozone system  114 . In some implementations, a small portion (e.g., 50 gpm, 100 gpm, 200 gpm, 300 gpm, 400 gpm or another volumetric flow rate) is circulated through the denitrification system  112  and then to the ozone system  114  is then circulated (with the main flow of the saltwater  101 ) to the deaeration system  120 . Once ozone has contacted the saltwater  101 , nitrate is removed from the saltwater  101 , and dissolved gasses in the saltwater  101  are balanced, the flow of saltwater  101  is returned to the saltwater source  102 . 
       FIGS. 2A-2B  illustrate schematic diagrams of example implementations of a denitrification system for a saltwater treatment system.  FIG. 2A , for example, illustrates an example implementation of a sulphur denitrification system  200  that, in some aspects, may be implemented as the denitrification system  112  shown in  FIG. 1 . For example, as shown in  FIG. 2A , the denitrification system  200  is fluidly coupled to receive a flow of the saltwater  101  from the saltwater source  102 , treat the flow of saltwater  101  (e.g., implement the denitrification process) and return the treated saltwater  101  to the saltwater source  102 . 
     As illustrated, generally, the denitrification system  200  comprises a recirculation loop that is fluidly coupled to the saltwater source  102  and, within the recirculation loop, includes multiple tanks  206 . The recirculation loop comprises an inlet header  202  that is fluidly coupled to the saltwater source  102  and, through valves  212 , to inlets of the tanks  206 . The recirculation loop also includes an outlet header  204  that is fluidly coupled to the saltwater source  102  and, through valves  214 , to outlets of the tanks  206 . The outlet header  204  is fluidly coupled to the inlet header  202  as well, e.g., by bypass header  220  and valve  222 . 
     In this example, at least some of the tanks  206  define volumes that can hold media  230  and bacteria used in the denitrification process. For example, in some aspects, at least some of the tanks  206  may hold sulphur media  230  that support bacteria (e.g.,  Thiobacillus denarificans ) used to transform nitrate to, e.g., nitrous oxide and/or nitrogen gas during SDN. In some examples, the media  230  may include sulfur media that is comprised of a 99.9% elemental sulfur “prill.” 
     Further, in this example, at least one of the tanks  206  may define a volume that can hold a calcium carbonate media  240 , such as calcium carbonite (CaCO3) or aragonite (i.e., naturally occurring CaCO 3 ). For example, as shown, the tanks  206  are coupled in parallel with respect to the inlet and outlet headers  202  and  204 , and several (e.g., four) of the tanks  206  hold denitrification media  230  while one tank  206  holds the calcium carbonate media  240 . In alternative implementations, there may be more or fewer tanks  206  that hold the denitrification media  230  and there may be more or fewer tanks  206  that hold the calcium carbonate media  240 . 
     In this example, the tank  206  that holds the calcium carbonate media  240  also includes a carbon dioxide (CO 2 ) injection system  228 . For example, in some aspects, SDN may be implemented more at a water pH of about 5.5 pH. The calcium carbonate media  240  (e.g., CaCO 3 , aragonite, or other media) may help maintain a specified or predetermined pH of the saltwater  101 , but may also affect the SDN process by providing dissolved carbonate and bicarbonate (and in some cases, an abundance of such material) into the saltwater  101 . In some aspects, such dissolved carbonate and bicarbonate may increase an efficiency (e.g., a rate at which nitrate is changed to nitrous oxide or nitrogen gas). In some aspects, the SDN reaction may be made more efficient by between 30-60% (e.g., increase a rate of nitrate to nitrogen gas or N x O x ) when CaCO 3  is added to one or more of the tanks  206 . 
     Further, in some aspects, the CO 2  injection system  228  may be fluidly coupled to the tank  206  that contains the calcium carbonate media  240  to deliver a flow of CO 2  gas into the tank  206 . The gaseous CO 2 , in some implementations, may dissolve or increase a rate of dissolution, of the calcium carbonate media  240  within the flow of saltwater  101 , thereby increasing an efficiency of the SDN (e.g., increasing a rate of nitrate to nitrogen gas). In some aspects, for example in RAS, the flow of CO 2  may be delivered from a gas exchange system (not shown in this figure) that removes dissolved CO 2  from the saltwater  101 . For example, harvested CO 2  (e.g., from a gas exchange system) may be introduced to the system  200  through the injected CO 2  in the tank  206  that contains the calcium carbonate media  240 . 
     In some implementations, a particular tank  206  (or tanks  206 ) may hold sulphur media  230  and the calcium carbonite media  240  together. For example, in some implementations, the sulfur media  230 , the denitrification bacteria, and the CaCO 3  (e.g., aragonite) may be mixed together within one or more of the tanks  206 . In some aspects, the bacteria reside on the sulfur granules naturally and may require no other form of addition or supplementation from an operator of the system  100 . In some aspects, multiple tanks  206  may include the media  230 , bacteria, and aragonite, while other tanks  206  may include one or the other of the sulphur media  240 /bacteria or the aragonite. In tanks  206  including both the media  240 /bacteria and aragonite, the calcium carbonate media  240  may be placed atop the sulfur media  230 , e.g., in a proportion of 8 to 1 sulfur to aragonite. The medias  230  and  240  may remain stratified and layered in this configuration, thus allowing the aragonite volume to be monitored and replenished when needed. In other aspects, the aragonite and sulphur media  230  may be blended together in the same tank  206  (or tanks  206 ) at proportions, e.g., as high as 1 to 1 sulfur to aragonite with no stratification or layering provided in this mix. 
     In this example, each tank  206  includes a backwash line  218  at an outlet of the tank  206  (e.g., between the tank  206  and the outlet header  204 ). In some aspects, for example, the backwash line  218  may be opened to allow flushing, and thus cleaning, of the sulphur and aragonite media within the tanks  206  (either individually or in combination). For instance, it may be desirable to backwash or increase the upwelling flow to each tank  206  separately and discharge a portion of the saltwater  101  to a recovery system (e.g., to reuse in the saltwater source  102 ) or a sanitary sewer, in order to reduce biological fouling of the media (e.g., media  230  or  240  or both) by bacterial sludge (e.g., heterotrophic). In some aspects, a backwash or discharge may help to undo channeling of sulfur media  230  that can often occur. For example, backwashing helps to keep the water moving consistently and in contact with as much of the sulfur volume as possible. Channeling can often lead to overly anoxic zones that can reduce efficiency and give off harmful hydrogen sulfide gasses. 
     In this example, each tank  206  also includes a gas release device  216  that is fluidly coupled to an inner volume of the tank  206 . In some aspects, the gas release device  216  may be automatically operated based on a predetermined or present pressure within the volume of the tanks  206 , to release a flow of gas  232  into an ambient environment. For example, the gas  232  may be nitrous oxide and/or nitrogen gas that results from the denitrification process within the tanks  206 . As another example, other gasses, such as CO 2 , may be released through the gas release device  216 . 
     Additional gas release devices  224 , as shown in this example, are positioned in the outlet header  204  downstream of the bypass conduit  220  and valve  234 . An orifice  234 , such as an expansion valve, is also positioned in the outlet header  204 , upstream of the gas release device  224 . In combination, the gas release device  224  and orifice  234  may operate to provide a pressure drop to further release gasses, such as nitrogen, into an ambient environment. For instance, the incorporation of the orifice  234  and adjacent gas release device  224 , which may be positioned at an elevated location in the system  200  (and in some aspects a highest elevation in the system  200 ), may create a low or reduced pressure area of the recirculation loop, relative to other portions of the loop (e.g., the tanks  206 , the inlet header  202 ). At this high elevation, low pressure area, nitrogen and other gasses within the saltwater  101  may bubble out of the liquid and be released into the ambient environment. For instance, due to a phase change as a result of the drop in pressure, newly release nitrogen and similar gasses can be immediately evacuated from the recirculation loop, thus preventing such gasses from dissolving back into solution (e.g., due to pumping of the saltwater  101  through the loop by pump  208 ). 
     As shown, another gas release device  224  may also be positioned in the bypass conduit  220  (in this example, downstream of the bypass valve  222 ). The bypass valve  222  and gas release device  224  may also be positioned at an elevated location in the system  200  (and in some aspects a highest elevation in the system  200 ). As noted, this elevated positioning may create a low or reduced pressure area of the recirculation loop, relative to other portions of the loop (e.g., the tanks  206 , the inlet header  202 ). At this elevated low pressure area, nitrogen and other gasses within the saltwater  101  may bubble out of the liquid and be released into the ambient environment located just downstream of bypass valve  222 , which is also operated in conjunction or in combination with the gas release device  224  in the bypass conduit  220 . 
     The example system  200  includes a pump  208  and valve  210 . The pump  208  (e.g., centrifugal or other form of pump and may be the same as pump  118 ) may circulate the saltwater  101  from the saltwater source  102  (e.g., as shown in  FIG. 1 ) and into the inlet header  202  to flow through the recirculation loop. In some aspects, the valve  210  may be controlled (e.g., modulated) to vary a flow rate of the saltwater  101  that is circulated to all of the tanks  206 . In alternative aspects, the valve  210  may be an isolation valve or a modulating valve, and the pump  208  may be controllably varied (e.g., with a variable frequency drive, step motor, or otherwise) to vary the flow rate of the saltwater  101 . In other alternative implementations, the pump  208  and valve  210  may provide a constant or substantially constant flow rate of the saltwater  101  through the recirculation loop. 
     As noted, valves  212  and  214  are positioned at inlets and outlets, respectively, of the tanks  206 . In some aspects, valves  212  and  214  may be shut-off isolation valves (e.g., able to fully close or fully open), which, when operated (e.g., manually or automatically), can fluidly isolate the tanks  206  (individually or in combination) from the recirculation loop. In alternative aspects, one or more of the valves  212  and  214  may be modulating control valves which, when operated, can modulate a flow rate of the saltwater  101  to the tanks (individually or in combination). 
     Other valves or flow control devices may be positioned in the recirculation loop as well. For example, as shown, modulating valve  222  (e.g., automatically or manually, or both) is positioned to control a flow rate of the saltwater  101  that is circulated from the outlet header  204  to the inlet header  202 . Thus, the control valve  222  may be controlled to vary a flow rate of the saltwater  101  recirculated within the recirculation loop for further treatment (e.g., for further removal of nitrate, carbonate addition or both), as well as provide the optimal hydraulic retention time (HRT) of the exhibit water in the denitrification process. Thus, according to the example denitrification system  200 , there may be two loops. A first loop comprises the inlet and outlet headers  202  and  204 , respectively, and the bypass header  220 . A second loop comprises the inlet and outlet headers  202  and  204 , respectively, but not the bypass header  220 . The first loop is pumped and recirculated (e.g., continuously) through the tanks  206 , while the second loop titrates a small amount of saltwater  101  (e.g., relative to a total volume of saltwater  101  in the saltwater  102 , or relative to an amount of saltwater  101  circulated through the saltwater treatment system  100 ) from the saltwater source  102  into the first loop and back out again to the saltwater source  102 . 
     In some aspects, the first loop contains the saltwater  101  circulated at particular parameter (e.g., pressure, velocity) to efficiently utilize a full volume and surface area of the denitrification media  230  (e.g., sulfur) and bacteria inside the tanks  206 . In some aspects, to ensure such parameters are met, a particular volume of saltwater  101  may be looped through the tanks  206  in multiple passes to achieve effective (e.g., achieving a particular nitrate concentration) denitrification of that volume of saltwater  101 . In some aspects, such multiple passes may better achieve effective denitrification relative to, for example, a quantity of media volume inside the tanks  206 . Thus, effective denitrification may be achieved in system  200  with smaller tanks relative to conventional denitrification systems. 
     In some aspects, a flow rate of the saltwater  101  through the second loop (e.g., from and to the saltwater source  102 ) may be controlled to not overwhelm the flow of the saltwater  101  in the first loop. For instance, if too much saltwater  101  is circulated into the second loop, effective denitrification may not occur in the first loop, a dissolved oxygen level may rise, and an overall nitrate concentration may rise or not decrease in order to account for, e.g., food and animal excretion in the saltwater  101 . 
     In an example operation of system  200 , saltwater  101  from the saltwater source  102  is circulated into the inlet header  202  and, in parallel, to the tanks  206 . In some aspects, a flow rate of the saltwater  101  circulated through each tank  206  is similar, substantially similar, or identical. Within at least a portion of the tanks  206 , the saltwater  101  flows through the media  230  and denitrification bacteria, and the denitrification process transforms the nitrate within the saltwater  101  to nitrogen gas as previously described. Nitrous oxide or nitrogen gas is released from the tanks  206  through the gas release devices  216 . As shown, within at least one of the tanks  206 , the saltwater  101  flows through the media  240 , through which a calcium carbonate (e.g., aragonite) addition process occurs, as well as a pH balancing process (e.g., pH is maintained at a minimum of at least 5.5). Further, as the media  240  comprises CaCO 3  or aragonite, bits of such media are dissolved into the saltwater  101 . In some aspects, the CO 2  injection device introduces gaseous CO 2  into the tank  206  that holds the calcium carbonate media  240 , thereby further dissolving or increasing a dissolution rate of the CO 2  and carbonate into the saltwater  101 . Treated saltwater  101  flows from the tanks  206  into the outlet header  204 . A portion of the treated saltwater  101  flows back to the inlet header  202  through the bypass header  220 , while another portion of the treated saltwater  101  flows back to the saltwater source  102 . The portion of the treated saltwater  101  that flows through the bypass header  220  may join an additional flow of saltwater  101  from the saltwater source  102  and be circulated back to the tanks  206 . 
     In an additional operation or aspect of this example, a temperature control system  242  may be included within the system  200 , e.g., within the bypass conduit  220 . For example, a heating source within the temperature control system  242  may operate (e.g., powered by gas, electricity, solar, or other energy form) to heat the saltwater  101  within the recirculation loop of the system  200 . For example, temperature control system  242  may be operated (e.g., automatically or manually) to maintain a temperature of the saltwater in the recirculation loop at or about 20° C. to 22° C. (e.g., as a setpoint temperature). The temperature control system  242  may include a thermostat that measures the temperature of the saltwater  101  in the bypass conduit  220  and a heating source. In some aspects, the temperature control system  242  may not include a heating source but may be communicably coupled to a controller that controls a heating source within the system  200  based on temperature measurements from the system  242 . 
     In another example operation or aspect of this example, an ozone gas injection and pressurized tank contact system  244  may be included within the system  200  (e.g., downstream of the orifice  234  and gas release device  224 ) to treat the saltwater  101  before returning to the tank  102 . In some aspects, this ozone gas injection and pressurized tank contact system  244  may be operated in addition to the ozone system  114  shown in  FIG. 1 . In some aspects, this ozone gas injection and pressurized tank contact system  244  may be operated alternatively to the ozone system  114  shown in  FIG. 1  and, in some cases, may replace the ozone system  114 . Thus, in some aspects, the denitrification system  200  may include an ozone system (in accordance with the present disclosure) as a single package. In operation, a sulphur denitrification process may create reduced chemical species that, if left untreated, may be detrimental to animals that live in the saltwater source  102 . Oxidizing the saltwater  101  before it returns to the saltwater source  102  may treat such effluent to ensure no harmful effects to, e.g., animals that live in the saltwater source  102  (e.g., aquarium or aquaculture habitat). The ozone gas injection and pressurized tank contact system  244  may include an oxidation-reduction potential (ORP) sensor that measures the ability of the saltwater  101  to act as an oxidizing or reducing agent in order to control the dosage of ozone from the system  244  to the saltwater  101 . The ORP sensor, in operation, provides feedback to a controller that, in turns, may control the system  244 , the ozone system  114 , or both, to maintain ORP values in the saltwater  101  above  200  millivolts but not exceeding  400  millivolts. 
     Turning to  FIG. 2B , this figure illustrates an example implementation of a sulphur denitrification system  250  that, in some aspects, may be implemented as the denitrification system  112  shown in  FIG. 1 . As shown in  FIG. 2B , system  250  generally includes two denitrification systems  200  that are fluidly coupled to receive a flow of saltwater  101  from the saltwater source  102 , treat the flow of saltwater  101  (e.g., through a denitrification process and pH balancing process), and return at least a portion of the treated saltwater  101  to the saltwater source  102 . Thus, within the denitrification system  250 , each system  200  may be a subsystem that can be operated and controlled independent or in tandem to provide for denitrification, pH balancing, or both, of the saltwater  101 . Further example denitrification systems may include three or more subsystems  200 , e.g., fluidly coupled in parallel with the saltwater source  102 . Also, each subsystem  200  within the system  250  may include more tanks  206  or fewer tanks  206 , including more or fewer tanks  206  with denitrification media  230  and more or fewer tanks  206  with calcium carbonate media  240 . Example operations of each subsystem  200  within system  250  may be similar to that previously described, and may be implemented independently per subsystem  200  (e.g., components of each subsystem  200  may be controlled to different parameters or desired outputs) or in tandem (e.g., components of each subsystem  200  may be controlled to the same or similar parameters or desired outputs). 
       FIGS. 3A-3C  illustrate an example implementation of a tank  300  for a denitrification system. In some aspects, the tank  300  may be similar or identical to one or more of the tanks  206  shown in  FIGS. 2A and 2B . Thus, tank  300  may be utilized in a denitrification process (e.g., with denitrification media  230 ), a pH balancing and aragonite addition process (e.g., with calcium carbonate media  240 ), or both.  FIG. 3A  shows a side cutaway view of the tank  300 .  FIG. 3B  shows a sectional plan view of a bottom portion of the tank  300  that includes a saltwater inlet manifold  312 .  FIG. 3C  shows a sectional plan view of a top portion of the tank  300  that includes a saltwater outlet  310 . 
     Tank  300 , in this example, includes a shell  302  that encloses a volume and is supported on a base  306 . In this figure, the tank  300  is illustrated as vertically-oriented, e.g., a long axis of the volume is oriented vertically with respect to a floor on which the tank  300  is supported. Thus, the long axis is aligned in a direction of gravity. In some aspects, each of the tanks  206  shown in  FIGS. 2A-2B  are also vertically-aligned. By vertically-aligning the tank  300 , a flow of saltwater to be treated within the tank  300  achieves sufficient contact, and sufficient time in contact, with a process media  322  (e.g., a denitrification media, such as sulphur, that supports denitrification bacteria, or a calcium carbonate media, such as CaCO 3 ). In the example tank  300 , the media  322  may be sulphur that supports denitrification bacteria. The sulphur  322  may also be supported on a bed of pea gravel  324 , positioned at a bottom portion of the shell  302 . 
     The shell  302  includes a number of appurtenances. For example, access panels  304  and  308  may be positioned, as shown, in lower and upper portions of the shell  302 , respectively. Such access panels  304  and  308  may be openable to provide human access or visual access into the volume of the shell  302  (e.g., for inspection, cleaning, or otherwise). Further, sample ports  320  may be positioned vertically along the shell  302  to perform manual water sampling, e.g., of the media  322  and saltwater that is flowing through the media  322 . 
     As shown more clearly in  FIG. 3B , the saltwater inlet manifold  312  includes an inlet  318  that is fluidly coupled with multiple outlet branches  314  through a header  316 . Ends of the outlet branches  314  that are proximate to the shell  302  include openings to allow the saltwater to flow from the inlet  318 , through the header  316 , and into the volume of the shell  302  through the branches  314 . As shown, the outlet branches  314  are dispersed (e.g., evenly) through the volume of the shell  302  within the pea gravel  324 . In some aspects, such dispersion may provide for even or substantially even flow of the saltwater from the bottom portion of the shell  302  vertically upward through the volume and to the saltwater outlet  310 . In some aspects, one or more of the outlet branches  314  may have perforations or finely machined slits arranged along a length of the branch  314 , thereby providing for saltwater output from the perforations without allowing media to fall into or enter the outlet branches  314 . In alternative implementations, there may be a single inlet for the saltwater into the shell  302 , rather than a manifold with multiple outlets. In further alternative implementations, an outlet distribution manifold may be utilized with perforations or finely cut slits that is similar to the inlet manifold outlet  312 . For example, an outlet manifold may prevent or help prevent media  322  inside the vessel  302  from escaping into the outlet  310 . Additionally, for this purpose, a screen, perforated plate, or other containment mechanism may be utilized to keep the media  322  from escaping the tank  300 . 
     In this example, the saltwater outlet  310  is positioned on the top of the vessel sidewall  302  and aligned with a vertical axis of the shell  302 . In alternative implementations, the saltwater outlet  310  may be positioned on the top of the shell  302  but offset from the vertical axis of the shell  302 , or from a lateral side of the shell  302  near the top. 
     In operation, saltwater is circulated (e.g., forcibly) through the inlet  318  and into the saltwater inlet manifold  312 . The saltwater exits, for example, branches  314  of the manifold  312 , such as through perforations or finely machined slits in the branches  314 , outlets at ends of the branches  314 , or both. The saltwater is circulated through the pea gravel  324  and into the media  322 . In the case of a denitrification tank  300 , the saltwater circulated through the media  322  that supports denitrification bacteria. The denitrification process, as earlier described, occurs in the media  322 . Nitrate is transformed, by the bacteria, to nitrite and then to nitrous oxide and/or nitrogen gas, which can exit the tank  300  through a gas release device that may be connected to a gas relief connection  326  positioned at a top of the shell  302 . In the case of a pH balancing tank  300 , the media  322  may include CaCO 3  or aragonite, which dissolves into the saltwater. The treated saltwater flows from the media  322  into the top of the shell  302  and exits the tank  300  through the saltwater outlet  310 . 
       FIG. 4  illustrates a schematic diagram of a control system  400  for a denitrification system. For example, control system  400  may be used to control, e.g., the denitrification system  112 , denitrification system  200 , denitrification system  250 , or other denitrification system in accordance with the present disclosure. As illustrated, the control system  400  includes a controller  402  (e.g., microprocessor based, PLC, electromechanical, pneumatic, or other form of controller) that is communicably coupled to a database  404 , as well as inputs (shown with connecting arrows into the controller  402 ) and outputs (shown with connecting arrows away from the controller  402 ). One or both of the controller  402  and database  404  may store instructions coded on non-transitory computer-readable media that, with the controller  402 , transforms inputs and setpoints (e.g., provided by an operator of the control system  400 ) into outputs to control one or more components of a denitrification system. 
     As illustrated, there may be multiple components of a denitrification system that provide inputs to the controller  402 . For example, a nitrate concentration sensor  406  may be positioned within a denitrification system (e.g., within a flow conduit that carries a flow of saltwater or freshwater that contains nitrate, within a denitrification tank, or elsewhere in contact with the saltwater) to measure a concentration of nitrate in a flow of saltwater (e.g., in parts per million or other units). The measurement of the nitrate concentration (e.g., as a percentage or absolute value) may be transmitted to the controller  402 . In some aspects, the nitrate concentration sensor  406  may measure a nitrate concentration differential between a flow of saltwater entering the denitrification system and a flow saltwater leaving the denitrification system. In some examples, the nitrate concentration sensor  406  may be a NITRATAX from Hach Company. As another example, the sensor  406  may be a Sea-Bird Coastal SUVA Optical Nitrate Sensor from OTT Hydromet. In some aspects, the sensor  406  may include a reagent based analyzer with photo cell for measuring nitrate. The sensor  406  may output a 4-20 mA signal with onboard 24 VDC relays for control of process equipment, e.g., through the controller  402 . 
     The controller  402  may also receive differential pressure (DP) measurements from one or more differential pressure sensors  408  (e.g., shown here as two sensors). For example, with reference to denitrification system  200  shown in  FIG. 2A , DP sensors  408  may be positioned to measure a fluid pressure difference across the pump  208  and between the outlet header  204  and the inlet header  202 . Such measurements may be used by the controller  402 , for example, to determine relative flow rates of the saltwater flowing through the recirculation loop of system  200 . 
     The controller  402  may also receive inputs from one or more flow meters  410  (e.g., five shown here but more or fewer are also contemplated) and  412  (shown as two but may be more or fewer). For example, in some aspects, a flow meter  410  may be positioned at an inlet or an outlet of each tank  206  of the system  200  shown in  FIG. 2A  in order to measure a flow rate of saltwater through each individual tank  206 . In some aspects, flow meters  412  may also be positioned in the recirculation loop at, e.g., one or more of the inlet header  202 , the outlet header  204 , or the bypass header  220 . Each flow meter  410 / 412  may measure a flow rate (in gpm or other units) of the saltwater at the particular location in which the flow meter  410 / 412  is positioned and provide the measurement to the controller  402 . 
     The controller  402  may also receive inputs from a thermostat or temperature sensor  428  that is part of a temperature control system (e.g., temperature control system  242  shown in  FIG. 2A ). The thermostat  428  may periodically provide temperature measurements of, e.g., a saltwater flow within the denitrification system. In turn, the controller  402  may also be communicably coupled to a heater system  424  of the temperature control system that is positioned in a recirculating loop of the denitrification system (e.g., within a bypass conduit of the loop or other location). 
     The controller  402  may also receive inputs from an ORP sensor  430  (e.g., as part of the ozone gas injection and pressurized tank contact system  244  shown in  FIG. 2A ). The ORP sensor  430  can measure the ability of the saltwater  101  to act as an oxidizing or reducing agent. In turn, the controller  402  is also communicably coupled to control an ozone system  426 , e.g., the ozone gas injection and pressurized tank contact system  244 , the ozone system  114 , or both in cases which include two separate ozone systems. 
     The controller  402  may also receive inputs from a dissolved oxygen (DO) sensor  432 . The DO sensor  432  can be positioned in the denitrification system to measure an amount of dissolved oxygen in the saltwater. For example, in some aspects, an oxygen sensor may be placed within the recirculation loop (e.g., at or near an inlet or outlet of one or more of the tanks, in the inlet header, in the outlet header, in the bypass header) to measure a concentration of dissolved oxygen in the saltwater. In some aspects, for example, such as when the denitrification system is an autotrophic denitrification system using facultative denitrification bacteria, the denitrification process may be most efficient when the oxygen concentration is kept relatively low. For instance, facultatively anaerobic bacteria, when located in an anaerobic environment, utilize alternative oxygen sources such as nitrate, as an alternative oxygen source. Thus, the bacteria in a sulphur denitrification system may more efficiently transform nitrate into nitrogen gas when the environment is anaerobic. Anaerobic conditions, however, may be difficult to achieve in a closed system aquaria due to the anaerobic conditions required of such habitats (e.g., due to animal requirements). 
     As illustrated, the controller  402  may be communicably coupled (e.g., wirelessly or wired) to control the operation of multiple components of a denitrification system, such as denitrification system  200 . For example, the controller  402  may be communicably coupled to a CO 2  injection device  414 , which, e.g., can be the CO 2  injection device  228  described in  FIG. 2A . The controller  402  may also be communicably coupled to control valves  416  that are positioned, respectively, e.g., in the bypass header  220  and inlet header  202  in the denitrification system  200 . The controller  402  may also be communicably coupled to control valves  418  and  420  that are positioned, respectively, at inlets of the tanks  206  and outlets of the tanks  206  in the denitrification system  200 . The controller  402  may also be communicably coupled to one or more pumps  422  (one shown here but more or fewer are possible). For example, pump  422  may represent the recirculation pump  208  shown in the denitrification system  200 . 
     In some aspects, valves  416  may be controllably modulated between fully open and fully closed to modulate a flow rate of the saltwater circulated therethrough. In contrast, valves  418  and  420  may be controllably operated to fully shut or fully open. However, in alternative implementations, each valve  416 / 418 / 420  may be a modulating valve (e.g., controlled to modulate between fully open and fully closed). 
     In some aspects, the pump  422  may also be controlled to be on (e.g., full or nameplate power) or off (e.g., no power). Thus, operation of one or more of the valves  416 / 418 / 420  may control flow rates of the saltwater circulating through the denitrification system. Based on the operation of each of the valves  416 / 418 / 420 , some portions of the saltwater may flow at different flow rates as compared to other portions of the saltwater within the denitrification system. In a specific example implementation, the pump  422  may be controlled to circulate the saltwater at a constant or substantially constant flow rate, while the valve  416  positioned in a bypass header (e.g., bypass header  220 ) is modulated to control the flow rate of the saltwater through the denitrification system. Valves  418 / 420  may be isolation valves in this example implementation, controlled (with control system  400  or manually) to be fully open or fully closed. 
     In another specific example implementation, the pump  422  may be controlled to vary a flow rate of the saltwater, such as by a variable frequency drive, step-motor, or other motor controller. In this specific implementation, all of the valves  416 / 418 / 420  may be isolation valves (e.g., fully open or fully closed) and the pump  422  is responsible for controlling and modulating the flow rate of the saltwater in the recirculation loop of the denitrification system. In some cases, even in this example, the valve  416  in the bypass header may be a modulating valve. 
     The controller  402  can receive the example inputs described previously and use one or more of the inputs and setpoints stored in the database  404  in one or more operational algorithms stored as instructions in the controller  402  to control the components of the denitrification system. In an example operation of the controller  402 , a nitrate concentration measurement may be sent to the controller  402  from the nitrate concentration  406 . In some cases, this measurement may be sent dynamically (e.g., in real time as saltwater is flowing through the denitrification system) and periodically (e.g., every minute, every hour, every day, or other time period). The controller  402  compares the measured nitrate concentration in the saltwater against a nitrate setpoint value (or setpoint range), e.g., set by an operator and stored in the controller  402  or database  404 . If the measured concentration is less than or equal to the setpoint, the controller  402  may take no action. 
     If the measured concentration is above the setpoint value or upper end of a setpoint range, the controller  402  may provide signals to one or more components of the denitrification system in order to decrease the concentration of nitrate within the saltwater. For example, the controller  402  may signal valve  416  in the bypass header (or any of valves  416 / 418 / 420 , independently or in combination) to increase a percent open state of the valve. In some aspects, for example, as the valve  416  in the bypass header opens, more saltwater from outlets of the denitrification tanks is recirculated back to inlets of the denitrification tanks. This may increase a hydraulic retention time of the saltwater within the denitrification system (e.g., within the recirculation loop of the denitrification system). As hydraulic retention time of the saltwater in the denitrification system increases (e.g., as a number of passes a particular volume of the saltwater flows through the denitrification tanks), more nitrate may be changed to nitrite and then to nitrogen gas, thereby decreasing a nitrate concentration in the saltwater. As the controller  402  receives additional nitrate concentration measurements from the nitrate concentration sensor  406 , the controller  402  may continue to open the valve  416  or cease opening the valve  416  (e.g., when the measured value is less than the setpoint). 
     The controller  402  may also or alternatively signal pump  422  to decrease a flow rate of the saltwater circulating within the recirculation loop of the denitrification system. For example, as the pump  220  slows down, the saltwater within the recirculation loop spends more time circulating (e.g., vertically from bottom to top) through the denitrification tanks. This may also increase a hydraulic retention time of the saltwater within the denitrification system (e.g., within the recirculation loop of the denitrification system), which, as described previously, may increase a rate of change of nitrate to nitrogen gas in the denitrification process. As the controller  402  receives additional nitrate concentration measurements from the nitrate concentration sensor  406 , the controller  402  may continue to decrease the speed of the pump  422  or cease changing the speed of the pump  422  (e.g., when the measured value is less than the setpoint). 
     The controller  402  may also or alternatively signal CO 2  injection device  414  to increase an amount of CO 2  injected into a pH balancing tank of the denitrification system. For example, as more CO 2  is injected into the pH balancing tank, the CaCO 3  or aragonite within the tank may more quickly dissolve in the saltwater. As the concentration of CaCO 3  or aragonite within the saltwater in the recirculation loop increases, the denitrification process may also increase. As the controller  402  receives additional nitrate concentration measurements from the nitrate concentration sensor  406 , the controller  402  may continue to increase the rate of CO 2  injected into the pH balancing tank or keep the CO 2  injection rate constant (e.g., when the measured value is less than the setpoint). 
     In another example operation, the controller  402  may compare the temperature measurements from the sensor  428  to a setpoint temperature and, based on the comparison, operate the heater  424  (or other component, such as one or more valves  416 / 418 ) to maintain the saltwater flow at or above the predetermined set point. For example, if the measured temperature is too low, the heater  424  may be actuated or increased in power, or one or more valves  416 / 418  may be adjusted to limit the flow, to heat the flow of saltwater. 
     In other example operations, the DO sensor  432  measures an amount of dissolved oxygen in the saltwater and provides the measurement to the controller  402 . The controller  402  may compare the measured DO against a setpoint value for DO, e.g., set by an operator and stored in the controller  402  or database  404 . In some aspects, the setpoint may be below about 3.0 mg/l or 40% saturation, which are considered minimums for denitrification to occur. In some aspects, the setpoint may be between about 0.10 mg/l to 2.0 mg/l for ideal SDN to occur. 
     The DO measurement may be sent dynamically (e.g., in real time as saltwater is flowing through the denitrification system) and periodically (e.g., every minute, every hour, every day, or other time period) from the sensor  432  to the controller  402 . If the measured DO is less than or equal to a setpoint (or within a setpoint range), the controller  402  may take no action. If the measured concentration is greater than the setpoint value or outside of a setpoint range, the controller  402  may provide signals to one or more components of the denitrification system in order to decrease the concentration of oxygen within the saltwater. For example, to reduce a DO concentration, the controller  402  may control a valve  416  in an inlet header to circulate less saltwater from the source into the denitrification system. By reducing a flow rate of the saltwater into the denitrification system, a hydraulic retention rate of the recirculating saltwater is increased and DO decreases. 
     In other example operations, a theoretical nitrate removal may be calculated by the controller  402 . For example, the controller  402  may calculate the theoretical nitrate removal by using measured values of a flow rate passing through the denitrification system and returning to the saltwater water source (e.g., the second loop) and a difference between an entering nitrate concentration and a leaving nitrate concentration (e.g., entering the second loop and leaving the second loop). Such a difference may be a delta nitrate for the SDN process and can be used by the controller  402  to calculate a net nitrate flux for the SDN process. This net nitrate flux can be displayed as an input and possibly entered as a set point for the controller  402 , thus creating outputs for the controller  402  to attempt to achieve or maintain. 
     The controller  402 , during these processes, may also periodically receive other values, such as differential pressure values from sensors  408  and flow rate values from sensors  410  and  412  and compare such values to corresponding setpoints (or setpoint ranges) of differential pressure and flow rate to ensure that such measured values are within acceptable ranges. For example, the controller  402  may implement a feedback loop that periodically receives such measurements as differential pressure values, flow rates, temperatures, and other measurements from sensors in the denitrification system and compares such values to stored setpoints (or setpoint ranges) associated with these values (e.g., set by an operator of the control system  400 ). If the measured values do not meet the setpoints (or are outside of setpoint ranges), the controller  402  may adjust one or more components (e.g., valves  416 / 418 / 420 , pump  422 , or other component) until the measured values meet the setpoints or are within the setpoint ranges. 
     In some aspects, the controller  402  may compare values of input devices such as flow, temperature, dissolved oxygen, pH, or ORP, and compare the measured values against predetermined alarm parameters that can be set in the database  404 . If the input parameters are measured as out of safe range, then an executable action can be taken to shut down or isolate the system and create a “lock-out” condition that must be cleared by an operator before normal running conditions can be re-established. The alarms can also be generated as an output device including visible, audible, or notification alarm sent out as a phone or text based message using wireless or IP connections. 
       FIG. 5  is a schematic diagram of a controller  500  for a control system, such as the controller  402  for control system  400 . The controller  500  can be used to carry out the operations described in association with any of the computer-implemented methods described previously, according to some implementations, such as implementations of an injectant detection system and process. In some implementations, computing systems and devices and the functional operations described in this specification can be implemented in digital electronic circuitry, in tangibly-embodied computer software or firmware, in computer hardware, including the structures disclosed in this specification (e.g., control system  400 ) and their structural equivalents, or in combinations of one or more of them. The controller  500  is intended to include various forms of digital computers, such as laptops, desktops, workstations, personal digital assistants, servers, blade servers, mainframes, and other appropriate computers, including vehicles installed on base units or pod units of modular vehicles. The controller  500  can also include mobile devices, such as personal digital assistants, cellular telephones, smartphones, and other similar computing devices. Additionally the system can include portable storage media, such as, Universal Serial Bus (USB) flash drives. For example, the USB flash drives may store operating systems and other applications. The USB flash drives can include input/output components, such as a wireless transmitter or USB connector that may be inserted into a USB port of another computing device. 
     The controller  500  includes a processor  510 , a memory  520 , a storage device  530 , and an input/output device  540 . Each of the components  510 ,  520 ,  530 , and  540  are interconnected using a system bus  550 . The processor  510  is capable of processing instructions for execution within the controller  500 . The processor may be designed using any of a number of architectures. For example, the processor  510  may be a CISC (Complex Instruction Set Computers) processor, a RISC (Reduced Instruction Set Computer) processor, or a MISC (Minimal Instruction Set Computer) processor. 
     In an implementation, the processor  510  is a single-threaded processor. In another implementation, the processor  510  is a multi-threaded processor. The processor  510  is capable of processing instructions stored in the memory  520  or on the storage device  530  to display graphical information for a user interface on the input/output device  540 . 
     The memory  520  stores information within the controller  500 . In one implementation, the memory  520  is a computer-readable medium. In one implementation, the memory  520  is a volatile memory unit. In another implementation, the memory  520  is a non-volatile memory unit. 
     The storage device  530  is capable of providing mass storage for the controller  500 . In one implementation, the storage device  530  is a computer-readable medium. In various different implementations, the storage device  530  may be a floppy disk device, a hard disk device, an optical disk device, or a tape device. 
     The input/output device  540  provides input/output operations for the system  500 . In one implementation, the input/output device  540  includes a keyboard and/or pointing device. In another implementation, the input/output device  540  includes a display unit for displaying graphical user interfaces. 
     The features described can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. The apparatus can be implemented in a computer program product tangibly embodied in an information carrier, for example, in a machine-readable storage device for execution by a programmable processor; and method steps can be performed by a programmable processor executing a program of instructions to perform functions of the described implementations by operating on input data and generating output. The described features can be implemented advantageously in one or more computer programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. A computer program is a set of instructions that can be used, directly or indirectly, in a computer to perform a certain activity or bring about a certain result. A computer program can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. 
     Suitable processors for the execution of a program of instructions include, by way of example, both general and special purpose microprocessors, and the sole processor or one of multiple processors of any kind of computer. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for executing instructions and one or more memories for storing instructions and data. Generally, a computer will also include, or be operatively coupled to communicate with, one or more mass storage devices for storing data files; such devices include magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; and optical disks. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, ASICs (application-specific integrated circuits). 
     To provide for interaction with a user, the features can be implemented on a computer having a display device such as a CRT (cathode ray tube) or LCD (liquid crystal display) monitor for displaying information to the user and a keyboard and a pointing device such as a mouse or a trackball by which the user can provide input to the computer. Additionally, such activities can be implemented via touchscreen flat-panel displays and other appropriate mechanisms. 
     The features can be implemented in a computer system that includes a back-end component, such as a data server, or that includes a middleware component, such as an application server or an Internet server, or that includes a front-end component, such as a client computer having a graphical user interface or an Internet browser, or any combination of them. The components of the system can be connected by any form or medium of digital data communication such as a communication network. Examples of communication networks include a local area network (“LAN”), a wide area network (“WAN”), peer-to-peer networks (having ad-hoc or static members), grid computing infrastructures, and the Internet. 
     The computer system can include clients and servers. A client and server are generally remote from each other and typically interact through a network, such as the described one. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other. 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any inventions or of what may be claimed, but rather as descriptions of features specific to particular implementations of particular inventions. Certain features that are described in this specification in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described previously as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the implementations described previously should not be understood as requiring such separation in all implementations, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
     Thus, particular implementations of the subject matter have been described. Other implementations are within the scope of the following claims. In some cases, the actions recited in the claims can be performed in a different order and still achieve desirable results. In addition, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. In certain implementations, multitasking and parallel processing may be advantageous. 
     Thus, particular implementations of the present disclosure have been described but others are contemplated by the present disclosure as well. For example, although the present disclosure uses saltwater as an example liquid to be treated, e.g., through denitrification, pH balancing, and other treatments, the concepts discussed herein can be applied to non-saltwater sources as well, including freshwater habitats that support fish and other animals. Other implementations are within the scope of the following claims.