Patent Publication Number: US-2021171375-A1

Title: Pretreatment to remove ammonia from high strength wastewater with memrbane aerated biofilm sidestream

Description:
FIELD 
     This specification relates to wastewater treatment and to membrane aerated biofilm reactors. 
     BACKGROUND 
     Many wastewaters with high ammonia concentration, for example those found in industrial effluents or from dewatering municipal sludge, tend to be alkalinity limited. When alkalinity is depleted the pH of the wastewater drops and further biological nitrification is inhibited. This limits the potential to remove ammonia by biological nitrification. 
     In some treatment processes, chemicals (for example caustic soda) are added to increase the alkalinity of wastewater. In other treatment processes, attempts are made to recover alkalinity biologically, for example by adding a carbon source to enhance biological denitrification or by trying to recover alkalinity through the anammox pathway. However, these processes may be expensive to operate or difficult to control. 
     In some cases, wastewater with a high ammonia concentration is discharged into a municipal wastewater treatment plant (WWTP). While the high strength wastewater is diluted by low strength wastewater such as municipal sewage also being treated in the WWTP, the ammonia loading to the WWTP increases. This can cause operational problems in the WWTP or cause the effluent ammonia concentration to exceed the effluent ammonia permit. 
     INTRODUCTION 
     The following is meant to introduce the reader to the invention, but not to limit or define any claimed invention. 
     In a process described herein, high strength wastewater (in particular, wastewater with high ammonia concentration) is pre-treated before discharging it into a wastewater treatment plant treating lower strength wastewater, for example an activated sludge plant treating municipal sewage. The high strength wastewater is pre-treated biologically by contact with a fixed film supported on gas transfer membranes. The pre-treatment may be a batch or continuous process. In some examples, the pre-treatment is controlled to remove ammonia to about the point of material alkalinity depletion. Optionally, one or more parameters such as alkalinity, pH, or membrane exhaust oxygen concentration can be monitored to determine if alkalinity depletion has occurred or is about to occur. In some examples, the high strength wastewater is blended with wastewater having less ammonia relative to its non-ammonia alkalinity, for example wastewater having much more than 1 mol of alkalinity per mol of ammonia (more than 3.57 mg CaCO 3  equivalent per mg NH 4   + —N), for example municipal sewage or primary effluent. Optionally, the high strength wastewater may include a liquid fraction of one or more of anaerobic digester sludge, primary sludge, or activated sludge, for example a liquid fraction of anaerobic digester sludge. 
     In a system described herein a membrane aerated biofilm reactor (MABR) is combined with an activated sludge system. The MABR includes one or more membrane aerated biofilm modules. The system is configured so that the MABR receives high strength wastewater and discharges an effluent to the activated sludge system. In use, oxygen is supplied to the modules and an attached growth is present in the MABR in the form of a fixed film supported by gas transfer membranes of the modules. Ammonia (i.e. ammonium or NH 4   + —N) is nitrified or otherwise oxidized in an attached biofilm and thereby removed from the high strength wastewater feed water before it enters the activated sludge system. Optionally, a portion of municipal sewage flowing to the activated sludge system may also be treated in the MABR before being treated in a process tank of the activated sludge system. Optionally, the system may be configured to convey a liquid fraction of one or more sludges generated in the activated sludge system or an anaerobic digester to the MABR as the high strength wastewater. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       The Figure is a process flow diagram of a wastewater treatment plant  10 . 
     
    
    
     DETAILED DESCRIPTION 
     High strength wastewater can have, for example, 1000 mg/L or more of ammonia. In some cases, the high strength wastewater may also be alkalinity-deficient, meaning that the wastewater does not have enough alkalinity to allow for biological nitrification of all of the ammonia in the wastewater. In a process to be described in more detail below, the high strength wastewater is treated by way of biological oxidation (i.e. nitrification) in the biofilm of a membrane aerated biofilm reactor (MABR) before being treated further in an activated sludge process. The high strength wastewater may be treated in a continuous flow or batch feed process in the MABR. Optionally, the MABR does not have collection and recycle of active solids and so does not have material suspended growth. Effluent from the MABR may be mixed with, for example, low strength wastewater such as municipal sewage, optionally by blending the MABR effluent with, for example, influent wastewater, primary effluent, mixed liquor or return activated sludge in an activated sludge plant. Optionally, the high strength wastewater may be diluted, for example with belt filter press washwater, before being treated in the MABR. 
     Optionally, the MABR can be operated so as to remove ammonia to about the point of alkalinity depletion. In a batch feed process, for example, the batch reaction can be ended when alkalinity is depleted to about a level at which pH depression becomes inhibitory to biological activity. The lack of sufficient alkalinity may be determined by directly measuring alkalinity and/or ammonia, or by measuring a related parameter such as pH or membrane exhaust oxygen concentration or both. For example, alkalinity depletion can be indicated by a pH of wastewater in the MABR at or below a specified threshold value, for example a value in the range of about 6 to 6.5. Alkalinity depletion can also be indicated by a rapid increase in membrane exhaust oxygen concentration, for example relative to an average concentration over a preceding hour or more., or by membrane exhaust oxygen concentration at or above a specified threshold, for example a value in the range of about 15% to 15.5% or a different range determined by site specific conditions. An increase in membrane exhaust oxygen concentration that is not attributable to a change in oxygen feed rate indicates that microorganisms in the biofilm are being inhibited from oxidizing (i.e. nitrifying) ammonia. A continuous MABR process may also be configured to operate at about the point of alkalinity depletion by adjusting one or more process parameters such as feed rate or residence time upon sensing alkalinity depletion. However a continuous process may be more difficult to control than a batch process while allowing for variations in process conditions (i.e. diurnal variations in influent sewage flow rates) and so might require a higher pH threshold or lower membrane exhaust oxygen concentration threshold. 
     Optionally, wastewater with a lower ammonia to alkalinity ratio may be mixed with the high strength water and treated in the MABR. Adding lower ammonia to alkalinity wastewater (for example municipal sewage or primary effluent of municipal sewage) allows for removal of more ammonia in the MABR. Optionally, the amount of lower ammonia to alkalinity wastewater added can be varied over time to reduce the effect of variations in other process conditions. Optionally, the blend of high strength wastewater to lower ammonia to alkalinity wastewater can be adjusted to achieve at least a sufficient ratio of alkalinity to ammonia in the combined stream such that ammonia removal in the MABR is not limited by lack of alkalinity. For example, the combined stream may be blended to have at least about 2 mol alkalinity per mol of ammonia (7.14 mg CaCO 3  eq per mg NH 4  as N). 
     The Figure shows a wastewater treatment system  10 . Wastewater  12 , for example municipal sewage, is fed into a primary clarifier  14 . The primary clarifier  14  produces primary sludge  16  and primary effluent  18 . Primary effluent  18  flows into one or more process tanks  20 . In the example shown, process tank  20  has a grid of aerators  22 , for example fine bubble aerators. The aerators  22  are supplied with air from a blower  36  in an amount sufficient to keep mixed liquor  24  aerobic at least while it is in the process tank  20 . Mixed liquor  24  flows to a secondary clarifier  26 . Secondary clarifier  26  produces plant effluent  28  and activated sludge  30 . The activated sludge  30  is split into waste activated sludge  32  and return activated sludge  34 . Return activated sludge  34  returns to process tank  20 . 
     Some suspended solids in the wastewater are removed in the primary clarifier  14 . In other examples, the primary clarifier  14  is not present and influent suspended solids pass through the process tank  20  to the secondary clarifier  26 . Some of the remaining organic compounds (i.e. biological oxygen demand (BOD)) are removed by suspended biomass in the process tank  20 . In particular, ammonia is converted to nitrate by nitrifying bacteria. In other activated sludge systems, more process tanks  20  are added. For example, adding an anoxic tank allows total nitrogen to be reduced by way of a denitrification process. The primary clarifier  14 , if any, one or more process tanks  20  and secondary clarifier  26  form an activated sludge reactor or system  8 . 
     An MABR tank  38  contains one or more membrane aerated biofilm modules (MABM)  40 . MABR aerators  48 , for example coarse bubble aerators or pulsing aerators, are added below the MABM  40  to periodically scour the biofilm and/or refresh water in or around the MABM  40 . Excess biofilm released form the MABM  40 , or solids that settle in the MABR tank  38 , can be removed as MABR solids  47 . MABR solids  47  can be removed directly from the MABR tank  38  as shown or from a downstream separator. Optionally, MABR solids  47  can be sent to an anaerobic digester  58 . 
     High strength wastewater  42  flows into the MABR tank  38 . Optionally, an equalization tank  44  may be added to allow the flow rate of high strength wastewater  42  to be adjusted. Optionally, a screen may be added upstream of the MABR tank  38  to protect the MABM  40  from large solids or excessive amounts of fibers or hair. High strength wastewater  42  has a higher ammonia concentration than wastewater  12 . 
     MABR effluent  46  flows to the process tank  20  optionally directly or by being blended with the wastewater  12  or primary effluent  18  or added to the primary clarifier  14 . 
     The flow of MABR effluent  46  to the process tank  20  may be continuous or intermittent. In another option, a portion of the MABR effluent  46  (which can temporarily include all of the MABR effluent  46 ) may be continuously or periodically recycled to the MABR tank  38 . This can be done, for example, to mix the contents of the MABR tank  38  or to adjust the residence time of the MABR tank  38 . 
     In the example shown, waste activated sludge  32  passes through a thickener  50 . The thickener  50  produces thickened waste activated sludge (TWAS)  54  and filtrate  56 . TWAS  54  and primary effluent  16  are treated in an anaerobic digester  58 . Digestate (alternatively called digester sludge)  60  is separated in dewaterer  60  to produce a centrate  52 . Centrate  52  and filtrate  56  make up high strength wastewater  42 . In other examples, the primary sludge  16  may also pass through the same thickener  50  or a different thickener, or waste activated sludge  32  might not be thickened, such that filtrate  56  could be a liquid fraction of one or both of the waste activated sludge  32  and primary sludge  16 . In other examples, only one or the sludges may be treated in digester  58  or there might be no digester  58 . In another option, digestate  60  may be thickened rather than dewatered. 
     Any type of solid-liquid separation device may be used for thickener  50  or dewaterer  60 . While sludge from the activated sludge process is typically thickened and sludge from an anaerobic digester is typically dewatered, the concentration of the solids fraction from either does not need to be within either a thickening or dewatering range. The words “filtrate” and “centrate” are used for convenience to refer to a liquid fraction of solid-liquid separation generally and are not limited to the specific products or a filter or centrifuge. In other options, only one of the filtrate  56  or centrate  52  makes up high strength wastewater  42 . In other examples, the high strength wastewater  42  might be entirely or partially supplied from a source outside of system  10 , for example from industrial (i.e. industrial process, agricultural or mining) wastewater. 
     MABR blower  64  supplies air to the MABM  40 . The MABR aerators  48  can receive air from blower  64  (or another blower) directly or in the form of exhaust air that has passed through the MABM  40 . The MABR aerators  48  typically receive air only periodically. The flow of air to the MABM  40  is typically on. The flow rate can be modulated or constant. The air flow rate can be selected to prevent oxidation (i.e. nitrification) in the biofilm from being inhibited by lack of oxygen. In the case of a modulated airflow, the rate can be varied considering one or more process measurements, for example ammonia concentration in the influent or water in the MABR tank  38 . An oxygen concentration sensor  66  can be provided on an oxygen exhaust line  68  from one or more of the MABM  40 . A pH sensor  69  can be provided in communication with wastewater in the MABR tank  38 . 
     The MABM  40 , MABR tank  38  and associated equipment such as blower  64  and MABR aerators  48  forms a membrane aerated biofilm reactor (MABR)  70 . The high strength wastewater  42  is treated first in the MABR  70 . Nitrification takes place on a fixed biofilm that grows on membranes of the MABM  40 . Since the biofilm is attached, the MABR  70  does not require capture of mixed liquor suspended solids in a clarifier or with a filtration membrane or other solid separation device. 
     In one option, the MABR  70  is operated in a batch process. High strength wastewater  42  maybe drawn periodically from equalization tank  44  and added to MABR tank  38 . Alternatively, high strength wastewater  42  may be added to MABR tank  38  as it is produced, for example in a WWTP that performs sludge dewatering for 4-12 hours per day. A corresponding amount of MABR effluent  46  is sent to the activated sludge reactor  8 . The amount of high strength wastewater  42  added to start a new batch may be about equal to or less than the volume of the MABR tank  38 . Ammonia oxidizing (i.e. nitrifying) bacteria are maintained in the MABR  70  between batches because they remain in a biofilm attached to the membrane media. 
     Optionally, a batch may be maintained until the alkalinity of the high strength wastewater  42  is materially depleted. Material depletion of alkalinity can be signaled, for example, by the pH decreasing to or below a specified threshold, for example a threshold in the range of 6 to 6.5. Alternatively, material depletion of alkalinity can be signaled, for example, by the membrane exhaust oxygen concentration increasing rapidly, increasing to or above a specified threshold for example a threshold in the range of 15% to 15.5%, or increasing relative to a plot of oxygen concentration to ammonia concentration produced without alkalinity depletion and at the same or a correlated oxygen feed rate. When material alkalinity depletion is imminent or has occurred to a small degree, the reactor is flushed and at least some its contents replaced with a fresh batch of wastewater. 
     In another option, the MABR is operated in a continuous process. In continuous mode operation, the high strength, alkalinity-deficient wastewater is blended with lower strength alkalinity-abundant dilution water. The blending ratio can be optionally adjusted to maintain an alkalinity to ammonia ratio within a selected range or above a selected threshold. For example, the blending ratio can be selected to provide at least 2 mol alkalinity per mol of ammonia in the wastewater blend. Alternatively, the blending ratio can be selected to be sufficient to avoid material depletion of alkalinity as signaled, for example, by a pH at or above a specified threshold for example in the range of 6 to 6.5; the membrane exhaust oxygen concentration being at or above a specified threshold for example a threshold in the range of 15% to 15.5%, or the membrane exhaust oxygen concentration being above a plot of oxygen concentration to ammonia concentration produced without alkalinity depletion and at the same or a correlated oxygen feed rate. 
     In the MABR, oxygen is delivered to the ammonia oxidizing (i.e. nitrifying) organisms in the biofilm through the supporting membranes in a “bubble-less” diffusion process. The diffusion based gas transfer process is more efficient than fine bubble aeration. De-nitrification can be provided in an anoxic outer layer of the biofilm, in bulk water outside of the biofilm, or in an anoxic zone of the wastewater treatment plant. Optionally, the process may use “shortcut” nitrogen removal pathways i.e. nitrite shunt, partial nitritation, or partial nitritation/anammox (deammonification) rather than conventional nitrification-denitrification. 
     Additional information regarding MABRs and their operation can be found in US Publication Number 2016/0009578 A1, Membrane Assembly for Supporting a Biofilm; US Publication Number 2017/0088449 A1, Wastewater Treatment with Primary Treatment and MBR or MABR-IFAS Reactors; and, International Publication Number WO 2016/209235 A1, Floating Apparatus for Membrane Biofilm Reactor and Process for Water Treatment, which are incorporated herein by reference. 
     US Publication Number 2016/0009578 A1, Membrane Assembly for Supporting a Biofilm, describes a cord for supporting a biofilm. The cord has a plurality of yarns, including at least one yarn having a plurality of hollow fiber gas transfer membranes. A module has a plurality of the cords potted in at least one header. A reactor has a module immersed in a tank of water to be treated. Air is supplied to the module and a biofilm forms over the cords. 
     US Publication Number 2017/0088449 A1, Wastewater Treatment with Primary Treatment and MBR or MABR-IFAS Reactors, describes various processes for treating wastewater. A gas transfer membrane is immersed in water and pressurized air is supplied to the membrane. In one example, the gas transfer membranes are added to a process tank in an activated sludge reactor. The membrane supports an oxygenated nitrifying biofilm, which is immersed in an anoxic suspended biomass in the tank. 
     International Publication Number WO 2016/209235 A1, Floating Apparatus for Membrane Biofilm Reactor and Process for Water Treatment, describes an apparatus having a plurality of gas transfer membranes. The apparatus floats in water with the membranes submerged in the water. The apparatus can be added to a process tank in a conventional activated sludge reactor. 
     EXAMPLE 
     A membrane aerated biofilm reactor (MABR) was operated at a municipal waste water treatment plant. The MABR was used to reduce ammonia in filtrate from a filter-press fed with anaerobic digester sludge. The MABR was operated as a batch process. 130 L batches of filtrate were added to the MABR and recirculated through the MABR tank which had a volume of 800 L. Each batch was operated by displacement, wherein 130 L was added to the existing 800 L, displacing 130 L out of the process. The filtrate contained 1960 mg/L ammonia, 4690 mg/L alkalinity, 2180 mg/L COD. After a batch of filtrate was added to the MABR tank, the concentration in the tank was about 740 mg/L of ammonia and 800 mg/L of alkalinity. Each batch was operated for three to five days, and during that time process air supplied to the membranes of the MABR modules was the sole oxygen source for nitrification. During each batch the ammonia concentration in the reactor was measured periodically, along with pH, alkalinity, nitrate, nitrite, and COD. The exhaust gas from the MABR modules was also measured for oxygen concentration to determine the mass transfer of oxygen through the membranes. A batch was considered complete when alkalinity was depleted to the point of inhibiting nitrification, which was indicated by the pH decreasing to about 6.5 or less. 
     Ammonia measurements from the batch testing resulted in observed nitrification rates between 3.5 and 17 g-NH 4 —N/d/m 2 , measured as the mass of ammonia nitrified per day per square meter of membrane surface area. The average observed nitrification rate was 8.4 g-NH 4 —N/d/m 2 . The percentage removal of ammonia during each batch was 20%, and was limited by the alkalinity available in the filtrate. Corresponding oxygen transfer rates ranged from 18 to 28 g-O 2 /d/m 2 , averaging 20.3 g-O 2 /d/m 2 . 
     As a secondary test, volatile fatty acids (VFA), sourced from an acid-phase digester on site, was added to the MABR at the end of a batch run, (i.e. when nitrification had slowed due to lack of alkalinity and pH was &lt;6) to provide carbon for denitrification. The purpose was to test the ability of the system to produce alkalinity by denitrification that could be used for further nitrification by the MABR. The test proved effective and produced further nitrification at rates averaging 6.2 g-NH 4 —N/d/m 2 . 
     Aeration efficiency rates for the MABR modules ranged from 3.3 to 5 kg-O 2 /kWh, averaging 4.5 kg-O 2 /kWh under batch test conditions. Typical fine bubble aeration efficiency rates range from 1-2 kg-O 2 /kWh. The MABR modules were more energy efficient than fine bubble aeration for removal of side stream ammonia loads, based on these test conditions. The amount of oxygen consumed per unit of ammonia removed was less than the amount required for nitrification indicating that at least some ammonia might be removed in the MABR by nitritation or another ammonia oxidation pathway. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.