Abstract:
The invention provides a process and apparatus for the recovery of heat energy from wastewater. Wastewater, for example grey water from a domestic residence, is introduced to a detention chamber, which provides effective decoupling between the introduction of new wastewater and the demand for heat energy from its ultimate application.  A heat exchange surface, in contact with the wastewater on one side and a working fluid on the other, extracts heat from the detention chamber through thermal conduction and the working fluid is transferred, via a heat pump, to a second heat exchange surface. The second heat exchange surface, in contact with the working fluid on one side and heat energy storage media on the other, transfers heat energy to the storage media through conduction. Heat energy can then be extracted from the storage media for applications including heating of potable water, or provision of building heating.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to a process and apparatus for the removal of residual heat from wastewater for reuse. The preferred embodiment described as an example herein is particularly suited to the removal of heat from grey water in domestic residences, for later reuse in supplying energy for water heating in the home. 
       BACKGROUND 
       [0002]    Globally, two key factors reducing the environmental sustainability of housing are water consumption and energy use. Consumption of these resources is increasing due to increasing population. Pollution is reducing the availability of fresh water, while efforts to reduce pollution increase energy production costs. The problem is compounded by localized population growth in greenfield developments in sunbelt areas (where water resources and infrastructure are scarce) and by the effects of a changing climate. With the energy embedded in potable water, through purification and distribution, accounting for a significant proportion of global energy generation—the ‘water-energy nexus’—opportunity exists to target both problems in an integrated way. 
         [0003]    Wastewater discharged from domestic premises is a notable loss of energy from the typical home. Reduced energy consumption through the recovery or elimination of waste heat has seen increasing adoption, through heat recovery ventilation (HRV) systems, low-e glass and improved insulation; yet wastewater heat has received little attention. It is a challenge for two reasons: the contaminant load will quickly block efficient, single pass heat exchangers, and the irregular pattern of waste production limits the amount of heat that can be easily recovered. Incumbent solutions simplify the process by targeting relatively clean shower water (where warm waste and a requirement for delivery of hot water coincide), but this does not tap the full potential for energy savings. 
         [0004]    In situations where wastewater treatment is also desirable , the removal of heat from the wastewater stream presents additional challenges for the treatment methodology. Biological systems, for example, rely on the action of microorganisms to digest contaminants, but these processes are slowed as the temperature is reduced. Similarly, low temperatures reduce the efficiencies of many filtration and chemicals systems. Some flocculation processes, for example, fail completely outside an optimal temperature range. 
         [0005]    Newly developed wastewater treatment processes (such as PCT2010902814) eliminate this temperature dependence, enabling the integrated approach described by this invention. 
         [0006]    Existing methods of high efficiency water heating technologies include solar collectors and air-sourced heat pump systems, but these technologies suffer major flaws that have prevented their widespread adoption. Solar applications operate only during daylight hours and are dependent on weather (producing reduced output during cloudy weather). While evacuated tube solar collectors are largely independent of outside temperature, they are vulnerable to hail. Alternatives can be subject to bursting during freezing temperatures. Air-sourced heat pumps present an alternative, but their efficiency is highly dependent on outside air temperature, making them unsuitable in cold climates. Passive systems are also available and, while cost effective, they do not preform reliably, place waste and potable streams in close proximity, and do not tolerate highly contaminated waste streams. 
         [0007]    There is, therefore, a need in many circumstances for a more reliable, robust form of heat capture device that can supply the domestic hot water needs in an efficient way. It must also be compatible with wastewater treatment technology to ensure that direct energy consumption for water heating, and the flow-on effects of supplying water, can be addressed. 
       SUMMARY OF THE INVENTION 
       [0008]    In one respect, the current invention resides in a process for the removal of heat energy from wastewater that comprises:
       a) Collection of wastewater in a reservoir;   b) Transfer of heat energy from the collected wastewater into a working fluid;   c) Transport of the working fluid;   d) Transfer of the heat energy from the working fluid to a heat energy reservoir; and   e) Transfer of the heat energy, on an as-needed basis, from the heat energy reservoir to the incoming potable water supply.       
 
         [0014]    According to the embodiment described, the preferred method of heat transfer is using a heat pump, where the working fluid is a refrigerant gas. Many commercially available refrigerant gases are suitable, including R-134a, R-417a, R-744, R-600a, R-410a; with R-417a being the most preferred according to this invention. 
         [0015]    Preferably, the process further comprises:
       a) Passing the influent wastewater over a filter to remove particles larger than approximately 200 μm;   b) Delivery of the wastewater, with heat energy removed, to a wastewater treatment system for water recovery.       
 
         [0018]    In another respect, the invention resides in an apparatus for the removal of heat energy from wastewater. The apparatus comprises:
       a) A device allowing seamless interconnection between the wastewater supply and the process described herein;   b) A heat exchange surface in contact with the wastewater;   c) A heat storage reservoir containing heat storage media;   d) Heat exchange surfaces in contact with the heat storage media;   e) Plumbing to transfer working fluid between the heat exchange surfaces;       
 
         [0024]    In the preferred embodiment, the apparatus further comprises:
       a) A compressor to move the working fluid between the heat exchange surfaces;   b) A thermostatic expansion valve, or a capillary tube, to promote a phase change in the working fluid;   c) Temperature measuring devices;   d) A thermostatic control system;   e) A supplemental, backup heating device;   f) Phase change media in the heat storage reservoir; and   g) Plumbing connecting the potable water supply to the heat storage device;       
 
         [0032]    Additionally, the interconnection device may further comprise one or more of the following:
       a) A detention chamber to buffer and smooth peak loads on the system;   b) A wastewater backflow prevention device;   c) A pump for the transfer of cooled wastewater to a treatment system;   d) A level sensor to control the pump;   e) An integrated filter capable of removing particles larger than approximately 200 μm;   f) A sealed lid to prevent the ingress of water;   g) A vent to encourage reliable flow of water;       
 
         [0040]    The process according to this invention is ideally suited to the treatment of domestic grey water. ‘Grey water’ is wastewater produced from fixtures including showers, hand sinks and laundry facilities. These fixtures are not designed for the collection of human excrement or discharges, and faeces or urine does not grossly contaminate the resulting wastewater. 
         [0041]    Grey water fixtures generally include major sources of hot water consumption, and have warmer resulting waste streams. It is important that these are captured to ensure the invention described herein operates at the highest efficiency. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0042]    The following drawings describe, in a non-limiting way, the invention with respect to a preferred embodiment: 
           [0043]      FIG. 1 . Describes the process of the invention and how it may be installed to capture heat energy from domestic grey water. 
           [0044]      FIG. 2 . Presents a detailed cross-section of an appropriate interconnection device. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0045]    The invention resides in a process for the recovery of heat energy from waste water. The invention is suitable to application to many types of wastewater; preferably this water is generated in domestic residences. The invention can also be applied to other waste streams where warm water is generated, and a need to heat incoming water coincide. Water processed according to this invention must be above 0° Celsius, and preferably wastewater should be generated between 15 and 65 degrees. 
         [0046]    According to  FIG. 1 , a preferred embodiment is depicted in which the invention is coupled to the grey water sources ( 100 ) in a domestic residence ( 101 ). Wastewater is directed, via separated plumbing ( 102 ) designed to maintain separation between grey water and more heavily contaminated streams, to an interconnection device ( 103 ). The interconnection device has a number of functions, including acting as a buffer to decouple the generation of wastewater from the treatment, heat capture and reuse processes. Each of these processes operates according to a differing (regular or irregular) schedule, and the decoupling effect allows them to be interconnected. Inside the interconnection device, a heat exchange surface ( 104 ) is placed in contact with the collected wastewater. The device additionally comprises connection to the conventional sewer system ( 105 ) or an alternative method of wastewater disposal such as a leach field, septic system, holding pond or aerated wastewater treatment system. A separate connection to a suitable grey water treatment system ( 106 ) is also included. This connection might alternatively be used to supply grey water for applications other than treatment and reuse. 
         [0047]    The heat exchange surfaces act as a barrier between the grey water, contained in the interconnection device, and a refrigerant working fluid contained by a network of refrigerant plumbing arranged in a closed loop ( 107 ). The plumbing can be made from a great number of materials, with copper being the most preferred. The loop operates in such a way that refrigerant is expanded from a liquid phase, by means of an expansion valve ( 108 ) or a capillary tube. The cold gas is passed through the heat exchange surface where its temperature is raised by energy transfer from the collected wastewater. This warm gas can then be compressed using the compressor ( 108 ), where it is passed to another heat exchange surface ( 109 ). This heat exchange surface is in contact with heat storage media ( 110 ) in a heat storage reservoir ( 111 ). The hot refrigerant is able to transfer energy, via the heat exchange surface, into the media. The remaining plumbing ( 112 ) then returns the refrigerant fluid to the expansion valve and the cycle is repeated. 
         [0048]    If a second heat exchange surface ( 113 ) is placed in contact with the heat storage media, energy can be transferred to cold incoming potable water ( 114 ). This system operates most efficiently when there is a high difference in temperature between the storage media and the incoming water supply. For this reason, the heat storage reservoir will be insulated and will preferably contain some form of supplementary heat source ( 115 ). This heat source may be of any available type, including: electric element, gas fire, solar, geothermal, wood fire, among others as appropriate. If the storage media is water, it is desirable that the temperature be maintained above 65° Celsius in order to prevent the growth of thermophillic bacteria. The water must also be prevented from boiling, by remaining below 100° Celsius, to prevent undue pressure on the structure of the reservoir. A reservoir containing water would require a pressure relief valve. Alternatively, the heat storage reservoir may contain a phase change material, such as paraffin wax, capable of storing larger amounts of heat energy for a given volume. 
         [0049]    In some circumstances, it is desirable that the heat energy storage media ( 110 ) is itself the potable water supply. In this instance, the heat exchange surface ( 113 ) is replaced by an open pipe allowing direct flow of potable water into, and out of, the reservoir. For applications of this type, a tempering valve ( 116 ) is placed between the heat energy reservoir and any residential fixtures. This ensures water supplied to the home is maintained at a temperature suitable for application to human skin. 
         [0050]    The invention further resides in an apparatus for the recovery of energy from wastewater. The apparatus facilitates the extraction of heat energy from water streams by providing an interconnection between heat transfer equipment and the waste that would be sent directly to sewer in ordinary circumstances. The apparatus also provides a connection between incoming, cold, potable water and heat stored in an energy storage reservoir, along with a mechanism to transport heat energy between the interconnection device and the reservoir. The heat transfer mechanism might comprise a direct heat transfer, dilution of one stream with another, conversion to mechanical or chemical energy, or conversion to electrical energy. In the preferred embodiment, energy is transported by means of a refrigeration cycle, or heat pump. 
         [0051]    According to  FIG. 2 , the interconnection device comprises a waste inlet ( 200 ), where wastewater, preferably including hot domestic grey water, enters the device. The interconnection device may be made of a wide range of suitable waterproof materials including plastic, metal, composites, or natural substances. Preferably, the material should be smooth, to reduce the adherence of wastewater contaminants and should have low thermal conductivity, to reduce the loss of heat energy to the surrounding environment. In many applications, the plumbing connection will be below the surface of the ground, so the design of the device must be such that it can withstand ground pressures. It must also have features that prevent it being lifted from the soil due to uplift pressures, frost, or a water table that rises periodically. The preferred embodiment described here would be constructed from an insulating plastic material ( 201 ) of sufficient thickness to attenuate heat losses to the surrounding soil. 
         [0052]    Water entering via the inlet is then optionally passed across a filter ( 202 ) where particles larger than a prescribed size are separated from the main flow. The filter may be of many configurations, and may be washable, disposable or self-cleaning. Preferably, a self cleaning filter incorporating wedge wires designed to separate particles larger than approximately 200 μm is used. Collected particles ( 203 ) are then directed, using a small amount of incoming wastewater, toward the outlet ( 204 ). 
         [0053]    Water, with larger particles removed, is then directed into a heat extraction chamber ( 205 ), where it is placed in contact with a heat exchange surface ( 206 ). The heat exchange surface is also in contact with a refrigerant working fluid in the preferred embodiment. The working fluid is connected to the rest of the process by means of plumbing with an inlet ( 207 ) and outlet ( 208 ). For best efficiency, it is preferable that the inlet is nearer the top of the chamber, and the hottest water, and the outlet nearest the bottom of the chamber. 
         [0054]    Alternatively, the heat exchange surface could be directly in contact with the potable supply, or could comprise an alternative method of energy transfer, like a Peltier device. The heat transfer chamber is designed such that newly incoming water contacts the heat exchange surface before being directed into the main body of the tank. This ensures that the temperature of water in contact with the heat exchange surface is at the highest possible temperature, which promotes efficient transfer into the working fluid, and thus efficient operation of the apparatus. 
         [0055]    Domestic wastewater is generated in a pulsatile fashion, for example the emptying of a bath. For this reason, the heat extraction chamber is sized such that it can contain pulses of wastewater for a sufficient period to extract the maximum amount of heat. Preferably, in a typical home, the size of the chamber is between 50 and 500 litres, with between 100 and 150 litres being most preferred. Other applications will require the chamber to be sized differently as appropriate. 
         [0056]    Temperature in the heat extraction chamber is maintained above a critical point through the use of a thermostatic control system ( 209 ) and a temperature-measuring device ( 210 ). In the case of water, the temperature must be maintained above the freezing point, 0° Celsius, to ensure that ice does not form. Ice reduces the efficiency of heat transfer by insulating the heat exchange surface from any incoming, warmer water. It can also block flows and disrupt the intended operation of the interconnection device. The low temperature threshold for the thermostatic control system is set between 0° and 4° Celsius, with between 2° and °4 degrees being the optimum range according to this invention. At temperatures below this threshold, the heat energy recovery process is stopped until further wastewater is gathered and the temperature returns above the threshold. To reduce starting and stopping frequency, it is desirable that the thermostatic control system includes a lag, for example: turning off the heat recovery equipment when the temperature falls to 2° Celsius, and activating it again only when the temperature rises to 4° C. 
         [0057]    Water in the chamber can be directed through the use of baffles ( 211 ) and weirs ( 212 ) to ensure that the hottest water remains in contact with the heat exchange surface for the maximum time possible. Optionally, it can be directed by mechanical means, such as a recirculation pump or mechanical mixer arranged in such a way as to maintain turbulent flow and circulation within the chamber. 
         [0058]    After the heat has been recovered, water is directed to a second storage chamber by passing over a final weir ( 212 ). This chamber ( 213 ) is designed to uncouple the pulsatile output of water generated by the residence, from the processing requirements of a downstream wastewater treatment system. In may cases, these systems operate in a continuous fashion, or have a defined batch size that would be otherwise incompatible with the output from a typical home. The size of the chamber will depend, in particular, on the specific treatment process being used, but will typically be between 50 litres and 2000 litres. Most preferably, the detention chamber will have a capacity between 100 litres and 300 litres. Water can be directed from the detention chamber to a suitable wastewater treatment system by means of a pump ( 214 ), optionally controlled by a level sensor ( 215 ), and interconnecting plumbing ( 216 ). 
         [0059]    In some cases, water may not be transferred from the detention chamber at the same rate it is being generated by the residence. In these cases, the detention chamber may become full and water will exit the chamber via an overflow port ( 217 ), traveling directly to the conventional sewer system, or other conventional method of wastewater disposal, via a plumbing connection ( 218 ). In these circumstances, the outgoing wastewater will carry away any remaining particulate matter that has been collected by the filter. 
         [0060]    Finally, the interconnection device is designed to prevent any discharge of malodorous gases to the local environment. A lid ( 219 ) with a tight seal ( 220 ) also prevents the ingress of ground water, or rainwater. Some ventilation is required to ensure the reliable flow of liquid through the device, so a connection to a standard plumbing vent is required. The device may also include a backflow prevention system ( 221 ), designed to prevent waste from the conventional sewer system from entering the system by the reverse path and contaminating the grey water with heavily soiled streams.