Abstract Rough Draft

The point of this study is primarily to determine the cost effectiveness of sewage effluant heat recovery and how this scales with the size of the plant. The carbon footprint and greenhouse gas emissions of the sewage effluent heat recovery system will also be investigated and compared to the carbon footprint as well as greenhouse gas emissions of a conventional system.

This report will not include research into the actual district energy distribution system as it is impossible to estimate costs for this and would be the same for a waste water heat recovery based system as for a conventional biomass or gas powered plant.

There are two typical types of district energy plants, ones with a series of large boilers that come on as demand increases. The second type consists of two plants, one to handle a base load which covers 100% of the heating load, the second part of the plant is called a peaking plant this would be a conventional gas boiler that only comes on when the outside temperature rapidly changes and the base load system is in the process of ramping up to match the heating load. The first system is done with boilers that have the ability to rapidly adapt to changes in load, this means they usually are conventional boilers that run off of natural gas, fuel oil, or in some cases syn. gas. The second system will usually use a type of biomass boiler, or cogeneration plant for the base load plant, and a conventional boiler for the peaking plant.

The sewage effluent heat recovery system would not be like either of these systems. Since it simply functions as a heat transfer from the effluent to the fluid in the district energy system it has the ability to respond instantly to load changes and has no need for a peaking plant. The system would simply consist of a copper plate heat exchanger submersed in the effluent outflow. There will be a three way valve to allow fluid to bypass the heat exchangers to maintain a constant loop temperature. downstream of the three way valves you would have pumps the circulate the fluid throughout the district energy system.

“More attention is now being paid to the high number of disease-causing germs in the

sewage treatment plant effluent. Micro and ultra filtration, combined with the activated sludge process, has turned out in recent years to be a suitable method for minimising the effluent load. Tightening discharge standards for sewage treatment effluents can thus be met” Coppen, J. (2004). Advanced wastewater treatment systems (Doctoral dissertation, University of Southern Queensland). As a result of this design considerations involved with sinking the heat exchanger directly into the effluent outflow are reduced however a secondary heat exchanger may be required to isolate the district energy system from the possibility of contamination.

The total heat available in a  system such as this is enormous. Sadohara, S and Ojima, (1991) State:“We have chosen Tokyo city area as an experimental field and seven kinds of exhausted heat sources which are power plants, sewage plants, incinerators, refrigerating storages, electricity converters, subways and underground cables. We indicate the locations of these heat sources and calculated the quantity of their exhausted heat. After we compared this waste heat with heat demand, it became clear that power plants, sewage plants and incinerators have enough exhausted heat to supply heat to the surroundings. The total exhausted heat is 18 295 Tcal/a, which corresponds to 47.3% of total demand (38 705 Tcal/a).”  That translates to 8.28 Billion BTU/h (47.3%) of the total heating load for Tokyo city being provided from reject heat from other processes including sewage heat recovery.

A simple methodology is presented which enables the sizing and performance analysis of heat pump systems in sewage effluent heat recovery applications. Using typical winter effluent temperatures from sewage treatment plants in the south of England it is shown that both gas engine driven and electrically driven heat pumps can provide substantial savings when compared to natural gas fired boilers. The recovered heat can be either used to satisfy the heating needs of the plant or exported to neighbouring agricultural or industrial complexes. Tassou, S.A.(1988).

Data will be used from the City of Prince George Sewage Treatment Plant to ascertain the quantity of heat available from the sewage effluent. A system will then be designed which will be capable of extracting that quantity of heat from the effluent. The carbon footprint of running this system will be calculated by useing the Climatic data for Prince George and assuming peak demand would be at the coldest design day then scaling that based on temperature throughout the rest of the year. This will be used to establish the energy needed and from there the total running costs for a conventional system would be and the total greenhouse gas emitted as well as the carbon footprint. This will then be compared to the sewage heat recovery system to see the energy savings and the greenhouse gas reduction.