A revised proposal.

Waste Water Heat Recovery for District Heat Recovery

Bradley Erickson

District Energy systems are becoming more popular in metro and urban areas. These systems are where you have a central plant for heating and cooling and the fluid for this system is piped around an area. The buildings in this area instead of having their own furnace, simply have a heating/cooling coil that uses the fluid from the district energy system as a source of heat and as a place to dump heat for air conditioning in the summer.

This system with a large central plant offers interesting opportunities for energy savings and greener source of heat. This study will look into the feasibility of waste water heat recovery/rejection systems as central plants for these district energy systems.

The effluence outflow from sewage treatment plants is quite warm and remains warm throughout the year. This is due mainly to the heat generated by the bacterial activity used to decompose organic material in the sewage. In systems with a large outflow this could provide the opportunity to extract a large amount of heat from the water.

Purpose

There has been a great deal of research into the feasibility of district energy systems. I aim to shed light on this and to establish an awareness of the feasibility of waste heat systems in the hope that this will give designers enough information to consider these systems as viable options in a wider set of design scenarios.

Goals

• Determine constraints to prevent negative ecosystem impacts downstream of effluent outflow.

• Find a correlation between the capacity of sewage treatment plant, and the amount of heat it can provide

• Design complications, and considerations that must be made due to the composition of the effluent.
  
  

• Determine the cost effectiveness of this system compared to more common plants such as biomass boilers, and natural gas boilers.

Methodology

For this study data will be gathered on the effluent outflow temperatures, flow rate , turbidity and where possible oxygen content from a couple sewage treatment plants in various cities around the province. The minimum and maximum allowable outflow temperatures will be determined based on values used at the sewage treatment plants. This information will be used to determine the maximum amount of thermal energy that can be rejected or extracted into or from the effluent outflow without affecting the downstream ecosystem.

To determine the cost effectiveness, the initial cost, running cost, and maintenance costs will be calculated. These costs will be compared to the costs of installing, running and maintaining a conventional combustion system that uses biomass or natural gas as a fuel source. The greenhouse gas emissions will also be calculated and compared between these systems.

Budget

Travel Expenses 150$

Citation

Cassitto L. (1990). District Heating Systems in Europe - A Review. Resources Conservation and Recycling, 4(4), 271-281.

Anna-Marie Tillman, Mikael Svingby, Henrik Lundstrom (1998) Life Cycle Assessment of Municipal Waste Water Systems, 3(3) 145-157

Funamizu N, Lida M, Sakakura Y, Takakuwa T. (2001). Reuse of Heat Energy in Wastewater: Implementation Examples in Japan. Water Science and Technology, 42(10), 277-285

Kristina Holmgren, Dag Henning (2004) Comparison Between Material and Energy Recovery of Municipal Waste from an Energy Perspective: A Study of two Swedish municipalities. 43(1) 51-73

Sadohara, S., Ojima, T. (1991) Study on Heat-source for District Heating in Tokyo. Lausanne 1, 571-575

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.

Explanatory Graphic

How the system works basically is that you copper plate heat ex-changer is submerged in the sewage outflow. the fluid circulating through the heat exchanger picks up the heat from the effluant and then is pumped through the district energy system.

After some initial research I've decided the System i'm going to use in my project is a Passive system with heat exchanges submersed in the effluent outflow. these will be hooked to a three way valve to control the flow through the heat exchanger to maintain a constant temperature in the loop. The pumps will circulate fluid through the district energy system. My reasoning for going with this system is that the cost of this system is far lower than an active heat recovery with a heat pump. This allows the system to be feasible for smaller communities, There is also less maintenance for this system allowing for deployment in more remote communities.

To get this information i used information and costing from various manufacturers as well as RS Means mechanical Cost Data.

RS Means (2013). RS Means Mechanical Cost Data 2013, Robert s Means Co.