Energy recovery from municipal sewage and greywater offers cities a new reliable resource. Erwin Schwartz of DDI Heat Exchangers Inc. reports on a novel application of rectangular, wide-gap heat exchangers that has worldwide potential.
The Canadian company DDI Heat Exchangers Inc. installed, with the support of the Quebec provincial government and municipal sewage authority, the first direct city sewage energy recovery system in the world using its rectangular channeled heat exchangers. In the city of Baie d'Urfe, located near Montreal, the system is used to heat an indoor soccer building's central boiler water and space in the winter and cool the building in the summer.
Municipal sewage that flows in subterranean pipes under city streets is warmer than the above air in the winter and colder in the summer. That huge energy, which is readily available in many cities, is not being recovered, yet holds great potential worldwide.
In North America and Europe, municipal regulations require double-wall separation between sewage or grey water and clean drinking water that (lows to the central boiler on the other side of the heat exchanger. Current technologies like spiral and tube-in-shell heat exchangers cannot have double-wall structure and also allow for efficient heat transfer between the two liquids. Those technologies would also have problems of plugging and screening costs requirements.
DDI's Rectangular Channeled Heat Exchangers have been in operation in many wastewater treatment plants and molasses processing facilities in North America for the last 15 years. The DDI rectangular patented structure allows adding another plate in between the two liquids without major heat transfer losses. This technology structure has very large wide gaps of five centimeters, which lower the risks of plugging, and controllable width, which allows for fast flows to lower the risk of baking to the surface. DDI reports that this technology design enables fluids with up to 26 percent solids to flow without plugging.
The Montreal project faced several challenges. One issue was how to connect the sewage pipe of 18' to 96' (or even more), directly to the heat exchanger. If the connection is made too close to the bottom of the city sewage pipe, then all the solid particles will risk plugging in the heat exchanger. If the connection made to the pipe is too high, then there will be no flows at all in dry weather. DDI tried different connection locations. A plastic transparent pipe section was installed to see if the flow fills up the pipe and the conditions under which it takes place.
A major obstacle to overcome was to pull in the sewage when the city sewage pipe is far away and deep under the pump. Testing was carried out over a distance of more than 98 meters away from the connection and six meters below street level.
Another issue was to find a pump that at startup can suck up air only in order to empty the long pipe between the heat exchanger and the city pipe from air, before pulling in sewage. That pump also had to allow reverse flow for unplugging blockage if necessary. DDI chose the German-manufactured Vogelsang Reliable pump that is capable of reverse-flow, emptying air, and pulling in sewage.
DDI conducted a study to determine the connection location of the sewage pipe. Based on the findings, the connection was made upstream to avoid the risk of closed-loop flow, yet not too far away in order to lower the cost of excavation and connection.
Multiple safety features are used to prevent any risk of sewage flowing outside of the indoor soccer building: air-liquid valves, flow detection floaters, and computerized control panel safety shut down procedures. In addition, DDl's Easy Cleaning Process System allows each two channels to be cleaned within seconds if plugging does occur.
DDI completed installation of the world's first direct city sewage energy recovery system in July 2014. The company's patented DDI heat exchanger with wide gaps, capable of using direct flow from sewage, helps avoid high capital and maintenance costs and inefficiencies incurred from indirect technologies (huge holding tank, screening, land purchase, losing heat etc) that would be necessary to screen sewage and clean holding tanks.