Heat Extraction from Plant Effluent: “Pumped-Up Heat Pumps”

High-temperature effluent-source heat pumps are able to meet the heating requirements of a wastewater treatment plant. The use of heat pumps can make anaerobic digester gas that might otherwise be used for heating available for other purposes. More than four times the heat required by a wastewater treatment plant is available with just a 1°C drop in effluent temperature. At least four U.S. heat pump manufacturers can provide heat pumps to meet digestion process temperature needs utilizing effluent at typical temperatures. The anaerobic digester gas that would otherwise be used for heating can be upgraded to natural gas quality using mature technologies.The alternative to install effluent-source heat pumps and upgrade digester gas to pipeline-quality biomethane can be economically attractive under certain scenarios. A number of states have favorable conditions for considering this alternative, namely low electricity prices and high natural gas prices. An analysis of a hypothetical 76 mld plant provides an approach to assess when this alternative may have a better net benefit than cogeneration. King County's South Plant in Renton, Washington, is an example of the use of effluent-source heat pumps with digester gas upgrading for sale. The plant has been operating for 25 years and could produce enough revenues from the sale of biomethane to provide a simple payback for the digester gas upgrading system and heat pumps in less than 10 years.