Analyzing digester heating requirements

By Ivan Drako, Michelle Albert, Ron Cariglia, Sean Hutchinson and Edwin Ayson

Anaerobic digestion is one of the key suspended-growth treatment processes widely used for the stabilization of organic materials and biosolids produced at a wastewater treatment plant.

In order to efficiently stabilize organic materials, an optimum sludge temperature must be maintained for a period of time. For mesophilic digestion, the temperature must theoretically be maintained between 30^oC – 38^oC. This is achieved by transferring heat from hot water to the sludge via a heat exchanger, followed by recirculation of the sludge back to the digester tank. This keeps it constantly suspended and prevents solids and temperature stratification in the digester reactor.

WSP recently completed the design of upgrades to some primary digesters at Toronto’s Ashbridges Bay Wastewater Treatment Plant. The project included digester clean out, sludge dewatering, rehabilitation of the digester tank structure, installation of new thermal insulation and membrane in the digester roof, new gas proofing process, and electrical, instrumentation and control upgrades to process and pumping equipment. Modification to the SCADA system for automated control of the sludge temperature in the digester was also part of the project scope.

In order to update the process control narrative and develop modifications to the SCADA system, an analysis of the digester heating requirement was conducted. This included understanding the seasonal trends of heat loss and sludge temperature variations, from the heat exchanger outlet port to the digester and back to the heat exchanger inlet port.

Summary of site findings

In order to estimate heating requirements, the digester’s technical data was reviewed and analyzed.

It is a primary digester with a fixed concrete roof and conical floor slab, 33.5 m diameter, 10.8 m operating water depth, and 12.2 m side wall height. The above grade wall height and below grade wall depth of the digester structure is approximately equal. The above grade structure is thermally insulated. The below grade structure has thermal insulation installed to the frost line depth, only between the structure and the soil. There is no insulation in the deeper digester walls section or beneath the base slab.

A recirculation pump continuously withdraws sludge from the digester tank and discharges to the heat exchanger for heat transfer from the hot water source to the sludge and returns it back to the digester.