Municipal sludge digester upgrade for biofuel production

A number of municipal wastewater treatment plants in New Zealand operate simple sludge digesters with floating roofs, biogas recirculation mixing, low volatile solids (VS) loads (about 1.5 kg VS. m-3 digester.day-1), long hydraulic residence times (15-25 days) and low biogas productivities (0.7 m3 biogas. m-3 digester.day-1). Typically these digesters suffer from inefficient sludge: biomass contact, poorly mixed dead zones, flow short circuiting and consequential build-up of sediment.

Waste Solutions has successfully designed, constructed and commissioned a number of industrial sludge digesters with high organic loading rates (4 - 5 kg VS m-3 digester.day-1). These systems have the capability to process approximately 3 times the organic load of comparable municipal digesters because they are well mixed (mixing energy: 10-20 W.m-3 digester) resulting in high biogas productivities (2 - 3 m3 biogas. m-3 digester.day-1) and short hydraulic residence times (10-15 days).

Here we report a hydraulic residence time analysis for the Palmerston North Totara Road primary sludge digesters. We present the application of the analysis for a digester mixing upgrade with the ultimate objective to generate additional digester capacity for co-digestion of additional trade waste materials. A simple process model is used to determine the improved biogas production and digester facility operating costs. Actual digester operation records show that the mixing system upgrade achieves more than 100 % biofuel (biogas) output improvement and an expected payback period of less than 2 years. The full biogas production is used to operate a generator for production of renewable electricity. The generator waste heat is used for digester heating. This configuration allows to generate additional revenue through higher utilization of existing capital assets.