OPERATION AND PERFORMANCE OF A HIGH RATE AUTO-HEATED AEROBIC DIGESTE

The Village of Plover, Wisconsin, operates an oxidation ditch activated sludge system for treating wastewater generated within the Village's sewer service area. The Village recently constructed a high rate auto-heated aerobic digester for stabilizing waste activated sludge. Digested sludge is belt pressed prior to land application.The need to reliably and safely dispose of biosolids, together with increased public awareness, resulted in the need to improve sludge quality for land application. The high rate auto-heated aerobic digester was constructed to achieve compliance with the vector attraction reduction requirements prior to the biosolids leaving the treatment plant site. The Village did not want to rely on incorporation of biosolids to achieve vector attraction reduction.Pre-thickening of waste activated sludge ahead of the aerobic digester, together with efficient fine pore aeration, results into auto-heating of the digester to temperatures up to 49 °C (120 °F). High operating temperatures in the digesters increases the rate of digestion and results in high volatile solids reduction and high pathogen reductions. Volatile solids reduction over the first year of operation has consistently been above the 38 % volatile solids reduction required to achieve vector attraction reduction. Historically, achieving 38 % volatile solids reduction for extended aeration activated sludge has been difficult to achieve using aerobic digestion technology. Fecal coliform densities have also been reduced to near Class A levels.High temperature aerobic digestion processes have frequently experienced a variety of operational challenges, including generation of odors, limitations in oxygen delivery, high recycle loadings, foaming, and poor dewaterability of the digested sludge. The Plover aerobic digester experienced many of these issues during the first year of operation. Each of these challenges was addressed by implementing operational changes, resulting in a process that produced a stable biosolids product without the associated operational issues.Many of these challenges were addresses by reducing operating temperature in the digesters. This resulted in improved belt press solids capture lower polymer demand and reduction in odors.This paper will present the operational challenges presented by the high rate auto-heated aerobic digester and discuss the operational changes considered to address these issues including the effects on performance of the system.