Energetics of advanced integrated wastewater pond systems

An energy balance is presented for a second generation Advanced Integrated Wastewater Pond System (AIWPS) prototype at the University of California, Berkeley, Environmental Engineering and Health Sciences Laboratory in Richmond, California. Modifications were made to the existing 1.8 ML facultative pond in order to further optimize methane fermentation and to demonstrate the recovery of methane using a submerged gas collector. Methane production rates were determined over a range of in-pond digester loadings and temperatures. The feasibility of submerged gas collection was proven, and the advantages of in-pond digestion in terms of BOD₅ and VSS removal as well as biogas scrubbing were quantified. Biogas methane concentrations increased by more than 50% as the biogas emerged through the overlying water column and most of the carbon dioxide fraction was utilized by microalgae. Electrical power requirements for mixing two 0.1 hectare algal High Rate Ponds (HRPs) were measured over a range of channel depths and velocities, and electrical power requirements for daily recirculation pumping were also measured. Oxygenation and total treatment energy requirements for the second generation AIWPS prototype at Richmond were compared with oxygenation and total treatment energy requirements for the first generation AIWPS at St. Helena, California and for two mechanical wastewater treatment plants of comparable capacity and effluent quality at Pinole and Brentwood, California. Using preliminary methane production and recovery rates achieved at Richmond, the cogeneration potential was estimated and projected for larger second generation AIWPSs of 2 MLD and 200 MLD capacities. By incorporating methane recovery and electrical power generation together with efficient HRP mixing using paddle wheels, full-scale second generation AIWPSs will be able to produce as much energy as they require for primary and secondary treatment. Additional energy would be required to produce a tertiary disinfected effluent suitable for unrestricted reuse in California, including recreational and indirect potable reuse. The additional power requirements for complete algal harvest using dissolved air flotation (DAF) and filtration were estimated for second generation AIWPSs based on data collected at Richmond and Stockton, California, and the additional power requirements for final UV disinfection were estimated.