Methane fermentation, submerged gas collection, and the fate of carbon in advanced integrated wastewater pond systems

Abstract

There are several basic reasons for concern regarding the fate of carbonaceous material in waste stabilization ponds: accumulation of solids; performance and useful life of the pond system; and, the control of methane emissions. In conventional ponds methane fermentation is minimal, and carbon-rich organic matter is integrated by bacteria and microalgae which grow and settle. The integration of carbon decreases pond volume and treatment capacity and causes the ponds to age prematurely, to produce odor, and to require frequent sludge removal; and, any methane produced escapes to the atmosphere. However, if carbon-rich organics are efficiently converted to methane or to harvested microalgae, the pond system will continue to treat wastewater effectively for an extended period of time. Advanced Integrated Wastewater Pond Systems (AIWPSs) developed at the University of California fully utilize methane fermentation and microalgal cultivation to treat wastewater and to reclaim energy and nutrients. First generation AIWPSs have provided reliable municipal sewage treatment at St. Helena and Hollister, California, for 28 and 16 years, respectively, without the need for sludge removal. However, these first generation systems lack the facilities to recover and utilize the carbon-rich treatment byproducts of methane and algal biomass. The recovery of methane using a submerged gas collector was demonstrated using a second generation AIWPS prototype at the University of California, Berkeley, and the optimization of in-pond methane fermentation, the growth of microalgae in High Rate Ponds, and the harvest of microalgae by sedimentation and dissolved air flotation were studied. Preliminary data are presented to quantify the fate of carbon in the second generation AIWPS prototype and to estimate the fate of carbon in a full-scale, 200 MLD second generation AIWPS treating municipal sewage. In the experimental system, 17% of the influent organic carbon was recovered as methane, and an average of 6 g C/m2/d were assimilated into harvestable algal biomass. In a full-scale second generation AIWPS in a climate comparable to Richmond, California, located at 37° N latitude, these values would be significantly higher--as much as 30% of the influent organic carbon would be recovered as methane and as much as 10 g C/m2/d would be assimilated by microalgae. These efficiencies would increase further in warmer climates with more abundant sunlight.