METHANE FERMENTATION, SUBMERGED GAS COLLECTION, AND THE FATE OF CARBON IN ADVANCED INTEGRATED WASTE-WATER POND SYSTEMS

There are several basic reasons for concern regarding the fate of carb onaceous 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 minim al, and carbon-rich organic matter is integrated by bacteria and micro algae 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 met hane produced escapes to the atmosphere. However, if carbon-rich organ ics are efficiently converted to methane or to harvested microalgae, d ie pond system will continue to treat wastewater effectively for an ex tended period of time. Advanced Integrated Wastewater Pond Systems (AI WPSs) developed at the University of California fully utilize methane fermentation and microalgal cultivation to treat wastewater and to rec laim energy aid nutrients. First generation AIWPSs have provided relia ble municipal sewage treatment at St. Helena and Hollister, California , for 28 and 16 years, respectively, without the need for sludge remov al. However, these first generation systems lack the facilities to rec over and utilize the carbon-rich treatment byproducts of methane and a lgal biomass. The recovery of methane using a submerged gas collector was demonstrated using a second generation AIWPS prototype at the Univ ersity of California, Berkeley, and the optimization of in-pond methan e fermentation, the growth of microalgae in High Rate Foods, and the h arvest of microalgae by sedimentation and dissolved air flotation were studied. Preliminary data are presented to quantify the fate of carbo n in the second generation AIWPS prototype and to estimate the fare of carbon in a full-scale, 200 MLD second generation AIWPS treating muni cipal sewage. In the experimental system, 17% of the influent organic carbon was recovered as methane, and rut average of 6 g C/m(2)/d were assimilated into harvestable algal biomass. In a full-scale second gen eration AIWPS in a climate comparable to Richmond, California located at 37 degrees N latitude, these values would be significantly higher - as much as 30% of the influent organic carbon would be recovered as m ethane aid as much as 10 g C/m(2)/d would be assimilated.