New Life for Old Drying Beds

Construction of San Antonio's Dos Rios Water Recycling Center (WRC) in 1987 included 132 biosolids sand drying beds of 8,400 ft2 each, for a total of 1,108,800 ft2 of available drying area. Until 1999 the beds were underutilized because of poor bed maintenance and neighbor complaints regarding odors and vectors. In 1999, introduction of parasitic wasps (Nasonia vitripennis, Muscidifurax zaraptor) and installation of Purple Martin (Progne subis) condominiums led to a successful, cost-effective, environmentally friendly solution. Using natural vector control instead of insecticides resulted in a savings of 50,000 per year while reducing risks to plant operations, reducing man hours expended for application, eliminating the need for a licensed applicator, and allaying employee health concerns. Successful vector control also created an opportunity to further reduce costs and improve solids handling operations through greater drying bed utilization. Operations and maintenance practices were scrutinized and optimized, and in 2001 an experimental program involving polymer addition to drying bed biosolids was initiated. Polymer addition was shown to greatly decrease drying time, increase total solids, and allow much greater solids loading. Currently, full-scale implementation of a polymerized bed program is underway. The goal is to naturally dry 100% of biosolids production, eliminating belt press operations and providing additional cost saving for San Antonio Water System ratepayers.