Treating a Problematic Sub-Micron Blue-Green Algae Dominated Pond Water with Aluminum Chemistry: An Economical Solution to Meet the NPDES Solids Permit Limit for the San Francisco Bay

The City of Sunnyvale Water Pollution Control Plant (WPCP) is a 12 MGD Average Dry Weather Flow tertiary wastewater treatment facility (1). As part of the secondary system, the plant operates oxidation ponds (2) for removal of organic matter and some ammonia. The ponds are rich in nutrients and as such propagates algae. Prior to 2021, the dominant algae species in the ponds were nano-, micro- and mesoplankton species that consisted of some cyanobacteria, protozoa, small algae, and some large algae ranging in size from 2 micron to 2 mm. These species served the plant well with photosynthesis providing needed oxygen for the oxidation reactions and nutrient uptake by algae. During the early part of 2021, the ponds became dominated by picoplankton species, namely a unicellular cyanobacterium (also known as bluegreen algae(3) ) named Synechocystis with a size range of 0.2 to 2 micron. Granted Synechocystis continued to serve the plant well with photosynthesis and nutrient uptake, however, it upended our plant by not allowing itself to be removed in our Dissolved Air Flotation (DAF) tanks where the pond effluent is treated for solids/liquid separations. As a result, the plant violated its NPDES permit limit on multiple occasions for solids discharging to the lower San Francisco Bay(4), resulting in possible fines to the City. This paper discusses the research work conducted this year in terms of water chemistry at the DAF in determining the most optimal condition to remove Synechocystis from the pond effluent. This effort not only prevented the plant from violating the NPDES permit limit for solids again but also provided the City with a cost-effective solution.