A quantitative microbial risk assessment of wastewater treatment plant blending: case study in San Francisco Bay
An investigation was carried out to evaluate the impacts of blending practices (i.e., a practice used to manage wet weather flows) on the effluent from the East Bay Municipal Utility District's (EBMUD) wastewater treatment plant in Oakland, California and water quality in the receiving water (San Francisco Bay). A static based quantitative microbial risk assessment (QMRA) was used to estimate the incremental risk to public health from recreational exposure to adenovirus and the protozoan Giardia spp. in San Francisco Bay for wet season (generally between October and March) blending and non-blending events. The mean risks of infection per recreational exposure event during the wet season for all of the modeled scenarios were more than an order-of-magnitude below the USEPA's illness level (36 illnesses per 1000 contact events) associated with recreational water quality. While the QMRA results showed discernible differences in per event estimated risks between blending and non-blending scenarios, the estimated incremental increase in the annual number of infections due to blending (based on median estimates) resulted in an estimated combined increase of less than one infection annually. These estimates are subject to various uncertainties, including the potential for secondary transmission, assumptions on the extent of exposures, and the number of blending days required in the future due to climate change, which are discussed in this paper.