Removal of nutrients from WWTP effluent by an algae-mussel trophic cascade

New treatment steps are needed to better remove nutrients from WWTP effluents to prevent further eutrophication of the receiving waterbodies. Algae can be used to remove nutrients from effluent, but the subsequent removal of the algae biomass remains a challenge. Employing the high filtration capacity of dreissenid mussels may offer an efficient tool to remove effluent grown algae. The aim of the present study was therefore to evaluate if an algae-mussel trophic cascade allows an efficient removal of both nutrients and effluent-cultured algae. To this end, we inoculated WWTP effluent with the alga Tetradesmus obliquus in a batch system and monitored algal growth and nutrient concentrations for 11 days. Subsequently, we fed this algal-effluent solution to Dreissena bugensis for 3 days, after which we determined the amount of remaining algae and produced (pseudo)faeces. The algae-mussel set-up was able to remove 99% of P and 99% of N from the initial effluent, and the combination of algae settlement and mussel filtration resulted in a 94% removal of suspended algae after only 24 h. A total of 67% of the algae were consumed by the mussels after 3 days. Most filtered algae were deposited as pseudofaeces. An even higher efficiency may be achieved by selecting algae with a high nutrient removal capacity, but also with a high palatability for the mussels. Nonetheless, this is the first research that shows that an algae-mussel cascade could already offer an efficient solution to reduce nutrient concentrations in WWTP effluent. This may reduce costs, energy consumption and coagulants use at WWTPs, and could offer an alternative for high-tech treatment techniques, which may be more suitable in countries lacking the infrastructure to build these high-tech treatment steps.