Adding value to the treatment of municipal wastewater through the intensive production of freshwater macroalgae

Abstract

Municipal wastewater treatment plants discharge large quantities of treated water that, in many regions, is not productively used and is instead released directly into the environment. In this study we examine the use of freshwater macroalgae as an in-line tertiary treatment process for existing municipal treatment plants. We examine the suitability of using the treated discharge water from a 29,000m3.day−1 municipal wastewater treatment plant as the sole source of water and nutrients for the intensive cultivation of the freshwater macroalga Oedogonium intermedium. A monoculture of algae was initially cultivated for a 3month period in which water quality and biomass productivity were quantified and the composition of the biomass characterized. These cultures were then maintained for a further 9months to determine the average monthly biomass productivity, and seasonal variation, over a 12month period. The cultivation of Oedogonium significantly improved the quality of the discharged water with a 36% reduction in total nitrogen and a 65% reduction in total phosphorous. The average monthly biomass productivity of Oedogonium ranged between a minimum of 8.9gDW·m−2·day−1 in June (austral winter – dry season) and a maximum of 15.8gDW·m−2·day−1 in January (austral summer – wet season) with an average annual rate of 12.5gDW·m−2·day−1. The biomass produced was of a high quality with a total protein content of 23g·100g−1 and a total lipid content of 10g·100g−1. Both the protein (10g·100g−1 of essential amino acids) and lipid (4.5g·100g−1 of polyunsaturated fatty acids) provide product opportunities for animal nutrition. This study demonstrates that the production of algae integrated with the operation of conventional wastewater treatment can complement and add value to existing processes by recovering residual nutrients and metals and, at the same time, create a high-quality biomass resource for product development.