Phosphorus Recovery from Wastewater through Enhanced Micro-algal Uptake

Phosphorus (P) is an important constituent for living organisms as an energy carrier and a component of important biomolecules, which cannot be substituted with another element. Also, it is considered as the main element inducing eutrophication in freshwater bodies. In order to control eutrophication, wastewater treatment works (WWTW) remove P from their final effluent mostly through chemical precipitation and biological bacterial uptake. However, in order to achieve nutrient recovery from wastewaters, it has been suggested alternative biological processes based on biological algal uptake. Microalgae can be used to recover P from wastewaters as they can assimilate high amounts of P for their growth and store any excess as polyphosphate (i.e., luxury P uptake). This study is aimed at examining the potential use of microalgae cultivation for P recovery from wastewater and to identify the implications for its implementation at large WWTW. Chlamydomonas reinhardtii 11/32C was chosen as a model organism for this study. With regard to the role of the Nitrogen (N) source (ammonium v. nitrate), results show that there is little impact on P uptake when either ammonium or nitrate is used as the N source; however, by using a mix of ammonium and nitrate, P uptake increases as a result of stress conditions as ammonium is consumed faster than nitrate. Controlling environmental factors for microalgae growth showed that luxurious intracellular P uptake can range between 0.3 and 3.6% dry weight; optimum conditions for algae growth and P uptake where controlled by N concentration (200 mg N L-1, 50:50 NH4+:NO3-), phosphate concentration (100 mg P L-1 ), light intensity (250 μEm-2s-1) and photoperiod (16 hr light: 8 hr dark), resulting in a maximum algal biomass production of 148 mg VSS L-1d-1 and intracellular P uptake of 2.8 mg P L-1d-1. This study proved that C.reinhardtii 11/32C able to store the excess of P as polyphosphate granules located in cell’s vacuole. A continuous flow mixotrophic microalgae cultivation system was operated to investigate its performance as a novel biological nutrient control and recovery process. Under tested conditions, the mixotrophic system was able to remove both nitrogen and phosphorus at 97% and 50% respectively, with average P uptake of 2.03 mg P L-1 d-1 and algal biomass production of 248 mg VSS L-1 d-1. This system in operation generated on average 2.6 g L-1 of harvested algal biomass per day with a P content of 2.1% dry weight. These results show that a mixotrophic microalgae system has the potential to be implemented as a tertiary wastewater treatment process for biological nutrient removal and hence, a desktop case of study was conducted using data from a full-scale WWTW to assess the feasibility to implement this approach for nutrient recovery.