Microalgae in Sewage Treatment

Abstract

Anaerobic digestion is a common method of treating sewage sludge; however the anaerobic digestate liquor (ADL) contains high nutrient levels that have to be treated. The aim of this study was to determine whether microalgae could be used to strip nitrogen (N) and phosphorous (P) from ADL produced at Bran Sands sewage treatment works (STW); whether the ADL contained toxins and/or missing nutrients; and identify the spatial and temporal parameters of a potential bioremediation system. ADL was collected from Bran Sands STW and Scenedesmus obliquus was grown on various dilutions of ADL at laboratory scale. It was found that microalgae grown on the ADL was dually inhibited by high ammonia (NH3) concentrations and limited trace element availability. Trace element limitation decreased the microalgae biomass productivity to 1/3 of cultures with trace element supplementation. It was also found that NH3 concentrations > 17-23 mg/l NH3-N completely inhibited growth. A long time lag observed in 10 % ADL solutions was found to be due to the pH decreasing in ADL flasks over time (from 9.30 to 8.60), leading to a decrease in NH3 concentration until the toxicity threshold was crossed (approximately 20 mg/l NH3-N at pH 8.80), after which exponential growth occurred. Using 17.62 mg/l NH3-N as an inhibition threshold, it was calculated that the highest concentration of total ammonia nitrogen TAN that could be remediated at pH 7.0 was 400 mg/l TAN (a 3.75 × dilution of neat ADL). Based on observed growth and nutrient uptake rates, it was further calculated that a microalgae remediation system could strip 400 mg/l TAN from the ADL within 5.66 days. If growth conditions were optimized and the growth rate could be increased to > 1.0 d-1 (reported by Ho et al. (2010)) the remediation time could be reduced to 3.31 days (although this does not consider other growth-limiting factors such as light inhibition). Due to fluctuating N:P ratios it was not possible to calculate predicted P uptake.