Biomass production and nutrient removal by Chlorella sp. as affected by sludge liquor concentration

The use of microalgae for biomass production and nutrient removal from the reject water produced in the dewatering process of anaerobically digested sludge, sludge liquor, was investigated. The sludge liquor was characterized by a high content of total suspended solids (1590 mg L−1), a high nitrogen concentration (1210 mg L−1), and a low phosphorus concentration (28 mg L−1). Chlorella sp. was grown in sludge liquor diluted with wastewater treatment plant effluent water to different concentrations (12, 25, 40, 50, 70, and 100%) using batch mode. The environmental conditions were 25 °C, a continuous lightning of 115 μmol m−2 s−1, and a CO2 concentration of 3.0%. The highest biomass production (0.42-0.45 g dry weight L−1 Day−1) was achieved at 40-50% sludge liquor, which was comparable to the production of the control culture grown with an artificial fertilizer. The biomass production was 0.12 and 0.26 g dry weight L−1 Day−1 at 12% and 100% sludge liquor, respectively. The percentage of nitrogen in the algal biomass increased from 3.6% in 12% sludge liquor and reached a saturation of ∼10% in concentrations with 50% sludge liquor and higher. The phosphorus content in the biomass increased linearly from 0.2 to 1.5% with increasing sludge liquor concentrations. The highest nitrogen removal rates by algal biosynthesis were 33.6-42.6 mg TN L−1 Day−1 at 40-70% sludge liquor, while the highest phosphorus removal rates were 3.1-4.1 mg TP L−1 Day−1 at 50-100% sludge liquor.