OPTIMIZATION OF MICROALGAE FOR SLUDGE LIQUOR TREATMENT, BIOMASS PRODUCTION AND RENEWABLE ENERGY RECOVERY

Abstract

The increase in world population resulting in increased global energy demand has consequently led to the extensive use and global depletion of fossil fuel sources. Environmental menaces resulting from the extensive use of fossil fuel as primary energy; such as the accumulation of anthropogenic carbon dioxide in the atmosphere giving rise to global warming stimulates the need for clean energy systems. Liquor from sludge dewatering processes holds a lot of potential for clean energy generation. Returning this liquor to the treatment plant increases the nutrient load of the system, thereby adding to the energy and operating cost requirement of wastewater treatment plants (WWTPs). In this research, Chlorella vulgaris (C.vulgaris) strain of the microalgae was cultivated in sludge liquor to achieve three purposes; treat sludge liquor, generate biomass and generate energy. C.vulgaris was first cultured in Bolds Basal Media (BBM) and after four days (during exponential growth phase) it was extracted and cultivated in liquor samples. All culture and cultivation were done using cotton plugged conical flasks. Liquor was obtained from four sludge samples using different chemical treatments. The liquor obtained was then used to cultivate C.vulgaris. Sample1 was obtained from sludge conditioned with zetag66 polyelectrolyte at 0.6g/l sludge, sample2a was obtained using 27.5g lime/l sludge, sample2b was obtained using 1.28g lime/l sludge and lastly sample3 was obtained using 35g ferric chloride/l sludge. Each sample was then characterised and necessary dilutions were made and used to cultivate C.vulgaris. 50ml of C.vulgaris was used for each media and the ratio of C.vulgaris: liquor was 5:1, 1:1, 5.6:1 and 10:1 for media1, media2a, media2b and media3 respectively. At the end of a 14 day cultivation period, the following results were obtained; Ammonia and phosphate removal of 97.4% and 91.5%, 84.2% and 89.3%, 88.9% and 88.3%, and 91.1% and 83.3% for media1, media2a, media2b and media3 respectively. Biomass yield of 1.36kgDS/m3 with 65%VS, 2.94kg/m3 with 24%VS, 1kg/m3 with 72%VS and 0.87kg/m3 with 60%VS was obtained from media1, media2a, media2b and media3 respectively. The energy and financial worth of 1m3 of each media was estimated based on the methane potential of C.vulgaris biomass obtained. Therefore media1 was worth 3.07kWh about £1.12, media 2a and 2b 2.51kWh about £0.91 each and media3 1.81kWh about £0.66. Additional digestate of 0.83kgDS, 2.51kgDS, 0.57kgDS and 0.56kgDS can be generated from media1, media2a, media2b and media3 respectively with ammonia within the range of 24.70 – 47.52 mgNH3-N/m3media. Therefore growing microalgae in sludge liquor will help to meet stringent effluent standards by WWTPs and generate income. The ammonia and phosphate required for the cultivation are readily available in the dewatered sludge liquor, but for liquor from the chemical treatment process, the nutrients might be insufficient. Dewatering liquor from raw sludge, therefore, provides a suitable option for energy generation using C.vulgaris and also saves the operating cost associated with the use of chemicals and aeration by conventional treatment methods.