Treatment of a dairy farm wastewater using an upflow anaerobic sludge blanket (UASB) process
Deerpaul A & Ragen A K
Paper Presenter: Ashveen Deerpaul, Esquel (Mtius) Ltd
Dairy production in Mauritius existed since the seventeenth century during the Dutch occupation of the island and has evolved positively up to the 1990’s. It has played a vital role in the agricultural diversification and the socio-economic development of the country. However, after the 90’s, due to urbanizations, emergence of the textile industries and the evolution of the tourist industry and services, the agricultural sector in Mauritius had taken a declining trend. Despite the decline in the local dairy sector, demand for milk and milk products has been increasing and is expected to increase with the growing population. Therefore, in an endeavour to promote local dairy industry, the government is encouraging the setting up of dairy enterprises to increase milk production and initiate the production of value added milk products. Development of this sector in Mauritius will provide an increased supply of fresh milk, thus reducing the need of foreign exchange.
However, dairy farm generates large amount of wastes in the environment. The wastewater generated by confined animal production contains a high amount of organic matter, solids and nutrients and pathogens. These pollutants are harmful, when they are discharged in water bodies without proper treatment. The disposal and treatment of such wastewater is a major concern to the environment. The aim of this study is to investigate into the treatment of a dairy farm wastewater using the Upflow Anaerobic Sludge Blanket (UASB) process. The UASB was selected because it not only removes organic pollutants, but also generates methane-rich biogas as a source of energy.
Raw wastewater from a dairy farm was characterised in terms of pH, temperature, chemical oxygen demand (COD), total suspended solids (TSS), total Khejedal nitrogen (TKN), Ammonia-nitrogen (NH3-N), ortho-phosphate (PO42-) and Total Coliform. A pilot scale UASB reactor of 14.5 L capacity was set up and operated for a total period of 96 days, during which the treatment of the raw wastewater was investigated. The performance of the UASB reactor was assessed in terms of COD removal, volatile fatty acids (VFA) conversion and volume of biogas generated. The pH of the effluent was also monitored. A preliminary design and costing of an industrial scale UASB reactor for the dairy farm were performed.
The average pH, temperature, COD, TSS, TKN, NH3-N, PO42- were 8.0, 25.9 °C, 9550 mg/l, 29123 mg/l, 448 mg/l, 261 mg/l, 62 mg/l, respectively. The wastewater was highly pathogenic, as indicated by the high value of total coliform at 1.7 ´ 108 CFU/100ml.
The reactor was inoculated with an anaerobic sludge, obtained from an operating UASB reactor. The methanogenic activity of the sludge was 0.95 gCOD/gVSS.d. The pre-treated wastewater had an average COD load of 6735 mg/l and was fed in the reactor at organic loading rate (OLR) ranging from 4.7 to 28.9 kgCOD/m3d during the period of investigation. The reactor was initially fed at a flow rate of 0.44 L/h, corresponding to a hydraulic retention time (HRT) of 33 h and upflow velocity of 0.06 m/h. The start-up period took 6 weeks. COD removal efficiency attained 65.6%, which indicated the end of start-up period.
The mean OLR was subsequently increased from 13.1 to 20.7 and 26.8 kgCOD/m3d, corresponding to a HRT of 12 h, 8 h and 6 h respectively. A mean COD removal of 78.7% was achieved during the post start-up period while the VFA conversion averaged at 72.9%. Moreover, the investigations indicated that the COD removal efficiency was almost independent of the HRT and remained invariably above 60%. The optimum HRT for this study was found to be at 8 h. the study also revealed the existence of a linear relationship between the COD loading and the biogas produced. The maximum volume of biogas produced was 8.89 L/d. The average CH4 and CO2 content in the biogas produced were found to be 68.1% and 31.9%, respectively.
The treatment proposed and design consists of an equalisation tank of 30 m3 prior to a UASB reactor with a capacity of 40 m3 for a 25 m3/day wastewater at an average COD load of 6735 mg/l. The total capital investment was calculated at Rs 2,690,509 and the annual operating cost was estimated at Rs 220,000.