Design, economic and environmental analysis of dairy waste management systems

The project studied the performance of dairy waste management (DWM) systems in order to determine their potential in producing energy and reducing environmental pollution. The project involved a series of laboratory studies on anaerobic digestion, effluent treatment and utilization, and an analysis of hypothetical DWM systems. It was observed that ambient temperature digestion of dairy manure was possible at organic loading rates (OLR) as high as 6.0 kg/m3-d and hydraulic retention times (HRT) as short as 5 days with average ambient temperatures in the range 15--25°C. Increasing the OLR from 1.0 to 6.0 kg VS/m3-d increased the biogas production rate but decreased the biogas yield and treatment efficiency. At an OLR of 6.0 kg VS/m 3-d, biogas production rates as high as 1.66 m3/m 3-d and 1.02 m3/m3-d were obtained at summer and winter average ambient temperatures of 25°C and 15°C respectively for a suspended-growth digester. The corresponding biogas production rates for an attached-growth digester were 1.88 m3/m3-d and 1.08 m3/m3-d in summer and winter, respectively. The stability of anaerobic digestion was never affected by changes in temperature, high OLR and concentrations, or short HRT.; Gravity settling of suspended solids was only effective for effluents with less than 2% total solids. Concentrated effluents required dilution with water. Both chemical and electro-coagulation were capable of removing over 90% of the suspended solids from digester effluents to produce clarified wastewater. Although both aluminum sulfate and ferric sulfate were effective coagulants for digester effluents, the unit cost of coagulation was low with aluminum sulfate. Electro-coagulation using iron electrodes was more effective than using aluminum electrodes and the treatment efficiency was dependent on electrode surface area and retention time. In hydroponics studies a good crop of lettuce was obtained when supplied with aerated digester effluents or a mixture of aerated and raw digester effluents. Raw digester effluents produced a poor yield of lettuce possibly due to ammonia toxicity. The hydroponics system removed over 75% of the nitrogen and soluble phosphorus from the wastewater.; The analysis of hypothetical DWM systems with AD systems revealed that their performance was dependent on the size of dairy herd, digester operating temperature and OLR, digester heating method, effluent treatment method, and end-use of treated effluent. Well-designed DWM systems are capable of producing surplus electricity for use outside the system. The 15-year life-cycle cost of electricity from DWM systems on farms with 50--200 cows was estimated to be between {dollar}0.06 and {dollar}2.00/kWh. A 100-year life-cycle greenhouse gas analysis showed that DWM systems with AD systems were capable of reducing global warming potential by about 81%. On the basis of this project dairy farmers should be encouraged to introduce AD systems into DWM systems not only for energy production but also for reducing global warming.