Effluent review on dairy

Identifying benefits for environmentA study conducted on the effluent stream at Department of Primary Industry's dairy, Ellibank in Victoria, has provided a better understanding on how to improve the potential for energy recovery, energy savings, water and nutrient recovery.

The dairy, which is a Victorian State Government entity, is run on commercial lines and milks in the order of 500 cows. The study was conducted by Active Research over a six-week period and used a test rig with a 2200 L fixed-film anaerobic digester operating at 38°C. A total of 53,000 L of effluent was treated during the study.

Effluent was collected from the existing flume and was pumped to a holding tank located beside the test rig. The influent was then pumped at a prescribed rate from the receiving tank to the bottom of the digester using a positive displacement pump controlled through an inverter. Before arriving at the digester, it passed through a magnetic flowmeter and an ultrasound unit for flock disintegration. The liquid then slowly rose through the various stages of decomposition and out through a spillway and on to storage.

The fixed-film system facilitates the faster production of methane gas, which can then be used to replace fossil fuels. According to the study, 1 kW/h of electricity generated from methane will produce 0.035 kg of greenhouse gas compared to 1.05 kg produced by coal-generated electricity. With an annual usage of 240,000 kW, the dairy could save 243,000 kg of CO2 using electricity generated from methane.

As fixed-film anaerobic digestion removes mainly carbon, nutrients contained in the organic matter are conserved and reduced to more soluble and biologically available forms. This provides a more predictable quick-release organic fertiliser that can be applied to pasture at optimum rates for maximum plant growth, with minimal loss to the environment. A dairy generating 30,000 L of wastewater per day would be creating a benefit of 9 kg of nitrogen per day.

The fixed-film system has another advantage in that there is an almost complete absence of residual sludge. Any biological sludge which may escape the fixed-film digester and accumulate in the final setting tank is progressively returned to the digester tank for mixing with fresh influent and reprocessing, thus ensuring maximum gas production.

The study concluded that the use of fixed-film anaerobic digestion for waste treatment produces benefits such as energy production and conservation, elimination of odour, pathogen control, minimising sludge production, conservation of nutrients and reduction of greenhouse emissions.