Real-time treatment of dairy manure: Implications of oxidation reduction potential regimes to nutrient management strategies

A pilot-scale sequencing batch reactor (SBR) was operated at a dairy farm to test real-time based control in winter operation conditions. A combination of high loading and low oxidation reduction potential (ORP) conditions in the aerobic stage of SBR treatment (an end value of −50 to −150 mV) inhibited nitrification while maintaining carbon removal. After a period of over-aeration over several cycles, the ORP at the end of the aerobic stage increased to values of 50–75 mV. Subsequently, nitrification was observed, accompanied by higher total cycle times. Significant increase in removal efficiencies of ammonical nitrogen (α < 0.0001) and chemical oxygen demand (α < 0.001) were observed for the high ORP phase. It is postulated that higher ORP regimes are needed for nitrification. In low ORP regimes, nitrification is absent or occurs at an extremely low rate. It is also noted that nitrifying systems treating high strength animal manure can possibly lead to unacceptably high levels of effluent nitrate + nitrite nitrogen (NOx-N). Two manure management schemes are proposed that give the farmer an option to either retain the nutrients, or remove them from the wastewater. Some advantages and disadvantages of the schemes are also discussed.