Carbon and nitrogen removal rates from a laboratory and pilotscale horizontal flow biofilm reactor

In Ireland, dairy washwater commonly comprises farmyard run-off and dairy parlour washings.  Land-spreading is the most widely used method for treating this wastewater.  However, this method can be labour intensive and can cause, in some cases, the nitrate contamination of groundwater. Partial remediation of such soiled water prior to landspreading is desirable In this study, a pilot-scale horizontal flow biofilm reactor (HFBR) with step-feed was constructed and tested at a dairy farm site in County Offaly, Ireland. The HFBR comprised an 810 mm high stack of 45 horizontal corrugated polypropylene sheets each having a top surface plan area (TSPA) of 1.92 m2 and a total surface area of 2.48 m2. At their ends, the sheets were alternately offset to allow the wastewater to flow horizontally along each sheet before dropping vertically to the sheet below.  Biofilms developed on the sheets and treated the wastewater as it flowed through the reactor. Soiled water, consisting of a mixture of water, washing chemicals, milk, manure and some rainwater was pumped from an underground storage tank onto the unit. During the 122-day study, the total hydraulic loading rate, based on the TSPA of the sheets was 50 l m2.day.  Soiled water was pumped for three minutes each hour, in a step feed arrangement at rates of 33.33 l m2.day onto the top sheet (Sheet 0) and 16.67 l m2.day onto Sheet 30.  The average influent concentrations were: 2904.2 mg total chemical oxygen demand (COD) l, 950 mg 5-day biochemical oxygen demand (BOD5) l and 177.9 mg total nitrogen (TN) l. The loading rates were 145.2 g total COD m2.day, 47.5 g BOD5 m2.day and 8.9 g TN m2.day, based on the TSPA.  From Day 45 the HFBR unit removed 74.9 % total COD and 69.6 % BOD5, equivalent to TSPA removal rates of 108.8g COD m2.day and 34.1 g BOD5 m2.day. At the end of the study the NH4-N on Sheet 29 had reduced from 123.1 mg l in the influent to 37.0 mg l, and NO3-N had increased from 0.0 mg l to 69.0 mg l indicating that nitrification was occurring. Figures 1 and 2 show typical COD and nitrogen profiles during steady state operation of the unit. The total nitrogen (TN) removal percentage was 56.0 % equating to a TSPA removal rate of 5.0 g TN m2.day.  The unit was simple to construct and operate, with little maintenance. The technology does not require any mechanical aeration system and could be installed into in-situ tanks on farms. Improved performance is readily achieved by increasing the number of sheets stacked or reducing the hydraulic loading rate. The HFBR provides a simple, robust and economical alternative for remediation of agricultural soiled water before further use.