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.