BioCycle December 2005, Vol. 46, No. 12, p. 54
The challenge of dealing with runoff from windrows when composting organic residuals is discussed by researcher Sean Tyrrel of Cranfield University.

WHEN RAIN FALLS on open wind-rows, compost liquor forms which percolates through materials, explains Sean Tyrrel of Cranfield University in the winter 2005 issue of the United Kingdom’s Composting News. As the liquor runs off – accumulating more contaminants from the hard surfaces it flows over, it is collected in a storage lagoon. Its strength can be comparable to raw sewage in terms of treatable parameters such as five day biochemical oxygen demand, ammonia and suspended solids. According to Tyrrel, options for safe disposal include recycling liquor back to the windrows to supply moisture during dry weather.
As explained in his report, research at Cranfield University is exploring the feasibility of on-site biological treatment which would use a low-tech biofilter system. The biological film which forms on the filter media surface would transform pollutants such as biodegradable organics and ammonia into less hazardous substances. Researchers wanted to determine if such materials as waste wood chips or packaging residuals could be used as a biofiltration medium.
Tyrrel writes that “oversized” materials reduced the pollution potential of the liquor. “Oversize is the coarse fraction removed during final screening of windrowed materials to produce finished compost – mostly twigs and branches that are too large. Findings suggest that oversize particles had the right size, shape, surface properties to encourage an active biological film to develop and to allow liquor and air to move freely through the filter.”
Further studies were conducted at a larger scale and longer period to confirm the potential of liquor biofiltration. Three types of biofiltration medium were tested: oversize, finished compost and granite chips as a control. Liquor from SITA UK’s site at Lount in Leicestershire, a typical green waste composting facility, was used to dose the filters. Biofilter performance was compared over a ten month period.
This larger scale trial confirmed the findings of the preliminary study. The oversize medium consistently outperformed both of the other test media in the removal of ammonia. The oversize filter typically removed > 80 percent of the ammonia in the liquor compared to < 60 percent removal in the compost filter and < 45 percent in the granite filter. This is described as “an interesting finding as it suggests that the organic filter media were more effective than the mineral medium.” A high quality treated effluent with an ammonia concentration of < 10mg/l was produced on 95 percent of sampling occasions by the oversize filter.
Untreated compost liquor generally has a very low concentration of nitrate (< 0.5 mgN/l) because of anaerobic conditions in the storage lagoon. Nitrate concentrations in the treated effluent from the oversize filter rose to around 10 mgN/l due to the activity of bacteria in the biofilter which are able to oxidize ammonia to nitrate. A limited survey of BOD5 indicated that the oversize biofilter was capable of removal efficiencies of 68 to 92 percent, giving final effluent concentrations in the range of 6-22 mg/l. Compost liquor tends to have a high chemical oxygen demand (COD) which is thought to be due to the leaching of soluble humic compounds from the compost. COD removal was not impressive in any of the filters, but oversize proved to have the biggest effect, typically reducing concentration by about 20 percent. As the COD is not readily biodegradable, it poses little deoxygenation risk to receiving watercourses.
In summary, these findings indicate that the oversize biofilter is capable of producing an effluent that could in theory be discharged to a watercourse if measures are taken to control suspended solids and polish occasional ammonia peaks. A recently completed study using a small, portable reed bed as a polishing device has produced very promising results. “The next step is to develop what is still an experimental system into a robust, self-contained, and easily maintained system capable of sustainable operation at a composting facility with low management inputs,” concludes Tyrrel.