The role of Constructed Wetlands in treating farmyard dirty water

The Code of Good Agricultural Practice defines dirty water as “a low dry matter waste made up of water contaminated by manure, urine, waste milk and cleaning materials”. The Nitrates Directive regulations further define dirty water for land spreading during the closed period in terms of Biochemical Oxygen Demand (BOD) concentrations of =2,000 mg/litre, total nitrogen of =300 mg N/l and with a dry matter content of <1%. These concentrations are considerably lower than would be expected from animal manures and silage effluent for which BOD typically exceeds 30,000 mg/l, total nitrogen >3,000 mg N/l and dry matter >6%.

However, when compared to many other pollutants and the standards required for maintaining rivers and lakes in a good water quality state, dirty water is highly

polluting. The BOD of raw domestic sewage, for example, would be expected to be between 200 and 300 mg/l, while the BOD of dirty water recorded at Greenmount Campus is typically over 1000 mg/l. For lakes and rivers, concentrations of phosphorus (P) should be less than 0.05 mg/l to lower the risk of eutrophication. However, the total P concentration of dirty water recorded at Greenmount Campus was 50 mg/litre. The Code of Good Agricultural Practice states that dirty water “should be collected and disposed of carefully and must never be allowed to enter a drain or waterway”.

As dirty water is generally produced in large volumes with minimal value as a source of fertiliser, the management of it poses considerable difficulties for Northern Ireland farmers. In particular for dairy farmers, who not only often have areas of yard that are contaminated due to the movement of cattle but who also generate significant volumes of “wash water” from milking parlours and dairies.

Constructed wetlands have been suggested as a possible treatment option for dirty water. They are widely used in municipal and industrial wastewater treatment schemes (especially in Europe and the USA) where they are claimed to offer a simple technology with a low environmental impact, based on the pollution treatment ability of natural wetland systems.

The Greenmount Campus wetland consists of five ponds constructed in the summer of 2004 and planted in autumn of 2004. To allow the wetland plants to become established, the wetland was not used to treat dirty water until November 2005. Sampling and analyses were jointly undertaken by staff from Agri-Food and Biosciences Institute (AFBI), Greenmount Campus, (CAFRE), the Queens University of Belfast and the Water Management Unit of the Northern Ireland Environment Agency (NIEA).

The Greenmount wetland is used to treat dirty water from the Campus dairy unit. The three main sources of dirty water treated are:

Results from the records of the various wetland parameters sampled and analysed (Table 1) indicate that during the monitoring period, the treatment capacity of the wetlands was amply sufficient to meet the requirements for the discharge consent limit of 40 mg/l BOD set by NIEA. Nutrient retention within the wetland was also of a very high level, and consistently much higher than those reported in studies of many other wetlands. Phosphorus retention, considered a key criterion for dirty water treatment, has been especially high, even allowing for seasonal fluctuations and P outflow concentrations which have increased each year.

Table 1. Performance of the Greenmount Campus Constructed Wetland

Bacterial pathogen removal performance was consistently excellent, regardless of initial coliform concentrations or pond water levels, with both faecal and total coliforms reduced to very low counts, equivalent to low background levels of natural wetland water.

The ratio of wetland size to dirty yard area of approximately 2:1 was sufficient to meet the discharge consent BOD of 40 mg/l on all sampling occasions. However, the discharge concentrations of all contaminants from pond 4 were on average twice those of pond 5, which indicates that any less than the 5 pond, 1.25 ha system to treat the dirty water would have been unlikely to have achieved acceptable results.

An open day to demonstrate the potential of Constructed Wetland to treat farmyard dirty water is being organised for Wednesday 15 September 2010 at Greenmount Campus, CAFRE, near Antrim town. Farmers and growers wanting to learn more about the potential of Constructed Wetlands to treat farmyard dirty water can join tours of the Greenmount Wetland starting at 30 minute intervals from 10.00am with the last tour starting at 3.00pm.

The Constructed Wetland at Greenmount Campus, Cafre