Farming-out reed technology
Stewart Moir of Scottish Agricultural College's consultancy department, Heartland Environmental, explores the potential for reed beds in treating agricultural effluent
The Scottish Agricultural College (SAC) is actively involved in the treatment of effluents which issue from rural communities and industries. As Scotland's largest reed bed designer, SAC has built a reputation over the years as being a UK leader in the development of reed bed technology.
Since 1985 SAC has been involved in an advisory capacity on many reed bed systems and since 1993 the company has been designing and building reed bed systems commercially. Reed beds, however, form only a part of its overall service to rural communities, industry and agriculture under its 'Effluent Treatment Services' programme.
SAC advises on and specifies a range of systems available for developments that do not have access to a local authority mains sewer. Reed beds form the bulk of its business, however, and to-date SAC has designed in excess of 90 systems. Around 70% of these systems are for domestic clients. SAC systems treat sewage from developments which range from between one house and several hundred houses. The other 30% are reed bed systems for rural businesses and industries. These reed beds serve developments such as caravan parks, hotels, visitor centres and effluents from various industries such as vegetable processing, lorry park run-off and leachate from landfill sites. Currently, SAC reed bed systems service over 8,000 people, which is about 1.5M litres of effluent every day. This equates to over 35,000m² of reed beds or about five football pitches in surface area.
One of SAC's largest reed bed systems is for Cardrona village, a new rural development in the Scottish Borders three miles south of Peebles. Cardrona Village will consist of over 200 houses and a 150 bed four star hotel. The treatment system is designed for a population equivalent of 1,400 and consists of four large septic tanks followed by four large reed beds. This configuration is designed to accommodate further development over the coming years. Two reed beds are now fully operational, the first completed in early 2000, the second in October 2001.
The design process was challenging as not only had the legal requirements of the discharge consent to be met, the system had to fit into the overall site layout. This meant consultation with several house builders and the golf course designer. The 340 acre Cardrona village site includes a new 18-hole golf course, which surrounds the development on three sides. The reed bed system had to be designed around several of the fairways, tees and greens. SAC also designed the storm water run-off system for the village and the treatment pond had to be landscaped within the golf course.
From the outset sustainability was a key design criteria for Cardrona, believed to be the first new rural village in the Scottish Borders for over 200 years. The system for Cardrona Village was designed for Scottish Borders Council in consultation with East of Scotland Water (EoSW). The latter will be responsible for operation and maintenance in the future. As well as providing an environmentally-sound treatment system, the reed beds are actually built using over 7,000 tonnes of recycled gravel, graded out from soil materials used in the construction of the golf course.
Down on the Farm
SAC is not only involved in consultancy but also has an active programme of research and development on the treatment of agricultural effluents and also using waste materials as growing media. In the past SAC's research has looked at nutrient removal and pesticide degradation within reed bed systems. In 1999 SAC secured funding from the Scottish Executive Environment & Rural Affairs Department (SEERAD) to investigate and develop a reed bed system which could treat effluents from the dairy unit at SAC's Auchincruive farm, near Ayr.
There are only a handful of systems in the UK designed for agricultural effluents and most of them were built for research purposes. The very high strengths of agricultural effluents have precluded them from being suitable for treatment by reed bed systems. When comparing a typical rural stream which has a BOD in the region of 3mg/l with silage effluent for example, with a BOD of anything between 20,000 and 80,000mg/l, the challenge is clear. Any release of agricultural effluent into a stream will result in a serious pollution incident.
The main aims of the research were to produce an effluent suitable for recycling on the farm, achieve an effluent suitable for discharging to a watercourse and ensure cost-effective construction, operation and maintenance.
Farms have many types of effluent which arise from many different activities but they can be grouped into washings and contaminated waters. Washings can, for example, arise from washing down machinery or milking equipment. Contaminated waters are a pollutant and unsuitable for discharge. These waters can be from yards where livestock is collected or moved across, or from clamps which store livestock manures prior to application on the land. The strongest contaminated waters come from silage clamps (where grass is stored for winter fodder).
These washings and contaminated waters are normally collected, stored and applied to the land with livestock slurries thereby recycling nutrients. However, these effluents can pose problems when stored and applied with slurries and removing them from the system, by treating them separately, could provide several benefits.
At SAC's Auchincruive farm they separated the washings from the dairy unit and fed them into a treatment system. These washings arise from the sterile part of the milking process. The cows are milked twice a day and after each milking the pipe work and tanks, in which the milk is stored prior to collection, are flushed and cleaned. The dairy washings, therefore, consist of water, milk residue and disinfectants. Treating these washings is a challenge as they are generally around pH12 and are sterile in addition to having a BOD of between 1,000 and 2,000mg/l. Dairy washings are a true waste product to the farmer as they have negligible nutrient content.
The dairy washings are collected and pre-treated in an aerobic sequence batch reactor (SBR). The washings are then discharged into a number of reed beds for final treatment. Both the SBR and the reed beds work in sequence with the scheduled milking of the cows. The farm runs a large herd of over 200 cows and each day the washings account for approximately 3m3 of wastewater which is equivalent to 1,100m³ per annum.
Pre-treating the washings means that the footprint of the reed beds can be kept to an absolute minimum so as not to use valuable land. Hence the reed beds have a total footprint area of only 52m².
The system has been running since January 2000 and results have been excellent. SAC has consistently achieved a standard issue from the reed beds below the typical UK watercourse discharge standard of 20mg/l BOD and 30mg/l suspended solids. This type of system is now ready to be used by dairy farmers all over the UK. This research is also applicable to food processors producing milk-based products.
Following on from this success SAC is now looking at treating silage effluent currently collected with other yard run-off and irrigated onto the land at Auchincruive separate from the livestock slurry. This effluent poses a completely different problem in that pH4 is an average value and BOD varies between 5,000 and 30,000mg/l. SAC is using similar technology in that it is aerating the collecting tank and discharging into the reed bed system which has been extended to a total footprint area of 130m2. Since May 2001 the extended reed bed system has been receiving 3m³ of dairy washings plus 3m³ of silage effluent each day. Preliminary results are encouraging but would not at this stage allow the discharge of the effluent into a watercourse. SAC is confident that this can be achieved in the future.
The benefits of treating agricultural effluents, rather than applying them to land, are significant. If washings and waters are not stored with the slurry more storage space is available. The slurry will be thicker and therefore less likely to run-off the land. This may go some way to reducing diffuse pollution from farmland.
Farmers will also get better value for money. Less volume to spread means reduced costs and each load of slurry that is spread will have a greater concentration of nutrients per m³ when not diluted by nutrient-deficient washings. Depending on the farm set-up other benefits such as reducing storage costs, reducing water bills by recycling treated water and freeing-up land to increase production are all possible spin-offs from treating effluents rather than storing them prior to land application.
This programme of research runs until March 2003. Both the dairy washings and the silage effluent systems are full scale, farm size demonstration units. Further information on the pollution of water from agricultural sources can be found in the Ministry of Agriculture, Fisheries and Food (MAFF, now Department of the Environment, Food and Rural Affairs, DEFRA) 1998 publication 'Code of Good Agricultural Practice for the Prevention of Water', commonly called The Water Code and also in the Scottish Office Agriculture, Environment and Fisheries Department (SOAEFD, now SEERAD) 1997 publication 'Prevention of Environmental Pollution from Agricultural Activity', commonly called The PEPFAA code.