Protecting Water Quality at Agricultural Composting Facilities

Author: Nick Andrews, Small Farms Program, Oregon State University

Publish Date: Summer 2011

Anyone who has composted at a medium to large scale (i.e. more than 100 tons per year) in a wet climate knows that leachate is easy to produce and can be difficult to manage. Saturated compost becomes anaerobic and is difficult to use, and the leachate can contaminate water. OSU Extension, experienced agricultural composters, leading composting consultants, and the Oregon Departments of Agriculture (ODA) and Environmental Quality (DEQ) are collaborating on a new project to promote high-quality composting on farms in Oregon and to protect water quality at agricultural composting facilities. In 2009 the DEQ implemented new rules for composting in Oregon, including agricultural composting operations processing more than 100 tons of vegetative or manure feedstock or more than 20 tons of mortality and other proteinaceous waste per year. The main drivers for these new regulations were the increasing potential to compost a larger portion of organic wastes produced in Oregon, and the need for adequate environmental standards to protect water from resulting leachate.

In response to these new regulations, OSU Extension is leading a project to promote good composting practices on farms, and to identify cost-effective methods that farmers can use to protect water quality near their composting facilities. OSU Extension and composting consultants will conduct an anonymous survey of agricultural composters to study the challenges posed by agricultural composters. We will also publish resources with information about agricultural composting and the protection of water quality near agricultural composting facilities. We will offer at least one workshop addressing agricultural composting and water quality, and a tour of agricultural composting facilities. The project website is under development at: Please contact Nick if you are interested in participating.

The new DEQ rules require that farms composting more than 100 tons of vegetative or manure feedstock or more than 20 tons of mortality and other proteinaceous waste per year from their farm or from off-farm sources must submit an application to DEQ for a composting facility site screening, including land-use compatibility statement (LUCS) signed by the local planning authority. Composting is generally recognized as an agricultural activity when done in conjunction with farm use, so it is expected that a farm composting operation will easily obtain a favorable planning decision on the LUCS. DEQ screens agricultural composting sites to assess the risks posed to surface water and groundwater, and the potential for odor problems. If your operation is expected to pose a low risk, you will receive a registration permit and report the volume of non-agricultural waste composted annually. This approval is good for 10 years; there is a one-time registration fee of $150. If your operation is expected to pose a high risk, you will be required complete a more detailed DEQ approved compost plan, receive an individual composting facility permit, and report the volume of non-agricultural wastes composted annually. Higher risk operations pay, in addition to the screening fee, a one-time plan review fee, and an annual compliance fee. During the screening process, DEQ will work with farm composting operations to help reduce the risk posed by the facility. Criteria used by DEQ to screen composting facilities are listed in the sidebar.

There are several steps farmers can take to prevent leachate generation, to manage leachate, and protect water quality. Choose a site for composting that is less likely to contaminate water or create odor problems. Heavier soils with slower infiltration rates (i.e. silty clay loam) are preferred because the risk of leaching is lower than on excessively well-drained sandy soils. Avoid sites with shallow aquifers, a high seasonal water table, or sites that are likely to flood. Fields with drainage tiles are more likely to transport leachate to surface water if drainage water is not treated. Whenever possible prevent stormwater run-off from mixing with compost piles and leachate. If compost piles can be actively managed to stabilize the feedstock before winter rains arrive, leachate will be minimized. In many cases, it is a good idea to leave piles undisturbed during the winter so that they can be covered and protected from heavy rain. Covered or not, composting material should be consolidated to reduce the surface area exposed to precipitation. Tall piles with steep sides are often better able to absorb moderate amounts of rain. The shallow edges of compost piles and relatively small piles of compost can produce more leachate. A good C/N ratio (i.e. 25-40/1), good moisture content (40-60%), and good porosity will promote thermophilic composting. The heat of composting in a well-managed compost pile evaporates water and may reduce leachate generation. In some situations, it is possible to build a slightly drier compost pile that can absorb moderate rainfall before becoming saturated and generating leachate. When leachate is produced, it must be well managed so that it doesn’t contaminate water. If the compost is on an impermeable pad, or the site is graded or has a slight natural slope, filter strips or bioswales on the downhill side or slope can help to disperse leachate across a larger surface area and allow it to infiltrate. Collected leachate can also be land applied at agronomic rates. Direct injection of leachate below the surface is not usually recommended and requires additional regulatory oversight.

In addition, leachate that is allowed to pool and stagnate on the site can be a primary source of odor. A well-graded site and adequate management including appropriate dispersal of leachate can significantly reduce odor potential.

The water protection strategies that make sense at a specific site vary depending on site characteristics, the types and volume of feedstock handled, rainfall patterns, and other management considerations. By late, 2012 new composting and water quality guides will be published and available online. Many of the partners in this project also teach an agricultural composting workshop at OSU’s North Willamette Research and Extension Center. Information about the Agricultural Composting Resources and Education Series is available at: