Science and Engineering

Water Quality Protection

Tom Richard

Composting has long been viewed as an environmentally beneficial activity. To maintain that positive reputation it is essential that compost facilities consider and mitigate any adverse environmental impacts. Water quality protection can be accomplished at most composting facilities by proper attention to siting, ingredient mixtures, and compost pile management.

The results of water quality monitoring studies at Cornell and elsewhere indicate that outdoor windrow composting can be practiced in an environmentally sound manner (Richard and Chadsey, 1994; Rymshaw et al., 1992; Cole, 1994). However, there are a few aspects of this process that can potentially create problems. For leaf composting, the primary concerns are BOD and phenol concentrations found in water runoff and percolation. Biochemical Oxygen Demand and phenols are both natural products of decomposition, but the concentrated levels generated by large-scale composting should not be discharged into surface water supplies. Additional potential concerns when composting nutrient rich materials such as grass, manure, or sewage sludge include nitrogen compounds such as nitrate and ammonia, and in some cases phosphorus as well. With manure or sewage sludge there may also be pathogen concerns. These concerns, while important, are readily managed, and can be mitigated through careful facility design and operation.

A water quality threat?

Facility Design

Selecting the right site is critical to many aspects of a composting operation, from materials transport and road access to neighborhood relations. From an environmental management perspective, the critical issues are soil type, slope, and the nature of the buffer between the site and surface or groundwater resources. Soils can impact site design in a variety of ways. If the soils are impermeable, groundwater is protected from nitrate pollution, but runoff is maximized which increases the BOD, phosphorus, and pathogen threat to surface water. On the other hand, highly permeable soils reduce the runoff potential but may allow excessive nitrate infiltration to groundwater. Intermediate soil types may be best for sites which are operated on the native soil. For some large facilities, or those handling challenging waste materials, a working surface of gravel, compacted sand, oiled stone or even asphalt or concrete may be appropriate. Such surfaces can improve trafficability during wet seasons considerably, but the surface or groundwater quality issues remain.

The buffer between the site and surface or groundwater resources is the first line of defense against water pollution. Deep soils, well above the seasonally high water table, can filter solid particles and minimize nitrate migration. Two feet of such vertical buffer are required by New York State regulations, and while a greater depth would be advantageous, such soils are rare in many parts of the state. Horizontal buffers are required to be a minimum of 200 feet from wells or surface water bodies and 25 feet from drainage swales in New York State. Although the nature of this horizontal buffer is not specified in the regulations, grass can help filter the runoff and minimize pollutant migration. Such vegetative filter strips are further described below.

Site design issues which may impact on water quality include the selection of a working surface (native soil or an improved surface), exclusion of run-on to the site by surface diversions, possible drainage of wet sites, and the possible provision of roofs over some or all of the composting area to divert precipitation and keep compost or waste materials dry. In all but fully roofed sites there will be surface runoff which may need to be managed as described below. Slope of the site a surface drainage to either divert uphill water away from the site or collect site runoff for management should be considered in the design process.

A number of factors combine to determine the quality of water running off compost sites. One obvious factor which is often overlooked is the excess water running onto the site from upslope. Diversion ditches and berms which divert that water around the site will minimize the runoff which needs to be managed. Siting the facility on a soil with moderate to high permeability will also significantly reduce the runoff generated on the site. For the runoff which remains, alternatives to surface discharge include such simple technologies as soil treatment, filter strips, or recirculation, so that sophisticated collection and treatment systems should not be needed.

These simple, low-cost treatment strategies have proven effective for a variety of wastewaters and organic wastes (Loehr et. al., 1979). Soil treatment forces the percolation of water through the soil profile, where these organic compounds can be adsorbed and degraded. Vegetative filter strips slow the motion of runoff water so that many particles can settle out of the water, while others are physically filtered and adsorbed onto plants. Recirculation would involve pumping the runoff water back into the compost windrows, where the organic compounds could further degrade and the water would be evaporated through the composting process. This last option should work very well during dry summer or early fall weather, when water often needs to be added, but would not be appropriate if the moisture content of the compost was already high.


The day to day operation of the composting site offers considerable opportunities to minimize water quality impacts. The proper selection, mixing, and management of materials can help control overall runoff, BOD, pathogen and nutrient movement. Assuring appropriate moisture and carbon to nitrogen (C:N) ratios throughout the composting process can be very effective at limiting these pollutants. A review of the basic principles of compost facility operations, with more detailed discussion of these issues as well as data on C:N ratios, water content, and bulk density of some common agricultural composting materials are provided in the NRAES On Farm Composting Handbook (Rynk et al., 1992) and the Getting the Right Mix section of these web pages.

Nitrate is most easily controlled by maintaining an appropriate C:N ratio in the composting mixture. Raw materials should normally be blended to approximately 30:1 carbon to nitrogen ratio by weight. The ratio between these key elements is based on microbial biomass and energy requirements. Inadequate nitrogen (a high C:N ratio) results in limited microbial biomass and slow decomposition, while excess nitrogen (a low C:N ratio) is likely to leave the composting system as either ammonia (odors) or nitrate (water pollution). In a nitrogen limited system microorganisms efficiently assimilate nitrate, ammonia and other nitrogen compounds from the aqueous phase of the compost, thus limiting the pollution threat.

The ideal ratio of carbon to nitrogen will depend on the availability of these elements to microbial decomposition. Carbon availability is particularly variable, depending on the surface area or particles and the extent of lignification of the material. Composting occurs in aqueous films on the surfaces of particles, so greater surface area increases the availability of carbon compounds. Lignin, because of its complex structure and variety of chemical bonds, is resistant to decay. For both of these reasons the carbon in large wood chips is less available than that in straw or paper, so greater quantities of wood chips would be required to balance a high nitrogen source like manure.

The data from experimental studies indicates low C:N ratio mixtures can generate nitrate levels above the groundwater standard (Rymshaw et al.; 1992, Cole, 1994) Much of this nitrate in runoff and leachate will infiltrate into the ground. While microbial assimilation and denitrification may somewhat reduce these levels as water passes through the soil, these processes will have a limited effect and are difficult to control. Proper management of the C:N ratio is perhaps the only practical way to limit nitrate contamination site short of installing an impermeable pad and water treatment system.

The other important factor to consider when creating a composting mixture is water content. From a microbial standpoint, optimal water content should be in the 40 to 60% range. This moisture content is a balance between water and air filled pore space, allowing adequate moisture for decomposition as well as airflow for oxygen supply. The ideal water content will vary somewhat with particle size and density, and fine, dense organic substrates should be drier if adequate aeration is to be assured. Excess water, in addition to increasing the odor potential via anaerobic decomposition, will increase the runoff and leachate potential of a composting pile during rainfall events.

With both C:N ratios and moisture content, the optimum water and nitrogen levels for rapid composting may create a greater than necessary water pollution threat. Increasing the C:N ratio from 30:1 to 40:1 and decreasing the water content from 60 % to 50% may slow down decomposition somewhat, but can provide an extra margin of safety in protecting water quality.

Once the materials are mixed and formed into a compost pile windrow management becomes an important factor. Windrows should be oriented parallel to the slope, so that precipitation landing between the windrows can move freely off the composting area. Pile shape can have a considerable influence on the amount of precipitation retained in a pile, with a flat or concave top retaining water and a convex or peaked shape shedding water, particularly in periods of heavy rain. These effects are most pronounced when the composting process is just starting or after a period of dry weather. In the early phases of composting a peaked windrow shape can act like a thatched roof or haystack, effectively shedding water. Part of this effect is due to the large initial particle size, and part is due to waxes and oils on the surfaces of particles. Both of these initial effects will diminish over time as the material decomposes. During dry weather the outer surface of even stabilized organic material can become somewhat hydrophobic, limiting absorption and encouraging runoff.

If a pile does get too moist, the only practical way to dry it is to increase the turning frequency. The clouds of moisture evident during turning release significant amounts of water, and the increased porosity which results from turning will increase diffusion and convective losses of moisture between turnings. This approach can be helpful during mild or warm weather, but caution must be exercised in winter when excessive turning can cool the pile.

Runoff management

Implementation of the preventative measures described above can considerably reduce the water pollution threat. However, some facilities may require additional management of runoff from the site. As indicated above, the runoff pollutants of primary concern are BOD and phosphorus, largely associated with suspended solids particles. Pathogenic cysts may either be absorbed on particles or be free in solution, and again the relative significance is not adequately researched. Four readily available strategies exist to help control these pollutants: vegetative filter strips, sediment traps or basins, treatment ponds, and recirculation systems.

This simplest runoff management strategy is the installation of a vegetative filter strip. Vegetative filter strips trap particles in dense surface vegetation. Grasses are commonly used, and must be planted in a carefully graded surface over which runoff can be directed in a thin even layer. Suspended particles flowing slowly through the grass attach to plants and settle to the soil surface, leading to a significant reduction in BOD levels.

Sediment traps operate by settling dense particles out of the runoff. Particles settle by gravity during passage through a basin of slowly moving water. This approach can be particularly effective for removing phosphorous associated with sediment. Because much of the BOD and nitrogen in compost site runoff will be in light organic particles, the effectiveness of this approach may be somewhat limited. However, it will help limit sediment movement off the site, and can be a useful adjunct to either a vegetative filter strip or a treatment pond, enhancing the effectiveness of each.

During dry periods of the year compost runoff can be recirculated to the compost piles themselves, or alternately used to irrigate cropland or pasture. The nutrients as well as moisture can thus serve a useful purpose, either by supplying needed moisture to the compost windrows or by providing nutrients and water to crops. However, a recirculation system requires both a pumping and distribution system and adequate storage capacity for prolonged wet periods. While this approach offers a closed system which appears ideal for pathogen control, care may need to be taken to separate runoff from the fresh manure to avoid contaminating finished compost or crops.

Storage requires the construction of a pond, which can also be used to treat the waste. Ponds can be designed for aerobic or facultative treatment of runoff water. In either case microorganisms continue the decomposition process started in the compost pile, but in an aqueous system. As the organic material stabilizes, the BOD levels will drop. Pathogen levels are also expected to drop, although the rate will be dependant on seasonal temperature variations and will be slow during winter in unfrozen portions of a pond. To be effective, ponds must be designed to contain the runoff from major storm events, with an adequate residence time for microbial stabilization. Details of pond design vary with climate, runoff characteristics, and pond effluent requirements. The Natural Resources Conservation Service (NRCS) has considerable expertise in adapting treatment systems to the local situation.

Runoff collection pond

All these treatment options will help with nitrogen and phosphorus removal as well as BOD and pathogens. Sediment basins and ponds will settle out particulate matter, which includes bound nutrients such as phosphorus. However, these sedimentation mechanisms are not likely to remove nutrients or BOD as well as soil adsorption and crop uptake in a land treatment system. For nitrogen removal, vegetative filter strips and irrigation systems can both be effective, and either is enhanced by alternating flow pulses with rest periods. Phosphorus removal is most efficient under aerobic conditions, and irrigation systems generally show higher removal rates than vegetative filter strips although either can be effective. Although little is currently known about the effectiveness of these approaches in destroying the pathogens of concern, increased opportunities for adsorption, desiccation, and other forms of environmental and microbiological stress are integral to the physical and biological treatment processes described. An appropriate combination of these removal mechanisms can be designed to address the pollution parameters of local concern.


Water quality protection at a composting site can be accomplished through proper site design, operations, and runoff management. Composting facilities vary widely in size, materials processed, and site characteristics, and all these factors will effect the design of appropriate preventative measures. Although the available evidence is limited, current indications are that runoff from composting windrows has BOD and nutrient levels comparable to low strength municipal wastewaters. Land treatment systems which have proven effective for these alternative wastewaters we can expect to be effective for windrow composting facilities as well.


Cole, M.A. 1994. Assessing the impact of composting yard trimmings. BioCycle 35(4):92-96.
Loehr, R. C., W. J. Jewell, J. D. Novak, W. W. Clarkson, G. S. Friedman. 1979. Land Application of Wastes, Vol. II. Van Nostrand Reinhold Co. New York, NY.
Richard, T.L. and M. Chadsey. 1994. Environmental Impact Assessment. In: Composting Source Separated Organics. Edited by BioCycle staff. J.G. Press, Inc. Emmaus, PA. pp 232-237. Also published in 1990 as: Environmental monitoring at a yard waste composting facility. BioCycle. 31(4):42-46.
Rymshaw, E., M.F. Walter, and T.L. Richard. 1992. Agricultural Composting: Environmental Monitoring and Management Practices. Dept. of Ag. & Bio. Eng., Cornell University, Ithaca, NY.
Rynk, R., M. van de Kamp, G.B. Willson, M.E. Singley, T.L. Richard, J.J. Kolega, F.R. Gouin, L. Laliberty, Jr., K. Day, D.W. Murphy, H.A.J. Hoitink, and W.F. Brinton. 1992. On-Farm Composting Handbook. NRAES, Cornell University, Ithaca, NY. 186 pp.


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