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.
Operations
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.
Summary
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.
References
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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Cornell Waste Management Institute ©1996
Cornell University
Ithaca, NY 14853
607-255-1187
cwmi@cornell.edu