Table 3 summarizes the more common problems at leaf composting sites, their causes, and recommendations for their remedy. Most problems can be prevented by proper facility siting, design, operation, and maintenance. Grass clippings present additional concerns, which are also addressed in the discussion below.

 A. Odor

The major problem encountered even at leaf only composting sites is odor. Those unfamiliar with handling large masses of leaves may be surprised at how serious a problem this can be. Starting with relatively innocuous leaves, it is possible to generate odors comparable to those of a barnyard or worse. Grass clippings greatly intensify both the odor strength and its unpleasantness.

In general, odor problems develop in four stages:

1) odorous compounds must be present initially or be produced during processing;
2) these odors must be released from the pile;
3) the odors must travel off-site; and
4) they must be detected by sensitive individuals (receptors).
An odor problem can be prevented by eliminating any stage.

With the minimal technology described previously (Section V.A), stages 1-3 all occur, but since no receptors are present (stage 4), there is not a problem. Except where very large buffer zones are present, however, this approach to odor "control" is not possible.

In most cases, prevention of odor problems can best be achieved by preventing odor formation in the first place (Stage 1). For leaf composting this means avoiding prolonged anaerobic conditions. Under anaerobic conditions, volatile organic acids (which have vinegar, cheesy, goaty, and sour odors), alcohols and esters (fruity, floral, alcohol-like), and amines and sulfur compounds (barnyard, fishy, rotten) can be produced. In contrast, with aerobic conditions only a mild earthy odor is expected. If excessive ammonia or urea-based fertilizer, grass clippings, or other high nitrogen materials are added, an ammonia odor also may be produced even under aerobic conditions. Prevention of anaerobic conditions is virtually impossible with grass clippings.

The major cause of odor production at leaf composting sites is making the windrow too large, especially when first assembled. Because of the initial high concentration of readily degradable material, there is a high demand for oxygen. If the piles are too large, sufficient oxygen cannot penetrate from the outside, and a large anaerobic core develops. Decomposition slows down, switching over to the odor-producing acid fermentation described above.

A second important source of odor production is failure to form windrows quickly enough once the leaves are collected. Leaves cannot be simply dropped at the site for later composting, or collected and stored elsewhere. Although the intention might be to store them, temporary storage of leaves, unless they are very dry, can result in vigorous decomposition within one to two days, leading to anaerobic conditions and the production of offensive odors. Grass clippings, as discussed earlier (Section VI.A), are almost always odorous already when they are delivered to the composting site.

If odors should be produced at a site, or if odorous materials are dropped off at the site (such as occurs with grass clippings, or previously stored leaves), the second line of defense is to prevent their release (Stage 2). This might best be accomplished by leaving the odorous mass undisturbed until oxygen has penetrated sufficiently to destroy the odors. However, this may take several months or even years. Shaving off thin (1-2 foot) layers from the edges as they become aerobic may help speed this process.

If a long wait is not practical, another approach may be possible. Since many of the odorous compounds in leaf composting are acidic in nature, raising the pH (neutralizing the acids) will convert them to an ionized (negatively charged, dissociated) form. In this form they cannot be released to the air and will remain in the pile. For example, with the most commonly formed organic acid, acetic acid (vinegar), the reaction is:


Application of pulverized limestone is probably the best way to raise the pH. Sprinkling the limestone in powdered form directly onto surfaces from which odors are escaping may be the simplest approach, although a liquid slurry of limestone in water might be more effective. A layer (1 foot) of finished compost spread over the odorous material also helps to reduce odor release, serving as a "bio-scrubber".

The use of limestone may be ineffective with odors generated from grass clippings or other high nitrogen wastes. Ammonia and amines are weak bases rather than acids, and raising the pH may therefore actually increase odor release:

NH4+ ö NH3 + H+

If odors are still produced and released despite these precautions, it may still be possible to minimize their off-site impact (Stage 3). This approach relies on timing odor-releasing operations to coincide with favorable wind conditions. A wind sock should be installed at the site to determine wind direction, and odor releasing operations performed only when the site is downwind of residences and other sensitive neighboring land uses. Also, higher winds are preferable to calm and light wind conditions because the higher the wind speed, the greater the dilution of any released odors.

Some commercially available products claim to mask or eliminate composting odors when sprayed onto windrows. Masking agents try to use another odor (lemon, pine, roses, etc.) to hide the objectionable odors. To our knowledge, they have not been successful at composting sites. Odor elimination agents, with the exception of limestone noted above, are also unsuccessful in our experience.

B. Leachate

One way in which leachate may pose a problem is by forming small pools or "ponds." Ponding is a concern because it can create an odor problem (since anaerobic conditions are likely to develop both in the pool and in the base of any water saturated piles), serve as a place for mosquito breeding, and interfere with operations on the site (soft, muddy areas). Prevention, by properly grading the site, is the best remedy. Also, windrows should run down slopes rather than across, making it easier for the water to run off rather than accumulate between windrows. If ponding occurs and odors are released from the pools, adding pulverized limestone may be helpful.

Pollution of surface waters (lakes, streams) is the other major concern with leachate. While leachate from leaf composting is generally not toxic, it may deplete the dissolved oxygen in the water, possibly even to the point where fish kills could occur. Because of its dark color, it might also lead to a discoloration of the water.

In order to prevent this potential pollution, leachate should not be allowed to enter surface waters without prior treatment. This treatment might consist of simple percolation down into or through the soil, or passage through a sand barrier constructed to intercept any horizontal flow. In passing through the soil or sand, the leachate is both physically filtered and biologically degraded to remove a substantial portion of the pollutants. Contamination of ground water does not appear to be a problem associated with leaf composting.

With grass clippings, however, leachate may contain high levels of nitrogen. This may pose a problem of nitrogen contamination for both surface and groundwaters, and may not be adequately treated with simple soil or sand filters. Such contamination must be prevented either by limiting the nitrogen in the leachate (through control of the carbon to nitrogen ratio - by minimizing the amount of grass clippings, for example), or by more sophisticated (and expensive) leachate collection and treatment systems. NJDEPE will consider both depth to groundwater and proposed treatment methods when reviewing permit applications.

Treatment of high nitrogen leachate on site is not a simple matter. Initially the nitrogen may be in a reduced form (either as ammonia or as organic nitrogen), but under aerobic conditions it will be converted to nitrate. Nitrate is the number one groundwater contaminant both in New Jersey and nationally, mainly as a result of agricultural practices. In theory the nitrate can be removed by cycling back and forth between aerobic and anaerobic conditions (nitrate may be converted to harmless nitrogen gas under anaerobic conditions); by taking it up as a nutrient in plants (either aquatic plants or algae, or through use of the treated leachate in controlled amounts for irrigation of crops); or by incorporating it into the composting of low nitrogen wastes (such as leaves). Full scale application of any of these alternatives may be problematic and seasonally limited, and may require large retention or treatment ponds. In some cases, discharge to a municipal sewage treatment plant may be another option.

C. Inadequate Composting Rate

Occasionally composting will progress too slowly in some windrows, usually because the material is too dry. Sufficient water should be added initially, either before or as the windrows are being formed. Other opportunities to add water are during the combining operation (low level technology) and scheduled or extra turnings. Adequate wetting usually cannot be accomplished simply by spraying water on the outside of the piles. Similarly, rainfall is not effective within the required time frame unless it is followed almost immediately by turning.

Another cause of slow composting is piles which are too large. Once acidic anaerobic conditions occur, the material tends to be preserved ("pickled") rather than decomposed. To avoid this problem, follow the recommendations in Sections V.

D. Bags

Some communities have found bagging of leaves and grass clippings to be a convenient collection method. (Collection of grass clippings should be discouraged - see Section I.B.1b.) Plastic bags, however, pose a major problem at composting sites because they must be removed from the compostable material. Even so-called "degradable" plastic bags are incompatible with composting because they degrade far too slowly. If plastic bags are to be used, it is very strongly recommended that they be removed prior to delivery of the material to the composting site (for example, by opening and dumping them curbside, at the point of collection). In New Jersey, facilities are required to empty plastic bags prior to windrow formation - and properly recycle or dispose of the plastic.

An acceptable alternative is the use of paper bags. These do not have to be removed, since they decompose at about the same rate as the composting material. Plastic bags have a lower initial unit cost than paper bags, but the expense of removing them may exceed the price differential.

 E. Aspergillus fumigatus

One relatively new concern with leaf composting is the release of spores of Aspergillus fumigatus. This is a common, widespread, naturally occurring fungus found in soil and on vegetative materials. Its airborne spores may produce an allergic response in some people, and in a few cases they are capable of causing infection in individuals with a compromised (weakened) immune system. Because these spores have been found to be of some limited concern in sludge composting, research has been conducted to examine their importance in yard waste composting.

Based on these findings, A. fumigatus appears to pose relatively little risk to neighbors of composting sites with adequate buffer zones (see Section IV.C). However, workers at a site may receive high exposures, and therefore some precautions seem warranted. Potential workers at the composting site should be screened for conditions that might predispose them to infection or allergic response. Such conditions include asthma, a history of allergic responses, a weakened immune system, the taking of antibiotics or adrenal cortical hormones, and a punctured eardrum. Workers having such conditions should not be assigned to the composting operation (nor any other tasks putting them at similar elevated risk) unless a health specialist is consulted.

Additionally, wearing an approved dust mask during leaf drop off, windrow formation and turning, screening, and similar dust-generating operations is recommended. Air conditioner filters in loaders and turning machines should be cleaned frequently. It also is expected that adequate wetting and minimum disturbance of the windrows, as recommended here, will help to reduce potential exposure.

 F. Toxic Contaminants

Leaves as collected may contain low levels of some toxic materials. Lead, for example, is present because of its former use in gasoline and paint. Limited testing to date, however, has found only low levels, and these appear to be dropping as use of lead has decreased. Lead levels in leachate typically meet drinking water standards.

Some pesticides also may be present, particularly in grass clippings, but again the levels ordinarily are expected to be too low to pose any concern. An unconfirmed 1991 study in Illinois found low levels of 19 pesticides in composting yard wastes. The concentrations were so low that all except one of the 44 samples met the levels allowed in raw agricultural commodities for the 13 detected pesticides for which such levels have been established. (No levels are set for the other 6, which were mostly banned pesticides no longer in use.) In samples from 6 sites in New Jersey only one pesticide, chlordane, was detected. Since chlordane is no longer used, and since it is not taken up by plants, it is believed that this came from the residential soil mixed with the yard waste during raking or bagging.

Based on these considerations and findings, yard waste compost is considered safe for residential use without specific testing. The only exception to this would be for composts containing materials from golf courses, where more intensive use of more toxic and persistent compounds is common. Such materials may require testing for specific metals and pesticides before general use.

We know of no cases in which hazardous wastes have been intentionally mixed with leaves for illegal disposal. If such an incident is suspected, NJDEPE's Emergency Response Hotline (609-292-7172) should be contacted.

G. Other Potential Problems

There are several other potential problems which may arise at leaf and yard trimmings composting facilities. These include noise, dust, and traffic.

Noise may be a concern depending on siting, buffers, and the equipment used. Noisy operations should be performed as far from sensitive receptors as possible, and at the least objectionable times. Berms can be effective in decreasing off site noise.

Dust from the windrows can be minimized by proper wetting. Dust from the roads and aisles, however, may be a problem during dry weather.

Traffic must be considered during original siting of the facility. Limiting hours of operation may be necessary in some cases.

 H. Technical Assistance

To assist municipalities and counties in the proper management of composting facilities, technical assistance is available through the County Cooperative Extension Offices. County Extension Agents are available to assist in the siting of composting facilities, to visit composting sites to identify problems and recommend remedies, to assist in marketing the final product, and to provide public education programs. They have received training and equipment to make the following measurements: temperature, oxygen content, combustible gas content, pH, and moisture content. Appendix D lists County Cooperative Extension phone numbers.

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