A. Grass Clippings

Grass clippings represent another significant seasonal solid waste. In some suburban New Jersey communities they may account for nearly one-third of the total municipal solid waste load during peak grass-growing periods. Although grass clippings are readily compostable, the odor problems they pose make this treatment option difficult to implement for most communities. State permitting requirements are more stringent, particularly with respect to buffer zones, staging, and odor control, and collection costs may be substantial.

Therefore, the best alternative for grass clippings is not to collect them at all (see also Section I.B.1 and Appendix E3). Residents and lawn care services should be encouraged to leave grass clippings on the lawn. Turf grass specialists, such as Dr. Henry W. Indyk (Professor Emeritus) and Dr. James A. Murphy at Cook College, recommend mowing frequently enough so that the short clippings filter through the growing grass and return their nutrients to the soil. This is best for the lawn, as well as for reducing collection and disposal costs. Clippings also can be incorporated in moderate amounts in backyard leaf composting piles or used as a garden mulch.

If grass clippings are to be composted at a municipal facility, extra care must be taken to ensure that the windrows do not become anaerobic. Grass clippings are still alive when first cut, and are relatively high in nitrogen, moisture content, and readily degradable organics compared to the fallen leaves collected in autumn. For these reasons they decompose more rapidly, have a higher oxygen demand, and quickly go anaerobic. They are often highly odorous by the time they are delivered to a composting site. Therefore, it is especially important to properly implement and strictly enforce the odor control measures discussed in Section VII.A. Additional precautions such as expanding the buffer zone and improved management of leachate also will be necessary.

If the grass clippings could be delivered to a composting site without causing odor problems, they could be incorporated (before the end of the day) into the partially composted leaf windrows. A ratio of no less than 3 volumes of partially composted leaves to 1 volume of grass clippings is recommended. Good mixing is essential and can be achieved with a front-end loader by working together 20-30 bucketfuls of material at a time, then forming a windrow with the mixture. The windrow should then be turned with a specialized windrow turning machine. Alternatively, a smaller amount of clippings might be placed on top of a windrow and then thoroughly mixed in with two passes of a windrow turning machine. In either case, it is preferable that a 1 foot layer of leaves without grass clippings be left on the bottom of the windrow initially, as grass clippings in direct contact with the soil have a greater potential for odor production. Windrows containing grass clippings should not be constructed to a height of greater than 6 feet or width of 12 feet, and 5 feet by 10 feet or less is preferable.

Since the leaves collected in the fall typically have lost half their original volume by the grass clipping collection season, the 3 to 1 ratio means that the amount of grass clippings which can be handled is only one sixth of the collected leaves. Generation ratios are often closer to 1 to 1. This serves to further emphasize the need to minimize the amount of grass clippings collected by educating residents about the benefits of recycling them in their own yards. There may be some potential for reuse of windrows already containing grass clippings, but this is probably limited to a minimum overall ratio of 2 to 1.

Once the leaves and grass have been mixed in this way, no further odor problem is expected. The partially composted leaves act as a bulking agent to improve penetration of oxygen to the grass clippings, and as a sorbent to trap small amounts of odorous compounds. Because of their high C to N ratio, the leaves also tie up ammonia as it is released from the decomposition of the clippings, minimizing both ammonia odors and the release of nitrogen to leachate and groundwater or surface waters. The grass in turn speeds the decomposition of the leaves by providing needed nitrogen. The end result is a higher quality compost product which is ready in a shorter period of time.

However, these benefits must be balanced against the increased potential for odor problems. Only facilities that can provide an adequate buffer zone (1000 feet or greater from the grass handling areas; a smaller buffer zone might be considered where demonstrated to be acceptable), and that have the flexibility to turn the windrows on a more regular basis than is required for leaf composting alone, should attempt to compost grass clippings. Facilities that compost grass clippings also should monitor nitrogen levels in leachate and groundwater, including background sampling both upgradient from the site, and on-site prior to receiving materials.

Other bulking agents have been proposed for composting of grass clippings, and might serve as a partial or complete substitute for partially composted leaves. However, in addition to providing bulking for better aeration, any such materials or mixtures also must maintain a sufficient supply of available carbon to tie up ammonia as it is released from the clippings. If not, ammonia odors (even under completely aerobic conditions) and nitrogen contamination of water may occur. Woody materials, in particular, do not normally supply sufficient available carbon for this purpose, even though their C to N ratios are high. (The C is relatively unavailable to microorganisms.)

 B. Woody Materials

Wood tends to decompose very slowly, making composting of woody materials impractical in most cases. Thus woody materials should not be intentionally incorporated in leaf or leaf/grass composting windrows unless there is an end use for a mixed wood/compost product. (Separation by screening usually is too expensive.) Small amounts of incidentally included branches and twigs pose little problem.

Tree trunks and large branches often can be given away or even sold as firewood if cut to reasonable lengths. For smaller diameter woody materials, chipping, alone or followed by composting (with or without leaves or grass clippings), may produce a usable mulch. Direct incorporation of woodchips or other woody materials into the soil is not recommended because of the slow rate of decomposition and the high C to N ratio.

 C. Other Organic Materials

Many other organics, such as most agricultural and food wastes, are potentially compostable. However, these materials may not be suitable for the composting technologies being used at yard waste composting facilities. Contact the NJDEPE DSWM Bureau of Resource Recovery Engineering (see Appendix D) for further information on specific materials.

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