V. APPROPRIATE LEVEL OF TECHNOLOGY
Four levels (minimal, low, intermediate, and high) of composting technology can be considered. The particular one which is most appropriate for a given application will depend mainly on the quantity and types of material accepted and the site selected, although the equipment and manpower available are also factors. Table 1 indicates that the lower the level of technology, the greater the requirements for available space and composting time, but the lower the cost.
The level of technology refers to the extent to which the ideal conditions for composting are met. Minimal technology meets the conditions poorly, leading to slow processing and a strong odor potential. However, if site conditions permit this type of operation, it can be highly cost effective. It might be considered for leaves only, or for some agricultural wastes.
The low-level technology provides somewhat better composting conditions, cutting the time requirements and odor potential. This is considered acceptable in many cases for smaller operations handling leaves only. It is discussed in the greatest detail below as a point of comparison for the other methods.
Intermediate-level technology invests more effort into speeding up the composting process. It is appropriate for larger sites, and is necessary where grass clippings are accepted at a site.
The high-level of technology approaches optimum processing conditions. While not used for leaves, it is normally required to handle other municipal solid waste fractions, sludges, and other highly putrescible wastes.
A. Minimal Technology
If a large area that is well isolated from sensitive neighboring land uses is available, a very low-cost approach to leaf composting is possible. Leaves brought to the site are formed into large windrows (for example, 12 feet high by 24 feet wide) using a front-end loader. Once each year the windrow is turned and reformed. An additional windrow is constructed with the new leaves each fall. After three to five years the material in a windrow is usually sufficiently well stabilized to be used as compost.
With this "minimal" technology the necessary conditions for rapid composting are not achieved. Much of the pile remains anaerobic for a full year at a time between turnings. The center of the pile will probably also reach inhibitively high temperatures, especially the first year. However, the greatly reduced rate of activity is compensated for by providing a prolonged composting time.
Using this approach, odors can be expected for the first year, and serious odors likely will be released during the first turning. Usually by the second turning, odors have diminished. Because of these odors, an extensive buffer zone is required. Up to a quarter mile distance or more to sensitive neighboring land uses is recommended.
The obvious advantage of this approach is that it is extremely inexpensive. Only a few days per year of front-end loader operation is required. Even wetting of the incoming leaves may not be necessary except in very dry years. The large piles will conserve moisture, and the long time period ensures the cumulative exposure to considerable precipitation.
A second advantage is that relatively little space is required for the composting itself because the piles are so large and little aisle space is needed. For example, using 12 foot high by 24 foot wide piles, a single windrow 60 yards long would contain approximately 1500 cubic yards of leaves. Even though the leaves must stay on site for at least three years, a one acre site (excluding buffer) is expected to be adequate for a yearly collection of 4000 cubic yards.
However, because of the odors produced, a large buffer zone is needed. Thus, a very large total area is required, although only a small portion of it is actually utilized for the composting. This might be useful in a wooded area, so that only a small clearing would be required, or at an isolated industrial site or public works yard.
B. Low-Level Technology
In densely populated New Jersey, siting of a minimal technology facility is rarely possible. Therefore, the necessary conditions for rapid and nuisance free composting have to be more nearly met. In particular this means that a better job must be done of ensuring adequate moisture content, oxygenation and temperature control.
The simplest way to achieve the desired temperature range would seem to be to build piles large enough to conserve sufficient heat, but not so large as to overheat. On the other hand, adequate oxygenation by passive diffusion of air from the outside of the pile could be achieved if the piles were small enough. Unfortunately no single pile size completely reconciles these conflicting goals. However, the desired conditions can be approached by starting with moderate size piles (6 feet high by 12-14 feet wide), then combining two piles after the first burst of microbial activity (which lasts approximately one month). Water addition at the outset is usually necessary to provide adequate moisture.
Using this approach it is possible to produce a thoroughly decomposed (finished) compost in 16-18 months. The compost is ready for use in the spring, which is the time of peak demand for the product. Slight odors may be produced early in the composting cycle, but these usually are not detectable more than a few yards away from the windrows. After 10-11 months large curing piles are formed around the perimeter of the site, freeing the original area to accept the new leaf collection. Costs are still quite low, as only three to four operations with a front-end loader are required after initial windrow formation (one combining, one or two turnings, and one curing pile formation). Despite the fact that more space is required for the actual composting (roughly 1 acre per 3500 cubic yards of leaves) compared to the minimal technology, less total area is needed overall because of the reduced buffer requirement.
Unless otherwise indicated, the low-level technology is recommended for small to medium size sites composting leaves only. This is the technology that NJDEPE/DSWM prescribes in its permit exemption regulations (N.J.A.C. 7:26-1.11). However, this technology generally is not acceptable at larger sites, or if grass clippings are accepted.
Table 2 summarizes the scheduling and estimates of manpower and equipment requirements for a moderate-sized leaf composting facility (15,000 cubic yards of leaves per year) employing the low level technology. The individual steps are discussed in more detail below. A summary sheet, meant for distribution to field personnel, is provided in Appendix B.
As indicated, a number of assumptions went into Table 2. The manpower and equipment time estimates, in particular, should be considered only as a general indication of the needs at a specific site, since they may be highly variable. More details are given below.
1. Site Preparation
Prior to each collection season the site must be readied to allow all necessary truck access and front-end loader operation. The one part of the operation which has little scheduling flexibility is delivery of the collected leaves. Once leaves are collected, they must be promptly processed through the staging area and formed into windrows (Section V.B.2-4). It is critical, therefore, to prevent operational bottlenecks, such as an area becoming so muddy that trucks get stuck trying to drop off their loads.
The yearly site preparation should include regrading and road and leachate system (if any) maintenance. All refuse and debris from the previous year's operation should be removed. Normally this step will require at most a few days work. It can be scheduled any time after the active site has been cleared of the leaves from the previous year (by formation of the curing piles), but before the new collection season begins.
2. Delivery of Leaves
It is recommended that trucks dump their loads of leaves in a staging area, rather than trying to form windrows directly. Although a staging area involves additional labor, its use is justified for several reasons.
Leaves are normally collected in several ways and delivered in a variety of trucks including garbage compactors, roll-offs, and vacuums. If the windrows are formed directly by dropping off loads from these trucks, highly non-uniform size, moisture, and compaction - and hence decomposition - results. This can be minimized by the operations performed in a staging area. Wetting is virtually impossible in directly formed windrows since most of the water simply runs off the outside. Use of a staging area also leads to a more uniform windrow size and shape, giving both a better appearance and more efficient composting. Keeping trucks on the firmer surfaces, rather than backing into windrows, decreases their chance of getting stuck during wet weather. Even in good weather the staging area can speed the delivery process. It may be feasible to move the staging area periodically (weekly for example) to minimize the distance to the active windrow forming area.
Windrow formation must take place immediately after leaves are received. If freshly dumped leaves are allowed to sit for more than a day or so in the staging area, odor problems may develop.
Supervision may be required to prevent dumping in undesired locations. Also, a monthly record of the amount of leaves delivered must be kept. The collection log must be submitted to the municipality of origin for the Tonnage Grant Program. A daily tabulation of the number of loads for each individual truck of known capacity may be the best accounting method. A copy of the collection log developed by the NJDEPE/DSWM is provided in Appendix C.
The staging area also provides an opportunity to inspect loads as they are dropped off, and to remove incidental contamination from litter or other refuse. Heavily contaminated loads should be rejected, if possible, to avoid sorting and disposal costs, and to encourage better collection practices in the future. As indicated earlier (Section IV.L), plastic bags should not be accepted at the site.
Wetting of the leaves will be required during much of the collection season. Adequate wetting can only be achieved prior to or during windrow formation, or when windrows have been opened up for turning or other purposes. Most of the water applied to the outside of a windrow is simply shed by the leaves, or quickly re-evaporated to the air. The water should be sprayed on the leaves with a firehouse (see Section IV.I) as the loader breaks the masses apart in the staging area, and/or as they are placed in the windrows.
If the moisture content of the delivered leaves is known, the amount of water that must be added to get any particular desired moisture content can be calculated (Appendix A). On a more informal level, the rule of thumb is that it should be possible to squeeze a few drops of water from a fistful of the leaves. As a rough approximation, perhaps 20 gallons of water will be required on average per cubic yard of leaves collected.
4. Forming Windrows
Once the leaves have been dropped off in the staging area, the front-end loader can be used to break apart and spread the compacted materials to facilitate wetting. Plastic bags, branches, curbing, and other incidental debris can be removed by hand. The front-end loader can then be used to place the uncompacted leaves in windrows.
The windrows initially should be 6 feet high by 12-14 feet wide. Any convenient length can be used. Two windrows can be formed side by side, with only 1-2 feet between, to conserve space. Sufficient aisle space between pairs of windrows (typically 12-16 feet) should normally be allowed for loader operation. Although in some cases it may be possible to have fewer aisles if space is limited, this may make turning operations awkward.
Neatly formed windrows with well maintained aisles give a professional appearance to the facility, while messy windrows give the impression of a "leaf dump." Care should be taken that equipment, especially the loader, does not ride up on the windrows, compacting them. Loosely piled leaves are required in order to ensure sufficient pore space for adequate air penetration into the windrows.
5. Combining Windrows
After approximately one month, much of the initial oxygen demand of the leaves has been exerted and the piles have been reduced to about half their original size through decomposition and self-compaction. At this point, two windrows can be combined to form a single one that is still only about 6 feet high by 14 feet wide (about the same size as each of the initial windrows). Combining the windrows will help conserve heat during the colder weather. Portions of the center of the new, combined windrow may go anaerobic temporarily, but significant odors and acidification are not expected because much of the readily degradable material has already been consumed by the microorganisms.
Combining should be done by moving and turning both piles, not by placing one on top of the other. The maximum degree of mixing and "fluffing" is desired.
To conserve space, combining may begin before leaf collection has been completed. In this way some of the space freed by combining windrows (formed with leaves collected in early November), can be used for new windrows made with leaves collected late in the season (mid-December).
6. Turning Windrows
As early as is practical in March or April, each windrow should be turned. Turning mixes the material, re-wets the dry outer edges, re-oxygenates the interior, and exposes the formerly cool edges to the hotter internal temperatures. The result is an increased rate of decomposition and improved destruction of any pathogens and weed seeds.
As with the prior combining operation, maximum mixing and "fluffing" is desired during turning. At this time additional water may be added if the material is too dry. (However, every effort should be made to provide sufficient water initially.)
Additional turnings throughout the summer would further enhance composting rate and product quality. At least one such turning is recommended in order to prevent any weed growth on the windrows from going to seed.
Using the low-level technology described here, much of the material will not be completely stabilized by the end of the summer, yet the composting area must be cleared to allow for site preparation for the next year's leaves. This does not represent a problem since the material is now moderately well decomposed, has little oxygen demand, and is unlikely to produce odors.
At this time, therefore, the material can be moved and formed into a large curing pile around the perimeter of the site. The curing pile may be made as large as 12 feet high by 24 feet wide to conserve space, but should not be compacted when formed. Moving the material also provides additional turning and mixing, while the large pile exposes a relatively small surface area to drying and freezing conditions. Additional weed and pathogen destruction is achieved at the temperatures reached within the large, well-insulated curing pile. This material is expected to be well stabilized by the following spring, but may be left in place longer if convenient.
8. Shredding or Screening
Once composting is completed (post-curing), shredding or screening is a final optional step to improve the physical quality and appearance of the finished compost, making it more acceptable for many home and commercial uses. Depending on the equipment, this step breaks up clumps and separates out rejects consisting of any uncomposted leaves, branches, rocks, plastic, and other extraneous materials. The "rejects" may be composted for an additional period, then re-shredded or screened to minimize the amount requiring disposal.
This step is fairly labor intensive. Leaf compost can only be processed at about half the rated capacity of some of the equipment. Typically, a front-end loader is required for filling the hopper, and at least one person is required to operate the shredder/screener itself.
Shredding and screening will proceed more rapidly if the compost is not too wet. Overly moist material to be shredded might be spread out to dry for a day or two beforehand.
The major advantage of using a shredder or screener is that it yields a more uniform and debris-free final product. In some cases it also can be used to mix finished compost with soil. Disadvantages include the labor and equipment requirements, the need to dispose of rejects, and the capital cost of the specialized machine. For amending final landfill cover or sale to top soil companies (where it will be shredded during blending), shredding or screening is not needed.
One way to reduce costs is to share a single unit among several sites or communities through an interlocal agreement. Sharing is possible since the specialized equipment is only needed for a month or two per year, and scheduling can be flexible.
C. Intermediate-Level Technology
More frequent turning of the windrows will speed the composting process through improved aeration and the physical mixing and grinding (particle size reduction). Because of the increased rate of biological activity, turning must be continued regularly once it is started. If not, acid-anaerobic conditions and odors quickly develop, and the next turning releases the odor. During the first few weeks, two turnings per week may be required. This later can be reduced to one per week, then one every two weeks. The need for turning should be monitored by measurement of oxygen content and temperature within the windrows. Turning should be scheduled to prevent oxygen in the windrows from dropping below 5% for prolonged periods, and to prevent temperatures from exceeding 60øC (140øF). Once the operator becomes familiar with his system, turning can be based on a schedule, with only periodic monitoring. Following this approach, finished compost can be produced in as little as 6 months or even less.
Except for very small sites, such frequent turning by front-end loader is impractical. The turning takes too much time, equipment and labor costs are too high, and the mixing and grinding is not very thorough. Also, compaction of the windrow is likely, and the soil at most sites would get too rutted or muddy. For these reasons, specialized turning machines must be used.
Several commercially available turning machines currently are in use in New Jersey. Some are mounted on a tractor or front-end loader, and driven first along one side of the windrow and then the other, turning half of it at a time. Others straddle the pile, turning the composting material all at once and displacing it backward. Another approach used by some equipment is to lift the material and displace it to the side.
Before investing in expensive dedicated equipment, careful thought should be given as to its advantages and disadvantages. A major advantage may be the shorter composting period. This results from faster biological action, due to more thorough aeration, mixing and grinding. However, this is really an advantage only if the site is required for another use (such as a beach parking lot) during the summer.
In addition to the expense, an important disadvantage to consider is that (perhaps surprisingly) this approach may require more land than the low level technology process. This is because in many cases windrow height is limited to only 5 feet or less for the turning machines. Some larger models can accommodate a 7 foot high windrow, but piles of this size are prone to odor generation. Also, paired windrows cannot be used with types that are tractor or front-end loader mounted because of the need to turn windrows from both sides. However, straddle type and side displacement turners may only require narrow aisles, saving space.
Contrary to what might be expected, turning may have the effect of reducing average overall oxygen levels within a windrow. Although the turning itself does incorporate additional oxygen, the higher rate of decomposition which results from the concurrent mixing and grinding can lead to rapid (a few hours) oxygen depletion and anaerobic conditions.
Specialized turning machinery may require a better graded surface for efficient operation. On the other hand, such equipment may create fewer ruts and less muddy conditions than front end loaders.
Another point is that some of this equipment is very noisy. This should be mitigated to the maximum extent possible if residences or other sensitive land uses are located nearby. Noise levels must not exceed 65 dB(A) at the receptor's property line as set forth in the New Jersey Noise Code (N.J.A.C. 7:29-1 et seq.). These considerations also apply to front end loaders.
A staging area is not as important for an intermediate level of technology. Turning can be used to help reduce differences in initial windrow size, compactness, and composition. Frequent turning also makes initial water addition less critical. If inadequate moisture is present, turning during or immediately after it rains (or snows) can be used to incorporate water. This may make an on-site water source unnecessary (if fire officials approve).
Grass clippings (other than those incidental to fall leaf collection) should not be accepted at a site unless frequent turning is available (see Section VI.A). Specialized turning equipment then is required.
Finally, the overall economic impact of accelerated composting should be examined. The increased turning efficiency (time, energy, and labor) of the specialized equipment may justify the initial expense at larger sites even if the shortened composting period is not a major factor. For sites of 10,000 cubic yards or less, they may be uneconomical unless shared, but at sites of 30,000 cubic yards or more they may be a necessity.
D. High-Level Technology
In order to approach a maximal rate of decomposition, near optimal levels of temperature and oxygenation are required. This also minimizes odors, as the putrescible (odor-causing) materials are quickly decomposed, and anaerobic conditions are minimized. These desired conditions can best be achieved by using an approach originally developed for sewage sludge composting, known as the Rutgers process control strategy. While this strategy has been successfully field tested for leaf composting, exact design and operation details for this application have not been developed.
Briefly, this approach consists of using forced pressure aeration of the composting pile, with the blower controlled by a temperature feedback system. When the temperature at a specific monitoring location within a pile exceeds a preset value, the blower automatically comes on to remove heat and water vapor and cool the pile. This control strategy ensures near optimum temperatures in the bulk of the material, and at the same time maintains a well-oxygenated condition. During the start-up period (and at other times, if needed), the blowers also come on under control of a timer (perhaps for 30 seconds every 15 minutes) to provide a minimal level of oxygen. After 2-10 weeks of composting, the aeration system would be removed, and the windrows turned periodically. Additional information on the Rutgers strategy is provided in some of the papers listed in the Bibliography.
An advantage of this approach is that large windrows can be formed initially, thus using less space, yet extensive anaerobic conditions do not develop because of the good aeration. Therefore, serious odors and slowed decomposition do not occur. The largest pile tested to date was 10 feet high by 20 feet wide, which may be close to the maximum feasible. A second advantage is that as a result of the rapid decomposition which occurs early in the season, composting can be completed within several months (perhaps one month of ventilation followed by a period of frequent turning).
Nitrogen addition may be beneficial, since the temperature and oxygen limitations are largely overcome with this control strategy (unlike the case with the minimal, low, and intermediate levels of technology). This would further speed decomposition without leading to odor problems. As a first approximation, 5 pounds of nitrogen per ton of wet leaves (about one pound per cubic yard) could be tried.
However, since the incoming leaves may themselves be odorous and since these odors may be released during initial windrow formation and start up, a moderate size buffer zone (as with the low-level technology) is still required. Also, the need for the blowers, timers, and controllers (several hundred dollars per set-up) and the additional labor for installation and security requirements will increase the cost of this approach relative to the low-level technology. Although the overall cost still is expected to be moderate, in general this approach does not seem to be warranted for leaves.
On the other hand, a high level of technology may be warranted for grass
clippings, and is probably necessary for food wastes. The high level of
technology lends itself well to enclosed systems, and in fact some of the
best composting "tunnel" systems developed in the mushroom industry are now
being adapted for solid waste applications. These systems, in addition to
employing temperature feedback control and being totally enclosed, are capable
of recirculating a portion of the blower air stream. This makes odor control
much more efficient, and will minimize any potential problem with
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