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May 1, 1999
When launching a composting operation, the last thing you want is an overflowing retention pond. Ditto for dry windrows, damaged turners and compost leachate. However, proper planning prior to construction - including allowances for collection and debagging, space management and water control - can help you avoid these problems.
Getting It In: Collection and Debagging A composting facility can't survive without yard waste, which can be debagged using manual or machine labor. Because of logistical issues and spatial requirements, manual debagging isn't very feasible for a facility accepting more than five packer truck loads per day, unless the site has a very large usable area.
Typically, a crew of two workers can handle about one to one and a half packer truck loads of bagged yard waste during an eight hour day if the loads are spread evenly throughout the day.
At a manual debagging site, material should be spread by a loader in a large area, allowing workers to pull out the bags and bundles of brush. Waste should be dumped in small heaps at each work station, then pushed by a loader into larger heaps at the end of the day.
The upside to manual debagging is the avoided cost of a mechanical debagging machine. However, reducing worker safety hazards when debagging manually is crucial [See "Mitigating Safety Hazards," page 215].
For some municipalities, a mechanical debagging machine - using an air blower or rotating spikes - may be worth the investment. There also are windrow turning machines that may be able to do some of the debagging, but these require attention. Plastic shreds can get stuck around the rotating parts of the turner, and logs and stumps mixed in the yard waste can damage the machine.
Another debagging option is to grind the bagged yard waste and brush it together, compost it and screen it later. Odor can be a factor if the grass inside the bags becomes anaerobic. But as long as the contents remain inside the bags, it should be minimal because the grinding operation will neutralize it through aeration. Note that shredded compost heats up immediately after windrows are formed, and it must be turned or aerated the following day.
In addition to debagging, choosing the collection method is important. For example, mixing yard waste and brush during collection will require more pre-processing, resulting in higher costs. However, contractual or political realities can make some collection methods unfeasible for some municipalities.
Space Management Allowing ample space for the receiving, windrow turning, curing, screening, stockpiling and drainage areas is critical.
Receiving. This is where yard waste is unloaded, piled, debagged and shredded. For manual debagging, 11/42 hectare is necessary to accommodate the loader spreading the material, work stations and a separate area if brush is collected with grass. Shredding requires approximately 11/44 hectare.
Windrow turning. Processed yard waste is formed into windrows in the turning area.
When a loader is used to turn compost, it is important not to undercut the pad because damage may occur during the winter and early spring when the ground between windrows may be frozen, but the bottom of the warm windrows won't be.
If a rubber-tired loader or a tow-behind machine is used for compost turning, allow 3 meters to 4 meters between windrows. Windrow spacing can be reduced to about 1 meter by using a straddle machine, which typically can handle windrow heights of 1 meter to 2 meters. Plan for sufficient machine turning space at the end of each windrow, and allow more space for a tow-behind machine. Required turning space can be reduced partially by skipping every other windrow when turning.
The high-temperature turning period will last three to four weeks, depending on the turning program's efficiency. Grass generally will take less time than leaves to reach the pathogen-kill temperature requirement and to compost, unless nitrogen content is increased or an additive is used. Mixing grass and leaves can accelerate the process by improving the ratio between carbon and nitrogen.
In northern climates where leaf and grass seasons don't overlap, using an additive such as urea may be needed. This can be expensive, though.
Curing. After meeting the pathogen-kill temperature requirement, compost must cure. During this time, windrow size will reduce.
Depending on local regulations, this may take two to six months. The quicker the compost cures, the quicker it can be moved off the pad. Consequently, it is important to determine whether the compost turning program can achieve a short turnaround time.
When designing a new pad, allow approximately 200 kilograms per cubic meter (kg/m3) for pure, freshly shredded leaves and brush, and 250 kg/m3 for shredded grass with brush. Six-month-old matured compost can weigh more than 500 kg/m3. If space is a problem, consider increasing or improving the number and types of turning equipment.
Screening. Matured compost may need to be screened for final marketing, depending on the feedstock, foreign contents, the rate of return and the end-users' expectations. Municipalities that give away their compost may not need to be concerned about achieving commercial grade quality because the compost is free and the end-users typically are not too demanding. Quality expectations increase and screening may be necessary if compost is sold.
The good news is that screening can be done off the pad.
The space requirement for screening matured compost is approximately 11/44 hectare, including the screener, conveyor belts, stockpiled screened compost and overs, or materials too large to fit through the screen. If plastic bags are in the feedstock, overs may account for 25 percent of the total matured compost because the plastic shreds may form nuclei that bind the compost together.
Stockpiling. Finished compost is stored here until it is removed from the site. Space will depend on the amount of finished compost. Keeping accurate records of each years' volume or pile size will make future planning easier. If the stockpile is in a cone shape, you can calculate the cone's volume if you know the height and the radius.
One quick way to estimate the height is by measuring the length of the conveyor belt, which is usually only 2 feet to 3 feet above the tip of the cone pile, and calculate the height. The radius can be calculated by measuring the circumference of the cone's base.
Retention ponds. Many jurisdictions require stormwater or compost leachate control, so a retention pond may be necessary.
An oversized pond requiring a lot of space may be needed to handle surface water collected on acres of flat land and to prevent a flat pad from overflowing.
Fortunately, soil excavated from the pond can be used for pad construction if your timing is right.
Water Control Compost requires a delicate balance of wet and dry conditions. Windrows need to be kept moist, but surface water build-up can lead to compost leachate and stagnant water flooding or pooling on the pad. To avoid ponding or pooling water, build a pad with proper slope.
It's better to have a slope that is slightly too steep than too flat. Although a slope of 2 percent to 4 percent generally will provide sufficient drainage, it may be difficult or expensive to construct a pad with a rise of 4 meters for every 100 meters in length.
Instead of constructing a pad based on sheet flow, consider building it with a crown line in the middle and a ditch along each side, sloping everything toward the retention pond.
This design will save material costs and allow a steeper fall from the crown to the side ditches and quicker drainage.
Keeping the pad dry also will reduce rutting caused by loader tires. Remember, it's easier and cheaper to repair ditches annually than to repair a large portion of the pad. WA
Not all municipalities can afford mechanical debagging machines. Manual debagging, however, requires facility managers to keep a sharp eye on potential safety risks, such as:
* Debaggers working in close proximity to moving loader and packer trucks;
* Trip and fall hazards from brush intertwined with yard waste;
* Accidental mixing of garbage or hazardous waste with yard waste;
* Eye injuries and respiratory problems from workers emptying dusty bags of waste;
* Overexertion or heat exhaustion; and
* Lack of training for temporary workers.
Giving away compost can be a wise business decision, as many municipalities have discovered. In addition to generating goodwill, free compost promotions can raise awareness of other disposal programs.
Take the Regional Municipality of Halton, Toronto, for example. When the first batch of compost matured in 1994, Halton held a two week compost giveaway program - with one catch. Only residents who dropped off household hazardous waste (HHW) at Halton's HHW depot could receive free compost.
As a result, the depot traffic increased dramatically and the combined event received positive press coverage and public acceptance. Now, this popular combined event has been running for four years.
Municipalities can combine any number of programs with a free composting promotion, including grasscycling, HHW, scrap tires, food bank donations, etc.
Compost must be kept moist at all times, which is not as simple as it sounds. In fact, managers at the 10-acre compost facility at the Halton Waste Management site in Toronto tried several methods before finding the right one.
The compost plant was constructed in the early 1990s as a complement to the Regional Municipality of Halton's landfill operation, which serves four local municipalities.
Halton's feedstock includes shredded grass, leaf, brush and plastic bags. When materials initially are shredded, their moisture content can be less than 35 percent, and a lot of watering is needed.
The facility managers' first attempt to moisten the compost included using a 75,000 liter (2,000 gallon) water truck to spray the side slopes of the windrows, using the front bumper nozzles. Because the nozzles could not reach the top of the windrows, most of the water trickled down the compost slopes going back into the pond. Only a few outside inches were wet, and the core still was dry.
Then, a 4-meter horizontal swing-pipe was installed at the top of the truck to dump water directly on top of the windrows. More than 75,000 liters of water were dumped on a 130 meter long windrow within 20 minutes, including loading of water at the compost pond. However, only the top half of the windrows were wet after the first run, and the operation needed to be repeated after every turn.
Next, facility managers used a bulldozer to level 2,000 tons of freshly shredded compost into a 1-meter high flat pile covering an 85 meter by 60 meter area. A farm-type spray irrigation cannon system watered the flat compost pile for 20 hours. The compost became wet, generating about eight drops of water when squeezed. The wet compost then was reformed into windrows. Almost 1 million liters of water were used on the 2,000 tons (2 million kilograms) of compost, amounting to 1 liter of water per 2 kilograms of compost.
The downside to this method was that some odorous leachate was generated from the windrows' bottom as a result of the weight from the top half of the windrows. The situation did not last long because the excess water evaporated when the windrows were turned daily. The leachate was flushed with clean water back into the pond.
A third method involved spraying water directly into the tub grinder during shredding by connecting a 111/42-inch fire hose to the tub grinder, then connecting the hose to a 3 inch pump, which pumped water from the pond. This increased the moisture content from the 35 percent range to the 45 percent range, and the compost did not get overly wet. More hoses can be used at the tub for higher moisture content. Using water from the compost pond also can reduce hauling and treatment costs.
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