BioCycle February 2006, Vol. 47, No. 2, p. 23
Washington State composting company finds most effective strategy is to capture and treat leachate separately from storm water, thus reducing contaminants and solids in the storm water to be treated.
Cedar Grove Composting operates two large-scale composting facilities in Washington State. Its first, in Maple Valley, processes yard trimmings and some commercial and residential organics on a 28-acre paved site. Its second facility, in Everett, opened in 2004 and processes yard trimmings and residential and commercial organics on a 26-acre paved site. The original Maple Valley site has operational capacity of 352,000 tons (120,000 tons of food residuals) and the new Everett site has 164,000 tons of food residuals and yard trimmings capacity.
In the fall of 2004, Metro Council, the Portland, Oregon area’s regional government, authorized Metro to enter into a contract with Cedar Grove for transportation, processing and composting commercial organics collected in Portland and the surrounding area. In the contract, Cedar Grove agreed to pursue development of a composting facility in the Portland region once the annual flow of food residuals reaches 10,000 tons/year on an ongoing basis. In the interim, collected organics are processed at both the Maple Valley and Everett, Washington facilities.
While storm water management is always a factor in facility design, it has become a critical issue in Oregon. The state Department of Environmental Quality (DEQ) is involved in a rulemaking for solid waste facilities, and as part of its rule revisions for composting sites, is developing new requirements for storm water management, upgrading the existing rules. The DEQ has drafted water quality discharge limits that cover pH, copper, lead, zinc, ammonia, biological oxygen demand (BOD), total dissolved solids (TDS), phosphorus, E. coli and oil. A committee comprised of composters and regulators in the state was established to evaluate and comment on the rulemaking. Cedar Grove has participated in the rulemaking process by giving comments and presentations on various issues.
Most yard trimmings composting facilities in Oregon utilize fairly low tech methods (e.g., large static piles); few have paved sites. Members of the committee were concerned about their ability to meet stringent water quality requirements, and the cost of upgrades to comply with the rules. After years of operating a composting facility using a variety of methodologies (starting with low tech windrow and static piles, then aerated static piles and most recently covered composting), the DEQ asked Cedar Grove to put together a presentation on storm water management options and the related costs. Our company is confident that the storm water and leachate management practices employed at our sites in Washington State will comply with the water quality discharge limits under development in Oregon (see Tables 1 and 2 discussed later in this article). This article, based on the presentation prepared for the DEQ, describes strategies at both facilities.
The cost of functional storm water treatment ranges from a low range that includes use of compost berms, bioswales and solids separators, to a higher range that includes lined ponds, pond aeration, and multiple units or treatment trains. The tools discussed in this article reflect both ends of the spectrum. Perhaps the most important “takeaway” message is to separate rain water draining from the receiving, grinding and active composting areas – the “contaminated side” – from the “treated side,” which includes curing piles and product screening and storage. Because many of the composting facilities in Oregon may have to pave their sites in order to meet the storm water discharge limits, operators must consider where the water will drain. In general, the contaminated side should be downstream from the treated side, both in terms of drainage and wind direction. This minimizes the chance of untreated material contaminating material that has met the pathogen and vector attraction reduction requirements. Figure 1 shows a site plan using the configuration just described.
The Everett composting facility is permitted to process 164,000 tons/year of all food residuals (pre and postconsumer) and yard trimmings using the Gore Cover System. All active composting piles are covered with the Gore™ Covers, which enables air to circulate but traps odor compounds (essentially they are entrained in water droplets that fall back into the active composting pile). The microbial action breaks down the odor compounds while under the cover. Fresh material is placed over trenches with aeration holes; piles are aerated in the positive mode.
Lessons learned from managing leachate and storm water in Maple Valley were applied to the design of the Everett composting facility. As the Maple Valley site evolved, a series of aerated ponds were installed to treat a combined flow of leachate and storm water. Essentially, the water quality in each pond improves (Figure 2) – due to microbial breakdown of the organics in the storm water – until it meets the discharge limits. A solids separator ahead of the first pond settles out some of the organics. By removing organics before the ponds, those organics can be reintroduced into the compost process saving valuable organic content and reducing the load of organics in the water treatment system thus reducing the cost.
When it came to designing leachate and storm water management at the Everett site, it was decided to: a) Minimize the amount of leachate from the contaminated side; and b) Treat that leachate separately from the overall site storm water. Keeping leachate from the overall storm water flow (and thus creating an opportunity to collect and recycle that leachate back into the fresh material piles) can have a significant impact on the cost of storm water treatment.
The first step was to cover the feedstock receiving area and have that floor drain to a dedicated sump that goes to an underground leachate collection system. In the high traffic area, e.g., where trucks idle while waiting to unload, runoff drains to an oil/water separator. Rainwater that falls in the grinding area also is classified as leachate and drains to the leachate collection system.
A third leachate collection point is the trench system under the covered aerated static piles. The primary purpose of the trenches is to introduce air into the piles. However, since the aeration system operates only 25 percent of the time to keep the piles aerobic, the trenches also act as a leachate collection system that drains to the leachate tank. Typically leachate is generated by the piles if the moisture level is too high in the beginning feed mix. Additionally, having covered active compost piles significantly reduces the amount of leachate that needs to be treated as a result of rain events – from thousands of gallons to just a few gallons per day per pile.
Leachate collected from the receiving and grinding areas and the aeration trenches ends up in a 46,000-gallon tank that is aerated and kept under cover. Aeration minimizes the generation of odors, which is critical as 100 percent of the leachate from the tank is applied to the feedstocks prior to composting. Because more moisture is needed in the initial mix than is available from the tank, captured storm water is used to supplement the recycled leachate.
Critical at all phases of outdoor composting is good housekeeping, especially keeping paved surfaces clean – on both the contaminated and treated sides of the facility. Using a sweeper truck reduces the amount of solids that end up in the storm water, reducing the organic load in the treatment pond.
STORM WATER TREATMENT TRAIN
The core components of the storm water capture and treatment system at Everett are a solids separator, two slightly aerated ponds (500,000 gallons each), a wet pond with plants to take up nutrients (500,000 gallons), and a bioswale. Storm water flows into the first basin of the solids separator, and then continues through four subcompartments or weirs. The separator is designed to slow down the water flow to give the solids an opportunity to settle out into these basins. Furthermore, the basins are designed so that a front-end loader can drive in and remove collected solids. Figure 3 is an overview of the solids separator and the 500,000 gallon aerated pond that storm water flows into, and shows the quality of the water flowing through the aerator. It is interesting to compare the quality of water from Pond 3 in the combined leachate/ storm water treatment system (Figure 2), with the quality of the water from the pond in Figure 3. This illustrates the effectiveness of separating leachate flow from storm water flow.
The wet pond (Figure 4) is designed for vegetation. The primary purpose of this pond is to reduce the nutrient content of the storm water to meet water quality discharge limits, especially phosphorus and ammonia (a nitrogen source). The wet pond drains into a bioswale (Figure 5), essentially a water “polishing” step. Vegetation in the bioswale consumes excess nutrients. This tool also slows down the flow of the water leaving the wet pond, providing a chance for any remaining solids to settle out. During heavy rains, the overflow from the pond flows through the bioswale more quickly, reducing the ability for solids to settle and nutrients to be absorbed. However, the greater volume of rain water mixed with the pond overflow reduces that potential contamination. Then, as the water flow slows down, the bioswale can once again assist in settling solids and consuming nutrients via the vegetation.
Figure 6 shows the final discharge point at the Everett facility. That drainage swale discharges into a stream that eventually flows to the Puget Sound. Conversely, at the Maple Valley facility, treated water from the aerated ponds gets discharged to an industrial wastewater treatment plant (for a discharge fee).
FINISHED COMPOST AS NET WATER CONSUMER
One of the beauties of compost – and why it is valued for its water retention capabilities in turf, horticultural and agricultural applications – is that finished compost piles (if sized appropriately) actually become net water users after rain events, versus leachate producers. After the first and second phases of covered composting, compost is cured in open piles over the aeration trenches. The goal of this phase is to reduce the moisture content of the compost from about 55 percent to 45 percent before screening. The aeration system helps to dry out the material.
These piles, which are roughly 164-feet long, 12-feet high and 26-feet wide, contain about 400 to 500 tons of material each. At this stage, when rain falls on the piles, it soaks in, and no leachate is produced. After a significant rain event, it may be necessary to keep the piles on the aeration trenches longer to achieve 45 percent moisture content, but no leachate is generated.
The piles of screened compost also are net water users. However the area around the piles, where loaders are driving in and out to load trucks, can become a source of solids that can end up in the storm water flow (Figure 7). The wheels of the loaders can drag compost onto the surrounding surface and there also may be spillage during truck loading. Having a sweeper truck keep the loading area clean reduces the potential of that material ending up in the storm water treatment system.
For the same reason, good housekeeping is critical in the screening area itself. In Everett, the outfeed conveyor of the screen drops into a concrete block containment bunker (Figure 8). This prevents material from blowing away – and it also keeps storm water from flowing into the screened material.
NUMBERS TELL THE STORY
To get a clear picture of the effectiveness of the various components of the storm water and leachate treatment systems at both the Everett and Maple Valley sites, Cedar Grove ran analyses of the water quality for the same parameters to be included in the Oregon DEQ composting facility regulations. Table 1 has the data from the combined storm water and leachate treatment train at Maple Valley. The pretreatment elements (ahead of the ponds) are the oil-water separator in high traffic areas and the solids separator. Table 1 lists the proposed Oregon standard and then notes the levels after each of the three ponds. By the end of the second pond (highly aerated), all parameters but ammonia, BOD and E. coli meet the proposed Oregon standard. After the third pond (slightly aerated), the water quality fell well within the Oregon proposed standards.
Table 2 provides the results from the Everett facility. The pretreatment elements are similar to Maple Valley (oil-water separator and solids separator). After the solids separator but before the lightly aerated pond, the storm water met the limits for pH, copper, lead, zinc and total dissolved solids (TDS). After the pond, all of the parameters fell well below the proposed DEQ standards. In fact, excepting for pH, ammonia and BOD, there were “nondetect” quantities on all of the parameters sampled. This highlights the effectiveness of separating leachate and storm water runoff, and demonstrates how the wet pond and bioswale function as water “polishers.”
The important takeaway lesson from the data in Tables 1 and 2 is that composting facilities can achieve the required level of water quality with combined or separate treatment systems for leachate and storm water. However, based on our years of operating experience and facility designs, it ultimately is easier and less expensive to manage them separately.
The final piece of the storm water management analysis was to look at the cost of these various treatment options. This was especially important as the composters in Oregon are very concerned about the investment required to comply with the new storm water quality discharge standards. In terms of basic housekeeping, a sweeper truck can be rented at a cost of $75/hour. Estimating 4-hours twice per week to clean the site, the cost is $600/week. Another critical housekeeping step is keeping the solid separators and the catch basin clean. That cost is estimated at $75/hour, and it takes about 2-hours/week to complete the task.
Table 3 provides the costs for each treatment unit at the Everett composting facility. The total cost is about $433,500; the system has the capacity to treat 1.5 million gallons of storm water. The aerated ponds, wet pond and bioswale use about 3 acres of the 26 acre site. (In comparison, the total treatment capacity in Maple Valley is 8.6 million gallons and takes up about 5 acres of the 28-acre site.) Table 3 includes a deduction of $7,000 if an 80 ml liner is used; the cost ($83,000) is for a 100 ml liner, which we used in Everett. In Washington State, the regulation only calls for 60 ml. The reason to buy a thicker liner is related to maintenance, and cleaning out the ponds periodically. The thicker the liner, the easier it is to clean. It minimizes punctures to the liner during clean out, which then require fixing or sealing. Cedar Grove has determined that using a thicker liner up front saves a significant amount of money in subsequent years.
To achieve the most effective storm water control for the least cost, we determined that the following components are necessary: Sweeper truck ($600/week); construction of a 1-foot high by 3-foot wide compost filter berm (using 1-inch screened compost produced at the site thus no out-of-pocket cost) to filter sediment out of storm water as it flows toward the treatment system; a 4-phase solids separator ($58,500); one unlined wet pond ($50,000 for one million gallon capacity); and one bioswale ($9,000 with vegetation). These five components should be capable of meeting the proposed DEQ standards.
The assigned prices are from our costs at the Everett facility; a smaller composting site would be able to scale down the size of these treatment components, taking into account annual rainfall in that local area and the characteristics of rain events. For example, Portland, Oregon receives about 36-inches of rain/year on average, and it tends to fall in drizzles. Western Washington, e.g., the Puget Sound region, receives 49- to 55-inches/year of rain, and much of that falls in heavy rain events that yield 2- to 3-inches per day.
Many other treatment options are available for storm water, however most have been developed for other industries such as sewage treatment plants. Although many of those treatment options are applicable, it seems that compost facilities face a unique problem with the concentrations of contaminants and the fact that most operations have to develop the most cost-effective system to stay in business. While the $433,500 price tag from the treatment train installed at the Everett facility may seem expensive, when compared with the volume of storm water to be treated over a 10-year period, it really is only 26 cents/ton. After employing many other technologies over the years and spending thousands of dollars on equipment and engineering services, Cedar Grove has learned the best treatment train for its yard trimmings and food residuals composting process.
Jerry Bartlett is Vice President for Cedar Grove Composting, Inc. He has 20 years of experience in the environmental industry and a thorough knowledge of solid and hazardous waste regulations.
February 17, 2006 | General
Storm Water Treatment Options At Composting Facilities
BioCycle February 2006, Vol. 47, No. 2, p. 23