BioCycle April 2008, Vol. 49, No. 4, p. 29
Segregating storm water flows at composting facilities to minimize generation of highly contaminated runoff goes a long way to reducing treatment costs. Part II
WATER quality regulations have expanded from individual end-of-pipe point sources to include runoff-generated area nonpoint sources of water pollutants. This has led to increased scrutiny of water pollution potential from rainfall-induced runoff at composting facilities. While all types and configurations of composting facilities are being examined, the majority of attention is on the mainstream technology of open-air turned windrow systems, where rainfall comes into contact with waste materials, composting piles and finished compost products.
Part 1 of this series (February 2008) looked at the quantity and quality considerations of this runoff. The amount and data quality of chemical and biological characterization data for storm water runoff is rather limited; however, available data suggests that compost pile leachate and runoff contaminated with leachate have levels of traditional water pollutants that can exceed levels found in standard domestic wastewater. These pollutants include oxygen-demanding substances, suspended solids, nutrients and bacterial contamination. Other pollutants of importance in compost runoff are heavy metals, oil and grease, and tannins and phenols. Tannins and phenols are derived from the woody materials used in composting, and are derivatives of aromatic hydrocarbons which, if discharged untreated, have potentially significant impacts on aquatic life.
Oxygen-demanding substances in wastewater are normally measured using two surrogates – Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD). BOD is normally defined as the amount of oxygen required by bacteria while stabilizing decomposable organic matter in water under aerobic conditions. COD is defined as the total quantity of oxygen needed to completely oxidize all organic matter to carbon dioxide and water. COD values are usually higher than BOD values and may be much greater when significant amounts of biologically resistant organic matter (like lignin in wood fibers) are present. Consuming the dissolved oxygen in waterways has significant adverse effects on aquatic life. Suspended solids washed into streams can blanket and smother bottom-dwelling aquatic life as sediments settle. These solids can also interfere with light transmission and aquatic vegetation photosynthesis if they remain suspended. In addition, they often carry adsorbed oxygen-demanding substances into the water, which cause odor problems as dissolved oxygen levels fall.
Nutrients of importance in water pollution control are the water-soluble forms of nitrogen (ammonia, nitrates and nitrites) as well as phosphorus. These can stimulate the growth of algae in water, which create their own oxygen demands on waters when they die, and ammonia is toxic to aquatic life in sufficient quantities. Contamination of waters with fecal coliform is one of the principal reasons why some waters have lost their “fishable, swimmable” status. Tannins, phenols and similar substances are not only toxic in their own way, but consume dissolved oxygen as they degrade to less complex forms.
Heavy metals such as chromium, copper, lead and zinc are a concern in storm water runoff from urbanized areas, and can be a concern in composting facility runoff, particularly from facilities handling industrial or biosolids feedstocks. As the majority of composting facilities process feedstocks with limited heavy metals, and multiyear sampling at some facilities indicates no migration of heavy metals in compost facility runoff, these are pollutants of largely secondary consideration in developing water pollution control strategies for composting facilities.
Part 3 of this series (to run in May 2008) will look at how storm water from composting facilities is being regulated. Most states in the U.S. have delegated authority from the U.S. Environmental Protection Agency to regulate point and nonpoint sources of water pollution. Permits issued by state environmental agencies regulate the quantities of pollutants allowed to be discharged. These allowable quantities are converted to concentrations in the discharge (which are easier to measure for compliance) based on allowable quantities of water flow. This approach works well with traditional end-of-pipe discharges from installations where water flow is predictable and relatively consistent.
Water flows in storm water runoff are considerably more unpredictable. There is a varying assortment of control strategies in use by states for controlling potential water pollution from storm water runoff at composting facilities. For example, Oregon has developed a customized storm water permitting program for composting facilities. North Carolina, on the other hand, has decided not to issue any storm water permits for composting facilities, instead making facilities get wastewater discharge permits. Several states use the Multi-Sector General Permit approach used for industrial facilities, choosing to regulate composters under the SIC Code for Fertilizer Mixing (SIC 2875). Others have no regulations at all. It is becoming increasingly clear, however, that composting facilities must plan for control and management of storm water through a combination of both structural and nonstructural management techniques.
REDUCE, REUSE, RECYCLE
Reducing the quantities of storm water to be managed is the first step. Under the conditional no exposure exclusion, operators of industrial facilities in any of the categories of “storm water discharges associated with industrial activity,” may have the opportunity to certify to a condition of “no exposure” if their industrial materials and operations are not exposed to storm water. At least one in-vessel composting operation in North Carolina is pursuing this to avoid permitting. Several water quality regulators interviewed for this series of articles expressed a desire to see new composting facilities enclosed in buildings, and existing facilities retrofitted with roofed structures to keep rainfall from compost piles.
Another method of reducing the quantity of “contaminated” storm water is through segregation of runoff flows to minimize the quantities of waters with the highest degree of contamination (i.e. leachates). Strategies include site grading to divert up-gradient runoff around a composting facility, using design and construction techniques to segregate leachate from storm water (see “Enzyme Producer Grows Greener With Composting,” BioCycle December 2006) and isolating vehicle and equipment washing stations with their own runoff containment. By segregating water flows, costs for treating highly-contaminated wastewaters can be reduced and less expensive Best Management Practices (BMPs) and pollution prevention mechanisms can be used for managing lightly contaminated storm waters.
Storm Water Pollution Prevention Plans (SWPPP) are another widely used mechanism to reduce pollutants in storm water. While more specifics are provided in Part 3 of this series, a SWPPP includes: a facility assessment; identification of areas of potential or past pollution discharge; a monitoring (sampling and visual inspection) plan; a schedule for implementing additional or enhanced BMPs; a list of operational and structural BMPs; and development of operation and maintenance procedures (Washington DOE, 2004).
The most feasible method of reusing collected storm water is to reintroduce it to the compost piles to keep moisture contents at the optimum 50 to 55 percent level. Windrows can be “irrigated” with hoses, sprinklers or water trucks. As compost piles have considerable water-absorptive capacities, a substantial amount of water can be reused this way. Table 1 shows a sample calculation of how much water can be reused for one irrigation event. Assumptions used in this example include: Facility captures and holds the 25-year, 24-hour storm (6.62 inches for coastal mid-Atlantic state); all up-gradient runoff is diverted around composting facility; capture and contain runoff from 8-acre compost pad; all windrows are covered with fabric and are impermeable; site soils are in Hydrologic Soil Group A or B.
In this example, the windrows are covered with fabric blankets. Open windrows will produce much less runoff to be managed due to absorption of rainwater into the windrow (see Part 1 of this series). While the above method is based on computing the weight of water to be added (and then converting that back to gallons), another formula for calculating the maximum volume of water that can be added to compost piles is (The Composting Association, 2007):
VL = VM x dM x (MCmax — MCM) x 1000
100 x dL
VL = volume of water to be added (litres)
VM = total volume of composting material (cubic metres)
dM = bulk density of composting material (tonnes/cu. meter)
MCmax= target moisture content, percent (usually 55%)
MCM= starting moisture content of pile, percent
dL= density of water (tonnes/cu.meter)
Because there is some risk that the collected storm water will have fecal coliform contamination, irrigation with storm water should not be done after a compost pile reaches the Process to Further Reduce Pathogens (PFRP) time-temperature standard unless that water has been disinfected. Otherwise, there is a risk of reinoculating a finished compost pile with viable pathogens. This is true even if only yard trimmings or vegetative debris are being composted, as monitoring data has shown elevated levels of fecal coliform in runoff from yard trimmings compost facilities. This fecal contamination is presumably from animal feces mixed in with the yard debris.
Disinfection of collected storm water is possible, using a dosage rate of 5 to 25 mg/L available chlorine (i.e. between 2 and 6.5 ounces of Clorox® bleach per 100 gallons of water). The effectiveness of this depends on the suspended solids content of the storm water, and it may have adverse effects on the composting process once used as irrigation water. In addition, some states may consider this a “wastewater treatment process” for permitting requirements.
Some composters are developing permanent water reuse systems to manage storm water. At Royal Oak Farm, a 500-tons/day open-air turned windrow facility in Evington, Virginia, a subterranean irrigation system was built as part of an upgrade. This system serves both water reuse and fire fighting purposes, and consists of a series of 3-inch stanchions, or standpipes, around the perimeter of the four compost pads, fed by a network of 6-inch PVC pipes. The system is charged by two 30-HP electric pumps, where irrigating windrows is done with one pump in service, drawing water from a lined storm water pond on site. Both pumps are used if fire fighting is needed; additional water can be drawn from a separate farm pond. Royal Oak uses a Backhus windrow turner and a windup hose reel to connect the turner to the irrigation standpipes.
One way to recycle composting facility runoff is to use it as irrigation water for crops. These can include row crops, pastureland, turfgrass or biomass silviculture crops (such as hybrid poplar trees). Irrigation methods include overland flow, a spray system, drip irrigation or using subsurface infiltration galleries. Hydraulic loading is a primary design tool when using irrigation to recycle purely storm water. Hydraulic loading, or how much water a field can absorb, is defined by site-specific soil conditions, depths to seasonal high groundwater tables and regional climate considerations (a water balance analysis of precipitation and pan evaporation).
Because composting facility runoff contains nutrients, it is more likely to be regulated as a wastewater and be subject to both nutrient and hydraulic loading constraints. Many states now require land application systems to be based on Nutrient Management Plans (NMPs), which are site- and field-specific assessments of the potential for nitrogen and phosphorus transport to surface waters.
Whether compost facility runoff is classified as a “wastewater” or a “storm water” is a legal question and has implications for recycling via land application. Many states require some degree of treatment of a “wastewater” prior to land application. For example, in Virginia, wastewater to be land applied must be pretreated to a maximum BOD level of 60 mg/L and predisinfected to a maximum fecal coliform level of 200 MPN/100 ml. The degree of disinfection often influences the size of the required buffer zone. In Washington, the setback requirement from property lines is 100 feet if the wastewater meets the disinfection pre-application limit of 200 MPN/100 ml, but climbs to 650 feet if it does not.
Traditionally, storm water management has been about managing the quantity of water more than the quality of that water. With the new focus on nonpoint source pollution, the emphasis is now on pollutant removal efficiencies of existing storm water quantity control measures, like detention ponds, as well on treatment devices now on the market, like storm water filtration systems that can be incorporated into a municipal storm drain network.
Best Management Practices
BMPs remain the cornerstone of storm water management strategies, and many are very suitable for use at composting facilities. BMPs are practices, procedures or structural controls used to prevent or reduce adverse impacts to receiving waters. BMPs do this by managing the quantity and quality of the storm water, the leachate from compost piles and equipment washdown wastewater generated at a composting facility. BMPs can be structural, operational or both. Structural BMPs are physical improvements and treatments that can control, treat and protect water quality. Examples include bioretention basins, vegetated filter strips and constructed wetlands. Operational BMPs focus on pollution prevention activities and on operation and maintenance of structural BMPs.
The Oregon Department of Environmental Quality retained the consulting engineering firm, CH2MHill, to evaluate suitable BMPs for composting facilities as part of the background research for development of the new Compost Facility Storm Water Permit program (see Part 3 for more information on this permit). This study ranked 27 BMPs in terms of space efficiency, odor control, cost, level of complexity, number of benchmark constituents potentially controlled and whether the BMP was beneficial for control of bacteria, lead and nitrates (Oregon DEQ, 2004). Table 2 lists the 27 BMPs evaluated. The study concluded that these BMPs were suitable for use by composting facilities, with some modifications of definitions to tailor them to composting. Oregon’s new compost storm water permitting program requires the use of one or more of these BMPs for runoff that has not been mixed with compost pile leachate. The Fiscal Impact Analysis estimated, for two hypothetical composting operations, total annual BMP costs (amortized capital plus operation) of $87,900 to $114,100. Estimated impact on tipping fees varied from $1.61/ton to $9.10/ton (Oregon DEQ, 2008).
On-site treatment alternatives are complicated by several factors. The degree of treatment needed depends on the ultimate disposition of the contaminated storm water. Another factor is the need for some form of flow equalization, as wastewater treatment systems operate most efficiently under a relatively constant flow. As runoff varies with storm intensity and amount of composting pad occupied by windrows, a retention basin of adequate size is needed upstream of any treatment processes.
If the water is to be reused for crop irrigation via a spray field, then pretreatment (as noted above) is usually sufficient. If it is to be discharged, treatment levels depend on discharge permit levels. In a nutrient-sensitive waterway subject to water quality-based effluent limitations, it is possible that storm water would have to be treated to advanced (or “tertiary”) levels. Tertiary effluent discharge concentrations are typically on the order of 3 to 5 mg/L BOD, 3 to 5 mg/L TSS, 1 mg/L Total Nitrogen and 1 mg/L Total Phosphorus.
The pollutants found in composting facility storm water can be treated by several different “unit processes” or in combination “package plants.” Table 3 lists some of the various unit processes used in wastewater treatment.
Aeration/oxidation is the process of reducing oxygen-demanding substances by raising dissolved oxygen levels. Most simply, this involves aerating a storm water pond. Numerous types of pond aerators are on the market, but composters should seek models with the highest oxygen transfer rate. For example, in Table 1, a pond could contain 1.25 million gallons after the 24-hour, 25-year storm. If that storm water has a BOD concentration of 100 mg/L, then the pond would contain 1,044 lbs of BOD. One pond aerator on the market has an oxygen transfer rate of 6.8 lbs/hour, so that aerator would have to run 153.5 hours to consume the entire BOD in the pond (neglecting microbial uptake and utilization of both oxygen-demanding organic materials and the dissolved oxygen in the water).
Biological conversion is the fundamental process used in activated sludge and fixed-film wastewater treatment systems (i.e. aerobic lagoons), as well as the processes at work in biologically-rich features like engineered wetlands, bioretention basins, bioswales, etc. Biological conversion will reduce pollutant concentrations of BOD, fecal coliform, some heavy metals and nutrients. Bioretention ponds (also known as rain gardens) are becoming widespread in areas adopting Low Impact Development policies, and have been shown to remove significant amounts of pollutants from storm water. Figure 1 illustrates a conceptual cross section of a bioretention pond.
Suspended solids are a common problem in compost storm water systems due to compost fines washed in with the runoff. Keeping solids out of storm water management systems provides several benefits: improves the efficiency of other treatment processes, such as disinfection; eliminates or reduces difficult maintenance tasks in lined ponds; reduces or eliminates the potential for anaerobic conditions to form in a pond or basin (with accompanying malodors); and minimizes the potential for discharge of solids in the event of a pond overflow. Use of Filtrexx™ compost-filled filter socks is an inexpensive way to keep solids out of ponds.
Disinfecting storm water for either reuse in the compost pile, for recycling via a spray field or for discharge permit compliance is difficult. The three primary methods are adding chlorine-containing compounds (like sodium hypochlorite), making and introducing ozone gas into the water or passing the water through a bank of ultraviolet lights for irradiation. All three methods are sensitive to the suspended solids levels in the storm water. A new process for disinfecting storm water is electrocoagulation, which was originally used for precipitating heavy metals out of wastewater. A 2004 pilot study on urban runoff in Los Angeles showed a 99 percent removal of total coliform, a 98 percent removal of chromium, a 96 percent removal of copper and a 98 percent removal of lead (Brzozowski, 2007).
Engineered wetlands may offer one of the best alternatives for management of composting facility runoff. Wetlands have a higher rate of biological activity than most ecosystems and, as a result, are capable of transforming the conventional pollutants found in storm water into harmless byproducts, or into nutrients that can be used to encourage higher levels of biological productivity (see sidebar).
Pump & Haul
In some cases, composting facilities may have to consider “pump-and-haul.” Due to capacity restrictions at the local treatment plant, and to extremely strict water quality standards in the watershed of a potable water reservoir, this is an alternative being considered for the Durham, North Carolina Yard Waste Composting Facility. North Carolina regulations require the capture and management of the runoff from the 24-hour, 25-year storm, which in the case of Durham, is about 1.8 million gallons. At $0.15/gallon cost to pump, haul and discharge at a treatment plant, this could be a cost of $270,000 each time the pond has to be emptied.
Water quality management – along with odor control and air emissions -may well be another powerful impetus for new composting facilities to consider in-vessel systems or enclosure in buildings. New open-air facilities likely will need to carefully engineer sites to segregate storm water runoff into manageable amounts to limit the quantity of highly contaminated and associated high-cost treatment requirements. Existing facilities facing permit renewals in states with aggressive storm water management programs likely will have to consider multiple management measures, including pollution prevention operational practices, and combinations of water quantity and water quality management facilities.
Craig Coker is a Contributing Editor to BioCycle and a Principal in the firm of Coker Composting & Consulting in Roanoke, Virginia (www.cokercompost.com). He can be reached at (540) 904-2698.
POLLUTANT REMOVAL VIA WETLANDS
THERE are two main types of engineered, or constructed, wetlands – the Free-Water-Surface (FWS) wetland, which has a standing pool of water, and a Subsurface-Flow (SF) wetland, in which water lies below the surface of the wetlands media (usually gravel). FWS wetlands don’t work well in northern cold regions, but SF wetlands have been shown to operate satisfactorily at subzero temperatures in Wyoming and Montana. Wetlands have measured pollutant removal efficiencies of 24 to 70 percent for phosphorus and between 31 and 84 percent for nitrogen.
Research underway at Virginia Tech (Virginia Polytechnic Institute and State University) suggests that combining engineered wetlands with conventional retention ponds can increase phosphorus removal above those levels. SF wetlands are being used to treat dairy feedlot runoff (high BOD and nutrients) in Vermont and are being used to treat airport deicing facility runoff in Buffalo, New York and Hartford, Connecticut. With the Buffalo project, the runoff has a BOD load in excess of 10,000 lbs/day and an effluent discharge permit level of 30 mg/L (Whitney, 2008).