February 25, 2008 | General

Managing Storm Water

BioCycle February 2008, Vol. 49, No. 2, p. 28
Is it storm water? Is it wastewater? What is the best method to calculate flows at the site? These questions, and more, are answered in BioCycle’s new Operator Insights feature. Part I
Craig Coker

ONE OF the clearly recognized benefits of compost when used as a soil amendment is its ability to increase the water holding capacity of the soil. This is due to the ability of compost to retain water. Yet, this same beneficial material, while being manufactured, is now believed to create water quality problems that require regulatory permitting and potentially expensive treatment systems. Unfortunately, there is a wide variety of state regulatory approaches to address this issue; some regulate storm water runoff from composting pads as a wastewater, and some regulate it as storm water.
The purpose of this two-part article is to examine how storm water is produced at a composting facility, what types of contaminants it might have, how is it being regulated and what types of treatment work to meet regulatory requirements. As a former composting facility operator and state organics recycling coordinator, and now as a consultant, I have worked first-hand on calculating storm water flows, identifying pollutants, and determining the optimum management strategy. Those lessons learned are reflected in these articles.
Part I also kicks off a new BioCycle editorial feature in 2008, “Operator Insights.” Appearing in every other issue, Operator Insights will examine topics that site managers face in their daily operations.
In a nutshell, contamination. By virtue of some of the data collected in annual storm water runoff monitoring requirements on composting facilities – imposed under the Phase 1 National Pollutant Discharge Elimination System (NPDES) program established in the 1990s – it is now apparent that rain falling on exposed composting windrows can pick up substantial amounts of pollutants. If discharged to receiving streams unmanaged, the pollutants have the potential to cause water quality problems in those streams. These off-site losses present a higher level of concern than a similar amount of the same compounds in the compost itself because water quality standards for some constituents (i.e. pesticides, metals) are set at much lower levels than any standards for land application of compost containing these compounds and elements (Cole, 1994).
Not surprisingly, the constituents in composting pad storm water runoff depend, to some extent, on the feedstocks being composted. Yard trimmings composting facilities have long been thought to have the least potential for contamination due to the relatively clean nature of the feedstocks, i.e., least amount of chemical and pathogenic contamination to start with (relative to manure, biosolids and food scraps). Yet runoff from these facilities has been shown to be highly variable, containing potentially significant levels of nutrients, soluble salts, Biological and Chemical Oxygen Demand (BOD/COD), tannins and phenols from decomposing leaves, herbicides, pesticides, fungicides and fecal coliform (probably from animal feces mixed in the yard trimmings). Animal manure, biosolids and food scraps composting facility runoff will likely have higher levels of nutrients, organic acids produced during decomposition and fecal coliform bacteria (Oregon DEQ, 2004). Little data is available on other pathogenic microorganisms in composting facility runoff.
Potential impacts of these constituents on stream and river water quality are the same as, and in some cases more severe than, untreated discharges of sanitary wastewater. BOD and COD exert an oxygen demand on the dissolved oxygen in water, which if depleted will cause significant aquatic ecological mortalities. Widespread fish kills are the most obvious of these problems. Nutrients contribute to the geomorphologic process known as eutrophication, where nutrients support the growth of algae, which also deplete oxygen upon their deaths, as well as stimulate the growth of vegetation (this is an acceleration of the natural process by which shallow watercourses become swamps, and in turn, eventually become dry land). Tannins and lignins are natural dissolved organic acids that give water the characteristic color of compost tea. Phenols are a group of related acidic compounds that are hydroxyl derivatives of aromatic hydrocarbons. These include such substances as cresol, catechol, quinol, xylenol, guaiacol and resorcinol. There are two effects apparent in phenol-contaminated waters: toxicity to aquatic life and the generation of an unpleasant taste in fish and shellfish.
There are different sources of water contributing to runoff from a composting facility and each is subject to different levels of potential contamination. Ordered from most to least potential for contamination, they are: leachate, process storm water, nonprocess storm water, wash water and run-on. “Runoff” is generally a mix of leachate, process storm water, wash water and nonprocess storm water. Each has different characteristics, as defined below, and should be managed to prevent potential environmental harm.
In most composting piles, water moves to the bottom under the influence of gravity and creates leachate if the moisture content of the compost exceeds its water holding capacity (Krogmann, 2000). Moisture content in a pile is affected by feedstock types, mixing procedures, incoming rainfall, decomposition rates of organic matter that release the water bound inside plant and animal cell walls, the presence or absence of forced aeration that tends to evaporate more moisture and whether the composting process uses supplemental irrigation during active composting. A more coarsly textured mix, like yard trimmings, will have less water holding capacity than a more finely textured mix, such as dairy manure bulked with sawdust. Leachate draining through a composting pile will pick up soluble materials (tannins, nutrients, salts) as well as small particulate matter created by the decomposition process. It is a combination of the tannic acids and the particulate matter that give leachate its characteristic dark brown color.
Process storm water is precipitation that falls on the site and contacts the composting material without flowing through the pile. This includes runoff from the sides of the pile as well as storm water that comes in contact with waste material and compost that has strayed from the pile. Nonprocess storm water is precipitation that falls on the compost site, but that doesn’t come into contact with wastes or compost. Wash water is generated by washing vehicles and equipment and contains materials dislodged from vehicle wheels and bodies, but the concentrations tend to be lower due to the high volumes of dilutive water used in washing. On the other hand, wash waters can contain surfactants and other chemicals from any washing detergents used. Run-on is rainfall runoff from uphill of the composting site that flows through the site and comes in contact with wastes or compost. Enclosed composting facilities also have condensate – water that evaporates from the compost and condenses on cooler surfaces such as building walls.
Figuring out how much storm water runoff will have to be managed at a composting facility is not an exact science. Rainfall varies in intensity both in space and in time, as evidenced by the intensity of a thunderstorm on one side of a road, but not on the other. Similarly, runoff quantities vary as a function of how much rain has fallen recently, possibly saturating the ground, as well as constructed conditions like composting pad construction materials; presence, orientation, spacing and age of windrows; and moisture content in those windrows. A 2004 Canadian study concluded that about 68 percent of the incoming rainfall became runoff at windrow composting facilities, and that there was a significant delay between rainfall and runoff as the compost detained the rainfall and released it slowly over a period of one to two days (Wilson, 2004).
Quantities Of Storm Water
There are several tools of hydrology and hydraulics available to calculate runoff quantities from various storm events. Storm water management regulations and programs have historically been focused on managing quantities of storm water. These rules are often based on what is called the “recurrence interval” of a storm; terms such as “25-year, 24-hour” and “10-year, 1-hour” storms are often used.
The recurrence interval of a storm is a statistical abstract, and mathematically is the inverse of its probability of occurrence in any given year. For example, a “25-year” storm has a statistical probability of occurring once every 25 years (the inverse is 1/25, which equals 0.04, or 4 percent). Thus the 25-year storm has a 4 percent chance of occurring in any given year. Similarly, a 10-year storm has a 10 percent chance of occurrence and a 100-year storm has a 1 percent chance of occurrence. It is worthwhile to note that these statistical abstractions can be misleading. Many people think that if a storm has only a 4 percent chance of occurring (i.e. once every 25 years), it is unlikely to occur more frequently. It is entirely possible that two 25-year storms can occur in the same year, or even in the same week or month.
A “24-hour” storm is the total amount of precipitation that falls in 24 hours, which, for example, is 6.5 inches here in western Virginia. Similarly, a one-hour storm is the amount of rain falling in one hour (about 2.2 inches here). However, storm water management systems are based on both volumes of storm water as well as flow rates of water to be managed, so a storm of one hour duration with a rainfall intensity of 1.5 inches per hour will produce the same volume of rain (1.5 inches) as a storm of six hour duration but only one-quarter inch/hour rainfall intensity. That 1.5 inches of rain will produce a volume of 54,450 cubic feet of rainfall (about 407,000 gallons) on a 10-acre concrete or asphalt composting pad. Not all of that rain becomes runoff.
Flow rates are measured in volumes per unit of time (for example, cubic feet per second, or cfs). Flow rates of runoff from a storm vary over the duration of the storm and are characterized by hydrographs, which plot the change in runoff flow rate over time for a given storm. Figure 1 is a runoff hydrograph for a 10-year, 1-hour storm of 2.2 inches/hour falling on 13.25 acres of gravel compost pad draining to a pond. This storm will produce a peak discharge of 14.13 cfs and a runoff volume of 18,274 cubic feet (136,690 gallons).
Calculating A Runoff Curve Or Coefficient
Figuring out how much of the rain falling becomes runoff to be managed in a storm water system requires an estimation of the absorptive capacity of the surface onto which that rain falls. In a forested watershed, rainfall is intercepted by leaves and infiltrates into the ground, so only a portion becomes runoff that reaches streams. In an asphalt parking lot (or composting pad), little is intercepted or infiltrated, so most of it becomes runoff (but not all, as even asphalt or concrete will intercept small amounts of rain in surface irregularities).
Dimensionless numerical coefficients have been developed to represent different surface conditions affecting runoff. The exact number used depends on the method of hydrological analysis – whether it is hydrograph-based or nonhydrograph-based. Table 1 presents runoff coefficients for different land uses (or “cover types”) for both nonhydrograph methods and for hydrograph-based methods (Virginia DCR, 1999). Generally speaking, land uses that have higher amounts of impervious surface have higher runoff coefficients, reflecting that more of the incoming rainfall is being converted to outgoing runoff and less is lost to interception and infiltration.
Hydrologic soil groups are assigned by the U.S. Department of Agriculture and reflect the “runoff potential” of a particular soil. For example sandy soils tend to be in Groups A and B (see Table 1 categories), while clayey soils tend to be in Groups C and D, which have higher runoff potential.
Calculating a runoff curve number or coefficient for a composting pad requires use of a weighted average approach (weighted by the percentage of the pad occupied by windrows and the percentage not covered with windrows). The total square footage of the pad occupied by windrows has a lower value than the total square footage of the pad between windrows. Not all composting windrows have the same runoff coefficient. Drier compost, subject to summer temperatures and lighter rain intensity, would likely result in a lower measured runoff coefficient. For an asphalt composting pad, assume a coefficient of 0.85 for the aisle spaces between windrows. For the windrows themselves, assume a coefficient between 0.50 and 0.70 (Kalaba, et al., 2007).
The most widely used nonhydrograph method for calculating runoff is the Rational Method. It was developed in 1889 as a method for calculating peak flows for sizing storm drains:
Q = C x I x A
where Q = maximum rate of runoff, in cubic feet per second
C = a dimensionless runoff coefficient (see Table 1)
I = the design rainfall intensity, in inches per hour, for a duration equal to the time of concentration of the watershed
A = the drainage area, in acres
The term “time of concentration” refers to the time it takes for runoff to move from the most hydrologically distant point in the drainage area to the point of interest, such as the inlet to a storm pond, a flow monitoring station on a stream or the design location for a new impoundment. For paved composting pad runoff calculations, time of concentration is measured in minutes, provided upgradient runoff is properly diverted around the pad. For natural watersheds, time of concentration is measured in hours.
There are a number of limitations with the Rational Method in determining runoff volumes and flow rates: it assumes the duration of the design storm is equal to the time of concentration in the drainage area; it fails to account for the fact that compost windrows can shed rainfall, absorb rainfall or act as a reservoir and detain rainfall; and it assumes that the fraction of rainfall that becomes runoff is independent of rainfall intensity or volume (with windrows of compost, runoff varies with rainfall intensity), along with others (Kalaba, 2007).
Using the Rational Method on the same composting facility illustrated in Figure 1 (i.e. drainage area = 13.25 acres, rainfall intensity = 2.2 in/hr) and assuming a weighted runoff coefficient of 0.75, the calculated peak discharge rate is 21.86 cfs, a 54 percent overestimate compared to the more rigorous and accurate hydrograph method shown in Figure 1. This might have resulted in expensive overdesign of a storm water management system.
Developing an accurate natural hydrograph of a storm event requires extensive real-time flow monitoring. Given the cost and difficulty of real-time monitoring of small watersheds, synthetic unit hydrographs were developed by Snyder in 1938 to establish a method of simulating a natural hydrograph by using watershed parameters (area, shape, slope and ground cover) and storm characteristics. The synthetic unit hydrograph method is the cornerstone of the hydrologic work done by the USDA’s Soil Conservation Service (now the Natural Resources Conservation Service), embodied in the National Engineering Handbook, Section 4, Hydrology (1985) and in widely-available computer models like TR-20, “Project Formulation, Hydrology” (1982) and TR-55, “Urban Hydrology for Small Watersheds” (1986).
TR-55 presents two general methods for estimating peak discharges from urban watersheds: the graphical method and the tabular method. The graphical method is limited to watersheds where runoff characteristics are fairly uniform and soils, land use and ground cover can be represented by a single Runoff Curve Number (see Table 1 for examples). The graphical method provides a peak discharge only and is not applicable for situations where a hydrograph is required. The tabular method is a more complete approach and can be used to develop a hydrograph at any point in a watershed.
There are a number of other computer-based storm water hydrologic and water quality models available, such as the U. S. Environmental Protection Agency’s Storm Water Management Model (SWMM), which is a comprehensive computer model for analysis of quantity and quality problems associated with urban runoff. Both single event and continuous simulation can be performed on catchments having storm sewers, or combined sewers and natural drainage, for prediction of flows, stages and pollutant concentrations. These models tend to be watershed-scale models, rather than site-scale models.
Sampling Methods and Tools
Sampling of storm water for laboratory analysis and characterization is considerably more difficult than sampling process wastewaters coming out of the end of a pipe, because of the lack of control over sampling times and conditions. Rainfall events often occur at night, on weekends and holidays, and sometimes with little advance notice. It is also difficult to obtain a truly “representative” sample. For example, the storm water discharge permit issued by the Missouri Department of Natural Resources to a private composter requires sampling “during a storm water event of 0.1 inch or greater and during the first 30 minutes of the discharge.” This is meant to capture the “first flush” of pollutants swept up by runoff, which is obviously meant to capture a “worst-case” situation. Taking a one-time sample of runoff (known as a “grab” sample) at the wrong time may result in pollutant concentrations that are not truly representative.
Sampling methods to offset this difficulty include flow-composited and time-composited samples, often collected with automated sampling equipment. A flow-proportional composite sample consists of discrete samples collected at a rate proportional to flow. Taking flow-composited samples requires knowing the flow rate of the watercourse, which is normally done by inserting a temporary calibrated V-notch weir in the channel. A time composite sample consists of discrete samples collected at constant time intervals.
Actual procedures used to obtain good quality representative samples are important. Recommended procedures include: wear disposable, powder-free gloves; grab samples with the storm water entering directly into bottles provided by the laboratory (don’t transfer them from other containers that may be contaminated with phosphorus-based detergent residue); sample where the water has a moderate flow and some turbulence, if possible, so that the sample is well-mixed; do not overfill the bottle so as not to wash out any sample preservative provided by the laboratory (normally used for ammonia and phosphorus); and cap and label the bottle as soon as the sample is taken (Washington DOE, 2005).
If samples are to be analyzed for biologicals (fecal coliform, other pathogens), the same sample preservation and shipping issues that affect compost samples will affect these water samples. Refrigerate the sample immediately after collection and ship it to the laboratory using an overnight service, packing the sample in iced gel-packs, or the equivalent.
Pollutant Concentrations
Pollutant concentrations in runoff from composting facilities vary widely. A 1997 study by the Clean Washington Center characterized runoff from four composting facilities in the Pacific Northwest, which is shown in Table 2 (CWC, 1997). Another study (Krogmann, 2000) monitored storm water quality for three years at a European composting facility handling residential source-separated organics. The results of that monitoring are shown in Table 3.
The BOD5/COD5 ratio is a measure of the biodegradability of a wastewater. A BOD5/COD5 ratio of 0.5 is the same order of magnitude as municipal wastewater and is considered easily degradable. A wastewater with a BOD5/COD5 ratio less than 0.1 is considered biologically difficult to degrade. The BOD5/COD5 ratio of the runoff from the large-scale open windrow facility in Krogmann’s study ranged between 0.02 (minimum) and 0.37 (maximum) with a geometric mean of 0.05, suggesting it is a wastestream that may be difficult to biodegrade.
Part 2 of this article, to be published in April 2008, will discuss structural and operational management options for managing storm water quality and the storm water permitting programs and issues affecting the composting industry.
Craig Coker is a Contributing Editor to BioCycle and a Principal in the firm of Coker Composting & Consulting in Roanoke, Virginia. He can be reached at (540) 904-2698 or by email at
Clean Washington Center, “Evaluation of Compost Facility Runoff for Beneficial Reuse,” prepared by E & A Environmental Consultants, May 1997, p. 11.
Cole, M.A., “Assessing the Impact of Composting Yard Trimmings,” BioCycle, Vol. 35, No. 4, April 1994, p. 92.
Kalaba, L., et al., “A Storm Water Runoff Model for Open Windrow Composting Sites,” Compost Science & Utilization, Vol. 15, No. 3, p. 142-150.
Krogmann, U., “Selected Characteristics of Leachate, Condensate, and Runoff Released During Composting of Biogenic Wastes,” Waste Management and Research, Vol. 18, 2000, p. 235-248.
Oregon Department of Environmental Quality, “Commercial Composting Water Quality Permit Development,” prepared by CH2MHill, May 2004.
Virginia Department of Conservation and Recreation, “Virginia Storm water Handbook,” 1999, Sec. 4, Hydrologic Methods.
Washington Department of Ecology, “How to do Storm water Sampling – A Guide for Industrial Facilities,” Publication 02-10-071, Dec. 2002, revised Jan. 2005.
Wilson, B.G., et al., “Stormwater Runoff From Open Windrow Composting Facilities,” Journal of Environmental Engineering Science, Vol. 3, 2004, p. 537-540.

Sign up