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Tollner, Ernest W.
Windrow compost pad runoff
Summary
Runoff volume and flow rate of windrow composting pads should be conservatively designed treating all rainfall as runoff. As pads age, runoff hydrology becomes comparable to moderately porous soils that retain and then evaporate significant water amounts, thereby reducing available and/or required water for land application irrigation.
Situation
Windrow composting pads enable degradation of organic wastes in a controlled manner while having minimal effects on the surrounding environment. The environmental benefits of utilizing windrow composting facilities include diversion of organic material from landfills and enabling soil improvement. In an open windrow facility, the runoff contains compost materials. Surface runoff from the compost pad can be high in Biochemical Oxygen Demand, suspended solids, and nutrients that may adversely affect receiving water bodies. Sanitary sewer systems are generally unavailable for treating runoff from remotely sited facilities. Instead, pad discharges must be directed toward a detention pond prior to subsequent use in land application or other approved irrigation schemes. Compost pads are constructed using porous gravel aggregate overlaying a compacted clay base. Design characteristics of the compost pad include a 1% to 2% surface slope, windrows oriented parallel to the flow direction, and a gravel thickness of 150 to 200 mm placed over a compacted clay base. A collection pond capable of containing the 25 year-24 hour runoff receives the pad runoff. It is difficult to determine the amount and flow rate of surface runoff. Variation in runoff impacts the design of potential conveyances to the pond from the pad. The variation also affects the water quantity to be available for irrigation or disposal in land application programs.
Response
The UGA Bioconversion Center compost pad in Athens, Georgia provided the data for the study. The pad has a total surface area of 7284.22 m2. Regulatory authorities mandate collection of rainfall-runoff data and pad utilization data. Availability of these data provided a basis for developing a solution to the runoff volume and runoff flow rate as a function of rainfall event. Because the NRCS Curve number method is by far the most common method for quantifying rainfall runoff, we estimated the Curve Number. To get an indication of runoff rate, we also modeled the pad using a commercially available groundwater flow model (HYDRUS 2D) and developed a Compartment model capable of yielding a linearized rainfall-runoff transfer function (instantaneous unit hydrograph).
Impact
The effective Curve Number was 75 plus or minus 5% after 10 years of operation. An earlier analysis found that the Curve Number was 98. This led us to conclude that the original gravel media became increasingly filled with compost material as time elapsed. Further, incorporated material was progressively changing the gravel media behavior from that of a rapid filter to that of a moderately porous soil. Using the HYDRUS and Compartmental models, the predicted runoff versus time over an 8 month interval from December 2010 through July 2011 was within 6% of the observed runoff.
State Issue
Conservation & Management of Natural Resources
Details
- Year: 2011
- Geographic Scope: National
- County: Clarke
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Program Areas:
- Agriculture & Natural Resources
Author
Collaborator(s)
CAES Collaborator(s)
- Radcliffe, David E.
- Rasmussen, Todd C.
Research Impact