Windrow compost pad runoff

Windrow composting pads enable degradation of organic wastes in a controlled manner while having minimal effects on the surrounding environment. 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. 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.The UGA Bioconversion Center compost pad in Athens, Georgia provided the data for a study by UGA biological and agricultural engineers. 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. Because the NRCS Curve number method is by far the most common method for quantifying rainfall runoff, they estimated the Curve Number. To get an indication of runoff rate, they 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). The effective Curve Number was 75 plus or minus 5 percent after 10 years of operation. An earlier analysis found that the Curve Number was 98. This led the engineers 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 percent of the observed runoff.