BioCycle Magazine

Calculating capacity at composting sites

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The addition of new composting facilities or expansion of capacity at existing ones is becoming more and more difficult as feasible sites are taken out of circulation. This has created interest in maximizing the unit production of compost at composting sites so that they perform as efficiently as possible. The need is especially acute if physical expansion is not possible, but increases in production are desired.

Calculating composting capacity involves much more than just analyzing the capacities and physical areas needed for active composting (e.g., aerated static piles or turned windrows) and curing piles and grinding/screening. Some of the more important ancillary and infrastructure requirements include: Buffer area to serve the purposes of noise and air emission abatement, and of visual screening; Weigh-scale and accommodation of the delivery vehicle queue; Fire lanes; Traffic control and monitoring/ contamination identification; Stockpiles to accommodate seasonal market demand for compost product; Air emission and odor control system (e.g., biofilter); Utilities; Leachate containment and treatment.

Business decisions that affect the capacity and area required for the facility primarily include the inventory volume that must be allocated to accommodate the seasonal demand for compost, and the acceptable level of odor and/or air emissions. Those levels may be defined by state and/or federal regulatory limits or, in their absence, by the facility operator based on best management practices and/or on good neighbor policy. Lower levels of air emissions may require more complex emission control systems.

Receiving raw materials and stockpiling of intermediate processed organics can occupy considerable percentages of the total facility land area. Traffic patterns and monitoring must be well planned so that the public health and safety (and delivery vehicles) are protected and flow of raw materials to downstream processing is not impeded.

Of the conventional composting technologies, turned windrows require a larger footprint than aerated static piles (ASPs) due to aisles between the windrows. While ASPs use physical space more efficiently, for the process to be operationally efficient, the material being composted must be able to be biologically degraded to a satisfactory degree without physical mixing.

A simple method of estimating the required total area of a turned windrow without the inclusion of aisles is to first estimate the unit volume for the cross section of the windrow (in cubic feet or yards) per foot of length of windrow. This can be calculated using the cross-sectional area of the pile (e.g., trapezoid) and assuming a unit pile length of one foot. (In this simple method, the effect of loss of mass from the piles is ignored as a first order estimation.) Next, divide the facility's volumetric production (equal to the daily capacity times the design residence time in the piles [in days]) by the unit volume per foot. The result is an approximation of the total length of the resulting windrow.

This calculated total windrow length is then divided into shorter lengths to fit within the site boundaries and operating schedule, thus yielding the number of rows and corresponding aisles. For example, for a 100 ton per day turned windrow active composting operation with 15-foot wide windrows, 15-foot fire lanes, 30-day composting period, and unit windrow volume of 3 cy/t, a pad area of approximately 2 acres is required.

The necessary capacities of processing equipment (e.g., grinders, screens) must be estimated based on the design throughput, the characteristics of the materials to be composted, and any anticipated daily or seasonal fluctuations in the flow of feedstock. The best location within the facility and area requirements for grinding and screening require consideration of many factors, including types of mobile equipment that will load the processing equipment, methods to be employed for protecting worker health and safety and procedures to be used to control noise and airborne emissions. Resolving these considerations takes significantly more land area than may be indicated by laying out the overall dimensions of the equipment on a site plan.

When all aspects governing the total land area needed for composting are considered, the active composting area may be 40 to 70 percent of the total area required for a facility employing turned windrows and 20 to 50 percent for a facility using ASPs. This means that the other area requirements are in the range of 30 to 80 percent; thus substantial real estate is dedicated to activities other than active composting. The wide range in these percentages reflects the impact of economies of scale. For example, size reduction, screening and conveying equipment must be of a minimum size to accommodate grinding and screening of yard waste, even if the design throughput of the facility is low.

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