BioCycle September 2006, Vol. 47, No. 9, p. 32
Analysis of windrows and aerated trapezoidal piles of varying depths highlights options to increase capacity on the same footprint.
Jan Allen and Will Bakx

THE ability to expand throughput capacity at composting facilities is often a space constraint issue. At sites with limited land area to grow, technology becomes the key factor in the search for increased capacity and expansion. Evaluating which approach is the most appropriate is a matter of finding the balance between optimizing space efficiency, technology, residence time, and cost.
For many composting operations, the initial size of the site is rarely the ultimate development size. When an operation starts, the owner often doesn’t realize how large the program will become or what the volume of the waste stream will be. New diversion programs come on line, and seasonal variations in the waste stream lead to larger waste flows that may not have been anticipated. As a result, many sites have had to grow reactively over the years, instead of in an organized or deliberate way. Critical components for successful facility management, such as adequate space for final screening and product storage, are sometimes compromised as operators scramble for room for active composting piles.
Sonoma Compost Company in Petaluma, California is a yard trimmings composting facility owned by Will Bakx and his partners. The site is located at the Sonoma County Central Landfill. Turned windrows are currently used. The facility has a set amount of real estate and Sonoma Compost needed to increase its processing capacity from its original size because the county and most cities it services began collecting yard trimmings weekly rather than biweekly, dramatically increasing the amount of incoming feedstock. In 2005, the site often processed over 250 tons/day. Sonoma Compost decided to work with CH2M Hill to conduct a pilot study to evaluate if aerated static piles could significantly increase the feedstock throughput on the same physical footprint. The company also wanted to monitor the impact on odors and leachate management at the same time.
SPACE EFFICIENT PROCESSING OPTIONS
Outdoor windrow composting remains one of the most common technologies, primarily because of its fairly low entry costs compared to methods using aeration systems and/or enclosures. The rate of composting typically is determined by frequency of pile turning, moisture addition, etc. Where windrow turners are used, the pile height and width are determined by the equipment design.
To evaluate other technology options based on the amount of material that can be processed on a set footprint, calculations were done using a turned windrow system as a baseline, and then compared to a trapezoidal pile and a deep static pile. The following are descriptions of trapezoidal versus deep static piles:
Trapezoidal Pile: A trapezoidal pile is usually an aerated static pile, built with conveyors, wheel loaders and in some cases, excavators. The footprint difference between a windrow and trapezoidal operation is significant, starting with the height of the piles, which can be upwards of 14 feet. A dairy using this method had trapezoidal piles built 12-feet deep by 60-feet wide and 120-feet long. A negative air system, combined with a biofilter, was used for process aeration and odor control. Approximately 2,200 cubic yards of material are in each pile. A windrow system with center-to-center pile spacing on the same footprint (60-feet wide by 120-feet long), with a pile height of 7-feet, can process 750 to 800 cy in total (provided the windrow machine has maneuvering clearance at each end of the 120 foot windrow). The assumption is roughly three windrows with two aisles (about 8-feet wide) in the 60-foot wide space. Each foot of windrow holds approximately 2.2 cy of material.
Deep Static Piles: Deep static piles can be built with or without aeration, and in any length and width. Facilities use front-end loaders, excavators, and conveyors to build the piles. If above ground piping is used, additional space is required for removing reusable pipes before moving a pile. Nonaerated systems and below grade aeration piping avoids this loss of active composting space. In one case, a yards debris facility that does no grinding initially begins the process with trapezoidal piles that are 25 feet deep. The material, as received, is blended with sawdust and bark dust and then the piles are built with excavators on a pad that has air plenums built into the floor. The piles are aerated under negative pressure, and turned about every three weeks with an excavator. Despite the 25-foot depth, a tremendous amount of air is pulled through the pile. Approximately 45,000 cy of material are in each pile; sections within the pile represent different batches.
Table 1 compares the volume of material that can be processed in a turned windrow, trapezoidal pile and deep static pile. A one-acre footprint is assumed for each composting method (calculated by computing the volume of a single pile and then expanding that to a full acre footprint with a series of piles to fill the acre). According to the assumptions shown in the table, 2,900 cy, 13,400 cy and 19,000 cy of raw material/acre can be processed using the turned windrow, trapezoidal pile and deep static pile methods respectively.
When looking at this data, one’s first impression may be that the differences in volume have to do with depth of the pile (7-feet, 12-feet and 20-feet for windrow, trapezoidal and static piles respectively). But volume differences are not only attributed to pile depth. Other factors include the amount of open pavement between the piles and the side slope to the piles. On a proportional basis, there is 6.6 times more pile volume on a given acre using 20-foot static piles versus 7-foot windrows.
This exercise can be done with any series of pile shapes. Figure 1 is a graphical comparison of the three configurations. The pile in the rear of the figure represents the deep cross section described above, where there is a sidewall to actually make the cross section larger.
ESTIMATED RESIDENCE TIME, RELATED CAPITAL ALLOCATIONS
The greatest capital investment for a composting facility is in the active composting and curing processes. In turn, processing capacity of those two phases is determined by pile volume and residence time. Different feedstocks require different residence times. In addition, residence time is determined by technology and the quality specifications for the product. Some technologies can compress the residence time more than others. Of course high quality products usually require more time.
Residence time for active composting and curing varies by feedstock (the wide range is primarily a factor of technology used):
Manure composting – 20 to 60 days; Food residuals composting – 30 to 60 days; Biosolids composting – 40 to 84 days; Yard trimmings composting – 50 to 100 days. Yard trimmings have so much wood and hard-to-degrade cellulose and lignin, that there is usually a longer residence time than with more biodegradable feedstocks. Generally speaking, active composting is usually the first two or three weeks of the process, and may be as long as four weeks.
Other parameters that come into play when evaluating the costs to expand throughput on the same footprint include storage, traffic and logistics (machine shop, storm water system, parking, etc.). Those elements require considerable space but do not have the same cost factor as active and curing phases of compost. In terms of storage, some facilities need capacity to store 270 days of compost production or longer. With everything taken into account, the area allocation for the active and curing area may only be 16 to 22 percent – when added together – of the entire site. And most of the capital budget will be spent on that 16 to 22 percent. (Storage typically is 45 to 60 percent of the acreage; traffic and logistics typically are 25 to 35 percent.) Given challenges with permitting, if capacity is really the issue for composting facilities, there are some parts of the process that can be exported (or relocated), specifically the storage of the finished product.
SONOMA COMPOST PILOT
The pilot at Sonoma Compost was set up to evaluate trapezoidal aerated static piles. All aeration pipes were above grade. Piles were run on negative pressure, with process air treated in a biofilter. Going into the pilot, Sonoma Compost decided to “hybridize” the trapezoidal aerobic static pile (ASP) process by creating three stages – ASP, windrow, ASP. The primary reasons for the hybrid approach were the anticipated higher cost of achieving PFRP (process to further reduce pathogens) in the initial aerated static piles, and the benefit of increased particle size reduction in the windrow phase. Sonoma Compost realized that meeting PFRP in an ASP system would require an insulating layer of ground yard trimmings – about 1.0 to 1.5 feet deep – on the piles in order for all material to be exposed to 55°C temperatures for three days. The owners reasoned that meeting PFRP during the windrow phase, while concurrently achieving the desired particle size for end product screening (to minimize screen overs), made more economic and operational sense.
In the primary phase, where the biological activity is the most active, 900 cubic yards of yard trimmings were placed in a trapezoidal pile. The airflow was set at 1,600 cfm. After three weeks, the pile was moved to standard windrows. Piles were turned once/week. After three weeks, the piles went through the third phase in the aerated static pile for curing and to actively bring down the moisture content to approximately 35 percent. The airflow for this phase was found to be sufficient at 900 cfm.
A key goal of the pilot was to find out if this approach improved throughput capacity. Clues to the answer involved two variables – the footprint of the piles, and the residence time. The loading capacity per running foot of an aerated static pile was 10 cy, and the windrows held about 2 cy. The aerated static piles also compressed the overall residence time. The total composting time for the aerated static piles was 70 days compared to 98 days for windrow composting achieving similar compost maturity.
When calculating the space required, clearances had to be factored in for pulling out the above ground piping. Allowing for that, the aerated static pile system could process roughly twice as much per day on a given plot of land – 30 cy/day versus 15 cy/day for windrows (in an area of 200-feet by 80-feet). And if the piping were put below grade with aerated pavement, that production rate would go up considerably because the open space needed for pulling out the pipes wouldn’t be necessary. The production rate would go up to 52 cy/day. However, below ground piping was not a realistic option as the existing cement pad would have to be torn up to install the pipes.
The finished compost quality (based on compost maturity) from the hybrid ASP system was equal to that achieved with the windrows – despite the shorter total composting time with the ASP approach. In terms of particle size reduction, using the trapezoidal ASP system, without the benefit of turning with the Scarab windrow turner, would result in a larger final particle size, and thus a much higher percentage of screen overs (and a loss of end product revenue). However, Sonoma Compost found that the initial ASP phase actually improved the ratio of desired screened product to screen overs. This was attributed to the initial ASP phase softening the larger woody particles so that they broke down better during agitation with the turner.
In terms of odor emissions and other environmental considerations, no malodors were generated when the aerated static piles were broken down after the initial active composting phase. Additionally, minimal leachate came out of the aerated piles. The aeration system was a positive factor in controlling excess moisture after rainstorms. (The pilot project was designed to capture any leachate in an underground trap; the leachate was pumped back onto the primary static piles that still had to meet PFRP temperatures as moisture addition.)
Ironically, shortly after Sonoma Compost committed to the pilot, it learned that the county could make an additional three acres of space available adjacent to the existing site. Sonoma Compost decided to conduct the pilot anyway in order to answer the questions about increased throughput, odor control and ammonia and CO2 emissions. The biofilter used in the pilot was considered to be effective in treating odors and reducing emissions.
At the present time, Sonoma Compost has decided to continue utilizing turned windrows for yard trimmings composting, taking advantage of the adjacent space. Although more material can be composted per acre using trapezoidal piles, not enough capacity is gained in total because of the need to use above ground piping (and there are increased labor costs to remove the pipes).
Capital And Operating Cost Comparisons
A further exercise was done by CH2M Hill to compare both capital and operating costs for windrow, versus 12-foot deep and 15-foot deep trapezoidal piles (aerated and both 100-feet in length). A facility designed with a volume capacity of 10,000 tons (estimated at 100 tons/day) was used as the baseline. The following basic assumptions were used to evaluate the windrow and two trapezoidal pile systems: Concrete slab surface under piles; 800 lbs/cubic yard density; $5/sq. ft. pavement cost; 25 gallon/hour of diesel use at $3/gallon; 50 kW electric rate at $0.06/kWhr; $30/hour labor rate; seven year loan at 8 percent interest; $100/hour front-end loader hourly cost; 350 cy/hour “reclaim and rebuild rate” for managing the trapezoidal piles. The windrow was assumed to have a 98 day residence time; the trapezoidal piles have a 70 day residence time.
Table 2 provides the capital and operating costs calculated for the three scenarios. In terms of capital costs, the windrow system has the highest capital cost ($9.62/ton) due mostly to the cost of concrete slab needed for the acreage (7.34 acres for the windrows vs. 1.85 and 1.50 acres for the 12-foot high and 15-foot high trapezoidal piles respectively). Conversely, machinery costs were significantly higher for the trapezoidal piles. Capital costs for those were $5.12 for the 12-foot pile and $4.12 for the 15-foot pile.
The windrow system has the lowest operating cost – $2.76/ton/day – versus $2.94/ton/day for both the 12-foot and 15-foot high trapezoidal piles. The windrow system had the highest labor cost ($80/ton/day versus $49/ton/day for the trapezoidal piles) whereas it had zero cost for pile “reclaim and rebuild” versus $204/day for the trapezoidal piles. When taken together, based on this particular exercise, total per ton production costs are calculated at $12.38/ton for windrow, $8.07/ton for the 12-foot high trapezoidal pile and $7.06/ton for the 15-foot pile.
When comparing these numbers, other factors need to be kept in mind. For example, in the Sonoma Compost trial, it was decided to add in an interim windrow stage primarily to reduce particle size (and achieve pathogen reduction). Without that step, screen rejects were significantly higher, which is a revenue loss for a company with high-end compost markets.
Jan Allen is a Principal Technologist with CH2M Hill in Seattle, Washington. Will Bakx is a partner in Sonoma Compost Company (www.sonomacompost.com) in Petaluma, California. This article is based on a presentation by Jan Allen at the 2006 BioCycle West Coast Conference in Portland, Oregon.