Solar Composting A Bright Idea?

Reducing the reliance of composting facilities on polluting diesel equipment is an attractive prospect in 21st century California Composting can solve many environmental problems, but facilities also pose daunting questions for environmental regulators. Concerns about odours and air emissions, water runoff from feedstocks and active piles, and diesel equipment use on site not only create environmental trade-offs for the operation, they weigh heavily on the bottom line. Could solar power be the answer to these challenges? A site in California believes so. by Robert Horowitz 'Traditional' open-windrow composting sites are becoming almost impossible to site anywhere near California's population centres, - where they are most needed. Air and water quality regulations have raised the bar in terms of facility construction and management. In places like California's San Joaquin Valley - a prime agricultural region with poor air quality - the adoption of best available control technologies and the purchase of air pollution offset credits are mandatory for all new or expanding composting facilities. This will add millions of dollars in additional costs for future facilities, and require new skills and methods. It appears the timing is right for a paradigm change in how organic wastes are handled. Lately, it seems a lot of momentum is behind anaerobic digesters for food waste management. While there are advantages to this approach, the solid digestate will probably need to be composted. Therefore, in order for California to reach its goal of 75% source reduction, recycling and composting by 2020, composting capacity is needed. An attractive concept for the 21st Century composting facility is one that uses less land, reduces reliance on heavy diesel equipment, consumes less water and produces fewer odours and emissions. This facility needs to be small enough to be sited near communities, but large enough to achieve efficiencies of scale. This facility needs to handle mixed feedstock, such as food or digestate, yet be a good neighbour. The 1.5 hp pumps used in the project were powered by these solar panels Is it possible? Thanks to a grant from the San Joaquin Valley Air Pollution Control District, one project points to a potential direction for 21st Century composting sites that yields benefits for both sustainability and the bottom line. A pilot project, managed by the Association of Compost Producers with help from CalRecycle, was conducted at a composting site in Tulare, CA owned by Harvest Power. The prototype system - known as the Solar-Powered Extended Aerated Static Pile (eASP) - reduces compost facility air emissions to levels at which permitting is affordable. It uses less heavy equipment for material handling, and solar energy in lieu of diesel for compost aeration. Furthermore, the eASP allows for low-cost irrigation systems, cutting the use of water trucks. The larger piles mean more composting can be done per acre, reducing investment in land or allowing for larger buffer zones. The Status Quo In the U.S. commercial compost operators typically form long, narrow windrows of feedstock in an open field. These windrows are aerated by the use of mechanical "straddle" turners, powered by diesel engines, usually larger than 400 horsepower. The new system replaces long windrows with taller, wider rectangular piles on a smaller field, aerated by forced-air piping. In this project 1.5 hp (0.9 kW) aeration pumps were powered with solar panels. In windrows, operators kill pathogens by implementing five passes with the windrow turner within 15 days, while keeping the temperature above 131°F (55°C). The new system, which is covered by a 1 foot (30cm) thick biofilter layer of finished, unscreened material that doubles as an insulation layer, achieves pathogen reduction in as little as three days, without any turning. Windrow systems typically achieve maximum degradation of materials in about 30 days. Most operators allow the material to cure for another 30 days or more, before it is tested for pathogens and heavy metals (required in California), and cleared for sale. As part of the project, identical materials were composted by the windrow and eASP methods using a U.S. Composting Council STA-approved lab. After 30 days, the two materials streams remained identical, meaning the eASP system does not entail a time disadvantage which would impact our calculations of land-use efficiency. The Extended Aerated Pile With an eASP, a new zone is laid up against the side of an existing aeration zone. The shared border reduces surface area, helping to retain water and heat. It also saves valuable space on the composting pad, which is at a premium because the pad is almost always either an impermeable or a compacted surface. With a straddle turner/windrow system, space needs to be left in between the windrows and at the end of the windrows to maneuver the turner; with the eASP more space can be used for composting. Solar panels were used to power the forced aeration, reversing the natural aeration cycle The Biofilter Compost Cap Micro-organisms living in the damp pore spaces of a biological filter medium consume odourous pollutants, such as volatile organic compounds (VOCs) or ammonia. Biofilters are very effective on ammonia and VOCs, but also reduce greenhouse gases. The biofilter compost cap places a 1 foot (30cm) thick filter layer on the top of every newly created compost pile, reducing the amount of piping, maintenance and blower power needed for biofiltration. Because the pile is not turned, one cap lasts the entire active composting phase. The cap materials, unscreened finished compost, are available at every composting site. A new way to build compost piles The concept of using a conveyor system to bring freshly ground materials is not often employed at composting sites because fixed conveyors limit operational flexibility. The project team, led by Kevin Barnes, compost site manager for the City of Bakersfield, adapted a mobile potato piler as the key element in a conveyor train which led from a slow-speed shredder to a home-made hopper, to an intermediate conveyor, and finally to the piler. Run remotely by a joystick, the piler outfall can move left and right, up and down, and can lengthen or retract. This allows the operator to build an entire 30 foot wide ASP section to 10 feet high by just lifting a few fingers. If the operator then wants to switch feedstocks and apply the biofilter cap made from finished materials, this can be done without moving the conveyor train. Once a 30 foot by 30 foot (9.1m by 9.1m) section of ASP is built, the conveyor train can be moved back 30 feet, and the process begins again. Three build sections produce a 30 foot by 90 foot (9.1m by 27.4m) ASP zone - a good operational size. For the pilot project, three eASP zones were built one week apart to facilitate emissions testing. Click to Enlarge Keeping Materials Moist A challenge identified early on was ensuring feedstocks remained moist enough to compost during a hot California summer. Windrow facilities in the San Joaquin Valley typically run water trucks constantly, applying a steady stream of liquids to piles baking under relentless sunshine some 300 days per year. In the winter, piles capture rainfall. Water trucks are another large diesel engine on a compost site, typically 400 hp (300 kW)or more. The process of filling the truck and spraying the water at piles results in spillage and ponding. At sites that pump groundwater from dozens or hundreds of feet below, this waste is expensive. For the pilot project, Barnes created a home-made irrigation system using 1.5 inch (3.8cm) fire hose and two sprayers attached to the potato piler discharge. On a permanent system, this function would be fine-tuned, but the DIY system wet materials to the upper end of the desired range (~60%). A low-pressure sprinkler system on top of the piles, made out of materials found at any home improvement store, kept the tops of the piles moist for a 30-day study period, during which the temperature exceeded 100°F (37.8°C) every day. The initial watering of ASP feedstock and 30-days of timed sprinkling of the eASPs used approximately 20% less water than the traditional windrows, which were watered by a 4000 gallon (15,140 litre) watering truck with a sprayer on the back. For a theoretical 100,000 ton (91,000 tonne) per year facility, this would save about one million gallons of water per year, with commensurate GHG reductions from eliminating the water truck fuel use as well as fuel or electricity and emissions savings from pumping 20% less groundwater. A homemade irrigation system used a 1.5 inch fire hose Solar powered aeration The pilot project used solar panels to power 'positive' aeration, meaning ambient air was sucked into a pump then forced underneath the pile using a manifold system and 4 inch (10cm) perforated pipes laid in a plenum of coarse-ground materials. From there the air filters upward into the pile, mimicking the natural convection flow. Potential odours and air pollutants move upward with the airflow, but once they reach the biofilter cap they are consumed by microbes. This is the core concept of biofiltration. Positive aeration uses less energy than 'negative' aeration, in which gases within the pile are sucked out the bottom, reversing the natural convection cycle. Once removed from the pile, the air must be delivered to the bottom of a biofilter for treatment. Organic acids and other compounds in the compost gases mean pumps in negative aeration systems must resist corrosion, adding to cost. The dual action means pump sizes are larger, as well. The solar powered aeration systems were designed by Peter Moon of O2 Compost. The systems come in 'plug-and-play' mode. The "guts" are contained in a white metal case containing an inverter and four deep-cycle batteries, as well as timers and switches to control the flow of electricity to the battery and to the aeration fans. The solar panels are mounted on an aluminium frame, and plugged into the box, which is then plugged into the aeration pump. A typical setting would be to run the fans two minutes out of every 20, though aeration can be adjusted as desired. The batteries ensure the cycle is not interrupted at night or during periods without sun. The solar systems were 'plug and play' and featured a deep cycle inverter and battery storage Cutting diesel use Diesel use was reduced during the first 30 days of composting by three primary methods: Reducing the use of loaders to move materials from the grinder to the windrow Elimination of active-phase windrow turning No trucks for moisture maintenance. Diesel emission reductions for this project do not consider the grinding of feedstocks or the trommeling of the final product, as these activities will occur regardless of composting method. Further, they also do not include windrow turning during the curing phase. Many operators will continue to turn materials that are past the active, high temperature composting phase - this turning may occur weekly or less frequently. In addition, the calculations for this project also do not count any diesel powered pumping equipment that may have been used to lift groundwater. The first passes with a turner on new windrows use the most energy. The feedstock particles are still large and may be clumped together; the turner must move slowly to maintain rotational speed in the cylinder. During this time, many of the contaminants in the feedstock will also be reduced in size, which can make them more difficult to remove later on. Barnes cites this as one of the unheralded benefits of the eASP method. Another benefit is odour reductions, because most odours occur during the active phase and turning events are quite often associated with odour complaints against compost facilities. The average California operator would turn a windrow seven times during the first 30 days, including five turns for the legally mandated pathogen reduction process, one turn right after pile construction, and another turn in the fourth week. When the replacement of turning with solar power was added to reduced use of water trucks and fewer loaders to build piles, the overall diesel use reduction for the active compost phase was calculated to be 87%. With diesel running $4+ per gallon in California, the savings are estimated at about $3000 per year for a 100,000 ton per year facility. One of the advantages of forced aeration is that contaminant particles are reduced in size Cutting odours and fugitive emissions The 87% reduction in diesel use during active composting means that NOx, diesel particulates and unburned hydrocarbons emissions were reduced by 87% for this phase of operations. This is important in the San Joaquin Valley, which is considered 'extreme non-attainment' for ground-level ozone pollution. The Valley is also considered 'NOx limited,' which means the formation of ozone can be most effectively prevented by reducing NOx emissions, which come mostly from internal combustion engines. The other half of the ozone equation is VOCs, which mix with NOx in the presence of strong sunlight to make ozone. VOCs are given off in the fugitive emissions of decomposing organic matter, whether the materials are on the forest floor or in your green can, but these emissions are concentrated at a composting facility. Because of this, the San Joaquin Valley air district has adopted regulations mandating fugitive VOC emission reductions at composting facilities using a series of best management practices which escalate with facility size. Biofilter compost caps are particularly effective at reducing VOCs, and are required of the largest facilities in the San Joaquin Valley. Another of the management practices mandated by the air district is saturating the top three inches of the windrow before turning. The top of the eASP was kept wet by the sprinkler system. Moisture is known to control VOC emissions, and we believe the moist condition of the biofilter compost cap contributed to the high levels of control. This project further validates the air district's use of the wet top as an emissions reduction practice. Emissions testing contractors Chuck Schmidt and Tom Card designed and implemented a program of 84 samples covering three weeks of active composting. These were sent to an accredited lab and analysed for VOCs, ammonia and greenhouse gases. The fugitive emissions were compared to the baseline windrow made of the same feedstocks on the same day. Conclusion The proliferation of low-cost, high-efficiency and durable solar panels makes it possible to set up solar-powered composting systems that reduce reliance on alternative forms of power and avoid very expensive grid hook-up costs. This type of technology lends itself to a more stable site layout, which can then utilise conveyors for material movement, further reducing reliance on diesel-powered equipment. A potato piler, a standard piece of equipment in the potato storage industry, is an ideal prototype for compost conveyors. Static piles can easily be watered using simple irrigation systems that save water and further reduce diesel reliance. When combined with the already proven technology of a compost cap, reductions of VOCs and greenhouse gases can be substantial. Acknowledgements This project was funded by the San Joaquin Valley Air Pollution Control District under Agreement C-15636. The full report can be downloaded at: www.tinyurl.com/kufga2n Robert Horowitz is supervisor of organics management and construction & demolition unit at California Department of Resources, Recycling & Recovery (CalRecycle). This article is on-line.