BioCycle October 2007, Vol. 48, No. 10, p. 50
Pilot testing successfully demonstrated extracting heat from the “hot” zone of the compost pile to melt snow and ice, free the compost cover from the slab, and preheat another pad as well as blower air.
CHALLENGES OF winter operations, where temperatures drop as low as 25°C, led the Greater Moncton Sewerage Commission (GMSC) in New Brunswick, Canada to develop an innovative energy recovery and reuse system as part of its new Biosolids Composting Facility. The GMSC composting process, carried out on an outdoor concrete pad, combines bottom aeration and the GORETM Cover System. The first phase is designed to process 12,000 wet tons annually with bark and wood chips in roughly a 1:1 ratio by weight. The GMSC wastewater treatment plant services a population of about 95,000. Located in Riverview, this 25 MGD facility provides chemically-assisted primary treatment and incorporates sludge dewatering, alkaline treatment and handling facilities. The composting facility is located about six miles from the treatment plant.
Over the years, the GMSC has developed several land-based resource programs utilizing lime stabilized biosolids, including sod farming, landfill cover, open pit mine site rehabilitation, golf courses, tree farming and as a soil additive in agriculture. The limited time frame during which direct land application can be used, due to seasonal conditions, posed a real challenge to the day-to-day operation of the plant. The GMSC needed to further stabilize the material for storage and handling to be nuisance-free. Consequently, over the years, GMSC developed composting techniques to the point that it currently processes all of its biosolids into compost. Today, this compost is used in horticulture as a mulch, in the manufacturing of topsoil and in land reclamation projects.
To address wintertime operational challenges of the new composting system, a pilot testing program was set up in 2006 and 2007 to gather data on recovering and reusing heat generated by the composting process. The pilot was designed to test the practical applications of this system, and determine the best configuration for equipment, piping, tanks and pumping. This article summarizes the findings of work done to date, which demonstrate that the concept can become a practical feature of an overall energy management system for a composting facility.
COMPOST HEAT UTILIZATION SYSTEM
The GMSC biosolids composting facility has a large pad laid out to have four compost windrows on the north portion of the pad (184 feet by 164 feet) and four windrows on the south portion of equal size. The space between the two sets of windrows is 49 feet wide. This is a modular design so that additional pads of equal size could be constructed as needed. Two air trenches per compost windrow provide air and allow drainage of excess leachate and water. These extend the whole length of the windrow and are spaced 4.1 feet apart.
The north and south portions of the pad end with a push wall (retaining wall) that facilitates loader operation. Aeration blowers are installed on the back side of the wall, and one fan is connected to its two respective air trenches through a wye. At the back of the wall, an enclosure shelters the electrical and control equipment of the composting process. At ground level there is an open grating platform, while the below ground level houses piping, headers and valves associated with the glycol heat recovery and reuse system. Covering and recovering of the piles with the GORETM Cover are done with a mobile winder (the Power Winding Monster).
During the planning phase of the composting facility, the GMSC design and operational team recognized challenges that needed to be addressed in the cold Canadian climate:
1. Snow, as well as melting snow and ice, can build up on the GORETM Cover, and in particular on the perimeter held down with weights. When the cover has to be removed to turn the pile, this ice and snow become a serious challenge to operators. It has to be broken off and removed manually without damaging the cover material. The cover perimeter also bonds to the surface due to the moisture.
2. Weights normally used to seal the perimeter of the cover to the surface consist of a fire hose filled with water. In a cold climate, this is not practical once frozen.
3. Wind is variable and of higher intensity in Eastern Canada, as compared to many sites where this system has been used.
4. The blower normally installed on the back of the end wall would draw outside cold air. Cold air can be as low as -30°C in winter. Aside from impeding the composting process, this can result in freeze-up of the aeration trenches.
5. Following short durations of mild conditions, melting snow and/or rain can result in a sheet of ice forming on pad surfaces that are not in use. This makes it difficult to operate the equipment and retards the composting process.
The solution developed by the GMSC involved the installation of a network of pipes in the concrete slab carrying a glycol/water solution. The network of pipes is made of NPS 1 Polyethylene Brine type pipes, normally used in arena ice surface construction. These are spaced four inches apart and are installed in groups of two, connected at the far end with a loop connector section. These pipes are connected to headers in the pipe trench located behind the push wall.
Under each windrow, these pipes are connected to three sets of double headers. The central header and piping can be used to preheat the pad; however, its primary role is collecting heat from the core of the compost windrow. On either side of this central system, there are two zones with individual headers and piping to provide coverage from the edge of the compost heap to beyond the edge of the cover. These two edge sections of piping are primarily used to heat the perimeter for melting snow and ice prior to removing the cover during winter months. The overall design principle is that the heat produced from the composting process itself will be used to melt snow and ice on the edges of windrows.
The headers are connected in the enclosure to four carrier pipes that separate the hot supply, cold return, cold supply and hot return. The design allows any zone to be active or inactive by integrating motorized valves, pumps and a central storage system consisting of two large storage tanks. The glycol header and carrier pipe system within the enclosure dissipates heat from the composting process itself and reduces the need for preheating the blower air from external sources.
The GMSC design team also worked with GORE representatives to improve the weigh down system used on the perimeter of the cover. Straps have been incorporated in the manufacture of the covers with special tie-down hooks installed in the slab at 16-foot intervals. All of the features just described have been effective in dealing with the challenges identified.
Prior to completing all the piping installations, header connections and integrating operator controls, a pilot was set-up to evaluate the true potential of this concept and provide information that could be used in the final design. The testing would help determine if using the unique header configuration, combined with the intended flow controls and storage, would be effective at extracting heat from the “hot” zone of the compost pile. This recovered heat could then be used to melt snow and ice, to free up the cover from the slab, to preheat another pad and to preheat blower air. An intermediate storage system was simulated to determine if heat could be extracted on an intermittent basis while usage would be on the operational schedule. Preliminary pilot work was carried out in the winter of 2005-2006. This was followed by a new elaborate setup for the winter of 2006-2007.
During the initial installation of the glycol system, piping was connected in series so that pressure testing could be done. This temporary arrangement allowed the design team to configure one windrow in two zones. One pump would circulate glycol from the center line of the windrow toward the west edge and return to the pump, while a second pump would circulate glycol from the center toward the east edge. From the center, heat would be extracted from the first 9.8 feet (very hot zone) while heat would be lost on the last 9.8-foot strip before returning to the tank. There was no intermediate storage during this test.
During a testing period from May 1-8, 2006, it was observed that the exposed concrete surface and the area where the GORETM Cover was sealed to the ground became free of snow and ice, and was dry. Enough heat from one pile can be extracted and used along the edge to achieve the objective. Water circulating at 37°C in the cold zone is more than adequate in melting snow and the temperature dropped by only 9°C to 28°C. The flow rate during this pilot phase was low at 2.1 US gal/min, providing a retention time of 3.3 hours in the heat recovery zone and 3.3 hours in the cold zone.
The second test using the header configuration was carried out from March 3-27, 2007. The circulation pump ran for 3.5 minutes to completely extract the hot water from the central zone of the pile and to replace it with tank water that had cooled down. The pump off time would allow the water to reach maximum temperatures. The off time was varied from one hour to 24 hours to determine trends.
The results show that the longer the off time, the higher the temperature will rise or the closer the glycol/water temperature will be to the compost windrow temperature. The maximum glycol temperature reached for off periods up to 24 hours is approximately 20°C less than the core of the compost pile. For this particular test, maximum closed loop glycol temperature reached 40°C, while the compost pile remained at approximately 60°C and seemed unaffected by the heat extraction. At four hours and six hours off time, the maximum temperature reached was consistent for every cycle, while the tank temperature also remained stable. At 12 hours off time, while the tank temperature remained stable, there was a gradual rise in the overall maximum temperature of the glycol.
Based on these preliminary results, the heat that can be extracted was calculated for eight windrows on the active compost pad at 60°C. It has to be noted that temperatures normally remain above 65°C for extended periods. While higher temperatures are achieved at longer off time intervals, maximum BTUs will be recovered at relatively frequent pump operation that is sustainable (Table 1). It also appears that the one and two hour off time results in maximum temperatures decreasing, indicating this would not be sustainable if the energy extracted was used at the same rate. However, these results also indicate that four to six hours between pump cycle time allows enough time for water to reach maximum achievable temperatures considering the compost pile temperature. At this rate, there seems to be no effect on compost pile temperature due to heat extraction.
This information has been used to develop design guidelines for the overall system of storage, piping, valving and control. It also will be used in the planning of pilot work to be continued in the winter of 2007-2008.
Conrad Allain is Operations Manager at the Greater Moncton Sewerage Commission in Riverview, New Brunswick, Canada. This article is based on a technical paper presented at the International Conference on Wastewater Biosolids Sustainability, organized and hosted by GMSC, June 24-27, 2007 in Moncton.
October 25, 2007 | General
Energy Recovery At Biosolids Composting Facility
BioCycle October 2007, Vol. 48, No. 10, p. 50