BioCycle August 2006, Vol. 47, No. 8, p. 44
A pilot study in November, 2005 at the Rapid City, South Dakota composting facility shed some light on the best way to design an agitated bay system to meet regulatory requirements in the UK.
Barbara Petroff and Paul Gormsen
ALL communities in the United Kingdom (UK) will be expected to establish solid waste recycling projects that comply with the European Commission’s Council Directive 1999/31/EC on the Landfill of Waste (EC Landfill Directive) to reduce disposal of organics. Private generators of organic residuals (i.e., food processors, commercial operators, universities and other large-scale institutions) are also responsible for treating and reusing or disposing of organic wastes according to the established regulations. Authorities estimate that as much as £5 billion (US $8.5 billion) will be invested in capital on these projects.
The EC Landfill Directive, in part, aims to reduce the amount of methane or greenhouse gas emitted from landfills as a measure to fulfill the 1997 Kyoto Protocol to the United Nations Framework Convention on Climate Change. In 2002, the European Union ratified the protocol and agreed to reduce emissions by 8 percent of the 1990 level, and the UK agreed to reduce its portion in phases by 2008 to 2012. Recycling and landfill diversion of biodegradable waste is intended to aid in that commitment.
Strategies to achieve the biodegradable reduction milestones include composting, paper recycling and energy recovery. For the UK, authorities estimate that by 2020, more than 500 new alternative facilities will be necessary to divert approximately 19 million metric tons/year of organic materials from landfill disposal.
While the EC Landfill Directive is clear, the means to comply is more complex. Cases of swine fever in 2000 and foot-and-mouth disease in 2001 were attributed to contaminated catering waste. With increasing concern for diseases transmitted from animals to humans, the UK’s Department of Food, Environment, and Rural Affairs (DEFRA) and other delegated administrations in countries of the UK have charged the State Veterinary Services with approving treatment facilities and processes, including composting, where animal by-products and catering wastes are involved. Animal by-products approvals involve a two-stage authorization – prevalidation of the proposed process system and site validation. Approvals are provisional until the facility is built and operations demonstrate meeting compliance standards for several consecutive months.
ANIMAL BY-PRODUCTS REGULATIONS
The Animal By-Products Regulations in countries of the UK (ABPR UK) establish treatment standards on a matrix approach. Depending on the nature of the material, the treatment standards govern the maximum particle size and minimum retention time and temperature parameters for specific types of systems. Waste materials fall within three specified categories. In descending order of risk from an animal health perspective, they are: Category 3: Catering waste and food processing industry wastes; Category 2: Slaughtered animals; and Category 1: Specified risk materials and catering waste from international transport. Domestic kitchen waste “arising in kitchens, including domestic kitchens,” is considered Category 3 catering waste (where meat or products of animal origin are handled on the premises). Green waste from gardens and parks is not affected by the rule unless it is mixed with catering waste or animal by-products. Notes the DEFRA rule: “Where any two or more of these waste streams are mixed, the entire mixture must be considered to be the highest risk material involved.”
According to EU Regulation 1774/ 2002, the treatment criterion for Category 3 animal by-products is 70°C for at least one hour with a maximum particle size of 12mm (0.5 in). For Category 3 catering waste operated as per the ABPR UK, composting systems must meet one of the sets of standards shown in Table 1, and must undergo an additional barrier treatment. The additional barrier involves one of the following options: Assurance that meat was excluded at the source in the catering waste and at least 18 days of storage following treatment; or treating meat-included material a second time using any of the methods in Table 1. If the windrow system is used as the second treatment, it does not need to be enclosed.
Another condition of the ABPR UK is that the compliance methods use an “all in, all out” approach for housed windrow systems (enclosed in a roofed building). For example, when the time and temperature for a housed windrow starts, no new material can be introduced. The windrow is treated as one batch. When that batch achieves the 8-day minimum for time and temperature, the windrow can be transferred to the location where it will meet the second stage time and temperature requirements. Then, a new batch can be introduced into the enclosed windrow hall. Continuous flow agitated bed systems such as IPS Composting Systems (IPS) must incorporate additional steps that meet the ABPR UK criteria. To evaluate ABPR UK compliance, a pilot study was conducted at a facility in Rapid City, South Dakota, which uses the IPS system.
WHY RAPID CITY?
The Rapid City cocomposting facility processes household and restaurant waste along with liquid biosolids from the City’s wastewater treatment plant for a community of about 85,000 people. (See “Rapid City Closes The Loop On MSW Management,” November 2003 for a complete description of the facility.) These feedstocks contain materials that meet the definition of “catering waste with meat products” as described in the ABPR UK. Siemens Water Technologies, owner of the IPS technology, chose the Rapid City plant for the pilot study because it offered conditions that closely simulated a conceptual IPS composting facility in the UK. Of the approximately 196 tonnes (216 tons) of material it receives daily, about 136 tonnes (150 tons) constitute the organic fraction to be composted, and this material is similar to the organic residuals to be recycled in many communities in the UK.
Trucks transport and off-load the material containing the organic fraction onto the tip floor inside the enclosed receiving area. A front-end loader places the material onto a conveyor for transfer through a picking line and into two parallel rotating Keppel Segher Dano Drums. Inside the rotating drums, liquid biosolids are introduced and the material is mixed for six hours before being discharged through a dual trommel screen. The remaining organic fraction is less than 6cm (2.4-inch minus) and has a 40 to 42 percent solids concentration. The organic material is conveyed into the composting building where a level screw distributes the material along a push wall, and a front-end loader loads the agitated bays.
Once each working day, two IPS agitators mix and move the material about 3 meters (10 feet) down the nine bays so that each load requires 29 days to move from the front end of the bay to the off-loading end. At the off-loading end, a front-end loader transfers the composted material to a covered building with an aerated floor for 45 days of curing before entering the finishing process.
In the active composting bays, temperatures are automatically recorded and controlled via a computer system. Thermocouples are built into the bay walls, and positive aeration is used. To simulate conditions proposed for the UK, one of the nine bays was selected and the material was agitated and moved twice each working day for a retention time of 14 days, rather than the standard 29 days.
THE PILOT TEST
The test was designed to demonstrate the ability to meet one of the following three UK DEFRA criteria in a closed reactor (see Table 1) after 14 days retention time in the IPS Composting System:
o < 6 cm (~2 inch) particles at 70°C for at least one hour for two separate periods
o < 12 mm (0.5 inch) particles at 70°C for at least one hour
o < 12 mm (0.5 inch) particles at 60°C for 48 hours
To simulate a representative winter temperature scenario for a facility in the UK, the study took place in October and November. Four post-IPS system composting trials were conducted over a four-week period. Material was composted in the bays for 14 days, randomly tested for fecal coliform and salmonella, and transferred from the discharge end of the bays into an insulated roll-off container with a fabricated insulated lid.
To prepare for the pilot, the plant staff fabricated panels to insulate the top and sides of a 23 cubic meter (30 cubic-yard) roll-off container. They cut and wedged two layers of 5cm (2 inch) thick insulating sheets into the sections between the outside ridges of the container. The lid sections were composed of double insulating sheets, secured between two layers of 12mm (0.5 inch) thick plywood. Twelve holes were drilled into the lid at predetermined places to accommodate the temperature probes. The position of the holes ranged from 12cm (0.4 feet) to 1.2 meters (4.0 feet) from the sides of the reactor, and from 1.1 meters (3.6 feet) to 3.5 meters (12 feet) from the front of the reactor. Reotemp thermometers with automated recordings were set to read temperatures every half hour. Each probe was numbered and assigned a designated hole in the lid and depth into the reactor. The depths ranged from 30cm (1.0 feet) to 1.3 meters (4.5 feet). The thirteenth probe was placed on the outside of the front left corner of the container to measure the ambient temperatures.
At the end of the trial period, the material was transferred to a curing pile where temperatures were recorded for one week. The latter step was done in accordance with a principle embraced by the US Environmental Protection Agency to demonstrate that the nonpathogenic bacterial community survives to adequately suppress pathogenic bacteria.
At the conclusion of the four post-IPS system trials, one pre-IPS system trial was performed. Raw MSW material at the loading end, prior to composting, was transferred into a roll-off container and measured at intervals over a 7-day period to track the temperature gradient and to determine the approximate length of time it takes to heat the material to 70°C and hold for one hour. Since Siemens will have the option in the UK of treating the material either before or after composting, information from the pre-IPS system trial was equally valuable to the post-IPS trial.
Each trial – the four post (Trials 1-4) and one pre (Trial 5) – was modified to measure the results of specific intervention and conditions. Trial 1 and Trial 4 compost was Each morning, the temperature probes were read manually and recorded to compare to the automated data. Daily recordings were also made for temperatures in the last zone of Bay #9 and in the pilot compost curing pile. Random tests were performed for salmonella and fecal coliform at the off-loading end of the bay that composted the trial material.
After each trial was completed, the compost was transferred from the closed reactor to the covered curing structure. Compost from each trial was added to the single curing pile. Manual temperatures were taken over seven days, and remained elevated above 50°C.
PILOT TEST RESULTS
Although several of the probes recorded 70°C temperatures, the vast majority did not (see Table 2), as required by the DEFRA criteria for ABPR UK. Information gained from this demonstration, however, was pertinent to the design of a closed reactor concept that would meet the compliance criteria. When the materials were removed from the end of the IPS composting process, screening and reducing the material gradient to 12mm minus (0.5 inch minus) appeared to have an adverse effect on the biological activity and subsequent temperature of the composting materials in the simulated closed reactor. It is reasonable to deduct that low ambient temperatures were conducted through exposed metal surfaces of the container to the compost. In addition, the oxygen level inside the container may have been compromised and therefore may have affected the outcome.
Throughout the trial period, the composting material in the last zone of the designated trial bay ranged from 57°C to 62°C at the bay wall (coolest point in the bay). Manually probed temperatures in the center of the compost pile ranged from 63°C to 71°C. (The Rapid City facility sets the automated blowers to turn on and cool the compost when the bay wall thermocouples register >60°C in the bays. This set point is used because the USEPA requires temperatures greater than 55°C for a minimum of three consecutive days.) Also, throughout the entire trial period, the trial compost that was moved from the closed reactor to curing achieved and maintained temperatures between 50°C and 64°C. These thermophilic temperatures indicate strong biological activity before and after retention time in the closed reactor.
Unscreened compost fared better than screened compost. This could be due to the larger particle size (< 6cm or 2.4 inches unscreened vs. < 12mm or 0.5 inches screened) and to less disturbance or exposure to low ambient temperatures. Unscreened larger particles could have allowed better oxygenation than the smaller particles that would have been more compact. Screening the compost prior to Trials 2 and 3 took 1.5 to 2.0 hours outdoors, where ambient temperatures ranged from 10°C to 12°C. Therefore, screening resulted in heat loss.
Low oxygen levels could have contributed to the inability of temperature levels to recover in the closed reactor. Compared to the aerated compost in the designated trial bay, gas measurements revealed low oxygen levels in the screened compost that had been in the closed reactor for three days. The closed reactor oxygen was between 1.1 and 10.3 percent, compared to 20.4 percent in the aerated designated trial bay. Oxygen measurements were not taken on the trial curing pile; however, temperatures in the curing piles registered 50°C to 64°C, suggesting that reduced oxygen level and lack of agitation could be a factor in the closed reactor.
There were a number of surface points on the trial closed reactor that would have allowed conduction of the cold ambient air temperatures. Metal ridges on the sides and the bottom of the reactor were not insulated.
The pilot test suggested that it would be more economical to achieve 70°C for one hour after, rather than before, the IPS system. During the pilot study, compost processed in the designated IPS trial bay for 14 days consistently demonstrated a temperature elevation to a thermophilic range in the last zone of the bay. The raw material used in the pre-IPS system trial (Trial 5) maintained significantly lower average temperatures than the post unscreened compost trials (Trials 1 and 4). Therefore, the heated processed compost at the end of the IPS system would appear to have an advantage over raw unprocessed materials in the length of time it would take to achieve 70°C in a closed reactor.
Based on the pilot test, Siemens concluded that it would be difficult to achieve the ABPR UK time and temperature requirements adequately throughout the composting material in a closed reactor without engineering and process modifications to the system. Therefore, the company determined that the best approach for a closed reactor would be one that follows the agitated bays and allows the material to be turned and mixed. Since the compost coming out of the IPS system would already have a particle size of less than 6cm (< 2.4 inches), screening would not be required prior to the closed reactor. In addition, screening before the reactor results in a substantial loss of heat in the material and would, therefore, require substantially greater time and cost to bring the temperature of the material up to 70ºC in the enclosed reactor.
The temperature of the interior reactor wall and the void space in the reactor itself, needs to be maintained at greater than 70°C. This would insure that all of the compost material does not fall below the required standard at any time during the compliance process. Moisture control should also be considered and carefully monitored and controlled. Once these factors are implemented in the IPS Composting System design, the system should be able to meet DEFRA criteria for ABPR UK.
The purpose of the overall IPS approach is to produce a high quality homogeneous compost that is not compromised during the process of achieving ABPR UK standards. Meeting these criteria at the end of the agitated bays maintains the sequence, continuity, and integrity of the typical technology train for preprocessing, composting, curing, and finishing. Preprocessing would still involve screening the organic fraction rather than shredding the raw materials. Active composting would involve 14 days of daily agitation with controlled temperatures, aeration, and moisture addition to produce a homogeneous material. This composting material would then enter the ABPR UK compliant closed reactor. Curing, screening, and additional product finishing after the reactor would be designed in accordance with the end market disposition.
Barbara Petroff is Project Manager and Paul Gormsen is Business Manager with IPS Composting System, part of Siemens Water Technologies.
August 20, 2006 | General
Meeting DEFRA Time And Temperature Standards For Composting (United Kingdom)
BioCycle August 2006, Vol. 47, No. 8, p. 44