BioCycle April 2009, Vol. 50, No. 4, p. 20
Pretreating Msw Prior To Anaerobic Digestion
Research assesses feasibility of using rotary drum reactors for preprocessing municipal solid waste before anaerobic digestion, to produce renewable energy as well as compost.
R. Zhang, J. Rapport, P. Gikas, B. Zhu, B. Jenkins, J. Lord and C. Choate
ROTARY drum reactors (RDR) have been used since the early 1970’s by the composting industry for pretreating and separating organic fractions from municipal solid waste (MSW) prior to composting. Different designs and operational conditions have been developed over the years for the RDR processes employed in the U.S., Canada, Australia, Japan and several countries in Europe. About a dozen composting plants in North America currently use rotary drums in their facilities as a pretreatment process for composting solid wastes, such as MSW, biosolids, paper or animal manure (Figure 1). Facilities in the U.S. that use rotary drums span the climate range from Arizona to Florida to Alaska, and treat from 3 to 300 tons/day of waste.
The RDR process consists of a long, horizontal rotating vessel followed by screens for separating the organic fraction. In some RDR systems, the end of the drum is lined with an inner drum perforated with 1- to 3-inch holes for materials separation. Other systems employ a trommel screen after the drum. The drum design depends on the desired retention time, the amount of waste treated, and the drum’s slope and rotational speed. Typical drums used in the U.S. range from 100 to 200 feet long and 10 to 15 feet in diameter. They rotate at 1 to 5 rpm, and most include forced air blowers, although a few do not.
RDR systems not only separate organic materials from mixed MSW, but they also increase the speed of composting. Gasses and odors are treated using biofilters. In most systems, waste materials remain in the drum for two to three days, and biological degradation begins almost immediately. Without adding heat, temperatures rise to 135° to 150°F inside the drum due to the biological activity. At a South Dakota facility that uses a retention time of six hours, the internal temperature still rises to 70°F in the winter. The capital and operating cost of RDR systems can be quite high, especially considering the energy required to operate them. The drums in the U.S. require 100 to 400 HP per drum, depending on size and rotational speed. This translates to 80 to 110 kWh per ton of waste treated. The economics of the RDR system could be improved by recovering the energy from the waste prior to composting. This could readily be accomplished using anaerobic digestion (AD).
At least two waste treatment facilities in France and Belgium use rotary drums as a pretreatment for AD, and then compost the residual solids from the digesters. At the SIVOM composting facility in Varennes-Jarcy, France, 80 tons/day of source separated organics (SSO) and 190 tons/day of mixed MSW pass through two rotary drums before going to a Valorga-brand anaerobic digester. In three days, the drums recover 80 percent and 50 percent of the mass of the SSO and mixed MSW, respectively, as feedstock for the anaerobic digester. In Brecht, Belgium, 200 tons/day of source separated vegetable, kitchen and garden wastes pass through two rotary drums in series. The drums are used primarily to break open bags and bottles, as the total residence time is only six hours. The recovered waste then goes to a Dranco-brand anaerobic digester for 2 to 3 weeks before being pressed and conveyed to an enclosed aeration bed for a condensed composting treatment.
BIOGAS PRODUCTION POTENTIAL
Considering the increasing interests in energy recovery from organic residuals, researchers at the University of California, Davis (UC Davis) partnered recently with Norcal Waste Systems, Inc. to evaluate the materials coming out of rotary drums at six MSW composting plants in the U.S. The anaerobic digestibility and biogas production potential of the treated MSW were studied with an aim to integrate the rotary drum reactor with AD and composting processes. This research was funded by the California Integrated Waste Management Board (CIWMB). A summary of the research results are presented below. Detailed descriptions of research methods and results are presented in articles by Gikas et al. (2008 and 2009) and Zhu et al. (2008 and 2009).
The six composting plants surveyed treat MSW and biosolids in their rotating drums. Five of the six facilities aim primarily to treat the MSW, using the biosolids to balance the moisture content at 50 to 55 percent. However, the sanitary district that owns the facility in Pinetop-Lakeside, Arizona converts the biosolids to compost, while using MSW (mainly paper and cardboard) for moisture content control.
All the plants accept MSW with marginal or no source separation. Three plants presort the waste at the recovery facility to recycle aluminum, ferrous and plastics and to reduce the volume of materials loaded into the drum. They also manually remove materials prior to loading that created problems during the rotating process, such as cables, wires, ropes, hoses and cloth. Drum retention times vary from 2 to 5 days, with the exception of the facility in Rapid City, South Dakota, where the retention time is approximately 6 hours and MSW is loaded daily in single batches.
Three random samples (Figure 2) were taken at least one week apart from each facility. In the lab, the samples’ chemical compositions were analyzed before placing them in one-liter batch anaerobic digesters for 20 days at thermophilic temperature of 135°F to determine the biogas production potential. As shown in Table 1, total solids (TS) contents of the samples ranged from 35 to 55 percent. Volatile solids (VS) contents were 71 to 81 percent of TS. The carbon-to-nitrogen ratio (C:N) ranged from 25 to 43. The high biogas yield (0.48 to 0.61 m3 kg-1 VS) and methane content (58 to 60 percent), shown in Table 2, indicate that the majority of the solids in the samples were digestible. Thus, RDR-treated solid waste can be digested anaerobically with similar energy yields despite widely varying treatment conditions.
Most notably, waste from the facility in Rapid City yielded the most biogas, which can be attributed to the lowest retention time and hence less time for the readily degradable organics to decompose in the drum reactor. If the biogas were converted to electricity at 30 percent efficiency, assuming the higher heating value for methane, the facility could recover about 360 kWh of electricity per ton of waste treated (almost three times as much as is consumed by the drums). About 1,200 lbs of residual solids would be recovered from the digesters. An estimated 1 to 3 weeks of additional composting (e.g. aerated windrows) would be required to obtain stable compost, but most of the volume reduction would have already occurred.
BIOGAS YIELD AND DRUM RETENTION TIME
Operating the RDRs with a reduced retention time can potentially have both positive and negative effects. On the one hand, fewer organics may be recovered due to insufficient time for the larger organic materials to break down enough to fit through the screen holes. On the other hand, smaller drums require significantly less capital expenditure and energy input. They can also produce more degradable feedstocks for AD, as less retention time results in less loss of biodegradable matter in the drum. This trade-off suggests that the drum could be sized to optimize the energy balance if the biogas yield were determined as a function of the retention time. Based on this principle, the effect of retention time on biogas production was tested at one plant by removing samples from ports placed at one- and two-thirds of the length of the drum as well as the drum end.
The composting facility in Pinetop-Lakeside has a 125-foot long RDR, 10 feet in diameter. The plant transitioned from mixed MSW to various paper fractions over the course of 2007. Average retention time is 3 days. Air is blown into the foam-insulated drum to maintain marginally aerobic microbial activity and keep the temperature at 115° to 155°F. Material discharged from the rotary drum passes over a trommel screen with 1.25-inch openings. For the research study, the RDR was operated for a week from February 26 to March 4, 2007, with four different waste types: MSW; a mixture of MSW, cardboard and paper waste; a mixture of MSW and biosolids; and a mixture of cardboard, paper waste and biosolids. Each type of waste was sampled after 1, 2 and 3 days in the reactor by accessing sampling ports at different points along the length of the drum. The samples were analyzed for chemical composition before being placed in one-liter batch anaerobic digesters for 20 days at thermophilic temperature of 135°F.
In general, the biogas yields were similar for all treatment conditions. However, the biogas yield of MSW and biosolids tended to increase slightly when the retention time in the rotary drum was shortened, as expected. For the wastes containing papers, the two-day retention time in the drum resulted in the highest biogas yield. This suggests that the aerobic treatment process in the drum may increase the bioavailability of the cellulose in the paper when it is anaerobically digested. The methane content was about 60 percent for all of the samples tested.
The waste samples from the Pinetop-Lakeside facility were further tested in a continuous two-stage Anaerobic Phased Solids (APS) Digester in the laboratory at 12-day retention time, thermophilic temperature (135°F), and organic loading rate of 9.2 kg VS m-3 d-1. The biogas production rate was determined to be 3.5 m3 (biogas) m-3 (reactor volume) d-1 and the biogas and methane yields were 0.38 and 0.19 m3 kg-1 VS, respectively. Anaerobic digestion resulted in 38 percent TS reduction and 53 percent VS reduction in the organic solids. The residual solids recovered from the digesters had a high heating value of about 14.7 MJ kg-1 TS and could be used as a fuel for thermal conversion processes, such as combustion or gasification. The biogas yield of the RDR treated waste as obtained from the APS Digester was close to the biogas yield of grass clippings (0.44 m3 kg-1 VS) digested under similar conditions.
The results of the UC Davis study suggest that existing MSW composting plants that utilize RDR treatment processing could install anaerobic digesters and recover the energy they consume without significantly affecting the compost output. Furthermore, the elevated temperatures achieved in the RDR reduce the energy input to the digester and increase the energy output. However, a full financial analysis should be conducted to determine whether AD is a feasible option for both existing and new waste treatment plants. Composting facilities that plan to install RDR systems may wish to consider a smaller RDR in conjunction with an anaerobic digester as an alternative to the larger RDR required for aerobic composting alone.
Ruihong Zhang, Josh Rapport, Petros Gikas, Baoning Zhu and Bryan Jenkins are with the University of California, Davis. James Lord and Chris Choate are with Norcal Waste Systems, Inc. The authors wish to thank Phil Hayes, Pinetop-Lakeside, AZ; Whitney Hall, Nantucket, MA; Susan McIntyre, Delaware County, NY; Mike Oyler, Rapid City, SD; Tom Leonard, Sevierville, TN; and Stefan Hreniuc, Cobb County, GA, for providing the waste samples and RDR plant design and operational information. They also appreciate the funding support from CIWMB and the project management and guidance from Ronald Lew.
April 27, 2009 | General
Pretreating MSW Prior To Anaerobic Digestion
BioCycle April 2009, Vol. 50, No. 4, p. 20