November 22, 2006 | General

Evaluating AD System Performance For MSW Organics

BioCycle November 2006, Vol. 47, No. 11, p. 35
Operations were monitored and outputs sampled to determine the biodegradability of source separated organics, quantify biogas generation and composition, and examine constituents in the SSO pulp. Part I
Brian Van Opstal

THE CITY of Toronto’s Dufferin Organics Processing Facility (DOPF) receives approximately 25,000 metric tons/year of source separated organic (SSO) material from Toronto’s residential Green Bin and the commercial Yellow Bag collection programs. The DOPF utilizes the patented BTA process that includes a wet pretreatment system and a single stage, mesophilic anaerobic digester. The purpose of the DOPF is to separate the film plastic bin liners and contaminant materials fractions of the SSO from the organic material, and to convert the organic fraction into a digested solids material that is a suitable feedstock for the production of an unrestricted use compost product (i.e. that satisfies Ontario’s quality standards for unrestricted use compost). As a secondary purpose, the DOPF also produces renewable energy in the form of biogas from the anaerobic digestion of organic material. Presently, the DOPF does not have a biogas energy conversion system and all biogas is combusted in an open flare.
The facility receives SSO from 6:00 am to 6:00 pm, Monday to Friday. SSO processing operations continue until approximately 10:00 pm. The DOPF does not receive or process SSO on weekends or statutory holidays. Material inputs are: SSO and city water for processing and cleaning operations. Major material outputs are solid processing residue (e.g. film plastic and contaminant materials), sent for landfill disposal; digested solids, delivered to an off-site composting facility; biogas, which is flared; and, effluent, which is discharged without pretreatment to the city’s sanitary sewer system.
Parts 1 and 2 of this article series are based on a report from the City of Toronto’s Solid Waste Management Services titled “Full-Scale Anaerobic Digestion of Source Separated Organic Material for Methane Production.” The report was prepared following an evaluation of the suitability of the DOPF for the residential SSO stream. Part 1 describes the DOPF operations, material balances, and composition of the SSO pulp and biogas. Part II reports on how the data obtained was used to evaluate the suitability of the DOPF AD system for source separated organic material, and compares performance of the DOPF based on operating results and published results for other, similar facilities.
Figure 1 shows the two sequential processing systems at the DOPF. After tipping, large contaminants are manually removed from the SSO and form part of the DOPF’s solid processing residual waste that is landfilled. Following bulky item removal, the SSO is transferred to the BTA wet pretreatment system, which is a batch process with two objectives: Separate the film plastic and contaminants from the organic fraction of the SSO; and Transform the SSO into an organic pulp. Recycled process water and city water are added to the BTA wet separation system prior to the pulping cycle in order to dilute the organic pulp material to a total solids content of 6 to 7 percent. After pulping, film plastic and floatable contaminants are automatically removed from the top of the separation vessel by raking. Heavier contaminants settle to the bottom of the vessel and are removed via a trap. Both the film plastic and contaminants are landfilled.
Next, the clean SSO pulp is transferred to a temporary storage tank that acts as a buffer between the wet separation system and the anaerobic digestion system. A BTA grit removal system is operated in parallel with the pulp buffer tank in order to remove grit (sand, small stones, small pieces of glass, egg shells etc.) from the SSO pulp.
The AD system is comprised of an anaerobic digestion vessel (the “digester”), a solids recycle system, a dewatering system, a biogas flare and a process water recycle system as shown in Figure 1. There also are ancillary systems for digester heating and mixing that are not further discussed in this article series.
When operating at its design capacity, the anaerobic digestion system receives approximately 300 m3 per day of SSO pulp from the receiving and preprocessing system (equivalent to approximately 3 m3 of pulp per metric ton SSO received). The digester is a completely mixed reactor with a solids recycling system (i.e. a “contact” digestion process). The liquid capacity of the digester is 3,600 m3 and the hydraulic retention time is approximately 17 days. The temperature is maintained at approximately 37°C, which is in the mesophilic temperature range.
A solids recycle system operates in parallel with the digester. Its purpose is to maintain the digester at 4 to 5 percent total solids (average 4.7 percent). At this proportion of solid material, the digester contains a microbial population large enough to process material at the design loading rate. The mean solids retention time within the anaerobic digestion system is approximately 27 days.
The dewatering system is comprised of screw presses that separate the contents of the digester into a solid fraction, i.e., a digested solid material with a solids content of 22 to 25 percent, and a liquid fraction that is temporarily stored in the process water tank before being either added to the hydropulper (i.e. recycled through the process) or discharged to the sanitary sewer.
On weekdays, SSO pulp is delivered to the anaerobic digestion system continuously from 6:00 am until the plant has processed the maximum allowable daily limit of 100 metric tons of SSO (equivalent to approximately 300 m3 of SSO pulp). This is typically achieved between 8:00 pm and 10:00 pm. Digested material is withdrawn from the digester concurrent with feeding to maintain a constant liquid volume in the tank of 3,600 m3.
During the first six months of 2005 (January 1 to June 30), the DOPF operated reliably at or near its design capacity of 100 metric tons of SSO per day (equivalent to 25,000 metric tons/year based on a five day per week operating schedule). Based on operating records during that time, the average weekly material inputs (in metric tons) to the DOPF were: 483 tons of SSO and 590 tons of fresh water (a total 1,073 tons). Average weekly material outputs for the same period were: 77 tons of biogas, 148 tons of digested solids, 133 tons of solid processing residue, and 715 tons of effluent discharged to the sanitary sewer (a total 1,073 tons). These numbers, except for output effluent, were obtained directly from facility operating records. Output effluent was estimated from the positive difference between total inputs and the sum of all other outputs (including the recycle of 657 tons of water recycled back to the front-end of the process).
Average daily biogas production for the period January 1 to June 30, 2005, is presented in Figure 2. Measurements were taken at approximately 7:00 am, Monday-Friday prior to the commencement of processing operations. Intermittent SSO pulp delivery to the anaerobic digestion system results in daily and weekly cycles of biogas methane content, as shown in Figure 3. The highest biogas methane content occurs on Monday mornings, prior to the commencement of SSO pulp delivery to the anaerobic digestion system.
Daily reductions in biogas methane content correspond to periods of pulp delivery tothe anaerobic digestion system. During pulp delivery, hydrolysis reactions and microbial conversion of the feedstock to volatile fatty acids (VFAs) outpaces the microbial conversion of the VFAs to methane (methanogenesis). As a result, the pH within the digester is reduced causing a release of dissolved carbon dioxide. The proportion of carbon dioxide in the biogas is thus increased and a corresponding decrease in the proportion of methane is observed. During the nightly and weekend interruptions in pulp delivery to the anaerobic digestion system, the VFAs continue to be converted into methane causing the pH within the digester to increase. Carbon dioxide goes into solution and an increase in the proportion of methane in the biogas is observed.
Because of its sensitivity to pH conditions within the digester, biogas methane content is commonly used as a process control parameter for anaerobic digestion processes. A rapid reduction in biogas methane content signals the onset of process upset conditions and signals to the operator the need to implement contingency measures, such as a reduction in organic loading.
Material stream inputs to and outputs from the DOPF anaerobic digestion system were sampled and analyzed. Grab samples of pulp, effluent and digested solids were collected at intervals during the first operating shift of the facility (6:00 am – 3:00 pm). The grab samples were combined and four composite samples were taken for each material stream. Grab samples of biogas for analysis of matrix gasses and reduced sulfur compounds were collected near the beginning, midpoint and endpoint of the daily schedule of pulp delivery to the anaerobic digestion system (approximately 8:00 am, 3:00 pm, 9:00 pm respectively). Matrix gasses include carbon dioxide, methane, carbon monoxide, hydrogen, oxygen and nitrogen. Reduced sulphur compounds include hydrogen sulfide, dimethyl sulfide, dimethyly disulfide, methyl mercaptan.
Analysis of the SSO pulp samples revealed the physical and chemical properties of the SSO pulp to be as summarized in Table 1. In addition to the macronutrients listed in Table 1, the organisms involved in the anaerobic digestion process require lesser amounts of the elements and compounds (micronutrients) listed in Table 2. The DOPF experienced periodic digestion process upsets during its first 18 months of operation. The City commissioned an assessment of BTA’s anaero-bic digestion process that, among other observations, identified micronutrient deficiency as a possible contributor to the process instability. Specific mention was made of the possibility of cobalt (Co) deficiency as methanogenic organisms are particularly sensitive to this element. The digestion process assessment estimated micronutrient concentrations in the digester are reported in Table 2.
Micronutrient requirements can be satisfied by careful selection of feedstock materials. Where the feedstock is limited to a single material stream, as is true for the DOPF, and that feedstock is micronutrient deficient, a “vitamin” of specific micronutrients can be added directly to the digestion system. The DOPF operator began monitoring soluble concentrations of key micronutrients – cobalt (Co) and nickel (Ni) – in the digester and in June 2004, began adding Co as cobalt chloride (CoCl2). An almost immediate improvement in the performance and stability of the digestion process was observed. Digester micronutrient concentrations reported by the operator are summarized in Table 3. Lower Co concentrations correspond to the periods between CoCl2 additions. The operator reports typical total and volatile solids concentrations in the digester for this period to be 4.7 percent and 57 percent respectively.
The biodegradability of a complex organic feedstock material like SSO pulp is a function of the proportion of solid material that is volatile, the chemical composition of the volatile material fraction, i.e. relative proportions of readily degradable vs. partially or nondegradable volatile materials, and the environment of the anaerobic digestion process which is itself determined by the system’s design and operation. The complexity and variability of a feedstock material, like SSO pulp, makes detailed investigation of its chemical composition impractical for full-scale operating systems like the DOPF.
A practical and useful measure of the overall performance of the feedstock and digestion system is a measure of the proportion of volatile material destroyed, i.e. mineralized into nonvolatile solids or converted into biogas. The quantity of volatile material destroyed can be compared to the quantity of biogas and methane produced, and useful performance parameters for the feedstock and system can be calculated. Performance parameters include the Specific Biogas Potential (SBP) and the Specific Methane Potential (SMP) are further discussed in Part 2 (to appear in December 2006).
The proportion of volatile material destroyed by the DOPF anaerobic digestion system was determined from the average weekly digestion system mass balance, and the composition of the input and output material streams as determined from analysis of samples collected as part of this study. Estimated average volatile solids loading on the anaerobic digestion system is approximately 82 metric tons/week (1437 m3 x 56856 ug VS/g). Estimated average volatile solids outputs from the anaerobic digestion system (all in metric tons) are: Digested solids – 25.2 tons (140 tons digested solids x 137,500 ug VS/g); Effluent to sewer -3.7 tons (784 tons effluent x 4,748 ug VS/g); Process water recycle – 2.2 tons (462 tons effluent x 4,748 ug VS/g); Total volatile solids output – 31.1 tons. Therefore, an estimated 50.9 metric tons of volatile solids are destroyed per week resulting in a volatile solids destruction rate of approximately 62 percent.
Cumulative biogas generation at the DOPF is measured continuously by an inline flow meter. Flow meter readings are recorded manually at the start of every weekday operating shift. Daily biogas generation is determined by comparing morning to morning measurements. Total biogas generation during the study period was 90,577 m3, equivalent to an average daily biogas generation rate of 7,548 m3 per day.
Biogas methane generation during the study period wasestimated from average daily measurements of biogas methane and daily biogas generation. Total biogas methane generation during the study period was 45,216 m3 equivalent to an average daily rate of 3,768 m3 methane/day (approximately 50% v/v). Daily biogas methane generation during the study period is presented in Figure 4.
While taking samples to determine biogas composition, ambient air contaminated six of 12 biogas samples for analysis of matrix gasses, as revealed by the high proportions of nitrogen and oxygen in the samples. The three uncontaminated samples confirmed the accuracy of the twice daily on-site methane and carbon dioxide measurements taken by the DOPF operator. Based on the close agreement between analytical sample results and on-site measurements, it was determined that the on-site measurements of methane and carbon dioxide were sufficiently accurate for the objectives of the study.
The composition of the biogas was measured at intervals over the study period as described earlier in the article. Average concentrations of carbon dioxide, methane and other matrix gasses are presented in Table 4. Average concentrations of reduced sulfur compounds are presented in Table 5. Concentrations of siloxane compounds in the biogas are presented in Table 6. The typical daily and weekly variation in biogas methane content were observed over the study period.
Part 2 of this article series will discuss the anaerobic digestion system process parameters as determined from the steady state operating conditions and material stream sampling data discussed in Part 1. In addition, the article will cover the suitability of the SSO material for anaerobic digestion, as well as the suitability of the digested solids as a feedstock for composting. Further analysis of biogas generation also will be discussed, along with a comparison to typical values for other AD systems processing an SSO stream.
Brian Van Opstal is Senior Engineer in the Solid Waste Management Services division of the Works and Emergency Services agency at the City of Toronto. He can be reached at bvanops@ The author acknowledges the assistance of Anne Wheatly, Supervisor of Organics Processing, City of Toronto, and Doug Beattie, Plant Manager, CCI-TBN Toronto Inc. for his assistance with sample and data collection. The City of Toronto also acknowledges the support, financial and otherwise, of the Canadian Biomass Innovation Network (CBIN), an organization formed to coordinate the Federal Government’s research and development activities in the area of bioenergy, biofuels and industrial biotechnology.

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