BioCycle December 2006, Vol. 47, No. 12, p. 39
Toronto program is a first step towards a comprehensive assessment of the potential for biogas production from the anaerobic digestion of municipal organic materials. Part II
Brian Van Opstal
THE FIRST part of this report on Toronto, Canada’s waste management program – “Evaluating AD Systems Performance for MSW Organics,” appeared in the November 2006 issue of BioCycle. It described the Dufferin Organics Processing Facility (DOPF) operations, materials balances and composition of the source separated organic pulp and biogas. This concluding portion reports on how the data obtained was used to evaluate suitability of the anaerobic digestion system for source separated organics. The DOPF utilizes the BTA process that includes a wet pretreatment system and a single stage mesophilic anaerobic digester.
Key process parameters for the DOPF digestion system are: Hydraulic Retention Time (HRT), Solid Retention Time (SRT) and Organic Loading Rate (OLR). Figure 1 shows the equations used to calculate these process parameters for the DOPF. The HRT reports the ratio of digester volume (m3) to the volumetric rate of feeding (m3 per day). For the DOPF, the HRT is 17.5 days.
For a completely mixed digester, the SRT is the ratio of the mass of solid material in the digester (kg total solids) to the rate at which solid material is removed from the digester (kg total solids per day). For the DOPF, the SRT is 27 days.
The OLR reports the ratio of the mass rate at which volatile solids are fed to the digester to the volume of the digester. For the DOPF, the OLR is 3.3 kg VS/m3/day.
DOPF anaerobic digestion process parameters are compared to values reported for similar systems in Table 1.
SUITABILITY OF SSO MATERIAL FOR ANAEROBIC DIGESTION
For anaerobic digestion to run optimally, certain nutrients (nitrogen and phosphorus) and micronutrients (sulfur, vitamins, and trace minerals including Fe, Ni, Mg, Ca, Na, Ba, Tu, Mo, Se and Co) must be present. An average ratio COD/N/P of around 600/7/1 is recommended for a substrate to be anaerobically digested. Using earlier results reported, the organic material pulp was found to have a COD/N/P ratio of 24/6.6/1.
A prior assessment of the digestion process at the DOPF identified a possible deficiency of the micronutrients Co and Ni. Estimated and recommended nutrient concentrations in the digester are presented in Table 2. Estimates of digester concentrations of Co and Ni were confirmed by actual measurements provided by the operator. The operator began adding CoCl2 to correct a Co deficiency in June 2004, and immediately noticed an improvement in the stability and performance of the digestion process.
Average NH3-N concentration in the digester exceeded 100 mg/L; the reported threshold for the onset of inhibition. High ammonia concentrations result from the recycling of the liquid fraction of the digester contents as process water, and from the low COD/N ratio of the organic pulp feedstock. Removal of excess nitrogen from the anaerobic digestion system, by increasing the rate at which process water is wasted to the sewer and replaced by fresh water, was recommended by the previous process assessment. Other facilities not constrained to processing only SSO feedstock should consider selecting complementary feedstocks with a high COD/N ratio.
DIGESTED SOLIDS AS A FEEDSTOCK FOR COMPOST PRODUCTION
The digested solids material produced by the DOPF has the physical and chemical characteristics reported in Table 3. Because the material is biologically unstable and processing conditions have not satisfied the regulatory requirements for pathogen destruction, it is necessary that the material be subjected to secondary processing at an off-site location. Thus far, and for the foreseeable future, the preferred method of secondary treatment of the digested solids is aerobic composting. Therefore, the suitability of the digested solids as a feedstock for an aerobic composting process is of interest.
In Ontario, provincial composting guidelines for the production and use of an unrestricted use aerobic compost product require that all composting feedstock materials have concentrations of elemental contaminants below the maximum concentrations allowed in the finished compost product. Optimum characteristics for a feedstock for aerobic composting are a carbon to nitrogen ratio (C:N) of 30:1 and a moisture content in the range of 55 percent. DOPF digested solids contain approximately 55 percent volatile material and have a C:N and moisture content in the range of 2 and a moisture content of 75 percent. Therefore, successful composting of DOPF digester solids will require blending with other high C:N and dry feedstock materials in order to achieve optimum characteristics for composting.
Three key biogas generation parameters are monitored – gas production rate (GPR), specific biogas production (SBP) and specific methane production (SMP). The GPR reports the ratio of the daily quantity of biogas generated (m3/day) to the volume of the digester (m3). The SBP reports the quantity of biogas produced (m3) per mass volatile solids (kg VS) fed to the digester. SMP reports the quantity of biogas methane produced per mass volatile solids fed to the digester. Figure 2 shows the equations used to calculate these three parameters for the DOPF BTA’s anaerobic digestion system as determined from steady state operating conditions and material stream sampling results.
BTA’s anaerobic digestion system at the DOPF appears to produce biogas at a rate similar to that measured at other facilities processing a source-separated municipal organic material feedstock as shown in Table 4. Variability in the biogas production rate and methane content resulting from daily and weekly digester feeding cycles present operational challenges for biogas utilization. Two general approaches are available: (1) adopt a schedule of constant digester feeding; and, (2) addition of temporary gas storage to buffer changes in quantity and methane content.
The average methane content of the biogas is consistent with reported results for other facilities processing a source-separated municipal organic material feedstock. The DOPF biogas contains hydrogen sulfide (H2S) at the low end of reported values; 1050 ppmv for DOPF biogas vs 100 – 20,000 ppmv for industrial wastewater and municipal sludge digesters. However, because of the corrosive nature of the products of H2S combustion, which are major problems for internal combustion engines, microturbines and fuel cells, pretreatment of biogas for H2S removal is typically recommended when concentrations exceed 500 ppmv.
The DOPF biogas was found to contain 5,900 ug/m3 of siloxane compounds in the octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) species. Biogas siloxanes result from the volatilization of these compounds present in the feedstock material. Siloxanes are used in a variety of personal hygiene, health care and industrial silicone products and as a result are present in biogas from the anaerobic digestion of wastewater sludge and in landfill gas. Reported values for combined D4 and D5 siloxanes in digester and landfill gas vary but are typically in the range of 4 – 60 mg/m3
When siloxanes are combusted they are converted to silicon dioxide (SiO2) precipitants which deposit in the exhaust stages of the combustion equipment. These precipitates are corrosive, which causes equipment failure, and, through their accumulation, reduce the efficiency of heat transfer processes. Pretreatment for siloxane removal should be considered for future biogas energy utilization systems either at the DOPF or other facility processing source separated municipal organic materials.
While anaerobic digestion of municipal organic materials is relatively well established in other jurisdictions, especially in western Europe, it remains an undeveloped technique in North America. This project provides some information of the performance of a large-scale, operating anaerobic digestion facility processing a source separated municipal organic material feedstock, specifically the City of Toronto’s Green Bin program material stream. The general conclusions that can be derived from this project are listed below.
The organic pulp derived from the City’s source separated organic material stream, after pretreatment to remove film plastic and contaminant materials using the BTA wet pretreatment process, is a suitable feedstock for anaerobic digestion. Where the SSO material is the only feedstock stream, such as for the DOPF, digestion process management needs to address the potential for NH3 accumulation in the digester, partly as a result of low COD/N ratio of the pulp feed material. Management strategies include a high wasting rate for the liquid fraction of the digester contents and co-digestion with other feedstock materials with complementary characteristics, i.e. a high COD/N ratio.
Micronutrient concentrations within the digester should also be monitored to ensure sufficiency. For the DOPF it was suspected that low concentrations of cobalt (Co) and nickel (Ni) could be impeding the digestion process. An immediate improvement in the performance and stability of the digestion process was observed after the addition of CoCl2.
Digested solids produced by the DOPF’s BTA anaerobic digestion system is a suitable feedstock for the production of an unrestricted use aerobic compost product. The wet pretreatment process produces an organic pulp feedstock material that is virtually free of film plastic and contaminant materials. The BTA grit removal system effectively removes small pieces of dense inert materials, such as gravel and glass and other ceramics, from the organic pulp material. Effective contaminant removal during feedstock preparation ensures that composting options for the resulting digested solids material are not limited by the presence of inert, foreign contaminant materials. Also, concentrations of regulated elemental contaminants in the digested solids materials are below regulatory limits (As, Hg, Se not reported) and therefore do not restrict or otherwise limit the use of the digested solids as a compost feedstock.
Digested solids produced by the DOPF’s BTA anaerobic digestion system have a high moisture content and low C:N ratio, and must be blended with a dry, high C:N material in order to achieve physical and chemical conditions optimum for aerobic composting.
Main digestion process parameters for the DOPF (HRT, SRT, OLR, VS feed and VS Removal) are similar to those reported for other facilities processing a source separated municipal organic material feedstock. The digestion process achieves a VS removal rate of 62 percent and produces approximately 0.64 m3 of biogas per kg of VS feed, which is approximately equivalent to 120 m3 per tonne of SSO material received at the DOPF for processing.
The biogas has an average methane content of 56 percent however, because of the intermittent digester feeding schedule, the methane content varies daily and weekly, ranging from a high value of 70 percent typically measured on Monday mornings prior to commencement of feeding, to a low of just over 40 percent measured on weekdays at the end of the feeding cycle. Total biogas generation is expected to follow a similar daily and weekly pattern. Variability in biogas generation and methane content challenges biogas utilization systems and approaches for minimizing variability should be explored.
The biogas will require pretreatment prior to utilization for energy recovery. Specifically, pretreatment for H2S and siloxane removal will be required. The type and removal efficiency of the pretreatment systems will depend upon the type of utilization system selected.
FIRST STEP TO ASSESSMENT
The limited results of this project are a first step towards a comprehensive assessment of the potential for biogas production from the anaerobic digestion of municipal organic materials. Subsequent investigations conducted at, or in combination with, the DOPF can address important questions beyond either the scope or ability of this project to answer.
Suggested areas for subsequent investigations include: On-site methods of removing NH3 from the digester to minimize loading on the municipal wastewater treatment system; Continuous monitoring of biogas generation and methane content; Effect of continuous feeding on the performance of the anaerobic digestion system and biogas production; Effect of adding high C:N feedstock or amendment materials; and Effect of size reduction of the organic pulp prior to digestion.
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 e-mailed at email@example.com.
December 14, 2006 | General
Managing AD System Logistics For MSW Organics
BioCycle December 2006, Vol. 47, No. 12, p. 39