2012 Coleman Anaerobic Digestion SSO vs WWTP

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WEAO 2012 Conference. Anaerobic digestion is a well understood biosolids treatment technology. The primary benefits are the production of renewable energy and the reduction of material to be disposed of. However, biosolids are a minor source of digestible material when compared with organic solid wastes, industrial wastes, agricultural wastes (e.g. manure) and energy crops. This paper will present the design of thenew Disco Source Separated Organics (SSO) Anaerobic Digestion ProcessingFacility in the City of Toronto and contrast it with the design of a typical Biosolids Digestion Facility. Issues such as pre-processing options, digester design, end product quality, process water balance and biogas production are discussed


<p>ANAEROBIC DIGESTION - COMPARISON OF DESIGN CONSIDERATIONS BETWEEN A BIOSOLIDS AND ORGANIC SOLID WASTES DIGESTION FACILITY P. Coleman, PhD PEng, AECOM J. Blischke, MS, AECOM</p> <p>ABSTRACT Anaerobic digestion is a well understood biosolids treatment technology. The primary benefits are the production of renewable energy and the reduction of material to be disposed of. However, biosolids are a minor source of digestible material when compared with organic solid wastes, industrial wastes, agricultural wastes (e.g. manure) and energy crops. This paper will present the design of the new Disco Source Separated Organics (SSO) Anaerobic Digestion Processing Facility in the City of Toronto and contrast it with the design of a typical Biosolids Digestion Facility. Issues such as pre-processing options, digester design, end product quality, process water balance and biogas production are discussed 1. INTRODUCTION</p> <p>On vacation, in 1176, Allesandro Volta collected gas released from Lake Maggiore (Italy) and showed that it was flammable. Volta experimented with this marsh gas building a pistola which was an early precursor of the internal combustion engine. A century later Bechamp demonstrated a microbial origin of this gas using an ethanol-based media inoculated with rabbit feces (Speece 2008). In 1860, Louis Mouras built a closed container with a water seal in which organic matter (excrement) disappeared. This system, later referred to as the Mouras Automatic Scavenger, was considered an elegant solution to the treatment of sewage solids. However, evidence suggests that many people already relied on this method of treatment. It is believed that the first purpose-built anaerobic digester was built in 1859 for an Indian leper colony (Speece 2008). Thirty one years later, W. D. Scott-Moncrieff constructed a tank with an empty space at the bottom and a submerged bed of stones on the upper part creating probably one of the first anaerobic filters. The first noteworthy linking of liquefaction of sewage solids and gas production/utilization did not occur until 1895. Cameron constructed a septic tank in Exeter (UK) that produced gas that powered nearby gas lights. Today, anaerobic systems are used to produce renewable energy from high strength industrial effluents, agricultural material, municipal wastes, organic</p> <p>WEAO 2012 Technical Conference, Ottawa, Ontario</p> <p>Page 1</p> <p>industrial byproducts and biosolids. Anaerobic digestion provides a means to reduce pollution from organic wastes while producing a renewable fuel biogas. Biogas can used to produce heat, steam, electricity and replace fossil-based vehicle fuels. 2. MICROBIOLOGY</p> <p>Anaerobic digestion (see Figure 1) can be divided into four steps (Henze, van Loosdrecht et al. 2008): 1. Hydrolysis: Enzymes excreted by fermentative bacteria break down complex undissolved material into less complex, dissolved compounds that can pass through cells walls and membranes of fermentative bacteria. When digestion source separated organics, approximately 15% of ammonia is released into solution at this step (Zhang, Walker et al. 2010). 2. Acidogenesis: Dissolved compounds within fermentative cells are converted to simpler compounds and excreted. These include volatile fatty acids, ammonia, alcohols (e.g. ethanol), lactic acid, carbon dioxide and hydrogen sulfide. 3. Acetogenesis: (intermediary acid production) digestion products are converted into acetate, hydrogen, and carbon dioxide. 4. Methanogenesis: Acetate, hydrogen, carbon dioxide plus carbonate, formate or methanol are converted into methane and carbon dioxide (i.e. biogas). Typically, 70% of the methane is produced from acetate (by aceticlastic methanogens) and 30% from carbon dioxide and hydrogen (by hydrogenotrophic methanogens).</p> <p>3.</p> <p>FEEDSTOCK</p> <p>There are a number of organic feedstocks suitable for anaerobic digestion (Figure 2). In a typical North American city, the responsibility for these feedstocks is spread across city departments (e.g. solid waste vs. water and sewage). Therefore, the management of these feedstocks is fragmented and opportunity to cooperate to obtain the best solution for a city is more often than not missed. A typical SSO digester produces about 110 m3 of biogas per wet tonne of material processed which is about 80% of the biogas potential of the SSO. This is about 0.8 to 1.0 m3/kg volatile matter destroyed which is similar to the biogas yield for sewage sludge. The yield will change depending on what organics are present in the SSO and how the SSO is prepared for digestion (e.g. hydrolysed).</p> <p>WEAO 2012 Technical Conference, Ottawa, Ontario</p> <p>Page 2</p> <p>Unlike SSO, the organic make up of sewage will only vary significantly from one site to another if there is a significant industrial discharge to the sewer system. Apart from the variability in the feed characteristics, the other differences between SSO and sewage sludges are (1) water content and (2) contaminants in the feedstock. Sewage sludge consists of three types of materials: raw volatile solids (VS), biomass solids (e.g. waste activated sludge), and inert material. The inert material consists of material that arrives with the sewage and material created in the process (e.g. iron phosphates). Screenings and grit, if not removed from the raw sewage, create problems in the digester. Screenings form rafts on the water surface and interfere with rotating equipment (e.g. pumps, mixers). Plastics, if they make it into the final digested product, can limit the use of the end product. Grit increases equipment wear and settles out in the digester reducing the volumetric capacity. The primary processing challenge for sewage treatment sludges is water. Sewage sludge is dilute. In North America digesters are fed a sludge mix at about 4% solids (Krause 2010): A typical design sustained-peak loading rate for mesophilic digesters is 1.9 to 2.5 kg volatile solids/m3/d (0.12 to 0.16 lb volatile solids/d/cu ft). The upper limit of the volatile solids loading rate typically is determined by the rate at which toxic materials particularly ammonia accumulate or methane formers wash out. A limiting value of 3.2 kg volatile solids/m3/d (0.20 lb volatile solids/d/cu ft) is often used. The more dilute the sewage sludge, the more energy is required to heat the feed and to maintain the temperature in the digester. Thickening the sludge to about 6% decreases the heat demand. In some circumstances, it is possible to heat a properly insulated digester fed at 6% solids using only the waste heat from a properly sized biogas co-generation engine. In Europe, where mechanical thickening of primary sludge is more common, digesters are fed with sludge at 5% to 8% solids. This is because a thickening belt or rotary drum can reliably achieve 6% and 9% dried solids when thickening waste activated sludge and primary sludge respectively. The average digester VS loading is between 3 and 4 kg VS/m3/d. The experience in the wastewater industry with high solids digestion started with the first thermal hydrolysis plant installed at Hias, Norway in 1995 (Fjrgrd and Sander 1999). Attempts to load conventional digesters at higher rates without pasteurizing or hydrolyzing the sludge failed because of foaming or souring (Brown and Sale 2002).</p> <p>WEAO 2012 Technical Conference, Ottawa, Ontario</p> <p>Page 3</p> <p>A number of thermal hydrolysis installations have followed including one to be built for District of Columbia Water and Sewer Authority (Washington, DC). The feed concentration is about 10% solids. The volatile solids load to the digester is between 5 and 6 kg VS/m3/d. The constituents of the collected SSO (e.g. each household has a separate bin) and the organic fraction of municipal solid wastes (OFMSW) (e.g. what is left after the material is mechanically processed at a material recovery facility) varies from jurisdiction to jurisdiction. Even the SSO collected in neighboring communities can be different due to different rules as to what can be put into a green bin. For example, the Region of Peel and the City of Toronto both have green bin programs. The City of Toronto allows the use of plastic bags and accepts diapers in the green bin. The Region of Peel only allows certified compostable plastic bags in the green bin and insists that diapers are disposed with the regular garbage. 4. CITY OF TORONTO SSO ANAEROBIC DIGESTION FACILITIES</p> <p>The City of Toronto has one operating anaerobic digestion processing facility (Dufferin) and is currently building a second one at the Disco Road Transfer Station site. In the City of Torontos Dufferin Facility, the waste arrives as raw SSO on the tip floor with plastic bags, heavy debris (e.g. rocks) and grit (Figure 3). The moisture content varies by season. The moisture content is typically around 68%. The material is pulped using a mix of fresh and recycled water as a preconditioning step for wet digestion (with a total solids [TS] content of typically less than 15%). At the Toronto facilities, the pulp is digested and the digestate is dewatered. The maximum loading rate is about 5 kg VS/m3/d. At other non-BTA facilities, the pulp is dewatered and only the liquid is digested. 5. USE OF WASTEWATER SIMULATION SOFTWARE TO DESIGN SSO PROCESSING FACILITIES Wastewater treatment plant simulation software (e.g. BioWin, GPS-X) was developed to model the processing of liquid wastes (see Figure 4 as an example). Because the Disco Road Facility includes a wastewater treatment plant and anaerobic digesters, AECOM modeled the facility using both BioWin and GPS-X. Both these packages include sophisticated pH and digester models that involve complex water chemistry calculations. AECOM also recently modeled a facility that received, digested and dewatered raw sludge cake delivered by truck to the facility. In this case, in order to set the inputs necessary for the anaerobic digestion model, AECOM modeled</p> <p>WEAO 2012 Technical Conference, Ottawa, Ontario</p> <p>Page 4</p> <p>the wastewater treatment plant and then imported the output from the WWTP plant model to the biosolids processing facility model. For the most part, this approach worked. However, this approach cannot be used for an SSO facility. The designer therefore must start with solids, volatile solids and moisture content information. The nitrogen and phosphorus content is estimated from literature values (e.g. %N/wet solids) and from historic sewage and cake discharge data from an existing facility. Measuring the characteristics of the feedstock directly is expensive and labor intensive (Jansen, Spliid et al. 2004) The two most difficult decisions to be made by an SSO facility modeler are (1) how to set the dissolved solids so that the anaerobic digester model functions and (2) how to characterize COD, N and P waste fractions when entering the feedstock into the model. The knowledge of SSO and OFMSW characteristics is limited compared to what is known about domestic sewage. Commercial simulation software packages need to allow the modeler to dumb down the model to reflect the limited information on the model inputs. For example, it may not be necessary to model the digester pH. What is more important is modeling the ammonia concentration in the digester, centrate and wastewater treatment plant as well as the varying levels in the process water buffer tanks. 6. PREPROCESSING</p> <p>Most wastewater treatment plants screen and de-grit the raw sewage at the head of the treatment process. However, in jurisdictions where there is a strict rule about plastics in biosolids used on agricultural land, the sludge is screened a second time through a 5mm to 10mm screen prior to thickening. SSO also contain containments that need to be removed before the material is digested. These contaminants are more difficult to manage than screenings in sewage sludge. Consequently, there are a number of proprietary wet anaerobic digestion processes on the market (Figure 5) each with its own approach to managing contaminants. This paper will focus on the BTA process which is installed at one Toronto facility (Dufferin) and will be soon installed at a second Toronto facility (Disco Road). The Dufferin SSO Facility, commissioned in 2002, was originally designed to process 25,000 wet tonnes per year (TPY). The facility is currently processing close to 40,000 wet TPY. The facility is to be expanded to process up to 55,000 TPY. The new Disco Road facility is designed to process 75,000 wet TPY. Both sites use the wet BTA Process (BTA 2011). Figure 6 illustrates in a simplified flow diagram the BTA Process.</p> <p>WEAO 2012 Technical Conference, Ottawa, Ontario</p> <p>Page 5</p> <p>Other preprocessing options include pasteurization (to meet strict disinfection standards) and/or physical/chemical treatment of the sludge to make it more digestable. There are comparable options for other organics feedstocks (e.g. SSO). The wet BTA pre-treatment step generates a waste suspension in the Pulper (Figure 7) by adding primarily recycled water to the raw SSO. During the pulping process the plastic bags are broken open and removed along with other light/floating material using a rake. The so-called Light Fraction that includes the plastic bags are washed and pressed/de-watered. Heavy material (e.g. stones, glass, batteries, cutlery) also called Heavy Fraction sinks to the bottom of the pulper where it is removed through a heavy fraction trap. The pulp, at about 8-10% solids, is then passed to a de-gritting step. The pulp is passed through hydrocyclones as a key component of the grit removal system to remove finer particles. The pulp is then fed into the digester directly or temporarily stored in a suspension buffer tank before being fed to the digester. 7. DIGESTER DESIGN</p> <p>Sewage digesters come in one of three shapes (Figure 8): cylindrical (height &lt; diameter), double cone (height &gt;diameter) and egg shaped. Most digesters for wet SSO digestion are cylindrical in shape. This is the most economical shape for the size of these types of digesters. The digesters are normally constructed from coated steel or concrete. This is because solid waste facilities differ from wastewater treatment plants in two ways: (1) expected asset life is shorter and (2) the feedstock can be diverted during facility shutdown because the feedstock can be diverted to another site or stockpiled. This is not the case for raw sewage. Unconfined gas, confined gas, draft tube, pump, linear motion and big blade are used in sewage digesters. This said, as the thickness of the feed increases, mixing moves away from turbulent towards laminar systems. In the latter case, the mixer tends to fold the material much like a cook uses a spatula when mixing cake batter. SSO digester designers tend to shy away from mixers involving rotating equipment when the SSO contains contaminants that will wrap around rotating shafts. For example, Dufferin and Disco Road both use gas lances to mix the digester. 8. PROCESS DESIG...</p>