Anaerobic Digestion in Canada

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<p>ANAEROBIC DIGESTION IN CANADAAnna M. Crolla, M.A.Sc. Christopher B. Kinsley, M.Eng., P.Eng. Collge dAlfred - University of Guelph, Canada Kevin Kennedy, Ph.D. University of Ottawa, Canada 1.0 Introduction</p> <p>Around the world, pollution of air and water from municipal, industrial and agricultural operations continues to grow. Governments and industries are constantly on the lookout for technologies that will allow more efficient and cost-effective waste treatment. One technology that can successfully treat the organic fraction of wastes and wastewaters is anaerobic digestion (AD). When used in a fully-engineered system, AD not only provides pollution prevention, but also allows for energy, compost and nutrient recovery. AD is growing to become a key method for both waste reduction and recovery of a renewable fuel and other valuable co-products. This chapter will describe: Various feedstocks for anaerobic digestion Pre-treatment options for sewage sludge treatment Anaerobic digester options for agricultural wastes with Canadian examples Methane production from organic solid waste treatment using landfill and designated anaerobic digestion systems Anaerobic digestion options for industrial wastewaters</p> <p>ATAU Course Notes Anaerobic Digestion in Canada</p> <p>1</p> <p>2.0</p> <p>Feedstock for Anaerobic Digestion</p> <p>2.1 Sewage Sludge Digestion of sewage sludge provides significant benefits when recycling the sludge back to land. The digestion process provides sanitisation and also reduces the odour potential from the sludge. Typically between 30 and 70 % of sewage sludge is treated by AD depending on national legislation and priorities (IEA, 2001). The energy generated from the AD process is usually used to power the sewage treatment works and where applicable (larger facilities) the excess biogas is exported from the plant. 2.2 Agricultural Wastes Farm scale digestion plants treating principally animal wastes have seen widespread use throughout the world, with plants in both developing and technically advanced countries. In rural communities small scale units are typical; Nepal has some 47,000 digesters, while China estimates they have 6 million digesters (IEA, 2001). These digesters are generally used for providing gas for cooking and lighting for a single household. In more developed countries, farm scale AD plants are generally larger and the biogas produced is used to generate heat and electricity to run the farm and for off-farm export. The small farm scale digestion plants are based on simple stirred tank designs that use long retention times to provide the treatment required. Whereas modern developments in agricultural waste digestion have developed the concept of centralised anaerobic digestion (CAD) where many farms co-operate to feed a single larger digestion plant. 2.3 Municipal Solid Wastes Organic wastes from households and municipalities provide potential feedstocks for anaerobic digestion. Wastes can be treated to gain the biogas from the waste as well as stabilising it to prevent further problems in the landfill. 2.4 Industrial Wastes Organic solid wastes from industry are increasingly being controlled by environmental legislation. Breweries, vegetable and meat processing industries are increasingly using anaerobic digestion in their waste management strategies.</p> <p>ATAU Course Notes Anaerobic Digestion in Canada</p> <p>2</p> <p>3.0 3.1</p> <p>Treatment of Sewage Sludge in Canada Introduction</p> <p>Municipal wastewater treatment plants (MWWTP) produce primary and secondary sludge streams that are high in organic content. Mesophilic anaerobic digestion of these sludges is often employed to reduce the mass of solids for disposal, reduce their pathogen content and to generate biogas for energy recovery. Presently in Canada there is a dual focus pertaining to the treatment of MWWTP sludges. Reducing green house gas emissions and utilization of alternative energy sources has created an interest in evaluating options that are available for maximizing the generation of biogas from anaerobic digestion for energy recovery. Additionally, pre-treatment options that increase the potential of producing Class A biosolids have a tendency to have more public support than those that only increase biogas production. The centre of Disease Control (USA) has recommended all sludges be treated to class A standard because of the risk that disease could be transmitted through Class B sludge. Recently, in Ottawa, Canada a biosolids management program that included spreading of mesophilically anaerobic digested municipal sludges on agricultural land was stopped as a result of public health concerns. The various emerging sludge treatment technologies are either reduction processes or digestion processes (Kelly, 2003) that can be coupled with some type of pre-treatment. Generally, sludge reduction technologies such as incineration, gasification, pyrolyis, wet air oxidation, fuel from sludge and supercritical water oxidation tend to operate at high temperatures, low pressures and short retention times. On the other hand sludge digestion processes and associated pre-treatment processes such as Microsludge and high pressure hydrothermal tend to operate at lower temperatures but higher pressures. On the macro-scale ultrasound seems to operate at low pressure and low temperature. However at the micro-scale the opposite is in fact true (discussed below). This chapter focuses on high powered ultrasonication, chemical and pressure cell disruption (Microsludge) and thermal and pressure (SUBBOR) pre-treatment that are now available commercially. More experimental pre-treatment options such as electropulse are not addressed. Additionally, these options do not include modifications to operating practices or implementation of alternative digestion technologies that also may lead to enhanced sludge stabilization and biogas production. Examples of other options not described include enhanced primary settling, submerged combustion, sludge</p> <p>ATAU Course Notes Anaerobic Digestion in Canada</p> <p>3</p> <p>thickening, improved digester mixing (pancake versus eggshape design), increase solids residence times incorporating sludge recycle, temperature phased anaerobic digestion (TPAD) or aerobic thermophilic pre-treatment and dual digestion.</p> <p>3.2</p> <p>Sludge Production</p> <p>Production of biogas at municipal wastewater treatment plants will be a function of many factors including the sources of the sludge. Sources of sludge in most wastewater treatment plants consist of the primary clarifier sludge and waste sludge from secondary biological treatment processes. The generation and characteristics of primary sludges are influenced by the sources of wastewater and other waters (i.e. infiltration) to the sewer system, by climatic conditions (i.e. dry weather versus wet weather) and also operation of the primary settler (i.e. surface loadings, use of precipitants). Secondary sludges are influenced by the type of the secondary process employed (i.e. activated sludge versus trickling filters or extended aeration activated sludge versus conventional activated sludge) and by the operation of the secondary processes (i.e. solids residence time (SRT) in activated sludge). These will impact on the quantity of sludges produced and their biodegradability. For example, short SRTs generally produce more sludge that is more readily degradable.</p> <p>It must therefore be recognized that the subsequently described pre-treatment options for enhancing biogas production at municipal wastewater treatment plants will be impacted by the sludge properties and direct comparison of options may be difficult. Hence, the impact of implementing a pre-treatment option on biogas generation will vary from plant to plant and may also vary temporally. 3.3 3.3.1 Pre-treatment Options Microsludge Process</p> <p>Microsludge (US Patent No. 6,013,183, international patents pending) is a chemical and pressure pre-treatment process that significantly changes both the rate and the extent that waste activated sludge (WAS) is degraded in a conventional mesophilic anaerobic digester. The patented process uses alkaline pre-treatment and an industrial scale homogenizer to provide an enormous and sudden pressure change to burst the cells. The resulting liquefied WAS is readily degraded in an anaerobic digester to form methane and carbon dioxide. 4</p> <p>ATAU Course Notes Anaerobic Digestion in Canada</p> <p>Conventional municipal wastewater treatment typically involves mesophilic anaerobic digestion of both primary solids and secondary solids (WAS) from aerobic biological treatment to produce methane and carbon dioxide. The rate limiting step is digestion of the WAS. The rate-limiting step for anaerobic digestion of WAS is the destruction of the cell membrane of each microbe (Parkin and Owen, 1986). Anaerobic digestion of WAS is both slow and incomplete because the individual cell membranes are not significantly degraded in conventional mesophilic (35 C) anaerobic digesters that rely on enzymes to promote cell lysis. Anaerobic digestion of WAS without pre-treatment falls short of an ideal biosolids management system for the following reasons: 1. Large quantities of undigested sludge still require disposal. 2. Partially digested sludge generates offensive odours and greenhouse gases. 3. Incomplete pathogen kill necessitates additional sludge processing before biosolids are safe to use as a fertilizer. 4. Undigested sludge is a wasted resource since the methane generation potential is not fully realized. Description of Microsludge Process Operations The Microsludge process (Stephenson and Dhaliwal, 2000) utilizes alkaline pre-treatment to weaken cell membranes, mechanical shear to reduce particle size, a self-cleaning screen to remove oversize debris, and an industrial scale homogenizer to provide an enormous and sudden pressure change to burst or lyse the cells. Figure 1 illustrates how Microsludge can be integrated into a WWTP. The heart of the Microsludge process is an industrial scale homogenizer that provides a large and abrupt pressure drop. At 12,000 psig (82,700 kPag), WAS in the cell disruption homogenizing valve is accelerated up to 305 meters per second in about 2 microseconds. This high velocity flow then impinges on an impact ring, disrupting the cell membranes and producing a liquefied WAS homogenate.</p> <p>ATAU Course Notes Anaerobic Digestion in Canada</p> <p>5</p> <p>Figure 1: The Microsludge process (Stephenson and Dhaliwal, 2000)</p> <p>Table 1: Optimum Microsludge Operating ConditionsPROCESSING STEP Chemical Pre-treatment OPTIMAL SETTING 1.</p>