Estudo de avaliação e optimização do potencial de biogás das lamas mistas da ETAR de Chelas

Mestrado em Engenharia do Ambiente - Instituto Superior de AgronomiaIn the last few years, the number of municipal wastewater treatment plants (WWTP) in Portugal increased significantly resulting in the production of large quantities of “mixed sludge” (primary sludge + activated sludge) that should undergo stabilization. Aerobic stabilization of “mixed sludge” has been shown to cause poorer dewatering properties and, at the same time, increases the biopolymer needs. Anaerobic digestion of sewage sludge has been applied to WWTP for decades. It is a well known, efficient and environmentally sustainable technology which enables energy production as heat, electricity and/or vehicle fuel, as well as, stabilization and volume reduction of sludge. The management and handling of wastewater sludge is a global challenge and represents for most of the wastewater treatment plants a considerable cost. The solid part of wastewater sludge consists in general of 60 to 70% of organic matter; the rest is inert or inorganic. The organic matter in the sludge binds large parts of the water, resulting in increased sludge volumes. Many wastewater treatment plants in Portugal (serving more than 40 000 inhabitants equivalents) use an anaerobic digestion for stabilization of the generated solids, i.e., to degrade the organic matter and to minimize the sludge volumes. On the other hand it is important to increase the production of renewable energy. Typical sewage sludge comprises of primary sludge separated from wastewater during pre-settling and biological excess sludge from the activated sludge system. Characteristics of sewage sludge differ somewhat in different countries and areas e.g. due to water consumption and local industry. Total solids (TS) content is usually low and sludge volume high unless some of the water is removed prior to sludge treatment. Biological stabilization of sludge aims at degradation of volatile solids (VS), the organic content of the sludge, and subsequent decrease in sludge volume. Moreover, nitrogen and phosphorous contents are important, especially when the stabilized sludge is being reused as fertilizer or soil improver. Sewage sludge contains easily biodegradable materials and it is typical methane production potential is approximately 300–400 m3/ t VS added. Lipid-rich materials are known to have high methane production potentials, but their degradation products, long-chain fatty acids (LCFA), may be severely inhibitive to methanogenesis. The inhibition was long assumed irreversible, but lately it has been proved reversible with increasing consumption of acetate and butyrate indicating the recovery. Moreover, when digesting lipid-rich materials, a lag phase is usually noticed between LCFA and methane production. Grease trap sludge comprises of greasy materials separated from wastewaters of e.g. restaurants and institutional kitchens. Composting may be applied for grease trap sludge, but its high moisture content makes it technically unsuitable. Anaerobic digestion offers a better option as it is able to treat wet materials. Digestion of grease trap sludge alone is not feasible due to relatively small amounts produced and high likelihood of LCFA inhibition.In the last few years, the number of municipal wastewater treatment plants (WWTP) in Portugal increased significantly resulting in the production of large quantities of “mixed sludge” (primary sludge + activated sludge) that should undergo stabilization. Aerobic stabilization of “mixed sludge” has been shown to cause poorer dewatering properties and, at the same time, increases the biopolymer needs. Anaerobic digestion of sewage sludge has been applied to WWTP for decades. It is a well known, efficient and environmentally sustainable technology which enables energy production as heat, electricity and/or vehicle fuel, as well as, stabilization and volume reduction of sludge. The management and handling of wastewater sludge is a global challenge and represents for most of the wastewater treatment plants a considerable cost. The solid part of wastewater sludge consists in general of 60 to 70% of organic matter; the rest is inert or inorganic. The organic matter in the sludge binds large parts of the water, resulting in increased sludge volumes. Many wastewater treatment plants in Portugal (serving more than 40 000 inhabitants equivalents) use an anaerobic digestion for stabilization of the generated solids, i.e., to degrade the organic matter and to minimize the sludge volumes. On the other hand it is important to increase the production of renewable energy. Typical sewage sludge comprises of primary sludge separated from wastewater during pre-settling and biological excess sludge from the activated sludge system. Characteristics of sewage sludge differ somewhat in different countries and areas e.g. due to water consumption and local industry. Total solids (TS) content is usually low and sludge volume high unless some of the water is removed prior to sludge treatment. Biological stabilization of sludge aims at degradation of volatile solids (VS), the organic content of the sludge, and subsequent decrease in sludge volume. Moreover, nitrogen and phosphorous contents are important, especially when the stabilized sludge is being reused as fertilizer or soil improver. Sewage sludge contains easily biodegradable materials and it is typical methane production potential is approximately 300–400 m3/ t VS added. Lipid-rich materials are known to have high methane production potentials, but their degradation products, long-chain fatty acids (LCFA), may be severely inhibitive to methanogenesis. The inhibition was long assumed irreversible, but lately it has been proved reversible with increasing consumption of acetate and butyrate indicating the recovery. Moreover, when digesting lipid-rich materials, a lag phase is usually noticed between LCFA and methane production. Grease trap sludge comprises of greasy materials separated from wastewaters of e.g. restaurants and institutional kitchens. Composting may be applied for grease trap sludge, but its high moisture content makes it technically unsuitable. Anaerobic digestion offers a better option as it is able to treat wet materials. Digestion of grease trap sludge alone is not feasible due to relatively small amounts produced and high likelihood of LCFA inhibition.Most often, sewage sludge is digested alone, though co-digestion with other substrates could be beneficial regarding the increase biogas production and income through gate fees. Anaerobic co-digestion is reported to offer several benefits over stand alone material digestion, such as, increased cost-efficiency (one plant for several materials), increased degradation of the treated materials due to synergistic effects, optimization of moisture and nutrient content. It also promote the dilution of inhibitive compounds, such as ammonia and degradation products of lipids, as well as increased biogas production. As it may also offer acceptable alternative to current disposal strategies because it reduces volume of “mixed sludge” stabilized producing a residue that can be use for soil conditioning. This study was performed to examine the feasibility of mesophilic anaerobic co-digestion of municipal sewage sludge and the grease trap sludge removed from the pre-treatment of the wastewater treatment plant. This issue is of primary importance considering that carbon addition (“grease trap sludge removed”), performed to allow more efficient nutrient removal, accounts to a significant extend for the cost in wastewater treatment plants. With this approach the organic fraction of “the fat removed” is diverted from landfill disposal and can be used as a carbon source to enhance the biogas production. The co-digestion of sludge from grease trap and sewage sludge was performed in laboratory “batch” reactors. The addition of 5% of organic matter from grease trap sludge to sewage sludge digesters was seen to increase the biogas yield of 4 times more