Stringent emission limits, population growth and increasing urbanisation continue to drive the advancement in wastewater treatment (WWT) technologies and waste management frameworks. Today in Ireland, over 96% of the sludge generated during WWT is spread on agricultural land; however, restrictions set by agricultural quality assurance schemes are encouraging the search for new alternatives. Plants with a capacity greater than 100,000 population equivalent (p.e.) can implement anaerobic digestion (AD) as a means of sludge treatment and energy recovery. Pilot and WWT plant scale studies have reported biogas yields between 4 and 10 GJ t-1 (1-3 kWh kg-1) of dry sludge through AD of municipal sewage sludge (Qiao et al., 2011). However, large scale biogas plants can consume about 40% of this energy for their operation, diminishing energy efficiency (Berglund and Börjesson, 2006). Thermal technologies for the conversion of either sewage sludge or digestate represent a potential route for both sludge volume reduction and energy recovery. In particular, sludge combustion and/or gasification could provide either thermal or chemical energy for combined Heat and Power (CHP) generation and could be readily integrated with WWT plants.
This project explores the state-of-the-art of combustion (incineration) and gasification technologies used for biomass and waste conversion. This study evaluates not only the technical performance of these technologies, but also investment and operational costs, and waste generation, treatment and valorisation through recovery of materials and chemicals. Using a pseudo-thermodynamic approach for modelling thermal conversion, the performance of combustion and gasification of sludge and digestate was evaluated under various operational conditions and solid material properties (moisture, composition). The model evaluated the technical performance of the thermal conversion processes, as well as the integration of energy carriers for power generation and heat recovery through different available technologies, such as steam and gas turbines, and combustion engines. In order to support local and governmental authorities in the consideration of these alternatives, different techno-economic indicators, including energy recovery efficiency, treatment costs, levelised cost of electricity generation and carbon footprint of the operation of the WWT-sludge management plant were included and thoroughly compared among the different potential process alternatives.
Gasification and AD-gasification integrated with CHP generation was technically feasible and offered means to reduce final waste disposal costs and improve energy efficiency of the WWT plant. The most efficient process concept for energy recovery used internal combustion engines to generate power from energy carriers produced from gasification and AD-gasification, i.e. biogas and syngas. Conditions under which electricity generation was maximised were met with extensive sludge pretreatment, i.e. drying, and thus with low heat recovery efficiencies. On the other hand, AD integrated with gasification gave a greater thermal recovery flexibility leading to conditions in which net surpluses of both electricity and heat were achieved. The combination of AD and gasification offered competitive costs of electricity generation (20-50c kWh-1), with low carbon footprint (<300 kg CO2 t-1 dry sludge). Internal combustion engines offer great flexibility and competitive power efficiencies at typical scales for energy recovery in WWT facilities (>10 MWe).
When gasification was used as the only sludge conversion additional energy for heat generation was required in the process. This additional heat was proposed to be generated via co-processing with renewable solid fuels and wastes, i.e. biomass, animal slurries, organic-fraction of municipal solid waste. Biomass rates between 0.8 and 1 times that of the sludge feed rate were required to meet energy demands at a reduced carbon footprint.
It is important to emphasise that the scale of the facility is vital in meeting sustainability criteria, especially in terms of operational and capital expenditures. The evaluation presented in this study was applied to the largest WWT scale in Ireland (1.6 Mp.e.), such as that of the Ringsend WWT plant, which currently produces approx. 85 t d-1 (tpd) of dry sludge (digestate). However, most existing anaerobic digestion facilities have capacities between 40,000 and 150,000 p.e., with sludge generation rates between 20 and 100 dry tpd, depending on the influent wastewater. Combustion engines offer sufficient flexibility to operate with the power capacities expected for these scales (>100 kWel). However, costs of installation can make the implementation of gasification challenging at small scales. Raw sludge generation rates under 130 tpd led to levelised costs of electricity generation slightly above the national costs fossil-based electricity (24c kWh-1). However, gasification and AD-gasification as sludge treatment was economically competitive with generation rates above 25 tpd. This scale challenge may still be overcome through other approaches that are suggested for future research. On-site thermal pretreatment can facilitate sludge transportation to a centralised facility, where energy recovery may offset overall treatment costs in terms of energy and carbon footprint. A centralised gasification facility would offer the possibility of implementing biomass and/or waste co-processing with greater economic and technical efficiencies, while reducing operational challenges. It is also important to note that sludge transportation and biomass co-processing will have additional energy penalties, transportation costs and carbon footprint, which must be taken into account in the evaluation of an optimal sludge transportation and treatment network at a county or national level. Biomass has direct and indirect carbon emissions linked to harvesting, use of fertilisers, land use change, importation, and transportation that were not considered in the present study. These may affect the carbon footprint of large scale plants with high biomass-to-sludge co-processing ratios.