Self-sufficient wastewater treatment: energy recovery from sewage sludge
29 August 2017
One of the most common practices for sewage sludge disposal involves its application on agricultural land. Over 80% of the sludge generated during WWT is spread on agricultural soil in Ireland . However, EU countries such as Austria, Germany, Switzerland and the Netherlands currently incinerate over 36% of the sewage sludge as an alternative to tackle land availability and avoid soil pollution (Figure 1).
The presence of pharmaceutical and personal care products, metals and pathogens within sewage sludge may contaminate soil and water bodies and end up affecting food products – and, ultimately, human and animal health . The current EU Sewage Sludge Directive 86/278/EEC regulate the utilisation of these biosolids in agriculture by mandating biological, chemical or heat treatment and limiting application rates to comply with metals uptake in soil.
It is important for the WWT sector to find viable alternatives to agricultural landspreading that could also improve energy efficiency of the process chain of waste management. Plants of >100,000 population equivalent (p.e.) of capacity have the cost-efficient opportunity of using 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) dry sludge through AD of municipal sewage sludge .
Biomass and waste conversion
However, biogas plants have large heat demands for AD operation and thermal treatment of the digestate residue before disposal/final use, consuming over 40% of the biogas energy . Thermal technologies for the conversion of either sewage sludge or digestate, i.e. combustion and gasification, are potential routes for waste volume reduction and energy recovery on-site.
This research, funded by the Environmental Protection Agency of Ireland (EPA), explored the state-of-the-art of combustion (incineration) and gasification technologies used for biomass and waste conversion and evaluated their potential in reducing the energy and carbon footprint of WWT plants in Ireland.
The study focused on a WWT plant of the scale of the Ringsend facility in Dublin (1.6 Mp.e.) assuming a daily generation of 130 tonnes of dry sewage sludge . Eight scenarios were considered using different technologies including either thermal conversion and combined heat and power (CHP) generation, or combined anaerobic digestion with thermal conversion prior to CHP generation (Figure 2).
The energy efficiencies and economic performance factors of each scenario were determined using thermodynamic modelling tools and heuristic knowledge of the technologies. The energy recovery technologies were compared on the basis of several factors, including electricity and heat generation efficiencies of the system, the coverage of electricity and heat demands of the operation of the WWT and sludge treatment facility (Cel, Chr), and the levelised costs of treatment and electricity generation (COT, COE).
The reference WWT facility was assumed to have an annual energy footprint of 26 kWh per p.e. (electricity), in addition to heat and power demands of the operation of anaerobic digestion units and the thermal conversion and CHP components of the scenarios.
Various operational conditions were varied simultaneously within typical ranges of performance to evaluate their effect on the performance indicators as well as overall performance between the different scenarios.
Key findings and outcomes
The factors of electricity and heat demand coverage can offer an idea of the relative performance of each scenario prior to optimisation of each configuration (Figure 3). The scenario involving the combustion of sewage sludge or digestates (Scenarios 1 and 5) and the recovery of energy in a steam turbine could cover only 25-45% of power demands on site. Nevertheless, a sludge conversion facility of this type can potentially generate enough energy to heat circa 5,000 household units, following adequate heat integration of the systems.
Scenarios 2 and 6 which involved gasification to generate gas fuel (syngas) to feed a boiler led to lower power generation and heat recovery efficiencies than the corresponding combustion systems (Scenarios 1 and 5), due to the additional energy requirements of the gasification reactor.
A gas turbine system using syngas as fuel (Scenarios 4 and 8) could theoretically provide larger power efficiencies and coverage than the previous scenarios. Anaerobic digestion combined with gasification and gas turbines for electricity generation could cover electricity and heat demands on-site in excess by up to 20 and 75%, respectively.
However, this technology at the scale studied herein presents disadvantages due to intensive auxiliary energy demands, leading to prohibitive costs of electricity generation (>50c kWh-1). Furthermore, gas turbine technologies today are still limited to large scales (>100 MWel) in the use of fuels with low energy contents, such as syngas and combinations of syngas-biogas, so future development is still needed to adapt this system at medium to small sites.
The use of gas turbines in waste-to-energy facilities can be suitable for co-processing of high volumes of waste and/or biomass (>500 tpd).
Systems that used a combustion engine for electricity generation using syngas as fuel (scenarios 3 and 7) were the most efficient and technically feasible energy recovery approaches found in this study.
However, gasification as the sole sewage sludge management process offered a smaller range of conditions under which both electricity and heat demands of the WWT and sludge management facility were fully met. Conditions under which electricity generation is maximised (surplus of 15-35%) required extensive sludge pretreatment, i.e. thermal drying, and thus they were associated with low heat demands coverage (<100%).
Nevertheless, an integrated system of anaerobic digestion and gasification gave larger thermal recovery and power generation efficiencies leading to complete coverage of both electricity and heat under all operational conditions explored (Scenario 7). These scenarios also gave the lowest operation costs (€150-170 t-1 dry sludge) and electricity generation costs (20-50c kWh-1). Furthermore, internal combustion engines are more flexible and efficient for small scales as expected for decentralised energy recovery systems in WWT facilities (>10 MWel).
Co-processing of other organic-fraction wastes and/or biomass could potentially reduce the specific capital investment of these projects and increase the overall energy balance by addressing the limitations of energy efficiency and economic feasibility associated with process scale.
Furthermore, co-processing has also the advantage of reducing operational challenges due to seasonal variations in sludge generation and its properties by offering a steady renewable alternative fuel supply.
For the case study, it was found that processing 130 tpd of dry biomass, such as energy crops (e.g. Miscanthus) or other organic wastes (e.g. poultry litter, forestry residues), in addition to the nominal sludge capacity (130 tpd dry sludge), could neutralise the operational carbon footprint of the WWT plant and sludge treatment facility, as well as it could reduce the levelised cost of electricity generation by 8-35%.
Facility scale is one of the most important aspects in meeting feasible operational and capital expenditures. The largest WWT facility in Ireland (circa 1.6 Mp.e.) currently produces about 50 tpd of dry digestate or approximately 80-100 tpd dry sludge. However, most WWT facilities with secondary or biological treatment in Ireland have capacities between 10,000 and 50,000 p.e., producing between 3 and 15 tpd of dry sludge.
At these scales, operational costs and levelised costs of electricity become prohibitive. Even though combustion engines and boilers are flexible to operate with small nominal capacities (100-700 kWel), operational costs and levelised costs of electricity generation will make the implementation of these technologies undesirable.
Sludge incinerators process typically 30 to 700 tpd, depending on the reactor configuration, e.g. fluidised beds operate at 30-200 tpd, moving grates at 120-700 tpd. On the other hand, gasifiers are restricted to throughputs above 250 tpd .
Strategies to improve the economy of scale of sewage sludge-to-energy facilities may rely on decentralised conversion with on-site thermal treatment and collection. Thermal drying and/or torrefaction of sewage sludge on site can reduce waste volume and improve solid fuel properties for transportation to a centralised facility where the waste can be converted to energy under more efficient conditions.
Electricity generation and heat recovery at a centralised site could potentially offset the energy and carbon footprint of the decentralised treatment and collection facilities. Future endeavours should look at feasibility and design of a decentralised treatment and collection scheme of sewage sludge and other wastes and the effects that direct and indirect energy penalties may have on costs of implementation and carbon footprint of the waste management system.
Currently, various members of our research team are involved in the bioenergy research area of the Centre for Marine and Renewable Energy (MaREI) dedicated to the design of a sustainable Irish biomethane production system relying on centralised thermal conversion sites processing organic wastes, such as forestry residues.
Within this project, resource assessment and the economic and environmental footprint of a national waste collection scheme are vital components of the design of sustainable indigenous bioenergy products in Ireland.
Karla Dussan and Rory Monaghan, Mechanical Engineering Department, School of Engineering and Informatics, National University of Ireland Galway
The authors acknowledge the EPA for its financial support (Grant Number 2014-RE-DS-3) and the contributions of the Project Steering Committee members, Fiona Lane (Irish Water), Mick Henry (EPA Ireland), Aisling O’Connor (EPA Ireland) and Eamonn Merriman (EPA Ireland). Further information of this project can be found on the SAFER-Data section of the EPA website and on the Therme Research Group’s website. For more information on the ongoing research projects within MaREI Research Centre, visit Therme Research Group’s website.
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