Resource consumption and process performance of wastewater treatment plants
14 March 2017
Wastewater treatment is a resource-intensive process that utilises several inputs, such as energy, chemicals and water, to produce an effluent that meets designated environmental standards. Driven by environmental regulations, the focus of wastewater treatment plants (WWTPs) has traditionally been the quality of the effluent and not necessarily the energy or resource efficiency of the plant.
Regulations and penalties provide incentives to meet environmental effluent standards; however, to date, there are no such analogous penalties or incentives to expedite a focus on resource efficiency. It is imperative to recognise that resource utilisation and, indeed, sludge management also have significant environmental consequences, and therefore WWTP performance should be viewed holistically.
This research, funded by the Irish Environmental Protection Agency, sought to address this challenge by adopting a multi-pronged approach to audit and benchmark the resource efficiency of Irish WWTPs, including the use of key performance indicators (KPIs), life-cycle analysis (LCA) and exergy analysis (Figure 1).
Ten representative Irish WWTPs were audited in detail. The plants varied in scale, with regard to their design capacities (which were quantified in terms of units of population equivalent [PE]), from 600 PE to 186,000 PE.
Simultaneous energy and resource consumption and water quality audits were undertaken, resulting in the development of benchmarking tools and auditing methodologies, and the detailed performance evaluation of the plants in order to support better resource management and to provide baseline data on the holistic performances of the WWTPs.
This work involved several key considerations:
- The selection of representative plants;
- The development of appropriate auditing and benchmarking methodologies;
- Issues surrounding data availability and data accuracy within a WWTP, particularly flow data;
- The identification of essential data requirements for each of the individual benchmarking and auditing processes and the subsequent development and implementation of data acquisition strategies; and
- The determination of metrics that allow a fair comparison across WWTPs despite the many variables, such as scale, influent characteristics, discharge requirements, technology and nutrient removal requirements, that exist.
Summary of key findings and outcomes
It was found that resource consumption varied significantly, both across the range of audited WWTPs and as a result of the chosen metric (Figure 2). In general, economies of scale were evident, with the larger WWTPs consuming less energy per cubic metre of treated wastewater and per unit mass reduction in pollutants.
The research demonstrates that the performance of a WWTP is a function of many complex variables, and, therefore, assessing plant performances over a range of metrics can provide more holistic insights into potential optimisation strategies. Specific comparisons between two WWTPs, which were of a similar scale and use similar technologies, demonstrated that the energy consumed in the WWTPs (WWTP E and F) averaged 1705 kWh/day and 1451 kWh/day respectively (WWTP F consuming 14.9% less energy).
However, when the metric compared was energy consumption per unit 5-day biochemical oxygen demand (BOD5) removed, these energy consumption was 4.68 kWh/kg BOD5 for WWTP E and 7.3 kWh/kg BOD5 for WWTP F. Thus, WWTP F was found to consume 56% more energy per unit BOD5 removed when compared to WWTP E.
It should be noted that there were significant differences in the quality of the influent for these plants, which highlights the complexity of comparing and benchmarking WWTP performance.
Energy audits and life cycle assessment
While energy monitoring can be expensive and can require significant calibration and maintenance, detailed energy audits provide accurate baselines for energy management and optimisation. Furthermore, they can highlight and pinpoint specific issues that may otherwise go unnoticed, may be relatively easy and inexpensive to remedy and can have very short pay-back times.
The energy audits highlighted, in some cases, poor power factors, blower control issues (e.g. switching from automatic to manual, which results in increased and unnecessary energy consumption), equipment breakdowns and poor equipment reliability. Addressing these issues can have short payback times and can, in some cases, be relatively simple.
Other important issues that became apparent throughout this project were related to flow measurement and the determination of appropriate sampling regimes. For example, some WWTP flow meters were not installed in the correct locations, some were impeded by other upstream and downstream WWTP elements and some were infrequently calibrated.
The life-cycle impact assessment (LCIA) methodology used in the study is the CML (Centre for Environmental Science) 2001 (Nov. 10), which is compliant with the ISO 14040 series pertaining to life cycle assessment (Figure 4). The LCA studies confirmed the importance of assessing WWTP performance holistically and reinforced two important considerations that are often overlooked in the assessment of WWTP environmental performance: (1) the energy required to operate WWTPs; and (2) the management of the sludge produced by plants.
A software tool to assist WWTP benchmarking and performance management was developed and tested: Key Performance Indicator Calculator (KPICalc). KPICalc is easily accessible, highly automated and suitable for implementation in WWTPs with varying treatment processes, PE capacity, staffing numbers and resource consumption. In addition, this toolkit can assist stakeholders with the identification of faults in data acquisition methods, offers users an incentive for improving data acquisition methods and is flexible in terms of data availability (Figure 4).
Summary of key recommendations
This study showed that the performance of WWTPs is a function of many variables, including some that the plant manager has little control over, such as influent concentrations and discharge requirements. Therefore, common, simple benchmarking metrics, such as kWh/m3 or kWh/PE, are unlikely to allow fair comparisons across plants.
Similarly, energy audits or water quality testing alone are not sufficient for comprehensive audits and benchmarking plant performance. Effectiveness and efficiency should not be considered separately, and the ultimate goal should be to operate WWTPs that are both effective and efficient.
In general, this is best achieved at the design phase, during which the longer term life-cycle costs and performance of the WWTP can be anticipated and optimised, rather than by solely focusing on the initial capital costs. The key recommendations are as follows:
- Assess plant performance using multiple criteria and key performance indicators;
- Design for efficiency at the outset, including the provision of adequate data management tools;
- Specify and provide adequate monitoring instrumentation and equipment;
- Adopt a holistic approach to the evaluation of environmental performance;
- Introduce and implement preventive maintenance schedules for process equipment, and ensure that monitoring equipment is calibrated regularly;
- Review power factors and control strategies regularly;
- Ensure minimum data requirements are in place to allow for efficient implementation of benchmarking, life cycle analysis and other management tools. Such analyses can then, in turn, identify significant operational efficiencies.
Intersectoral and multidisciplinary projects of this nature can increase understanding of the challenges faced to increase the resource efficiency of wastewater treatment in Ireland and the tools developed can inform process and operational efficiencies. This research complements and supports existing and future work to improve WWTP efficiency.
The authors acknowledge the contribution of the Project Steering Committee, comprising John Gray (consultant), Thomas Griffin (DECLG) and Dr Edmond O’Reilly (Irish Water). The authors also acknowledge the environmental assessment and testing work carried out by Niall Durham. Finally, the authors would also like to gratefully acknowledge the support of and the information provided by various local authorities and wastewater treatment plant operators without whom this research would not have been possible.
The authors would like to acknowledge the EPA for its financial support (EPA Grant Numbers 2012-W-MS-10 and 2014-W-DS-16). Further details on this study are available on the SAFER-Data section of the EPA website. The results of a follow-up project (2014-W-DS-16) which focuses on resource efficiency in small-scale WWTPs will be published towards the end of the year. This publication will contain details of life cycle and performance benchmarking assessment tools developed over the course of these studies. The project team will present the various tools developed at an event at the end of this project.http://www.engineersjournal.ie/2017/03/14/process-performance-irish-wastewater-treatment/http://www.engineersjournal.ie/wp-content/uploads/2017/03/Wastewater-treatment-1024x580.jpghttp://www.engineersjournal.ie/wp-content/uploads/2017/03/Wastewater-treatment-300x300.jpgChemEPA,Irish Water,wastewater,water