Wastewater treatment plants (WWTPs) are a key user of energy (electricity, gas etc) and resources in both the public and private sectors. Despite recent advances in sensor and control technology across many industries, many WWTP processes, including aeration, pumping, and mixing, still operate at a constant rate with limited control.
Such systems can often be at a basic stage of evolution in terms of control capability and efficiency, resulting in inefficient energy and resource consumption. It is well recognised that the automation of WWTP monitoring and control using sensors can optimise performance by improving operations, reducing energy consumption, and enabling desired effluent standards to be met reliably and efficiently.
Real-time control (RTC) or quasi-RTC (eg control of individual treatment cycles) offers a means of improving resource efficiency; however, it is broadly underused in operational WWTPs (1).
Why do we need to save energy at WWTPs?
Water and wastewater treatment are essential elements of global energy consumption. In Europe, WWTPs contribute to about 1% of the total electricity consumption in cities (2, 3).
Electricity usage could rise by more than 680 TWh between now and 2030 if cities adopt the conventional technological blueprint for centralised wastewater capacity.
If cities instead deploy a range of economically viable energy efficiency technologies in all new wastewater facilities, the increase in electricity consumption could be reduced by roughly 10% (4).
The existing plants feature a broad variability of electric energy use. Shifting the least efficient plants to an average level of efficiency could save 5,500 GWh annually, while compliance with the standards of the most efficient plants would save 13,500 GWh annually (5).
In Ireland, the national utility for water and wastewater services accounts for 22% of all electricity consumption in the public sector (6). As a public sector body, Uisce Éireann has committed to improve energy efficiency by 50% compared to its 2009 baseline.
By 2020, it achieved a 33% improvement in its energy efficiency performance, equating to a cumulative saving of more than 212 GWh of primary energy: enough energy to power more than 50,000 homes for an entire year, saving an equivalent of more than 120,000 tonnes of carbon (6).
Globally, WWTP energy usage is predicted to rise by 60-100% to meet environmental standards over the next 15 years (7). Due to several variables, population growth and the need to connect more global populations into clean water and sanitation to align with SDG6 objectives – along with rising standards for effluent discharge – energy demand for wastewater services is expected to continue to rise globally.
Energy-saving strategies
Wastewater treatment compliance is the priority of a WWTP, but energy efficiency is also essential. A delicate balance exists in managing wastewater compliance needs with energy efficiency and emissions goals.
By not providing enough resources, even a well-designed WWTP will not achieve discharge compliance. Meanwhile, using excessive energy can ensure that discharge compliance is achieved, the environmental costs of this excessive energy usage may potentially outweigh the benefits of discharge compliance.
Furthermore, if a process is not controlled effectively, it may be possible to overconsume energy and still not meet discharge compliance. For many WWTPs the current best-case scenario is that discharge compliance is achieved, and a certain amount of excess energy consumption is accepted. However, by providing advanced process controls, it may be possible to improve this best-case scenario. Figure 1 summarises the various scenarios.
Figure 1: Balancing energy consumption, process control and discharge regulations.
Uisce Éireann reported proposed energy-efficiency projects (applicable to both the water and wastewater sector) to the Sustainable Energy Authority of Ireland (SEAI) in its 2020 energy report (6). A breakdown of these projects, and the estimated savings, as total final consumption (kWh TFC), is shown in Figure 2.
Figure 2: Breakdown of proposed energy-efficiency projects in Irish Water (6)
Process optimisation (which would include process control) and energy efficient design are defined as categories in Irish Water’s report (shown in purple above and yellow), equating to more than 10% and 30% of the expected energy savings.
RTC benefits and challenges
Wastewater treatment plants can employ RTC by using physical or virtual sensors to monitor wastewater quality, equipment, and other system parameters to enhance treatment outcomes and system operation.
Activated sludge treatment (including pumping, mixing and aeration) is a key process that has the potential to benefit greatly from more advanced control due to it typically being the largest energy user in a WWTP.
Many WWTPs are designed to rely on basic RTC systems such as dissolved oxygen (DO) monitoring and ramping up/down of blowers to achieve DO set points.
However, substantial developments in multiparameter sensors, instrumentation, and automation provide an opportunity to increase energy efficiency even further by employing intermediate or more advanced RTC methods, without jeopardising discharge compliance.
Innovation in the wastewater sector has been hindered by long-standing worries about licensing non-compliance and a lack of faith in instrumentation, control, and automation (ICA) ADDIN EN.CITE.DATA (8). However, current advances in ICA has made it possible to choose more advanced controls systems (9, 10).
Stages of real-time control
In general, RTC approaches can be divided into three categories namely, basic, intermediate, and advanced RTC.
Basic methods
Basic RTC methods are conducted without the use of advanced numerical modelling methods. Basic RTC includes closed-loop control systems that have the flexibility to change some process parameters to maintain predefined conditions. Manual intervention is still prominent at this stage.
Figure 3 depicts some examples of basic RTC options for WWTPs. Using DO set-points to regulate the aeration of activated sludge is a frequent application of basic RTC (8).
Figure 3. Examples of Basic RTC.
Intermediate methods
Intermediate methods of RTC can involve upgrading equipment that is already in use, enhancing data collection and analysis and employing additional RTC (eg) intermittent aeration (Figure 4).
Based on the specifics of the WWTP, the installation of new plant (eg aeration and pumping equipment) that utilises RTC could achieve a payback period of three to 10 years (11).
In addition, the savings that can be realised by increasing efficiency can allow initial investments to be recovered faster than previously predicted given current energy costs.
After the payback has been reached, the savings will last for years, allowing finances to be invested in other areas of improvement and allowing the treatment plant to meet future demands (12). Intermediate methods also assess WWTP loading and adjust some aspects of operation to optimise treatment and energy/resource efficiency.
Figure 4 Examples of Intermediate RTC.
Advanced RTC methods
Advanced RTC provides a highly automated level of process control, with manual intervention in specific cases. By installing advanced RTC capabilities in the aeration process, the system will be managed by real time sensors and controllers which can monitor variables like DO, temperature, ammonium, oxidation reduction potential, pH, energy, etc, thus increasing WWTP performance and capacity (Figure 5).
Figure 5 Examples of Advanced RTC.
Advanced RTC methods presents a further step towards the intelligent use of data (13). Advanced RTC can be made possible by the diversity of parameters sensors that are currently available in the market and the inclusion of operational processes such as feed forward and feed backward control, ammonium-based control, full secondary process system integration, and dynamic loading.
To deliver the appropriate level of treatment, it is important to design plants to meet the maximum expected treatment capacity with an additional safety factor and incorporate a process control system.
In addition, RTC allows us the assurance to systematically depart from the conventional strategy and optimise performance during periods of changing flow or pollution concentrations.
Other examples include detailed numerical models of WWTPs to enable scenario analysis and concepts such as demand response, which can both save energy and reduce energy costs in aligning WWTP operation with electricity utilities.
Demand response can improve system operation, planning, and economic efficiency by reducing the variability of the system's marginal costs. Customers are effectively introduced to the idea of demand response through a time-based pricing structure, such as time-of-use tariffs, peak demand charges, real-time pricing, and extreme day pricing.
Time-of-use rates, which are enabled by smart meter data to move demand consumption away from peak hours when electricity prices are highest, are anticipated to incentivise large energy consumers. (14-17).
Case studies
At a small Midwestern municipal WWTP in the US serving 16,000PE, including the town’s large local food processing industry, operators had to meet a 1.0 mg/L total phosphorous limit.
To achieve this, operators were manually adjusting the ferric feed based on results from samples taken during the previous week. The result was, they were treating the previous week’s phosphorus load, not the current one.
After deploying a phosphorus controller RTC101P Chemical Phosphorus Optimisation Solution, the WWTP was able to cut its average ferric chloride feed from 0.0002 m3/h to 0.0009 m3/h, a 56% reduction. The annual ferric chloride cost savings amounted to about €114,262, resulting in a payback period of less than one year (18).
In another study, Jiaxing Honghe Sewage Treatment plant in China that treats 20,000 m3/d, which includes effluent from printing and dyeing analysed impacts of equipment and process upgrades to coagulant dosage.
By using an intelligent coagulant management system, the effluent total phosphorus concentration decreased from 0.5 mg/L to 0.3 mg/L and the stability of the suspended solid removal rate also increased. With automated multiparameter dosing, the treatment costs decreased from €0.025/kg to €0.017/kg (equivalent), saving 31.6% on coagulant use (19).
The Minnesota River Valley Public Utilities Commission (MRVPUC) provides wastewater treatment for the city of Le Suer and the city of Henderson in south-central Minnesota.
Its WWTP serves a domestic population of about 4,500 and the facility processes domestic and industrial wastewater at a rate of 3,785 to 5,678 m3/day, and it receives at least 45 kg of phosphorus each day was upgraded from stabilisation ponds to an extended air-activated sludge. Orthophosphate levels in the clarifier effluent were first continuously monitored owing to the initial installation of an online phosphate analyser.
The resulting data showed that the treatment stream can experience spikes of up to 0.60-0.64 m3/d of influent phosphorus and phosphate levels of up to 50% daily. An RTC platform for Phosphorus Removal (RTC-P) was installed.
The platform integrated fully with the in-place phosphate analyser and with the utility’s specific treatment control process. Based on post-treatment analyser measurements, the RTC-P uses a calculated feedback loop to adjust ferric chloride dosing as needed to maintain target effluent phosphate concentration reducing ferric chloride usage from 1.90- 2.30 m3/day to a level of about 0.76 -1.13 m3/day, and saved €3.15 per m3(20).
Introduction of the ACET method
The ACET project is a collaboration between the University of Galway and VorTech Water Solutions to develop a framework of control strategies that can be applied in a standardised/universal way to WWTPs.
The framework is built on three pillars: equipment provision, process design, and sensors and monitoring, as shown in Figure 6. The ACET project groups WWTP control methods into four tiers namely, tier 0 (no control), tier 1 (basic RTC control), tier 2 (intermediate), and tier 3 (advanced). On-site case studies will be used to show how varying levels of RTC can enhance WWTP performance and operational costs.
Figure 6 ACET Tier system.
The project will utilise life cycle assessment (LCA), benchmarking other analysis tools to consider not just the environmental impact of poor compliance, but also the environmental impact of energy inefficiency through carbon emissions. In this way, ACET can transform how WWTPs are designed and operated.
The potential benefits of ACET
The ACET framework concept has the potential to benefit WWTP operation in a variety of ways. The main benefits are reduced energy consumption and improved discharge regulation compliance.
However, additional benefits include how the ACET tier system can be implemented in operational WWTPs without significant process downtime or financial investment, which may make this approach a more favourable process upgrade measure than other options.
This is particularly evident when RTC is used in the activated sludge process to improve efficiency as RTC can offer a lower-cost alternative to replacing aeration equipment such as blowers and pipework, which not only has significant capital costs, but can also have significant lead times and/or requirements for lengthy and complicated process shutdowns.
Conclusion
What are the take home points?
Energy neutrality in WWTPs can be achieved through the following:
- Implementing RTC at facilities can reduce energy consumption in WWTPs.
- The ACET approach is a novel framework for control strategies which is divided into the three pillars: equipment, process, and monitoring. This article identifies that the integration of all three pillars into a novel, universal control framework may be a competitive option for accelerating the deployment of energy saving measures in many existing WWTPs.
Energy neutrality in WWTPs can be achieved through the following:
- Implementing RTC at facilities can reduce energy consumption in WWTPs;
- The ACET approach is a novel framework for control strategies which is divided into the three pillars: equipment, process, and monitoring. This article identifies that the integration of all three pillars into a novel, universal control framework may be a competitive option for accelerating the deployment of energy saving measures in many existing WWTPs;
- By offering a tiered approach, it is possible to overcome some of the challenges with implementing RTC by incrementally upgrading treatment facilities, in a capital efficient manner;
- By combining these approaches, WWTPs can minimise energy consumption, enhance performance, and strive towards energy neutrality while ensuring compliance with effluent standards.
Project website: https://www.universityofgalway.ie/acet/
Partner: https://vortechws.com/
References
WaterWorld. (2012). Examining Payback on New Generation High Speed Blowers https://www.waterworld.com/home/article/16193132/examining-payback-on-new-generation-high-speed-blowers [ Accessed 06/01/2023]. Journal of Environmental Chemical Engineering.
1) Wexglobal. How Automated Real-Time Controls Can Provide More Consistency in Wastewater Treatment Operations https://wex-global.com/how-automated-real-time-controls-can-provide-moreconsistency-in-wastewater-treatment-operations/. 2022.
2) Haslinger J, Lindtner S, Krampe J. Operating costs and energy demand of wastewater treatment plants in Austria: benchmarking results of the last 10 years. Water science and technology : a journal of the International Association on Water Pollution Research. 2016;74(11):2620-6.
3) Maktabifard M, Zaborowska E, Makinia J. Achieving energy neutrality in wastewater treatment plants through energy savings and enhancing renewable energy production. Reviews in Environmental Science and Bio/Technology. 2018;17(4):655-89.
4) Walton M. The energy sector should care about wastewater https://www.iea.org/commentaries/the-energy-sector-should-care-about-wastewater. iea. 2018;267:121989.
5) Ganora D, Hospido A, Husemann J, Krampe J, Loderer C, Longo S, et al. Opportunities to improve energy use in urban wastewater treatment: a European-scale analysis. Environmental Research Letters. 2019;14(4):044028.
6) SEAI. Irish Water Annual Energy Statement 2020. Sustainable Energy Authority of Ireland (SEAI https://psmrseaiie/Reports/PublicAnnualReportForPublic?customerId=4058&query=undefined [Accessed 20/12/2022]. 2020.
7) Walker NL, Williams AP, Styles D. Pitfalls in international benchmarking of energy intensity across wastewater treatment utilities. Journal of Environmental Management. 2021;300:113613.
8) IWA. Sewage Treatment Plants Stamatelatou K, Tsagarakis KP, editors: IWA Publishinghttps://iwaponline.com/ebooks/book/246/Sewage-Treatment-Plants [Accessed 06/01/2023]; 2015.
9) Fox S, Clifford E. Small-scale and decentralised wastewater treatment plant optimisation through real-time control. Journal of Applied Microbiology. 2018;124(2):336-67.
10) Vieira A, Marques R, Raposo R, Martins M, Alves R, Povoa P, et al. The impact of the art-ICA control technology on the performance, energy consumption and greenhouse gas emissions of full-scale wastewater treatment plants. Journal of Cleaner Production. 2019;213:680-7.
11) WaterWorld. Examining Payback on New Generation High Speed Blowers https://www.waterworld.com/home/article/16193132/examining-payback-on-new-generation-high-speed-blowers [ Accessed 06/01/2023]. 2012.
12) Sulzer. Tackling rising energy costs. https://wwwsulzercom/-/media/files/products/compressors-and-aeration/technical_articles/tackling-_rising_energy_costs_a10560pdf?la=en 2022.
13) Green So. Real-time combined optimisation of the sewer network and wastewater treatment plants. State of Green WATER MANAGEMENT. 2020;20:101168.
14) Casey E. Energy-saving UCD technology breathes life into wastewater treatment, https://www.engineersireland.ie/Brexit/energy-saving-ucd-technology-breathes-life-into-wastewater-treatment, https://www.ucd.ie/eacollege/t4media/Eoin-Casey.pdf 2018.
15) Kaan DR. Opportunities for Energy Savings and Generation in WWTPs https://esipp.ie/images/upload/2.%20Recep%20Kaan%20Dereli_online.pdf. 2021.
16) Palensky P, Dietrich D. Demand Side Management: Demand Response, Intelligent Energy Systems, and Smart Loads. IEEE Transactions on Industrial Informatics. 2011;7(3):381-8.
17) Liu Q, Dereli RK, Flynn D, Casey E. Demand response through reject water scheduling in water resource recovery facilities: A demonstration with BSM2. Water Research. 2021;188:116516.
18) White BDaM. How Automated Real-Time Controls Can Provide More Consistency in Wastewater Treatment Operations https://wex-global.com/how-automated-real-time-controls-can-provide-more-consistency-in-wastewater-treatment-operations/. Wexglobal 2022.
19) DOSCON. DOSCON – Innovation for water and wastewater treatment-CASE STUDIES. https://dosconno/#start[Accessed (1/1/2023). 2022;128.
20) HACH. WASTEWATER TREATMENT FACILITY COMPLEMENTS ONLINE MONITORING WITH AUTOMATED CONTROL file:///C:/Users/0128738s/Downloads/L2166%20(1).pdf. 2012.
Authors: Emmanuel Alepu Odey BE PhD is a postdoctoral researcher at the University of Galway in Civil Engineering. Edelle Doherty BE PhD CEng is a research fellow in Civil Engineering and lectures in electrical and electronic engineering. Sean Mulligan BE PhD CEng is the founder and CEO at VorTech Water Solutions Ltd. Fergus Clifford BE PE CEng is the operations lead at VorTech Water Solutions. Peter Leonard BSc is the technology lead at VorTech Water Solutions Ltd. Eoghan Clifford BE PhD CEng is a professor of engineering in civil engineering at University of Galway.