Innovative Storage Options
As highlighted within the road map, four out of the top five operation and process contributors for electricity consumption are part of WWTW. These are wastewater aeration (20%), wastewater pumping (29%), water treatment (21%) and wastewater sludge treatment (14%). As such, understanding these processes and operations’ ability to flex is paramount in optimizing demand management and maximizing the efficiency in which individual site locations use the renewable energy generated. However, rethinking how we use operations and processes of WWTW opens up the prospect of potential solutions to demand management and energy storage.
An additional option applicable specifically to wastewater treatment plants has been considered and an attempt has been made to quantify the parameters needed for this method to be included in the decision matrix. The method looks at storing wastewater in flow equalisation basins for processing at times of increased renewable generation or at times of low tariff rates to reduce demand on the grid and reduce the water utility’s operational energy costs.
We made use of data provided by Scottish Water for a WWTW. We considered whether this concept is possible and have attempted to quantify values for the criteria within the decision matrix by creating a model. Difficulties however emerged in attempting to model this concept given the data provided which will be discussed in detail, along with the merit of this concept and its potential advantages, disadvantages and application.
Academic Concept
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The idea for this concept was taken from a paper published by Musabandesu and Loge in 2021 (2) that presented a case study on load shifting at a WWTW in California where grid stability is becoming problematic with occasional rolling blackouts. (3) Significant financial incentives are available for companies that can remove large loads from the grid when requested. As well as traditional battery storage this case study also examined employing this concept of storing wastewater. (2) Although this is based on moving demand away from known peak times with the aim of utilising low tariff rates it is equally as applicable to matching demand with the supply from renewable generation, albeit with a few additional difficulties that will be discussed.
Case Study Site: WWTW Capacity
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For this method to work the plant must spend a significant time below its operational maximum capacity to allow loads to be shifted. Without knowing the maximum flowrate of wastewater the WWTW can process, we aim to verify if there is the potential for load shifting and will then estimate the capacity of this particular site using the data provided. For this analysis, the energy demand for the site will be assumed to be representative of the volume of wastewater treated. A higher energy consumption will be taken as indicative of a higher flow rate of wastewater through the plant. Although this will not be exact with the composition of the wastewater along with a series of other factors affecting the energy required to treat incoming wastewater, it has been taken as an acceptable approximation for the purposes of this concept study.
Innovative Storage Conclusion
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Using flow equalisation basins to store wastewater can act as an alternative energy storage solution giving WWTWs increased flexibility in their operations and giving an increased ability to match demand with supply, something that will become increasingly valuable in the future as the country and the world moves to a zero-carbon energy system. Although attempts to model this at the WWTW case study site were unsuccessful a rough estimate of what this system would look like were made. A maximum potential flexibility of around 144.8kWh for a 6-hour period was found with an average of 86kWh. This would require a 273,400-litre tank with a radius and height of 5.03m. Comparing this with conventional battery storage as a retrofitting solution to energy storage at existing sites this is likely a more expensive and difficult solution to install and would be operationally more difficult to run. However, when designing and constructing new plants designing with this method in mind could be advantageous. Additionally, WWTW are designed to operate at significantly higher levels in winter and rainier months and so designs could be made to maximise the use of this available extra capacity during drier periods. This may already be possible given the current infrastructure, but further data and site-specific information is required. A potential solution for future WWTWs could be a design that incorporates equalisation flow basins as well as battery storage with both working in unison. Despite not likely being a solution to implementing energy storage solutions at current sites there may be potential for this method to be applied at future WWTW, but further work and very detailed site-specific study would be needed to assess the potential benefits and to address any technical problems that may arise.
Just last month BBC Panorama found that water utilities in England and Wales released raw sewage into rivers 400,000 times last year. As a result, they are now committing to spending over £1bn to tackle this issue. Flow equalisation basins could provide a solution to this environmental problem and if we rethink how this storm surge infrastructure is implemented there is a significant opportunity to increase the operational flexibility of these sites.
Although more difficult to implement than simple battery storage, it would give treatment plants the additional flexibility to be assets for a country’s energy system by finding a secondary use of this much needed infrastructure.