Quantifying the site’s capacity
In order to model this method for increasing renewable integration, the capacity of the treatment plant must be known. Access to this data was unavailable so an estimation of the capacity must be made. Without access to wastewater flow data the energy consumption of the plant will be used and taken to be representative of the wastewater flow rate. It will be assumed that the plant uses a set quantity of energy to treat every litre of wastewater and variations in the quality of the wastewater and other factors will be ignored for simplicity and to work within the constraint of the available data. An initial estimate was made by considering the hour with the highest energy consumption, with a value of 75.4kWh being found. To verify if this is a reasonable representation or an outlier a histogram of the hourly energy consumption over the year was considered, as shown in Fig.4. This shows the 75.4kWh value to be a clear and significant outlier. Immediately following this point is the second highest energy consumption for any hour of the year, 73.4kWh, and prior to these points is the year’s second lowest hourly energy consumption at 15.8kWh. This can be seen in the data sources tab. These points are clearly linked but the exact reason is difficult to conclude and therefore it is difficult to ascertain if this is the plants’ capacity to which it could regularly operate or whether this was an anomaly or freak event. One possibility considered was this being a result of active management to match demand with the renewable supply. However, Fig.18 in Data sources shows this was not the case as very little energy was being generated by the wind turbine during this time. A more probable explanation is that a mechanical problem occurred, or maintenance was required, hence the energy consumption dropped significantly to only 15.8kWh whilst the fault was fixed with the energy demand then climbing to above 73kWh for two hours as the plant processed a backlog of wastewater. Whatever the reason, although the plant can operate at these levels it is unusual as shown in Fig.4 and it can be assumed Scottish Water would look to minimise running at such high levels to guarantee the quality of effluent discharged is not compromised. (2)
Histogram of hourly energy consumption across a year for the case study WWTW.
Analysing Fig.4 there is a clear drop after 51kWh and another noticeably drop after 61kWh with the plant only operating beyond this point for 60 hours of the year. To better gauge an estimate of the plant’s capacity the top 1% of energy consumption hours will be discarded. This gives a maximum value of 60kWh, which when considering Fig.4 seems reasonable. It can also be assumed that Scottish Water would want to avoid operating at full capacity, again to ensure the quality of the effluent being discharged, therefore for this model a 10% buffer will be included. This gives a capacity of 54kWh per hour for the site in question with the ability to increase to 60kWh and even beyond if so required.
The plant operates below this 54kWh capacity 97.2% of the time with an average hourly energy consumption of 39.7kWh, 14.3kWh below capacity with this shown in Fig.5. It can be seen that load shifting may be possible by storing and changing the times wastewater is treated and sections on Wastewater time energy profile and Modelling Problems will attempt to quantify the potential and the possible value of this method to Water Utilities clients
Hourly energy consumption across a full year for the case study WWTW site with the estimated 60kW capacity given as a solid red line and the 54kW capacity assuming a 10% buffer that will be used for modelling given as a dotted red line.