Like other water companies across the UK, NI Water is striving to capture data on the performance of its sewerage networks – in particular discharges from Combined Sewer Overflows (CSO) and Wastewater Pumping Stations (WwPS) to Shellfish and Bathing Waters – in a bid to provide greater visibility to environmental stakeholders.
Over 70% of the public sewer system in Northern Ireland is ‘combined’ and during periods of intense rainfall the capacity of the system is often overwhelmed resulting in discharges to the environment.
NI Water has made significant investment in Event Duration Monitoring (EDM) to establish, in line with environmental regulatory requirements, when spills are occurring from CSOs and WwPSs and for how long.
An early CSO monitoring project carried out by NI Water – which involved the installation of data loggers at CSOs; rain gauges and telemetry outstations – provided a valuable insight into the whole process of EDM and highlighted some of the difficulties associated with the process. These included:
n Installation of the data loggers presented health and safety and maintenance issues with equipment being damaged during desilting of the CSO chamber and through water ingress.
n Battery life was not as robust as expected,
n Data loggers only communication was via GSM – as some of the CSOs were remote GSM signal was not as reliable.
n Data loggers were not integrated into the existing NI Water telemetry protocols, data was transmitted to a stand-alone monitoring system requiring additional resources.
EDM in bathing and shellfish waters
The requirements of the revised Bathing Water Directive have been introduced to local legislation by the Quality of Bathing Water Regulations (NI) 2008. Under the new directive NI Water must provide all the following information:
n Location of wastewater outfall pipes.
n Location of consented storm overflows.
n Date and times of when CSOs release stormwater into the sea.
NI Water’s Asset Management team prioritised WwPS and CSOs within 2km of Bathing and Shellfish Waters which require overflow monitoring equipment. This was to be installed to monitor each overflow event and, via telemetry, collect data such as dates and times as requested by the ‘Revised Bathing Water Directive’; and, where possible, flows and volumes of spills.
An additional driver for monitoring of the CSOs, is to assess the performance of the wastewater network and efficiencies derived from minimising routine maintenance. In addition, the monitoring of CSOs would also reduce the impact of pollution events due to blocked sewers.
Lessons learnt from the initial CSO monitoring project have allowed NI Water to hone its installations and processes, develop smarter IT systems and invest in more reliable methods of data gathering.
On this basis, in 2017 the team from NI Water’s Integrated Capital Delivery undertook a pilot study of 20 WwPS and 10 CSOs in a bid to trial other technologies available and incorporate them into NI Water’s existing data collection systems.
The objective of this subsequent monitoring project is to achieve:
n In-day monitoring of overflow event, level and duration.
n Estimate of volume of spill based on level, duration and characteristics of application.
n Agglomeration of Shellfish and Bathing Water volumes to be estimated.
n Monitoring of rainfall.
n Alarm raised on an overflow with no rainfall.
n Trends of sewer levels to advise desalting operations.
n Signature design to achieve the above.
Integrating intelligent IT systems
Unlike any project before, this latest EDM aims to calculate estimated volumes of spillages from WwPS overflows where possible. Since this had never been done before within NI Water, a fundamental element to achieving this key objective was developing intelligent IT systems to support the EDM process.
Over the last year, NI Water’s Business Analytics Team have been working closely with colleagues from Integrated Capital Delivery to develop bespoke programmes to gather and process a range of relevant EDM data.
For the wastewater pumping stations involved in this latest project, flow rate is estimated based on the rate of rise prior to the overflow beginning. A capacity curve is stored for each wet well and associated network, and when an overflow occurs, the code checks back to the previous two values and takes the volume filled in that duration divided by the time between points to estimate flow rate. The estimated volume is simply calculated based on the duration of the overflow event and the estimated flow rate.
A ‘perseverance’ has been set for overflows to try and remove false positives, which were being received. The project is in its infancy, but it is intended to include a sanity check against a theoretical maximum flow rate based on the potential head and pipe diameters in the sewer network. Other future work includes:
n Agglomeration to Shellfish and Bathing Water sites, developed through the intelligent IT system.
n Cross-referencing overflows with rainfall data, pump trips, power failures and possibly tide times to try and help identify causes.
n Dynamically calculating and automatically updating capacity curves for pumping stations as data for the wastewater network is amended in corporate systems.
For CSOs, overflows are measured based on an ultrasonic on the weir. The level recorded from this is converted into an estimated flow rate based on a calculation using the length of weir. Other factors have been developed into the IT system such as: the type of weir i.e. single sided, double sided or screened weirs, with consideration for size and spacing of perforations along with an approximation for blinding. Whilst this is currently done on site, going forward it is planned to send back only the level and perform the calculation at the IT end.
For both WwPSs and CSOs, NI Water is working towards developing scripts to perform these calculations automatically, once the relevant data has been found in both telemetry and CAR (NI Water’s Corporate Asset Register). This has a number of benefits including:
n The fact calculations do not need to be set up individually on a site-by-site basis.
n If the network or assets on it are subsequently changed (i.e. a new/different screen is fitted to a CSO, or additional sewers are added to a WwPS affecting the capacity curves) the calculations will automatically update without a site visit being necessary.
n If the assumption used in the calculation methodology ever needs to be changed then this can be done for every site in one place without any site visits being needed.
While it is early days, NI Water is confident that by integrating these intelligent IT systems, the company will be in a better position to accurately inform key stakeholders on the performance of the wastewater network and allow potential impacts to be proactively managed.
The valuable asset data collected can also be fed into a range of modelling projects that are currently underway in Northern Ireland.