By Rika Law and Wayne Stiver
Peterborough Utilities Commission (PUC) owns and operates the Peterborough Water Treatment Plant (WTP), which consists of a conventional filtration system, a chlorine contact tank and clearwell.
Given that the chlorine contact tank and the clearwell were constructed in the 1920s, and that there was no operational redundancy for the chlorine contact tank, a new chlorine contact tank and clearwell had to be added. The PUC was also aware that the existing reservoir was leaking. However, they couldn’t quantify or determine the location and extent of the leak. Once the new reservoir was constructed, the existing reservoir could be taken out of service for permanent repairs of the leaks.
The project was designed so that, when both the new and existing reservoirs are constructed or repaired, they can operate in parallel, providing supply security, flexibility in operation, and additional storage.
Existing reservoir leak and shutdown
Excavation was underway for the new chlorine contact tank and clearwell, some distance from the existing chlorine contact tank and clearwell. Slope protection was in place and all seemed well on October 3, 2014. But, by October 6, the excavation site turned into a lake, as the existing reservoir developed a sizeable leak and flooded the site.
The PUC quickly switched the Peterborough WTP to bypass mode. This involved prechlorination at the beginning of the WTP to achieve the required disinfection. Treated water then went through a bypass pipe in the chlorine contact tank into the distribution system. This allowed PUC to drain down the existing chlorine contact tank and clearwell. Once this was done, assessment of the leak and repair plans could start.
R.V. Anderson Associates Limited (RVA) structural staff observed that numerous cracks along the floor of the reservoir in both cells allowed water to leak. The cracks were not new, but the water had found a new path of least resistance when the adjacent site was excavated. It was suspected that the water made channels/voids underneath the floor slab and that these would also need to be addressed.
Based on the structural investigation, the following emergency repairs were recommended by RVA:
- Polyurethane injection (with NSF-61 approved material for potable water application) for all observed floor cracks;
- Pressure grouting of the voids underneath the floor slab;
- Potable water dive into the operational inlet and effluent chambers of the existing reservoir to try to patch up cracks at the floor/wall interface.
This plan would allow the existing chlorine contact tank and clearwell to be put back into service as soon as possible. However, several issues complicated and hindered the emergency repairs, including: frigid winter temperatures; leaks from the operating inlet and effluent chambers (which were in use for the bypass operations); and, leaks from the isolating sluice gates and valves.
Challenges, strategy and lessons learned
There were several things that were done to prepare for this possible leak. PUC had the foresight to implement construction of the new reservoir, and to undertake permanent repairs to the existing one, prior to the major leak event. They knew that the reservoir was leaking, but were not able to quantify it because they could not stop the use of the existing chlorine contact tank and clearwell without going on emergency bypass operations. PUC had budgeted for the two projects in order to address the leaking existing reservoir issue and the lack of redundancy in their chlorine contact tank.
During the unexpected, emergency shutdown of the existing chlorine contact tank, PUC’s operations staff was also able to maintain the supply of potable water. PUC operated the WTP with prechlorination at the
beginning of the conventional filtration system, and emergency bypass of the chlorine contact tank and clearwell for five months. Monitoring for water quality, including trihalomethanes, was
PUC was proactive and informed the Ministry of Environment and Climate Change of the emergency situation and maintained good relations with them. Lessons learned from this unexpected turn of events included:
- Cultivation of a cooperative environment amongst all parties (client, consultant, contractor, approval agencies, etc.) is important throughout the project, but especially when trouble arises;
- Updating and informing key stakeholders of the issue throughout the process helps to keep the cooperative spirit;
- Knowledge of the water system, its limits, capabilities, and redundancies/contingencies, is necessary for troubleshooting under extreme circumstances.
Connecting the new system
Plant operators planned for a 36-hour window to perform a critical piping connection between the new chlorine contact tank and clearwell system and the existing distribution network. The situation was complicated due to: space constraints; night-time work; installation of new tee between two existing fixed points; installation of several large 1,200 mm valves and pipe pieces; and, “live” water system at both ends of the connection.
During the shutdown, PUC and RVA had several tools to continue distribution of potable water:
- Keep the water levels in the City reservoirs high;
- Overland bypass pumping between the existing clearwell (which had been repaired and put back into service) to another one for the high lift pumps to feed Zone 2 water reservoirs in the City;
- Back feed Zone 1 via Zone 2;
- Closely monitor the water levels and water demand;
- Review and revise the shutdown plan, complete with contingency plans, with the contractor three times;
- Conduct shutdown planning meetings to coordinate details of who, what, when, where and how.
Despite planning efforts by PUC, RVA and the contractor, accidents and unexpected events still occurred. These included warping of the stainless steel pipe during installation and the breaking of a specially measured and fabricated gasket on an important coupling. In the end the shutdown lasted 21 days! During that time, all parties worked together to try to find alternative solutions, including calling different suppliers, contractors and municipalities for replacement parts.
• Cultivate a cooperative environment between parties
• Update and inform key stakeholders
• Know your water system’s limits, capabilities and redundancies.
A shutdown can never have too many contingency plans, as redundant and pessimistic as it may sound. Also, reliable “as-built” information can save a lot of headaches. Due to the quick thinking, cooperative spirit and collective experience of Peterborough Utilities Commission, operations staff, contractors, and the consultant, both incidents at the plant had a positive ending.
Rika Law, P.Eng., PMP, is with R.V. Anderson Associates. This article appears in ES&E Magazine’s August 2016 issue.