Large Water Treatment Plant Scenario for Urban Supply

About Large Water Treatment Plant

SCENARIO A large water treatment works (WTW) treats water from a river, storing it in reservoirs that supply drinking water to a nearby city. This WTW alone supplies a third of the drinking water required by the city’s population. The WTW site consists of a mixture of 150-year-old buildings and a 30-year-old building extension. Plant rooms within the extension were built to contain the water treatment and disinfection equipment, while the older buildings now contain support services, including offices and laboratories. The site also includes four reservoirs, chemical and fuel storage areas, and extensive pipework. A high brick wall forms part of the site boundary, with one entrance on the site. The city’s built-up residential housing, schools, and a railway line are located on one side of the site boundary. Along the other side is the river that the WTW draws water from. The river is a protected ecosystem containing a range of vulnerable species and plants. A 50-mile (80km) system of large underground pipes connect the WTW to the wider water distribution network within the city.

River water is treated at the WTW through multiple stages: screening (mechanical removal of larger solids), filtration (removing smaller particles to improve water quality), and disinfection (killing micro-organisms that cause disease). Disinfection involves several methods, including exposure to ultraviolet (UV) light and the addition of sodium hypochlorite solution (a hazardous chemical, often called ‘liquid bleach’). Due to the large quantities (250 tonnes) of sodium hypochlorite and other hazardous chemicals stored on site, the WTW is considered a major hazard site.

Approximately 80 people work at the WTW, including process operators, maintenance workers, technicians, engineers, office and security workers. Although the process is largely automated, there are workers on site 24 hours a day, 7 days a week to carry out monitoring and maintenance tasks

Bulk deliveries of sodium hypochlorite arrive in road tankers twice a week. The road tanker drivers make deliveries to a signposted off-loading point. This off-loading point is along the only main two- way route around the site and is used by all vehicles, including delivery vehicles and those being driven by visitors and workers. The sodium hypochlorite deliveries take place early in the morning when workers arrive for the morning shifts, which can cause traffic congestion along the route. Occasionally, visitors have parked in the off-loading point as the dim morning light can make it difficult to see the sign indicating the area. The off-loading area is an uncovered, concrete hardstanding area, with a shallow kerb and surface drain that connects to the site drainage system.

WTW refurbishment The WTW is currently undergoing a major refurbishment to modernise the water disinfection system. As part of the upgrade, a new chemical dosing system is being installed within the building extension. The existing chemical dosing system is electrically powered and has been in place since the extension was built.

Flocculation is one stage of the water treatment process. Chemicals are added to the water and mixed using electrically-powered agitators mounted on the open-topped flocculation tanks. The agitation and mixing process is vigorous and regularly causes water and the chemical solution to splash up and over the sides of the tank. The agitators and mixers are worn and damaged, so water sometimes penetrates and cuts off the electricity supply. The electricity supply is automatically cut-off through a trip system if there is an electrical fault.

Some of the buttons on an existing control panel are discoloured, worn, and do not always work first time when pushed. As part of the upgrade, this panel will be relocated away from the flocculation tanks. Workers have also noticed evidence of rodent infestation, with droppings and chewed electrical cables. Electricians have temporarily repaired the cables with insulating tape.

The refurbishment also involves laying 5 000m of new dosing pipework. Therefore, multiple contractors access the site daily. The upgrade work inside the building extension initially involved draining all of the liquid disinfection chemicals from the existing dosing pipework. Then, the system was flushed using large quantities of water. The pipework was then isolated and made safe (decommissioned), then unbolted and dismantled using spanners, pipe cutters, and angle grinders.

Water trunk mains upgrade project In addition to the work at the WTW over the past six months, parts of the wider water distribution network in the city are also being upgraded. Regional maintenance teams are improving ageing cast-iron trunk mains (underground water pipes, approximately one to two metres in diameter) in busy areas of the city. The work involves installing new polyethylene (PE) slip liners inside the old cast-iron trunk mains, a method known as slip lining.

At one of the water trunk main upgrade sites, the maintenance team is installing a new slip liner and reconnecting the pipework. To carry out tasks, workers must enter a narrow trench that has been shored with timber walls and steel supports. A 360° excavator is positioned next to the trench on firm, level ground. It is fitted with a spreader bar lifting attachment that is used to move materials and equipment into and out of the trench. It is also used to lift and lower materials and equipment within the trench. Fixed pedestrian barriers are in place around the edge of the trench.

Complex underground structures, identification of existing underground services, and traffic congestion have made progress slower than expected. A public road traffic management plan is in place for the areas surrounding the water trunk mains work sites. This includes road and junction closures.

The maintenance teams work six days a week on eight-hour shifts. For the last three months, they have been offered an additional two hours of overtime each day to help with project delays.

Workers A and B from one of the regional maintenance teams, travel together to and from their work site in a work vehicle, departing from a regional depot just outside of the city. They share the driving to the work site between them. Both workers travel from their homes to the depot in their personal vehicles (journeys taking between 15 and 30 minutes). The workers leave their personal vehicles at the depot while they are working, and load up their work vehicle with the equipment needed for the day.

Late in the afternoon, with daylight fading, Workers A and B are nearing the end of an overtime shift. Worker A enters the trench to complete slip lining using the electrofusion welding unit. On entry, they realise that they have left their gas monitor and respiratory protective equipment (RPE) at street level next to the 360° excavator. They know that the work will only take a short time to complete. They are feeling very tired and want to finish the task before rush-hour traffic. They agree with Worker B to continue without the gas monitor and RPE to save time. During the task, Worker A begins to feel unwell and loses consciousness within the trench.

Task 1: WTW refurbishment 1

The refurbishment inside the WTW building extension involves replacement of existing pipework. All site operations will continue as normal during this work.

(a) Explain the possible significant health and safety hazards associated with this work. Note: You should support your answer, where applicable, using relevant information from the scenario.

(b) What management arrangements should be in place to protect people on-site before and during this work? Note: You should support your answer, where applicable, using relevant information from the scenario.

(16)

Task 2: Dangerous substances 2 Explain how the delivery and storage arrangements of sodium hypochlorite could be improved at the WTW.

Notes: Your answer must relate directly to the scenario.

(18)

Task 3: Electrical equipment and electrical supply system deterioration 3

Explain what may have contributed to deterioration of the electrical equipment and electrical supply system at the WTW site.