The City of Medicine Hat Water Treatment Plant draws raw water from the South Saskatchewan River, which is part of the South Saskatchewan River Basin (SSRB), and through the treatment process, potable water is released into the water distribution system for our consumption. In a nutshell, water from the South Saskatchewan River is drawn into the Treatment Plant through intakes at the bottom of the river. Raw water pumps transfer the water through screens, Solids Contact Units (SCUs) and dual media filters. The filtered water is then transferred to clearwells where the chlorination process is completed. The chlorinated water is then moved through the Ultraviolet Light Disinfection system before being pumped into the distribution system. An explanation of each step is discussed below.
The City of Medicine Hat is investigating building a mechanical de-watering plant to treat the SCU blowdown and filter backwash water. Currently this water is de-chlorinated and discharged back to the river.
The water treatment plant is rated for a maximum output of 160 ML (160 million liters) per day. During winter months, the average plant effluent is in the range of 20-25 million litres per day. During summer months, this will increase to anywhere from 85-130 million litres per day.
The raw water quality of the river fluctuates on a seasonal basis, meaning the treatment plant process must adapt with each fluctuation. Turbidity is one of the basic indicators of water quality. Turbidity is measured in ntu. NTU stands for Nephelometric Turbidity unit, i.e. the unit used to measure the turbidity of a fluid or the presence of suspended particles in water. The higher the concentration of suspended solids in the water is, the dirtier it looks and the higher the turbidity is. The turbidity of the South Saskatchewan River can vary between 1 to 10,000 NTU. Alberta Environment requires that the water leaving our filter must be below 0.3 ntu. The City of Medicine Hat Water Treatment Plant’s “in-house standard” for filtered water quality is below 0.1 NTU 100% of the time. We have both laboratory and on-line turbidimeters, which are calibrated periodically as per regulatory requirements. Other measures of water quality, such as chlorine residual, are measured in milligrams per liter (mg/l). Operators monitor online analyzers through our SCADA system. As well, they perform manual lab tests on a regular basis to ensure the City of Medicine Hat Water Treatment Plant meets and exceeds all regulatory requirements.
Raw Water Intakes
There are a number of different intakes located along the bottom of the South Saskatchewan River, with the first being constructed in 1912. The purpose of an intake is to draw water from the lowest possible depth in the river to ensure we will be able to continue to draw water in the event of drought conditions.
Raw Water Pumps
Raw Water Pumps, aka low lift pumps, transfer the untreated water from the intakes into the Water Treatment Plant for processing. There are multiple low lift pumps located in close proximity to the river to allow for flexibility where we draw river water into the plant. Intakes and pumps are equipped with screen mechanisms to prevent the passage of trash, logs or other foreign materials into the plant.
Solids Contact Units
Designed to treat the river water entering the plant, the Solids Contact Units (SCU’s) act as the primary treatment mechanisms for the City‘s Water Treatment Plant. The Plant is currently equipped with multiple SCU’s to accommodate seasonal flow variability and redundancy.
River water entering an SCU slows down as it passes through the various chambers housed within the unit. Aluminum Sulphate (Alum), referred to as a coagulant, is added to assist particulates found in the river water to clump together becoming heavier. These heavier particles, referred to as floc, settle while the clear water rises to the top of the unit around the perimeter where it is sent on to the filtration part of the treatment process.
Dual Media Filtration
The filters used in the Plant are called dual media because they consist of anthracite and crushed quartz. These filters are supported by an underdrain system to allow the water to drain from the filter while retaining the media. The purpose of filtration is to remove very fine particulate impurities from the water being treated as these particulates have the ability to shield microorganisms from chlorine disinfection. A common misconception of filtration is that particles are removed mainly via a “straining” process; however this is not the case. Filtration is essentially both a physical and chemical process.
In 2000, the Plant was retrofitted to allow for the feed of a polymer filter aid. As a result, filter effluent (discharge) particulate quality is among the best in Western Canada with turbidity levels consistently in the 0.03-0.05 NTU range with the regulatory requirement being below 0.3 NTU. The City of Medicine Hat Water Treatment Plant’s “in-house standard” for filtered water quality is below 0.1 NTU 100% of the time, so before any filter is put back into service, it is “filtered to waste” until effluent quality is lower than 0.1 NTU which is well below the regulatory requirement.
Baffled Clearwell System
The filtered water is transferred to the Baffled Clearwell system next. The clearwell is a multi-chambered storage system, much like a maze, where chlorine is administered and given enough contact time with the filtered water to disinfect prior to sending it onto UV disinfection. The contact time with the chlorine is essential to ensure that the potable water supply remains free of any disease-causing microorganisms as it continues through the distribution system. A minimum chlorine residual of 0.1 milligrams per litre (mg/L) is required to remain in the treated water distribution system at all times.
Ultraviolet Light Disinfection System
Part of the 2008 expansion was the installation of three Ultraviolet (UV) reactors in place of the Secondary Clearwell to enhance the disinfection and treatment of the clear water. UV disinfection is a revolutionary new concept in water treatment. It is a flexible, safe and cost-effective technology that effectively inactivates any disease-causing microorganisms found in raw water.
During the winter months, one reactor is in service, whereas during the peak summer demand a second reactor is used as needed. The third reactor unit is kept as a back-up should it be required. UV is particularly effective in the deactivation of Giardia and Cryptosporidium. UV disinfection systems are completely safe for human use and have been proven through many years of scientific study.
Chemicals used in the City of Medicine Hat Water Treatment Process
Aluminum Sulphate (alum) – a coagulant, added in the Solids Contact Unit, which causes very fine particles found in the raw water to clump or floc together into larger, heavier particles to settle out.
Caustic Soda (Sodium Hydroxide) – used primarily to adjust pH to prevent excessive corrosion within the distribution system. When large amounts of coagulant chemicals are used, like alum, the pH of finished water may be suppressed enough that it warrants the feed of caustic soda to bring the pH back up. In general, caustic soda is only required during periods of high spring runoff. Spring run-off water is generally higher in turbidity which means additional alum is used in the treatment process, therefore requiring the addition of caustic soda.
Chlorine – a disinfectant used to destroy pathogenic organisms in water. Prior to chlorination practices (early 1900’s), outbreaks of cholera, typhoid and dysentery were regular occurrences resulting in tens of thousands of deaths annually. Chlorination of water supplies has been the most significant public health accomplishment during the last 100+ years.
Polymer – a medium chain cationic polymer is used primarily as a filter aid. The function of a polymer is to bind small suspended particles to larger chemical flocs for easier settling and removal from water.
Potassium Permanganate – a strong chemical oxidizer used primarily for the treatment of taste and odour. It is used to oxidize iron, manganese and sulphide compounds in the raw water. The addition of this chemical must be very precise and quantified through laboratory tests prior to adjusting the feed rates. An overfeed of the chemical will produce an intense purple or pink colour in the water whereas an underfeed may result in taste and odour episodes.
Powdered Activated Carbon (PAC) – removes contaminants from water via “adsorption” and is also effective in taste and odour control. Feed rates must be monitored closely as PAC has the ability to penetrate filter beds in an overfeed situation. PAC is also effective in removing organic contaminants from raw water.
Sodium Bisulphate (SBS) – used to remove chlorine from Plant wastewater prior to discharge back into the South Saskatchewan River. Residual Chlorine can have a negative impact on aquatic life.