WATER QUALITY MATTERS
HOME	SOURCES	USES	TREATMENT	TRAINING	ABOUT	CONTACT
WATER TREATMENT
Depending on the planned use of water its quality may be suitable without treatment. Very often this is not the case and treatment is needed to reduce or eliminate contaminants. There are a variety of treatment methods available that range from simple filtration to complex multi-stage processes. It is very important to know what contaminants need to be reduced or removed from the water in order to successfully treat it. Treatment processes can be physical, chemical or biological or a combination of these three. A treatment system may use only one of these methods or a combination of them depending on the quality of the source water and the required quality of the treated water.
CLARIFICATION
Clarification is a process that involves letting gravity do the work of making particles (or sediment) settle out of the water. This treatment step is very common for surface water sources as this water source will generally have more suspended material (solids) than ground water. Ideally this is done while the water isn’t moving, or moving very slowly in a large container, like a tank or an open basin. The larger the particles that need to be removed, the quicker they can be removed by this physical process. AERATION The addition of air to water can help to make a dissolved contaminant change to an insoluble compound (solid particles in the water), especially iron and to a lesser extent manganese. Stronger sources of oxygen than air (called oxidants) added to the water can be used to specifically change contaminants in solution into particles than can then be removed by clarification or filtration. Aeration of a surface water pond can also help to improve water quality. COAGULATION The larger the particles are in the water, the easier they are to remove. Adding a chemical that helps attract smaller particles to it to form a larger mass is called coagulation, with the chemical added called a coagulant. Mixing the water while adding the coagulant is called flocculation, and speeds up the process of forming the larger particles. Like aeration, this process can take place in a pond or in an open or closed tank. CLARIFCATION Once any pre-treatment steps have been done, such as aeration or coagulation as described above, the water passes through a basin or tank that is designed to allow the particles in the water to fall to the bottom. This is the clarification (or sedimentation) process. Generally, the water flows in one side and the clarified water is taken from the top of the basin or tank on the other side with the sediment falling to the bottom.
FILTRATION
Filtration is the process of removing particles by straining them through a filter where the empty spaces (voids or pores) in the filter are too small for the particles to pass through. A filter can be as simple as a screen or a layer of sand or as complex as an ultrafine man-made membrane. In large treatment plants filters may sometimes be in large concrete basins and just use gravity to make the water flow through. In small systems filters are almost always in tanks and pressure is used to force the water through. Filters will eventually get plugged and are cleaned by reversing the flow through the filter, a process called backwashing. RAPID SAND The term rapid sand filter describes the filtration process – designed to have water to be rapidly be filtered and were originally designed to contain sand. Modern rapid sand filters contain multiple layers of filter media including sand, gravel, garnet and anthracite (hard coal). Due to the different densities of the filter materials, after backwashing they settle back out into separate layers ready to function like a brand-new filter. SLOW SAND (BIOFILTER) A slow sand filter is a sand or multi-media filter on which biological organisms (biomass) grow and feed on the contaminants in the water. The filtration process includes both some physical filtration as well as the biological removal of contaminants like iron, manganese and ammonia. Water flows much slower through the filter so that the biological process can remove contaminants. Granular Activated Carbon (GAC – see below) is sometimes used as a filter media because the jagged surfaces of the particles give much more surface area for the biological component to grow and consume the contaminants. MANGANESE GREENSAND Greensand is a bluish-green material called glauconite that is taken from the ocean floor and processed. It has a chemical coating that works to remove iron and manganese from water through a process called oxidation. The resulting particles are physically filtered by the greensand bed. Like any filter it needs to be backwashed, but in addition the chemical coating is regenerated by adding a chemical called potassium permanganate to the backflush water. GRANULAR ACTIVATED CARBON (GAC) GAC filters are used to remove dissolved organic matter, and colour, taste and odour from water. They are made from a carbon source (coal, wood, coconut shells) that is heated in the absence of air and then crushed. The resulting particles is very jagged and has a high surface area. The contaminants are removed by a process called adsorption, where the particles are attracted to and stuck to the surface of the GAC. In most cases when the GAC is no longer removing contaminants it must be removed and replaced. MEMBRANES This family of filters includes (from coarsest to finest) Microfiltration, Ultrafiltration, Nanofiltration and Reverse Osmosis (RO) membranes.
ION EXCHANGE (SOFTENING)
HARDNESS SPECIFIC ION REMOVAL
DISINFECTION
CHLORINATION ULRAVIOLET (UV) RADIATION
CUSTOM
CATALYTIC REMOVAL Example - arsenic removal ...