Whatever the source, be it surface water or municipal wastewater, whenever water might be used for drinking, it must meet high standards for quality, no matter whether the water is used for laundry, watering the garden, flushing the toilet, or simply drinking. To consistently reach those standards, treatment plants process source water through a series of specific steps.
Coagulation
Coagulation is usually the first step when treating water, whether it is agricultural runoff, stormwater, or withdrawals from a lake or reservoir. The goal is to remove tiny solids from the water, whether those are oily substances, soil particles, algae and organic compounds, dissolved substances, and gravel and sand. To accomplish this, salts like ferric sulfates and aluminum sulfates are added to the water and mixed. Since these salts carry a positive charge, they neutralize the natural negative charge of dissolved and suspended particles, like soil, and allow them to form clumps, or coagulate.
In a water treatment facility, the coagulants are added to the water and vigorously mixed, to ensure the chemicals are fully distributed throughout the water, which promotes particle collisions. Once the clumps have begun to form, the water may be run through a filtration medium to capture the largest clumps, or moved to a settling tank, where the heaviest particles have a chance to sink to the bottom. Even after coagulation, many clumps are invisible to the naked eye and are difficult to filter out. At this point, the water continues to subsequent steps, namely flocculation and sedimentation.
An important secondary benefit of the coagulation process is that it can remove large amounts of Natural Organic Matter (NOM) and Dissolved Organic Carbon (DOC). Not only do these substances make it more difficult to disinfect the water, but they can actually produce harmful byproducts when they react with chlorine.
Flocculation
In coagulation, the negative charges on dissolved and suspended particles in the water were neutralized, allowing them to move together and form small clumps. Flocculation is the next step, where the fine neutralized particles are encouraged to bind together into larger clumps (flocs) making them easier to remove from the liquid.
The flocculation step involves gently mixing the water with either natural or synthetic polymers. The gentle motion of the water allows long chain polymers to entangle small particles and clumps, which then collide and form even larger clumps, called flocs. The aggregation process continues until the flocs are large and strong enough to endure mechanical separation, whether that means sedimentation and filtration, or alternate methods like centrifugation.
Like coagulation, flocculation removes even more organic compounds and suspended particles, including inorganic precipitates. This process may, in fact, remove some of the viruses and bacteria that were present in the source water, but that only happens if the microorganisms were directly attached to the substances that were clumped together and removed.
When coagulation and flocculation steps are effective, smaller amounts of chlorine are required to thoroughly disinfect the water.
Sedimentation
Coagulation and Flocculation encourage suspended solids to form clumps which can be separated from the water. The sedimentation step allows the largest, heaviest flocs to settle to the bottom of the container for removal. This is a simple physical process which relies entirely on gravity to draw the flocs to the bottom. Since this step doesn’t remove flocs which have a specific gravity similar to water, filtration is a necessary next step.
When sedimentation is used effectively, the overall treatment process costs less: fewer chemicals are needed in subsequent steps, and the quality of produced water is more reliable.
Filtration
During the sedimentation step, the largest and heaviest clumps of solids naturally sank to the bottom of the tank where they could be removed. But a significant amount of clumped solids remain, even if the water appears clear. They may be too small or too light to sink, but they still must be removed.
In the filtration process, the clear water is passed through different materials with gradually decreasing pore sizes, like gravel, sand, and charcoal. These filters can remove dust, additional dissolved particles, certain chemicals, and some microorganisms. Activated carbon filters can also be used to remove unpleasant odors.
Some treatment plants also use ultrafiltration, where the filters have pores so tiny that only water and other small molecules (such as salts and certain tiny charged molecules) can pass through. Reverse osmosis is a similar filtration process and can be used for water treatment. It’s often used, for example, when processing salt water for human consumption.
Reverse osmosis, nanofiltration and ultrafiltration all engage pores that are small enough to eliminate all bacteria, parasites and even viruses from the filtered water, rendering it sterile. However, disinfectant chemicals still need to be added to protect the water from picking up more pathogens as it travels through pipes on its way to the final consumer.
Some disadvantages of reverse osmosis and ultrafiltration are the high energy costs to repeatedly force water through such fine membranes, as well as the cost of the specialized membranes themselves.
