Coagulation
Coagulation is a process where various chemicals are added to a reaction tank to remove the bulk suspended solids and other various contaminants. This process starts off with an assortment of mixing reactors, typically one or two reactors that add specific chemicals to take out all the finer particles in the water by combining them into heavier particles that settle out. The most widely used coagulants are aluminum-based such as alum and poly aluminum chloride.
Sometimes a slight pH adjustment will help coagulate the particles, as well
Flocculation
When coagulation is complete, the water enters a flocculation chamber where the coagulated particles are slowly stirred together with long-chain polymers (charged molecules that grab all the colloidal and coagulated particles and pull them together), creating visible, settleable particles that resemble snowflakes.
Sedimentation
The gravity settler (or sedimentation part of the wastewater treatment process) is typically a large circular device where flocculated material and water flow into the chamber and circulate from the center out. In a very slow settling process, the water rises to the top and overflows at the perimeter of the clarifier, allowing the solids to settle down to the bottom of the clarifier into a sludge blanket. The solids are then raked to the center of the clarifier into a cylindrical tube where a slow mixing takes place and the sludge is pumped out of the bottom into a sludge-handling or dewatering operation.
The dewatering process takes all the water out of the sludge with filter or belt presses, yielding a solid cake. The sludge water is put onto the press and runs between two belts that squeeze the water out, and the sludge is then put into a big hopper that goes to either a landfill or a place that reuses the sludge. The water from this process is typically reused and added to the front end of the clarifier
Filtration
The next step is generally running the water overflow into gravity sand filters. These filters are big areas where they put two to four feet of sand, which is finely crushed silica sand with jagged edges. The sand is typically installed in the filter at a depth of two to four feet, where it packs tightly. The feed water is then passed through, trapping the particles.
On smaller industrial systems, you might go with a packed-bed pressure multimedia filter versus gravity sand filtration. Sometimes, depending on the water source and whether or not it has a lot of iron, you can also use a green sand filter instead of the sand filter, but for the most part, the polishing step for conventional wastewater treatment is sand filtration.
Ultrafiltration (UF) can also be used after the clarifiers instead of the gravity sand filter, or it can replace the entire clarification process altogether. Membranes have become the newest technology for treatment, pumping water directly from the wastewater source through the UF (post-chlorination) and eliminating the entire clarifier/filtration train.
Disinfection
After the water flows through the gravity sand filter, the next step is typically disinfection or chlorination to kill the bacteria in the water
Sometimes this step is done upstream before filtration so the filters are disinfected and kept clean. If your system utilizes this step prior to filtration, you will need to use more disinfectant . . . this way the filters are disinfected and kept free from bacteria (as well as the filtered water). When you add the chlorine upfront you’re killing the bacteria and have less fouling. If bacteria sits in the bed, you might grow slime and have to backwash the filters more often. So it all depends upon how you’re system operates . . . whether your system is set up to chlorinate upstream (prior to filtration) or downstream (after filtration)
Distribution
If the wastewater is being reused in an industrial process, it’s typically pumped into a holding tank where it can be used based on the demands of the facility. If for municipal use, the treated water is usually pumped into a distribution system of water towers and various collection and distribution devices in a loop throughout the city.
Lime softening
In waters where you have high hardness or sulfates or other constituents, you need to precipitate or take out, and a lime, and/or a lime soda process is used. It raises the pH, causing hardness and metals in the water to precipitate out. Cold, warm, or hot lime processes can be used, and each will yield a different efficiency. In general, hotter water removes more hardness
Ion exchange softening
In some industrial and municipal applications, if there’s high hardness, there may be post-treatment for the removal of the hardness. Instead of lime, a softening resin can be used; a strong acid cation exchange process, whereby resin is charged with a sodium ion, and as the hardness comes through, it has a higher affinity for calcium, magnesium, and iron so it will grab that molecule and release the sodium molecule into the water
Special processes
As we stated above, wastewater and effluent regulations differ everywhere you go. We have discussed some of the most common steps in a wastewater treatment plant. Typically there are special process steps to treat a specific issue, such as the removal of certain metals or organics, or to reduce TDS for recycling, etc. For these various problems specific to your individual needs, careful consideration must be given to the proper method of treatment.
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