What is clarification in waste water treatment?
Clarification is an important stage in the treatment of water or wastewater because it removes suspended particles by gravity settling, resulting in a cleared liquid effluent. To begin, knowing primary and secondary clarification is required in order to select the best technology to suit treatment needs.
What is the difference between primary and secondary clarification?
Primary and secondary clarifiers are two separate but equally important components of any wastewater treatment system. Secondary clarifiers are built downstream of the biological treatment or activated sludge facility to differentiate the treated wastewater from the biological mass used for treatment. Primary clarifiers are built downstream of the screening and grit chambers of the plant to separate settleable solids from the raw wastewater influent. Both of these clarity techniques use gravity to extract particulates from the feed water that enters the clarifiers.
a) Primary Clarification
The first step in the water treatment process for eliminating suspended solids (TSS), oil, and grease is primary clarity, also described as sedimentation. This phase eliminates surface sediments and other large particles from the water or wastewater flow prior to biological treatment. Sludge settles at the bottom of the clarifier basins, where it is raked and eliminated by a sludge separator. Grease and oil, on the other hand, float to the surface and are skimmed off. A typical main clarifier eliminates 60% of suspended particles and 30% to 40% of Biological Oxygen Demand (BOD).
b) Secondary Clarifications
Secondary clarification follows the biological treatment, with the primary goal of returning activated sludge. During the secondary clarifying phase, microbial biomass sinks to the bottom as activated sludge. After settling for a length of time, the microbe biomass is returned to the aeration tank, and the process will be repeated until the effluent is clean before being transported for filtering and/or disinfection. Waste sludge is extracted and thickened before digestion.Based on the quantity of various pollutants present, more complex procedures may be necessary, such as simple filtration with or without chemicals, coagulation, flocculation, sedimentation, or flotation filtration.
c) Reagent-Free Filtration
When the raw water includes just a small amount of non-colloidal suspended particulates (as is very seldom the case with surface water), treatment will consist of straight filtration without reagents, potentially followed by pre-oxidation (chlorine, ozone) if necessary.
d) Coagulation is then followed by filtration
In the event of raw water with a minor colloidal turbidity and/or slight colour and/or containing no more than 1,000 to 2,500 micro-algae per mL, a coagulant and, if necessary, a flocculant is used based on the prior scheme (thus close to the A1 quality grade). Low concentrations of these chemicals should be used because they shorten the duration between backwashes in the filtering cycle. The coagulant will eliminate a particular number of organic molecules. Water that is mildly turbid but richly pigmented or loaded with algae, on the other hand, cannot be properly treated using this procedure; instead, a larger dosage of coagulant is required, and a sedimentation or flotation stage is required.
This deceptively simple solution is available in a range of applications that use single or multi-media filters.
e) Coagulation on filters
The coagulant is injected in line (static mixing) without any coagulation or flotation phases in this system, also known as contact filtration (or in line filtration). A flash mixer can be used between reagent injection and filtration in some cases, particularly with cold water; the water then arrives at the filters coagulated, but flocculation is not complete until the water percolates through the upper layers of filter media.
In the case of direct filtration after flocculation, one or more flash mixers (with contact periods in the range of one minute) and one or even more contact basins functioning as a flocculator or intermediate reactor will be used to ensure that the required time intervals between reagent additions are met.