لماذا نعالج المياه كيميائيا2/why treated water chemically2

b]Process Equipment
The process equipment does have some impact on the choice of coagulant and flocculant, however the raw water quality is the predominant facto

Settling Lagoon
The presence of a pretreatment settling lagoon will allow the water plant to maintain, on a year round basis, a consistent and much better quality of raw water. A settling lagoon will enable the water plant to let any temporary poor quality surface water by-pass the plant in-take, while concurrently permitting all the settleable solids to be naturally precipitated from the raw water prior to treatment. The choice of chemical treatment should therefore be consistent on a year round basis, as should be the overall treatment regime. A chemical programme can be tailored to meet specific needs and can be far less forgiving than programmes that have to satisfy fluctuating raw water quality situations

Direct filteration
A direct filtration plant will require a coagulant/flocculant combination that will readily generate a robust yet filterable floc with the raw water contaminants. The first inclination is to believe that the larger the floc the better, but this is often not the case. A large floc will be easily filtered from the water but all the filtering action will take place on the top of the filter bed leaving the majority of the media unused A smaller, denser, and more robust floc will also filter well but will use a greater percentage of the filter bed resulting in an improved quality of finished water and longer run lengths before breakthrough Those coagulants that hydrolyze quickly to form a sweep floc will show up well in a jar test, however the sweep floc is very fragile and may easily break up into fines that will be difficult to filter. The prepolymerized inorganic metal salts are the best option in direct filtration units There will be a temptation to consider the use of organic polyelectrolytes as filter aids, especially the pDADMAC's if there is colour to remove. The use of these products can be beneficial, but only as an aid to the primary coagulant. Should the dosage of these materials be above 4.0 mgL there will be a very real tendency to cause filter blinding and large "mud balls

Sedimentation and Filtration
The configuration of the settling basin process will have to be considered especially the chemical feed point and the intensity of the flocculators. If there are good flocculators, or mixing chambers with several minutes detention time (>5 minutes) then the best choice is a non catalyzed prepolymerized metal salt Such a chemical regime will generate a dense floc that will expel water naturally This floc will settle well, and depending on the retention time in the main settling basin may or may not require a flocculant. If the mixing energy or detention time is insufficient, the best option is to consider a sulphated alumina salt since the catalyzing characteristic of the sulphate will allow for reasonable results. Since a sweep floc will surface adsorb the raw water contaminants without any real coagulation there will always be a need for an organic polymer flocculant and carryover may become a concern Again there may be a temptation to consider the use of either a pDADMAC or a polyepiamine, they will show well in a jar test. As with the direct filtration process care has

to be taken since if any unreacted short chain polymer is carried over to the filter plant, mud balls and filter blinding could result. There is also the possibility that the cationic solution polymers may cause an overdose of positive charge that may result in poor settling and extra loading on the filters Solids Contact Clarifier A well designed and operated solids contact clarifier followed by a properly sized filter plant will present few challenges to any one chemical treatment regime that it will not present to another. There is typically good mixing time and intensity in the centrewell, where there is also the opportunity for floc collisions and growth A "good" sludge bed will provide a straining and filtering medium, which can be complimented by the inclined baffle plates that many clarifiers are now equipped with. If there is any unacceptable carryover, the filter plant will usually separate the pin floc from the water
There are several process variables that can enhance or detract from the performance of a chemical treatment programme. Raw water temperature chemical feed point(s), turbine speed sludge bed height, and clarifier blowdown can all be adjusted so as to optimize the performance. All chemical treatment programmes, if it is the correct choice for the raw water and the treatment goals should be able to be optimized in a solids contact clarifier