why treating water chemically/لماذا نعالج المياه كيميائيا 1

Select a Chemical Coagulant and Flocculant

Why do we Chemically Treat Water
today's increasingly complex society the demands of the consumer, the medical and scientific communities, and therefore the Municipal, Provincial and Federal regulators, have caused the quality guidelines for safe drinking water to bereviewed. In many cases what was considered acceptable by all segments of society just a decade ago would now be thought of as unsafe

In an era of changing regulations and guidelines it is difficult to define what is considered to be( the Guidelines for Drinking Water Quality) as well as the Provincial Licensing Authority will allow for a decision to be made
At the present time it can be anticipated that changes will be made with respect to the following parameters :-
Various pesticides, organic compounds and metals,cyanide dioxins and furans, fluoride nitrilotriacetic acid (NTA), total dissolved solids, trichloroacetic acid trihalomethanes

Once the community at large has made the decision as to the delivered quality of the finished water that will be enjoyed by the consumer, the practical considerations have to addressed Invariably mechanical means will be employed to help achieve the treatment objectives, but since no mechanical process is 100% efficient, chemical enhancement will often be necessary Another reason for appraising the utility of chemical treatment is when capital costs are being balanced against operating costs

Either the size/expense of a piece of equipment can be reduced if efficiency can be increased by implementing chemical treatment, or else physical space limitations do not allow for the installation of a large process addition and therefore optimization of existing equipment has to be considered first Chemicals typically find utility in the removal of suspended, colloidal and dissolved solids from water, including calcium and magnesium hardness mineral turbidity, organic colour and other organic substances, and undesirable microbiological species that can cause health concerns in humans

The four broad categories of chemicals used are lime for precipitation softening coagulants and flocculants for the removal of suspended and colloidal solids, powdered activated carbon for taste and odour, and disinfectants for the removal of pathogens Selection of Chemical Species There are three fundamental variables in water treatment, all three of which will have a significant influence on the type of chemical that could be usefully employed in a particular applicatio

The three variables are:-
Raw Water Quality.
Process Equipment
Treatment Objectives
An understanding of how the variables effect water chemistry will allow the operator to make sensible pre-screening decisions and let him/her focus on optimizing the process to achieve the treatment goal

Raw Water Quality
Clearly the quality of the raw water and the contaminant classification, has to have a significant impact on the type of chemicals used for liquid-solids separation.
There are however several factors to consider:-
The amount of alkalinity present in the water may eliminate some coagulants from consideration.
The amount of turbidity present may only determine the amount of coagulant that may be required.
One also has to be aware of how the raw water quality will change as a function of the time of the yea

Alkalinity
Alkalinity is of critical importance when selecting a metal salt coagulant such as polyhydroxy aluminum chloride (PAC) aluminum sulphate (alum), or ferric sulphate. All these materials need some alkalinity to drive the hydrolysis reactions that allow the coagulants to function. If the water has a low alkalinity, below 50 mgL , then the use of some of the more acidic metal salts may be precluded. In these instances there are two options either add supplemental alkalinity (as NaOH, Ca(OH)2 or Na2CO3), or use a
high basicity coagulant (>50% basicity such as PACl or ACH. If the water to be treated carries a very low alkalinity loading then the use of artificial alkalinity will always be necessary. In such a case it might be useful to try a combination of acidic and basic aluminum salts, PACl ACH or alum, together with sodium aluminate. Should the alkalinity be >50 mgL then, in general, there will be sufficient present to drive most coagulation reactions. However, if the coagulant dosage has to be higher (by a factor of two) than the raw water alkalinity it may be necessary add some alkalinity to drive the hydrolysis reactions to completion

pH
The pH of the water could also determine/eliminate many treatment options. If the pH is higher than 8.5 and Dissolved Organic Carbon (DOC), often referred to as colour, has to be removed a highly acidic coagulant that will drive the pH down to ± 7.0 will have to be considered. It may be necessary to add some soda ash in order to bring the Langlier Stability Index back to zero after such treatment. If the pH is acidic great care will have to be taken to ensure that the chemical reactions occur as desired and that the finished water is stable removal of colour will be easy. Ferric salts often perform well in acidic conditions. The most challenging conditions occur when colour has to be removed from a water that has a high pH and a low alkalinity. Careful depression of pH without alkalinity destruction can be realized if gaseous CO2 and Ca(OH)2 are added together. The choice of coagulant will determine the extent to which pH hasto be depressed. This is a somewhat recommended for a smaller community sophisticated approach and would not be with a restricted capital budge

Turbidity
The precipitation of mineral turbidity by the classic coagulation and flocculation process is well defined and reasonably straight forward. Turbidity can be classified as being anionically charged silica particles. Often the effect that turbidity has is dependent on the amount present rather than the classification. In low turbidity waters (<10 NTU) an organic polyelectrolyte should not be considered. The choice of inorganic coagulant should be one that quickly generates the Al(OH)3 sweep floc and will form a stable sludge bed. In moderate turbidity waters (<100 NTU the use of a general purpose inorganic salt is preferred, and most will be successful if the other conditions are right. In high turbidity situations, or in those instances where surface water turbidity can increase very rapidly, a
PACl blended with a polyepiamine is often the best choice. Sludge bed height, sludge volume, dewatering efficiency, and pH depression are all reasons to consider the PACl blend over large additions of alum or ferric sulphate Organic polyelectrolyte on its own will be effective, but the cost is high, and there is the potential to blind downstream filters with a high dosage of epiamine or pDADMAC (> 4.0 mgL

Colour
Dissolved Organic Carbon, DOC, colour is the parameter around which a chemical treatment regime is built. Hydrophillic colour is invariably more difficult to separate from water than is hydrophobic mineral turbidity. The complexing of colour is dependent on the pH of the water, the classification of the colour colloid, and the ability of the coagulant to break the hydrogen bonds present. The choice of chemicals must be one that will create a water in which the colour will be least stable (usually at a pH between 5.5 and 7.0), the alkalinity will be preserved for turbidity precipitation, and the finished water will be neither corrosive or scaling In many applications it is difficult for one material to be completely successful by itself, especially the inorganic metal salts In these instances cost performance economics dictate that a small amount of an organic short chain polymer, usually from the pDADMAC family be utilized. If alum or ferric sulphate is the primary coagulant, then the supplemental addition of pDADMAC will have to be via a separate feed system. If any of the PACl preparations are used, a one product blend can be selecte

Temperature
Temperature can affect the performance of the inorganic metal salts that rely on a chemical reaction. The colder temperatures (<5 C) have a profound effect on alum and iron salts to the extent that performance is often unacceptable during the winter. It is not unusual for a water plant to have to heat the raw water to a minimum of 8 C in winter to maintain adequate finished water quality. In industrial applications carryover of alumina or iron flocs can cause process non-conformities and off specification production. The non-sulphated polyhydroxy aluminum chloride choice does not appear to be as temperature sensitive and is therefore a good first choice coagulant for cold water applications. Almost all the coagulants will perform well in warmer waters, 10 C,25

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