How to Select a Chemical Coagulant and Flocculant.
In many water treatment processes the selection of the chemical regime is of critical importance.
The mechanical equipment will remove water contaminants to a reasonable level, but to meet the increasingly stringent Federal and Provincial licensing requirements chemical coagulation, flocculation, and disinfection are necessary.
This paper will address several topics that will help the water treatment plant operator select the most appropriate chemical treatment programme for the needs of the community that the plant services.
Why do we Chemically Treat Water?
Water is essential for life as we presently know it and in North America we have become accustomed to receiving good quality water at a reasonable cost (on world wide terms Canadian drinking water is provided at an extremely low cost).
In 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 be reviewed.
In many cases what was considered acceptable by all segments of society just a decade ago would now be thought of as unsafe.
At the present time it can be anticipated that changes will be made with respect to the following parameters
[1]; 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 application.
The three variables are;
1) Raw Water Quality.
2) Process Equipment.
3) Treatment Objectives.
These three variables can be further categorized as shown in the table below;
Raw Water Quality Process Equipment Treatment Objectives
Alkalinity Settling Lagoon
Potable Application
Partial Softening
Full Softening
pH Direct Filtration
Turbidity Sedimentation + Filtration
Colour Solids Contact Clarifier
Industrial Application
General Use
Ion Exchange
Temperature Dissolved Air Flotation
Hardness Mixing Intensity
Taste and Odour Sludge Disposal
flocculant selection process a considerable amount of time and needless effort can be saved.
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 goals
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;
1) The amount of alkalinity present in
the water may eliminate some coagulants from consideration.
2) 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 year.
Alkalinity
Alkalinity is of critical importance when selecting a metal salt coagulant such as polyhydroxy aluminum chloride (PACl), 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-1, 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 ahigh 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-1 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 sophisticated approach and would not be recommended for a smaller community with a restricted capital budget.
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-1).
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 selected.
Temperature
Temperature can affect the performance of the inorganic metal salts that rely on a chemical reaction.
The colder temperatures (<50 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 80 C in winter to maintain adequate finished water quality.
Inndustrial 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, 100 C T 250 C.
Hardness
Calcium and magnesium hardness are present in all waters to some degree or another.
The amount of CaH and the end use of the water will determine the strategy required to handle the presence of these minerals.
Typically lime is added to allow for the precipitation softening process to take place.
Lime sludges are dense and will tend to settle, however, it is recommended that 10 mgL-1 of an alumina coagulant be added to capture the lime fines.
It should be stressed that the coagulant is present only to capture the lime fines and not to coagulate raw water turbidity.
Lime sludges cannot be returned to the environment, so dewatering or lagoon storage is required, all coagulants should be evaluated with respect to their ability to dewater on the equipment in the water plant.
The major criteria for efficient lime softening is pH control, pH should be maintained at 10.0 ± 0.2. A metal based coagulant will consume alkalinity, especially in a well buffered high pH water, which could compromise the softening process.
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 waterfine particles that have to be well coagulated.
Most of the coagulants will settle PAC/organic particles, however because of the very fine nature of the species it is better to choose a coagulant that generates the densest sludge.
The correct choice will result in minimal pin floc carryover, while the incorrect choice will be characterized by a low consistency sludge bed and observable pin floc carryover. applications.
Almost all the coagulants will perform well in warmer waters, 100 C T 250 C.
Hardness Calcium and magnesium hardness are present in all waters to some degree or another.
The amount of CaH and the end use of the water will determine the strategy required to handle the presence of these minerals.
Typically lime is added to allow for the precipitation softening process to take place.
Lime sludges are dense and will tend to settle, however, it is recommended that 10 mgL-1 of an alumina coagulant be added to capture the lime fines.
It should be stressed that the coagulant is present only to capture the lime fines and not to coagulate raw water turbidity.
Lime sludges cannot be returned to the environment, so dewatering or lagoon storage is required, all coagulants should be evaluated with respect to their ability to dewater on the equipment in the water plant.
The major criteria for efficient lime softening is pH control, pH should be maintained at 10.0 ± 0.2.
A metal based coagulant will consume alkalinity, especially in a well buffered high pH water, which could compromise the softening process.
The best coagulant is therefore a pre-hydrolysed species with a high basicity. PACl has been found to be very suitable for lime softening applications.
A flocculant is seldom needed, but filtering is always recommended.
The only major problem encountered with a lime softening programme is if there is a need to soften at a high pH and remove organic colour at a low pH.
The only real solution is to make a capital investment in two clarifiers, arranged in series.
Initially the raw water is treated in a conventional way, at pH 7.0, a low basicity coagulant should be added to ensure that a good sludge bed is maintained for the straining and filtering action.
This is followed, in a separate clarifier, by the lime softening step, at pH 10.0 with a high basicity coagulant.
In municipal applications, where the requirement is to reduce hardness to<100 mgL-1 flocculants are not recommended, however the water should always be filtered.
In those industrial applications where the water is sent to an ion exchange stage, hardness is reduced to [1]40 mgL-1 and a flocculant is always used. Filtering is still required prior to the ion exchange equipment.
The advantage of a low basicity coagulant is even more pronounced in full softening applications, and the non-sulphated PACl is the coagulant of choice.
Taste and Odour
Taste and odour can be controlled in a variety of ways, but one of the most common is with the addition of powdered
محطات معالجة الصرف الصناعى والصحى بالطرق البيولوجية والكيميائية
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