Worksheet 1c – Different water types
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Type 1: High turbidity with high alkalinity
Most coagulants, including polymers, are effective making this type the easiest to treat. The large
numbers of colloids (indicated by the high turbidity) and the high alkalinity make flocculation easy.
Type 2: High turbidity with low alkalinity
Metal based coagulants need alkalinity available to be effective. Low alkalinity means additional
chemical dosing is required to maintain the pH within its and to boost alkalinity to ensure coagulation is
effective. Lime (in various forms) is often used for this purpose. Polymers are also effective and can be
used alone or in conjunction with the metal coagulants.
Type 3: Low turbidity with high alkalinity
The low numbers of colloid particles make it harder for the coagulants to come work effectively. This also
means that polymers by themselves are not always effective. To avoid excessive mixing and flocculation
times usually requires much higher coagulant doses, the use of polymers to aid them and possibly long
flocculation mixing times. Sometimes, adding a coagulant aid (such as bentonite clay) to provide
additional particles is effective enabling it to act like Type 1 water.
Type 4: Low turbidity with low alkalinity
This is a difficult type of water to coagulate. It has all the problems of a type three water, but just adding
more coagulant is not always enough. Low alkalinity makes it harder to prevent the coagulation pH
dropping so low that the necessary floc precipitates will not be formed effectively. Usually substances,
such as lime, soda ash or sodium hydroxide, are added to provide sufficient alkalinity. As with type three
water it may be easier to add a coagulant aid such as bentonite clay and polymers cannot be used
alone.
Worksheet 1c – Different water types
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Type 5: Colour with turbidity
If there are sufficient colloid particles in the water, the true colour causing substances are adsorbed onto
the clay particles and are removed with the turbidity in the same manner as type one and two waters.
Increased coagulant doses will usually be needed depending on the actual amount of substances
measured as part of true colour.
Type 6: Colour with low turbidity
When there is a high level of substances causing colour in the raw water in relation to the number of
colloid particles, two things can happen. One is that some colour is adsorbed onto the clay particles in a
similar way to type five water. The other is that the remaining free colour-causing substances stay in
solution.
When the coagulant is added it first reacts with the colour in solution and forms colloidal precipitates.
After this is completed, the coagulant then reacts with these precipitates and the clay particles, on which
the remaining colour has been adsorbed, in a normal turbidity removal process.
Because there are two separate reactions, higher coagulant doses are usually needed to for the removal
of either the colour or turbidity alone.
The affect of pH on the colour removal process is important. Low pH associated with colour removal
cause problems in controlling the levels of iron and manganese in the treated water when ferric-based
coagulants are used.
What type of water is normally processed by your treatment plant?
Does the type change at all, and if it does how often and when?
Match the normal treatment approaches used in your plant with the type of water that has to be
processed.
Worksheet 1c – Different water types
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