You may login with either your assigned username or your e-mail address.
The password field is case sensitive.

Shortened Filter Runs: Troubleshooting Turbidity Levels - Trinidad, West Indies

I live in Trinidad West Indies. I have a problem with a water treatment plant that treats water from two surface water intakes. During the rainy season, the turbidity levels often exceeds 150 NTU's. The plant cannot treat this level of turbidity because of the shortened filter runs. The plant  consists of a raw water intake followed by a clarifier, filter and a clear well. The water is chlorinated in the clear well before pumping into distribution. Water is treated with Alum to reduce particualte matter. How can I modify the plant to treat the higher turbidity levels and reduce shut downs at the plant and optimise production.?

Production at the plant has also fallen from 190 cu.m per hour to 154 cu.m per hour. The raw water pumps at the plant are to be changed shortly. Kindly suggest a suitable intervention.

J. Warner




Generally the turbidity level at the inlet of the Rapid Sand Filter needs to be controlled to maintain normal filter runs . The alum dose can be increased when your turbidity level increases. You can carry out jar tests to arrive at an optimal alum dose.


Dear J. Warner,

1. Your plant has been correctly designed to handle feed water with high turbidity of over 150 NTUs during the rainy season. A clarifier can handle such high turbidities of as much as 500 or more NTUs. A properly operated clarifier would reduce the turbidity down to approximately 20 NTU on an average which can easily be handled by the filter which would then produce filtered water with an average outlet turbidity of < 5 NTU. Obviously this is not happening.

2. From your description of your plant, it is not really clear what is the sequence of treatment in your plant. As an expert on water treatment, I would expect the sequence as follows: Raw water intake> Coagulant dosage> clarifier> clarified water tank (clear well)>chlorination> pressure filtration> distribution of water. I need your clarification whether your plant follows the above sequence and if not what is the actual sequence.

3. Alum is a coagulant which aids the turbidity to settle by forming 'floc' which then settles at the bottom of the clarifier as sludge which is removed by opening a sludge 'bleed off ' valve for this purpose. Normally I would expect alum to dose at 20 ppm in the clear water intake going to the clarifier which should have a flash mixer which ensures that alum and raw water are intimately mixed as they enter the clarification zone.

I would like you to clarify what is the dosage of alum in ppm and at what point it is dosed, and how is the alum and raw water mixed before entry to clarifier?

4. Clarified water from the clarifier should be stored in a clear well/clarified water storage tank from which it should be pumped through a filter or filters to points of use or to a filtered water storage tank. Can you give the turbidity of clarified water before it is before it enters the filter? What is the turbidity at the filter outlet?

If you can give me replies to each of my questions, I am sure I can give you a permanent solution.

Looking forward to your reply.


Ion Exchange


Dear J. Warner,

The conventional treatment process is adequate to treat surface water with higher turbidity. The coagulation- flocculation and sedimentation process need to be improved. Add optimum dose of alum (take help of jar test) and keep pH between 7 and 8. There must be good rapid mixing in flush mixer. In flocculation zone of clarifier there must be proper slow mixing system. Please check weir loading rate of clarifier.

Please try to keep turbidity of the clarifier water within 15 NTU. I think you have Rapid Sand Filter. The cleaning interval of filter would be one day. The turbidity of treated water should be less than 1 NTU.

The design parameters of your WTP need to be checked.

You can slightly overload provided design parameters are well within the permissible limit or range.


Dr Arunabha Majumder
Indian Water Works Association (IWWA)


Dear J. Warner,

The operational modification that possibly can solve the problem is to improve the performance of clarifier to bring it as near as possible to the non-rainy season level. For this, jar tests need to be carried out on rainy season water and accordingly coagulant dozing needs to be changed so that clarifier effluent quality is acceptable to the following filters.

If there are difference in jar test results and actual clarifier outlet quality then sludge removal pattern should also be looked into and shortcomings, if any, leading to deterioration of clarifier outlet quality due to inadequate withdrawal of sludge should be addressed.

Views are expressed in personal capacity.

With regards,

Environmental Engineer
Pollution Control Implementation Division - III
Central Pollution Control Board
New Delhi


Dear J. Warner,

This is the standard problem face by people who directly lift water from surface sources.

Had this surface water been subjected to pass through a natural filter system such as gravel pits before it comes to the point of abstraction you would not have been facing this problem.

I would advice you that you create an abstraction sturcture wthich is tapping groundwater which is being charged by the surface water sources. The problem of variation of turbidity will be taken care of automatically.

Best of luck

Dr. Anil Lalwani


Dear J. Warner,

More information is required for taking some decision. My inputs are as follows:

You have to introduce sedimentation facility with longer detention period for raw before entering clarifier.

You may try with some coagulant aids to increase efficiency.

One should go for Jar test, if not done, to determine the dosing of chemicals.
If it is a conventional RSF, scope for upgrading the same to dual media may also be considered.

Kindly let me know about better solutions, if possible.

With regards,

Diponkar Bordoloi
Assistant Engineer
Public Health and Engineering Department (PHED)
Guwahati, Assam


Dear J. Warner,

Probably you can try using poly aluminum chloride (PAC) in the place alum. Other option is to try a combination of alum and permitted coagulant/flocculants aids. For either of these options, laboratory experiments are needed prior to field application for fixing the doses.
These approaches may help in significantly reducing the turbidity of inlet water to the filter, thus resulting in longer filter runs. I have attached a paper on PAC, which may be of your interest.

Access paper here: Malhotra.pdf


Leela Iyengar

8. Need help - to study comparison of flocculants

I am work ing on project in my college on comparative study, uses, advantages, disadvantages of all the coagulants/ flocculants (organic as-well inorganic) used in present day's water / waste water treatment. In above regards I am considering flocculants like, 1. Alum, 2. PAC, 3. PFS, 4. PGa21Ca. Can some one please guide me.

Best regards,
N.P. Prashanth

Post new comment

The content of this field is kept private and will not be shown publicly.
This question is for testing whether you are a human visitor and to prevent automated spam submissions.
7 + 0 =
Solve this simple math problem and enter the result. E.g. for 1+3, enter 4.
Creative Commons License
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 2.5 India License.