Hello, I am working with municipal water treatment facility in Delhi, India.
We are facing a huge problem at our water filtration plant as sand filters are clogging/choking every 4-6 hours.
This problem is making our water filtration vulnerable and we need to backwash the filters multiple times per day. This is harming our water treatment efficiency.
Can anyone suggest a reason behind it and, if possible, a solution?
I have faced the same problem several years ago and it was due to the fact that suspended solids were escaping from the settling tank, along with clear water, as a result of not frequent operation of the filter press.
So the reasons for your problem could be :
1. High concentration of suspended solids, much higher than the initial filter design.
2. Suspended solids escaping from your settling tank, if you've got any.
3. Poor SAND QUALITY. If the sand is not QARTZ type- special sand for sand filters- but it's normal building sand then the susdpended solids stick with the sand particles. This blocks the water routes and as a result you get high pressure drop and frequent backwash.
Please note that if No3 is your problem then when you backwash the filter only a small part of the suspended solids are rejected so only a small part of the filter works. The rest
is blocked.
N.B. You must remember that the filtration speed should never exceed the value of 15 m/h. To calculate that you need to divide your flowrate by the cross-sectional area of the filter.
Lokesh- I concur with George. First - check your source water and your pre-treatment processes. Then - the filter material itself. One annoying problem we faced was the angular grain shape of the filter sand, which had orders of magnitude lower k, than would have been expected based only on grain size. Look at your filter material under a microscope - if it is moderately sub-rounded to well-rounded, then that is not the issue. However, if it is sub-angular to angular, that may be an important part of the issue.
Greetings from Oz: What is the constituency of the backwash - is it primarily organic or inorganic - installation of baleen with/without flocculation as a pre-filter will dramatically reduce your backwash concerns. Please visit www.baleen.com for background information. I am happy to advise further at yuri@baleen.com
I suppose this can be the potential reason of Filter clogging.
Actually, because of algae mass pH of raw water (river water) has increased to a large (pH About 8.2-8.4). Coagulant PAC (Poly Aluminium Chloride) does not work efficiently in this pH range. Nor high Chlorination was giving good results.
So, we Applied Alum as Coagulant with high dose.
However, turbidity of water is between 20-26 NTU Low.
Many things could results in Rapid gravity filter blockage but most all mud balls formation leading to decline in the efficiency of the filter. With such a problem, you will not only backwash several times a day but often it will over flow and if the plant is located in temperate region might lead to mud balls formation further deteriorating the filter. What causes this phenomenon is mostly the efficacy of the coagulation and flocculation process. First you need to make sure you find the optimum dose of your coagulant also within that you have to make sure the flocculation is taking place within the available settlement time. Often times we tend to forget there are two forces acting on the flocs; Vander walls forces of attraction and Electrostatic repulsive forces and until these two are in equilibrium the charges will repel thereby resulting in suspended flocs that will later be carried forward to the filter and that carried over flocs is the genesis of your filter blockages, mud balls formation, backwash and a resultant turbidity greater than 5.0 NTU.
I suppose this can be the potential reason of Filter clogging.
Actually, because of algae mass pH of raw water (river water) has increased to a large (pH About 8.2-8.4). Coagulant PAC (Poly Aluminium Chloride) does not work efficiently in this pH range. Nor high Chlorination was giving good results.
So, we Applied Alum as Coagulant with high dose.
However, turbidity of water is between 20-26 NTU Low.
Rapid head loss increase may be caused by several factors:
1. Poor raw water quality, high coagulant dosages, i.e. high sludge production/high solids load to the filter
2. Too high filtration rate combined with too high solids load (too high solids load). The higher the filtration rate, the shorter the filter runs...
3. Too high polymer doses (if used), may cause depositions in the upper part of the filter bed only
4. Poorly designed filters - with poor distribution of filter deposits over the filter depth (dual media filtration performs much better than single medium sand filters)
5. Inadequate pre treatment - if used (e.g. poor flocculation: too large, and too strong (polymer aided) flocs for the filter pores); poor sedimentation), etc
"There is no such thing as a good filter - it depends on the water too be filtered...."
I suppose this can be the potential reason of Filter clogging.
Actually, because of algae mass pH of raw water (river water) has increased to a large (pH About 8.2-8.4). Coagulant PAC (Poly Aluminium Chloride) does not work efficiently in this pH range. Nor high Chlorination was giving good results.
So, we Applied Alum as Coagulant with high dose.
However, turbidity of water is between 20-26 NTU Low.
Now, it is working well better than before.
You must open the filter and take a look about the sand. Microbiological developments can foul the sand. In this case you must make a strong backwash with hipoclorite to remove the biofilm.
In some cases the sand acts like a nucleation agent and a mineral scale could be seen.
I suppose this can be the potential reason of Filter clogging.
Actually, because of algae mass pH of raw water (river water) has increased to a large (pH About 8.2-8.4). Coagulant PAC (Poly Aluminium Chloride) does not work efficiently in this pH range. Nor high Chlorination was giving good results.
So, we Applied Alum as Coagulant with high dose.
However, turbidity of water is between 20-26 NTU Low.
Now, it is working well better than before.
Dear Lokesh, sorry to hearing that. Are You using chemicals prior to SF? If it is so, have You made any Jar test to assess the right amount to be metered? Maybe some post-coagulation in the SF due to excess of chemical? Maybe poor backwash? Is it air+water, right? Let's tune up the cycles and see.
I suppose this can be the potential reason of Filter clogging.
Actually, because of algae mass pH of raw water (river water) has increased to a large (pH About 8.2-8.4). Coagulant PAC (Poly Aluminium Chloride) does not work efficiently in this pH range. Nor high Chlorination was giving good results.
So, we Applied Alum as Coagulant with high dose.
However, turbidity of water is between 20-26 NTU Low.
Now, it is working well better than before.
There is a lot of relevant advice above. One point I would like to add about the coagulant and/or flocculant dosing is to check the mixing energy and how well the reagent stream is mixed in to the entire inflowing water body. I have seen a single small diameter pipe dropping reagent at one point into a water stream that is 1meter or more wide. That will not work effectively. It is better to add the coagulant/flocculant to the water stream using a perforated spreader bar for distribution across the entire flow width and it is important that there is a hydraulic jump, drop, baffles or some other form of hydraulic or mechanical mixing at that point to make sure the reagent is mixed throughout the water body.
Treatment works in India are normally well enough designed hydraulically, if they follow the CPHEEOO manual. If you have enough kinetic and/or potential energy at the dosing point you can create turbulent mixing energy by adding baffles. If not a mechanical mixer may be required.
I suppose this can be the potential reason of Filter clogging.
Actually, because of algae mass pH of raw water (river water) has increased to a large (pH About 8.2-8.4). Coagulant PAC (Poly Aluminium Chloride) does not work efficiently in this pH range. Nor high Chlorination was giving good results.
So, we Applied Alum as Coagulant with high dose.
However, turbidity of water is between 20-26 NTU Low.
Now, it is working well better than before.