Dear Lokesh, Unfortunately, ...
Published by Arindom Borah, NEPACK NE/Freelance - Chief Consultant
Dear Lokesh,
Unfortunately, RG sand bed filters clog quite rapidly. Where is the upstream water coming from? Is it coming from, or even from sedimentation tanks, which are basically tanks without the mechanical rotating machinery which helps to coagulate the impurities. The clearwater from the sedimentation tank top spills over and enters the RG sand bed. Here, the micro-organisms in the water are trapped in between the sand particles and the final clear water is obtained which is chemically treated and sent to the distribution reservoir. This is how a RG filter bed works. It is normal for a RG filter to clog in 8 hours or even faster. Therefore, the design is done in such a way that a number of filter beds are filled with water alternately i.e. when one lot is being back washed, another is working at full efficiency. That way you get continuous water.
When the Filter beds are built, the contractor has to guarantee a fixed output per day which is usually tested before the plant is taken over. Check with that figure. If there is a discrepancy, your upper water source i.e. the sedimentation process is not working as it should. If there is normal discrepancy taking into account efficiency loss of the system over the years, then you are getting the design flow. No use crying over it, in that case
You can occasionally change the top sand layers and loosen the sand at the bottom so that during backwash, compressed air is cleaning the passages between the sand particles thoroughly. It is also possible that your backwash pumps are too old and lack the head required to backwash properly? Perhaps you are just letting the impurities gather in the bed after each filtration cycle, rather than sieving them off the surface? Anyway, I've given you all the possibilities. All the best and happy 'Sherlocking'!!
2 Comments
Thanks for reply.
Regards
I think what you're describing above is the Slow Sand Filter not the Rapid Gravity Filter. The slow sand filter is that filter that forms a dirt layer called Schumutdecker that traps and forms a colony of bacteria. Contrary to the slow sand filter, the RGF in a COAGULATION and FLOCCULATION water treatment process filters the carry over Flocs from the Clarifiers, just before Disinfection. The idea is to make sure the water passing out of the RGF has a turbidity less than 5.0 NTU/FTU and that is needed in order to achieve efficient disinfection. The consistent clogging/blockage of the RGF is as a result of those carry over flocs and that can be solved by checking the dosage rate of the Aluminum Sulphate or whatever coagulant they are using perhaps because of changes in the raw water quality seasonally certain changes needed to be done periodically. Another way is to look at the Alkalinity of the raw water if the alkalinity is low and pre-lime is used then secondary precipitation might result in that excess flocs carried over to the filters.
Published by Abba Ahmed