2 replies to this topic

[alokjaiswal]

Dear All,

In our plant, we have conventional water treatment plant of 908 m3/hr to treat Raw water and then sent the same either for Cooling Tower make up water, or to DM water purpose. The process is
Inlet Chamber ------> Parshall Flume ------> Mixer ------> Clarifloculator ------> Rapid Gravity Sand flitration ------> Clear Water Sump.
The PAC (Poly Aluminium Chloride) is dosed in Inlet chamber. Pre-chlorination is also done at inlet chamber whereas Post-chlorination is carried out before Clear water storage Sump.
Th Raw water which is having Turbidity of less than 0.1 NTU is fed at inlet chamber. At inlet chamber, the backwash of Sand filter, which is stored in a different sump, is also pumped at inlet chamber. The reason which I have recieved from operating personnel to add backwash water at inlet is to maintain turbidity so that Clarifloculator can perform better.
Now the problem part is that Rapid Gravity Sand filter gets exhausted very quickly which is evident from high pressure drop across the filter as well decrease in flow rate. As soon as they are regenerated by blowing first air and then backwash water, the filter performs better. In 8 hours, we encountered 2-3 bed exhaustion in such a low turbidity water. The rapid gravity sand filters are designed for 1000 mg/L of suspended solids and 8-9 bed regeneration in a day.
My queries are:
1. Why the sand filter are getting exhausted very quickly at such a low turbidity?
2. Is it a good practice to introduce also the backwash water at inlet chamber?

Some more info:
1. We have also observed some white material undissolved (usually PAC) on sand filters.
2. The Backwash water is taken in a different sump and from there water is pumped at inlet chamber as indicated by consultant.
3. The turbidity is normally observed at less than 0.1 NTU at Raw water inlet and more than 2-3 NTU at Sand filter inlet.

Please provide assistance as this sand filter bed exhaustion is creating a lot of problem for plant operator.

[breizh]

Hi ,
1) Did you talk to your supplier about your problem ?
2) Did you open the equipment to check the sand and internal parts inside ? lack of sand ,channeling in the bed , corrosion ?
3) Back wash is full of rubish , dirt , must be drained .

Here is my understanding .
Hope this helps
Breizh

[alokjaiswal]

Reply is as follows:

1) Did you talk to your supplier about your problem ?
Reply: No. We have not talked to our supplier as the concerned person is on long leave.

2) Did you open the equipment to check the sand and internal parts inside ? lack of sand ,channeling in the bed , corrosion ?
Reply: Treated water requirement is such that the filters have not been checked yet. But it will be planned very soon.

3) Back wash is full of rubbish , dirt , must be drained .
Reply: Backwash may be full of rubbish but it is the consultant only that has suggested since turbidity is very low at Raw water inlet. Hence, to fulfill the requirement of turbidity in Clarifloculator, backwash water is recirculated.

But still some facts remains unexplained like why the sand filters gets exhausted at such a low turbidity of 2-3 NTU whereas it is designed for 1000 mg/L of suspended solids.