shortened filter runs after cl2 entry point change 1

[h2orange]

We recently moved our cl2 injection point from the head of our plant, to the top of our filters in order to help our disinfection by-products{tthms and haa5s).This has succesfully lowered our haa5's, but our filter run times have been reduced from 50-60 hours down to 15-20 hours.We have tried adjusting our alum to lime ratio in order to lower the ph,but this has had basically no effect.We also started feeding potassium permanganate at the head od the plant, still no change.Our plant is a 2 mgd, class two, conventional sand filter plant.

 

[bimr]

The change must have increased the amount of suspended solids that is being captured on the filter.

Unless you provide additinal information on the treatment plant, it is difficult to understand. For example, is this surface water?, is there a clarification?, is the lime for pH adjustment?, what are you trying to ozidize with potassium permanganate?, etc.

 

[h2orange]

the plant is a 2.0 mgd, class two, conventional sand filter, surface water plant.we use aluminum sulfate as a coagulant, and lime for ph control. To avoid algae growing in our sedimentation basins, we add potassium permanganate at the head of the plant right after alum and lime.There is a 4 hour detention time before it reaches the filters.As i said the filters are convential sand filters which filter about 700,000 gallons a day.The problem appears to be excessive air binding.

 

[ch81pc]

When you say on top of the filters, what is the retention time? Could you compromise by dosing on the baisin outlet thereby compared to dosing pre-basin? One other thing to try is adding the Permanganate before the pH control and Coag.
How long have you left the system to aclimatise to the new dosing arrangements? Some strange things can happen as the various chemistries and bio chemical systems adjust.

 

[bimr]

Why do you think the filters are air bound?

Was the transition to the shorter filter runs spontaneous or did it occur over a period of time?

From what you are describing, it seems that you are now growing algae in the sedimentation basins and filtering out the algae in the filters. Is it possible to schock chlorinate in the basins?

 

[h2orange]

I believe air binding is the problem because, when we backwash a filter it releases air and bubbles.....alot, like if it was boiling.

The transition was almost instantaneous, within 4-5 hours, which is our detention time from the head of the plant to the filters.

We have'nt tried "shock chlorination", but certainly plan on investigating it !

 

[bimr]

You probably need to review your operating log to make sure that nothing has changed (operating parameters) in the operation of the filters.

Your backwash duration should not have changed. One would expect it to be about 7-8 minutes.

Has the effluent turbidity changed?

 

[sjohns4]

What kind of process do you have ahead of your filters? I'm working with a client who has problems with their filters boiling as you have described. My problem is lack of historical info and conflicting stories from the operators.

Their filters take about 4-6 hours worth of operation before boiling, but are getting around 20 hours worth of run times. If a filter that has only been in service for a few hours is backwashed there is no bubbles.

Has your settled water trubididty changed since changing the location of the chlorine feed?

ch81pc - what would you anticipate adding the pot.per. before the lime and coagulant do?

If you find a cure please do post, and I'll do the same.

Mike

 

[bimr]

Boiling may occur during the air scour or other backwash steps as a result of an upset filter bed, uncontrolled air in the backwash water, or incorrect sequenching. Boiling during air scour can occur anywhere. Boiling caused by uncontrolled air in the backwash is usually near the flume area.

 

[sjohns4]

The filters in the plant I'm working on don't have air scour. They will actually boil shortly after both influent and effluent valves are closed, under static conditions, before opening backwash valves. Backwashing increases the boiling however.

 

[ch81pc]

Adding the KMnO4 before dosing coag/pH can help as the oxidised material will floc better. Adding it after the coag means you'll also be oxidising that too. Not a huge problem/improvement but every little bit helps.

With regards the boiling I have seen this on a small scale when we altered dosing points. We generated much finer particulates which got carried deeper into the bed. After a 2-3 day period of being in service, the back wash would liberate all of the accumulated gases generated by the bugs eating the Particulate BOD.

 

[sjohns4]

I wonder if increasing the chlorine concentration would help to kill the bugs?

 

[ch81pc]

It will, but may give you higher THM's.

 

[gbuch]

We are having the same issues with our filters at one of our plants. After about 4 hours air will begin to collect in the filter and we have to backwash each filter within 24 hours due to the flow thru the filters dropping off so much. Our turb. readings are still ok but our filters just cannot pass any water thru them. If anyone has any ideas we would love to hear them.

 

[bimr]

Has the amount of solids increased in the raw water?

It would seem that you have a build up of particles in the upper region of the filter. This in turn may cause a zone of negative pressure. This is undesirable because the negative pressures may cause air pockets to develop.

Examine your backwash procedures to ensure the backwash is being done properly.

If the backwash procedures are correct, then take a core sample of the media and evaluate the media.

You also may not have enough water head on top of the filter and/or the filter effluent is free draining to the atmosphere.

 

[sjohns4]

gbuch,

The name looks familiar, you talking about the Glade Ck. plant?

 

[justanoperator]

Adding KMnO4 to water adds Mn to water and that comes in two states soluable and insoluable. Insoluable Mn will flocc and settle with alum or most any other coagulant. But any soluable Mn in water will bypass settling and convert to insoluable Mn when chlorine (or other oxidant) is added to water (even flocced and settled Mn can revert back from insoluable to soluable in an anaerobic sludge). The reaction is slow at pH below 8 (see/search Mn soluability curve). The Mn will be introduced to filter in a small size paticle(colloidal). It will clog filter and cause air binding along with quick increase in loss of head. The Mn will also change the the effective size of filter media (by adhereing to media). The media will return to a normal size/shape after shock chlorination of media (may take several rounds of shock/backwash) I am starting to consider change to hydrogen peroxide as replacement for KMnO4. Send more questions if I have not covered all bases on this subject

 

[bimr]

Manganese from KMnO4 is not soluble, it is in the form of MnO4. Under typical water conditions, iron and manganese are
oxidized.

Water treatment plants should not have anaerobic sludge conditions except for the residual treatment processes.

Overfeeding of KMnO4 may cause excess manganese (insoluble) to pass through the treatment plant. Overfeeding of KMnO4 will be evident since the water will be pink and have brown/black solids. Other than that, the use of potassium permanganate has little impact on other treatment processes at the water treatment facility.

http://www.epa.gov/ogwdw/mdbp/pdf/alter/chapt_5.pdf

 

[bimr]

A number of operators have recently reported that when the chlorination was stopped, manganese oxide that has built up on the filters over the years has sloughed off.

This has occured at plants removing manganese with chlorine prior to filtration and where the filter media has become coated with manganese oxide.

If the chlorine is stopped, the "greensand" effect is stopped and operational problems begin.

This can cause significant manganese slough off. The manganese will have to be removed with backwashing, filter-to-waste, or other procedure.