hi rate sand filters in a municipal water treatment plant 1

[wpr]

Standard sand/anthracite filters are typically rated at 4 gpm/sf by state agencies. I have read several references from other consultants that indicate that these filters may be converted to "high rate", thus increasing plant capacity (but no real details on how to do this). My question is: What steps can be taken to realize this "high rate" system? I don't think it depends on upstream flocculation methods, as I can't see where any one process would be superior to another. FWIW, I have seen references to as much as 6gpm/sf.

 

[bimr]

If it is a gravity system, there is not much that you can do to push the water through.

If it is a pressure system, then all you have to do is to increase the influent pressure to force more water through.

There is no science that will allow you to improve a water filtration process so that it will function at higher throughput. It is just operating at the margins with less factor of safety. Your water quality may suffer slightly, your backwash frequency may increase, and your filter run time may decrease. The concept is akin to operating a vehicle at 95 mph as compared to 60 mph.

Industrial plants operate at the higher flow rates. There is little reason for a municipal water treatment plant to operate at the higher rates, except some equipment manufacturer is attempting to low ball a bid.

With all of the recent emphasis on water quality, it is talking a step backwards.

 

[QualityTime]

Having designed a number of municipal water treatment plants you actually can get more water to be filtered through a gravity filter. It all has to do with the sizing of the filter effluent rate control valve which is typically a butterfly valve. You typically determine what the minimum and maximum desired flow rate through your filter. These are your boundary operating conditions.

1. The best control range of a 90 degree butterfly valve is to size it so that it operates between 20 degrees open and 70 degrees open. In this range, the response is close to being linear (i.e. for a slight change in valve position your flow meter will pick up a change in flow)

2. In a standard rate filter you want to be able to filter water between, for example, 1 gpm/ft2 to 4 gpm/ft2.

3. Use the centerline of the effluent rate control valve as your elevation datum. You know what the operating water level is in the filter is in relation to the centerline of the effluent rate control valve. Say that is 14 ft

4. A freshly backwashed filter may have an initial head loss of 3 ft at 4 gpm/ft2 flow. Therefore at 4 gpm/ft2 flow, with a clean filter, you need to kill 11 ft of head. In the old days we used the BIF butterfly valve graphical tables that would show what the valve position would. At 1 gpm/ft2 flow, with a clean filter, you need to kill 11 ft of head the graph will tell you what the valve position will be.

5. A dirty filter may have a terminal head loss of 6 ft. Therefore at 4 gpm/ft2 flow, with a dirty filter, you need to kill 5 ft of head. In the old days we used the BIF butterfly valve graphical tables that would show what the valve position would. At 1 gpm/ft2 flow, with a dirty filter, you need to kill 11 ft of head the graph will tell you what the valve position will be.

6. Therefore (5) and (6) are your boundary conditions. Your valve should be working in the 20 degree to 70 degree open position range for these extreme boundary conditions. It stands to reason that the valve will be more open with a 4 gpm/ft2 flow with a dirty filter when you need to kill 5 ft of head compared to 1 gpm/ft2 flow with a clean filter when you need to kill 11 ft of head.

7. Getting back to achieving 6 gpm/ft2 filter rates. This means your filter effluent rate control valve will be bigger. An analogy would be your kitchen sink. If you want to drain it faster you have to have a bigger drain pipe. The more flow through the filter the more head loss you will have in the filter when it is clean. The filter runs will be shorter because the 6 ft terminal head loss will be reached faster.

8. Having high rate filters are probably okay for turbidity floc situations. It is definitely not okay for color floc. You may need to use polymers to toughen up the floc

9. There are implications upstream in the floc tanks. The increased flow reduces residence mixing time. Will that affect your floc formation

Nowadays, people don’t use graphs. You enter numbers into a valve supplier’s computer program to tell what the valve position will be. Hope this helps

 

[QualityTime]

One other thing. You are more susceptible to drive the floc through the filter at higher flow rates. All of these standard numbers such as 4 gpm/ft2 are tried and true numbers and they all have saftey factors built into it. Yes you could probably run the process at 6 ggm/ft2 but you just cut into your factory of safety. If something went wrong with your alum metering pumps or your polymer system or you just plain screwed up on some calculation you don't have that factor of safety cushion.

I have briefly read about high rate filters but I never really paid attention to them from a "what type of water it works best with and what problems can occur"

 

[QualityTime]

One more thing, in addition to the head loss in the filter you also have to take into account the loss in the filter effluent piping up to the filter effluent rate control valve. I think you now have enough to figure it out

 

[QualityTime]

I made some corrections to my post. I have been having problems with vision in my left eye and I am not picking up obvious errors. So I am reposting with corrections. My eyes are not what they used to be:


Having designed a number of municipal water treatment plants you actually can get more water to be filtered through a gravity filter. It all has to do with the sizing of the filter effluent rate control valve which is typically a butterfly valve. You typically determine what the minimum and maximum desired flow rate through your filter. These are your boundary operating conditions.

1. The best control range of a 90 degree butterfly valve is to size it so that it operates between 20 degrees open and 70 degrees open. In this range, the response is close to being linear (i.e. for a slight change in valve position your flow meter will pick up a change in flow)

2. In a standard rate filter you want to be able to filter water between, for example, 1 gpm/ft2 to 4 gpm/ft2.

3. Use the centerline of the effluent rate control valve as your elevation datum. You know what the operating water level is in the filter is in relation to the centerline of the effluent rate control valve. Say that is 14 ft

4. A freshly backwashed filter may have an initial head loss of 3 ft at 4 gpm/ft2 flow. Therefore at 4 gpm/ft2 flow, with a clean filter, you need to kill 11 ft of head. In the old days we used the BIF butterfly valve graphical tables that would show what the valve position would. At 1 gpm/ft2 flow, with a clean filter, you need to kill 11 ft of head the graph will tell you what the valve position will be.

5. A dirty filter may have a terminal head loss of 6 ft. Therefore at 4 gpm/ft2 flow, with a dirty filter, you need to kill 9 ft of head. In the old days we used the BIF butterfly valve graphical tables that would show what the valve position would. At 1 gpm/ft2 flow, with a dirty filter, you need to kill 9 ft of head the graph will tell you what the valve position will be.

6. Therefore (4) and (5) are your boundary conditions. Your valve should be working in the 20 degree to 70 degree open position range for these extreme boundary conditions. It stands to reason that the valve will be more open with a 4 gpm/ft2 flow with a dirty filter when you need to kill 9 ft of head compared to 1 gpm/ft2 flow with a clean filter when you need to kill 11 ft of head.

7. Getting back to achieving 6 gpm/ft2 filter rates. This means your filter effluent rate control valve will be bigger. An analogy would be your kitchen sink. If you want to drain it faster you have to have a bigger drain pipe. The more flow through the filter the more head loss you will have in the filter when it is clean. The filter runs will be shorter because the 6 ft terminal head loss will be reached faster.

8. Having high rate filters are probably okay for turbidity floc situations. It is definitely not okay for color floc. You may need to use polymers to toughen up the floc

9. There are implications upstream in the floc tanks. The increased flow reduces residence mixing time. Will that affect your floc formation

Nowadays, people don’t use graphs. You enter numbers into a valve supplier’s computer program to tell what the valve position will be. One more thing, in addition to the head loss in the filter you also have to take into account the loss in the filter effluent piping up to the filter effluent rate control valve. I think you now have enough to figure it out. Hope this helps

 

[operatorjoe]

I work at a surface water plant that was taken from 4gpm/ft.[sup]2[/sup]/min to 6gpm/ft.[sup]2[/sup]/min. The engineer neglected to increase size of openings at end of flocculators and in the baffle walls at beginning of sedimentation basins. We have a high seasonal shift in demand so during low flow in winter all worked fine. When we went to high flow rates the high velocities would shear flocc and carry over to filter and made for some very short filter runs. We have done a lot of mods to relieve this (cutting and coring concrete). We also started feeding an iron product followed by a PAC which doubled our chemical cost but dropped our settled turbidity to <.2 ntu. so filters have delivered 100-200 hours at the higher rates.

 

[bimr]

operatorjoe: Thank you for your observations. You point out that it is not as simple as just raising the throughput. There are operationals issues, costs, and quality issues when trying to go beyond the tradional operating pflow rate when using traditional equipment.