hi rate sand filters in a municipal water treatment plant
hi rate sand filters in a municipal water treatment plant
(OP)
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.





RE: hi rate sand filters in a municipal water treatment plant
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.
RE: hi rate sand filters in a municipal water treatment plant
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
RE: hi rate sand filters in a municipal water treatment plant
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"
RE: hi rate sand filters in a municipal water treatment plant
RE: hi rate sand filters in a municipal water treatment plant
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
RE: hi rate sand filters in a municipal water treatment plant
RE: hi rate sand filters in a municipal water treatment plant