Combined Water/Air Backwash of RGF

[tipp79]

Based on the Water Authority's design guideline, the water backwash flowrate during the combined water/air portion of the backwash regime is between 6 -12 m3/m2/h. The backwash flowrate during the water only portion is 21 m3/m2/h. Our mechanical engineer is finding it to diffiuclt to source a pump with such a range (the rfiltartion area is 3.14 m2 so the required flow range is 5.24 to 18 l/s). The lowest flow possible on the current pump selected is 6.9 l/s or 7.9 m3/m2/h. I've not being able to confirm the design basis for these flowrtaes but assume it is to cover varying temperatures with the actual flowrate optimised during commissioning.

My questions are

(1) does the above backwash flow range seem reasonable

(2) is there a way of checking whether we wil need to get the backwash rate down to the lower backwash rate

Many Thanks

 

[rcooper]

Has your mechanical engineer considered using two pumps for the higher flow rates and one pump for the lower flow rates?

 

[tipp79]

Yes. Our mech engineer is looking at that possibility and at trying to lower headlosses. I'm just trying to find out whether the pump/pumps is actually required to pump the entire range of flows.

I should have said earlier that the RGFs are second stage filters for removal of manganese.

Cheers

 

[rcooper]

Is the clean backwash water storage tank at a similar level or higher than filters? If so finding a pump will be a real challenge.

The water only rate seems about right.

The combined air/water rates seem low, but they depend upon the media being used. It does vary with temperature. Selection of media tends to be down to experience of the water being filtered or pilot trials. Have these been carried out for this plant?

 

[bimr]

If the sewer does not have the capacity to take the backwash flow away, then it is common to have the backwash collected in a sump. Then a small pump is used to pump it out to waste.

 

[sbush]

Answer: (1) Backwash flow range is reasonable for combined air-water method.

Answer: (2) Combined air-water method must have lower backwash rate. Check with the manufacturer of the air-water backwash collection trough. They should be the ones feeding design information to your mechanical engineer. Since the trough design is proprietary, it would be risky to rely on general design guidelines.

Is this a retrofit where the combined air-water feature is being added to an existing filter system?

S. Bush

http://www.water-eg.com

 

[Impdrew]

A practical limit does exist:

It is important not to exceed the designed backwash flow rate during a combined was-air scour stage due to collapse-pulsation leading to RGF media being carried over the launders. Depending on the wash sequence that you are employing, there are a number of factors that must be taken into account (remembering that you are filling the RGFs towards the backwash launders as you backwash):

Mainly the following:
Water level prior to combined air-scour-wash stage. Media level and expansion coefficient.
Level filled to after combined-air scour wash stage.
Level of the collection launders weir over.
Time that it will take the final high rate wash stage to bring the water level up to the launder, taking the end of the combined stage level as your starting point.

In general you want to ensure a time, such as 60 seconds (dependant on nozzle/flooring type), exists between the end of the combined stage and the point at which the high rate wash carries the oxidised manganese particulates over the launder. Not all the air cavities will immediately collapse when the air blowers stop, the 60 seconds is a good general rule to ensure that air-cavities don't collapse near the launder weir level which would lead to media being "thrown" over the weir.

The high rate wash stage has its flow dictated by the choice of media and water temperatures to achieve the required expansion

 

[scwaterguy]

It's best to provide a flow range greater than you think you will need. This becomes obvious during startup. Is your mechanical engineer trying to vary the speed of the pump for the flow range? Normally, this done with a constant speed pump and flow control valve. In backwashing, selecting a pump for best efficiency goes out the window.

 

[brucedaengineer]

The backwash flow rates sound very low to me for say a sand filter or even a multi-media filter with some filter coal as second layer - have you converted these from other units? If the manganese filter is a green sand filter or some "rare earth" media or very low SG media - that may explain it - I don't know these filter types - we would pre-oxidise manganese (KMnO4)and remove in clarifier/DAF stage before normal RGF multi-media filters - but we probably have cooperative Mn.

A normal filtration rate for rapid gravity (I assume that's RGF) sand filter or multi-media filter would be 10 m/hr so you'd expect a significantly higher Backwash rate. Simple water wash of a RG sand filter would be 40 m/hr plus, unless it is a very old design with very low launder height over the sand - say 500 mm or so, then still 35 m/hr. Air and water makes a large difference and has much lower rates but 6 m/hr does not sound right to me - way too low.

(I come from where 10 degrees C water is COLD, and over 22 degrees C water is HOT - i.e we are temperate)

We need to be clear what air-water and what water wash rates we are talking about. If air-water continues during backwash discharge over the launder and you have a low SG filter media then these low rates (12 m/hr) may be correct. If however the air-water phase stops before the water gets to the launder and the wash phase is then water only a wash rate of 21 m/hr is again very low - must be very low SG media - less than coal??.

I would be sceptical of the "Water Authority's design guideline" - better to contact a reliable water treatment contractor the Authority uses and discuss issues with them - better still contact the designer of the filter - do you have the O&M manuals - should all be in there. And of course consult a good WTP text book - inter-library loan? - say Kawamura/Montgomery/the US AWWA operations series etc

Any way let's assume the rates are right- how to get the pump to do both duties? It is very common to use an electrically actuated butterfly valve to regulate the low backwash rate flow and open for the higher rate flow. The valve position is controlled by PID/feedback loop from an electronic flow meter signal in the backwash supply pipeline. Some energy conscious authorities are now using VSD control for the same effect - not cost effective for one backwash per day for less than 10 minutes - go for the modulating, actuated butterfly valve and flow meter. We commonly do this for say a change of rate from 25m/hr air/water (not over launder) then stop air - say 60 seconds before water goes over launder and then open valve to control at higher flow rate - say 45 m/hr - these are sand or multi-media RGF after clarifier or DAF; also used in direct filtration.