By experiment, though it needs to be shorter than the blocking time divided by the number of filters in service.

I use the sight glass provided for this purpose to visually determine when to stop backwashing my pool filter.

Thank you everybody for your help.. I want to know does any change occur in flow or pressure? is it possible to determine the backwash time by using a pressure indicator or flow meter before or after the filters?

Anthracite

Sand Filters

sand filtration

No need. Usually it's done on backwash time, Boss. You'll work it out.

No, not really. A pressure drop (or raised feed pressure) only occurs in forward filtration when the media is compacted and the suspended solids are caught forming a filter cake. When the filter is in backwash, the flow is reversed, the media lifts is agitated and releases the catured solids.

The backwash flow is also normally flow controlled in some way, to prevent ...

a. Insufficent flow not removing the solids.

b. Excess flow from blowing all the media out the top of the filter.

So, no instrumentation are generally not used to control the duration of backwash. This is always done (in my experience) by time, which is determined during commissioning, then rarely looked at again (unless there is a problem).

Hope this helps.

Cheers

Do you mean the duration of the backwash (which is what most of the replies so far have assumed), or the time between successive backwashes?

yes exactly .

Not with you. Exactly what? Which is it?

The need for backwash is determined by the gradual increase in pressure drop through the filter and lower flow at constant pressure. At some point this flow rate is unable to keep up with the demands of the upstream process = backwash time.

Unless you have two filters in parallel and can switch to the other while this one is backwashed, you will need to backwash as fast as possible - backwash to the required clarity, and resume, with an accumulation tank taking the process flow while you wait.

In water systems, the water tank usually holds enough water to maintain flow, and water systems also have a number of filters in parallel so one can be backwashed and reformed as a dual media filter bed while the others serve.

It has been many years since I installed a backwash control system, but things may be the same now.This was an upgrade to an existing manual system.It still required the operator to open and close the reverse-flow valves and drain valves.

The actual backwash time was determined empirically, this was used as the starting point for the automatic system timer, which was adjustable.

The backwash began with flow at maximum, for the per-determined interval,then the flow was reduced in several stages, each stage controlled by a timer, and flow control to the main valve.This allowed the media to settle back into the proper layered profile.

Once initiated, the system ran to completion without intervention.

However, it did not start till the start button was pressed by the operator.The operator determined when to start the backwash cycle by lab analysis of the water, including NTU monitor,Ph, the flow thru the filters, which were individually monitored, and other factors.These factors will vary according to conditions and variables in the supply source, such as rain runoff into the source,etc.

I am sure there is a totally automatic method of doing this, but it may be impractical

($) to retrofit an existing system for totally hands-off operation.

It would require compiling and digitizing all the variables and automatic control of all valves,which is not difficult, but is expensive,even more so for retrofitting an existing system.

A lot of good answers here and all very valid.

Your duration of backwashing will be empirical and you must test the system and gauge the results accordingly.....based on SS concentrations, turbidity levels, and color in the raw water vs. the filtrate. Also, consideration must be made regarding the pressure head drop across the filter, top to bottom.

There are no fixed rules regarding the durations as all surface water are different. Same can be said about the composition (chemical makeup and acid solubility, etc etc) of your filter media.

Be careful to take into consideration seasonal "turnover" in your raw water reservoir(s) as that will greatly affect the performance of your mixed media filters, their runtimes, and the duration and rate of the filter backwash.

If memory serves me correctly, I installed 5 Eagle Timers,each one controlling a different aspect of the cycle.

#1 Controls start of process,and opens control valve wide open, and starts #2 at end of cycle.

#2 Starts,and reduces flow to stage 1, adjusts set point in flow controller to approximately 75 % max.and starts #3 at end of cycle.

#3 Starts, reduces flow to 50% of max, and starts #4

#4 Starts, reduces flow to 25%, and starts #5 at end of cycle.

#5 Starts, adjusts flow to 10% and times out till next cycle.The valve had a flow restrictor to prevent rapid closing, so it gradually closed the valve, and the filter media layered perfectly.

Electrical-Mechanical timers were chosen because of high lightning incidence in the area.

This proved to be a very reliable system.

The timers were each adjustable to allow for variations in the process as required.

I am sure there are much more sophisticated solid state computer controlled systems available now, but at the time(1980's) this was the way to go.

Wallace & Tiernan controllers?

Wonderfully said!

You should not have self-marked it OT, and I have started to correct the imbalance.