OK to pipe clearwell overflow to plant influent?

[gt5879c]

I have a situation where the design engineer provided a clearwell overflow box on the side of the clearwell at ground level (the clearwell is below grade), but the owner wants to pipe it somewhere rather than have the water just flow out on the ground. Can I pipe this to the influent raw water wet well? The overflows should be rare, and if they do happen, the dilution of the chlorinated water by the raw water would be great.

 

[alexcmmi]

Seems to meet you would not want any kind of connection from your finished water to your raw water. Check your states local health code (regulatory agency that oversees WWater Treatment Plants) and possibly even 10 States Standards.

If you pipe the clearwell drain anywhere, I would think you would want (need) an air gap to where ever it drains, and probably a backflow preventer in the drain line as a secondary precaution.

Also, might be a good idea to have a visual if the well is overflowing, you know, so the operator knows something is up / not right. That becomes the problem when you pipe these kind of overflows. You might have the worst case scenario where transducer fails, SCADA fails, altitude valve fails, whatever fails, and no one realizes it until they see water flowing on the ground....

 

[SteveWag]

I could envision a situation where 100% of the plant flow could be returned to the front end. In plants I have done, treated water is pumped from the clear well and if the pumps fail, and if the controls fail to take the plant off-line, then the clear well overflows at the treatment rate.

Is this a filter plant? If so, where does the backwash water go?

Steve

 

[JedClampett]

Where's your meter? You don't want to have to pay for the water twice.

 

[gt5879c]

Yes it is a filter plant. The backwash water goes to a pond. I was thinking if we piped the clearwell overflow to the raw water wet well, we would have to put in a check valve to keep raw water from entering the clearwell. I would also be discharging the pipe in the wet well above the emergency overflow of the raw water wet well.

 

[bimr]

One would think that the clearwell would never overflow. So you will be installing something that is a waste of time and money.

However, if the operator insists, it should not be a problem. I don't see any problem letting the water overflow back to the raw water wetwell, as long as you have an air grap.

You should install an air gap at the clearwell, so that the water will not siphon out of the clearwell.

 

[semo]

It also depends upon your source water. If you are treating ground water, it is likely that the DNR/DEQ would not allow the exposed water enter the headworks.

If you are treating surface water, I don't know why it couldn't be piped back to the headworks, except as alex mentioned, you don't want a cross connection.

A check valve also will more than likely not serve as a backflow prevention device. The DNR/DEQ will probably want an air gap or some other sort of device. A PRV valve will probably not work due to gravity situation.

Many states allow backwash water to be returned to the headworks provided it is at a reduced rate.

Note that depending on what you are treating, doing this will dilute your source water causing chemical feed dosages to be wrong. Don't know what kind of problems (if any) that may cause in your situation.

 

[ch81pc]

You need to be careful of a situation where the main inlet pumps stop (for what ever reason) but flows through the plant carry on for a period until datum level is reached. This could put a slug of chlorinated water back into which ever water body you are taking your raw water from. Fish don't like chlorine