A client of ours is having a pretty bad FOG problem at its WWTP, which is causing a number of headaches. One such headache is the build up of a floating sludge layer that shows up in the anaerobic selector tanks (see attached picture: http://files.engineering.com/getfile.aspx?folder=b...).

In the picture you can see a hose spraying the surface of one of the tanks. The operator does this to help break up and prevent the surface build up. The idea makes sense, I've just never seen it done before.

We're thinking of recommending a non-potable system to do what the hose is doing more efficiently. Essentially, we would run small diameter lines to the anaerobic tanks from the plant's effluent and install something similar to a building sprinkler system above the tanks.

Has anyone seen this done? Any ideas/advice?

Thanks in advance.

I have seen such systems in Asian WWTPs and they were called "scum spraying systems". I do not know the manufacturers but a quick Google search brought me to this site:
http://www.gilltrading.com/page/products/

One would have to question whether there is adequate mixing in the anaerobic tank. If the mixing was improved, you would not have the materials floating up and stagnating.

Physical control of foams is sometime practiced using enlarged surface scum traps and forceful water sprays. However, the water spray may not work with the heavy FOG that you have.

Looks familiar! We used to do a lot of scum destruction spray systems. We mounted a 2-inch pipe about a foot of the wall and 6-inches down and drilled and tapped every 2 – 3 foot with ¼” NPT taps. Then after fitting spray nozzles and using effluent water we clogged every nozzle. That led to the use of these:

http://www.usabluebook.com/p-303422-flat-spray-noz...

These use ½ NPT taps and the weighted lever can be opened with a broom handle to flush out clogging solids. We used brass fittings while these are plastic.

Your problem is that the scum (?) is static, that is, there is nothing to move the scum into the spray pattern. When we began using full-floor aeration (or in your case no aeration) the scum destruction systems were abandoned.

The two problems we found were freezing and the addition of dissolved air into the basin.

Steve

Would agree with SteveWag that the spray would probably not work.

Most people are using submersible mixers for these applications.

http://www.absgroupusa.com/scripts/getdownload.asp...


"Typical sizing of a selector for this application involves hydraulic sizing for 30 minutes at the design flow, with detention times to be no less than 10 minutes under peak flow conditions. In addition, the selector should be compartmentalized into three or more equal volume tanks, each with a mixer capable of maintaining complete mix conditions."

https://fortress.wa.gov/ecy/publications/publicati...

Actually, I don’t think you have an FOG problem, at least in the anaerobic/anoxic tank shown. It looks to me as if you are trying for some BNR operation, and if so, then you probably have a biological “scum”. Microscopic examination may reveal either nocardioformsa or microthrix parvicella filaments. The first one can be treated by application directly to the foam using bleach and a garden sprayer. The second is treated the same way but with polyaluminum chloride based flocculent sprayed on the surface. Doing this twice a day for a week should give you a handle on treatment.
Steve