Presently I have 2 blowers online that supply aeration to the basins.  They run for 50 minutes and shut off for 10.  I'd like to keep my D.O. between 1 and 2 mg/L.  Usually it swings but today both basins are <1 but none at 0.  It is hot and humid outside which I know makes for lower D.O. due to higher biological activity.  I was just wondering, at this point should I be running the blowers constant?

I would continue cycling the aeration as you are unless you loose nitrification (sometimes this is just temporary- they go into hybernation) with the low DO. The reason being if nitrification is lost there's no/ less nitrate to worry about denitrifying.

The low DO may or may not be a problem depending on how much oxygen is available within the floc. (only about 0.2 mg/L is needed in actuality- 1-2 mg/L in the bulk liquid is usually a safe number to keep the inteior of the flocs aerobic.) Also the lower the F/M the less DO will be needed in general to discourage the growth of low DO filaments. Knowing the microscope helps during these times because you can look for indicators of low DO and septicity.

Supplemental oxygen during the aeration portion may help (programing this will be tricky). Liquid oxygen, an additional rented mechanical aerator, or hydrogen peroxide addition (this breaks down to about 0.5 lb oxygen to lb peroxide added)- the peroxide has a little risk 50-100 mg/L would be as high as I'd feel comfortable going.

Currently I'm dealing with the same thing at my plant- high organic loading and over 100 degrees for much of the last week. ww temp is 85 and doesn't like to hold oxygen.  

best regards,

Ryan

I'd want to run the bloweres nonstop until water temp comes back down in order to maintain enough DO in the aeration basins.  Playing chicken with 0 DO levels is a losing game.

~Eric

Kimmy,

Some plants that run part or all of their process at very low DO's have reported to us more frequent diffuser cleaning requirements than when they operate at higher DO's.  There may be a correlation between low DO and lower alphaF values.  So if the reason is to save energy by turning blowers off 10 mins an hour it could be an elusive goal.

-Tom

It really depends on what you are trying to accomplish. If you have a nitrate limit it may be necessary to cycle. If not, you won't need to unless you want to try to recover some oxygen and alkalinity and have less risk of denitrification in the clarifier.