Can increase of Oxygen supply break down more COD in an AST plant? 3

[munich]

We treat effluent of a fast food restaurant in an activated sludge treatment plant. Our effluent is quite turbid, TSS levels around 50 mg/L and COD levels of the effluent are around 300 - 500 mg/L. COD inflow is around 1,500 mg/L.
BOD5 levels are very low, around 1-3 mg/L. MLSS is about 200 ml/L.

We use Venturi aeration and as BOD levels show this system is quite fine. However due to excessive use of detergents and disinfectants in the restaurant we believe that non-biodegradable matter increases the effluent COD. Hydrolyzed oil/grease might contribute as well.
Question: will doubling of the oxygen intake by addition of one Venturi aerator help to reduce effluent COD?

 

[TerraSouth]

Common sense says yes. I'd also look into the detention time of the effluent, as a longer detention time would give you better treatment and higher dissolved oxygen.

I'd also look at a very long anaerobic process before going to the activated treatment unit. A grease trap followed by a large anaerobic tank, this way you settle as much of the solids as possible and give the chemicals time to decompose somewhat.

Interesting question.

Keep us updated and good luck

 

[munich]

TerraSouth,
thanks for your message. I agree with what you say. The problem of the aeration tank is that it's too small thus detention time insuffucient. We have/had massive space problem with the location. Also cost matters played a role.

Since the stand-by aerator is there already, we definitively will install as it adds on to operational safety anyway. This alone is a good argument.

We will report, what the results were.
Cheers
munich

 

[bimr]

Your operating parameters indicate that the COD is not biodegradeable (or very slowly biodegrabeable). You have already oxidized all of the BOD5 down to 1-3 mg/l. If your DO is maintained above 2 mg/l, adding more oxygen is not going to do anything for you.

Anaerobic process are not economical except for very strong wastewaters.

 

[TerraSouth]

bimr,

excellent points!

 

[munich]

rbcoulter of the water treatment forum suggests increased MLSS. I like this.

Thanks again for all replies. I am glad I found this site.
Cheers
munich

 

[bimr]

rbcoulter is correct. You need to have at least 1500 mg/l to operate an activated sludge process.

It would be helpful to understand what you are doing if you posted more details such as the detention time, etc.

 

[munich]

bimr,
thanks again. Detention time is at the lower end, it's about 10 d, however we have very limited space, cost pressure etc. (site is in Savanna la Mar, Jamaica, near Negril).

Well, with respect to your MLSS figure of 1,500 mg/L:
this converts to an Imhoff volume of about 190 ml/L at an SVI of about 125. As we operate at 150 ml/L momentarily and with respect to your and rbcoulter's suggestion I will go up to about 200-250 ml/L. I don't want to go above this one since as of about 250 ml/L settleability becomes limited and SVI will increase.
To maintain all of this I will install the stand-by aeration to accomodate additional Oxygen demand plus to increase operational safety. Of course, I will keep you guys posted, which will take some time (re-orientation of sludge, analyses etc.).
Cheers
munich

 

[bimr]

You are describing a lagoon system which is operated differently than the conventional activated sludge process.

The problem with lagoons is they are commonly deficient in mixing energy and the solids settle out. If you have an extra aerator, you should use it, not for the oxygen input, but for the mixing energy. You should locate it towards the discharge end of the lagoon.

You should also investigate an extended aeration activated sludge process if you little land available. The extended aeration process only needs 1 day of retention.

The 1500 mg/l mixed liquor number is for activated sludge systems and is not appropriate for a lagoon system.

 

[munich]

bimr,
why do you think its a lagoon system? Its a small TP behind a restaurant.
Cheers
munich