Residual COD problems in treated fast food wastewater

[munich]

COD/TSS problems at effluent from fastfood WW treatment in Jamaica W.I.

We have the following problem with one treatment plant:
The customer is a fastfood restaurant, we treat about 5 m3/d of the combined kitchen and sewage WW after extensive grease/scum and primary treatment/removal in an Activated Sludge System, MLSS equivalent is around 250 ml/L. Aeration is Venturi, BOD5 of effluent continuously below 5 mg/L. Inflowing COD is around 1,200 - 1,500 mg/L, at effluent COD levels still around 300 mg/L. Turbidity off effluent high, coinciding with TSS levels around 50-80 mg/L.
Obviously there is an about 10x overdosage of detergents and disinfectants at the restaurant and we understand that the high COD is related to detergents and therefore interreation with turbidity, TSS and COD.

We were advised by the local authorities to install effluent polishing by sand filtration.
Following questions arrive:
1. Any opinion if that makes sense, if so, what are the effects one can expect?
2. If sand filtration, we would require a compact unit because of space problems. Any suggestions with regards to type of sand filtration (slow, upflow, downflow, grain size etc.)?

Thanks in advance
munich

 

[bimr]

There is no need to get fancy when you are dealing with such a small system.

Your best option is a down flow pressure filter with a dual media - anthracite sand. The anthracite will be 0.3-0.5 m deep and the underlying sand layer is 0.2-0.25 m deep. Use a production rate of 120 m3/m2/d. This will produce an effluent turbidity of 5-10 mg/l.

The pressure filter is recommended because it will allow you to backwash when you desire rather than when the filter gets dirty.

You should think about collecting and equalizing the backwash water as the volume may disrupt the activated sludge system.

Your MLSS of 250 mg/l is way too low. If you are operating a conventional activated sludge process, the mixed liquor should be at least 1,500 mg/l. The low number that you posted indicates that you have too much solids going out with the effluent.

The filter is not going to remove any non-biodegradable COD. You should investigate the use of alternative cleaning agents (or bio-degradable cleaning agents) if you want to eliminate the COD.