COD Removal 3

[Lan123]

What could be the impact of receiving industrial wastewater from a food processing factory that has high COD ~ 5000 mg/l and BOD5 = 2500 mg/l to a municipal wastewater treatment plant that treats domestic wastewater with BOD=200 mg/l?. The factory discharge is about 5% of the total plant flow and at present the plant has no COD limit to meet. The plant operates as a conventional activated sludge treatment plant.

Thanks in advance.

 

[bimr]

The impact is just extra BOD/COD.

Several issues will develop.

1. The industrial wastewater may use up the remaining treatment capacity of the municipal treatment plant.
2. Another issue is that municipal plants typically have pretreatment requirements. If there are pretreatment requirements, the food plant will be surcharged for extra strength wastewater when the BOD is above the pretreatment levels.
3. The municipal wastewater treatment plant will produce additional biosolids.

If the municipal treatment plant has adequate (generally with 20% excess after your contribution) you may strike a deal with the municipal wastewater treatment plant to handle your contribution. Some companies contribute capital as part of the necessary improvements when starting such an arrangement.

 

[ashtree]

Lan123

Although only 5% of the treatment plant flow it could well be 50% or more of the plants biological treatment capacity.
The municipal plant is probably designed for a COD of around 500mg/l(this design number varies from site to site) but your waste is 10 times that strength. Based on the rough maths even though it represents only 5% of the flow it will actually impose a biological load equivalent to 10 times that much.

If the municipal plant is 50% loaded or less it will probably cope. If its more than 50% loaded.....well its overloaded.

Bio solids production will definitely increase a lot and maybe beyond the system capacity.
Power consumption will increase a lot.
Maybe the aeration system will be too small and problems with permit exceedances may result. Even though there is no COD limit, the under aeration might cause problems with ammonia and nitrogen removal , phosphorus uptake, settleability of the sludge , inadequate BOD removal. However if the overload is only moderate the process may keep going okay and just have less in reserve.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[Lan123]

Thanks bimr and ashtree. We own the plant that has some excess capacity and want to accept the wastewater to make some money. based on the information provide both of you I will do some calculation to see whether it is a okay to do accept the waste or not.

 

[Watthour]

While much of what was offered is generally true, the answer lies within the analysis of the waste stream. Depending upon the contents, high COD may or may not contribute to solids production. Consider a recent SBR design I completed where the CBOD5 equivalent was over 20,000 ppm and TSS was only about 600. Sludge production has not been substantially higher than even a moderately loaded municipal waste plant.

What source(s) are being considered?

 

[bimr]

Not sure what point that you are making. The mass of biosolids produced in a wastewater treatment plant will be in proportion to the mass of BOD being treated. The mass of biosolids produced is in addition to the influent TSS. Food plant wastewater is readily biodegradable as demonstrated by the COD/BOD ratio.

If the wastewater strength is 20,000 mg/L BOD, one would investigate the use of an anaerobic process.

 

[Watthour]

My specific point was to caution against using general assumptions. The mass balance for solids is clear, and while assumptions such as that may be true with biosolids the OP did individually mention COD and BOD separately as load for the plant, and not simply biosolids as the sole source of COD loading. "Food processing" can also produce wastes with purely chemical conversion which must be satisfied before any of the bacteria present can thrive, not unlike ammonia/nitrogen load in a typical municipal plant where nitrogenous bacteria will be consuming almost all available oxygen before carbonaceous BOD is consumed. Furans, mercury, dioxins, chelated surfactants, and other high strength load can stream from "food processing" operations in amounts which will kill a municipal treatment plant faster than a pharmaceutical waste stream, so understanding the load before accepting it might just be important.

As for the second suggestion, one cannot easily apply an anaerobic process with high VOCs like benzene, toluene, hexane, trichloroethylene, and some of the constituents in the waste stream from a chemical processing operation.

Again, the most practical resolution may be dependent upon the source(s) of waste which is why I asked, and why the OP should ask his potential contributor for analysis.

 

[ashtree]

Yes you should always get some analysis of the proposed industrial waste before accepting it and i don't think anyone was suggesting otherwise.
Yes some of the waste that would contribute to a high COD number may not be able to be biologically treated or at best maybe be difficult.
Yes the waste stream may contain substances that are toxic or detrimental to the biological process.


bimr is absolutely correct with regards to the conversion of BOD5 to bio-solids(sludge, biomass etc). Whilst there will be some variation based on process, SRT , exact nature of the waste , temperature and whole range of other factors the conversion of BOD to cell mass is pretty well established within a certain range. A rough rule of thumb is that about 60% of BOD5 gets converted to cell mass. That is 100kg of BOD5 becomes 60kg of MLSS but there is of course some variation around those numbers.

Lan123 was asking for some general advice and obviously had done some initial investigation to know that COD was about 5000mg/l and BOD5 is about 2500mg/l and that was what we had responded to.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[Lan123]

Folks thanks again. After reading the comments posted here I have a clear understanding as to in what direction I should be going.