The EU urban Wastewater Treatment directive has a limit for COD of 125 mg/l, and for BOD 25 mg/l.
The BOD value, from memory, is related to the UK Royal Commission standards from 1904, which also had a (usually ignored) additional requirement for at least 8-fold dilution in the river, to ensure that the resulting in-river BOD was at a level that would not produce a sustained DO sag.
But what is the rationale behind choosing 125 mg/l for COD?
In New Zealand in some regions BOD is restricted to 10 mg/L at 90th percentile. While such restrictions apply mostly in sensitive receiving environment, consideration to limiting waste water contaminants is based on best practicable options. Most advanced waste water treatments systems can achieve 10 BOD mg/L.
As we are aware, COD measures total oxidisable-C by using chemical oxidation methods whilst BOD is measured biologically by promoting microbial activity by incubation. For the above reason for treated waste waters COD>BOD and BOD better represents rapidly oxidisable organic-C. If the BOD component of the COD is subtracted, the remaining organic-C is considered as recalcitrant or not-rapidly oxidisable-C. While such organic-C is not an immediate threat to the receiving environment, cumulatively large accumulations could cause long-term O2 depletions. It was for this reason I suspect EU had limited COD discharge and accorded higher value than BOD owing to practical reasons.
Selva - yes.
But why 125? Why not 120? 130? 100? 200?
What is the rationale behind those numbers?
There exist an equation used in determination of COD concentration for final effluent:
COD = 5 X BOD approx. (https://workspace.imperial.ac.uk/environmentalpolicy/Public/EPC2014/WASTEWATER%20HANDOUT%202014.pdf)
Äccording to this equation 5 * 25 mg*L-1 (5 * BOD) = 125 mg*L-1 (COD).
Regards
Vit
Jeremy, If I remember right the late Poul Harremoes was consultant on this and he actually consulted the industry to ask what would be realistic values that can be achieved. This value has to be seen in relation with the other to values:
TSS < 35 mg/l
BOD < 25 mg/l
Here the 35 mg TSS if we assume 80% VSS will represent approx 40 mg COD/l as particular COD this means that we allow approx 85 mg /l of Soluble COD. The BOD:TSS ratio can be assumed to be approx 2 giving 8 mg Soluble degradeable COD. Assuming approx 5% of COD in the in-fluent to a plant to be COD Soluble non biological degradable the max COD inlet concentration that can be accepted if the effluent criteria shall be fulfilled is approx. 1500 mg/l.
This is about the maximum value of COD that can found for Municipal Waste water treatment plant with Separate sewers systems, hence the value made good sense since it would allow most Municipal plants to avoid additional treatment stages after the biology like Ozonation or Carbon adsorption.
Dear Jeremy Dudley, don't rely on COD as it is a measure of organic and inorganic oxidation while BOD5 is a measure of organic oxidation inside the bacteria. Effluent standards are always in terms of BOD5 and SS.
In the US, the Clean Water Act allows for the development of site-specific conversion equations for BOD to either COD or TOC; so there are no legally mandated equivalents that are broadly applied. I would be curious if any institution has been able to validate the Effluent COD = 5 (Effluent BOD) that is the basis of this question.
I think its because always COD is higher than BOD but for this specific number 125 mg*L-1, I agree with Vit Mateju and there is an equation in the attached file from Vit Mateju :-
For domestic crude sewage COD = 2 X BOD approx.
For final effluent COD = 5 X BOD approx.
so according to this equation it could be 125 mg*L-1
kind regards,
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