Your question is not clear. But according to my opinion the  digester inlet is not municipal wastewater, but something else.

Regards

Vit

Normally the digester inlet is having a mixed content of both primary sludge and biological sludge. Where the original TSS in the inlet was approx. 300 mg/l,  at the bottom of the thickener it will reach 30000 mg/l ore more and the thickened Biological sludge might be even more. all this TSS also contains COD if we assume 65% VSS and 1.6 mg COD/g VSS and a TSS content of 30000 mg/l will this alone rise the Total COD to more than 30000 mg/l. Now if we look at Soluble COD there will always be some hydrolysis taking place both at the bottom of the primary settler and the secondary settlet further if you have gravity thickening of the primary sludge Further Hydolysis will take place. How much this hydrolysis will go is depending on the retention time of the sludge in the different thickening zones. the longer retetion time the more anaerobic condition and the more the Hydrolysis will speed up and your risk to see even the 3000 - 5000 mg/l that you mention.

Digester inlet consists of higher solids content (also higher organic solids content). This is much more concentrated and as such, possesses a higher COD content. For inlet to WWTP, this is mostly liquid with a lower concentration of solids, meaning a lower COD content. Besides, WWTP accepts a large volume of wastewater (lots of water involved) which 'dilutes' the COD content of the inlet.

Thank you Kim Helleshoj Soerensen and Zumar Muhammad Ali Bundhoo.

In practice; at most of the municipal WWTP, it has been found that the sludge is being collected in tank and then pumped into the digester as per time permits (may be  Indian Scenario). Will this affect the COD? and ultimately on Biogas production?

As already mentioned when ever the sludge is standing somewhere un-aerated - Anaerobic processes starts - first stage is Hydrolysis releasing Soluble COD as VFA = Increase in soluble COD. Now if the sludge stand several days the Process will get to stage of Methanisation and if this happens before the sludge id brought into the digester you will off-course have lost some Potential Gasproduction.

You should look at your question from the simplest viewpoint - basic mass balance:

The solids flow rate, QW is only a  small fraction of the flow at the treatment inlet Q. Therefore the COD load in  QW, although much smaller than the the inlet of the treatment plant, gives a much higher COD concentration in a much smaller flow stream.

Simple!..look at  corresponding COD loads and divide by respective flow rates.

You get the reason why COD concentration is much higher at the digester inlet.

My friend, firstly is this treatment plant  an activated sludge process or not ? If so, what type?  Conventional, extended aeration, ......etc? Remember that for a conventional activated sludge process with recycle having a thickener and digester with a primary clarifier the answer by Kim Helleshoj Soerensen is perfect. Please refer to Metcalf & Eddy Inc. (Wastewater Engineering :Treatment, Disposal and Reuse. I am ready for any cooperation with you. Thank you.