An obvious observation is ...

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An obvious observation is your net COD load is about 11 percent greater than your design load, but your contact time increased by 300 percent, providing a longer time for biological treatment.   If your design was appropriate for biologically treating the constituents that make up the COD , and had a reasonable factor of safety incorporated into the design with respect to meeting the oxygen demand, the additional 11 percent loading will likely be offset by the much longer hydraulic retention time available for COD uptake and treatment.  That being said, if the COD includes constituents that are toxic, and the toxin concentration is proportional to the COD concentration increase, there may be some inhibitory effects that will negatively impact treatment.  Unfortunately, it is impossible to confirm the plant will operate as intended with the little information you have provided.  

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Dear Mr.Vassos

we haven't any problem with COD in the outlet of aeration, but the problem is in the settling tank and turbidty increased, I think with an increase of retention tank the property of bioflocculation  will be weak, I want to know the effect for clarification properties .

thanks a lot 

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Thanks a lot for your answer, I am ready to provide information that needed for better decisions.  but despite COD and toxic constituents, I want to know increasing of 300 percent in contact time couse what problems in biologecal  treatment.

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Increasing the contact time means the amount of time to digest the available biodegradable carbon is three time longer.  Generally, that is a good thing.  The impact also depends on your reactor design.  A completely mixed reactor may take a 3:1 increase in load with a 3:1 increase in digestion time better than a plug-flow reactor.  If you have, for example, a plug flow reactor that receives 3 times the concentration of substrate, the oxygen demand at the head end of the process could overwhelm and exceed the oxygen supply.  Also, you need to determine the nature of the increased COD.  Whether it is organic or inorganic can greatly affect the process response to the high concentration.  Hope that makes sense.

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