Influence of Omitting the Primary Sedimentation in A2O Process

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Hi all,

If we eliminate the primary sedimentation in an A2O process, does the excess TSS in the influent cause a problem?

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9 Answers

  1. You can design the process with or without, usually the inflow characteristics will influence the decision as you may want to reduce organic load going to aeration to reduce oxygen demand and subsequent energy demand

    also can use primary settling to increase volatiles that can be concentrated up as a fermented feed to help with process requirements

    and of course primary settlers present an opportunity to digest sludge that can produce biogas for beneficiation

    but commitment should be made at design stage as process needs to be operated accordingly

    hope this helps

  2. You do not want to fill up aeration basin with inorganic solids resulting in reduced detention time, short circuiting, and poor air distribution.

  4. I agree with Amit Christian. One should also take note of the following:

    1. Depending on the raw influent characteristics, primary settling tanks remove a large portion of the organic load which now passes to the reactor.
    2. The higher organic load in the reactor will exert a higher oxygen demand and if your aeration capacity will not cope resulting in lower than required dissolved oxygen levels. This can be counteracted by shortening the sludge age but may not have a desired effect on the effluent quality.
    3. The higher organic load will also result in a higher MLSS to cope with the higher load. This can also be counteracted by shortening the sludge age but may not have a desired effect on the effluent quality.
    4. I would not recommend excluding the primary settling tank in an existing system without doing the calculations. If it is a new system it can be designed for the unsettled influent quality without a problem.

  5. You need to maintain the design parameters mlss, mlvss, do, nutrient. It is fibf if yyo are able to maintain.  However it is better to have a clarifuer to remove chemiche sludge   before aeration  tank so that sludge after aeration tank xac be used as organic manure to be used in fields. 

  6. The answer to your question will be dependent on your inlet TSS concentration and VSS/TSS ratio of the influent. If there is low VSS/TSS ratio meaning higher inert solids present in the influent then certainly your MLVSS/MLSS ratio in the aeration tank would not be as optimal as 0.65-0.75 which We consider as a standard rule to design Extended Aeration Tank . It will be on lower side but highly dependent on your inlet TSS concentration and its composition for inert solids or iTSS. If that is low then there might not be an issue. 

    you need to understand that when you dont put a primary clarifier then from process point of view, the MLVSS/MLSS ratio would not be as optimum as you think on paper because inert solids will be accumulated in the MLSS decreasing your volumetric biomass (read here MLVSS) fraction for the same level of MLSS. 

     

    I mean if you operate one plant with MLSS levels of 3000 ppm and if that plant is with Primary Clarifier then MLVSS/MLSS ratio would be naturally higher due to removal of both inert solids and lot of TSS at Primary stage. While without Primary Clarifier, everything will be going to your Aeration Tank and thus inert solids will contribute to

    your MLSS and thus reduce MLVSS/MLSS ratio. That means your MCRT would be lower for same MLSS without a Primary clarifier. 

     

    Hope that helps from process point of view.       

     

  7. The answer to your question will be dependent on your inlet TSS concentration and VSS/TSS ratio of the influent. If there is low VSS/TSS ratio meaning higher inert solids present in the influent then certainly your MLVSS/MLSS ratio in the aeration tank would not be as optimal as 0.65-0.75 which We consider as a standard rule to design Extended Aeration Tank . It will be on lower side but highly dependent on your inlet TSS concentration and its composition for inert solids or iTSS. If that is low then there might not be an issue. 

    you need to understand that when you dont put a primary clarifier then from process point of view, the MLVSS/MLSS ratio would not be as optimum as you think on paper because inert solids will be accumulated in the MLSS decreasing your volumetric biomass (read here MLVSS) fraction for the same level of MLSS. 

     

    I mean if you operate one plant with MLSS levels of 3000 ppm and if that plant is with Primary Clarifier then MLVSS/MLSS ratio would be naturally higher due to removal of both inert solids and lot of TSS at Primary stage. While without Primary Clarifier, everything will be going to your Aeration Tank and thus inert solids will contribute to

    your MLSS and thus reduce MLVSS/MLSS ratio. That means your MCRT would be lower for same MLSS without a Primary clarifier. 

     

    Hope that helps from process point of view.       

     

    1 Comment

    1. Thanks a lot because of your complete answer.

      Commented on by

  8. If you have good pretreatment that will ensure removal of grit non-biodegradeable plastics and rags, it is often of advantage to eliminate the primary sedimentation as the anaerobic and anoxic stages need sufficient organic carbon for the slower digestion processes.  Some process designers like to still incorporate primary sedimentation and then digest the primary sludge to produce volatile fatty acids to feed into the anaerobic process for the phosphorous uptake bacteria to metabolise more efficiently.  However having a primary sedimentation stage does mean the primary sludge needs to be further treated as it is still very active as compared to an activated sludge.  So a more simple process excludes primary sedimentation with the advantage of simpler sludge management, but perhaps at the expense of less efficient phosphorous removal (unless the anaerobic stage and anoxic-anaerobic recycle is designed to improve this).  I hope others can contribute as I am not 100% certain of the precise dynamics.