Activated Sludge

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I designed an activated sludge aeration basin. Qmax=3Qavg.

if i don't want to use an equalization tank so the basin volume (V) should be design according Qmax. and for calculating HRT, we have HRT=V/Qavg. HRT &SRT calculated from this method is much more from standards (also SRT may negative).

I Think SRT must be under 30  for activated sludge. Can anyone help?

Qavg= 5000 m3/day

Qmax= 14400 m3/day

F/M=0.3 1/day

MLSS=3000 ppm

BODin=240 BODout=20 ppm

-Calculate V: F/M=(Qmax.BODin)/(V.MLVSS)  

v= 5485 m3

-Calculate HRT= V/Qavg=1.09 day

Calculate SRT using Y=0.4 and kd=0.04 1/day: MLSS=SRT×Y×(BODin-BODout)/(HRT×(1+kd×SRT))

SRT=-75 day !!!!! 

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19 Answers

  1. Dear Ahmad,

    If we use:


    Qavg= 5000 m3/day (to calculate the aeration tank volume)

    Qmax= 14400 m3/day

    F/M=0.3 1/day

    MLSS=3000 ppm

    BODin=240 BODout=20 ppm

    Calculate SRT ​using Y=0.4 and ​kd=0.04 1/day

    The minimum calculated volume must be near to 1184 m3 as Rafael Gay suggest (1300 m3). This value will give you a minimum SRT near to 6,1 d and HRT near to 0,24 ​days.

    On the other hand if you want use a factor due to the Qmax (in a day or in a spacial season) without the use of a equalization tank, you must use the right flow pattern and calculations to obtain a bigger volume.

     

    Regards,

     

    Orlando D. Gutiérrez Coronado

  4. Hi Ahmad,

    Here are my comments

    CALCULATIONS:              
      Site Parameters              
      Qmax Qavg BODin BODout          
      m3/d m3/d mg/L mg/L          
      14400 5000 240 20          
                       
      Design assumptions taken            
      f/m MLVSS Y Qufl/Qavg Qwaste/Qufl        
      1/d mg/L MLVSS/BOD 2ry clrf ratio of flows        
      0.3 3000 0.5 0.26 0.03        
                       
      Calculated  Parameters            
      V HRT Qufl Qwaste MLVSS ufl SRT for V     
      m3 d m3/d m3/d mg/L d    
      1333 0.3 1300 39 11538 7.28 6.67    
                On wasted MLVSS On cells generation  
    COMMENTS:                
    1.- In above calc, additional volume (2xV) is not "active", is equalization volume  
    2.- When assuming additional 2xV is with MLVSS, many things may happen: anaerobic decay first
    3.- IF 2xV is aeration provided (i.e. DO or respirometry) sludge aging may occur  
    4.- If Q  increses significantly over design outflows and wasting at 2ry Clrifier would go wild
    5.- In above conditions on 4. the plant will not comply         

  5. Dear Mr Bahlake,

    I saw only this morning your answer where you clarify that the plant is municipal and the peak flow duration it’s only 2 hours/day.

    All municipal wastewater treatment have once or  twice per day peak flow that can reach, in function of the city dimension, up to 3 times the average flow.

    F/M is based on kg of BOD/day so the peak must be included in the kg of BOD entering in the biological reactor every day.

    Plant section like sedimentation basin must be dimensioned on the basis of m3/h entering in the plant so it’s necessary to use the max hourly inlet flow.

  6. Equalization basin is very important to feed the followings treatments steps with constant wastewater flow and  quality. The benefits are not only functional but also economic for management facility and investment cost reduction.

    The majority of industrial wastewater treatment plant are equipped with equalization tank but the majority of municipal wastewater treatment plants work without equalization tank.

    I asked more information regarding the flow pattern to give more appropriate indication.

    The case submitted by Mr Bahlake can applied for our skiing areas in the Alps where during the weekend the flow and organic load, in few hours, increase 3-4 time for about 48 hours.

    To maintain constant F/M in this case requires one storage tank able to accumulate the extra flow and to calculate the average flow on weekly basis and not on daily basis.

    To apply multi tanks system or variable volume tank cannot solve the problem because there aren’t enough biomass in the plant when the overload arrives.

    The plant section dimensioned on the hydraulic load basis must be calculated with the maximum flow.

    The biological reactor is much more elastic, the outlet BOD is very similar if you work with F/M 0,1 or 0,2 of course you can lose nitrification and to have in some cases bulking problem.

    I think not necessary to work with the same F/M in average condition and in peak condition.

  7. It is dangerous to try to run an activated sludge plant without a balance or equalisation tank.  The only plant, which hasn't has a capacity of 22 million L so it acts as a balance tank.  

    1 Comment

    1. I disagree! Most activated sludge plants in Europe and the Middle East (where I've gained my 26 years' experience) operate without balancing or equalisation tanks. They are, however, used on perhaps half of the AS plants in S Africa.

      Equalisation tanks do allow savings in the AS plant size, but the penalty is another tank to operate and maintain, and often an additional pumping stage.

  8. Dear Ahmad,

    I suppose, in this case, it is not recommendable design the system without an equalization tank. The equalization tank is used to reduce the size of the system and/or to have an easier operation of the system. 

    Liang have gived to you a very good suggestion. 

    On the other hand if you want have a numerical solution of this design I try with 19 days in SRT.

    Regards,

    Orlando D. Gutiérrez Coronado


  9. Dear Mr Bahlake,

    To give you an appropriate answer we need more information.

    What is the peak flow reason ? It’s only flow increase? (rain water in the sewer) It’s flow and organic load increase? ( population increase, typical for touristic area) It’s related to industrial  activity? (production increase or seasonal productions) What is the flow pattern? What is the time to pass from average and peak flow?

    1 Comment

    1. thank you for you  comment. actually this is municipal ww of touristic town (population increase). peak flow duration is 2 hr in a day. BOD of flow will be same as average flow

  10. Dear Ahmed

    For the question of SRT it is not a negativa value (see Table below).

    For the Problem of a pick factor of three, may be this area is a tourist one. To maintain a good performance of the bilogical process it is recommanded not apply different hydraulic and organic loads and so as a hydraulic system is needed befor the process.

     

    Parameter / Rate Parameter Unit Vaue
     Qavg   -  m3/d 5000
     Qmax    m3/d 14400
     F/M   Cm  kg DBO5/KG MLVSS/d 0,3
     MLVSS/MLSS    - 70%
     MLSS   X  mg MLSS/L 3000
      mg MLVSS/L 2100
     BODin   So  mg BOD5/L 240
     BODout   Se  mg BOD5/L 20
     Y   Y  1/d 0,4
     kd    kd  1/d 0,04
     DESIGN       
     V    =Qavg x (So-Se)/(Cm x X)  m3 1905
     =Qmax x (So-Se)/(Cm x X)  m3 5486
     HRT   HRT_avg = V_Qavg/Qavg  d 0,38
     HRT_max = V_Qmax/Qmax  d 0,38
     SRT   = (X x HRT_avg  x (So-Se))/(Y x (So - Se) - k x X x HRT_avg)  d 14,29
     = (X x HRT_max  x (So-Se))/(Y x (So - Se) - k x X x HRT_max)  d 14,29

    1 Comment

    1. thank u dear Driss. so you recommend to design an equalization tank. can we utilize a first sedimentation clarifiers as an equalization unit? (if we design first clarifiers hydrolically for maximom flow). i think this idea can reduce costs.

  11. If there is vast variation in flow and BOD then one should not avoid the Equalization tank.

    If volume of Aeration tank is more then it will increase the energy foot print.

    Rajendrakumar V Saraf

    1 Comment

    1. thank u dear Saraf. so you recommend to design an equalization tank. can we utilize a first sedimentation clarifiers as an equalization unit? (if we design first clarifiers hydrolically for maximom flow). i think this idea can reduce costs.

  12. Yes, you can design an activated system without balance tank. But you must select a VDR (variable depth reactor), which can have a relevant constant outflow with a variable inflow. Google check the website and find out how to design a VDR. Good luck! Let me know if you have any problem.

  13. Firstly, it's a BAD idea to design a wastewater plant without an equalization basin, especially if you have peak flows up to 3 times the average flows!

    Secondly, even if you were to design this w/o equalisation, depending in the expected flow profile (maybe the peak occurs only short while in a day), I would still size the aeration basin based on the average flow rather than the peak. I think if you do this, your -ve SRT should go away. However, it will be advisable to size the clarifier using peak flow.

    1 Comment

    1. thank u dear Idzham. so you recommend to design an equalization tank. can we utilize a first sedimentation clarifiers as an equalization unit? (if we design first clarifiers hydrolically for maximom flow). i think this idea can reduce costs.

  14. For plants designed for treatment of municipal wastewater  SRT values are 20 to 30 d range and F/M may range from 0.1 to 0.05 gBOD/gVSSd .

    At SRT in the range of 5 to 7d, F/M is 03. to 0.5gBOD/gVSSd

     

    SRT can be related to F/M as per following eq

    1/SRT = Y (F/M) *E/100 - kd

    Y  typical 0.6 , range 0.4 - 0.8

    kd   typical 0.1, range 0.06 - 0.15

    E% = ((BOD inlet - BOD out)/BODinlet)*100

    Source Wastewater Treatment, Metcalf & Eddy

  18. Dear Ahmad

    For AS F/M Ratio up to MLVSS and MLSS Ratio as well need to concern , if you base on theory to much this will make you confuse. All you design is right and can be success from your operation and operater. You can go this way but the good thing of EQ for the huge gab Vol of waste water is to ensure that your sludge still stable. If you do not have it you have to learn on site  about the high mid low flow then you can find the optimum for your AS operation. AGGIE , aggasit2012@gmail.com

    1 Comment

    1. thank u dear AGGASIT

      . so you recommend to design an equalization tank. can we utilize a first sedimentation clarifiers as an equalization unit? (if we design first clarifiers hydrolically for maximom flow). i think this idea can reduce costs.

  19. Dear Ahmad,

    Is this a Domestic Sewage? If yes then you do not need to have Equalization Tank only a lift basin would be sufficient. But if this is is related to Industrial Waste then you must have an Equalization Tank. 

    For 24 hrs HRT for Aeration quite good with respect to Domestic Sewage Treatment.

    But the system could be easily more optimized then you are designing.

    1 Comment

    1. thank u dear Ahmed.

      what is lift basin? i think that is a simple form of equalization basin without airation or instrumentation.

      so you recommend to design an equalization tank. can we utilize a first sedimentation clarifiers as an equalization unit? (if we design first clarifiers hydrolically for maximom flow). i think this idea can reduce costs.