UASB Effluent Becomes High pH Upon Aeration

I have an industrial effluent containing high COD, BOD and VFA. Raw effluent with BOD 25,000 ppm, COD 50,000 ppm, VFA 400 - 500 ppm.

After treatment by UASB, the total alkalinity is in the range of 4000 ppm as CaCO3 with pH 7.6 - 7.8 level, BOD = 2250 ppm, COD = 8000 ppm. 

Biological aeration is used to further treat the effluent to reduce the BOD to less than 20 ppm (no control on COD for final discharge in this case).

The problem that I am facing now is that the pH seems to increase too much upon biological aeration and will increase to pH 9.5 and this makes the aerobic process rather inefficient. 

I would appreciate if anyone can share similar experience and advice on how to improve the situation.  

Taxonomy

• Anaerobic Digestion
• Effluent
• Treatment Methods
• Biological Treatment
• Biological Treatment
• Water Treatment Solutions
• Activated Sludge

10 Answers

1. I think there is a problem with the nitrification in the aeration tank, as there should be the decrease in the pH, please do check the MLSS and MLVSS concentration.

2. Hello,Please check DO.Air flowing criteria for aeration please.Also BOD/COD ratio found low.less than 0.3.Then you have to modify the system with good biological inventory storage with recycling and use bio aerated filter to slow down PH rise issue.

   1 Comment

   1. What should be the ideal BOD/COD ratio to run the aeration tank smoothly.

3. I'm too with the calco-carbonatic-equilibrium reason for pH fluctuations, unless You have some denitrification occurring and accounting for pH raise.

4. good morning, what is the sustenance to conclude that your process is being damaged by its pH? normally due to the effect of the alkalinity provided by the uasb this value rises considerably, however not always having a pH above 9 translates into a loss of efficiency

5. Sounds like planktonic algae. Have they done a microscopic exam?

    

   Do you know the nutrient concentrations, ammonia and phosphorus? What is the DO?

   1 Comment

   1. DO is high, in the range of 4 - 6mg/L. Ammonia in the feed is around 500 - 1000mg/L. No data on Phosphorus but I think it is more than enough for nutrient. 

6. One way forward could be a low level of acid dosing at an earlier stage to reduce some of the alkalinity.  If nitric acid were to be used then this would also provide a source of nitrate for the bacteria.  I am not talking about a high level of acid dosing but enough to drop the pH to approx 7.0.  It may be worth a try.

7. What is the ammonia concentration after the UASB?

   1 Comment

   1. Dear Ciaran Gillen, ammonia concentration is expected to be 500 - 1000mg/L

8. CO2: Suggest not to use classic bubbling technology, you need carbonic acid system. I´m around if you need me

   1 Comment

   1. Thanks Miguel for your help. Do you think instead of aeration by air blower/diffuser combination, surface aerator will reduce this problem? 

      1 Comment reply

      1. CO2 will reduce your pH without destroying alkalinity. But yes, it should 

9. You may try with chlorination in aeration tank to reduce ph. It has worked in few plants. Also we used defoaming agent to ttedre foam.  

10. The alkalinity level you have is probably all bicarbonate at the initial pH.  Air contact continually will remove the CO2 (pseudo-equilibrium partial pressure due to hydrolysis of bicarbonate).

    Over time the balance of the alkalinity will shift away from all three species, to just two, namely bicarbonate and carbonate, which reaches equilibrium at a much higher pH.  If there is any chloramine in this system, it too will upset the pH balance as COD, and BOD reacts with this, leaving chloride and ammonia behind.  This sort of pH shift has been observed in water softener vessels where chloramine containing source water stagnates in the offline vessel(s), and decomposes to ammonia and chloride.  It only requires the 2-3 ppm chloramine to cause a shift from 7.8 pH to 8.5 pH. 

    1 Comment

    1. Thanks James Stewart. Noted on the removal of CO2 causes the hike in pH. Just wonder what can be done so that the pH does not increase so much in order to improve the aerobic process efficiency.

       1 Comment reply

       1. Carbonic acid addition, perhaps, also chlorination (slight) was suggested.  I lean toward carbonic acid, chlorine might just produce chlorinated organics you do not desire?