High Sulphide Content in WWTP Effluent

Published on by in Technology

We recently realized that one of our municipal wastewater treatment plants has effluent with high sulphide content of about 20 mg/l.

With 2 million cubic meters the plant treats wastewater from a mall.

The treatment process is designed as flows:

  1. Two screen systems
  2. Grit chamber
  3. Flow equalization tank
  4. Aeration( MBBR)
  5. Clarifier
  6. Discharge

We suspect that the detergents used in cleaning of the mall might be cause for increased sulphide.

What do you think are the possible causes for increased sulphide levels and what can we do to reduce the sulphide content to the permissible level of 2 mg/l before discharge?

Taxonomy

12 Answers

  1. Hillary

    I suspect the hydrogen sulphide presence is mainly due to conditions such as temperature and length of sewer, particularly if you have long pumping mains.  I would measure the H2S concentration at locations in the sewer to determine where it is generated.  You can then target the problem by dosing either calcium carbonate from prevention or oxygen for treatment.  Good luck,

    Matthew

     

  2. Dear Hillary,

    Good question,

    Biological methods for the removal of sulphur containing compounds from wastewater In the case of biological treatment, sulphate, sulphite and other sulphur compounds are reduced in an anaerobic step to sulphide, which in turn can be oxidized to elemental sulphur by way of limited oxidation For reducing sulphur compounds to sulphide, an electron donor is necessary, as follows from the reaction:

    SO4---  + 5 5H2O + 8e     ==HS-  +  H20-

    Sulphide is ​oxidised very ​readily in ​aerobic ​conditions Biotechnological processes for sulphide removal consist in the conversion of sulphide into elemental sulphur by colourless sulphur bacteria  according to the following reaction:

    2HS +  O2   =  2S  +  2OH

    or by genera of anaerobic photosynthetic bacteria from the families Chlorobiaceae and Chromaticeae that catalyse the photosynthetic van Niel reaction.

     

    2n H2S  +  n CO2     =  2n S   +  (CH2O)n   +  n H2O

     

    light radiated to a photosynthetic reactor is coupled to the conversion of sulphide to elemental sulphur using the reverse citric acid cycle . The advantage of such a method is that only small waste streams remain because the sulphur, that is, formed can be reused. However, the disadvantage is that, especially when the effluent contains little organic matter, electron donors (methanol, ethanol, glucose and other saccharides, organic acids, H2 and CO have to be added in order to provide sufficient reducing equivalents for the SRB. This, as a result, increases the costs of this method substantially . Organic compounds that have more than two carbon atoms that degrade under anaerobic conditions give H2 and acetate. H2 can be used as an electron donor for the reduction of sulphate and sulphite.

    Oxidation Reduction Potential (ORP) can be a valuable tool for seeing what is happening in unit processes like anoxic and anaerobic tanks.

     To finally remove the nitrogen, facultative bacteria to consume organic matter (raw wastewater) under anoxic (no free dissolved oxygen) conditions. Facultative bacteria can use free dissolved oxygen, nitrate, sulfate and carbon dioxide as a sort of oxygen source. They prefer free DO, since it’s not combined with anything like nitrate or sulfate.

  4. I, like others, suggest that you look at identifying the cause of the sulphide in the treated effluent. It is most likely something to do with the process operating programme. Then when you have identified the cause, apply a corrective action to the operating programme rather than adding more treatment at the end of the line.

    An increase in ORP in the Biological Section could be very effective.

    Regards

    Tom Keenan

  5. Usually in MBBR ,little is known about microbial communities,unlike in activated sludge systems.

    Community composition need to be investigated.

    Samples comprise both biofilm and suspension biomass.

    Characterization by molecular methods might be important.

  6. If your MBBR operated correctly, the sulfide would be oxidized to sulphate. Make sure the DO setpoint in the reactor is maintained well and BOD is reduced to a low level!

     

    If the raw water contains a lot of sulfide, it could easily controlled by adding metal ions to precipitate the sulfide, such as FeCl3, CaCO3 etc. 

  7. You wrote "effluent", that means post MBBR treatment, if there are sulfides in the effluent that means the desulfivibrio bacteria are not being inhibited from forming sulfides. An adjustment to increase ORP to >-50 mV (the quantifiable value at which inhibition starts) can be achieved by adding alkalinity or by adding aeration. If this is truly effluent--aerate the effluent to convert H2S into SO4.

    Related article in TPO magazine on using ORP as a treatment predictive analytic:

    http://www.tpomag.com/editorial/2011/10/its_not_black_magic

     

  8. In addition to the previous excellent responses, may I suggest the approach to review the sampling and analysis program across the plant - Attached as indicative idea. Based upon the data analysis it will be possible to suggest some options

  9. Dear Hillary 

    2,000,000 m3 in what time ?

    Polishing the effluent is like treatment of symptoms and not treating the reason. 

    First start analysing the overall conditions of your plant and the sewage source.

    After that check the design of your plant and the real conditions.

    If you have a single source like a mall and in each and every toilet unit is a disinfectant/germicide used than your STP will never work propperly.

    Start your job at the beginning not at the end, good luck.

    1 Comment

  10. Normally sulphide enters the water termed as MIC (Microbe Induced Corrosion). Bacteria are always looking for a food source and this is how sulphate reducing bacteria enter the water source. 

    The best way to treat sulphides is to carry out final polishing of the wastewater with chlorine dioxide. As regards to the dosage to be applied you can start at 3 ppm, that amounts to nearly 19 cubic meters of chlorine dioxide every day.

    Please confirm whether the volume of wastewater is 2 million cubic meters per day or 2000 cubic meters per day. As treating the wastewater with so much of chlorine dioxide would be expensive. 

    In case the volume is indeed 2 million cubic meters then lower the chlorine dioxide dosage to 1 ppm, which would work out to 6 cubic meters of chlorine dioxide every day. 

    As chlorine dioxide is very effective you should be able to get down to 2 mg/l before discharge.

  11. That does seem very high after an aerobic system. It might be worth sampling across the entire works to identify sulphide levels after each unit. If there is a long pipeline/ retention after treatment then sulphide can be produced if there is enough BOD and anaerobic conditions. Also for safety it would be worth checking for H2S across the works .

  12. Sulphide is oxidised very readily in aerobic conditions, so my first thought is to check the sampling and analysis. Was the sample stored for a while, especially in warm conditions, before analysis? If so, any biological activity in the sample would deplete the dissolved oxygen and then begin converting nitrate into nitrogen gas and, when the nitrate is all gone, sulphate into sulphide.

    Check your clarifier. Is the sludge blanket too high (i.e. is sludge allowed to accumulate in the clarifier and go septic?) Do you have enough treatment/aeration capacity or is there a very high biodegradable organic load remaining in the clarifier influent?

    1 Comment

    1. I especially agree with Jeremy because usually MBBR process is highly aerated for biomass needs but also for mixing plastic media so you should not observe such H2S concentrations at the outlet except if you have high SRT in your clarifier => if it is the case, you should see bubbles at the surface (especially downstream the bridge move) and also black sludge on the surface of the clarifier due to anaerobic conditions at the bottom.