Sulphur removal

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We experience problems with deposits of elementary sulfur in our waste water treatment.

For years we have used sulfuric acid for pH balancing of our influent water. We never had serious problems. Now we start noticing heavy deposits in our mixing tanks, both before and after de anaerobic digester itself.

We have no view on any deposits in the reactor since it is a closed tank, but it is possible that there are deposits there as well.

 

Although in the past this sulfur was largely removed from the water stream via our biogas, now this seems to accumulate more in the water, not in the gas.

This is to the point where metal parts are corroded away, although our pH measurement is still relatively neutral (average 6.91 with standard deviation 0.27).


Can anyone help me find the root cause of this phenomenon? Any idea's where to start?

Thank you for any advice or knowledge.

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10 Answers

  1. Biosol is used to minimise sewer odour and therefore corrosion in sewer systems. It has been shown in the field to reduce sewer odour from between 14% to 84% when compared with ferrous chloride and magnesium hydroxide dosing.

     

    Biosol removes the source of odour and hence corrosion.

     

    Biosol is different to all other technologies on the market.  Biosol acts at the microbial level blocking the communication signals that bacteria need to form and maintain slimes (biofilm). Sulphur reduction only occurs in microbial slimes, so if you remove the slimes and prevent them from reforming, sewer odour and hence corrosion cannot occur.

     

    Field experience across Australia has shown substantial reductions in the rate of sewer infrastructure corrosion. From initial surface pH readings of pH0.5 to in excess of pH3.5 and from pH readings of pH 2 to in excess of pH 4 after 2 years of Biosol dosing. This represents a massive savings from increased asset life, from vastly reduced infrastructure corrosion.

     

    Microbial slimes enable fat, oil and grease to adhere to the sewer pipes. Blocking the communication signals between the bacteria stops the production of the glues that enable fat, oil and grease to adhere to the sewer pipes. This fat and grease then transferred by normal sewage velocity to the treatment plant for process.

  2. We experience ​problems with ​deposits of ​elementary ​sulfur in our ​waste water ​treatment. ​

    Elementary ​sulfur deposits is strange.  Are you sure you are not getting a sulfur salt, not elementary sulfur.   Sulfate deposits used to be quite common in the 40s and 50s when sulfuric acid was used to lower the pH in cooling towers - that was before the use of some materials that allowed the cooling towers to operate at a higher pH.  Taking the pH down with the use of chromate and zinc allowed for good corrosion protection at a low pH.  It kinda sounds like you are taking the pH down to this extremely low level to control some other deposits (look at the Langelier index, and others).  The bacteria will function better if it is operating at a higher pH, up to 7.5 or even 8.2.  So, why are you at 6.91 pH?  Can it be raised?  "This is to ​the point where ​metal parts are ​corroded away, ​although our pH ​measurement is ​still ​relatively ​neutral (​average 6.​91 with ​standard ​deviation 0.27).​"  

     

    A 6.91 pH can be very corrosive.  This could be a problem. Look at Langelier and others. 

  3. The easiest fix for you where everything else downstream doesn't get screwed up is to use a blended lime slurry with calcium nitrate. Feed it to get the pH to 7.4 to 7.7. If it goes higher, you will just be wasting chemical. It will prevent H2S from building in the gas space and the nitrate will slow the reactions causing the H2S. Cheap and easy fix.  Install an H2S monitor and you can actually see the H2S level drop.  You should have an H2S monitor for employee safety anyway. You don't need anything fancy to see if it works. A BOD bottle, your WW and a small chemical sample plus an H2S probe.  No magic. H2S monitor and a small chemical metering pump.

  6. The primary forms of sulfur in a waste water system are hydrogen sulfide (H2S); elemental sulfur (S); sulfuric/sulfate (SO4-negatively charged); and organic sulfur (RSH).   Sulfur can move back and forth between these forms depending on microbes and redox potential and organic loading and pH.   The hydrogen sulfide species exists as a gas dissolved in water and it can evaporate from the water and condense on other surfaces where oxidation creates sulfuric acid and corrodes the surface.   The quickest way to limit this process is to add and iron salt such as ferrous chloride - FeCl2  (or ferric chloride - FeCl3 - more expensive) at the earliest point in the process.   This will react with hydrogen sulfide gas to form ferric sulfide salt which is insoluble and is not a gas.  This will turn the wastewater (or sludge) very black.  But it works well and works quickly.  That will buy you time while you study the source and form of sulfur and the oxidation  state of the system and perhaps develop a biological control plan using aeration.    Iron salts can be used long term but keep an eye on the pH because these are very strong acids.  Ferrous chloride is less acidic than ferric chloride.      

    1 Comment

    1. Great comment that goes straight to the chemistry and avoids the sales hype. 

  7. If the sulfur is entering your system as sulfate, you are having reduction occur within the system. If the sulfur is entering your system as sulfide, you are having oxidation occur. These are polar opposites so you must identify the influent S form 1st.  Chances are it is Sulfide and you are oxidizing it. That is the most common cause of elemental sulfur. Extremely difficult to stop the reduction at the S stage. Just run ORP's across your system. 

  8. You don't indicate the source of the wastewater, which makes a difference. Do you have an analysis of it prior to these changes? If nothing mechanically has changed, then something might have been introduced that has changed its chemistry. You should also look at the sulfuric acid that you're receiving to ensure that it has not changed. Over what period of time did this occur? The redox will tell you something has changed if you have historical data. Ensure that your pH reading is accurately monitoring the process.

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