Biocide Dosing Calculations

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I am currently working on redesigning a biocide pretreatment system for reverse osmosis.

Apart from the ​usual ​parameters, ​should I take ​into consideration any ​other ​parameters such as ​Oxidation ​Reduction ​Potential (ORP) ​levels? Or temperature for calculations?
How much do they affect the system and how good is the calculation estimation without them?

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9 Answers

  2. If you are using chlorine/hypochlorite prior to RO membranes, differential ORP is acceptable but I find marginal. It's ok for basic control. Most importantly, after you kill the bleach, measuring that it's all gone. ORP won't reliably give you a that reading. Several manufacturers make a chlorine meter that is much safer to use. I do not recommend bleach ahead of RO membranes. If you are, a GAC bed with sulfite feed is very important to kill the oxidant.  Chloramines or chlorine dioxide only for oxidizing biocides. Are your RO's on continuously, then there are several other treatments that are less dangerous to the membranes. slug fed non-oxidizers and IDE Technologies system are better. If they are on intermittently, a good low conductivity water flush and layup with a non-ox biocide is most effective.

    1 Comment

    1. Thanks for the answer Scott. Yes the system runs 24/365. I am not familiarise with EDI technology. I had problems of bacteria growing and fouling before so this option may be interesting for me.

  3. Of course you should take ORP and Temperature into consideration. Any biocide reaction is an OXIDATION REACTION. Also temperature affects the speed of the reaction.  

    1 Comment

    1. Thanks George. So, will I include as part of my calculation kinetic equations considering the grade of the reaction as well as the average temperature? But in terms of temperature, I think I should include temperature as an average value for a certain period of time I suppose. 

      2 Comment replies

      1. Yes I assume that, I took a constant temperature in my calculations due to the variation is small (sometimes are greater but I wont take that under consideration for the moment). Thank you for your reply George. If I need more advise I will contact with you. 

      2. Of course. In real world temp is never constant. If temp flactuations are small then you should take temp as constant. If not take the avarage. Normally temp increases the speed of the reaction because it increases the kinetic energy of the molecules which in turn increases the number of collissions per unit time. Good luck. If you need any further advise mail me at theschem@otenet.gr

         

  4. Are you referring to adding biocide before the membrane or to the permeate after the membrane? Before the membrane use chloramine (never free chlorine) at probably a couple of mg/l. Peracetic acid is also a possibility. Post RO you can use almost anything, since the  water will be very low in TOC and is should be already free of pathogens assuming the membrane and seals are in tact.

    1 Comment

    1. Thanks for your answer Joseph. Yes I am refering before membrane. Is paracetic acid good at bacteria control? Will that influence in the pH  (with the consequent pH control after)? 

  6. It depends on which desinfection agent you intend to use. But you have to find a way to protect your RO unit as already mentioned. 

    1 Comment

    1. His Ademar, thanks for answering. Effectively, my priority is protect membrane to extend their life and avoid bacteria growing either in the RO''s as well as maintain a good control of bacteria in the system (warm months are the most critically for me). 

  7. As with antibiotics, microbes have developed resistance to biocides. Therefore, dosing biocides into an RO system is not a good idea, especially if the treated water ends up in the aquatic environment. Instead, why not install an upstream bioprocess that will remove organic and inorganic electron donors and prevent or at least minimise biofouling in that way? For example, an expanded bed biofilm reactor, so that the place where biofilm forms can be controlled. If you want to know more about this technology, please contact me directly.

    2 Comments

    2. Thanks Mike. I used bed technology before but for other purposes. Will it be effective in this terms?

      1 Comment reply

      1. As far as I know, it's never been tried but, theoretically, it should work by removing nutrients that would otherwise reach the membranes and fuel biofilm growth on them.

  9. Dear María,

     

    Disregarding some "Chlorine resistant" membrane elements, oxidizing conditions must be avoided. In terms of efficiency, you should always choose those biocides with fastest effect on bacteria, being DBNPA the most chosen active. Remember that as long as the system is running, useful contact time is in the order of minutes to seconds. When you are thinking about a preservation program for standby systems, you would use long mid life products, like Isothiazolone.

     

    Best regards

    S. Cotic

    1 Comment

    1. Dear Maria, as mentioned above you may not have enough contact time with your Biocides. You might consider UV radiation with an exposure chamber sized for your required flow rates, and a bulb sized for those flow rates. Also keep in mind you may need other Pre-treatment to protect the quartz tube from fouling or particulate from eclipsing the UV rays from the bacteria. Of course these other pre-treatments would also protect your membranes on the RO as well, increasing their life and efficiency!