Increasing Hardness of RO Permeate

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We are implementing an RO system to reduce the TDS of a borehole supply for an industrial client.

The client, however, has a requirement for the hardness to be in a specific range of 4-5 °dH (deutsche Härte).

 What do you suggest as:

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7 Answers

  1. Reverse osmosis (RO) is a process that forces a feedwater stream containing dissolved impurities (salts and organics) through a semipermeable membrane into two separate streams: one of removed solids (concentrate or reject) and one of purified water, producing soft permeate water.

     

    As the feedwater passes through the membrane, the ions and organics are left behind in the reject and the permeate remains with 96 to 98 percent fewer impurities. Permeate recoveries are typically limited to 75 percent because of the limited solubility of the dissolved salts left behind in the reject.

     

    Normalized permeate flow is one of the most sensitive forecasters of trouble in an RO system. Fouling can reduce the permeate flow rate. However, just measuring the permeate flow rate is insufficient because it varies with the feedwater temperature, feed pressure, permeate pressure and feedwater conductivity (total dissolved solids, or TDS). Though all RO membrane manufacturers have their equations for determining the normalized permeate flow , several parameters are needed to develop this complex calculation: temperature, net driving pressure, pressure drop, permeate, feedwater TDS and permeate flow. This calculation corrects for temperature and pressure variations. The calculation, known as normalized permeate flow, adjusts the daily data readings to what they would be if the system were operating at startup pressure at 25°C. This allows for daily comparisons of RO performance. Seasonal feedwater temperature variations can make fouling trends difficult to detect if the normalized permeate flow calculation is not used. For example, RO feedwater derived from surface sources becomes warmer during the springtime. This increase in feedwater temperature affects membrane performance by increasing the permeate flow rate. If the membrane elements are fouling at the same time (fouling causes a decrease in permeate flow rate), it is unlikely that this will be noticed until a change in seasons. At this time, the membrane elements may be severely fouled, with the permeate flow rate severely restricted.

     

    Percentage rejection= (feed Conductivity-Permeate Conductivity)/

                                              Feed Conductivity

  2. You may consider using a side-stream process whereby about 2 - 5% of the final product is diverted and saturated with CO2, and then contacted with solid CaCO3. The CO2 acidified sidestream is able to take up considerable amount of CaCO3, increasing the alkalinity and the calcium concentration.  The CO2 is then stripped (and captured for re-use if required), and the side stream is blended with the mainstream at the proportion to achieve the desired hardness and pHs of the product.   Do a Google search for the paper entitled "Water stabilisation througha limestone mediated sidestream process" by Ciciyela Tsotsi.

  3. @Daniel Karwalski @Gunther Johne @Engr. Mansoor Ahmed PhD. Scholar  thank you for your input. The incoming TDS is between 650-700ppm. Unfortunately NF does not get me down to my required TDS of 80-120ppm. Using RO membranes with permeate blending I can only reach a projected 25ppm as CaCO3 (1.5 dH) Hardness before exceeding the required TDS, the pH would be in the region of 5.6SU at this stage. The required permeate flow rate is 20m3/hr.

    If possible, could you expand on the benefits and negatives of dosing Ca(OH)2 versus CaCl2?

  4. Both Gunther an David are correct. Permeate correction for hardness can be achieved in a number of ways, including those mentioned below plus dosing using NaOH and Calcium Chloride if RO permeate is low pH. The best method will depend on the daily permeate capacity of the system, along with the pH of the raw water and RO permeate. I assume the treated water TDS requirement is very low given the selection of RO instead of Nano membranes. For smaller flow systems, Calcite media filters work well and are more economical initially over lime and CO2 based systems. Calcite media dissolves at low pH until pH is neutralised, adding calcium and alkalinity to the water. Permeate blending will work only if the product water TDS requirement is higher, but in this case I would consider Nano membranes.  

  6. Dear Shandor,

    What is the Product TDS Requirement for your Client? I think might be 1000ppm or 1500ppm? 

    What is the Feed TDS?

    I recommend you to have Nano Filtration Membrane instead of RO if your Inlet TDS Vs Product TDS could give you required at 70% Salt Rejection Efficiency. As NF has Salt Rejection Efficiency up to 70% whereas RO has 99.8%, so the NF is better option in case your Feed TDS are that much to give required Product TDS at a Rejection Rate of 70%.

     

    Otherwise if the above is not the case, then I agree with Mr Gunther to choose that option.

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    1. What can be used in place of either the Nano filtration or the RO. 

  7. For the hardness increasing you can use two ways

    1 – blending of the RO permeate with raw water

    2 – using of calcium carbonate (granulated or lime milk) and carbon dioxide.

    For many industrial applications is the first way more than sufficing. But when your raw water has some contamination inside which your costumer will not have it´s better to use the second way.

    For online monitoring of the hardness and the regulation of my blending valve I use an ion selective sensor system – more information to this monitoring units you´ll find here http://water-monitoring.com/produktubersicht/?lang=en

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