Question: I read the article in WC&P regarding RO treatment removing 82 percent nitrate. What is the reject rate of nitrate if a nanofiltration membrane is used? Also please clarify the following: my understanding from the NSF guidelines is that the MCL for nitrate (measured as nitrogen) is 10 mg/L (IBWA and FDA) and 50 mg/L (WHO and EC), but some other organizations are measuring the nitrate (as nitrate). In that case, what is the MCL in mg/L? Are there any changes in the above levels of 10 and 50 mg/L?

With regards,
Thomas Patu K, Process Engineer
Makkah Water Co., Saudi Arabia

Answer: There was a study done in Spain on this very subject, “Performance of commercial nanofiltration membranes in the removal of nitrate ions”, Desalination, Volume 185, Issues 1-3, 1 November 2005, pages 281-287 by A. Santafé-Moros, J.M. Gozálvez-Zafrilla and J. Lora-García. (Find it online at http://www.science direct.com)

Andrew Warnes,
GE Water & Process Technologies

Answer: Though I have no data from testing specific products, generally one should not expect much reduction at all of nitrate ion utilizing nanofiltration. However, the 82 percent reduction you cite for RO seems reasonable. Nanofiltration is tailored for multivalent ion reduction; e.g., Ca++, Mg++, SO4=, at about 60-80 percent removal; not for monovalent ion removal (Na+, K+, Cl-). Monovalent ion removal would probably be expected to be less than 50 percent. Perhaps some of the other Tech Reviewers can give you specific examples from empirical data that they might have. You could try conducting a Google search to see if you can find such data.

Regarding the 10 mg/L US EPA MCL for nitrate, this is for nitrate nitrogen as “N”. To calculate or convert this to mg/L as nitrate (NO3-), you have to calculate the full mass or molecular weight including the oxygen atoms. The molecular wt. of nitrate is 62 (N = 14 and O = 16 x 3 = 48). The N represents only 22.58 percent of the weight of NO3. So to convert 10 mg N to mg NO3, divide 10 by 0.2258 = 44.3. Therefore, 10 mg nitrate-N/L = 44.3 mg nitrate as NO3/L. The WHO regulation rounds that up to 50 mg nitrate as NO3/L, which to converts back to N…50 x 0.2248 = 11.3 mg nitrate-N/L.

Gary Hatch, Ph.D.
Pentair Filtration, Inc.

Question: We are a pharmaceutical company manufacturing various medicines. We are thinking of using hollow fiber membrane ultrafiltration for the pretreatment of incoming water supplies (surface water, source lake). Sometimes we get high turbidity (10 NTU), especially in monsoon (our raining season), whereas normally we get < 2.0 NTU. We perform in-house chlorination to control microbial load in the form of pseudomonas, etc. Could you please advise if such a system (hollow fiber membrane ultrafiltration) is capable of combating the high turbidity and microbiological contamination issues? And whether this could replace our existing inline chlorination and multimedia filtration treatment processes? What are the limitations/disadvantages of the hollow fiber membrane ultrafiltration technique?

Adil Imtiaz, Assist Manager Safety
Health & Environmental Protection
Technical Division, Roche Pakistan
Limited

Answer: Hollow fiber MF and UF membranes are very effective in reducing turbidity in surface water supplies. They have become the technologies of choice to meet the Safe Drinking Water Act amendments for surface water supplies in the US. While extremely effective against particulates and requiring very little pretreatment, high concentrations of TOC can foul them. In this case, pretreatment will probably be required. The important thing is to get a worst-case water analysis and perhaps run a pilot test.

Peter S. Cartwright, P.E., CWS-VI

Question: We are putting ozone gas into distilled water and are wondering, if we are working at room temperature and normal ambient pressure, what concentration would be expected when the water is fully at saturation? How much more concentration could be expected if the water is chilled almost to freezing as the ozone is bubbled in?

Thanks for your help.

William C. Domb, DMD
Upland Calif.

Answer: The solubility of ozone in water is dependent on several factors, including: concentration of ozone in the gas; water temperature; absolute pressure of the water; ozone demand in the water; contactor efficiency. Given these variables, the only way to really know the concentration of ozone in your specific system is to measure it empirically using a dissolved ozone sensor.  That said, at gas phase ozone concentrations of one to three percent, you should expect 3.5 to 10.6 mg/L at 25°C. The concentration of dissolved ozone will be higher at a lower water temperature.  You can expect 7.4 to 22.2 mg/L at 5°C at gas phase ozone concentrations of one to three percent. (See Ozone, a Reference Manual, pg. 19-20; by the WQA Ozone Task Force, Joseph F. Harrison, Technical Editor; published by the Water Quality Association, 2004).

Bob Smith-McCollum
Pacific Ozone Technology

Answer: This is covered by Henry’s Law for Ideal Gases and is a function of water temperature and partial pressures (ozone gas phase concentration). As ozone is not an ideal gas, reactions take place that can lower the dissolved ozone level; however, if the water has little or no demand as would be expected with distilled water, you can use it.

Gases dissolve in liquids to form solutions. This dissolution is an equilibrium process for which an equilibrium constant can be written. For example, the equilibrium between ozone gas and dissolved ozone in water is O3(aq) <—> O3(g). The equilibrium constant for this equilibrium is K = p(O3)/c(O3). The form of the equilibrium constant shows that the concentration of a solute gas in a solution is directly proportional to the partial pressure of that gas above the solution.  Stating the pressure-concentration ratio as an equation and use of the usual modern symbol for the Henry’s Law constant on a concentration basis (K’c) gives the following form of Henry’s Law: p = K’cc

In this form, p is the partial pressure of the gas, c is its molar concentration and K’c is the Henry’s Law constant on the molar concentration scale. Henry’s Law is found to be an accurate description of the behavior of gases dissolving in liquids when concentrations and partial pressures are reasonably low. As concentrations and partial pressures increase, deviations from Henry’s Law become noticeable. This behavior is very similar to the behavior of gases, which are found to deviate from the Ideal Gas Law as pressures increase and temperatures decrease. For this reason, solutions which are found to obey Henry’s Law are sometimes called ideal dilute solutions.

Paul Overbeck
International Ozone Association

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