Feedback Questions and Answers

1. Can chorine dioxide prevent bromate formation in water plants using ozone?

Yes. Water plants using ozone can form bromates when the source water has bromide. At Ozone plants in Contra Costa California and El Paso Texas, chlorine dioxide doses of 1.0 mg/L to 1.5 mg/L reduced bromate formation from 50% to 78%. Also, at the Contra Costa Plant, it was demonstrated that 1.0 mg/L chlorine dioxide dose was superior to ammonia addition in reducing the bromates. For more information about the El Paso Study contact Dr. Doug Rittmann at drrittmann@elp.rr.com.

2. Can chlorine and quicklime be added in the same compartment and be dosed into a sedimentation tank by a dosing pump for the purification of water?

When chlorine is added with quicklime in the same compartment, the effectiveness of chlorine as a disinfectant is rendered practically useless. The effectiveness of chlorine as a disinfectant depends greatly on the pH level of the water. As the pH increases, there is less hypochlorous acid formed from the chlorine addition thereby making it less effective as a disinfectant. On the other hand, chlorine dioxide does not ionize in water like chlorine and therefore is "blind" to the pH effect up to about pH 10. Greater than pH 10 will cause chlorine dioxide to form chlorates but at less than pH 10, the chlorine dioxide is effective as a disinfectant.

3. What is the best stage for the introduction of chlorine into a treatment scheme?

The point of chlorine addition will be determined by the time necessary to meet disinfection requirements. In other words, the CT (concentration as mg/l times time in minutes) should be determined for meeting minimum Giardia sp. inactivation requirements. However, longer contact times and higher concentrations cause increased TTHMs formation potential, a suspected carcinogen in water. Often advanced oxidants such as chlorine dioxide or ozone are used in pretreatment to substitute for chlorine in order to reduce the contact time and dosage of chlorine, which reduces TTHM formation. Also, ozone and chlorine dioxide are able to meet a higher disinfection requirement by inactivating the more resistant Cryptosporidium sp. which chlorine is unable to inactivate. Normally, chlorine or chloramines (chlorine + ammonia) are always added in the clearwell reservoir prior to distribution to customers and the disinfectant residual is carried to the tap. Since drinking water regulations will limit TTHMs formation, advanced oxidants such as chlorine dioxide, ozone, or UV will likely be used within the treatment plant process prior to addition of chlorine. In my research, I have shown that mixing chlorine dioxide with chlorine at the ratio of 3:2 respectively will increase disinfection capability while lowering TTHMs formation potential. By doing this, the chlorine could be added at the raw water with the chlorine dioxide and avoid the TTHM formation potential from the chlorine and increase disinfection potential. You might want to read my paper titled: Impact of Mixing Chlorine with Chlorine Dioxide on TTHMs Formation in Drinking Water."pdf

4. One of our clients is having some problems with high pH. It happens when the inlet process water temperature gets high in one stage of the process. Would the temperature raise the pH? Have you ever experienced something like that? They have high levels of ammonia in the raw water and we dose ClO2 and Sodium Hypochlorite, so they have chloroamines and still have free ammonia too, because we don´t reach the breakpoint.

Since the raw water has high levels of natural ammonia, it is likely that the change in pH is directly related to the change in ammonia levels because increased ammonia levels will raise the pH. Higher temperatures are likely increasing the natural ammonia levels because of increased bacteriological action. For information about chloramines click the link, CHLORAMINES. When the pH of the water is >8.0, then only monochloramines are formed. The good news is that the chlorine dioxide will not react with the ammonia like the chlorine and is more effective as a disinfectant than chlorine at the higher pH level.

5. We are researching pre-chlorination or other disinfection methods for treating a wet well that is exhibiting reduced pumping flows attributed to biofilm. Any suggestions for a direction to go in?

If the problem is reduced flow due to biofilm, then the chlorite byproduct from chlorine dioxide addition is excellent for eliminating it. Water plants using chlorine dioxide in pretreatment at the raw water with a low level chlorite byproduct in the distribution system have prevented the effects from biofilms that cause nitrification from the chloramine treatment. The link, http://www.h2oc.com/pdfs/Chlorite.pdf, describes a study using low levels of chlorite to reduce nitrification problems. AWWA also sponsored studies in Texas about the benefits of the chlorite byproduct in distribution systems from chlorine dioxide treatment at the plant, which prevented the formation of di and trichloramines in distribution systems (odorous compounds). For more details read a paper by: McGuire, M.J. et al., 1999. "Using chlorite ion to control nitrification." Jour. AWWA 91:10:52-61.

6. How much ferrous is needed to reduce the chlorite concentration to the non-detectable level?

The amount of ferrous solution dosed at the flash mixer will depend upon the amount of chlorine dioxide applied, the % iron content of the solution, and the total residual oxidants (chlorite plus chlorine dioxide) level at the flash mixer. For example, if a plant is feeding 1.4 ppm of chlorine dioxide to the raw water and has a total residual oxidants' level of 0.98 ppm (1.4ppm X 0.70 = 0.98 ppm ClO2-) at the flash mixer and is using a 12% ferrous solution as Fe, then the amount of ferrous solution needed is 25.3 ppm as calculated in the following equation 1. See my paper "Advantages of Chlorite Reduction with Ferrous Ion". Equation 1: (ClO2 Dose, mg/l X0.70 X 3.1mg as Fe/mg ClO2-) / % Ferrous as Fe = mg/l Ferrous chloride solution

7. Is there a list of the typical DBPs from ClO2 + raw water? Meaning...what are the resultant molecules formed when ClO2 reacts with an organic molecule or something else?

When chlorine dioxide is added to raw water, the inorganic chlorite byproduct can vary from about 30% to 70% of the applied chlorine dioxide dose depending on oxidant demand, temperature, competitive side reactions and generation efficiency. Ferrous ions can be used to reduce the chlorite byproduct levels prior to entry in the distribution system. If the chlorite level is not reduced by ferrous, then it will decrease gradually as it leaves the plant and passes through the distribution system. Although chlorine dioxide does not form TTHMs, other organic by-products can be formed. Aryl and methyl derivatives of glyoxalic acid were the major chlorine dioxide by-products in aqueous solutions of humic and fulvic acids (precursors for TTHMs and HAAs formation) treated with chlorine dioxide (Rav-Acha, 1984). The aldehydes and carboxylic acids produced are usually oxydegradable products, and no adverse health effects have been associated with them. In contrast, quinones and hydroquinones produced from reactions of chlorine dioxide with humic and fulvic acids do cause undesirable biological effects (Rev-Acha, 1984). Quinones, which are formed by the oxidation of phenol by chlorine dioxide, are cytotoxic and will interact with native deoxyribonucleic acid (DNA)(Guttman-Bass et al. 1987). Acetaldehyde and formaldehyde, which are carcinogenic to some animals, form by oxidative deamination of tertiary amines and amino acids treated with chlorine dioxide (Condie 1986). Guttman-Bass et al. (1987) reported that treatment of humic materials with chlorine dioxide tended to decrease the mutation ratios (mutation : non-mutation), and they found also that chlorine dioxide can oxidize several known mutagens to inactivate products. Humic acids and fulvic acids (precursors of TTHMs and HAAs formation potential) are normally about 10% to 15% of the total organic carbon content of water. Since monitoring and characterizing humic acids content and fulvic acids content is very difficult and complex, it is more convenient and easier to evaluate the effect of chlorine dioxide dose on total organic carbon or (DOC) reduction and its corresponding effect on TTHM reduction (Rittmann, D. 1999).

1. Rav-Acha, C. 1984. The Reactions of Chlorine Dioxide With Aquatic Organic Materials and Their Health Effects. Water Res., 18(11):1329-1341.

2. Guttman-Bass, N. et al. 1987. Effects of Chlorine and Chlorine Dioxide on Mutagenic Activity of Lake Kinnereth Water. Environ. Sci. Technol., 21(3):252-260.

3. Condie, L. 1986. Toxicological Problems Associated With Chlorine Dioxide, Jour. AWWA, 78(6):73-78.

4. Rittmann, D. 1999. Impact of Chlorine in the Generation of Chlorine Dioxide on DBPs in Drinking Water., Doctoral Dissertation, Univ. of Texas at El Paso, El Paso, Texas.

The "cat urine" odor is caused by the reaction of low concentrations of chlorine dioxide gas with "carpet glue volatiles" in the air at the tap in a poorly ventilated room. The odor is not in the water itself but is formed in the air such as from a shower head near recently installed carpet. The odor will decrease when the "carpet glue volatiles" are removed by ventilating the room. It rarely occurs as compared to chlorine taste complaints from customers. Distribution systems using chloramines do not experience "cat urine" odor complaints. Also, water plants that remove chlorite with ferrous ion within the treatment plant do not experience this problem. See my paper "Advantages of Chlorite Reduction with Ferrous Ion."

The continuous removal manganese greensand process feeds a combination of oxidants such as potassium permanganate and chlorine to raw water prior to the manganese greensand filter bed. The chlorine will oxidize the iron and sulfide while the permanganate will complete the oxidation of trace amounts of iron and soluble manganese. A chlorine residual should be maintained throughout the process. Extended underfeeding of permanganate to the filter will eventually exhaust the oxidative capacity of the media causing manganese leakage. The media must therefore remain in a continually regenerated condition at all times. Although chlorine dioxide can be substituted for chlorine, the decision will depend upon the cost and treatment objectives desired by the water operator. Excess chlorine will form TTHMs while chlorine dioxide will not but the chlorine dioxide will still perform the oxidative requirements. However, it may require higher costs. If the water operator is already meeting TTHM levels with the use of chlorine then the use of chlorine dioxide may not be desirable. Also, the use of chlorine dioxide could reduce the amount of potassium permanganate which is expensive. An alternative idea is to accomplish multiple treatment capabilities by mixing some chlorine dioxide with chlorine in order to provide enhanced disinfection (AWWARF, 2000), more TTHM reduction (Impact of Mixing Chlorine with Chlorine Dioxide on TTHMs Formation in Drinking Water."pdf by D. Rittmann), improved taste and odor removal while having more reliable iron and manganese oxidation. Plants treating for iron and manganese normally have taste and odor issues as well. A pilot study is recommended before substituting chlorine dioxide for the chlorine.

10. If the raw water contains bromide, should we be concerned about bromate formation if we are using ozone or chlorine dioxide?

Chlorine dioxide does not form bromates in water unless the UV from sunlight comes into contact with the chlorine dioxide and bromide in solution. A cover can be used to block the sunlight. On the other hand, ozone can form bromates from the bromide in solution without the aid of sunlight or UV. The amount of bromates formed from ozonating the water is dependent upon the contact time and dosage. The current MCL for bromates is 0.010 mg/L. See the papers titled Bromate Control during Ozonation of High Bromide Drinking Water".pdf or Impact of Chlorine Dioxide Preoxidation of Ozone on Bromates.pdf

11. Which of the oxidants is the best for reducing taste and odors from the raw water?

Often, ozone is thought of as being the best oxidant for reducing taste and odors from the raw water. However, the evaluations have been done on an unequal dose basis in field studies. Laboratory studies performed by Lalezary et.al. in 1984&1986 on an equal dose basis have shown that chlorine dioxide is superior to the other oxidants in reducing the musty-type odors in raw water. Because the chlorine dioxide dose in the past has been limited by the MCL for chlorite, its capability to reduce musty type odors have been limited also. However, some plants are now adding more chlorine dioxide with the Chlorite Reduction capability using the ferrous ion in order to more effectively reduce taste and odors and disinfection byproducts.

12. I want to ask whether chlorine dioxide can be generated without a generator? Can we mix stable sodium chlorite with hydrochloric acid (at the time of application) in a tumbler (or something else) and then add it to water/waste for disinfection?

Chlorine dioxide can be generated from stable sodium chlorite reacting with hydrochloric acid but an EPA approved generator is a safer way of doing it. The best efficiency of conversion of the sodium chlorite to chlorine dioxide is 80% when using hydrochloric acid based on the following equation: 5NaClO2- + 4 HCl --> 4 ClO2 (gas) + 2H2O + 5NaCl. The chlorine gas phase chlorine dioxide systems reacting with chlorite or the Eka SVP Pure chlorine dioxide generator (chlorate based non-chlorine system) are capable of 95%+ conversion of the precursor chemicals of chlorite or chlorate. These systems are optimum in terms of efficiency and safety.

13. How will the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) impact chlorine dioxide treatment?

The next level of USEPA regulations (LT2ESWTR) will define the higher CT (product of concentration, mg/L and time, minutes) requirements for inactivation of Cryptosporidium sp. by chlorine dioxide. Since Cryptosporidium sp. is more resistant to inactivation, many water plants will need to increase their chlorine dioxide dosages. The higher dosages will likely require Chlorite Reduction by ferrous ion. However, by increasing the chlorine dioxide dosage, the water plants will also benefit from much lower TTHMs, HAAs, and taste and odor levels to meet Stage 2 D/DBP rules, which were not possible with the lower dosages under Stage 1 D/DBP rules.

14. Why would a water plant want to use ferrous instead of ferric coagulants?

There are a number of advantages of using ferrous instead of ferric ion coagulants. First, ferrous can be purchased in equivalent % iron content as ferric at a lower price. If the water plant process has an oxidant residual of chlorite or chlorine in pretreatment, then the plant can save coagulant cost by reducing or substituting the ferric coagulant dose with ferrous. Second, the water plant may use the less expensive ferrous to offset part or all of the cost of chlorine dioxide. Third, the ferrous ion could reduce or eliminate the chlorite byproduct level from chlorine dioxide dosages in excess of 1.4 mg/L. Fourth, when the ferrous ion eliminates the chlorite byproduct level prior to the distribution system that uses free chlorine, the possibility of "cat urine" odors at the tap is also eliminated.

There are six causes of higher TTHMs in drinking water. They are higher: 1. TOC levels, 2. chlorine doses, 3. pH levels, 4. temperature, 5. chlorine contact time, 6. Bromide . In order to lower TTHMs, you could: 1. lower or reduce the TOC level prior to chlorination by enhanced coagulation and/or granular activated carbon filtration or powder activated carbon treatment , 2. decrease the chlorine dose, 3. lower the pH level prior to chlorination, 4. postpone chlorine treatment to the finish water and substitute an advance oxidant such as chlorine dioxide in pretreatment, 5. lower temperatures but it is usually not controllable by the operation, 6. mix 3 parts of chlorine dioxide with 2 parts of chlorine (Impact of Mixing Chlorine with Chlorine Dioxide on TTHMs Formation in Drinking Water."pdf). 7. substitute chloramines for chlorine in the finish water, 8. Raise chlorine dioxide dose to 2 mg/L or higher in order to remove more TTHM precursors (humic and fulvic acids).

16. How does chlorine dioxide compare to other oxidants in reducing iron and manganese?

Chlorine dioxide and chlorite byproduct are superior to other oxidants in the oxidation of iron and manganese as shown in table 1 below (Aieta & Berg, 1986). When the soluble Ferrous (II) or Mn (II) is oxidized by the chlorine dioxide and chlorite, then the precipitated forms of ferric iron and manganese dioxide are removed by sedimentation or filtration and the chlorite residual is reduced to chloride.

Chlorine dioxide gas is readily soluble in water and remains in solution as a dissolved gas in the pH range of 2 to 10. It does not hydrolyze to any appreciable extent like chlorine, which is weaker as a disinfectant and oxidant especially with increasing pH levels. About 50 to 70 percent of the chlorine dioxide gas reacted in water treatment will result in chlorite formation by the following reduction half-reaction: ClO_{2 gas} + e^- ↔ ClO₂^- . Aieta and Berg showed that oxidizable material reduce the ClO₂^- formed in the previous equation by the following reaction: ClO₂^- + 4H⁺ + 4^-↔ Cl^- + 2H₂O

18. Hello. My name is Amanda and I am doing a science project on the hardness of water. Do you think you could help me with one minor detail? I would like to know the process that water goes through when it gets filtered. If you can help me that would be great. Thank you for your time.

The conventional water plant process is usually comprised of: Screening, pre-sedimentation (optional), primary disinfection, coagulation/flocculation, sedimentation, filtration and final disinfection. Primary disinfection could include the use of either ozone, chlorine dioxide, chloramines, or chlorination. Coagulation/Flocculation processes are where coagulants such as aluminum or ferric salts and/or polymers are added in order to help settle the suspended solids in the water. The sedimentation process that follows the chemical addition process is where the flocculated solids are settled. The final polishing stage is filtration where various filter media types are used such as sand, coal/sand, and/or granular activated carbon remove the turbidity and adsorb dissolved organic matter. The final disinfection phase uses chlorine or chloramines (ammonia mixed with chlorine). The chlorine residual is maintained through the distribution system to the customer's tap. For detail information, you might contact the websites at the USEPA or AWWA.

19. The effectiveness of chlorine residuals increases with higher temperatures within the normal water temperature range. Can you explain why?

The CT (concentration, mg/L times Time, minutes) required for disinfection decreases with increasing temperatures for all disinfectants (chlorine,chloramines, ozone, and chlorine dioxide) because with higher temperatures the respiration rate of microorganisms increase causing an increase in the ingestion rate of the disinfectant.

Chlorine dioxide is superior to other oxidants in reducing TTHMs’ formation on an equal dosage basis. It is readily soluble in water and remains in solution as dissolved gas in the pH range of 2 to 10. It does not hydrolyze to any appreciable extent like chlorine, which causes TTHMs and is weaker as an oxidant and disinfectant especially with increasing pH levels. Ozone increases the biodegradable dissolved organic carbon (BDOC) level in the distribution system, which may cause re-growth problems and higher TTHM levels. A consensus has emerged that ozone oxidation alone is relatively ineffective in controlling halogenated DBP precursors. Chloramines, the weakest disinfectant, form some TTHMs in pretreatment but chlorine dioxide will not. However, when chloramines are used in the distribution system, they are effective in minimizing TTHM formation where longer contact times are necessary. Therefore, chlorine dioxide use in pretreatment with chlorine/chloramines in the distribution system is an excellent combination for obtaining maximum disinfection capability with minimum TTHM formation potential.

EPA has established an alternative compliance criterion based on Specific Ultraviolet Adsorption (SUVA). It is a calculated value based on two analytical methods, ultraviolet absorption at 254 nm, and dissolved organic carbon (a filtered TOC sample). The SUVA calculation is as follows:

In some cases achieving compliance with the SUVA alternative is easier than achieving it through the TOC criterion. Drinking water systems are not required to perform enhanced coagulation or enhanced softening if the SUVA is < 2.0 L/mg-m, even when the TOC is greater than 2.0 mg/L. A low SUVA value indicates that the organic carbon content of the water consists primarily of non-humic substances and therefore has a lower TTHM potential. This can produce substantial savings in treatment costs.

22. Are chlorite-based, chlorine dioxide generators equal in performance to the chlorate-based, chlorine dioxide generators?

In a two-year lab and plant evaluation of the Rio Grande River water, the Rio Linda chlorite/chlorine gas type chlorine dioxide generator and the Eka SVP-Pure Chlorate-based, Chlorine Dioxide generator performed essentially the same in lowering TTHMs formation at 10 ppb per mg/L of chlorine dioxide for chlorine dioxide dosages between 3 and 7 mg/L, chlorine dosages from 3 to 7 mg/L, pH levels from 6 to 9, and TOC levels from 3 - 5 mg/L. At dosages less than 2 mg/L, the reduction of TTHMFP was from zero to less than 10 ug/L on each generator because the initial chlorine dioxide demand (1 minute contact time) of the Rio Grande River water was 1.5 to 2.0 mg/L which had insufficient contact time for lowering TTHM precursors. Research literature confirms that chlorine dioxide dosages less than 2 mg/L usually have minimum effect on reduction of TTHMs precursors but relies primarily on lowering TTHMs by replacing chlorine with chlorine dioxide in pre-treatment.

1. Chlorine dioxide can provide superior disinfection capability at economical cost as compared to lime. 2. Chlorine dioxide can provide superior fertilizer quality sludge (Class A) at neutral pH levels as compared to lime. 3. The chlorine dioxide solution should have better mixing and penetration of sludge solids than dry lime. 4. The chlorite by-product from chlorine dioxide addition should prevent bacterial re-growth and ultimately form chloride over time.See technical bulletin concerning treatment of wastewater sludge by chlorine dioxide.

"White water" is usually caused by entrapped air in the water. The source of the air can be from low water reservoir levels, groundwater levels below the well screen, air drawn through pump packing, or defective air relief valves in the system. These are maintenance issues which need to be remedied by the water supplier. However, customers often believe the problem is caused by high chlorine or other chemicals. It is important that the water supplier be notified of the problem but it is not a health related issue. Just stir the water drawn from the tap with a spoon and it should clear within a few seconds.

25. What is likely the direction of future water supply and pollution control efforts in the United States?

The likely direction of future water supply and pollution control efforts in the United States will be:

When water is described as having an occasional musty, soil, kerosene, or pesticide taste or odor, it is usually associated with an increase in geosmin, methyl-iso-borneal (MIB), or pyrazine compounds which reacted with chlorine to cause the odors described. Although the taste is unpleasant, they are not considered harmful. The water supplier should be notified. The odors can be eliminated at the water plant by a combination treatment of advanced oxidants and activated carbon filtration prior to chlorination.

27. Is it possible that when I use CLO2 in a swimming pool, I will generate too much ammonium in the water? Or is the ammonium generated by the people in the water?

Chlorine dioxide does not react with ammonia. The ammonia is generated from urination of the people in the swimming pool. Chlorine will react with ammonia to form chloramines which weakens its disinfection capability. On the other hand, chlorine dioxide is essentially “blind” to pH effects in terms of disinfection capability while chlorine will be less effective at higher pH levels. However, chlorine, bromine and sometimes iodine are normally the preferred disinfectants used in swimming pool applications because of their greater stability and lower cost than chlorine dioxide.

28. I am in great need of your help. I am trying to dechlorinate my tap water but I am not having success, even though I am using a known method, that is using ascorbic acid. The problem is that the municipal water treatment plant uses chloramines so when I neutralize the chlorine i have ammonia free in the water. Any suggestion on how to neutralize ammonia in DRINKING water ?

Regards, Rafael

There are a number of ways that ammonia can be removed from water. There are: 1). Physical Operations; 2. Chemical Processes; 3. Biological Processes. Since this is an end user application, I would recommend that you investigate a chemical process involving cation exchange. In this process a natural Zeolite called Clinoptilolite has been employed to remove ammonia. When the Clinoptilolite column becomes exhausted, it is regenerated by the use of a brine solution of about 2% sodium chloride, which can be renovated by electrolysis. The spent regenerant contains mainly NH4+, Ca+2, Mg+2, Na+, and Cl- ions. In municipal operations or large-scale operations, electrolysis is used to generate chlorine at the anode and hydrogen at the cathode. The chlorine reacts with the ammonia by breakpoint chlorination to produce nitrogen gas, and nitrogen and hydrogen gases are removed as off gases. After the electrolytic treatment, the regenerant has been renovated so that it may be reused. However, at the end-user application point, the sodium chloride solution would just become a waste discharge to the sewer that occurs in ion-exchange softening processes. In other words, you would not need to utilize the electrolytic process.