United States Environmental Protection Agency Municipal Environmental Research Laboratory Cincinnati OH 45268 Research and Development EPA-600/S2-82-051 August 1982 Project Summary Chlorine Dioxide Disinfection and Granular Activated Carbon Adsorption Ben W. Lykins, Jr., and Mark Griese A pilot plant study was designed to evaluate chlorine dioxide disinfection as an alternative to chlorine for drasti- cally reducing or preventing the pro- duction of trihalomethanes. Also in- vestigated was the effectiveness of post-treatment adsorption by virgin and reactivated granulated activated carbon (GAC) for removal of organic compounds that were either present in the source water or formed after dis- infection. The performance of a 0.38 m3/ min (100 gpm) pilot plant using chlorine dioxide disinfection for both raw and treated water was compared with that of a full-scale plant applying chlorine disinfection at similar locations. Two parallel post-filtration adsorber con- tactors consisting of virgin in one contactor and reactivated GAC in the other were continuously operated and evaluated until exhaustion. After about 90 days of operation, the GAC was educted and reactivated offsite by an infrared furnace. Composite samples of spent and reactivated GAC were collected during each run, and a representative virgin GAC sample was taken directly from the shipping bags for characterization. Various properties were determined on these samples to ensure that the spent GAC had been reactivated to essentially virgin conditions and to provide an opportunity to compare the performances of virgin and reactivated GAC. Analyses performed during the pro- ject consisted of the quantification of 14 volatile and 8 extractable organic compounds along with qualitative (detected/nondetected) determina- tions by mass spectrometer scans of 32 additional volatiles and 54 extrac- table organics. Additional data were collected for 9 inorganic metals, total organic carbon (TOC), and other pa- rameters such as turbidity, standard plate count (S PC), and total colif orms. This Project Summary was develop- ed by EPA's Municipal Environmental Research Laboratory, Cincinnati. OH, to announce key findings of the re- search project that is fully documented in a separate report of the same title (see Project Report ordering informa- tion at back). Introduction An experimental study was initiated at the waterworks facility in Evansville, Indiana, to help satisfy a basic need within the water treatment and supply industry for producing drinking water relatively free of organics. This location provided insight into the feasibility of implementing an alternative method of disinfection in a typical surface water plant. The specific objectives estab- lished for this project included: — to develop a water treatment process using chlorine dioxide as a disinfectant and to compare its ------- Table 1. Experimental Modes of Operation for Pilot Plant Run Chlorine Chlorine Dioxide Alum Polymer Mix Settle Lime Chlorine Filter Carbon Chlorine Dioxide 1 2 3 4 X XX X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X production of trihalomethanes with those produced during chlo- rine disinfection. — to determine whether any organic byproducts are formed when using chlorine dioxide as opposed to chlorine as a disinfectant. — to determine the disinfection ef- ficiency of chlorine dioxide and its ability to provide residual neces- sary for full-scale distribution use. — to determine the effectiveness of virgin and reactivated GAC for removing organic compounds that were either present in the filtered water or formed after chlorine dioxide disinfection. Treatment Method Selection The initial phase of the project con- sisted of a control study and three experimental modes of operation (Table 1). Forthe control study (Run 1), the pilot plant was operated for 2 weeks using pre- and post-treatment chlorine disin- fection and bypassing the GAC contactors to allow a performance com- parison with the full-scale plant. This study revealed that the average concen- trations of the two plant effluents were comparable. In addition to average values, the data for total instantaneous trihalomethanes (THM's) and TOC were subjected to the t-test at a 95 percent confidence level. This test showed no significant difference for either param- eter when data from the pilot plant were compared with those for the full-scale plant; the full-scale plant could thus be used as a control for subsequent experi- mentation. The other three runs were 3 weeks long and were evaluated to determine which treatment method would be used during the long-term operating phase. The method of disinfection used for Run 2 consisted of chlorine addition to the raw water and to the GAC effluent just before it entered the clearwell. For this and subsequent runs, the GAC con- tactors were placed in operation. The instantaneous average data for the second run showed an increase in the total trihalomethane(TTHM) concen- tration from 1.3/ug/Lintherawwaterto 31.3 ;ug/L in the GAC influent. After passing through the GAC contactors, the total instantaneousTHM concentra- tion decreased to 0.1 fjg/L. The average TOC concentration decreased from 2.6 mg/Lintherawwaterto2.1 mg/Lafter mixed media filtration, and it was further reduced to 0.3 mg/L by the GAC con- tactors. Total coliform counts were reduced from an average of 4,800/100 ml in the second run raw water to <1/100 mL after pre-treatment disinfection. Very little regrowth occurred through the carbon, with the average GAC effluent containing 1/100 mL. Subsequent disin- fection reduced the total coliforms to <1/100mL Mass spectrometer scans of both the volatiles and extractables showed only those instantaneous compounds previ- ously identified by gas chromatography (GC) evaluation and verified the removal of these compounds by GAC treatment. The third run consisted of a treatment method using no pre-GAC disinfection. Chlorine dioxide disinfection was used only after the GAC contactors. This method of treatment permitted the study of organic removal with GAC and with- out interference from those organic compounds attributed to disinfection; it could also be used to determine whether any byproducts were formed when chlorine dioxide was used after GAC treatment. The TOC concentrations were reduced by almost 40 percent (4.2 to 2.5 mg/L) after pilot plant treatment. Complete treatment, including GAC, reduced TOC concentrations by 80 percent (4.2 to 0.8 mg/L). Breakthrough occurred in TOC concentrations in about 1 week at GAC depths of 6 and 18 in. At 30 and 78 in. (with 7.7 min of empty bed contact time at 78 in.), no breakthrough was detected during the run. Various samples were stored for 3 days with chlorine and chlorine dioxide. The average concentrations showed a large increase in the raw water THM level when stored with chlorine (1.2 to 247jug/L)and a slight increase in those samples stored with chlorine dioxide (1.2 to 2.9 jug/L). The slight increase with chlorine dioxide is attributed to a small amount of unreacted chlorine present after generation. The final THM concentration after 78 in. of GAC was 38//g/L and 0.7/ug/L, respectively, for chlorine and chlorine dioxide storage. Although chlorine dioxide disinfection after the GAC contactors reduced the total coliform and SPC to acceptable values, slime growths were a problem in the treatment plant because no disinfec- tant was added to the raw water. The objectives of the fourth run were to determine whether any byproducts were formed when chlorine dioxide was used as a pre-treatment disinfectant, and to evaluate GAC removal of any organic compounds present or formed. Average concentrations for the instan- taneous THM's showed the raw water value of 3.6 fjg/L being reduced to 0.2 //g/L through the pilot plant clearwell. Very similar values were noted for the THM's produced when stored with a chlorine dioxide residual (3.7 //g/L for the raw water and 1.1 //g/L for the clearwell effluent). As in previous runs, the qualitative mass spectrometer scans did not indi- cate the formation of any byproducts relative to this mode of treatment. Also, no increase in the concentration of metals occurred after passage through the activated carbon contactors. Because a pre-disinfectant is required to control slime growth in the treatment plant. Run 3 was eliminated from con- sideration for further study. The second and fourth runs were evaluated based on an estimate of what the consumer might receive from each of the treatment modes. In both of these modes, adequate ------- disinfection was accomplished and mass spectrometer sea ns of the priority pollutants did not showthe formation of any byproducts relative to the disinfec- tant used other than the THM's. There- fore, a desire to adhere to the original study objectives resulted in selecting chlorine dioxide as a pre- and post- treatment disinfectant for long-term evaluation. Pilot Plant Operation After the pre- and post-chlorine diox- ide disinfection operational mode was selected for long-term studies, four additional runs were completed. Each run was approximately 3 months. For all runs, the performance of the pilot plant through GAC treatment was compared with the performance of the full-scale plant under normal operation (pre- and post-treatment chlorine disinfection). Also, two parallel contactors were used with one always containing virgin GAC, and the other one containing virgin GAC during the first run and once, twice, and thrice reactivated GAC for the remaining three runs. The effect of using pre-treatment chlorine dioxide disinfection for main- tenance of bacterial water quality and the reduction of THM's was evaluated by comparing the sand filter effluent's of both plants. Terminal THM reduction through coagulation and settling was determined by storing all samples for 3 days (estimated residence time in the Evansville distribution system) at am- bient temperature with a free chlorine residual.. The use of pre-treatment chlorine dioxide disinfection allowed substanti- ally more of the THM precursors to be removed by coagulation and settling than with pre-treatment chlorination. This was caused by less precursors being converted to THM's in the pres- ence of chlorine dioxide thereby allow- ing more to be available for removal by coagulation and settling. Therefore, the THM's formed during post-treatment chlorination and subsequently in the distribution system would be reduced. A comparison of the average terminal THM concentration for all runs of both disinfectants showed about 60 percent less concentration of THM's when chlo- rine dioxide was used for pre-treatment disinfection. Though THM precursors were reduced by using pre-treatment chlorine dioxide, no difference was noted in the averageTOC concentration, regardless of the type of pre-treatment disinfectant used. Because of the THM reductions noted by the use of pre-treatment chlorine dioxide disinfection, an additional study was completed to determine the feasi- bility and benefits of using chlorine dioxide in Evansville's full-scale plant. Although chlorine dioxide could be used as a pre-treatment disinfectant, chlorine was desired as the post-treatment dis- infectant because of the size of Evans- ville's distribution system and the need to maintain a sufficient disinfectant level throughout this system. Data generated from previous runs indicated that main- tenance of a chlorine dioxide residual over an extended period of time was difficult. The pilot plant GAC contactors were bypassed to simulate the full-scale plant as closely as possible. Chlorine dioxide was introduced into the raw water in sufficient quantities to maintain a slight residual through mixed media filtration (an average addition was 1.1 mg/L, and the average residual after filtration was 0.1 mg/L). An attempt was made to keep the total oxidant level below 0.5 mg/L. During this run, the average concentration of chlorine dioxide plus chlorite was 0.7 mg/L. The instantan- eous THM data after filtration again showed a reduction inTHM's when pre- treatment chlorine dioxide disinfection was used. No difference in the bacterio- logical quality of the two plant effluents was noticed when chlorinated effluents were compared. Each of the parallel contactors con- tained 545 to 590 kg (1,200 and 1,300 Ib) of a lignite-based GAC. During each of the four runs, one contactor always contained virgin GAC and the other contained reactivated GAC. Analysis of the GAC showed that the spent carbon in each reactivation was restored at least to its virgin state. Cumulative surface area distribution curves indicated that reactivated GAC for all runs had a total surface area greater than the virgin GAC. This addi- tional surface area was developed pri- marily in the small pore region (less than 30 Angstroms). Also, the iodine number and cumulative surface area correlated almost exactly indicating that the pores were open and maintained. Thus it was unlikely that any gradual degradation of adsorptive capacity would occur after subsequent reactivations. Ratios for Brunauer-Emmett-Teller (BET) surface area/iodine number for an average of two virgin samples and each of the three reactivation cycles were 1.08,1.06,1.08, and 1.03, respectively. Organic loading on the GAC was evaluated for TOC by comparing the sum of TOC removed to the sum of TOC applied per unit weight of carbon. This loading indicated that the virgin and subsequently reactivated GAC per- formed essentially the same (see Figure 1). The average TOC carbon use rate for both the virgin and the reactivated GAC was about 0.05 kg/m3 at the steady- state conditions that occurred after about 60 days of operation. Conclusions Pilot-plant evaluation of chlorine di- oxide disinfection and GAC treatment have provided some insight into this treatment scheme. The use of either chlorine dioxide or chlorine in raw water provides similar disinfection qualities through conventional treatment. Also, because chlorine dioxide has less de- mand and possibly strong oxidative properties, smaller concentrations were needed to produce effective disinfection. By using less applied chlorine dioxide, some of the difference between the cost of chlorine dioxide and chlorine can probably be recouped. Application of chlorine dioxide to the raw water produced less concentration of THMS'sinthefinished water than did chlorine addition (average of 51 //g/Las opposed to 130 fjg/L for all runs). Furthermore, chlorine dioxide disinfec- tion produced no organic byproducts other than those noted with chlorine disinfection (i.e., among priority pollu- tants). GAC was spent and reactivated three successive times and essentially re- stored to virgin state as indicated by several GAC properties and perform- ance comparisons with virgin GAC. Gross estimates of volumetric losses during GAC offsite reactivation were about 5 percent, and overall system losses (eduction, handling, transport, and reactivation) averaged about 8 percent. Production of chlorine dioxide did not proceed stoichiometrically, and thus some chlorite and chlorine were pro- duced. Out of the generator, on the average, the percentage of chlorine dioxide, free chlorine, and chlorite pro- duced was 64, 6, and 30, respectively. Both chlorine and chlorine dioxide were effectively removed by the GAC and the chlorite concentration was lowered. > US GOVERNMENT PRINTING OFFICE 1882-559-017/0751 ------- 5 5 5 I 0.05 0.04 0.03 0.02 o.o r 0.0 Virgin GAC Reactivated GAC 0.0 0.02 0.04 0.06 Sum of TOO applied, mg/kg GAC Figure 1. Comparison of virgin and once reactivated GAC for TOC removed vs TOC applied. 0.08 In some situations, the use of chlorine dioxide as the primary raw water disin- fectant is a viable alternative to chlorine disinfection when the THM concentra- tions must be reduced to meet promul- gated drinking water standards. The full report was submitted in ful- fillment of Cooperative Agreement No. CR804902 by the City of Evansville, Indiana, under the sponsorship of the U.S. Environmental Protection Agency. The EPA author Ben W. Lykins, Jr. (also the EPA Project Officer, see below) is with the Municipal Environmental Research Laboratory, Cincinnati, OH 45268; Mark Griese is with the Evansville Waterworks Department, Evans- ville, IN 47740. The complete report, entitled "Chlorine Dioxide Disinfection and Granular Activated Carbon Adsorption," (Order No. PB 82-227 737; Cost: $10.50, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Municipal Environmental Research Laboratory U.S. Environmental Protection Agency Cincinnati, OH 45268 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 Postage and Fees Paid Environmental Protection Agency EPA 335 Official Business Penalty for Private Use $300 RETURN POSTAGE GUARANTEED Third-Class Bulk Rate MERL0063240 LOU ft TILLEY RESIGN V EPA LIBRARIAN 230 S OEARBQRM ST CHICAGO IL 60604 ------- |