CASCADING BENEFITS: OPTIMIZING DISINFECTION FOR CHEMICAL SAVINGS, CONTROL NITRIFICATION AND DISINFECTION BY-PRODUCTS. ABSTRACT Ignacio Cadena, P.E., Project Manager Freese and Nichols, Inc. Bobbie Kidd, General Manager, Greenbelt Municipal and Industrial Water Authority KEYWORDS Loss of Chlorine Residual, Nitrification, Disinfection By-Products, Disinfection Strategy, Chemical Optimization INTRODUCTION The Greenbelt Municipal and Industrial Water Authority (GMIWA) operate an 8.0 MGD conventional Water Treatment Plant and a 120 mile linear system that supplies water to five primary customers and 23 small systems across 5 counties in northwestern Texas. The GMIWA operated a free chlorine system until high occurrences of disinfection by-products (DBP s) force them to evaluate disinfection alternatives. TCEQ approved the Greenbelt Municipal and Industrial Water Authority s (GMIWA) request to use chloramines as a disinfectant on November 3, 2003 to reduce formation of DBPs in the distribution system. The three year case study analyzed the loss of chlorine residual and nitrification in the distribution system; unusual DPS s issues; and evaluated the disinfection strategy at the Water Treatment Plant (WTP) to optimize the use of chemicals. LOSS OF CHLORINE RESIDUAL/ NITRIFICATION After two years of trouble free operation, during the fall of 2005 the GMIWA experienced difficulties maintaining chloramine residual at the end of the system. The total chlorine (chloramine) residual concentration recorded at the farthest point in the system at the time was 0.7 mg/l, with reports by the City of Crowell (farthest customer) of even lower concentrations in their distribution system. The loss of chlorine residual was attributed to a possible growth of nitrifying bacteria in the system. Additional sampling was conducted for HPC (heterotrophic plate count), free and total chlorine, free ammonia, nitrite and nitrate at the following sites: Childress (Site 3), Quanah (Site 4), Medicine Mound (Site 5), and Crowell (Site 6). The results showed an increase in the HPC, nitrates and nitrites in the last segment of the system from Site 5 to Site 6. Figure 1 shows a map of the Water system and the Site Locations. The Authority was advised that one effective means of controlling bacterial growth in the system was to periodically shock it by temporarily reverting from chloramines to a free chlorine residual. The Authority shut off the ammonia feed at the WTP on November 18, 2005 and reverted to free chlorine residual for approximately 6 weeks (until January 2, 2006). No taste and odor complaints were received during this period. The response after shocking the distribution system was sustained chlorine residual throughout the system. An annual free chlorine shock has been recommended as a result of the study to keep nitrifying bacteria from growing in the distribution system.
DISINFECTION BY-PRODUCTS - THM VIOLATIONS TCEQ performed the fourth quarter DBP sampling the first week in December 2005. At the time of the sampling, the Authority was implementing the action plan to address the nitrification outbreak and all GMIWA customers were receiving water with free chlorine. As a result of the action plan, the Authority s customers experienced unusually high TTHM s in the samples collected for this period. As a result, the Running Annual Average (RAA) was skewed to exceed the Total Trihalomethane (TTHM) limit of 80 ug/l for the next four quarters. This resulted in GMIWA wholesale customers having to notify their customers in writing of the water quality violations and provide a copy to TCEQ. The mandatory public notification was done on a quarterly basis for the duration of the violations. Recent records show that THMs were exceeding the MCL of 0.080 mg/l on multiple occasions between 2005 and 2007. However, efforts from the Authority have shown dramatic improvement in the THM and HAA s samples collected since the fourth quarter of 2007, with levels below the MCL for all customers. Table 1 shows the TTHM concentrations for the GMIWA customers from the 1 st quarter of 2006 to the fourth quarter of 2008. Table 1. TTHM Quarterly Concentrations for the GMIWA Customer Cities. GMIWA WTP TTHM City of Clarendon TTHM City of Hedley TTHM 1st Quarter 06 1st Quarter 06 71.30 1st Quarter 06 2nd Quarter 06 2nd Quarter 06 83.30 2nd Quarter 06 3rd Quarter 06 82.20 3rd Quarter 06 83.00 3rd Quarter 06 85.10 4th Quarter 06 70.70 4th Quarter 06 82.00 4th Quarter 06 125.10 1st Quarter 07 50.90 1st Quarter 07 52.10 1st Quarter 07 74.70 2nd Quarter 07 61.80 2nd Quarter 07 75.70 2nd Quarter 07 78.00 3rd Quarter 07 89.60 3rd Quarter 07 93.30 3rd Quarter 07 96.80 4th Quarter 07 79.70 4th Quarter 07 77.50 4th Quarter 07 84.20 1st Quarter 08 5.10 1st Quarter 08 5.10 1st Quarter 08 7.80 2nd Quarter 08 11.00 2nd Quarter 08 11.00 2nd Quarter 08 8.30 3rd Quarter 08 4.50 3rd Quarter 08 4.50 3rd Quarter 08 6.50 4th Quarter 08 10.90 4th Quarter 08 10.90 4th Quarter 08 26.70 City of Childress TTHM City of Quanah TTHM City of Crowell TTHM 1st Quarter 06 63.70 1st Quarter 06 61.00 1st Quarter 06 63.80 2nd Quarter 06 74.50 2nd Quarter 06 81.20 2nd Quarter 06 83.70 3rd Quarter 06 81.60 3rd Quarter 06 75.90 3rd Quarter 06 79.60 4th Quarter 06 146.00 4th Quarter 06 141.50 4th Quarter 06 161.50 1st Quarter 07 64.90 1st Quarter 07 65.90 1st Quarter 07 68.40 2nd Quarter 07 87.30 2nd Quarter 07 79.40 2nd Quarter 07 78.10 3rd Quarter 07 93.30 3rd Quarter 07 89.10 3rd Quarter 07 98.00 4th Quarter 07 NA 4th Quarter 07 65.70 4th Quarter 07 65.10 1st Quarter 08 12.10 1st Quarter 08 11.20 1st Quarter 08 9.90 2nd Quarter 08 13.40 2nd Quarter 08 10.60 2nd Quarter 08 12.10 3rd Quarter 07 10.40 3rd Quarter 08 8.70 3rd Quarter 08 13.20 4th Quarter 08 32.60 4th Quarter 08 29.30 4th Quarter 08 78.20
CHEMMICAL OPTIMIZATION The Greenbelt Water Authority disinfection strategy relies on free chlorine through the first two disinfection zones D1 and D2 (flocculators and clarifiers) and uses chloramines in disinfection zones D3 and D4 (filters and clearwell). The disinfection strategy was modeled to optimize the disinfectant residual needed to meet CT requirements. The model analyzed the optimum use of free chlorine and chloramines to meet a conservative inactivation ratio of 2. TCEQ s minimum inactivation ratio criteria is 1. Figure 2. Disinfection Strategy Schematic Rapid Mix + Flocculators Clarifiers D1 D2 Filters D3 Clearwell D4 Flocculator No. 1 Clarifier Cl 2 No. 1 Cl 2 NH 3 Cl Filter No. 1 2 RM Filter No. 2 Filter No. 3 Clearwell Flocculator No. 2 Clarifier No. 2 Filter No. 4 D1, D2, D3 & D4: Disinfection Zones Excess Ammonia Feed / Nitrification During the summer of 2006, the Greenbelt Water Authority continued to experience difficulties maintaining chloramine residual at Quanah (Site 4), Medicine Mound (Site 5) and Crowell (Site 6). The cause of the nitrifying bacteria growth coupled with chlorine residual loss was assessed, including an ammonia feed rate evaluation. Sampling similar to that done in the fall of 2005 was conducted again. The results showed a decrease of the total ammonia concentration from Site 2 to Site 3 and a total depletion of ammonia from Site 3 to Site 4. The results also showed a rapid increase in the nitrite/ nitrate concentration at Site 3, and a rapid increase in the HPC count from Site 3 to Site 4. An assessment of the chlorine and ammonia feed rate at the Water Treatment Plant (WTP) showed that between 60% and 80% of the chlorine fed at the rapid mix was depleted by the time the ammonia was fed following sedimentation. At the time, the chlorine:ammonia feed ratio was calculated based on the total pounds of chlorine and ammonia fed, a common practice. While the calculated (applied) ratio was targeted at 4.5:1, the actual ratio at the point of ammonia addition was approximately 2:1, resulting in a large excess of ammonia. An analysis of the operation from August 14, 2006 showed that the WTP flow was 4.785 MGD, the chlorine feed was a total of 262 lbs of Cl 2 and the Ammonia feed was a total of 55 lbs of NH 3 -N. From the MOR, the free chlorine residual reading at the second stage (D2) for that same day was 1.5 mg /L or a total of 59.9 lbs Cl 2 /day. The free chlorine concentration is the amount that will react with ammonia to form chloramines. The total chlorine concentration leaving the plant (D4) is 2.0 mg/l, meaning the chlorine: ammonia ratio is approximately 1.45:1 or 79.81 lbs Cl 2 : 55lbs NH 3 -N. An example of the ammonia feed rate calculation for a flow of 4.785 MGD is as follows: ( D2 free Cl 2 Residual + 2 nd Stage Cl 2 Dose) X 0.25 = NH 3 -N dose 1.5 mg/l 0.5 mg/l X 0.25 = 0.50 mg NH 3 -N/L 59.86 lbs Cl 2 /day 19.95 lbs Cl 2 /day X 0.25 = 19.95 lbs. NH 3 -N/day
It was recommended adjusting the ammonia feed rate to meet a 4.5:1 chlorine:ammonia ratio leaving the plant calculating the ration based on chlorine residual through the treatment process. It was also recommended sampling the parameters stated above at the same locations one week after the ammonia feed rate modification were made and one month after the changes. This will help us monitor the effect of the ammonia feed rate reduction with the growth of nitrifying bacteria in the distribution system. The adjustment to the ammonia feed rate reflect an approximate 50% reduction to accomplish a 4.5:1 chlorine:ammonia ratio based on the available chlorine left after the first disinfection zone. The ammonia feed rate reduction has helped the Authority prevent the growth of nitrifying bacteria in the distribution system to date. Approximately 800 lbs of ammonia are saved on average per month or $550 per month in cost savings. Disinfection Strategy Evaluation Plant records were reviewed to determine conditions likely responsible for the formation of excessive THMs. Although the disinfection strategy in place is appropriate, free chlorine residuals observed in zones D1 and D2 are significantly higher than required, and may be reduced to control THM formation without jeopardizing the disinfection performance. The existing disinfection strategy was modeled to optimize the disinfectant residual needed to meet CT requirements. The model analyzed the optimum use of free chlorine at disinfection zones D1/ D2, and chloramines at disinfection zones D3 and D4 to meet TCEQ s disinfection criteria. The plant parameters used for this evaluation are conservative values that will provide a significant safety factor for the resulting recommendations. Two temperatures were used to model summer and winter conditions. Summer conditions were modeled using a temperature of 20 ºC. For the winter conditions, a temperature of 5 ºC was used to model the effect of temperature in the inactivation ratio. The average flow used for this evaluation was 8 MGD with a ph of 8. A review of the monthly operating reports (MOR) from January 2004 through September 2007 showed an average flow of 3.2 MGD and an average temperature of 18.7 ºC during this three year period. The purpose of this analysis is to determine the preferred free chlorine residual concentration entering disinfection zone D3 (filter influent) given the Authority s desire to maintain 2.5 mg/l chloramine residual leaving the plant (D4). The results from the disinfection strategy analysis are shown on Table 1. Operating experience has indicated this 2.5 mg/l chloramine residual at disinfection zone D4 may not always satisfy TCEQ s minimum chlorine residual concentration criteria throughout the system without additional chemical boosting. The minimum disinfection inactivation ratio used in this evaluation was 2. TCEQ s criteria for minimum inactivation ratio is 1. In order for the Greenbelt Water Authority to meet the Stage 1 D/DBPR and TCEQ s disinfection criteria, the Authority will have to adjust the free chlorine feed rate to control the chlorine residual at disinfection zones D1/ D2. The target residual will require adjustment at least twice a year to accommodate flow and temperature seasonal variations.
Table 1. Disinfectant Optimization Table. DISINFECTION PROCESS DATA C Flow Temp Giardia Virus Inact. Date Disinfectant (mg/l) (MGD) ( o C) ph Log Log Ratio Time= FCL D1 1.5 4.000 5.0 8.0 FCL D2A 1.5 4.000 5.0 8.0 1 2 3 FCL D2B 1.5 4.000 5.0 8.0 1.63 63.83 3.27 CLA D3 1.5 2.000 5.0 8.0 (G) CLA D4 1.5 8.000 5.0 8.0 FCL D1 0.0 4.000 20.0 8.0 FCL D2A 0.0 4.000 20.0 8.0 FCL D2B 0.0 4.000 20.0 8.0 0.13 0.31 0.15 CLA D3 2.5 2.000 20.0 8.0 (V) CLA D4 2.5 8.000 20.0 8.0 FCL D1 0.5 4.000 5.0 8.0 FCL D2A 0.5 4.000 5.0 8.0 FCL D2B 0.5 4.000 5.0 8.0 0.67 21.36 1.34 CLA D3 2.5 2.000 5.0 8.0 (G) CLA D4 2.5 8.000 5.0 8.0 Winter Time Recommended Strategy 4 FCL D1 1.0 4.000 5.0 8.0 FCL D2A 1.0 4.000 5.0 8.0 FCL D2B 1.0 4.000 5.0 8.0 1.20 42.62 2.39 CLA D3 2.5 2.000 5.0 8.0 (G) CLA D4 2.5 8.000 5.0 8.0 Summer Time Recommended Strategy 5 FCL D1 0.3 4.000 20.0 8.0 FCL D2A 0.3 4.000 20.0 8.0 FCL D2B 0.3 4.000 20.0 8.0 1.17 36.38 2.34 CLA D3 2.5 2.000 20.0 8.0 (G) CLA D4 2.5 8.000 20.0 8.0
FINDINGS / RECOMMENDATIONS The recommendation was to continue with the same disinfection strategy. This strategy feeds free chlorine through disinfection zones D1/ D2 and chloramines through disinfection zone D3 and D4. It was recommended to adjust the free chlorine dose at the raw water meter vault (D1) feed point twice a year to accommodate the flow and temperature seasonal variations. A chart was developed (see Figure 3) to help the Greenbelt Water Authority determine the target chlorine residual concentration through disinfection zones D1/ D2 depending on the flow and the temperature in degrees Celsius (ºC). The plant flow range used to develop this chart is from 2 MGD to 8 MGD in 2 MGD increments. The temperature range used to develop this chart is from 6 ºC to 20 ºC in 2 ºC increments. How to use the Chlorine Residual Concentration Chart: Example: Daily average flow 5.0 MGD Water temperature at the Clearwell 15 ºC Approximate target free chlorine residual conc. 0.22 mg/l * Figure 3. Chlorine Residual Concentration Chart Example. Disinfection Zone D1/ D2 Free Chlorine Residual Concentration Chart Temperature Analysis at a Fixed Inactivation Ratio of 2 Water Flow Analysis at 2 MGD,4MGD, 6 MGD and 8 MGD 0.80 Free Chlorine Residual Concentration (mg/l) 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.22 mg/l 4 MGD 2 MGD 6 MGD 8 MGD 5.0 MGD 0.00 6.0 8.0 10.0 12.0 14.0 15.0 ºC 16.0 18.0 20.0 Temperature in C * The values from this Chart are conditioned to a 2.5 mg/l chlorine residual (chloramines) concentration through disinfection zones D3 and D4. In order for the GMIWA to meet the Stage 1 D/DBPR and TCEQ s disinfection criteria, the Authority adjusted the free chlorine feed rate to sustain a lower free chlorine residual concentration (between 0.3 to 0.6 mg/l) entering disinfection zone D2 (clarifiers). This adjustment not only provided a reduction in the formation of DBPs through the treatment process, but also a significant chemical savings. The optimization of the chemical disinfection strategy is saving the GMIWA approximately $20,000 per year in chlorine and ammonia chemicals since the changes were implemented (approximately a 50% reduction). Approximately 40,000 lbs of chlorine and 5,500 pounds of ammonia were saved in 2008.
Disinfection Zone D1/ D2 Free Chlorine Residual Concentration Chart Temperature Analysis at a Fixed Inactivation Ratio of 2 Water Flow Analysis at 2 MGD,4MGD, 6 MGD and 8 MGD 0.80 Free Chlorine Residual Concentration (mg/l) 0.70 0.60 0.50 0.40 0.30 0.20 0.10 Dis Zone 2 Cl2 Residual Conc at 2 MGD Dis Zone 2 Cl2 Residual Conc at 4 MGD 6 MGD 4 MGD 2 MGD Dis Zone 2 Cl2 Residual Conc at 6 MGD Dis Zone 2 Cl2 Residual Conc at 8 MGD 8 MGD 0.00 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 Temperature in C * The values from this Chart are conditioned to a 2.5 mg/l chlorine residual (chloramines) concentration through disinfection zones D3 and D4.