Peracetic Acid (PAA) versus Chlorination/De-chlorination A Disinfection Comparison

Transcription

1 Peracetic Acid (PAA) versus Chlorination/De-chlorination A Disinfection Comparison Martha S. Graham, P.E. *, George Lomax, Glabra Skipp Public Works Department, City of St. Augustine, Florida * mgraham@citystaug.com ABSTRACT Peracetic acid (PAA) is an approved disinfectant that has been used for decades in the food and medical industries, however it has historically been considered too expensive for use in wastewater disinfection applications. Recently, rising concerns around trihalomethanes (THMs) have opened the door to exploring alternative non-chlorine based disinfection technologies. The City of St. Augustine with the permission of Florida Department of Environmental Regulation performed a pilot test to compare the disinfection performance and costs of a PAA disinfection system versus a chlorination/de-chlorination system. The 5. Million Gallons per Day (MGD) St. Augustine, Florida Wastewater Treatment Plant (WWTP) was selected for the trial due to the fact that the plant s flow was split between two identical contact chambers. This configuration enabled a direct head-to-head evaluation between the PAA and chlorination/de-chlorination systems across three performance criteria that were critical to the plant; disinfection performance, effluent toxicity and chemical costs. The objective was to conduct a full-scale demonstration test to determine if a PAA disinfection system, using automated dose control technology could effectively reduce pathogen levels to achieve regulatory compliance while at the same time preventing harmful residuals from forming in the treated effluent. The test results demonstrated that PAA was consistently able to reduce pathogen loads to achieve regulatory compliance, prevent harmful residuals from forming in the treated effluent, and reduce its overall chemical consumption and cost. KEYWORDS: Peracetic acid, PAA, disinfection INTRODUCTION The wastewater industry has long relied on chlorine as a disinfectant. Chlorine has proven its effectiveness in destroying pathogenic organisms; however, it does not come without disadvantages. Chlorine residuals are harmful to aquatic life and regulations often require wastewater plants to dechlorinate prior to discharge. In addition, harmful disinfection byproducts, such as trihalomethanes (THMs) are produced from the chlorine disinfection process

2 (Dell Erba et. al 24). Concerns regarding chlorine residuals and THMs have prompted the industry to look for effective chlorine-free disinfection methods. Ultraviolet light and ozone are disinfection alternatives, but require large capital investments with high operation and maintenance costs. Peracetic acid (PAA) has been used to treat wastewater effluents in European countries for over a decade, but until recently, a lack of mass PAA production in the United States kept the cost too high for interest (Kitis 24). However, FMC s new PAA solution requires less chemical for disinfection, helping to lower the cost of PAA use. PAA is becoming an enticing alternative for wastewater plants in the United States that want to implement a chlorine-free disinfection method without a large capital investment. Aside from utilizing existing infrastructure for implementation, PAA has other benefits such as no dechlorination requirement, less chemical usage and lack of toxic chlorinated byproducts (Koivunen and Heinonen-Tanski, 25). In 29, the City of St. Augustine was approached by BIK Technologies, an associate of FMC Corporation, to conduct a pilot study using PAA disinfection. The City s wastewater treatment plant (average flow 2.7 MGD) consists of two identical 81,53 gallon chlorine contact chambers for disinfection, making it an ideal location for a side-by-side disinfectant comparison. The City entered into a partnership with FMC, where FMC provided all engineering support, pumping equipment and temporary storage containment for the pilot study. The City was responsible for purchasing chemical and providing laboratory analyses. The objective of the pilot study was to determine if a PAA disinfection system could be a cost effective method of reducing pathogen levels and preventing harmful disinfection byproducts from forming in the treated effluent. METHODOLOGY Pilot Study In April 21, the City of St. Augustine Wastewater Treatment Plant began a two month pilot study to compare its current chlorine disinfection with PAA disinfection. The plant utilized its two existing identical disinfection contact tanks with identical flow pacing equipment connected to the plant s supervisory control data acquisition (SCADA) system. One tank was treated with a 12% sodium hypochlorite solution dosed at an average of 1 parts per million (ppm), the other with VigorOx WWTII 15% peracetic acid formula manufactured by FMC Corporation dosed at an average of 1.5 ppm. After disinfection, both treatment tanks were blended together and dechlorinated with sodium bisulfite prior to discharge into the Matanzas River. Figure 1 shows the trial setup with sampling locations. Fecal coliform and enterococci samples were collected three times a week at points B, D and E. Autosamplers were set to collect flow

3 paced composite samples over twenty-four hours once a week at points D and E. Composite samples were analyzed for Carbonaceous Biochemical Oxygen Demand (CBOD) and Total Suspended Solids (TSS). Trihalomethane (THM) samples were collected once during the study at points A, B, C, D and E, with an additional sample collected at point D that was dechlorinated prior to analysis. Seven day chronic static renewal definitive toxicity tests were conducted with mysid shrimp (Mysidopsis bahia) and inland silverslide (Menidia beryllina) using sample water from each disinfection method (points D and E). During the week of chronic toxicity testing, PAA was dosed at the highest rate during the study, 2.5 ppm. The toxicity study was repeated in October 21 during routine semi-annual sampling for the plant s discharge permit. Full-Plant Disinfection Figure 1. Pilot study setup and sampling locations. In October 211 Florida Department of Environmental Protection (FDEP) issued the plant a permit to begin full plant peracetic acid disinfection, with the following conditions: dosing a minimum of 1.5 ppm PAA, monitoring effluent residual continuously and demonstrating through daily effluent grab samples that 1. ppm PAA residual is not exceeded. The option to use chlorination/dechlorination as a backup disinfection method was included in the permit revisions. Prior to implementation several facility modifications were made; these include: Two Flowmotion Systems Series 21 HEC peristaltic pumps installed to pump PAA to dosing point (Figure 2) Two roll top hardcover containment structures set up to house in-use PAA totes (Figure 3)

4 12 gallon PAA bulk storage location (Figure 4) PAA dosing point installed at the common discharge point of the plant s clarifiers to allow mixing prior to the contact basins (Figure 5) PAA residual analyzer installed just prior to effluent discharge Peracetic acid Single Analyte Meter (SAM) from CHEMetrics obtained for testing PAA residual of grab samples Figure 2. Peristaltic pumps installed to pump PAA to dosing point. Figure 3. PAA tote containment for feed to pumps.

5 Figure 4. The plant s existing chlorine storage was converted into a bulk PAA tote storage location. The PAA dosing point shown in Figure 5 was initially installed at the common discharge point of the plant s clarifiers (point a) but was relocated 47 feet downstream (point b) when it was discovered this pipe is reverse slope, allowing solids to accumulate in the pipe and thus placing a higher demand on the PAA. Figure 5. PAA dosing point and compliance sampling location. The recommended contact time for peracetic acid is 15 to 6 minutes. During the trial, only the contact basin was used and the contact time was within the recommended range. However, with full plant disinfection, the addition of the post aeration basins tripled the contact time and required the PAA dosage to be increased from the ppm used during the trial to ppm.

6 CFU/1 ml Fecal Coliform and enterococci samples were collected weekly at the plant s final effluent compliance sampling location (Figure 5, point c). An autosampler was set at the same compliance point to collect flow paced composite samples over twenty-four hours once a week. Samples were analyzed for CBOD and TSS. Seven day chronic static renewal definitive toxicity testing was conducted with mysid shrimp (Mysidopsis bahia) and inland silverslide (Menidia beryllina) using sample water collected from the final effluent sampling point. RESULTS Pilot Study Enterococci results are show in Figure 6. The PAA treated tank ranged from <1 to 6 colony forming unit per 1 milliliters (CFU/1 ml), well below the plant s discharge permit limit of 35 CFU/1 ml monthly geometric mean. Chlorine disinfection yielded similar results, ranging from <1 to 8 CFU/1 ml with an outlier at 29 CFU/1 ml. Pilot Study Enterococci Results /2/1 4/12/1 4/22/1 5/2/1 5/12/1 5/22/1 6/1/1 6/11/1 Date PAA Chlorine Figure 6. Pilot study enterococci results, PAA versus chlorine disinfection (Keogh and Tran 211). Figure 7 shows that the fecal coliform levels of the PAA treated tank ranged from <1 to 98 CFU/1 ml, with the highest count coinciding with a dissolved oxygen process adjustment in the Biological Treatment Units on May 14, 21. Chlorine disinfection ranged from <1 to 48 CFU/1 ml, with the highest count during the same May 14, 21 process adjustment. All fecal coliform counts were below the plant s discharge limit of 2 CFU/1 ml monthly geometric mean and maximum single sample of 8 CFU/1 ml.

7 CFU/1 ml 2 Pilot Study Fecal Coliform Results PAA Chlorine 4/2/1 4/12/1 4/22/1 5/2/1 5/12/1 5/22/1 6/1/1 6/11/1 Date Figure 7. Pilot study fecal coliform results, PAA versus chlorine disinfection (Keogh and Tran 211). Trihalomethane sample results from the chlorine treatment tank are shown in Table 1. Although concentrations slightly decreased after dechlorination, bromodichloromethane and dibromochloromethane still exceeded their respective surface water limits. PAA treatment tank trihalomethane sample concentrations (Table 2) were all less than laboratory detection limits and well below the surface water limits. Table 1. Trihalomethane results from chlorination/dechlorination treatment Volatiles Surface Chlorination Chlorination After Water Start End Dechlorination Limit* Bromodichloromethane (ug/l) Bromoform (ug/l) Chloroform (ug/l) Dibromochloromethane (ug/l) Total Trihalomethanes (ug/l) *Florida Department of Environmental Protection surface water limit for Class III marine waters Table 2. Trihalomethane results from PAA treatment Volatiles Surface After Water PAA Start PAA End Dechlorination Limit* Bromodichloromethane (ug/l) 22 <.6 <.6 <.6 Bromoform (ug/l) 36 <.6 <.6 <.6 Chloroform (ug/l) 47.8 <.64 <.64 <.64 Dibromochloromethane (ug/l) 34 <.75 <.75 <.75 Total Trihalomethanes (ug/l) - <.6 <.6 <.6 *Florida Department of Environmental Protection surface water limit for Class III marine waters

8 Seven day chronic toxicity results are presented in Tables 3 and 4. The initial test conducted during the pilot study showed a 25 percent inhibition concentration (IC 25 ) of 1% for all except the M. bahia in peracetic acid effluent receiving an IC 25 of 97.6%. The October toxicity results showed an IC 25 of 1% for both species. Table 3. Pilot study toxicity results May 23 28, 21 Chlorine/Dechlorination Peracetic Acid Disinfection Percent Effluent M. bahia M. beryllina M. bahia M. beryllina Control IC 25 >1 % >1% 97.6% >1 % Table 4. Toxicity results October 24 29, 21 Chlorine/Dechlorination Peracetic Acid Disinfection Percent Effluent M. bahia M. beryllina M. bahia M. beryllina Control IC 25 >1 % >1% >1% >1 % Full Plant PAA Disinfection The enterococci sample results from 12/27/11 through 6/13/12 are shown in Figure 8. The average enterococci count was 5 CFU/1 ml and the average monthly geometric mean from January through May 212 was 4 CFU/1 ml, well below the plant s discharge limit of 35 CFU/1 ml.

9 CFU/1 ml CFU/1 ml Full Plant PAA Disinfection: Enterococci Results 11/3/11 12/23/11 2/11/12 4/1/12 5/21/12 7/1/12 Date Figure 8. Enterococci results 12/27/11 through 6/13/12. Enterococci = 5 CFU/1 ml Results from fecal coliform samples collected 12/27/11 through 6/13/12 during full plant PAA disinfection are shown in Figure 9 and plotted against effluent PAA residual in Figure 1. All fecal coliform counts were well below the plant s discharge limit of 2 CFU/1 ml monthly geometric mean and maximum single sample of 8 CFU/1 ml. 6 Full Plant PAA Disinfection: Fecal Coliform Results 5 4 Fecal Coliform Count = 1 CFU/1 ml /3/11 1/22/12 3/12/12 5/1/12 6/2/12 8/9/12 Sample Date Figure 9. Fecal coliform results 12/27/11 through 6/13/12.

10 Effluent PAA Residual Full Plant PAA: Fecal Coliform Results vs. PAA Residual PAA residual =.28 ppm CFU/1 ml Figure 1. Fecal coliform results (12/27/11 through 6/13/12) versus PAA residual. The plant s effluent discharge limit for CBOD is a single sample of 6 mg/l, weekly average of 4 mg/l, monthly average of 25 mg/l and annual average of 2 mg/l. Figure 11 shows the highest CBOD concentration from January through June 212 was 7.5 mg/l while dosing 4 ppm PAA. The average effluent CBOD value was 3.5 mg/l and the average removal of influent values was 98 percent. The effluent discharge limit for TSS is a single sample of 6 mg/l, weekly average of 45 mg/l, monthly average of 3 mg/l and annual average of 2 mg/l. Figure 11 shows the highest TSS concentration from January through June 212 was 7. mg/l while dosing 3 ppm PAA. The average TSS concentration was 4.3 mg/l and the average removal of influent values was 98 percent.

11 CBOD and TSS (mg/l) PAA Dosage (ppm) 8 7 CBOD and TSS vs. PAA Dosage TSS CBOD PAA dosage 12/23/211 2/11/212 4/1/212 5/21/212 7/1/212 Figure 11. CBOD and TSS versus PAA dosage. Seven day chronic toxicity samples collected in April 212 during full plant PAA disinfection did not produce chronic toxicity to either species (Table 5). PAA dosage rates during the test period were ppm. Table 5. Toxicity results April 22 27, 212 Percent Effluent M. bahia M. beryllina Control Sal. Adj. Control IC25 >1% >1%

12 SUMMARY AND CONCLUSIONS Although the City has the option of using chlorination/dechlorination, the City has chosen to continue using PAA as the primary plant disinfection chemical. The effluent water now mimics the natural color of North Florida river water and lacks the bleaching of chlorine (Figure 12). Figure 12. Chlorine treated wastewater in 21 (left) versus PAA treated wastewater in 212 (right). Sample results have shown that PAA is able to achieve disinfection requirements for both fecal coliform and enterococci. Treated effluent passed toxicity studies and meets the plant s CBOD and TSS requirements. A benefit of PAA has been the absence of chlorinated disinfection byproduct formation. During the trial study, two trihalomethanes from the chlorine/decholorinated effluent were more than double their surface water limits while the PAA treated water was below laboratory detection limits. Regulators are pushing toward stricter control of disinfection byproducts, making PAA an attractive option.

13 The pilot study found PAA treatment could reduce chemical volume 9 percent and was 1 percent less expensive than chlorine/de-chlorination (Keogh and Tran 211). As of May 31, 212, the plant has maintained a dosage of 3. ppm, but is working to reduce the contact time and lower the PAA dosage to increase cost savings. Many other utilities from throughout the state have been touring the City s plant to see if PAA is a viable option for their operation. As more utilities begin to use PAA, the City anticipates increases in production/purity will drive the price down. ACKNOWLEDGEMENT The authors gratefully acknowledge the City of St. Augustine Wastewater Treatment Plant staff for their hard work and efforts in pioneering the use of PAA in wastewater disinfection treatment. The authors also wish to thank BKI Technologies and FMC Corporation for their support. REFERENCES Dell Erba, A.; Falsanisi, D.; Liberti L.; Notarnicola M.; Santoro D. (24) Disinfecting behavior of peracetic acid for municipal wastewater reuse. Desalination, 168, Mehmet, K. (23) Disinfection of wastewater with peracetic acid: a review. Environment International, Environment International, 3, Koivunen, J.; Heinonen-Tanski, H. (25) Peracetic acid (PAA) disinfection of primary, secondary, tertiary treated municipal wastewaters. Water Research, 39, Keogh, B.; Tran, M. (211) Old City, New Ideas: Peracetic Acid in Wastewater Disinfection at St. Augustine. Florida Water Resources Journal,