Session 311: Effects of TSS on Peracetic Acid Dosing for CSO Events

Transcription

1 Session 311: Effects of TSS on Peracetic Acid Dosing for CSO Events Authors: Robert Freeborn, Jacquelyn Wilson Background Peracetic acid has been used in the food and beverage industry since the 1950 s primarily for equipment sterilization and as a disinfectant for fruits, vegetables, meat and eggs, to reduce spoilage from bacteria and fungi. It is also used in the medical field for sterilization. Peracetic acid (PAA) is a very strong oxidizing agent and has a stronger oxidation potential than chlorine or chlorine dioxide. It is an equilibrium solution containing acetic acid, hydrogen peroxide water and peracetic acid. In municipal applications, it is typically applied at 12-16% and fed neat without the need for dilution. With a product density of 9.26, it is very close to water making dilution almost immediate without the need for extensive mixing. PAA was tested as an alternative disinfectant for wastewater over ten years ago. The longest use of the product has been in a waste water treatment plant Milan, Italy (specifically designed to feed PAA) where filtered plant effluent is treated with 12% PAA and used for irrigation of fruits, vineyards and other crops. It is land use approved and once in contact with the soil, degrades quickly as the equilibrium reaction consumes the hydrogen peroxide shifting back to acetic acid. PAA has been proven to be an economically viable alternative to liquid sodium hypochlorite and sodium bisulfite. Feed rates for secondary systems range from 0.6 to 1.5 ppm active PAA. Tests conducted at the Steubenville waste water treatment plant from April to June, 2012 concluded that maintaining a residual of only ppm active PAA could maintain fecal coliform levels less than summer limits of 200 CFU s/100ml. Once PAA levels dropped below the 0.35 ppm level, fecal coliform levels increased. At 0.4%, it also passes Acute WET testing with 100% survivability. Comparative cost savings of 27% were seen as compared to chlorine and sodium bisulfite.

2 GEOMEAN Fecal Coliform (CFU's/100ml) Fecal Coliform (CFU's/100ml) PAA Residual (ppm) Fecal Coliform vs. PAA Outfall Residual 1, , Daily Testing Fecal Coliform PAA Residual Even with spikes in fecal levels up to 290,000 CFU s/100ml due to secondary clarifier upsets, PAA dosage never changed from the 0.75 ppm level, still maintaining less than the 200 CFU/100ml discharge limit. Fecal Coliform Deactivation PAA Residual (ppm) Fecal Coliform

3 Combined Sewer Overflow (CSO) Testing Initial CSO tests were conducted in 2011 in the Southeast Michigan area. Samples were collected during an event every fifteen minutes and testing was completed on each of these samples. In order to determine what chemistry impacts PAA demand, background water testing was conducted as well as PAA uptake. Chemical analysis included: ph, conductivity, turbidity, total suspended solids, total iron, and total alkalinity. These tests were conducted to determine if there were any surrogate tests that could be performed to determine the required dosage to meet discharge pathogen limits. The tests results are shown below: Chemistry Summary Correct PAA PAA Cond "M" Alk Turbidity (FAU) Fe TSS TSS (ppm) Dose Demand (ppm) Date Site Status mmhos ph ORP ppm Settled Mixed ppm Hach Total Volatile ppm 1 min. 10 min PAA UPTAKE TESTING 2/21/2011 A C /17/2011 B C /17/2011 B C /17/2011 B C /17/2011 B C PAA Demand (ppm) 64 Dose <1 min 5 min 10 min 15 min 20 min 25 min 45 min 3 hrs. 2/17/2011 C C /17/2011 C C /17/2011 C C /28/2011 D C /28/2011 D C /28/2011 D C /28/2011 D C /28/2011 D C /28/2011 D C /28/2011 D C /28/2011 E C /28/2011 E C /28/2011 E C /28/2011 E C Based on the chemical analysis, it was concluded that typical chemical analysis saw no direct correlation to PAA uptake with the exception of total suspended solids. The uptake curves did follow the typical demand through the CSO event with higher dosages required at the first flush and peak, dropping down almost 6 ppm as the event approached tail off. The uptake slopes also demonstrated the demand variability throughout the event. In analyzing the demand data, it became apparent that as total suspended solids levels increased, so did PAA demand. This correlation helped determine a logical means to predict PAA demand based on total suspended solids level throughout the event. Theoretically, the

4 hydrograph of the event could be used to establish an algorithm so that the proper level of disinfectant could be applied to the CSO discharge stream without over or under feeding the product. Further testing would need to be conducted to determine the mechanism involved. Suspended solids levels and particle size impact on consumption of disinfectant. A paper authored by Dario Falsani, Ronald Gehr, Domenico Santoro, Adele Dell Erba, Michele Notarnicola1 and Lorenzo Liberti in 2006 (Kinetics of PAA Demand and its Implications on Disinfection of Wastewaters), it concluded, The PAA demand increases heavily when primary settled effluent samples were disinfected, suggesting that factors such as suspended solids content, COD and/or metals play a key role in the demand. In Secondary waste water systems these factors are minimized but in CSO applications, the most striking correlation in lab evaluations is with suspended solids. Peracetic acid demand was determined to demonstrate direct correlation to suspended solids levels not particle size. Straight line correlations were also observed throughout the Peracetic Acid demand tests. (Please refer to the CSO PAA demand test using Solvay s Proxitane WW-12 conducted in 2011 above)

5

6 Total Suspended Solids, Organic Material and PAA Consumption Observation 1 The data from 2011 indicates that the only surrogate test that could help establish PAA dosing during an event is suspended solids. As seen with varying pathogen levels in relatively clean secondary water, PAA is minimally affected and dosages are not changed as seen with other disinfectants. Testing done in 2013 and 2014 on stagnant lagoon systems as well as blended streams during wet weather events raises questions concerning total suspended solids levels as well as the relationship of TSS versus VSS (Volatile suspended solids). A 50,000 gpd lagoon system in Missouri was experiencing dramatic aseptic conditions in the third lagoon just prior to discharge. The State of Missouri is now requiring ALL dischargers to disinfect their effluent. The lagoon in question is oversize for the total flow and at times, there is zero flow even though there is still incoming flow. The surface area in combination with high temperatures results in a major water loss through evaporation. Subsequently, the third lagoon may not have any water flow resulting in stagnant flow. The first two lagoons are relatively clean but the third one has duckweed growing on top of the lagoon and is demonstrating the effects of an aseptic system with generation of hydrogen sulfite seen bubbling up though the duckweed as well as smelling it in the air. In addition, there is a green tint to the water which may be from algae or other organic material. When PAA uptake tests were conducted (non-filtered), it was apparent that there was an immediate demand on the product. A 2.36 ppm active PAA residual was consumed in less than 1 minute. Date: 7/24/2013 Background Testing Data Turbidity (FAU) Iron (ppm) 2.00 ph 7.41 Alkalinity (ppm) 230 Total Hardness (ppm) 400 (+)

7 The uptake of PAA and complete loss of product was due to either high pathogen levels or background interactions. To determine if the hydrogen sulfide or the organic material was causing the uptake, another sample was filtered and PAA uptake was measured. The color was removed but the sample still had a hydrogen sulfide odor. The results demonstrated that there was a 2.15 ppm uptake and a PAA residual could be maintained for over 20 minutes. The only conclusion was that the organic material was reacting with the product and consuming the PAA. Was it the PAA being consumed or the hydrogen peroxide reacting with the organic matter shifting the equilibrium and neutralizing any active PAA residual? Testing was conducted again in July Initial dosage tested was 7.81 ppm active PAA. The initial demand for PAA was 5.56 ppm. After 5 ppm of Hydrogen Peroxide was added to the sample, PAA demand dropped to 3.75 ppm. This indicates the consumption of hydrogen peroxide due to the organic material or hydrogen sulfide in the lagoon. This test indicates that

8 supplying a sacrificial oxidant reduces the demand on PAA so that it can effectively disinfect the bulk water. Observation 2 Testing conducted on blended discharge (primary and mixed secondary) revealed another anomaly with suspended solids at an Ohio waste water treatment facility. The initial uptake for raw primary was 6.18 ppm. A 75/25 blend of secondary and raw effluent produced an uptake of only 2.18 ppm. The samples were allowed to react for 20 minutes and then tested for e-coli. The uptake curves are shown below. It is evident that after the initial demand for PAA was met (due to hydrogen sulfide in the primary influent), the rate of PAA uptake was consistent. The primary sample contained solids which, when diluted, remained in the blended sample. The initial dosage for the blended test was only 3.47 ppm as compared to the starting dosage for raw primary of 6.93 ppm active PAA. When e-coli tests were conducted on the samples tested, a dosage of 3.5 ppm active PAA on the 75/25 blend for 20 minutes did not achieve the kill required to meet permit. After the initial uptake, a 4 log reduction in e-coli was seen after ten minutes. The slope of the PAA consumption mimicked the e-coli reduction. Due to the solids present and the consistent rate of kill, it could be concluded that the PAA was effective in killing the waterborne pathogens but the pathogens in the suspended solids remained and were allowed to regrow after the PAA was neutralized with sodium thiosulfate. The conclusion was that the suspended solids need more time or more mixing energy to allow the PAA to penetrate the mass to kill the pathogens.

9 Next Steps.. From the data collected, it can be concluded that the suspended solids in the raw primary influent, CSO influent or blended samples require either; 1) increased mixing, 2) higher PAA dosages rates or 3) increased holding times to effectively reduce pathogen levels in the discharge streams. To determine what mechanism is controlling the kill rate, a pilot scale study is being conducted in New Jersey in August 2014 which includes three solids removal systems and three disinfectant systems (PAA and two UV systems). The intent is to demonstrate how effective solids removal with disinfection is and how disinfection alone will perform in land locked situations where solids removal is not permitted. The testing will be conducted with background water analysis as well as sampling for VSS and TSS during wet weather events. The results of the testing should yield models where a wet weather event can be effectively treated to reduce pathogens while providing for elimination of solids being discharged into the receiving bodies of water (ponds, lakes, streams, rivers, oceans). A second pilot study will be conducted in Ohio to determine the effective performance of PAA versus chlorine during a CSO event. This test is being performed without solids removal to determine the efficacy of PAA and chlorine alone. Conclusion The tests conducted on CSO s over the past three years indicates that the dosage required to effectively control pathogen levels is dependent upon the total suspended solids levels during the event. PAA dosages change as the event unfolds from first flush to peak to tail off. There may be

10 an 8 ppm change in PAA dosage from the first flush to tail off. The intent of this study is to predict the event and properly treat the discharge water without over or under feeding disinfectant. Measurement of suspended solids is not totally reliable with the current technology. New technologies have been developed that may be able to predict the PAA demand based on suspended solids. The upcoming studies in New Jersey and Ohio should give supporting information as to the percentage of VSS vs. TSS in the CSO event. The hypothesis that hydrogen peroxide is consumed reducing the active PAA available is supported by the field tests conducted to date. Is a sacrificial oxidant required to overcome the initial demand? Is the size of the suspended solid relative? Is there any other background interference limiting the efficacy of PAA in CSO events?