A SIMPLE METHOD OF REMOVING DISINFECTION BYPRODUCT PRECURSORS IN WATER

Transcription

1 A SIMPLE METHOD OF REMOVING DISINFECTION BYPRODUCT PRECURSORS IN WATER Tony Robles Town of Kirkland Lake WTP Overall Responsible Operator Operator-In-Charge Abstract The formation of disinfection byproducts (DBPs) during water disinfection has long been a problem. The removal of natural organic matter (NOM), specifically DBP precursors in raw water is the most important step. Once DBP precursors are removed by coagulation, the use of expensive technologies to reduce DBP formation will not be needed. This paper describes a simple and new way of removing NOM before primary disinfection. The procedure involves the use of an acidified solution of activated silica (AS) and alum as a flocculant (AS-AL). Together with the coagulant alum, DBP precursor removal of 70 to 85 percent is achievable. The manufacture, use and effectiveness of AS-AL in reducing DBP formation are discussed. Key Words: Water treatment, THMs, DBPs, HAAs, Activated Silica, Chlorine Demand, DBP Precursors, NOM, Chlorination, Hach THM Plus INTRODUCTION The Kirkland Lake Water Treatment Plant (KLWTP) supplies potable water to a Northern Ontario town of about people. The plant has a rated output of 260 L/s or about 6 MGD. The shallow lake that supplies raw water to the plant is fed by rain and snow that accumulates in the watershed surrounding the lake. Like most small communities, the source water has small drainage basins and poor quality compared to the water source of large cities. The KLWTP shown in Figure 1 is a conventional surface-water treatment plant that uses alum as coagulant and activated silica as flocculant. Chlorine is used for disinfection after filtration. The plant had difficulties complying with Total Trihalomethanes (TTHMs) maximum allowable concentration of 100 ug/l, and maintaining stable chlorine residuals in the distribution system. Plant management has to develop a method of reducing DBPs formation to comply with regulations. Developing a method in-house and without using 1

2 outside resources, e.g., consultants, was challenging because the plant has limited resources of analytical equipment, manpower, and expertise. In addition, regulatory requirements in the Certificate of Approval and other government regulations do not allow modifications to plant processes without going through the costly and time consuming approval process. The only option left was to improve the chemical properties of activated silica using only existing plant chemicals, such as, sodium silicate, sulfuric acid, alum, chlorine and HFS. Related Work Most researchers tried to correlate the formation of DBPs to total and dissolved organic carbon, ultraviolet absorbance, and chlorine demand. The author believes that the formation of DBPs correlates well with chlorine demand. Past researches have concluded that some NOM are not DBP precursors shown by the fact that greater removal of dissolved organic carbon did not necessarily indicate greater reduction of the DBP formation potential (DBPFP). Alum, synthetic organic polymers, and resins can remove NOM from waters. No synthetic organic polymer used alone performed as well as alum. Resins can effectively remove charged NOM but not the uncharged NOM. Resin particles have to be very small to provide large surface area for NOM removal. Microfiltration (MF) and ultrafiltration (UF) have been used primarily to remove particles and microorganism from water. They remove less DBP precursors than alum, nanofiltration (NF) or reverse osmosis (RO). There are a number of technologies available to reduce DBP formation, such as, ozone, UV, MIEX, DAF, GAC, and chloramines. Most technologies deal with DBPs after they are formed, fewer deal with removing DBP precursors before disinfection and others replace chlorine as the disinfectant. All these DBP mitigation technologies are expensive add-on technologies to the conventional treatment technologies of coagulation, sedimentation and filtration. Most of them are not well suited to small systems with limited resources that often have poor quality source water. The ideal solution is to remove DBP precursors using conventional treatment before disinfection. The method of removing DBP precursors would then be available to both small and large water treatment plants and would not require large capital expenditures. Preferably, the method would be equally effective in treating the clean water source of cities as well as the poor water sources of rural communities. Ones DBP precursors are removed before disinfection then other DBP mitigating technologies will not be needed. 2

3 System Model Many researchers have shown that NOM have a neutral to slightly negative charge. Alum has a positive charge and is effective in removing NOM up to 50%. The activated silica used in water treatment has a negative charge and therefore not as effective as alum in removing DBP precursors. However, activated silica improves the effectiveness of alum in the coagulation process by producing a dense and coherent floc. Enhanced coagulation has demonstrated that using high dosage of alum and low ph improves NOM removal. However, significant lowering of the ph of raw water is expensive. Making the activated silica polymer acidic instead of the raw water is more economic. This requires lowering the activated silica ph from about 9.5 to below 2.5. An acidic activated silica polymer is less stable than a basic polymer. Addition of a polyvalent metal ion such as aluminum or iron, or both to the acidified activated silica sol may be enough to stabilize the sol. The adsorbed polyvalent metal ions increase the acidity of the polymer and should enhance the removal of DBP precursors. This acidic solution of AS and alum is hereby called AS-AL. It has been communicated to the author that the AS-AL carries a small positive charge. Solution Changing the plant activated silica manufacturing process from AS-XP (basic) to AS-AL (acidic) required minor plumbing changes. The caustic line that was used to make AS-XP is now used to deliver the alum. The SCADA activated silica batching program also required a minor revision. Table 1a and 1b summarize the steps in making AS-AL. Table 1a Steps in making AS-AL Step What to do. No. 1 Open water line and fill batching tank with water until total volume is liters. 2 Add 100 liters of Sodium Silicate with mixer operating at full speed. 3 Open water supply line and add 11 liters of 93% sulfuric acid into water line while water is added to the tank. Continue water addition until total volume in the tank is liters. Mixer is at high speed. The ph of the batch is about 7. 4 Pause (optional) for about 7 minutes for activated silica to form. This step allows different degrees of polymerization to take place. A longer pause favors the formation of higher molecular weight AS. 5 Open water supply line and add another 9 liters of 93% sulfuric acid into water line while water is added to the tank. Continue water 3

4 Step What to do. No. addition until total volume in the tank is liters. Mixer is at high speed. The ph of the batch should be about 1.5 to 2. 6 Add 50 liters of alum to the activated silica batch. Mixer at high speed. 7 Switch mixer speed from high to low. AS-AL batch is ready for use. Table 1b shows the typical timeline for the batching process. Not included in the estimated time is the extra 5 minutes mixing time after each step to make sure the mixture is homogenous before proceeding to the next step. The volume of ingredients added to the batch is measured using Milltronics liquid level sensor. Water, alum, and sodium silicate volumes are measured using the AS batch tank sensor. The acid volume is measured using the acid day tank sensor. Table 1b Timeline in making AS-AL Step What to do. No. 1 Estimated time: 13 minutes at 150 L/min water addition rate. The water is added at the bottom of the tank and mixed with leftover activated silica from the previous batch. It is advisable to keep the volume of leftover activated silica to a minimum (<50 L). 2 Estimated time: 15 to 35 minutes. The time required to add 100 liters of sodium silicate, depends on pump capacity and viscosity of the silicate. The silicate is added through the top of the tank to prevent it from contacting acidic solution that will cause gelling. The silicate should be stored at all times above 23 degrees Centigrade to keep the silicate fluid enough to pump. After silicate addition, the batch ph is greater than 11 and SiO 2 concentration of about 2%. 3 Estimated time: 5 minutes The water supply is first opened and after 1 minute the concentrated acid is injected into the water line for dilution and mixing before it comes in contact with the silicate solution inside the batch tank. The agitator provides vigorous mixing of the acid and silicate solution. After acid addition the ph of the batch is about 7, SiO 2 concentration about 1.5%. Batch volume about 3 cubic meters. 4 Estimated time 7 minutes. A pause (optional) or aging time for the formation of activated silica at about ph 7. 5 Estimated time: 5 minutes The water supply is first opened and after 1 minute the concentrated acid is injected into the water line for dilution and mixing before it comes in contact with the silicate solution inside the batch tank. The agitator provides vigorous mixing of the acid and AS solution. Add water until the total volume in the tank is 4

5 Step What to do. No liters. SiO 2 concentration about 1%. After acid addition the ph of the batch is about 1.5 to 2. 6 Estimated time: 10 minutes. Like the silicate, the alum is added through the top of the tank. 7 Switch mixer from high to low speed. Batch is ready for use. DBPs may be produced in the treatment plant and the distribution system as long as the water is in contact with free chlorine residual. The formation of the DBPs is influenced by chlorine contact time, chlorine dose and residual, temperature, ph, precursor concentration, and bromide concentration. The DBPFP is used to calculate the percent removal of DBP precursors. Because DBPFP depends on experimental conditions, data obtained using this method must only be compared to data obtained from methods using the same conditions. DBPFP will be defined here under specific conditions of temperature (20 C), incubation time (24h), darkness, and residual free chlorine (1-3 ppm), and then analyzing the resulting DBPs by the Hach THM Plus method. The Hach THM Plus analytical method provides an inexpensive, fast and operator-friendly method of finding the concentration of DBPs in water samples. The THM Plus method reacts with trihalogenated disinfection byproducts formed as the result of the disinfection of drinking water with chlorine in the presence of NOM. The concentration is reported as ug/l chloroform. Monitoring the performance of the plant using AS-AL requires the use of unconventional methods due to limited resources available. Emphasis is given to improving plant performance economically and in a short time period. There are three visual (qualitative) indicators to help monitor the performance of the new flocculant. The first are the glass windows at the side of the three clarifiers below the waterline as shown in Figure 2, the second is the Particle Index Monitor for the filter effluent as shown in Figure 3,, and the third is a white porcelain water fountain located near the plant control room as shown in Figure 4. The three equipment provide important clues on the direction of the test; going better (+) or going bad (-). The Degremont upflow clarifier is more sensitive to the quality of the sludge in the clarifier than a conventional clarifier. It requires a dense and coherent sludge to run properly. The glass window provides information on how well the sludge blanket handle designed rise rate and shows the sharpness of the sludge/clarified water interface. 5

6 The Particle Index Monitor gives two indications, numbers going up (negative results) and numbers going down (positive results). The particle index monitor provides an early warning and prevents process upset. It is more sensitive to particles above one micron than a turbidimeter. Staining on the surface of a water fountain is an indicator of the presence of organic matter in the treated water. The more often the water fountain requires cleaning the higher the organic content of the water. Cleaning frequencies are measured in days, weeks, or months. Quantitative means of monitoring the process are: DBPFP using Hach THM Plus, and the plant online and lab chlorine analyzers. The daily plant chlorine dose or consumption also provides quantitative indication on the progress of the test. Implementation On October 14, 2005, all plant operating parameters were kept unchanged except for the addition of AS-AL (6 ppm as silica) after the alum. The first indicator of the removal of DBP precursors would be a decrease in chlorine consumption. A significant drop of chlorine daily dosage was observed the days following the AS-AL addition. Based on three years of data ( ) for the same period, about 50% savings in chlorine is realized as shown in Figure 5. A DBPFP procedure was used to measure percent removal of DBP precursors. DBPFP may vary from day to day. For consistency, the samples of raw and treated water were taken the same hour of the day. The percent removal of the DBP precursors may be calculated from the data. For example, samples taken November 12, 2005 of the raw and treated water were subjected to a 1-day and 2- day DBPFP test as shown in Figure 6. The amount of DBP precursor s removed on the 1 st and 2 nd day of incubation was 78 and 79 percent respectively. It appears that the longer the incubation period the higher the percent removal. On the same day, the DBPs of the treated and distribution water samples were 46 ppb and 88 ppb respectively. The 2-day chlorine demand of the raw water was 8 ppm. The author prefers to use 1-day incubation period to closely simulate what is happening in the distribution system. The historical level of DBPs in the distribution samples are shown in Figure 7. The data shows that the plant could pass the present Canadian MAC for TTHM by just using AS-XP. The US EPA standards for DBPs are more stringent than the Canadian standards and include haloacetic acids. Only by using AS-AL can the KLWTP easily pass the USEPA standards which the Canadian regulatory body may tend to adopt in the future. The following visual observations can be made after one month of using AS-AL at the plant: 6

7 Sludge characteristics are about the same as using AS-XP and way better than using standard AS; The particle index is about the same as using AS-XP. The particle count of the treated water remains very good and may be equal or better than ultrafiltration; Even without prechlorination, the clarifier settling tubes are cleaner compared to using standard AS, shown in Figures 8 and 9 respectively; The floating organic scum after a filter backwash, shown in Figure 10, disappeared; and The water fountain test results are shown in Table 2. Table 2 Frequency of cleaning the water fountain Flocculant used Standard Activated Silica (AS) AS-XP (Basic) AS-AL (Acidic) Frequency of cleaning Every other day Once a week Still clean after one month and waiting CONCLUSIONS AND RECOMENDATIONS The flocculant AS-AL can remove 70 to 85% DBP precursors in raw water. At this level of DBP removal, any plant can easily pass the USEPA limits for DBPs. There are other benefits of having low chlorine demand water at the plant and distribution system. Some of these benefits are: (1) more stable plant operation, (2) more stable chlorine residual in the distribution system, (3) less instrument maintenance, and (4) lower chemical consumption. This process of removing DBP precursors can be used by small and large water treatment plants regardless of the concentration of NOM in the source water. The flocculant AS-AL will help bring the conventional water treatment process into the 21 st century. The results after one month of plant operations are very encouraging. There is a need for further research to investigate the mechanism of DBP precursor removal using AS-AL. Plant optimization is still on going to find the optimum dosage, the ratio of alum to silica in AS-AL, and the optimum dosage of alum. 7

8 ACKNOWLEDGEMENT The author is grateful for the cooperation and support provided by the Superintendent of Works, Frank Spinato, and plant operations staff members. The author would also like to acknowledge Brian Whitehead, Drinking Water Unit, Water Monitoring Section, Environmental Monitoring & Reporting Branch, of the Ministry of the Environment for providing analytical services during the test work. REFERENCES K. R. Lange; R. W. Spencer; Environmental Science and Technology, v. 2, n. 3, pp , (1968); ON THE MECHANISM OF ACTIVATED SILICA SOL FORMAION R. K. Iler; THE CHEMISTRY OF SILICA (1979), John Wiley & Sons, Inc. A. D. Nikolaou, et. al.; ORGANIC BY-PRODUCTS OF DRINKING WATER CHLORINATION, Gloval Nest: the Int. J. v. 1, n.3, pp , D. Bursill; DRINGKING WATER TREATMENT UNDERSTANDING THE PROCESSES AND MEETING THE CHALLENGES, Water Science and Technology Supply; v.1, n. 1, pp

9 Figure 1. Schematics of the Kirkland Lake WTP Figure 2 Clarifier sludge viewing window 9

10 Figure 3 Particle Index Monitor Figure 4. Water fountain 10

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12 Fig. 8 - Clarifier settling tubes using AS-AL 12

13 Fig. 9 - Clarifier settling tubes using standard AS Fig Floating organic matter after a filter backwash 13