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 City of South Bay - Emergency Evaluation, Example Technical Report

EXAMPLE: CIty of South Bay Emergency Evaluation

At the request of the Government Services Group, Inc. (GSG) and the City of South Bay, U.S. Water Services Corporation (USWSC) provided emergency evaluation, repair and troubleshooting of the disinfection system at the South Bay Water Treatment Facility. U. S. Water continues to provide operating services, system oversight, and engineering support.

As an example of a Technical Evaluation Report, following are excerpts from the initial South Bay System Evaluation:

The report is designed to provide general information on the water disinfection process and is also intended to identify specific deficiencies and necessary repairs within the water disinfection system. Additionally, this report outlines certain corrective actions and recommendations to address the operation of the water disinfection system.

2.0 BACKGROUND 2.1 Facility Overview The City of South Bay water treatment plant is a 2.0 MGD lime softening plant, consisting of three surface water withdrawal points that draw water from the Lake Okeechobee rim canal. The water is then pumped through a control valve into a cascading aeration tower that flows via gravity into a rapid mix tank where ferric chloride is injected. The water then flows via gravity into a floculator tank for further mixing. Polymer and lime are injected as the water flows via gravity into two precipitators that are operating in series. As the water gravity flows into the three filters, fluoride and gaseous carbon dioxide are injected. From the filters, the water gravity flows into a series of four clearwells. Chlorine is injected into the first clearwell; from there the water flows via gravity into the second clearwell. Ammonia is injected after the second clearwell prior to the third clearwell. The third and fourth clearwells are interconnected through an equalization pipe. From the third clearwell, the finished water is pumped into a 1 MG ground storage tank located at the plant. At the ground storage tank, there are three high service pumps that pump into the water distribution system. There are two additional transfer pumps connected to the fourth clearwell that can bypass the high service pumps and pump directly into the distribution system. Within the distribution system, there is an additional 1 MG ground storage tank that is fed from system pressure. At this storage tank, there are two high service pumps that draw water from the ground storage tank and pump into a hydropneumatic tank which also feeds the water distribution system.

2.2 Disinfection and the Use of Chloramines One of the most important stages in the water treatment process is disinfection. Disinfection is the treatment process used to destroy or inactivate disease-causing (pathogenic) organisms, usually through the addition of chlorine, or a combination of chlorine and ammonia, to the water. Diseases caused by pathogenic organisms in water are called waterborne diseases. [1,2] Chlorination is the most common means of disinfecting drinking water and, when it is properly applied, is safe, practical, and effective in destroying pathogens. Most pathogens are accustomed to living in the temperatures and conditions found in the bodies of humans and animals and do not survive well outside the body. Nevertheless, significant numbers can survive in potable water. Some pathogens, particularly certain viruses and those organisms that form cysts, can survive for long periods even under the most adverse conditions. As such, all drinking water should receive adequate disinfection to help ensure that pathogens are not present. [1]

As a general practice, most utilities employ free chlorine as the traditional type of water disinfection; however, there has been a growing trend in the past two decades to seek alternatives to free chlorine. This trend has been driven by the need to control the formation of disinfection byproducts, formed by the reaction of chlorine and naturally occurring organic substances that are measured as total organic carbon, TOC. Some utilities have continued to use free chlorine as the primary disinfectant in their water treatment plants, but have converted to chloramines for the secondary disinfectant in their distribution systems. Since chloramines persist longer and do not react as quickly to form the main DBP components, their use has become more popular. Besides satisfying a compliance objective for DBPs, many utilities find that chloramines provide a more persistent disinfectant and reduce the chlorine taste and odor of the product water.

Since surface water supplies are more susceptible to contamination than ground waters, they should always receive treatment before disinfection and delivery to consumers as required by the USEPA Surface Water Treatment Rule and the FDEP Drinking Water Rule under 62-555, F.A.C. Most systems with a surface water source must use sedimentation and filtration to ensure adequate removal of microorganisms. However, all surface water systems, whether they provide filtration or not, must practice disinfection under very specific conditions. The disinfection process must ensure adequate inactivation of Giardia cysts and viruses. The effectiveness of chlorination for inactivating Giardia cysts and viruses depends on the following factors [2]: (List of factors provided).

2.3 Reaction of Chlorine with Water When chlorine gas is added to water a chemical reaction occurs between the molecules of chlorine and water. Chlorine reacts rapidly with water to form two separate and distinct chemicals that are in solution in the water. In this reaction, which is called hydrolysis, chlorine combines with water to form two components, hypochlorous acid (HOCl) and hydrochloric acid (HCl).

At pH levels above 4, the above hydrolysis reaction goes to completion within a few tenths of a second at 18 °C, and within a few seconds at 0 °C [10]. The reaction of chorine with water reduces the alkalinity slightly. One (1.0) mg/l of chlorine causes the alkalinity to decrease by 1.4 mg/l as CaCO3 [6]. In the above reaction, the product of interest is hypochlorous acid (HOCl). Hypochlorous acid may undergo an additional reaction by dissociating into two components, hydrogen ion (H+) and hypochlorite ion, 

The disinfecting ability of free available chlorine depends on the relative concentration of the HOCl and OCl species in solution. HOCl is generally considered to have greater disinfecting ability than is OCl [7]. The ratio between these two species depends on the pH and temperature of the solution. At lower pH, the proportion of HOCl is relatively higher and, therefore, the disinfection efficiency is higher.

2.4 Reaction of Ammonia with Water Ammonia (NH3) is considered a weak base that is highly soluble in water. When it dissolves in water, it attracts a hydrogen ion (H+) from the water molecule to produce an ammonium ion (NH+4) and hydroxide ion (OH).

The relative concentration of NH3 and NH4+ in solution depends on the pH of the solution. At 25 °C, approximately equal concentrations of NH3 and NH4+ exist at pH 9.2. Above pH of 9.2, NH3 predominates while NH4+ predominates below pH of 9.2. The reaction of water with ammonia increases the alkalinity of the solution slightly: 1.0 mg/l of NH3 causes the alkalinity to increase by 2.9 mg/l as CaCO3 [6]. Increased alkalinity provides increased buffering capacity and thus helps to maintain consistent pH.

2.5 Chloramine Residual Disinfectant Level: For water systems using chloramines, it is important that chloramination is practiced such that excess ammonia is avoided. AWWA recommends that a minimum combined chlorine residual of 1.0 mg/l be maintained throughout the distribution system. One study has found that monochloramine residuals greater than 2.0 mg/l were successful in reducing biofilm in iron pipes [8]. AWWA also recommends a minimum combined chlorine residual of 2.0 mg/l for finished water leaving the treatment facilities. Most utilities establish a target chloramines residual for the end of the distribution system. If the chloramines decay rate is high in the distribution system, the initial chloramines dose required to meet the target residual at the end of the system may cause the system to exceed maximum residual disinfectant levels. Reducing the rate of chloramines decay is a key strategy in meeting the chloramine minimum residual disinfectant level. Factors influencing chloramines decay rate are discussed below.

2.6 Chloramine Decay When chloramines decay, the total chlorine concentration decreases and free ammonia is released. Chloramines decay in water distribution systems due to the autodecomposition of monochloramine and the reaction of monochloramine with reactive constituents in water and the pipe wall. This decay is accompanied by a release of ammonia, which can have considerable impact on the growth of ammonia-oxidizing bacteria. For every 1.0 mg/l decrease in monochloramine concentration, a 0.21 mg/l increase in ammonia concentration occurs. The amount of ammonia released by chloramine decay is often larger than the excess amount of ammonia following chloramines formation. Therefore, removing chloramines-demanding substances will help to minimize a significant cause of unnecessarily high ammonia residuals in the distribution system. The autodecomposition of monochloramine produces primarily nitrogen gas, chloride ion, and ammonia, although a small amount of nitrate is also produced. A decrease in pH occurs due to the production of hydrogen ions. The following reactions summarize the auotodecomposition of monochloramine:

 

The primary constituents in water and on the pipe wall that accelerate decomposition of chloramines are bromide, ferrous iron, and nitrite. Studies have shown that higher temperature, higher alkalinity, and lower pH yield faster decay rates. The studies have also confirmed that a higher initial total chloramines dose of 3 mg/l resulted in a faster decay rate compared with a lower chloramines dose of 1 mg/l. In addition, the studies indicated that a lower chlorine:ammonia-N ratio (3:1) resulted in excessive amounts of free ammonia. Higher chlorine:ammonia-N ratios between 4:1 and 5:1 are recommended due to the fact that at these ratios monochloramine is the predominant species and will minimize the initial amount of free ammonia-N in the distribution system [12].

2.7 Chloramines and Nitrification The implementation of chloramines disinfection can ironically lead to a loss of residual disinfectant in water systems where nitrification is not controlled. Adverse effects associated with nitrification in chloraminated water distribution systems include: (List Provided)...........

As mentioned previously, ammonia levels can reach high values in chloraminated systems due to overfeeding of ammonia at the treatment plant and ammonia released during chloramine decay. Ammonia is successively oxidized to nitrite by ammonia oxidizing bacteria, and to nitrate by nitrite oxidizing bacteria.

It is NO2 that can represent a significant sink for residual chloramine, as chloramines oxidize nitrite. Nitrite is formed by nitrifying bacteria when there is free available ammonia present. Nitrite is of concern because it is not very stable under typical conditions, and should be oxidized to nitrate NO3 provided there is adequate monochloramine present. This oxidation reduces the concentration of monochloramine and studies have shown that the residual may be depleted entirely especially in water storage tanks under long detention times and hot climates. Factors other than excess ammonia may foster the growth of nitrifying bacteria within the water distribution system, and include........(list provided)......

Ammonia Doses and Free Ammonia Residuals discussed........

3.0 CITY OF SOUTH BAY DISINFECTION SYSTEM EVALUATION 3.1 Recent Developments At the request of the City of South Bay, U.S. Water Services Corporation staff have recently inspected and cleaned the ammonia feed system. During the inspection, they observed significant amounts of calcium and magnesium precipitation in the ammonia solution feed and delivery system, including clogging of the injection and solution lines. The clogging has also adversely impacted the reliability of the disinfection system to meet the Viral and Microbial Inactivation Requirements of Rule 62-555.320, F.A.C. To improve operation of the system, a dual downstream ammonia injection system has been installed to allow for easy removal, inspection, cleaning and reinstallation without disruption of the system. In addition, ammonia solution- feed systems using vacuum-operated units use injectors to draw ammonia gas from the storage cylinders and mix with dilution water. However, one problem with solution-feed ammonia systems is that the ammonia hydroxide formed when ammonia gas is commingled with the solution at the venturi causes rapid increase in pH which consequently results in the precipitation of calcium and magnesium hardness. The photo below was taken for a ball valve in the ammonia feed system before the water softeners were installed. As indicated, due to the high levels of hardness in the water, extensive amounts of calcium and magnesium hardness precipitation occurred. One of the most common methods that is normally pursued in mitigating this phenomenon involves the use of water softeners in the injector water supply line to reduce the hardness of the feed water to 35 mg/l or lower as recommended by the AWWA guidelines under AWWA Manual M20. Accordingly, after discovering this problem at this facility, USWSC installed a dual water softening unit in the injector water supply line for the ammonia solution-feed system, which has resulted in correcting the hardness precipitation problem. It was discovered during the inspection that Clear Wells #1 and #2 contained monochloramines and free ammonia. Based on current design, Clearwells #1 and #2 are supposed to be used for primary disinfection using free chlorine only. However, the presence of monochloramines in these tanks is an indication that a hydraulic connection exist between Clear Well #3 and Clearwells #1 and #2. This connection may have been caused by the transfer piping between the tanks. Following that discovery, Clearwell #1 was taken out of service for proper inspection to determine the cause of the hydraulic connection between the tanks. As part of the procedure, boil water notice was sent to all customers and the proper bacteriological sampling was performed.

3.2 Troubleshooting 3.2.1 Events of July 12 and 13, 2007 The following is offered in further clarification of the events surrounding the emergency repair and troubleshooting of the disinfection system at South Bay Water Treatment Plant in the late evening of July 12, 2007, and through the early morning of July 13, 2007.

USWSC staff responded by visiting the site and finding the system temporarily feeding liquid ammonia and gaseous chlorine.

USWSC set up new softening unit for solution-feed water supply and began measurements of free ammonia, total and free chlorine, and monochloramine at various locations.

USWSC determined that a free chlorine residual was not present at any concentration at the outlet of Clearwell #2, as required, to meet required CT value and ensure Giardia and Virus inactivation.

USWSC requested GSG to issue an immediate “boil water” notice on the system. As you are aware, absence of coliforms in the finished water does not necessarily indicate adequate disinfection of other pathogens such as viruses, Giardia and Cryptosporidium, which is why maintenance of the required minimum CT is essential to ensure microbiologically safe drinking water. This is especially true for surface water supplies which are more susceptible to waterborne pathogens. Due to the system’s inability to meet the required CT value, USWSC staff advised GSG to issue the “boil water” notice, since no such notice was in effect even though the disinfection system had not been functioning properly for some time. (The system was not maintaining free chlorine residual after the contact time at the outfall of Clearwell #2. The reported values for free chlorine in the presence of monochloramine are suspect due to the known interference of monochloramine in the chemical analysis for free residual chlorine using the DPD method.

USWSC continued to investigate the cause of the disinfection system failure including testing for total and monochloramines and free ammonia in Clearwells Nos. 1, 2, and 3 and the finished water. It was determined that significant amounts (2 to3 mg/l ) of monochloramine were present in Clearwells #1 and #2. Also, free ammonia was present throughout all clearwells and the finished water in concentrations greater than the instruments ability to accurately measure (i.e.,more than 0.55 mg/l), due to excessive dosage of ammonia.

Investigation continued by terminating the raw water flow and all chemical feeds to the facility and then pumping down Clearwell #3, utilizing existing transfer pumps, to near the floor. The USWSC staff then observed an equalization of water levels in Clearwells #1 and #3 and a decline in the operating water level of Clearwell #2 to 54 from the top floor elevation.

This clearly indicated that a hydraulic link existed between Clearwells #1, #2 and #3. The link could be a leaking wall section of Clearwells #1 and #3 or possible pipe connection unknown to staff. The clearwell layout at this facility has been modified a number of times and the piping configuration is extremely confusing and not intuitive by visual observation.

As such, it was decided that since the system was already on boil water and, because of the serious nature of not meeting the minimum requirements of disinfection, and in consideration for public health and safety, a physical inspection of Clearwell #3 was necessary to investigate the problem and restore required disinfection.

USWSC staff discussed entry into the tank relative to ambient atmospheric conditions as well as personal safety issues and believed the environment to be safe for the following reasons: (reasons listed)......

USWSC continued the investigation over the next several days and ultimately as of July 19, 2007, corrected the deficiencies in the disinfection system.

3.2.2 Description of the Clearwell Operation and Valving There are a total of five clearwells located at the City of South Bay water treatment plant. Two of them (Clearwells #4 and #5) were part of the old treatment plant and the other three clearwells (#1, #2 and #3) were added as part of the treatment plant expansion and are designed to run in series. Clearwell #1is located underneath the filters and receives filter effluent via gravity. Clearwell #2 contains three baffles and is located beneath the ozone building. From Clearwell #2, water flows via gravity through a 20” transfer pipe and butterfly valve (Valve 5) into Clearwell number 3. There is also a bypass pipe and butterfly valve (Valve 4 - Ozone bypass) that allows Clearwell #2 to be bypassed, thus allowing flow from Clearwell #1 to flow directly into Clearwell #3. There is an equalization pipe and butterfly valve (Valve 2) that exists between Clearwell #3 and Clearwell #4, which is located at the old water treatment plant. Clearwell #4 does not receive any flow other than through the equalization pipe from Clearwell #3. Clearwell #5 is the old water treatment plant filter effluent tank, which is located beneath the old water treatment plant filters. Clearwell #5 is connected to the transfer pipe between Clearwell #1 and Clearwell #2. Clearwell #5 does not receive any flow other than backflow from Clearwell number 1 and 2. There is a 16" butterfly valve (Valve 1) located at the western corner of the old water treatment building that allows Clearwell #5 to be isolated, etc.........(a valve position diagram was provided in this report).

During the investigation and the plant inspection, the following was discovered:

A hydraulic link between the clearwells and backmixing exist.

Inability to obtain a free chlorine residual at the transfer pipe between Clearwell #2 and Clearwell #3.

Presence of free ammonia and chloramines in Clearwell #2. Chlorine is injected in Clearwell #1 prior to the transfer pipe to Clearwell #2. Clearwell #2 is designed for primary disinfection using free chlorine only; therefore, it should not contain chloramines or ammonia. In addition, ammonia is injected in the transfer pipe between Clearwell #2 and Clearwell #3; which is an indication that free ammonia should not be present either Clearwell #1 or #2. Both of these findings (i.e., presence of chloramines and free ammonia in Clearwell#2, and presence of ammonia in Cleawell#1) indicate that a backflow connection exist between the clearwells.

Clearwell #1 receives flow from the filters situated above the clearwells. This is the only source of flow for the clearwells. Additionally, the control weir elevation in Clearwell #2 should not allow the water level to drop in Clearwells #1 or #2, below the weir elevation, regardless of the water level or pumping condition in Clearwell #3. When the water level dropped in Clearwells #1 and#2 as Clearwell #3 was pumped down, it was evident that there was a hydraulic connection between the clearwells.

After investigating the available as-builts and observing the clearwell level fluctuate, the following testing procedures were followed:

1) Turned off the influent flow and turned on transfer pump to the ground storage tank.

2) Observed water level in Clearwell #1, drop to 96” from rim elevation.

3) Observed water level in Clearwell #2, drop to 54” from rim elevation.

4) Clearwells #1 & #2 were supposed to be operated at a fixed capacity and should not be affected by transfer pump operations and water level in Clearwell #3.

5) A hydraulic connection exist between Clearwells #1, #2, #3, #4 and #5.

6) Plant is currently operated by transferring water from Clearwell #3 to the ground storage tank. This operation causes routine low water level in Clearwell #3, in turn creating back mixing from Clearwell #3 and #4, which contains chloraminated water, to enter Clearwells #1 and #2.

7) Additional Cl2 is added in an attempt to achieve free residual, but backmixing renders this ineffective.

8) Each time Clearwell #3 is pumped down to transfer to the ground storage tank, and then refilled, backmixing was occurring.

Through this series of testing it was determined that the hydraulic connection between the old treatment plant and the expanded treatment plant was still present and that the ozone bypass was open. These valves have since been located and the valve positions confirmed: (List of located valve positions were provided).........

4.0 SUMMARY AND CONCLUSION The evaluation of the disinfection system at this facility has revealed that this water treatment facility was not capable of achieving the minimum required microbial disinfection at all times as required by the USEPA Surface Water Treatment Rule and the FDEP Drinking Water Rule under 62-555, F.A.C. These rules require a minimum of 3-log inactivation for Giardia cysts and 4-log inactivation for viruses. A table was provided showing a comparison between the log inactivation for Giardia and viruses before and after the adjustments and corrective actions that have been implemented by USWSC staff.......

The failure of the water disinfection system was primarily caused by the following factors: 

a) Hydraulic link between Clearwells 1, 2, 3, and 4, which has resulted in the lack of free chlorine primary disinfection in Clearwell #2;

b) Improper positioning and operation of the valving between the clearwells;

c)  Lack of experience, knowledge and training of the South Bay operation staff in the system design and operating requirements;

d)  Lack of analytical instruments to monitor the concentrations of monochloramines and free ammonia at the plant and throughout the water the distribution system;

e)  Unreliable ammonia and chlorine feed and metering equipment;

f)  Lack of field testing to determine the effectiveness of the disinfection system. USWSC noticed, through field measurements and testing, that the chlorine and ammonia are not added at the proper ratio and the excess ammonia was fed without any control.

5.0 RECOMMENDATIONS

1. Research old as-built drawings to determine potential hydraulic links which could cause backmixing of Clearwells, resulting in absence of free chlorine and presence of total residuals in Clearwells #1 and #2.

2. Determine if transfer pumps will operate in automatic position.

3. Determine the function of Clearwell #4. Possible stagnant water holding area and increased microbiological growth.

4. Create flow schematic drawings of Clearwells and all piping. (Completed)

5. Determine if ozone building bypass valve is closed and holding tight.

6. Determine if 16" valve of Clearwell #4 is closed and holding tight.

7. Measure actual INF flow rates with each combination of raw transfer pumps in operation so that interim CT value calculations can accurately be performed.

8. Install permanent INF flow meter with 24 hour chart recorder.

9. Determine minimum operating level of Clearwell #3.

10. Install free chlorine analyzer and 24-hour chart recorder at the Clearwell #2 outlet for calculating CT values.

11. Purchase necessary test equipment for monitoring free ammonia and monochloramine.

12. Install online pH analyzer at same location as chlorine analyzer. Facility has the potential for varying pH levels, greatly affecting the CT values.

13. Provide on-going operator training relative to existing Clearwell configuration and CT value calculations.

14. Review baffling factors used for all Clearwells and determine the most appropriate factor to be assigned to each one.

15. Determine if facility is equipped with level monitor for 24- hour recording of Ground Storage Tank (GST).

16. Determine if finished water meter is accurate and capable of recording peak hourly flow rates. (USWSC calibrated and will record peak hourly flow)

17. Perform in-depth hydraulic profile using level transite to determine tank levels and weir elevations.

18. Assist operations staff to determine correct chlorine to ammonia ratio.

19. Schedule free chlorine burn for plant and distribution system to control microbial growth that may have been enhanced due to overfeed of ammonia.

20. Check for nitrification in the distribution system. Monitor for NO2 and NO3 concentrations at various locations in plant and distribution system.

21. After all piping and Clearwell configurations are verified, determine how required CT values can be achieved.

22. Perform engineering study to determine alternative approaches for feeding of ammonia including types of ammonia additives and best type of injection.

23. Conduct follow-up testing for compliance with disinfection by-product requirements.

24. Develop baseline for Heterotrophic Plate Count (HPC), Nitrites and Nitrates and begin monthly testing.

25. Evaluate plant chemistry and TOC removal efficiency.

26. Evaluate the gas chlorine feed system.

27. Complete flushing of the water distribution system and identify dead-ends and sampling stations. Excessive amounts of ammonia nitrogen in the water distribution system can result in adverse water quality impact due to bacteriological growth and high nutrient concentration. Therefore, intermittent and routine flushing of the water distribution system, using free chlorine, is necessary at a minimum of 6-month intervals.

28. Establish procedure for remote storage tank turnover. Possibly implementing night time fill and day time use.

29. Complete a comprehensive assessment of entire water treatment and distribution system to ensure equipment reliability, liability protection, proper operation and maintenance, and public health and safety.

REFERENCES - Notable Literature References were provided in this report.......