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Managing milk-plant odors by NJ Pinto PE, RA Straut PE, and EA Pond PE
O&M with UV disinfection by R Hill, PE and J Succow
Mitigating sewer odor and corrosion by RJ Pope, PE and N Ettele
Odor dispersion: models and methods by RJ Pope, PE and P Diosey, Ph.D., QEP
First step to effective odor control by RJ Pope, PE
Implementing a P2 program by BM Veith, PE
Featured facility: Kodak's King's Landing Plant by D Beecher, C Popen, D Taylor, D Wolf, R Regelsberger
President's message by AJ Zabinski, PE
Executive Director's report: Leadership, what can it do for you? by P Cerro-Reehil
NYWEA scholarship fund by R Hennigan
Summer 2000 — Vol. 30, No. 2by Ronald Hill, P.E. and June Succow
Quick reference- Plant facilities
- Route 7 Plant
- Observations
Ridgefield, Connecticut, a town of 25,000 people in Fairfield County, has two municipal wastewater treatment plants. Both plants have been in operation for over 10 years and are operated by a private contractor, and both plants use ultraviolet radiation (UV) systems to disinfect the effluent and reduce fecal coliform colonies.
Disinfection is the selective destruction or inactivation of disease-causing organisms such as bacteria, viruses, or amoebic cysts. UV either kills microorganisms or inactivates their reproductive capability—effectively disinfecting wastewater effluent. UV damages a cell's RNA, which is one reason for inactivation. In the process of disinfecting with UV, wastewater flows past light bulbs that emit ultraviolet radiation. The bulbs are covered by quartz sleeves called "crystals." An advantage of UV disinfection is that no chemical addition is required and there is no possibility of overdosing.
In Ridgefield's plants the UV disinfection is used seasonally because the NPDES permit requires disinfection only from May 1 to Sept 30; the Connecticut Department of Environmental Protection (CTDEP) requires no disinfection during the rest of the year. CTDEP does require dechlorination, however, if chlorine compounds are used for disinfection. The UV eliminates the need for dechlorination.
The designs of Ridgefield's UV systems have caused difficulties with operation and maintenance (O&M). The experiences that operators have had with these plants can be used to design UV disinfection systems that are more easily managed.
Plant facilities
The South Street Plant, a 1-mgd facility with peak flow of 4 mgd, uses activated sludge. This Plant underwent a major upgrade from 1989 to 1992. It serves Ridgefield's downtown area, mostly commercial establishments and residences. A utility supplies the water used in this area. This Plant's effluents properties are presented in Table 1.
| Year | Flow (mgd) | Turbidity (ntu) | Temp (°C) | Coliform colonies/100 mL - Average | Coliform colonies/100 mL - Geometric mean |
|---|---|---|---|---|---|
| 1997 | 0.50 | 1.47 | 18.3 | 64.55 | 2.6 |
| 1998 | 0.62 | 1.53 | 19.5 | 68.90 | 3.9 |
| 1999 | 0.48 | 1.17 | 20.0 | 103.96 | 2.0 |
The Plant discharges to a swamp which flows into the Norwalk River. The Plant has a sand filter with automatic backwash in place before the UV system. The effluent from this system is of good quality. After the disinfection, flow is measured with a Parshall flume. The waste water is then discharged.
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The disinfection system
Two ultraviolet banks of bulbs are used. Each bank contains eleven modules, and each module contains eight bulbs. The total number of bulbs is 176. For the system to operate effectively the bulbs must be submerged. The system contains an automatic water level controller which maintains the minimum water elevation suitable for operation.
A major consideration when transmitting light through water is the amount of suspended solids in the water. Light cannot penetrate beyond a solid, and solids can diminish the effectiveness of UV disinfection. The South Street Plant's systems have UV detection probes to sense a reduction in transmittance. The intensity of the ultraviolet light is monitored by two detection probes in the channel on each bank of bulbs. Readout of this intensity is provided in the logic control panel. If the intensity falls below a user-selected set-point an alarm is activated at the annunciator panel. The ultraviolet system for the Plant has the specifications shown in Table 2. The system is capable of meeting 200 fecal coliform-counts/100 mL for any 30-day consecutive geometric mean of daily samples and 400 fecal coliforms/100 mL for any 7-day period.
| Design average flow rate | 1.19 mgd |
| Peak flow rate | 4.5 mgd |
| Minimum flow rate | 0.2 mgd |
| Minimum UV transmittance | 65% |
| Maximum TSS | 20 mg/L |
The UV light's intensity must be adjusted to insure it is sufficient to inactivate or kill the microorganisms present. The intensity is modified by manually removing bulbs or racks from use.
The effectiveness of disinfection is determined by the number of fecal coliform bacilli survive after disinfection. Analysis for fecal coliform is performed at the Plant's laboratory. Testing is performed once a week as required by the NPDES Permit. Effluent samples are taken by a composite sampler over 24 hr. The samples are placed in a sterile sample bottle, which is then passed through a 0.45-micron filter. The colonies are counted on the filter after 24 hr.
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| Working with electrical equipment over water makes maintenance risky. |
O&M experiences
At this Plant the ballast for the UV system has been a major problem. The ballast was very difficult to obtain, and it failed shortly after being installed. Moisture also proved to be a problem for the controls even though they are contained in a sealed box.
Although the channel system is flow-paced, experience has shown that operation was more effective with both UV banks on. The rate structure for electricity was partly responsible for this measure because flow-pacing causes electric spikes which increase electric costs. Continuous use of both banks of UV lights resulted in lower electric costs—a total of $1000/month. Continuous operation, however, wears the bulbs more quickly. The manufacturer claims the bulbs will last 7500 hr or 2 yr, but they last only about 1 yr. For these units bulbs cost $50 each and each crystal costs $31.
The bulbs and crystals are cleaned every 2 weeks on a schedule of preventive maintenance. This job takes three people 2 hr or two people 4 hr. An operator cleans the crystals with a rag dipped in muriatic acid. (A citric acid cleaner recommended by the manufacturer proved to be ineffective.)
If the UV system fails during planned maintenance the effluent is diverted to a 225,000-gal clarifier. If the failure is not planned, a high-test hypochlorite, HTH, (CaOCl) or sodium hypochlorite system is installed. If the UV system goes down when disinfection is required, the CTDEP is notified.
Specific maintenance problems
The design of this system was not well thought out. Each rack of bulbs weighs 50 lb. For operators to lift the racks from the channel requires them to straddle the channel and lift the rack. A lifting system is needed to remove the racks. The units are installed in a channel, and the combination of electricity, metal, and water makes maintenance workers uncomfortable. In addition, any electrical components dropped during maintenance fall into the effluent channel.
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| A device should have been provided for lifting the bulbs for maintenance. |
To replace a bulb the rack has to be pulled and the crystal unscrewed. If the crystal is not installed correctly, water leaks into the bulb causing the bulb to fail. If the crystal is tightened incorrectly the crystal will crack causing leakage of water into the bulb causing the bulb to fail. The crystals get brittle over time. If they are not installed carefully they scratch and reduce the system's effectiveness. Although UV systems can be flow-paced, experience has shown that bulb life is longer when the bulbs are operated at constant power.
The electrical controls are mounted directly on the unit, placing them in a humid and corrosive area. It is preferable to mount electrical controls in a separate box and in a better location for maintenance. The electrical controls have no maintenance override. To work on the electrical components the bulbs must be turned off; hence there is no disinfection when servicing these components. A manual or maintenance override would be helpful.
A separate channel for each unit would have been advantageous. Then one channel could be dewatered when work has to be performed on a UV system and the other unit could remain in operation.
Route 7 Plant
A smaller Plant is located off US Route 7 and serves a nursing home and commercial and industrial facilities. This 120,000-gal/day Plant was built in 1985 to replace an industrial facility. It has rotating biological contactors, followed by secondary tanks, followed by UV disinfection. Water used in this area is primarily supplied by wells. This Plant discharges to the Norwalk River. This plant has higher turbidity than the South Street Plant. Its effluent's properties are given in Table 3.
| Year | Flow (mgd) | Turbidity (ntu) | Temp (°C) | Coliform colonies/100 mL - Average | Coliform colonies/100 mL - Geometric mean |
|---|---|---|---|---|---|
| 1997 | 0.066 | 3.2 | 18.1 | 53.5 | 1.3 |
| 1998 | 0.067 | 4.8 | 17.6 | 200.1 | 181.2 |
| 1999 | 0.077 | 3.8 | 18.7 | 66.9 | 26.5 |
The disinfection system
This Plant has two units, each built by a different manufacturer, that work in parallel. One UV system is a 3-inch Teflon tube with bulbs mounted on the outside. Solids do not accumulate on this unit, and cleaning takes 2 hr. Originally there were two of these units, but parts from one were cannibalized to leave one working unit from the original two. The other unit is a steel tank with bulbs inside the tank. The effluent flows into this unit and goes down past the bulbs and then out the top. Solids can deposit at the bottom of this unit. Cleaning this unit takes 1 hr.
The units are left on because they cannot be flow-paced. They are supposed to have the same design capacity but do not. Both manufacturers have gone out of business, and maintenance is problematical. One of the units has an UV monitor, but it is no longer used because no spare parts are available. The monthly power requirement for UV disinfection at this Plant is 760 kWh, $73/month.
For the in-line units the inlet and outlet are above the bulbs causing solids to settle out in the tube area. This increases routine maintenance. Having the outlet on the bottom would reduce this problem. Alternatively, a drain valve on the bottom of the unit would be helpful in removing these solids.
O&M experiences
Cleaning these units requires 4 man-hr twice a month. Preventive maintenance takes 1 man-hr/week. There are four bulbs in each unit for a total of eight bulbs; they cost $33 each. Each bulb in the steel unit has a power meter which gives an indication of when the bulb is wearing out. The Teflon unit has no power meter, but since the bulbs are mounted on the outside, operators have a visual indication of how they are working. In this facility all the water is enclosed; hence the building is not humid and corrosion of the electrical controls is not a problem.
At this Plant the ballast failed, and all bulbs are replaced every other year. The systems are turned on one week before the start of disinfection to insure that they are working and to resolve any problems before disinfection is required.
The peak flows to these units are limited to 110,000 gal/day despite the Plant's design capacity of 120,000 gal/day. Excess flow causes a backup before the disinfection unit, and there is insufficient head to get more flow through these units. Maintenance is performed during low flow periods. Work is performed on one unit while the other unit operating. To date there have been no unscheduled stoppages due to insufficient disinfection.
Specific maintenance problems
For these units the manufacturer recommended citric acid to clean the crystals. Citric acid was found to be unable to remove the hard deposits which formed on the crystals. Therefore, muriatic acid is used for cleaning. Crystals are fragile and get brittle from the heat. Each sleeve costs $38. The crystals are held together with a lock nut, and have to be treated with care. The intensity of the UV in these units cannot be adjusted. Lack of photocells or UV detectors means there is no feedback.
Suspended solids settle in these units. This requires flushing to remove the solids deposited. The unit has to be taken off line for 1/2 hr to perform this flushing. The Plant discharge is 8 ft higher than these units thereby aggravating the settling problem of solids.
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| Suspended solids collect in the inline unit at the Route 7 Plant as the unit is lower than the discharge. |
It takes 1 week to get each UV system operational after being off-line for the winter.
Observations
In Connecticut if a plant chlorinates to disinfect, it must dechlorinate. This requires the use of sodium bisulfite, sulfur dioxide, or sodium thiosulfate. Using UV eliminates chemicals for disinfection and dechlorination and eliminates the need to handle chlorine gas, sodium hypochlorite, or dechlorination chemicals. Another advantage of UV is that the effluent cannot be overdosed.
A major disadvantage of UV disinfection is there is no residual to measure. With a chlorine-based system, the chlorine residual indicates how the disinfection system is operating. A UV system can use fecal coliform tests, but information from the tests is available only after a 24-hr time lag. If there are problems with the disinfection system, it will not be known immediately.
The people who operate these plants have run both
chlorine disinfection systems and UV disinfection
systems. They feel that the advantages of UV
disinfection are greater than other disinfection
systems.
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Ronald Hill, PE
is with Hazen and Sawyer, PC and is a member of the
Ridgefield Water Pollution Control Authority. During the summer of 2000
he is on active duty with the U.S. military at Vaques,
Puerto Rico. June M. Succow, is a Project Manager for
OMI, Inc.
