Black & Veatch's Frank Rogalla takes a look at the ultraviolet bluesIn response to EU bathing water and shellfish directives, ultraviolet (UV) radiation has become the preferred method of wastewater disinfection. When a UV system functions as intended, it is easy to achieve effluent requirements with an efficient reduction of the coliform counts, yet UV tends to have a reputation for unreliability - because there is no way to quickly verify if the system achieves its result.
Also, UV tends to be sold on the basis of minimum maintenance, but some critical checks have to be carried out regularly to achieve a consistent performance. When a UV system fails to achieve the desired results, its electrical, mechanical and process controls must be carefully examined. This article summarises some helpful hints on troubleshooting UV systems. Simply stated, UV disinfection involves running wastewater through a confined chamber containing rows of UV lamps. The UV energy from the lamps inactivates the micro-organisms (viruses, protozoa, and bacteria) in the wastewater. A UV system consists of a power supply, an electrical system, a reactor, lamps, a mechanical system to hold the lamps, a cleaning system (if applicable) and a control system.
The UV lamps are enclosed in individual quartz sleeves for protection and reactor chambers (either open or enclosed channels) hold the lamps in either a horizontal or a vertical configuration. In an open-channel system the lamps are kept submerged by effluent weirs or automatic level control devices. The UV system may also be housed in a building to shield it from the elements. The UV system can be divided into three:
- electrical component - lamps, ballasts or transformers, wiring and the electrical control system,
- mechanical component - quartz sleeves, frames, cleaning mechanism and reactor configuration,
- process component - involves effluent quality, hydraulics, headloss and the level of disinfection to be attained.
Operators must undergo training, which is often provided by the manufacturer. Plant staff should review the operation and maintenance manuals furnished with the equipment before and after training to determine whether any information is missing or requires clarification.Electrical System Checks
Operators should check the following if problems with UV disinfection appear to be related to the electrical system:
- check lamps are energised - one of the first things to check when troubleshooting a UV system is whether the lamps are turned on. While this may seem obvious, it is often overlooked. A hard-wired audible alarm to indicate when the lamps are de-energised is the easiest way to avoid this pitfall,
- check lamps are connected - during replacement, the UV lamps are disconnected from the power supply, if they are not reconnected, the system cannot operate. All electrical connections need to be checked to ensure power is flowing and the system is delivering the correct amperage,
- useful lamp life - most UV control systems track the lamps' time in operation. When a lamp approaches the end of its useful life, system performance must be carefully observed to avoid compliance problems. Operating staff should continue to use the lamps as long as they are producing sufficient UV light, while closely monitoring system performance after the manufacturer's estimated service life has been exceeded,
- ballast output - all UV systems use electronic ballasts to energise the lamps. The ballasts are considered the heart of the UV system because if they fail to operate, the lamps will not turn on. Each ballast's electrical output should be checked to verify proper system settings. If the output is too high, the lamps may need to be replaced more frequently. If the output is too low, the lamps will not become energised. Mechanical System Checks Properly maintaining a UV system's mechanical components is critical for successful operation. The primary maintenance task is cleaning the quartz sleeves. However, all of the following areas should be checked regularly:
- system cleaning - as noted, the quartz sleeves will become coated with solids deposits, which impedes the transfer of UV radiation to the wastewater. When radiation intensity drops below a set point, the control system indicates the need for cleaning. Deposits are removed by either mechanical or chemical cleaning. After cleaning, operators should examine the sleeves to ascertain all deposits have been removed. In addition, the quartz sleeves should also be cleaned manually a couple of times per year,
- proper chemical delivery - if a chemical cleaning system is empty or plugged, it will not effectively remove the solids deposits. Some UV cleaning systems incorporate both mechanical wiping and chemical cleaning. Medium-pressure systems require both mechanical and chemical cleaning because of their high operating temperatures (up to 1,100°F). For these systems to be cleaned effectively, the chemical must be delivered during mechanical wiping of the sleeve,
- online transmittance measurements - flowmeters, UV intensity and transmittance probes must be cleaned, calibrated and maintained. In addition, online measurements should be verified on bench-scale to ensure accuracy. Sensor locations must also be checked to ensure representative measurements are collected,
- online intensity measurements are used to help control the radiation dose, as well as determine when the quartz sleeves need cleaning and lamps replacing. Intensity sensors need to be checked frequently and may need to be replaced annually,
- to measure the applied UV dose, flow information must be transmitted to the PLC. If the flow information is incorrect, the dose may be too high or too low. The flowmeter should be calibrated in accordance with the manufacturer's recommendations,
- ballast cooling system - some UV systems use a closed-loop ballast cooling system that pumps antifreeze through a radiator. The ballasts are attached to the radiator and the antifreeze transfers the heat from the ballasts to the water or the air, depending on the system design. The seals and connections in the cooling system need to be checked for leakage.
- when a UV system fails to meet the requirements for inactivating micro-organisms, one of the first conditions to check is transmittance. Transmittance is defined as the ability to transmit UV light. It is measured with a spectrophotometer using a lamp producing 254nm (UV254) light, using online or bench instruments. Transmittance can vary widely between treatment plants and from hour to hour. This variability should have been taken into account during the design of the UV disinfection system but is not always foreseeable,
- high effluent TSS concentrations can adversely affect system performance by shielding the bacteria from UV light. One way to evaluate the effect of solids on the UV system is to use the solids concentration in the treatment plant effluent as a baseline. High or increasing TSS concentrations may necessitate more frequent cleaning of the UV system,
- a newer method of identifying problems with UV systems is examining the UV spectra, the actual wavelength of the compound being tested. Results are compared with a referenced solution at UV254. If the spectra results indicate a considerable difference, either as a result of being depressed or because it occurs at a different wavelength, disinfection may be impacted,
- algae can grow on the sides of a UV reactor and begin to block flow through the system. This is especially a nuisance at plants using medium-pressure UV systems because some of the spectra from a medium-pressure lamp are in the visible light range, potentially promoting the growth of algae in the reactor,
- effluent colour can become a problem if the WwTW receives wastewater from a paper or textile mill because coloured water can absorb UV light. Designers and operators need to work closely with the industrial pre-treatment staff to identify the cause and source of the colour compounds and ways to control its discharge. If the online UV transmittance monitor reports a low value and the laboratory verifies the reported value, the plant staff may want to examine the types of industrial dischargers connected to the facility. In some cases, the discharge from an industry may be colourless but still may impact UV transmittance,
- iron residuals from chemical pre-treatment for phosphorus removal also absorb UV light, therefore the compatibility of iron addition and UV disinfection has to be carefully examined. The operating temperatures of UV systems, especially medium-pressure, create favourable conditions for deposits of iron, manganese and calcium carbonate (hardness) on the quartz sleeve. These deposits can interfere with the performance of the system if they are not removed frequently,
- microbial testing is key to monitoring the performance of any disinfectant. If appropriate sampling procedures (40 CFR 136) are not followed, results can be misleading. Samples should collected using grab techniques, placed in certified, clean containers and kept at a temperature of 4°C during transport to the laboratory. Filled sample bottles must be kept in the dark at all times, which can be accomplished by wrapping them in aluminum foil. There are many reasons why UV disinfection systems may fail to meet performance requirements, making troubleshooting such systems a complicated undertaking. Operators at facilities which use UV disinfection should be well trained in the operation and maintenance of such systems.