Treatment and disinfection of wastewater is routinely practiced as a means of protecting both the environment and public health. During the past 20 years ultraviolet (UV) technology has become accepted as an alternative to chemical disinfection. The main drivers for acceptance of UV disinfection are the public concern for safety and an increased awareness of the impact of chemical disinfectants on the environment.
In the mid-70s and -80s various UV reactor designs were assessed for the disinfection of wastewaters. The early acceptance of UV disinfection of wastewater was hindered by reports from some installations that rapid system fouling required frequent cleaning to guarantee disinfection performance. The systems appeared to be more suitable for small treatment plants since many of the original reactor designs and cleaning mechanisms, such as wipers and ultrasonic devices, were not capable of providing the low maintenance and long-term reliability required by large wastewater treatment plants.
An EPA review of UV installation maintenance indicated that while some of the cleaning mechanisms were able to maintain sleeve cleanliness for a time, eventually hand cleaning with an acid solution was required in order to restore the original UV transmittance of the quartz sleeves (US EPA 1986).
More than 2,000 UV systems are installed in wastewater treatment plants in North America. Experience over the last 20 years indicates that while equipment continues to foul, effective cleaning options are available. Manufacturers of UV disinfection systems either provide cleaning equipment or recommend methods for chemical cleaning that include out of channel cleaning tanks, manual wiping or acid recirculation systems. Automatic cleaning systems are also available that provide a combination of mechanical and chemical cleaning that eliminates the need for hand cleaning and significantly decreases maintenance costs.
Impact of Fouling
Wastewater effluents contain various organic and inorganic compounds that eventually foul the quartz sleeves protecting UV lamps. Fouling of the sleeves can decrease the transmittance of UV light through the sleeve into the surrounding wastewater. A sufficient reduction in UV dose delivery results in poor disinfection performance that is often indicated by increased fecal coliform counts.
Process, temperature, pH and the concentration of organic and inorganic compounds are factors contributing to the rate and composition of the fouling material. A review of plant maintenance routines indicates that the fouling rate and the composition of the fouling material often is seasonal and site specific. For conventional low-pressure systems the average cleaning rate is once a month, however some plants clean weekly and others only once or twice a year.
Inorganic compounds dissolved in effluents form a deposit on quartz surfaces that resembles the scale formed by hard water. The most common scale components are naturally occurring or may be chemicals added during the treatment process. Organic compounds that can cause fouling include colloids, proteins, nucleic acids, lipids, oil and grease, extracellular polymers and microorganisms.
Cleaning Methods
A combination of organic and inorganic material on the sleeves often requires a combined mechanical and chemical cleaning techniques. Inorganic scale and light deposits may be removed by soaking in an acidic solution (pH < 2). Heavier deposits may require hand wiping for complete removal. The sleeves should be air-dried to check for complete removal of deposits and films.
Treatment of sleeves with wetting agents or protective coatings should be avoided since many commercial compounds absorb UV light and will result in poor disinfection. Creative solutions such as car wax and windshield protective solutions leave a transparent film that completely blocks the passage of UV light through the quartz. Contact your UV manufacturer for a list of recommended cleaning solutions.
Mechanical Cleaning
UV systems may be equipped with mechanical wipers that can slow the rate of fouling in some effluents. Frequent passes of the wiper over the quartz surface slows the deposition of scale, but deposits eventually will form and chemical cleaning will be required to remove the scaling deposits completely.
Chemical Cleaning
Acid solutions such as phosphoric or industrial descaling liquids (Lime-A-Way) are used in cleaning tanks or for hand wiping. The acid concentrations are generally 20-25 percent with an effective pH of less than 2. As the pH increases above 3 to 4 the cleaning efficiency decreases and phosphates tend to form deposits in cleaning tanks and a white film on sleeves. Spent cleaning solutions should be disposed of at the head works of the plant. Bases such as sodium or potassium hydroxide should not be used since these solutions eventually will etch quartz surfaces resulting in decreased UV transmittance.
Combined Cleaning Technologies
Although many plants are operating successfully using manual cleaning methods, maintenance and labor costs must be considered when selecting the most appropriate UV disinfection system.
The combination of chemical and mechanical cleaning in an automated system provides the most efficient cleaning method available. Trojan Technologies offers two such UV systems. The company's UV4000TM and UV3000PlusTM feature automatic self-cleaning systems designed to function while the lamps remain in operation.
For some effluents, mechanical wiping may be sufficient. However, in the case of high intensity UV systems using either low-pressure or medium pressure lamps, the increased flow volume treated by fewer lamps often translates into more rapid fouling. The properties of these deposits may require the combination of chemical and mechanical wiping to ensure efficient UV dose delivery and optimum power consumption.
The automated chemical/mechanical system has a sealed device containing a small volume of acidic solution. This design combines mechanical wiping with chemicals for more effective cleaning. The fully automated cleaning cycle can be programmed to accommodate the fouling rate for each specific plant. Cleaning cycles are activated by a timer and are programmed to clean modules sequentially within each operating bank while the lamps remain submerged in the effluent channel. Disinfection is not interrupted during the cleaning process.
