March 2014, Vol. 26, No.3

Operator Essentials

What every operator should know about ultraviolet disinfection

Dave Murray

 

 Knowledge 

Principle  

Practical considerations  

History 

While the first mercury vapor lamp with ultraviolet (UV) output was developed in 1903, practical use in wastewater disinfection has occurred only within the past 25 years. 

Design principles, lamp technology, and process control electronics have solidified UV disinfection as the primary alternative to chlorine disinfection of wastewater. 

Benefits 

UV disinfection provides substantial environmental benefits compared to the use of chlorine or sodium hypochlorite for wastewater disinfection. 

Benefits include the following: 

n   no residual chlorine in the effluent, 

n   no need for dechlorination, 

n   no byproduct formation, 

n   no onsite chemical storage, 

n   no risk management plan requirement, and 

n   improved worker and community safety. 

Wavelength 

UV light is electromagnetic energy within the range of 190 to 400 nanometers (nm) — these wavelengths are shorter than visible light. UV-C, within the range of 200 to 290 nm, is strongly absorbed by the genetic material within biological cells. 

Low-pressure UV lamps produce most of their output at a wavelength of 253.7 nm, which is very close to the optimum wavelength of 260 nm for inactivation of genetic cell material. 

Microbial inactivation 

Microbes are inactivated by disruption of cells’ genetic material, mainly deoxyribonucleic acid (DNA). DNA is a complex double-chained molecule containing the genetic code for biological replication. Replication is stopped when the DNA strand is disrupted by UV light. 

Increased UV dose improves the rate of microbial inactivation. 

  

Microbial regrowth is possible under some conditions including low UV dose. 

Lamp types 

There are several types of UV lamps used for wastewater disinfection. These include low-pressure; low-pressure, high-output (LPHO); and medium-pressure lamps. 

Low-pressure lamp types are very energy-efficient compared to medium-pressure lamps. 

  

Low-pressure and LPHO lamps produce most of their output at 253.7 nm and are about 35% to 45% efficient at converting input energy into germicidal UV radiation. 

  

Medium-pressure lamps are polychromatic and are about 10% to 16% efficient at converting input energy into germicidal UV radiation. 

Lamp ballasts 

Lamp ballasts regulate the power applied to each UV lamp. Ballasts can be mounted in a separate panel near the UV channel or in the individual lamp modules depending on manufacturer design. 

Ballasts can be electronic or electromagnetic. 

Electronic ballasts enable the power supply to each lamp to be lessened for energy conservation. 

UV system arrangement 

UV lamps can be arranged in a number of ways within a system. They can be mounted either horizontally or vertically in an open channel. They also can be located within a closed reactor and be either parallel or perpendicular to flow direction. One manufacturer’s configuration has flow-through lamp sleeves with the water on the exterior. 

Lamp arrangement usually is a function of the manufacturer’s design and does not greatly affect system performance. However, some systems are easier to maintain because of better lamp access and cleaning mechanisms. 

UV transmittance 

UV transmittance (UVT) is a measure of the clarity of an effluent to UV light and is measured using a spectrophotometer set at 253.7 nm. It is the percentage of UV light applied to the water sample that passes through a 1-centimeter (cm) path. The actual measurement is absorbance units (a.u.) per cm and transmittance is calculated as follows: 

  

UVT = 100 × 10 –(a.u./cm)  

Low UVT may result from to dissolved chemicals, colloidal particles, or suspended particles in the water. 

  

It is important to remember that visual clarity is not always an indicator of UVT. Some industrial wastes are high absorbers of UV light, but have no noticeable color. 

  

Typical UVT values for secondary effluent range from 55% to 75%, but each effluent is unique and must be tested. 

UV dose 

UV dose is determined by the following equation: 

  

UV dose = I × t, 

where 

I = UV intensity in milliwatts per square centimeter (mW/cm2), and  

t = exposure time in seconds (s). 

  

The units for UV dose are mWS/cm2 or millijoules per square centimeter (mJ/cm2). 

Typical UV doses for secondary effluent are between 15 and 30 mWs/cm2. UV dose rates for reclaimed water can be between 80 and 120 mWs/cm2 if granular media filtration is provided. UV doses for reclaimed water can be between 40 and 80 mWs/cm2 for membrane filtered water. 

Water quality considerations 

Water quality can affect UV disinfection system performance in several ways. These include lamp-sleeve fouling, reduced UVT, absorption of light by colloidal or suspended particles, and absorption of UV light by chemical contaminants. 

Water quality parameters of concern for UV system design include the following: 

n   UVT, 

n   total suspended solids, 

n   calcium and magnesium hardness, 

n   iron and manganese content, and 

n   organic constituents such as polycyclic organic 

    compounds. 

Lamp life 

Lamp life is a factor of lamp design and the number of on/off cycles. The UV output of UV lamps decreases during their useful life span, but the rate of decrease varies among lamps types and suppliers. 

Low-pressure and LPHO lamps typically should last up to 12,000 hours before lamp output falls below acceptable standards. 

  

Medium-pressure lamps have a useful life that typically is much shorter, approximately 4000 to 8000 hours. 

Sleeve fouling/cleaning 

UV lamps are contained within quartz sleeves to separate them from the water environment. 

Lamp sleeve fouling can be a major source of operating problems since disinfection effectiveness decreases substantially when the light cannot reach the process flow. 

  

Many currently available UV systems have either mechanical or chemical/mechanical cleaning mechanisms to keep the sleeves in good condition. Even with this equipment, manual or chemical cleaning may be needed occasionally. 

  

Lamp sleeve deposits may be due to calcium or magnesium scaling, iron fouling, or organic deposition. 

Process control 

Process control can be relatively simple or very complex depending on system size and design requirements. 

The minimum control level is flow proportioning to allow lamps to be turned on or off or dimmed in response to changes in flow rate. Many systems also use UV transmittance to proportion intensity as transmittance changes. Many systems have controls that increase UV power as the lamps age to maintain the UV dose. 

Potential operating problems 

Potential operating problems include frequent lamp sleeve fouling, algae buildup, and hydraulic distribution problems. 

Manual lamp sleeve cleaning and removal of algae buildup is sometimes required to ensure system performance. 

Performance 

UV disinfection performance can be measured using testing of upstream and downstream microbial indicators. 

Removal frequently is calculated as the log inactivation rate. Permit standards frequently rely on geometric mean values to minimize the effect of a single bad result on long-term system performance. 

Reuse applications 

UV disinfection is used commonly for wastewater reclamation and reuse. 

UV dose rates for water reclamation usually are about three times higher than for discharge applications. Filtration upstream of disinfection is required to achieve very low bacterial levels needed for reuse. 

Advanced oxidation 

UV irradiation is used for advanced oxidation with or without chemical catalysts. Catalysts include hydrogen peroxide. Advanced oxidation is used for indirect potable reuse when organic contaminants are present. 

Typical trace contaminants include N-nitrosodimethylamine (NDMA) and 1-4-dioxane. UV doses for advanced oxidation are several orders of magnitude higher than for disinfection. The measure of energy for advanced oxidation applications is electrical energy per order of contaminant destroyed per unit volume of water or EE/O ( i.e., 0.2 kWh/1000 gal treated per order of contaminant destroyed). 

Dave Murray is an environmental engineer in the Portland, Ore., office of Brown and Caldwell (Walnut Creek, Calif.).