WASTEWATER TREATMENT
General
Capital costs may be two to three times higher for wastewater when compared to potable water treatment of similar flow rates.

Usually need to install medium pressure lamps to meet disinfection standards. The higher temperature of the lamps result in greater fouling, scaling, ect., when compared to low pressure lamps.

UV is generally regarded as a site specific design, more so in wastewater treatment than in potable water treatment. Most all references recommend a full pilot study over several different seasons of the year, such as winter and summer; that it be preceded by filtration to guarantee lower suspended solids concentrations through the UV disinfection process. (<2NTU and not to exceed 5 NTU more than 5% of any 24 hr period of time) filtration upstream of the UV disinfection process appears to be the only way to meet stringent water quality objectives (Total coliform less than 2.2 MPN/100mL).

Cleaning
It is critical that operators clean the lamp sleeves on a routine basis to maximize UV light transmittance.

Remember, scaling of the quartz sleeves reduces light transmittance in the proper wavelength. Adjust the water pH to minimize scale.

Impacts
The effectiveness of the disinfection process is impacted by turbidity, total suspended solids, UV absorbance (ie: color, organic matter right), dissolved ions, hardness, alkalinity, and temperature. Wastewater, with its higher turbidity and suspended solids, tend to protect microbes from UV light more so than a potable water, especially in the larger wastewater flocs (which by definition, should NOT have them in the first place!). With a wastewater, suspended solids is probably more important than turbidity as a pre-disinfection goal, as the suspended solids particles REALLY shelter microbes from the UV light.

Decreasing water quality has a corresponding increase in both capital costs (number of lamps, ballasts, etc.) and operational costs (electrical power, labor, maintenance, lamp replacement, etc.).

Reliable and predictable upstream process performance is an operational key to for consistent wastewater disinfection goals to be met.

The number of flock particles, the sizes and percentage of each size of particle will affect disinfection. Wastewater with smaller floc particles, and lower numbers of floc particles, are much easier to disinfect. UV is not as effective for a secondary effluent with total suspended solids at or above 30 mg/L. The suspended solids absorb the UV radiation, and shield the embedded microbes.

WATER REUSE
Vast majority, if not all, filter this secondary effluent prior to disinfection by ultraviolet radiation. Filtration helps insure complete disinfection goals by decreasing the number of pounds of solids that are passed to the disinfection process, insures lower turbidities, and very small particles sizes. (Majority all are low pressure lamp sites.)

BOTH WATER AND WASTEWATER
Age of lamps: as the lamps age, there is a loss of UV light intensity.

As water hardness increases, carbonate scale on quartz sleeves increases. The scale is hard to eliminate - high hardness waters are problematic for cleaning.

Remember, UV is classified as a "physical process."

Optimization of upstream processes will more often than not produce favorable reductions in disinfection costs. This is especially true when using UV as a disinfection process.

Maintenance
Require redundancy-keep the process fully operational! Many installations recognize the critical nature of this process and have enacted " required overtime" to insure complete operation of the process.

Safety
Lockout - blockout - tag out is required on the electrical systems as a safety precaution. Enact and adhere to safety procedures to avoid over exposure to UV light.

Cleaning
The most cited cause for UV systems not meeting disinfection requirements is the lack of proper cleaning of the sleeves.

Chemical cleaning: citric acid, vinegar, sodium hydrosulfite solutions are usually used.

Sodium hydrosulfite is often used for non-contact cleaning systems.

Insure proper operation of mechanical wipers, or ultrasonic cleaning systems.

References:
"Wastewater Engineering, 3rd Edition;" Metcalf & Eddy c 1991 McGraw Hill, Inc

"Handbook of Chlorination & Alternative Disinfectants" 4th Edition; c 1999, Geo. Clifford White, John Whiley & Sons, Inc.

"USEPA Design Manual: Municipal Wastewater Disinfection." 1986. EPA office of research in development, Cincinnati, OH

Water Environment Federation. Wastewater Disinfection. WEF Manual of Practice FD-10. Alexandria, VA: WEF. 1996.

"UV Disinfection Costs For Inactivating Cryptosporidium," Cotton et al, AWWA Journal, June 2001

"USEPA, Wastewater Technology Fact Sheet," UV EPA 832-F-99-064 Sep 1999

"Impacts of a wastewater quality on UV disinfection of reclaimed wastewater," Carins, et al

National Water Research Institute. UV Disinfection Guidelines for Wastewater Reclamation in California and UV Disinfection Research Needs Identification. Fountain Valley, CA: NWRI. 1993.

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