UV Waste Water Treatment

In September 1999, the Environmental Protection Agency (EPA) published a technology fact sheet describing ultraviolet disinfection of wastewater. Disinfection is the primary method of destroying organisms in wastewater to ensure waterborne diseases are not spread into the environment. Diseases such as typhoid fever, cholera and infectious hepatitis are just a few dangers associated with untreated or improperly treated wastewater.

Description

• A UV disinfection system destroys an organism’s ability to reproduce. The system consists of three main components: mercury arc lamps, a reactor and ballasts.
  The optimum wavelength for destroying microorganisms is between 250 and 270 nm. The mercury arc lamp emits a monochromatic wavelength of 253.7 nm, making them greatly effective for this task.
  In the reactor, a series of mercury arc lamps are configured together so that the waste water can flow by them. The lamps will have one of two types of configurations: contact and noncontact. In a contact type of reactor, the lamps, enclosed in a quartz tube to minimize cooling, are immersed in the flowing wastewater. In a noncontact configuration, the lamps are suspended outside a transparent tube, which contains the flowing wastewater.
  The ballast in this system is simply a control box that provides a starting voltage for the lamps and maintains a continuous current.

Advantages and Disadvantages

• UV disinfection has several advantages. It uses no chemicals, eliminating the dangers of transporting, handling or storing them. This also makes the process friendly to the environment.
  The disadvantages to UV disinfection can determine if this system is viable for a wastewater treatment facility. If the level of turbidity and total suspended solids (TSS) is too high, UV can be rendered ineffective. Levels above 30 mg/L are considered too high. Another consideration is cost. Standard chlorination disinfection is less expensive.

Applicability

• Three critical factors come into consideration when deciding whether to use a UV disinfection system.
  The reactor must possess enough radial mixing to ensure all the organisms are properly exposed to the UV.
  The intensity of the UV radiation deteriorates over time. The quartz sleeve in a contact system can also get dirty due to particulates in the wastewater, causing a reduction in UV strength reaching the organisms.
  The waste water composition affects the effectiveness of the UV radiation. As stated above, a higher concentration of TSS increases the amount of UV radiation needed to neutralize the organisms.

Operation and Maintenance

• A proper maintenance program is necessary to ensure a UV system works with maximum efficiency.
  The surfaces between the lamps and the wastewater must stay clean. Common cleansers include citric acid, vinegar solutions and sodium hydrosulfite.
  System components must be replaced on a regular schedule: lamps every 12,000 working hours, ballasts every 10 years and quartz sleeves (in contact systems) every five years.

Costs

• Even though UV disinfection is more expensive than chlorination, when dechlorination is included, UV disinfection is competitively priced. The EPA states that costs have also decreased due to improvements in lamp and system design.