Ultra violet disinfection is now a standard feature in most waste water disinfection systems. UV has also been adopted by the drinking water community, as a barrier against the chlorine tolerant species such as Cryptosporidium and Giardia. The technology is widely favored due to its non- chemical nature, the fact that no subsequent de-chlorination process is required, and its ability to be unselective in disinfection performance.
Many consulting engineers that routinely incorporate UV technology into their treatment streams overlook the progress that has been made in recent years to understand the impact that hydraulics have on UV system performance, and continue to place UV lamps in open channels. A more efficient approach is to contain the waste stream in a pipe and disinfect the fluid in a closed vessel.
UV light works by causing permanent damage to the DNA found in all living species. Once the DNA becomes damaged, or dimerized, the organism is unable to carry out the routine cell functions of respiration, the assimilation of food, and replication. Once the cell is rendered non- viable the organism quickly dies. The difference in efficiency of UV systems from different UV manufacturers was made transparent with the advent of UV system validation using Bioassay techniques.
A bioassay involves the introduction of a non- pathogenic organism (biodosimeter) into the fluid stream before the UV system. The entire procedure is performed under controlled conditions, and each of the system variables: flow, transmittance, power loads and lamp intensity are carefully recorded, as samples are taken pre and post the UV system. Once the sample data is returned from the analyzing laboratory the actual system ability to disinfect can be compared to the manufacturer's claims. Of course such bioassays should be carried out under the auspices of a credible third party.
As bioassay validations became the standard, water treatment engineers started to notice how water hydraulics play a vital and often over looked role in system performance. In essence if a UV system design allows short circuits, or poor turbulence, then the water will receive differing degrees of UV dose. In extreme cases, the water can short circuit straight thru a UV system, rendering it grossly inefficient.
Most UV systems need to cope with a variety of flow rates, and usually an operating flow range is considered when designing the UV system.
Typical problems associated with open channel UV systems
- Erratic or reduced inactivation performance caused by poor hydraulics - Density currents can be created that cause the incoming wastewater to flow along the top or bottom of the lamp banks, resulting in short circuits, and poor disinfection. Often the entry and exit conditions are inappropriate; these lead to the formation of eddy currents that create uneven velocity profiles, which lead to short circuits.
- Dead zones or spaces can be formed within the channel which leads to short circuiting.
- Flow straighteners can introduce new problems - it is not unusual for a submerged perforated diffuser to have an open area of less than 20% of the cross sectional area of the open channel: headloss and overflow problems can then exist. Sometimes corner filets are needed to direct the flow back towards the lamps in rectangular shaped channels.
- Undersized channel width and depth can create very high velocities, and so reduce the residence time required for adequate UV dose delivery - This can be made worse if the open channel is designed for average dry weather flows, and not peak wet weather flows, at which time the headloss will impact upstream processes, and can breach the channel walls.
- Large open water surfaces - This can lead to fly and mosquito nuisance, as well as cause corrosion of electrical components due to the elevated humidity. Operators routinely lose tools that get dropped into the water. Sunlight causes algae to grow, and also stimulates an enzyme that can repair DNA damage caused by the UV system. This phenomenon is called photo-repair. Inhalation risk for operators from aerosols containing pathogenic organisms is little understood.
- Level Control is vital, but fragile - The level of the fluid in the channel must be carefully controlled: this can be achieved by a sliding gate mechanism; however these are prone to blocking. Counter weighted gate systems require frequent hinge lubrication, and often struggle to meet height tolerances.
- Safety considerations - UV light will burn exposed skin in seconds, causing erythema (sunburn). Burns to the inside of the eyeball, sometimes called arc eye or welding flash are extremely painful and can lead to retinal lesions, cataracts and yellowing of the lens on prolonged exposure.
- Cold weather maintenance considerations - Design engineers often don't consider that whilst the waste-stream will not freeze, the air above the waste-stream can be well below freezing. Consequently UV racks that are removed for maintenance in colder climates (such as northern US states or Canada) will freeze in the frigid air, making maintenance time consuming and uncomfortable for operators.
- Construction risk and overall cost - Open channel systems require precision alignment of floor and walls of the channel. This is labor intensive, expensive and slow. Typically open channel systems are being covered over, so grating and significant lengths of safety rails are also required.
A persuasive case can be made to put the UV system for waste water disinfection into a closed pipe. This ensures optimised hydraulics, and keeps the operators from exposure to the wastewater. atg UV Technology have reactors up to 30 inches in diameter designed specifically for waste water disinfection.