Closed-Vessel vs. Open Channel UV Disinfection?
UV Disinfection systems have become much more advanced in recent years, but should you adopt a closed-vessel or open channel disinfection technique?
Ultraviolet disinfection is now a standard feature in most wastewater treatment 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 favoured 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 cellis rendered non- viable, the organism quickly dies. The difference in UV system efficiency from the various UV manufacturers was made transparent with the advent of UV system validation using Bioassay techniques.
Above: 800W Closed Vessel Installation v.s. Open Channel.
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 analysing 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 overlooked 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 through 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. A persuasive case can be made to put the UV system for wastewater disinfection into a closed pipe. This ensures optimised hydraulics and keeps the operators from exposure to the wastewater. atg Evoqua has reactors up to 30 inches in diameter, designed specifically for wastewater disinfection.
CLOSED VESSEL ADVANTAGES
– Solutions for wastewater, water-reuse and drinking water
– Medium pressure and amalgam lamp technology
– Validations up to 5,000 m3/hr per single system
– Small footprint required
– Vertical or horizontal installation
– Access hatch on WW and water-reuse reactors
– Reduced/eliminated exposure to UV and effluent
– Minimised components = less maintenance
– Installed inline in the pipe vs. constructed open channel
TYPICAL DISADVANTAGES OF OPEN CHANNEL
Dead Zones – Dead zones or spaces can be formed within the channel which leads to short-circuiting and untreated water.
Poor Hydraulics – 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.
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: head loss 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 – This 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 head loss will impact upstream processes, and can breach the channel walls.
Large open water surfaces – This can lead to fly and mosquito nuisance, and 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. Counterweighted 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.
Above: Associated problems with open channel UV Systems.
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. This is labour intensive, expensive and slow. Typically open channel systems are being covered over, so grating and significant lengths of safety rails are also required.
CAPEX Advantages of Closed Vessel UV Treatment
The reduced number of lamps, quartz and reduced footprint of the closed vessel design, will considerably reduce the CAPEX (capital expenditure) costs of a project. The ‘end-feed’ closed vessel chamber design removes the requirement for large civil structures, whilst the high output 800 Watt amalgam UV lamps provide a significantly increased treatment capacity.
- UV chambers install directly into the pipe
- No requirement for concrete trenches
- No requirement for large civil structures
- No requirement for penstocks
- No requirement for level control
- Improved hydraulic performance
- Simple to install indoors or outdoors
- 100% Duty & 100% standby operation easily achieved
- Design for retrofitting into redundant open channels
- Significantly reduced installation footprint
OPEX Advantages of Closed Vessel UV Treatment
The 800 Watt Amalgam design offers the highest UV output with the fewest number of lamps, in the smallest footprint currently available in the UV market (low-pressure Amalgam systems). Typically, operational costs including power, lamps, quartz and maintenance can be 15% – 20% less when compared to traditional open channel systems.
- Reduced power consumption
- Increased disinfection efficiency
- Extended lamp life of 16,000 hours
- Significantly fewer lamps and quartz & ballasts
- Increased electronic ballast life (new modern ballast design)
- Significantly reduced maintenance time
- Automatic self-cleaning reduces required maintenance cycles
- Exclusive atg UV Datastream service reduces the required number of site visits
- Significant reduction in corrosion and structural damages
- Improved health & safety – no open water sources, or risk of UV exposure