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 favored due to its non-chemical nature, the fact that no subsequent de-chlorination process is required and the ability to be unselective in disinfection performance. Many consulting engineers which 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 will quickly die. The difference in UV system efficiency from the various 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 (bio dosimeter) 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 3rd party.
When 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 be able 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 UV Technology has reactors up to 30 inches in diameter which have been designed specifically for wastewater disinfection.
The advantages of Closed Vessel are:
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 cause by poor hydraulics. Density currents can be created which 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 and these lead to the formation of eddy currents which create uneven velocity profiles, which then 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 loose tools that are dropped into the water. Sunlight causes algae to grow which stimulates an enzyme that can repair DNA damage caused by the UV system. This phenomenon is called photo-repair.
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.
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.
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.
The key benefits are:
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.
