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News Release from: Hanovia | Subject: In-pipe UV disinfection of tertiary effluent
Edited by the Processingtalk Editorial
Team on 17 December 2003
In-pipe UV disinfection of tertiary
effluent
Chemical treatment of tertiary effluent is decreasing in popularity as it can produce unwanted disinfection by-products such as Tri-halo methanes, especially with highly organically-loaded effluent
Chemical treatment of tertiary effluent is decreasing in popularity as it can produce unwanted disinfection by-products such as Tri-halo methanes (THMs), especially with highly organically-loaded effluent Ultraviolet (UV) disinfection is finding growing acceptance as a viable, 'clean' alternative that effectively reduces pathogen levels in tertiary effluent prior to discharge
This article was originally published on Processingtalk on 24 Feb 2003 at 8.00am (UK)
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Types of UV systems.
UV disinfection systems are generally split into two distinct types: low pressure (LP) and medium pressure (MP).
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LP systems have a monochromatic UV output (limited to a single wavelength at 254nm), whereas MP systems have a polychromatic UV output (with an output between 240-310nm).
Many of the older UV installations use LP UV lamps to disinfect effluent in-channel.
While effective, these systems can require thousands of UV lamps and take up a great deal of space.
Many of these LP installations are now being replaced by smaller footprint, more compact and lower life-cost MP systems which can treat the waste stream in-pipe.
In addition, advances in monitoring and automated wiping of the MP systems allow the equipment to be remotely operated, with well-designed equipment lasting up to 12 months between servicing.
UV works by fusing Adenine and Thiamine molecules within a micro-organism DNA, rendering it unable to replicate.
DNA absorbs UV between 230-310nm, but it is generally accepted that peak absorbency occurs at 265nm.
LP lamps contain up to 150mg of mercury and are usually up to 1.5m in length.
The most powerful LP systems currently available are amalgam lamps that use amalgamated mercury and are available up to 280watts in power.
These lamps are efficient and can deliver up to 30 percent of useful UV output.
MP lamps usually contain less than 300mg of mercury and have a number of advantages over LP technology.
Firstly, they have a smaller footprint than LP lamps, allowing them to be utilised in many situations where larger, LP lamps cannot be used.
Secondly, MP lamps in use today have up to 10kW of power output, and although these lamps are generally less efficient than LP lamps they have, as mentioned above, a broader spectral output which can deliver up to 20 percent useful UV output - often a single medium-pressure lamp will treat a comparable amount of water that more than 30 conventional low-pressure lamps or up to 10 amalgam lamps would be required to treat.
Monitoring.
An important aspect of tertiary effluent that needs to be taken into account if UV treatment is to be effective is its transmissivity (its ability to transmit UV energy), which can vary dramatically.
Suspended or dissolved solids can absorb, scatter and shade the UV, affecting overall system performance.
Micro-organisms and organic contaminants need to be exposed to a specific intensity of UV light, for a given time, to ensure they are deactivated or reduced to harmless constituents.
Changes in transmittance can affect UV dose and reduce the effectiveness of the disinfection process, so an accurate transmittance monitoring system is vital.
An effective monitor should measure transmittance continually and feed the data to a control unit.
The Hanovia on-line transmittance monitor, for example, uses a number of monitor cameras situated at different, fixed distances from a common source.
The device continuously measures water transmittance, enabling the intensity of UV emitted by lamps in the disinfection chambers to be increased or decreased in response to changes in effluent quality.
In this way, the effluent is irradiated with the optimum UV dose at all times, regardless of its UV transmittance.
An additional benefit is energy saving, since the new system will prevent unnecessary overdosing with UV.
In-pipe vs in-channel UV treatment.
Effluent streams are either gravity fed or pumped - the larger installations are usually gravity fed and, historically, UV disinfection has normally been achieved in-channel, with thousands of UV lamps immersed horizontally, parallel to the fluid flow.
As treatment plants have become smaller and more numerous, it is now possible to contain the fluid stream in-pipe.
The increased velocities and non-laminar flows of pumped in-pipe flows are well suited to UV technology and, for all but the very largest flows, in-pipe treatment is therefore probably the most efficient and cost effective method.
In-pipe treatment is best achieved using a closed-vessel UV system.
The orientation of the inlet/outlet can be varied to suit specific site conditions, but the chambers should not be permitted to drain, so top-entry bottom-exit or bottom-inlet/outlet orientations should be avoided.
Some of the main advantages of treating effluent in-pipe rather than immersing lamps in an open channel are summarised as: Safety - operators cannot be exposed to UV, or to the risk of inhalation of effluent in aerosol.
In addition, automatic mechanical wipers keep the quartz sleeve which surrounds the UV lamp free from fouling, meaning that acids are not required.
Small footprint - in-pipe solutions are designed to be retrofitted between Cl2 contact tanks, with a footprint below 6m2.
In-channel systems need to have lamps arranged sequentially and can be more than 40m long to compensate for the laminar flow found in the channel.
Reliability - statistical failure of process plant is a function of the number of components used.
Ironically, most in-channel UV systems fail because of the failure of ancillary equipment such as level controllers, walkway interlocks or wiper over-loads.
Cost of Ownership - well designed equipment should not be cheap to procure and expensive to run; life cycle costs for an assembly with a high component count are higher than those for a system with a smaller number of components.
Automation also reduces the need for expensive operator interface.
Conclusion.
Several thousand municipal UV installations are now in use around the world, treating a wide variety of effluent streams.
As operators have become more sophisticated, plants more remote, and labour more expensive, UV equipment has been correspondingly improved, automated and generally simplified.
There are now over 100 in-pipe UV effluent treatment systems in use worldwide, treating over 100 million litres/day, and the number is growing rapidly.
Established in 1924 and based in Berkshire, England, Hanovia is one of the world's leading providers of UV technology for water, air and surface disinfection.
It is the only UV system supplier with its own R and D and lamp manufacturing facility and is therefore uniquely positioned to provide custom-made solutions for most disinfection problems.
Part of Halma plc, Hanovia has over 40, 000 UV systems installed in over 100 countries.
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