UV Irradiation vs. Chemical Disinfections
Whichever way you look at it, bacteria are a problem that must be controlled. Is ultraviolet irradiation a suitable replacement for chemical disinfection in industrial water treatment?
Power generation is still heavily dependent on the steam turbine. Even gas turbine power stations usually are provided with a waste heat boiler and steam turbine to maximize the efficiency with which fuel is converted into electricity.
Steam turbines operate in the Rankine thermodynamic cycle: Work is put into the system to pressurize water to boiler pressure. Then, heat is put in to evaporate the water to produce high pressure steam. The high pressure steam then is expanded through the turbine to obtain a useful work output. Finally, heat must be rejected to condense the low pressure steam back to water. This takes place in a condenser, which usually is water cooled. The cooling water may be seawater (if the power station is on the coast) or fresh water from a river, borehole or town supply. Turbines designed with fresh water cooling can have a single-pass, once-through system design with direct discharge. Alternatives include single-pass designs with evaporative cooling prior to discharge or a design where the cooling water is recycled via evaporative cooling towers.
The condenser cooling water is heated up, and this gives rise to scale and corrosion problems, depending on the source of the water and its chemistry. One thing you can be certain of is that wherever the water came from, it will contain microbiological contaminants, principally bacteria. Cooling systems provide an ideal temperature (typically 68 to 86°F [20 to 30°C]) for the growth of mesophilic bacteria. Some — for instance, as Legionella pneumophila, the causative agent of Legionnaires’ disease, and Pseudomonas aeruginosa — are pathogenic and present a public health risk. Most, however, are just troublesome.
Cooling towers are efficient at aerating the water passing through them, and such systems generally support aerobic bacteria. However, cooling systems are complex, and there often are dead-legs that can become oxygen depleted, allowing facultative and anaerobic species to thrive. Once established, the bacteria proliferate, and many excrete extracellular polymeric substances (EPS) that stick the cells together to form adherent slimes or biofilms. The slimes and biofilms foul the heat exchange surfaces and cooling tower packing, resulting in a loss of heat transfer efficiency. In extreme conditions, they may form a biomass that can completely block pipework.
Bacteria also are a major cause of microbiologically induced corrosion (MIC) in cooling water systems (figure 1). Some, like Ferrobacillus ferrooxidans, actually consume iron directly. Others, like the anaerobic sulfate-reducing Desulfovibrio desulfuricans, generate corrosive chemical conditions in the water. Biofilms prevent oxygen from getting to metal surfaces, setting up local differential aeration cells that result in pitting corrosion.
Whichever way you look at it, bacteria are a problem that must be controlled.
The obvious solution is disinfection. The biocide of first choice usually is an oxidizing chemical such as chlorine, chlorine dioxide or bromine. Chlorine gas dissolves in water to form hypochlorite ions, and these can be simply and economically produced in once-through seawater systems by electrolysis (electrochlorination).
There is, however, a problem with chemical disinfection: It does not kill all of the bacteria. Natural genetic variation in bacterial cells means that some are naturally immune to specific chemicals. Bacteria multiply at alarming rate — given the right conditions, they will double in number every 20 minutes — and this makes them fiendishly adaptive. In a short time, the resistant cell strain will become dominant, and the biocide becomes ineffective.
Changing to an alternative biocide is the obvious solution, but this leads to escalation in this war of attrition. The bacteria become resistant to the next biocide, necessitating another alternative. There are many chemicals that can be used — quaternary ammonium compounds (quats), glutaraldehyde, acrolein — but they are toxic. They also present health and safety issues related to storage and handling. Their use also limits the disposal routes for cooling tower bleed-off.
UV Radiation for Disinfection of Heat Transfer Surfaces
Ultraviolet radiation in the UV-C band has a wavelength around 250 nm. This is is close to the absorbance wavelength of the amino acid bases that form the rungs of the DNA double-helix. UV radiation fuses adjacent amino acid groups, making it impossible for the molecule to replicate and permanently damaging the thymine strand of the DNA helix. This means that bacteria exposed to UV radiation are not actually killed, but they cannot reproduce and cannot evolve to produce a resistant strain. As far as heat transfer disinfection is concerned, by blocking the bacteria’s ability to duplicate, UV irradiation is as effective as killing the bacteria for heat transfer disinfection.
UV is a broad-spectrum disinfectant that inactivates a range of microorganisms. This includes many that, like cryptosporidium and mycobacterium, are resistant to chlorine. It even inactivates mussel larvae, which can cause blockage in cooling water intakes.
The UV radiation technique is used in applications for pharmaceutical and healthcare water systems, food and drink manufacturing, and drinking water treatment. It is chemical free and does not create health and safety or environmental issues. All that is required is an electricity supply. The water flows through a reaction chamber that houses lamps that generate UV-C radiation. The hydraulic design of the reaction chamber ensures that all the water is exposed to equal intensity of radiation without short circuiting.
A single pass through a standard UV system (figure 2) delivering a UV dose of 40 mJ/cm2 typically will achieve better than a 4 Log reduction (99.99 percent) of microorganisms, including 17 known chlorine-resistant microorganisms such as the biofilm-forming bacteria Mycobacterium Intracellular. This performance can be validated by type testing using independent third-party bioassay with live, surrogate microorganisms (bacteriophage T1 and MS2) against U.S. EPA protocols.
Of course, UV radiation, like chemical disinfectants, is only effective if it is delivered to the microorganisms at the right dose. Impurities in the water can absorb UV radiation, and suspended particles can shield bacteria from the UV light. It is important, therefore, to measure the UV transmittance of the water; that is, measure the amount of UV energy that passes through a 1 cm pathlength of the water. It usually is reported as a percentage, so a 95 percent transmittance means that over the pathlength, 5 percent of the energy has been lost. This allows the UV system to be rated with a set capacity and biological log reduction performance to compensate for this loss.
As a chemical-free technology, UV disinfection provides a much needed solution for cooling applications where water is used in a single pass before being discharged into the environment. This is particularly common in coastal-based power stations. Due to increasing regulations such as the Priority Substances Directive and other industrial regulations such as HOCNF, residual chemicals such as chlorine and biocides such as glutaraldehyde can no longer be discharged into the environment, necessitating expensive treatment such as dechlorination. As a chemical-free solution, UV does not require the removal of residual chemical or toxic compounds. In addition, UV does not produce any disinfection byproducts, making it a holistic, environmentally friendly solution.
A well-designed UV system (figure 3) will incorporate:
- A hydraulically optimized UV reactor design to increase reactor efficiency.
- CFD-optimized lamp positioning to ensure UV intensity fields from individual lamps overlap to irradiate potential dead spots.
- UV probes to monitor the UV intensity (UV dose) delivered inside the reactor.
- Real-time measurement of flow rate and transmittance (UVT).
In addition, using a specially developed algorithm and local control panel (microprocessor or PLC), the UV system will vary the power output (100 percent to 40 percent power) to suit both flow rate and inlet water quality. This ensures the UV system operates efficiently at the optimum power setting, extending the life of UV lamps and lowering the required operational power.
In conclusion, experience in cooling system operation has demonstrated that UV disinfection is a cost-effective method of controlling biofilms. Effective against Legionella, the disinfection method is a compact, chemical-free, environmentally friendly process with minimal health and safety risk and little operator involvement.