UV disinfection - an emerging technology.
That chlorine is well known as a toxic substance poses challenges to its safe use as a disinfectant in water and wastewater treatment plants. In addition to increased EPA attention on chlorine utilization, safety regulations such as the Uniform Fire Code and OSHA standards are expected to tighten. The Uniform Fire Code presents additional specific requirements for chlorine handling that would require construction of high-cost scrubber and containment systems at water and wastewater treatment plants using chlorine for disinfection.
More stringent disinfection and water quality requirements, coupled with the need to protect the receiving waters and their inhabitants, make research into alternative forms of disinfection increasingly important. Ultraviolet (UV) disinfection is an effective method for killing bacteria found in the water and wastewater. UV systems are relatively inexpensive to install, safe and easy to operate, and offer operating costs comparable to chlorination systems.
Chlorine's disadvantages revolve around four key concerns: the chemical's inability to inactivate certain pathogens, excessive toxicity to fish and other aquatic species, worker safety, and carcinogenic by-products.
Chlorine's ability to disinfect stems from its capacity to chemically interfere with the microorganisms' life functions. The solution of chlorine in the wastewater forms hypochlorous acid, and the chemical reaction of this acid with bacterial or vital cell structure inactivates the elements necessary to maintain life. Though chlorine's effectiveness as a germicidal oxidant has been well documented, new concerns have arisen about its ability to inactivate two microorganisms found in water supply sources: giardia lamblia and crypto-sporidum. At the same time, more sophisticated analytical equipment has led to identification of a new class of water contaminants resulting from the use of chlorine: disinfection by-products. The organic compounds that form when chlorine reacts with naturally occurring organic matter in the water or wastewater include monochloramine, dichloramine, or trichloramine (depending on pH), and trihalomethanes, known as THMs. THMs such as chloroform and bromo-form have been shown to be carcinogenic and are strictly regulated.
Chlorine in wastewater discharges is toxic not only to pathogens, but to aquatic species as well. As a result, strict limits on the concentrations of chlorine in the plant's effluent have been set, requiring wastewater utilities to dechlorinate treatment plant effluent before discharge, and adding to the costs for wastewater treatment. One common approach is to add sulfur dioxide to the wastewater stream, which reacts instantaneously with chlorine to produce a nontoxic compound. The chlorination/ dechlorination process using gaseous or liquid chemicals will increase the dissolved solids in the plant effluent, adversely impacting reuse of the plant effluent for groundwater recharge and irrigation of sensitive crops.
In addition to the negative environmental impacts, water and wastewater plant operators are also concerned about the safety of plant workers handling chlorine and other associated chemicals. Chlorine gas leaks could endanger employees and nearby residents. The potential leaks of sulfur dioxide gas used for dechlorination are also a concern where it can combine with moisture and air to form sulfuric acid. Chlorine releases can occur from a variety of sources including startup operations, maintenance, equipment malfunction, or component failure. Minor losses can be attributed to gasket failures and valve packing adjustment whereas major leaks result from container rupture, pipeline breaks, broken connections, and possible combustion due to exposure of the containers to excessive temperatures. Accidents at water and wastewater treatment plants have occurred primarily due to mishandling of chlorine containers or leaks attributed to faulty gaskets or connections. One of the more recent accidents occurred at & water treatment plant in Morristown, Tennessee. Approximately 3000 pounds of gas escaped, forming a chlorine cloud that was five miles long, one mile wide, and 30 ft thick, and forcing the evacuation of 4000 people. Several plant personnel and emergency response team members suffered respiratory irritation and chemical burns from the escaped gas.
The UV Alternative
The use of UV irradiation for disinfection emerged in the mid-1970s. Currently, ultraviolet disinfection systems are being widely considered for application to treated wastewaters for both new plants and for retrofitting existing plants in lieu of conventional chlorination facilities. The UV light alters the genetic material in the cells of microorganisms. Because the genetic material carries information for reproduction, damage of these substances effectively sterilizes the cells, leaving them unable to reproduce.
Typically, the UV disinfection facilities consist of UV lamp banks installed in disinfection channels, and an acid cleaning system. Low pressure mercury vapor lamps are currently used in most of the existing UV installations in the USA. Important factors in the effectiveness of UV disinfection include the UV dose, the residence time in the disinfection channels, and the clarity of the wastewater. Required UV dose, normally expressed in milliwatt-seconds per square centimeter (mWS/sq.cm), depends on wastewater characteristics and varies with the species of microorganisms. UV dose is directly proportional to exposure time and UV intensity within the disinfection channels. Flow through the UV treatment process dictates the exposure time and, thus, the required dose. UV lamps lose 30 to 40 percent of their intensity with age, and the surface of the lamp quartz sleeve develops film from constant contact with the wastewater. This film absorbs the UV light and lowers the actual UV irradiation intensity delivered to the water. The tubes must be cleaned regularly to remove this film. Required minimum UV dose, based on the results of recent pilot studies, was established at 120 mWS/sq.cm. UV disinfection system is typically designed to provide the minimum dose at the design peak flow rate after 8000 hours of lamp operation.
To ensure that all water receives adequate exposure to UV irradiation, the network of lamps provided is quite dense, with lamps spaced 3 in. on center. To ease cleaning and maintenance, a bridge crane is usually provided to allow transferring of an entire lamp set to a cleaning tank where a dilute acid solution and air agitation is provided for cleaning the lamp sets. With UV, only a few seconds of exposure of treated effluent are required resulting in a very small volume needed for the UV channels as compared to the contact time needed for chlorine basins. Short circuiting can occur due to improper system hydraulic design or high water level in the channels resulting in passage of the flow through the system without sufficient exposure to UV irradiation. To prevent hydraulic short circuiting, multiple UV lamp sets in series are typically provided in each disinfection channel. UV disinfection channels are designed with high length-to-width ratio, and flow is distributed equally among channels using automatic level control gates and water level indication devices. UV disinfection system redundancy is usually provided by installing an additional standby disinfection channel. Plant operators can activate the standby channel during maintenance and system cleaning.
Typical concerns with the operation and design of UV disinfection systems are suspended solids and absorption characteristics of the plant effluent. The impact of these parameters on UV system performance could be estimated in a pilot study. The pilot study would serve to generate performance data for effective process design as well as to demonstrate the effectiveness of this technology for the site-specific plant conditions. Usually, the pilot study encompasses a period of three to six months.
In wastewater treatment facilities where chlorination-dechlorination is required, UV systems are less expensive to install and their operating costs are comparable. A disinfection system in a typical 3.5 mgd plant is approximately $100,000 with UV, versus more than $200,000 for a chlorination-dechlorination system. The operating costs for both systems are comparable at $0.10 per gallon treated. Although UV disinfection is an energy-intensive process, several steps can be taken to control operating costs. Since the effectiveness of UV disinfection depends on the amount of suspended solids in the plant effluent, treatment and filtration prior to UV disinfection would ensure optimum energy efficiency.
Given the wastewater quality, public health, aquatic protection, and worker safety issues from the use of chlorine in water disinfection, many water and wastewater utilities are looking for alternatives to this traditional method. One option, fast becoming widely accepted in the treatment of wastewater is UV disinfection. The total annualized cost for UV irradiation systems and gaseous chlorine disinfection systems are comparable. Instituting more stringent future regulations (e.g. effluent toxicity limits, Uniform Fire Code, and air emission control) could escalate the cost of chlorination systems beyond the costs associated with UV disinfection. Therefore, the feasibility of UV disinfection is worth exploring in detail when planning on future plant expansion or upgrade.
NIKOLAY VOUTCHKOV is Senior Project Engineer, Malcom Pirnie Inc., Newport News, Virginia
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|Date:||Aug 1, 1995|
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