UV Disinfection Process

A UV disinfection system transfers electromagnetic energy from a mercury arc lamp to an organism’s genetic material, the chromosomes which contain DNA and RNA. When UV radiation penetrates the cell wall of an organism, it destroys the cell’s ability to reproduce. In the disinfection process, UV radiation, generated by an electrical discharge through mercury vapor, penetrates the genetic material of microorganisms and causes molecular rearrangements that retard their ability to reproduce. Das gave a detailed review of the mechanisms of germicidal action, how does UV light works, how UV‐damaged DNA is repaired, and the effects of wastewater quality parameters on disinfection efficiency (Das 2004).

Table 6.10 Summary of results of brief exposures of fish to residual chlorine.

Species	Effect endpoint	Time	Measured residual chlorine concentration (mg/l)
Chinook salmon	First death	2.2 h	0.25
Brook trout	Median mortality	90 min	0.5
Brook trout	Mean survival time	9 h	0.35
Brook trout	Mean survival time	18 h	0.08
Brook trout	Mean survival time	48 h	0.04
Brown trout	Depressed activity	24 h	0.005
Rainbow trout	Slight avoidance	10 min	0.001
Rainbow trout	Lethal	2 h	0.3
Fingerling Rainbow trout	Lethal	4–5 h	0.25
Trout fry	Lethal	Instantly	0.3
Yellow perch	TL50a	1 h	<0.88
Yellow perch	TL50	12 h	0.494
Smallmouth bass	Median mortality	15 h	0.5
White sucker	Lethal	30–60 min	1.0
Largemouth bass	TL50	1 h	<0.74
Largemouth bass	TL50	12 h	0.365
Fathead minnow	TL50	1 h	<0.79
Fathead minnow	TL50	12 h	0.26
Miscellaneous	Initial kill	15 min	0.28
Miscellaneous	Erratic swimming	6 min	0.09

^a TL₅₀, median tolerance limit (50% survival) (Brungs 1973).

UV disinfection systems are not mass‐produced. Because their efficiency strongly depends on effluent characteristics that act to decrease the UV intensity in wastewater, each application must have a custom‐designed system. Table 6.11 shows the major parameters that must be taken into consideration when a UV disinfection system is being designed for wastewater. In comparison to chlorination/dechlorination, UV disinfection offers a reduction of potential chlorinated hydrocarbons (including potential carcinogens) in the receiving waters, as well as considerably greater safety to wastewater treatment plant operators and to nearby populated areas.

UV Transmission or Absorbance

UV light’s ability to penetrate wastewater is measured in a spectrophotometer at the same wavelength (254 nm) that is produced by germicidal lamps. This measurement is called the percent transmission or absorbance and it is a function of all the factors that absorb or reflect UV light. As the percent transmission gets lower (higher absorbance) the ability of the UV light to penetrate the wastewater and reach the target organisms decreases.

It cannot be estimated by simply looking at a sample of wastewater with the naked eye. The range of effective transmittances (T) will vary depending on the secondary treatment systems. In general, suspended growth‐treatment processes produce effluent with T varying from 60 to 65%. Fixed film processes range from 50 to 55% T and lagoons 35–40% T. Industries that influence UV transmittance include textile, printing, pulp and paper, food processing, meat and poultry processing, photo developing, and chemical manufacturing. A discussion on other major parameters affecting the UV disinfection efficiency of wastewater is given by Das (2004).

Disinfection Standards

The level of disinfection required to obtain an EPA National Pollutant Discharge Elimination System (NPDES) permit is commonly less than 200 fecal coliform unit per 100 ml as a 30‐day geometric mean. In general, a UV dose of 20–30 mW·s/cm² is required to achieve this level of disinfection in secondary‐treated wastewater with a 65% transmittance and total suspended solids (TSS) below 20 mg/l. The UV dose requirement to meet specific limits depends on the nature of the particle with respect to numbers, size, and composition. Therefore, UV dose requirements will vary (Table 6.11). A more stringent limit (<2.2 total coliform units per 100 ml) is required for water reuse in California and Hawaii. In such cases, filtered effluents with TSS of 2 mg/l or less and 65% transmittance may require UV dose as high as 120 mW·s/cm² to achieve this level of disinfection. The concentrations of solids, bacteria in the particles, and the particle size distribution are the main limiting factors in the design of systems that must meet stringent disinfection limits. It appears that the UV dose required to achieve the traditional coliform limits will achieve better virus inactivation than the comparable chlorine dose. Figure 6.11, which illustrates the relative doses of UV and chlorine required to inactivate selected organisms compared to fecal coliform indicator, also shows that the chlorine doses required to inactivate most organisms are much higher than comparable UV doses that would achieve the same level of disinfection (Das 2002, 2004; Trojan Technologies Inc. 2010).

Table 6.11 Major parameters affecting the UV disinfection of wastewater.

Parameters	Acceptable values/conditions
Percent transmittance (T) or absorbance	35–65
Total suspended solids (TSS) mg/L	5–10
Particle size distribution (PSD) μm	10–40
Flow rate or hydraulics design	Ideal plug flow
Iron (mg/l)	<0.3
Hardness (mg/l)	<300

Operation, Maintenance, and Worker Safety

The proper operation and maintenance (O&M) of a UV disinfection system ensures that sufficient UV radiation is transmitted to the organisms to render them sterile. All surfaces between the UV radiation and the target organisms must be cleaned, and the ballasts, lamps, and reactor must be functioning at peak efficiency. Inadequate cleaning is one of the most common causes of a UV system’s ineffectiveness. In all cases, the quartz sleeves or Teflon tubes need to be cleaned regularly by mechanical wipers, ultrasound equipment, or chemicals. The cleaning frequency is very site‐specific: some systems needing to be cleaned more often than others.

Chemical cleaning is most commonly done with citric acid. Other cleaning agents include mild vinegar solutions and sodium hydrosulfite. A combination of cleaning agents should be tested to find the agent most suitable for the wastewater characteristics without producing harmful or toxic by‐products.

UV is generated on‐site and poses no significant safety concerns to surrounding communities. Worker safety requirements are directed to protection from exposure (primarily of the eyes to skin) from UV light, as well as to strict monitoring of electrical hazards and safe handling and disposal of expended lamps, quartz, and ballasts (US EPA 1996; US EPA 1998).

Costs

Specifically, consideration must be given to how environmental matters affect our economic thinking and, conversely, how economic decisions affect the environment. Cost considerations are an integral part of the decision‐making process at the stage of identifying potential improvements to a process, a product, or an activity.

Estimated capital costs for 1, 10, and 50 million gal per day (mgd) (1 mgd = 3785 m³/day) plants are $150 000, $700 000, and $3 000 000 respectively. O&M costs are estimated to be 2.6, 1.4, and 0.8 US cents/1000 gal (or 3.785 m³), respectively, at an average dose of 6 mg/l. These increase to 4.0, 3.0, and 1.5 US cents/1000 gal at a dose of 10 mg/l.

Chemical costs of chlorine gas and hypochlorite vary considerably depending upon the locality, demand, and availability. Current prices for Cl₂ are varying in the range: $0.0425–0.06/lb (or 454 g) for 90 T (or ~90 000 kg) tank cars; $0.10/lb for 55 T (or ~55 000 kg) rail cars; $0.25–0.275/lb for 1 T (or ~1 000 kg) cylinders; and $0.50–0.55/lb for 150 lb (or ~68 kg) cylinders. Liquid sodium hypochlorite (12.5%) prices quoted were $0.50–1.75/gal.

A brief cost analysis for both chlorination/dechlorination and UV disinfection processes treating a wastewater treatment facility is presented in Figure 6.12, capable of processing 18 mgd. These costs can vary considerably depending on the locality, demands, and supply. The annualized cost values for alternatives were based on the design average plant flow of 18 mgd over a 20‐year period and 5.5% interest rate (F. Soroushian, personal communication; Temmer et al. 2000).

Figure 6.12 indicates that, although the construction cost for chlorination system is considerably less than the UV, annualized costs are about the same for both systems. There is no more real economic benefit for pursuing chlorination disinfection system.