Figure 1.
UV inactivation of MS2 phage
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UV disinfection in Florida The Florida Administrative Code 62-600 provides rules for reuse and land application specifying secondary treatment and filtration for all reuse applications. Effluent quality is regulated for solids (<5mg/L) and turbidity (<2NTU). Non-restricted reuse applications require non-detectable fecal coliform/100mL in 75 percent of samples. Proposals for UV disinfection are evaluated on a site-by-site basis using the NWRI guidelines for UV Disinfection. |
In North America, states with high population densities and heavy irrigation demands are steadily depleting ground water supplies. In coastal areas these conditions often result in salt-water intrusion of aquifers. In Florida, heavy tropical rains, typical of Florida's semi tropical climate, are squandered due to runoff and evaporation. In contrast, much of California is arid and suffers low annual rainfall, depleted ground water supplies and periodic droughts. Reclaimed wastewater or recycled water is now viewed as a valuable resource that can be used to augment limited water supplies. States like these actively promote wastewater reclamation and have already implemented comprehensive regulations covering a range of reuse applications.
In the past decade, Florida reuse sites have increased dramatically from 118 to 444 plants, representing a total flow capacity of 826 million gallons per day (MGD). California presently has over 250 plants producing one billion gallons per day (BGD), with a projected increase of 160 sites over the next 20 years.
Wastewater reuse requires effective measures to protect public health and to ensure that the impact on the environment is sustainable. To prevent the transmission of waterborne diseases, disinfection of reclaimed water is controlled by stringent regulations. In North America, more than 22 states have adopted regulations pertaining to specific reuse applications. These regulations specify wastewater treatment processes, nutrient removal, final effluent quality and disinfection criteria based upon the specific reuse applications. As a rule, the resulting effluents have low turbidity - a cloudy condition in water stemming from suspended silt or organic matter - and suspended solids. For such results, ultraviolet (UV) technology can economically achieve the most stringent disinfection targets as required by the states of California and Florida for restricted and unrestricted reuse applications.
UV disinfection
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Figure 1. |
UV is a non-chemical disinfection technology for wastewater that can protect the public against pathogenic microorganisms including protozoa, bacteria and viruses.
Chlorination, traditionally used for wastewater disinfection, generally provides reliable results. However, the benefits of chlorine are outweighed by environmental concerns and safety issues, including:
Reuse definitions
Treated wastewater for reuse applications is also referred to as reclaimed or recycled water. Unrestricted or non-restricted water reuse is a term used by many states to describe a range of applications where human contact may occur. For these applications the highest quality effluent and most stringent disinfection are generally required. Restricted reuse refers to applications where the risk of human exposure is slight, as in the case of drip irrigation of tree plantations.
UV disinfection guidelines - California reuse requirements
Figure 2. |
The State of California Wastewater Reclamation Criteria (Title 22) for unrestricted reuse requires primary treatment, secondary biological treatment, coagulation and filtration followed by chlorination/dechlorination.
Acceptance of UV disinfection technology in California was preceded by extensive pilot and full-scale studies that established:
This research confirmed the equivalency of UV and chlorine for inactivation of coliforms and formed the basis of the 1993 National Water Research Institute's (NWRI) "UV Disinfection Guidelines for Wastewater Reclamation in California".
The NWRI guidelines for unrestricted reuse effluents require a turbidity of <2 nephelometric turbidity units (NTU), TC <2.2/100 milliliters (mL) and a 4-log inactivation of poliovirus. The operational average UV dose of 140 mWs/cm2 is based on continuous monitoring of lamp intensity, UV transmittance and flow rate. Design specifications include reactor chambers, lamp orientation and a minimum number of UV banks, monitoring with alarm systems and system redundancy. The California Department of Health Services adopted the NWRI Guidelines specifying design and performance of UV systems.Wastewater reuse requires effective measures to protect public health and to ensure that the impact on the environment is sustainable.
Currently the NWRI Guidelines are being revised and expanded to include test protocols for new UV technologies and also to account for the improved effluent qualities produced by process technologies such as reverse osmosis and membrane filtration.
Effectiveness against viruses
Figure
3. |
UV systems in reuse applications
Figure 4. Flow capacity for reuse plants |
UV has been used for reuse disinfection since 1987. Presently, UV systems treat more than 500 MGD produced by 80 reuse sites in 12 states. The majority of the UV installations are based on conventional low-pressure lamp technology. The lamps are arranged horizontally in open channels. Larger plants have medium-pressure UV lamp systems. These high intensity lamps can supply the required high UV doses using 90 percent fewer lamps than a low-pressure system. The significantly reduced space requirements combined with an automatic cleaning system make this technology suitable for the delivery of high UV doses in large flow reuse plants (Figure 3). Many of the reuse sites process comparatively lower flows. Figure 4 illustrates the range of flow volumes treated by UV. The disinfection criteria are site specific, ranging from non-detectable FC to <200 FC/100mL and <2.2 TC to 23 TC/100mL. As suggested by the variation in disinfection targets, these sites represent a variety of restricted and non-restricted applications, ranging from deep well injection to irrigation of landscapes, golf courses and crops.
All of the sites filter before disinfection, however only 71 percent of the plants are required to monitor and achieve turbidity levels <2NTU. The regulated discharge permits for allowed solids levels showed the widest variation ranging from <5 mg/L to <30 mg/L (Figure 5). For many plants, these allowed TSS levels are inconsistent with the requirement to meet the low turbidity levels. Filtration, often required for reuse effluents, decreases the solids, particle size and number and ensures a consistent effluent quality. These factors provide reliable disinfection performance and a decreased UV dose demand.
Figure 5. Discharge permits for TSS |
UV transmittance, a measure of UV dose demand, is a parameter used to size UV systems. The transmittance of reuse effluents after coagulation and traditional filtration is often higher (60-75 percent) than the range observed in secondary effluents (55-65 percent). A more significant increase (80-90 percent) is observed after reverse osmosis or membrane filtration.
The UV design dose for installations with stringent coliform limits averages 100 mWs/cm2 (range 60-170). Applications with fecal coliform limits of 25 to 200/100mL have design doses ranging from 29 to 60 mWs/cm2.
Conclusions
In North America, urban centers are experiencing decreasing ground water tables, land subsidence, saltwater intrusion and chemical pollution. Reuse of wastewater, now recognized as an ecological and economic necessity, is practiced in many states.
Since 1987, ultraviolet radiation has been successfully used to disinfect reuse effluents. UV is a non-chemical disinfection technology for wastewater that can protect the public against pathogenic microorganisms including protozoa, bacteria and viruses. As an alternative to chemical disinfection, UV does not produce harmful by-products and is non-toxic to the environment.
G. Sakamoto is a senior biologist at Trojan Technologies Inc., London, Ontario, Canada. She can be reaced via e-mail at gsakamoto@trojanuv.com
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