Evaluating the Life Cycle Environmental Performance of Two Disinfection Technologies

With increasing emphasis on promoting a sustainable ecological future and concern over introducing toxic chemicals in water, disinfection process design is leaning toward technologies that destroy pathogens while balancing the effects of the disinfected wastewater aquatic biota or on a drinking water supply. Since ultraviolet (UV) irradiation is not a chemical additive, it does not leave or produce toxic by‐product in the wastewater, unlike traditional chlorination and dechlorination processes; therefore, the use of UV does not affect a drinking water supply or the aquatic biota in receiving waters.

The objective of this work is to present streamlined environmental (LCAs) between two competitive disinfection processes: chlorine vs. UV. This section discusses the use of LCA to quantify the environmental and public health benefits of UV disinfection technology as opposed to chlorination and dechlorination methods. LCA tools are used to evaluate the short‐term and long‐term environmental effects of both processes and to select the best sustainable process. Our approach applies environmental LCA to these disinfection processes, incorporating economic criteria and all aspects of the environment: chemical use, electricity use, and releases to water, air, and land.

The Challenge

With increasing emphasis on promoting sound ecological practices and concern over introducing toxic chemicals into water, designs for disinfection processes are increasingly leaning toward technologies that destroy pathogens while balancing the effects of this disinfected wastewater on aquatic biota or a drinking water supply.

In the United States and Canada, the use of ultraviolet light irradiation for the disinfection of wastewater has become the accepted alternative to chlorination or chlorination/dechlorination. There are several reasons for this move away from a proven technology. For example, because of current Uniform Fire Codes, containment requirements for volatile gases like chlorine, and public health concerns, municipalities are limited in the amounts of chlorine that can be stored in a water treatment plant. Moreover, the dechlorination process uses yet another chemical pollutant, sulfur dioxide, to remove chlorine from the effluent before discharged into the receiving water. Thus, and because chlorination/dechlorination of wastewater produces possible carcinogens in addition to destroying aquatic biota in the receiving waters, USEPAs started to look for an alternative wastewater disinfection system.

Various governments, municipalities (Das 2002, 2004; Das and Ekstrom 1999; Ecology 1998; LOTT 1994; USEPA 1986, 1992b), and corporations have sponsored research (Loge et al. 1996; Scheible 1987; Scheible and Forndran 1986; White 2010) that shows that the UV disinfection of wastewater was effective and economical. Another most important development was the parallel flow open channel modular UV system. This new design of the UV system for wastewater in the early 1980s opened up UV disinfection for both the retrofit market and new wastewater treatment plants. To promote a friendlier discharge to the marine environment, designers have begun to prefer alternative disinfection technologies, which emphasize sustainable and clean ecological disinfectants – such a clean technology is UV disinfection.

The Chlorination (Disinfection) Process

Despite the acknowledged advantages of disinfection by means of UV irradiation, however, chlorine continues to be the most widely used chemical for the disinfection of wastewater in the United States and elsewhere. The major advantages of chlorine over alternative disinfectants are cost‐effectiveness, reliability, and efficacy against a host of pathogenic organisms. We turn now to a detailed examination of the chlorination process.

When chlorine (Cl₂) is dissolved in freshwater, a mixture of hypochlorous acid (HOCl) and hydrochlorite ion (OCl⁻) is formed (Eq. 6.2). Chlorine exists predominantly as HOCl below pH 7.6 and as OCl⁻ above pH 7.6. HOCl and/or OCl⁻ is defined as free available chlorine, with the hypochlorous acid being the primary disinfectant.

(6.2)

Mono‐, di‐, and tri‐chloramines (NH₂Cl, NHCl₂, and NHCl₃) are formed when chlorine reacts with nitrogen present in secondary effluent in the form of ammonia. Municipal effluents usually contain all these forms of chlorine in some proportion and taken together they are known as “total residual chlorine” (TRC). Because saltwater contains bromide, the addition of chlorine to saltwater will also form hypobromous acid (HOBr), hypobromous ion (OBr⁻), and bromamines.

Chlorine is typically supplied as liquefied gas in cylinders. Chlorinators apply gaseous chlorine to a feed stream which is then injected into a mixing zone in the chlorine contact chambers. Initial mixing and effective contact times are essential for good process performance. Generally, contact periods of 15–30 minutes are required at peak flow (USEPA, 1985).