In the past decade, concerns over chlorine toxicity and protection of fish and wildlife have led to a dramatic growth in the practice of dechlorination to remove all or part of the chlorine residual and halogenated organics remaining after chlorination. Dechlorination also reduces or eliminates toxicity harmful to aquatic life in receiving waters.
Sulfur dioxide gas successively removes free chlorine, monochloramine, dichloramine, nitrogen trichloride, and polychlorinated compounds. When sulfur dioxide is added to wastewater, the following reactions occur:
(6.3)
(6.4)
(6.6)
(6.7)
(combined chlorine) (6.8)
For the overall reaction between sulfur dioxide and chlorine (Eq. 6.5), the stoichiometric weight ratio of sulfur dioxide to chlorine is 0.9 : 1. In practice, it has been found that about 1.0 mg/l of sulfur dioxide will accomplish for the dechlorination of 1.0 mg/l of chlorine residue (expressed as Cl2). Because the reactions of sulfur dioxide with chlorine and chloramines are nearly instantaneous, contact time is not usually a factor and contact chambers are not used; however, rapid and positive mixing at the point of application is an absolute requirement.
The ratio of free chlorine to the total combined chlorine residual before dechlorination determines whether the dechlorination process is partial or proceeds to completion. If the ratio is less than 85%, it can be assumed that significant organic nitrogen is present and that it will interfere with the free residual chlorine process (Metcalf and Eddy 2003).
Since dechlorination removes most of the TRC from disinfected wastewaters, it reduces the toxicity of disinfected wastewater effluent to aquatic wildlife. In most situations, sulfur dioxide dechlorination is a very reliable unit process in wastewater treatment, provided the precision of the combined chlorine residual monitoring service is adequate. Excess sulfur dioxide dosages should be avoided not only because of the chemical wastage but also because of the oxygen demand (BOD and COD) exerted by the excess sulfur dioxide.
Limitations
Chlorination/dechlorination is more complex to operate and maintain than chlorination alone. Major difficulties are the inability to measure residual SO2 and problems in the continuous measurement of a zero or low chlorine residual. Many halogenated organics are also rapidly formed upon chlorine addition and are unaffected by application of SO2. Heltz and Nweke who examined many plants, reported that the amount of residual chlorine in dechlorinated effluents considerably exceeded EPA criteria for receiving waters (Heltz and Nweke 1995).
Environmental Impact
Sulfuric acid and hydrochloric acid are products of SO2 dechlorination in small amounts but are generally neutralized in the wastewater. Based on laboratory experiments, residuals of sulfite dechlorination are at least three orders of magnitude less toxic than chlorine or ozone.
No cases of sulfur compounds affecting dissolved oxygen consumption or pH change in receiving waters or in dechlorinated effluents have been reported. In pilot studies, no significant oxygen depletion occurred until sulfur dioxide overdoses exceeded 50 mg/l. It is not uncommon, however, to find plants with post aeration after dechlorination to assure that dissolved oxygen requirements are met. Dechlorination with SO2 would significantly reduce toxicity due to chlorination disinfection process.
Sulfur dioxide, which is used for dechlorination purposes, is stored on site in small quantities. The maximum anticipated storage inventory is quite small, and there need be no concerns about threats to the health of workers or of residents of nearby populated areas.
Effects of Chlorine on Aquatic Life
The potential environmental and chemical effects of chlorine toxicity on 14 aquatic species are summarized in Table 6.9. Rainbow trout was the most sensitive of the species tested, followed by the golden shiner and three‐spined stickleback. A calculated chlorine residual of 0.03 mg/l, based on dilution of a measured concentration of 2.0 mg/l, reduced plankton photosynthesis by more than 20% of the value obtained with a dilution of effluent having no chlorine residual. Dechlorination with sodium bisulfite also eliminated chlorine‐related toxicity. Dechlorination with sulfur dioxide also greatly reduced the acute and chronic toxicity to fish and invertebrate species.
In considering these data, it should be borne in mind that the toxicity of chlorine wastes in rivers depends not on the amount of chlorine added but on the concentration of chlorine remaining in solution (Merkens 1958). The toxicity of this residual chlorine will depend on its composition (i.e. the relative proportions of free chlorine and chloramines). Arriving at this ratio, which in turn depends on at least five other variables, is a complex undertaking. Although free chlorine is more toxic than chloramines, research that has stood since the 1950s suggests that the difference between the toxicity to fish of free chlorine and chloramine is not very great (Dondoroff and Katz 1950; Merkens 1958; Hart 1973; Kozloff 1974).
Table 6.9 Select summary of acute and chronic toxic effects of residual chlorine on aquatic life.
| Species | Effect endpoint | Measured residual chlorine concentration (mg/l) |
| Coho salmon | 7‐day TL50a | 0.083 |
| Pink salmon | 100% kill (1–2) | 0.08–0.10 |
| Coho salmon | 100% kill (1–2) | 0.13–0.20 |
| Pink salmon | Maximum nonlethal | 0.05 |
| Coho salmon | Maximum nonlethal | 0.05 |
| Brook trout | 7‐day TL50 | 0.083 |
| Brook trout | Absent in streams | 0.015 |
| Brown trout | Absent in streams | 0.015 |
| Brook trout | 67% lethality (4) | 0.01 |
| Brook trout | Depressed activity | 0.005 |
| Rainbow trout | 96‐h TL50 | 0.14–0.29 |
| Rainbow trout | 7‐day TL50 | 0.08 |
| Rainbow trout | Lethal (12) | 0.01 |
| Trout fry | Lethal (2) | 0.06 |
| Yellow perch | 7‐day TL50 | 0.205 |
| Largemouth bass | 7‐day TL50 | 0.261 |
| Smallmouth bass | Absent in streams | 0.1 |
| White sucker | 7‐day TL50 | 0.132 |
| Walleye | 7‐day TL50 | 0.15 |
| Black bullhead | 96‐h TL50 | 0.099 |
| Fathead minnow | 96‐h TL50 | 0.05–0.16 |
| Fathead minnow | 7‐day TL50 | 0.082–0.115 |
| Fathead minnow | Safe concentration | 0.0165 |
| Golden shiner | 96‐h TL50 | 0.19 |
| Fish species diversity | 50% reduction | 0.01 |
| Send | Safe concentration | 0.00.34 |
| Send | Safe concentration | 0.012 |
| Daphnia magna | Safe concentration | 0.003 |
| Protozoa | Lethal | 0.1 |
a TL50, median tolerance limit (50% survival) (Brungs 1973).
Results of Laboratory Bioassays
Several studies (Dondoroff and Katz 1950; Merkens 1958; Thatcher 1977) indicated that salmonids were the most sensitive fish species. Laboratory bioassays support this generalization. A residual chlorine concentration of 0.006 mg/l was lethal to trout fry in 2 days, and the 7‐day median tolerance limits, or TL50, for rainbow trout was 0.08 mg/l with an estimated median period of survival of 1 year at 0.004 mg/l (Merkens 1958). The maximum nonlethal (in 23 days) concentration of residual chlorine for pink and coho salmon in seawater was 0.05 mg/l (Holland 1960). Rainbow trout were killed at 0.01 mg/l in 12 days, and they avoided a concentration of 0.001 mg/l (Sprague and Drury 1969). Brook trout had a mean survival time of 9 hours at 0.35 mg/l, 18 hours at 0.08 mg/l, and 48 hours at 0.04 mg/l. Mortality was 67% after 4 days at 0.01 mg/l (Dandy 1967, 1972). Fifty percent of brown trout were killed at 0.02 mg/l within 10.5 hours and at 0.01 mg/l within 43.5 hours (Pike 1971). Trout, salmon, and some fish‐food organisms are more sensitive than warm‐water fish, snails, and crayfish (Tables 6.9 and 6.10).
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