Factibilidad Económica

- Wastewater Chlorination

Over the past century, chlorination has evolved as the most commonly used method of disinfecting both water and wastewater.

The extensive use of chlorine has come about as a result of its

bactericidal effectiveness, ease of application, relatively low cost, and relatively persistent residual (Aieta, 1980). It has not been until the past decade, however, that its possible

detrimental effects have been considered. Two of the most

prevalent areas of concern pertain to the formation and discharge of potentially carcinogenic halogenated organic compounds which may impact downstream water users, and the toxic effects of chlorine and chlorinated effluents on aquatic organisms. It is

for these and other reasons that the indiscriminate practice of

wastewater chlorination for the purpose of disinfection is

presently undergoing review in the United States.

Prior to the 1970's, regulations governing wastewater

disinfection were at the discretion of the individual states.

Some states required either year-round disinfection or no

disinfection at all, whereas other states allowed either seasonal

disinfection or mandated disinfection on a case-by-case basis.

The implementation of mandatory effluent disinfection

drastically changed in August 1973 when the Federal Government,

through the USEPA, assumed regulatory control with the passage of

Public Law 92-500, the Federal Water Pollution Control Act

(FWPCA). The vast majority of wastewater treatment plants could

not meet the imposed maximum concentration of 2000 fecal

coliforms per 100 mL and were thus forced to employ a separate

disinfection step in the treatment process. Because chlorination

was the cheapest and most common disinfection technique available

at that time, the USEPA had in effect mandated the chlorination

of all wastewater discharges in the United States (Singer, Brown,

and Wiseman, 1988).

Following the enactment of the FWPCA, concern over the

impact of chlorinated effluents on receiving waters led to

initiation of considerable research. Thus by 1976, in response

to these concerns, the USEPA dropped the fecal coliform

limitations from the FWPCA. The states, therefore, reassumed the

responsibility for setting and enforcing their own water quality

regulations (Singer, Brown, and Wiseman, 1988). - Chemistry of Chlorine

When chlorine gas (C12) is added to water, two reactions occur: hydrolysis and ionization. In the hydrolysis reaction,

hypochlorous acid (HOC!) is formed. During the ionization

reaction, the hypochlorous acid will partially dissociate to form

the hypochlorite ion (0C1-). Chlorine existing as either HOCl or OCl- is referred to as free chlorine or free available chlorine

(FAC). The two reactions and the corresponding equilibrium

constants (at 25 degrees Celsius) are given below (Tchobanoglous and Schroeder, 1985). Hydrolysis reaction: -4 C12 + H20 = HOCl + H+ + CI- Kh = 4.5 X 10 Ionization reaction: -8 HOCl = H+ + OCl- Ka = 3.7 x 10

At 25 degrees Celsius and pH = 7.43, the activities of HOCl

and OCl- are equal. At pH values below 7.43, HOCl predominates,

whereas at pH values above 7.43, OCl- is the predominant species.

This pH relationship is significant as far as disinfecting

ability is concerned. It is reported that HOCl is approximately 80 to 100 times more effective at killing Escherichia coli than OCl- (Snoeyink and Jenkins, 1980).

Chlorine is a relatively strong oxidant and will

consequently react with various reduced chemical species present in water and wastewater. These reduced chemical species include ammonia (NH3), sulfide (HS-), and a vast array of organic

in a reduced level of disinfecting effectiveness.

In dilute ammonia solutions, hypochlorous acid (HOCl) reacts with ammonia (NH3) to form monochloramine (NH2C1),

dichloramine (NHC12), and trichloramine (nitrogen trichloride) (NC13). In addition, chlorine reacts with organic nitrogen

compounds to form organic chloramines. These species (inorganic and organic chloramines) are referred to collectively as combined chlorine or combined available chlorine (CAC). Inorganic

chloramines are much less effective than FAC as a disinfectant, but their disinfectant capability is more persistent (Brungs, 1973). In contrast, most organic chloramines possess little or no germicidal power although they titrate as combined chlorine in the iodometric and DPD procedures (Singer, Brown, and Wiseman,

1988). In fact, some of these species titrate as FAC, giving false and misleading FAC residuals.

- Chlorine Toxicity

Prior to 1970, little consideration was given to any

possible deleterious effects that might accompany the discharge of chlorinated wastewater to an aquatic system. Thereafter, early investigators of chlorine toxicity focused their attention

on the relative toxicities of free and combined chlorine.

Duodoroff and Katz in 1950, and Merkens in 1958 determined that FAC was more toxic and acted more rapidly than combined chlorine. However, they also both concluded that the toxicity of each class was probably of the same order of magnitude. In 1984, Wolf came

to similar conclusions and stated that a measure of total

residual chlorine would be sufficient to express the relative toxicity of a wastewater (Singer, Brown, and Wiseman, 1988). However, due to the presence of residual ammonia and organic

nitrogen in wastewater, and since chlorination is seldom carried to the point necessary to produce free chlorine, the residual chlorine typically exists in a combined state. (Brungs, 1972). Therefore, the major source of chlorine to which freshwater organisms are exposed to in wastewater effluent is residual

combined chlorine.

A search through the pertinent literature has shown that residual combined chlorine is extremely toxic even in dilute

concentrations. Chloramine concentrations of a few tenths of a

mg/1 are lethal to warm water fish (sunfish, bullheads, minnows). In addition, average concentrations of 0.16 mg/1 to 0.21 mg/1 residual combined chlorine caused complete kills of fathead minnows (Zillich, 1972). Daohnia magna. one of the more sensitive invertebrate species, died at a residual chlorine concentration (defined by Brungs as the summation of free chlorine, dichloramine and monochloramine) of 0.014 mg/1 and

acceptable reproduction occurred at 0.003 mg/1 and below (Brungs, 1973). Brungs goes on to report that for continuous

chlorination, the total residual chlorine concentration should not exceed 0.002 mg/1 (this should protect most aquatic

organisms). For intermittent chlorination, the total residual chlorine concentration should not exceed 0.04 mg/1 for a period of 2-hrs/day (this should protect most species of fish). Table

chlorine, dichloramine and monochloramine) for selected test

organisms (Brungs, 1973).

Table 3.2

Residual Chlorine Levels Toxic to Aquatic Life

Species Tested Yellow perch Largemouth bass Fathead minnow Rainbow trout Black bullhead Fathead minnow Golden shiner Fathead minnow Scud Daphnia magna Measured Residual Chlorine Cone (mg/1) 0.494 12-hr LC50 0.365 12-hr LC50 0.26 12-hr LC50 0.14 - 0.29 96-hr LC50 0.099 96-hr LC50 0.05 - 0.16 96-hr LC50 0.19 96-hr LC50 0.0165 Safe Cone 0.012 - 0.0034 Safe Cone 0.003 Safe Cone

It appears that the principal toxicant in most municipal secondary wastewater treatment plant effluent is residual

chlorine (Paller, et al, 1983). A number of field investigations support this statement (Zillieh, 1972; Esvelt, Kaufman and

Selleek, 1973; Environmental Research Laboratory, 1975; Bellanca and Baily, 1977; Ward and Degraeve, 1980).

From the results of the five field investigations previously referenced, it can be concluded that total residual chlorine in

final effluent, even at low levels, is extremely toxic. However, dechlorination, with either sodium bisulfite or sulfur dioxide,

can significantly reduce chlorine-induced toxicity. As a result, some nearby Atlantic Coast States (e.g., Maryland and Virginia) have enacted dechlorination policies.

Municipal wastewater treatment plants in the State of

Virginia are required to meet fecal coliform counts equal to or 33

less than 200 colony forming units (CPUs) per mL. If chlorine is

utilized as the disinfectant, dischargers are required to

maintain a minimum chlorine residual (after 30 minutes of contact

time) of 1 mg/1. However, the maximum allowable chlorine

residual in the receiving stream (for freshwater streams) is

0.011 mg/1. Dischargers failing to meet this maximum allowable

limit must dechlorinate. In addition, Virginia does not allow

chlorinated or dechlorinated final effluent discharged into its

trout streams. Consequently, dischargers must use an alternative

disinfectant (e.g., ultra-violet light, ozone).

Maryland requires its discharges into shellfish waters to

meet fecal coliform counts less than or equal to 14 CFUs/ml, and

200 CFUs/ml for all other waters. Maryland is even more

stringent than Virginia in that all municipal WWTPs are required

to dechlorinate. However, a chlorine residual prior to

dechlorination is not specified.