Wavelength Effects in Photochemical Oxidation of Organic Pollutants in Waste Water C. Y. Cha and J. M. Smith1 University of California, Davis, CA 95616
Kinetic measurements in a batch recycle reactor show that the rate of photooxidation of organic pollutants in secondary effluents increases severalfold as the wavelength of radiation is reduced from >2537A to 2537A. The same reduction in wavelength increased the rate of disappearance of free available chlorine only 404/, in a Sacramento area secondaiy effluent. The late of lemoval of pollutants depended only on the radiation absorbed by pollutants, while the chlorine rate was a function of radiation absorbed by both pollutants and chlorine.
U l t r a v i o l e t radiation has been found (Schorr et al., 1971) to accelerate the air oxidation of organic pollutants in secondary effluents from municipal waste water treatment plants. Using the same polychromatic radiation from a high-pressure mercury lamp, Ilaiicil and Smith (1971) observed an order of magnitude increase in rate of pollutant removal when chlorine was added as a sensitizer in concent,rations of a few mg/liter. From conversion measurement's, llleiners (1970) observed a n increase in conversion when the shorter wavelength radiation of a low-pressure mercury lamp was used. Since the electrical energy requirement would be a major cost in a commercial process, it is important to obtain as high a rate of reaction, or quantum yield, as possible. -iccordingly, rates have been measured using a low-pressure lamp in the same flow reactor apparatus as employed earlier (Hancil and S'mith, 1971). The data at the lower wavelength provided additional insight into the rate of disappearance of free available chlorine, and these results are also reported. Chemical Characteristics of Secondary Effluent-Chlorine Mixtures
Chlorine in water may be present as free available chlorine (as HOC1 or C10-, or both) or as combined available chlorine (chloramines and ot'her chloro derivatives). Combined available chlorine is negligible in distilled water but finite in secondary effluent after adding chlorine. Hancil and Smith (1971) showed that only free available chlorine contributes to the removal of organic carbon from secondary effluent. The initial chlorine demand of a secondary effluent is the difference between the chlorine added and the sum of free and combined available chlorine. Chlorine can react with ammonia and ions present, such as ferrous, manganous, nitrate, sulfide, and sulfite. Hence, the chlorine demand depends on the composition of the secondary effluent, amount of chlorine added, reaction time,. pH, and temperature. Severtheless, the free available chlorine appears to be a linear function of the total chlorine added, ( & I ) $ . For example, the relationship for a n effluent from the Howe .%venue secondary treatment plant (Sacramento, CA) , containing 1.23 mg/l. of X H B (as N) and 6.3 mg/l. of total organic carbon (TOC), is =
1.38 X
+ 1.52 Ccl
(g-mol/cma)
To whom correspondence should be addressed
(1)
The pH of this effluent was about 7.2 and remained constant during photochemical treatment because large amounts of bicarbonates served as a buffer. Both chlorine and pollutants absorb uv radiation, and the rate of disappearance of both depends on this absorption. The rate of energy absorption (per unit reactor volume) for the reactor used is
It is convenient to introduce the term A , which is the ratio of energy absorbed to intensity Ztot a t the reactor wall, as indicated in Equation 2. Foro the high:pressure lamp the summation extends from 22248 to :358A, with most' of th,e radiation a t 4358, 3660, and 31308, and 13y0 a t
