OZONATION OF NATURAL WATERS
4.3 Ozone-consuming substances and surrogates
The complexity and variety of the natural water constituents play a great role towards the seemingly unpredictable nature of water characteristics. Since it is not practical to analyse for every compound present in natural water. the studies ofozonation of natural waters always included many analytical data.
Yet il is less often discussed which info""ation should be considered as the key parameters (Hoigne, 1994).
Hoigne (1994) proposed a list of data (see Table 4-1) as standards to be reported when disclosing the results of an ozonation experiment.
Although this list was well·intended to provide a common basis for comparison purpose with other ozonation studies, in practice this lisl is nol always complete due to experimental constraints and time.
Most investigators omit the parameters whose effects on the type of water tested are negligible.
Table 4-1: Minimal data set to report in olonation study (Hoigne, 1994) No. Data name
effect of temperature 2 effect of pH
3 effect of alkalinity
4 dissolved organic material (DaM) as dissolved organic carbon (DOq
5 UV absorbance at ). - 255 nm 6 nitrite. iron and manganese 7 chloride
8 bromide 9 ammonia 10 turbidity 11 chlorine residual 12 hydrogen peroxide
13 cllaracterisation of the lifetime of dosed Olone 14 further parameters to be included
The exact nature of DeS in natural waters is variable and cannot be easily detennined. Natural waters contain varying concentration of numerous organic and inorganic compounds (Bablon et al., 1991). Table 4-1 lists Ihe ozone kinetic rate constants of selected organic and inorganic compounds. following Eqn. (4-5) as the rate expression. It is evident that the rate constants encompass a wide spectrum of values.
4-4
Chapter 4 Ozonation of natural waters
Table 4-2: Second-order kinetic rate constant of selected organic and inorganic compounds
LimitfRange ko,
Substance M·I
.,
·1 ReferenceINORGANIC
NO;: T=22°C 3.7x\Os Hoigne et al. (1985)
SO~' T=22°C I.O±O.I x
to
9 Hoigne et al. (1985)8,- T=20DC 160±20 Hoigrie et al. (1985)
Cl- T=22°C <3xlO'} Hoigne et al. (1985)
CI02/HCI02 T=20°C 1.0±0.1 x
to
1 Hoigne et al. (1985) Fe2• T=22°C > 5x 105 Hoigne et al. (1985)H2S T=20°C J±2 x 104 Hoigne et al. (\985)
HOCI T=20°C < 0.002 Hoigne and Bader (1983b)
HS- T=20°C 1.l±0.4 xlO· Hoigne et al. (1985)
T=25°C _ 2x 109 Garland et al. (1981)
Mnh pH = 5.5 to 7.0 3X\0)102xI04 Reckhow et al. (1991)
NH, T=25°C 20±1 Hoigne and Bader (1978)
ORGANIC
benzene pH"" 1.7 to 3.0 2.0±0A Hoigne and Bader (1983a)
toluene pH"" 1.7 14±3 Hoigne and Bader (1983a)
phenols 105 to 109
t
Bablon et al. (I991b)IOs to 109
t
tetrachloroethylene pH = 2.0 < 0.1 Hoigne and Bader (1983a)
ethanol pH = 2.0 O.37±0.4 Hoigne and Bader (1983a)
acetic acid pH = 2.5 to 5.0 < 3x 10.5 Hoigne and Bader(1983b) atrazine pH=3.4;T=24°C 2.6±0.4 x 109 Haag and Yao (1992)
acetone pH =2 0.032±0.006 Hoigne and Bader(1983a)
glucose eH - 2 0.045±D.05 Hoisne and Bader ~ 1983a~
Notes:
t for dissociated or protonated fonn t for non-dissociated fonn
While the rate constants for the reactions of most inorganic species are known, il is difficult to assess the ozone stability in natural waters due to the unknown effect of natural organic matter (NOM) (van Gunten, 2003). NOM is a term used to describe the complex matrix of organic material present in natural waters (Owen et aI., 1995). In particular. surface and ground waters contain substantial organic matter that may be detrimental 10 the water quality (Camel and Bermond, 1998). Many investigators have examined the effects of NOM on the ozonation performance (Owen el al., 1995: Westerhoffet al., 1998; Park et al., 2000:
Yavich and Masten. 2001). In other words, NOM has commonly been chosen to represent the amount of OCS. Although in principle, any of the compounds in Table 4-2 and many more in literature can be
4-5
Chapter 4 Ozonauon of nllual waters
considered as a representative compound for oes, the choice of such compound will depend on the aim of ozonation and the availablc analytical mcthods/equipment.
In the water treatment process, the NOM present in natural walers is generally transfonned into less recalcitrant compounds or removed through chemical oxidalion. The influences of NOM include Ihe biological re-growth in the distribution network, colour, taste and odour as well as the precursors 10 the DBP fonnation.
NOM can be divided into humic and non-humic fractions: the humic fraction is more hydrophobic in character and consists of humic and fulvic acids, whereas the non-humic fraction is less hydrophobic in character and comprises hydrophilic acids, proteins, amino acids and carbohydrates (Owen et al., 1995;
Swietlik et al., 2004). The presence of humic substances is a potential concern as several functional groups (e.g.: R-OH, R-COOH or R-NH1) have strong reactivities with halogens, complexation with metals and association with organic micro-pollutants (Westerhoff et aI., 1998; Bablon et aI., 1991a; Yavich and Masten, 2001; Swietlik et al., 2004). This may, for example, lead to the formation ofTHMs upon the final chlorination.
The reduction or transformation of NOM has been evaluated using various surrogates including (but not limited to) the apparent molecular weight distributions, TOC, DOC, UV absorbance at 254 nrn, THMFP or other DBP formation potentials (Pryor and Freese, 2000; Owen et al., 1995).
The humic fraction is generally less soluble and of higher apparent molecular weight than the non-humic fraction (Singer and Harrington, 1993) although the cut-off molecular weight differentiating the two fractions is not well-defined. The typical experimental approach is to fractionate NOM into a number of fractions according to their molecular weights and observe the shift in fractions due to ozonation.
The DOM is the portion of the total organic load in dissolved form. The dissolved and the total organic materials can be represented by measuring DOC and TOC respectively, where almost 90% of TOe is in dissolved fonn (Bablon et al., 1991). The UV absorbance characteristics due to molecular size or functional groups also render itself as an indicator for the dissolved organic material. There are often linear correlations between DOe values ofa water and the UV absorbances at 254 nm (Eaton et aI., 1995;
Hoigne. 1994) (Hoigne (1994) stated 255 nm; however, the value has been updated to 254 nm in Standard Methods.). The relative effect ofozonalion on NOM can be observed from the trend ofDOC, TOe or UV absorbance at 254 nm.
The measure of the DBPs or the DBP fonnation potential is another possible surrogate to assess the effect of ozonation on NOM. This method is targeted at specific functional groups of NOM especially where an ozone residual is present prior to chlorination or distribution network (Camel and Bermond, 1998).
The surrogate for NOM (and hence OCS) was chosen to be TOC and UV absorbance at 254 nm for the experimental work to come. The choice was based on the relative simplicity of the respective analytical methods. The equipment was readily available at Darvill Wastewater Works for measuring TOe and UV
4·6
Chapter 4 Ozonation of natural waters
absorbance as it forms part of the routine analyses at Umgeni Water (Pryor et aI., 2002; Umgeni Water, 2003).