should at least be combined with a biological treatment step for removal of AOC/BDOC (Chapter 3).
Nowadays, ozonation is mostly used for treatment of surface waters and is integrated into multi-barrier treatment systems (Kruithof & Masschelein, 1999). The development of the integration of ozonation into water treatment trains is shown schematically in Figure 5.1(a) for the evolution of lake water treatment in Switzerland from the 1950s to 2005.
This is representative for the development of treatment trains in other industrialised countries as well.
Figure 5.1(b) shows the evolution of the phosphate concentration in Lake Zurich which is an indicator of the degree of eutrophication with a peak in the early 1970s. Drinking water treatment had to follow this development (additional treatment steps) to cope with the various problems related to eutrophication (turbidity, high DOC concentrations, taste and odour issues, etc.). The phosphate concentration in Swiss lakes was reduced by rigorous measures in water pollution control (phosphate elimination in wastewater treatment plants, phosphate ban from textile washing detergents). In the 1950s, lake water treatment started with conventional treatment (sand filtration followed by chlorination) which was supplemented with a pre-chlorination followed by a flocculation process. The introduction of activated carbon was a consequence of increasing taste and odour problems but in other contexts also a barrier against micropollutants which started to emerge in the 1970s. In the mid-1970s, the discovery of trihalomethanes and the formation of chloro- and bromophenols [potent taste and odour compounds (Acero et al., 2005)] which are formed during chlorination was a motivation for moving away from chlorination to ozonation. First, an intermediate ozonation was introduced followed by biological activated carbon filtration (see also Chapter 3). Then, pre-chlorination was replaced by pre-ozonation.
The combination of ozone with biological activated carbon filtration is also known as the Mülheim process which was developed in 1974 (Sontheimeret al., 1978; Heilker, 1979). Mülheim is one of the cities in the most densely populated industrial area in Germany, the Ruhr area (ca. 4 million people), and draws its water from the river Ruhr. A rigorous and efficient treatment scheme was necessary to provide high-quality drinking water to the population (Figure 5.2 and discussion below).
Table 5.1 Estimated number of drinking water plants using ozone in Europe, North America and Japan (numbers from period 1997−2011)
Country Number
of plants
Number of plants per million capita
References
Switzerland 108 13.8 von Gunten & Salhi, 2003
France 700 10.6 Langlaiset al., 1991
Canada 68 2 Larocque, 1999
Germany .100 1.2 Böhme, 1999; Loebet al., 2011
United Kingdom 50 0.8 Lowndes, 1999
BENELUX ca. 20 0.72 Kruithof & Masschelein, 1999
USA 200 0.64 Rice, 1999
Japan .50 .0.39 Loebet al., 2011
When lake water quality in Switzerland improved in the 2000s and membranes became affordable for drinking water treatment, treatment trains were simplified including a membrane filtration step (ultrafiltration, UF). Hence, treatment schemes could be reduced to three steps, always including the combination of ozone with biological activated carbon filtration and membranes. This combination guarantees a high drinking water quality with respect to hygiene, aesthetic properties and chemical contaminants and allows a distribution of drinking water without residual disinfectants such as chlorine, chloramine and chlorine dioxide. This has the advantage that no disinfection by-products are formed in the distribution system (Sedlak & von Gunten, 2011).
Figure 5.1 (a) Evolution of lake water treatment for Lake Zurich water from 1950 to 2005. (b) Evolution of the phosphate concentration as an indicator for the trophic state of the lake. According to von Gunten, 2008, with permission.
As mentioned above, the implementation of the Mülheim process in 1974 was a considerable break-through in chlorine-free drinking water treatment. In a first step, the water passes through a slow sand filter. Thereby, suspended particles are retained and part of the organic matter is consumed by microbial processes. Subsequent ozonation oxidises micropollutants and transforms part of the remaining DOM to AOC/BDOC (Chapter 3), which leads to a further reduction of DOC in the following biofiltration step with multi-layer filters containing activated carbon (AC). The water is then UV-disinfected prior to distribution. In case of emergency, chlorine or chlorine dioxide dosing is possible.
Since the 1990s, membrane filtration, in particular UF, has become an interesting alternative to deep bed sand filtration processes. UF is an efficient barrier against micro-organisms (viruses, bacteria and protozoa) but does not retain organic micropollutants (Jacangeloet al., 1997). Therefore, a combination of UF with ozone oxidation and adsorption processes leads to a drinking water with good hygienic and chemical qualities.
Figure 5.3 shows a conventional process combination including ozonation and deep bed filtration processes and two possible process combinations including UF, ozonation and AC filtration (Pronk &
Kaiser, 2008).
All three process combinations are currently used for the treatment of Lake Zurich water in Switzerland.
Combination C may require a final disinfection with UV, because AC filters lose significant numbers of micro-organisms. In a pilot study with combination B, the total cell count determined by flow cytometry was ≈103cells/mL after ozonation and .105cells/mL after AC filtration (cf. Figure 5.4) (Hammeset al., 2008).
In the AC filter, bacteria can grow on AOC/BDOC, which leads to a significant increase in the total cell count (Figure 5.4). In combination B which is reflected in Figure 5.4, bacteria are removed by UF to below detection limit of flow cytometry, whereas in combination C, where AC filtration is the last treatment step, they would be released into the distribution system if not properly disinfected.
Figure 5.2 The Mülheim process with the characteristic combination of ozonation and biological filtration.
With permission of RWW Rheinisch-Westfälische Wasserwerksgesellschaft mbH.
Figure 5.4 Total cell concentration determined with flow cytometry as a function of the treatment step for process combination B in Figure 5.3. According to Hammeset al., 2008, with permission.
Figure 5.3 Conventional multi-barrier treatment with ozonation (Combination A) and two possible alternative process combinations (B and C) including ozonation and ultrafiltration. Adapted from Pronk & Kaiser, 2008, with permission.
5.3 MICROPOLLUTANTS IN WATER RESOURCES, DRINKING