Table 7.7Reductions of bacteria, viruses and protozoa achieved by water treatment technologies at drinking-water treatment plants for large communities

Treatment processEnteric pathogen groupMinimum removal (LRV)Maximum removal (LRV)Notes
Pretreatment
Roughing filtersBacteria0.22.3Depends on filter medium, coagulant
Storage reservoirsBacteria0.72.2Residence time > 40 days
Protozoa1.42.3Residence time 160 days
Bank filtrationViruses> 2.18.3Depends on travel distance, soil type, pumping rate, pH, ionic strength
Bacteria2> 6
Protozoa> 1> 2
Coagulation, flocculation and sedimentation
Conventional clarificationViruses0.13.4Depends on coagulation conditions
Bacteria0.22
Protozoa12
High-rate clarificationProtozoa> 22.8Depends on use of appropriate blanket polymer
Dissolved air flotationProtozoa0.62.6Depends on coagulant dose
Lime softeningViruses24Depends on pH and settling time
Bacteria14
Protozoa02
Filtration
Granular high-rate filtrationViruses03.5Depends on filter media and coagulation pretreatment; filtered water turbidity of ≤ 0.3 NTU in 95% of samples (and none to exceed 1 NTU) associated with 1–2 log reduction of viruses and 3 log reduction of Cryptosporidiuma
Bacteria0.24.4
Protozoa0.43.3
Slow sand filtrationViruses0.254Depends on presence of schmutzdecke, grain size, flow rate, operating conditions (mainly temperature, pH); filtered water turbidity of ≤ 1NTU in 95% of samples (and none to exceed 5 NTU) associated with 1–2 log reduction of viruses and 2.5–3 log reduction of Cryptosporidiuma
Bacteria26
Protozoa0.3> 5
Precoat filtrationViruses11.7If filter cake is present
Bacteria0.22.3Depends on chemical pretreatment
Protozoa36.7Depends on media grade and filtration rate
Membrane filtration: microfiltration, ultrafiltration, nanofiltration, reverse osmosisViruses< 1> 6.5Varies with membrane pore size (microfilters, ultrafilters, nanofilters and reverse osmosis filters), integrity of filter medium and filter seals, and resistance to chemical and biological (“grow-through”) degradation; maximum reductions associated with filtered water turbidity of < 0.1 NTUa
Bacteria1> 7
Protozoa2.3> 7
Primary disinfectionb,c
ChlorineViruses2 (Ct99 2–30 min·mg/l; 0–10 °C; pH 7–9)Free chlorine × contact time predicts efficacy; not effective against Cryptosporidium oocysts. Turbidity and chlorine-demanding solutes inhibit this process; hence, turbidity should be kept below 1 NTU to support effective disinfection. Where this is not practical, turbidities should be kept below 5 NTU with higher chlorine doses or contact times.a In addition to initial disinfection, the benefits of maintaining free chlorine residuals throughout distribution systems at or above 0.2 mg/l should be considered
Bacteria2 (Ct99 0.04–0.08 min·mg/l; 5 °C; pH 6-7)
Protozoa2 (Ct99 25–245 min·mg/l; 0–25 °C; pH 7–8; mainly Giardia)
Chlorine dioxideViruses2 (Ct99 2–30 min·mg/l; 0–10 °C; pH 7–9)
Bacteria2 (Ct99 0.02–0.3 min·mg/l; 15–25 °C; pH 6.5–7)
Protozoa2 (Ct99 100 min·mg/l)
OzoneViruses2 (Ct99 0.006–0.2 min·mg/l)Viruses generally more resistant than bacteria
Bacteria2 (Ct99 0.02 min·mg/l)
Protozoa2 (Ct99 0.5–40 min·mg/l)Depends on temperature; Cryptosporidium varies widely
UVViruses4 (7–186 mJ/cm2)Effectiveness of disinfection depends on delivered fluence (dose), which varies with intensity, exposure time and UV wavelength. Excessive turbidity and certain dissolved species inhibit this process; hence, turbidity should be kept below 1 NTU to support effective disinfection. Where this is not practical, turbidities should be kept below 5 NTU with higher fluencesa
Bacteria4 (0.65–230 mJ/cm2)
Protozoa4 (< 1–60 mJ/cm2)

Ct, product of disinfectant concentration and contact time; LRV, log10 reduction value

a

See Turbidity: Information for regulators and operators of water supplies (Annex 1)

b

Chemical disinfection: Ct values are given that achieve 2 LRV.

c

UV irradiation: UV dose range is given that achieves 4 LRV.

Sources: Chevrefils et al. (2006); Dullemont et al. (2006); Hijnen, Beerendonk & Medema (2006); see also the supporting document Water treatment and pathogen control (Annex 1).

From: 7, Microbial aspects

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Guidelines for drinking-water quality: Fourth edition incorporating the first and second addenda [Internet].
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