Residual Chlorine
Free chlorine residual is an indication of the efficiency of the disinfection process and is thus a rapid indicator of the probable microbiological safety or otherwise of the treated water.
Effect and possible implications of failure
- Indirect health
Absence of residual chlorine means either that the water was not treated with chlorine, or that insufficient chlorine was used to successfully disinfect the water. Where the untreated water contains pathogenic micro organisms, the absence of free residual chlorine indicates that there is a risk of microbiological infection. In distribution systems, the free chlorine residual protects against secondary contamination. However, if the concentration of chlorine is too high then irritation of mucous membranes, nausea and vomiting may occur.
SANS 241 Standards
- SANS 241 Table C.3 (Operational water quality alert values) Column 3, Alert level (Upper Limit): <0.5 mg/L
Possible reason/s for failure
- No chlorination process at the treatment plant (e.g. no disinfection, use of UV or ozone for disinfection)
- No chemicals for chlorination (e.g. chlorine gas, hypochlorite, etc not available)
- Poor process control (e.g. ineffective chlorine dosages, no monitoring and remedial intervention, problem with process control/SCADA system)
- Long retention times (e.g. in reservoirs, distribution network)
- Contamination (e.g. infiltration or seepage)
- Lack of maintenance (e.g. broken chlorine dosers)
- Poor design (e.g. poor mixing, retention time not optimised)
- Sabotage/vandalism
Turbidity
The turbidity is a measure of the suspended particles or degree of cloudiness of a water.
Effect and possible implications of failure
- Aesthetic
- Operational
- Indirect health
Although the consumption of turbid water per se does not have any direct health effects, high turbidities imply a high concentration of suspended particles. These particles can shield bacteria and other micro-organisms from the disinfecting properties of, for example, chlorine resulting in ineffective disinfection.
SANS 241 Standards
- SANS 241 Table C.3 (Operational water quality alert values) Column 3, Alert Level: 5 NTU
Possible reason/s for failure
- No filtration at the treatment plant (e.g. no slow sand or rapid filters)
- Poor process control (e.g. inappropriate filter runs, no monitoring and remedial intervention, problem with process control/SCADA system)
- Contamination (e.g. from pipe breaks and bursts, from repairs to network, infiltration or seepage)
- Lack of maintenance (e.g. filters not backwashed, reservoirs and pipes not cleaned/flushed, foreign objects not removed from open reservoirs)
- Poor design (e.g. inappropriate filter for water type and quality, open reservoirs, dead ends in network)
- Sabotage/vandalism