The Virus Problem in the Windhoek Waste Water Reclamation Project
137
The resistance of the infectious hepatitis (HI) virus appears to be quite high, if the free
residual chlorine of 0.4 ppm for 30 min at room temperature (Neefe et al 1947) is used as
a criterion of satisfactory disinfection, but this is not really appreciably higher than the
case of other hardy viruses, especially in the light of lack of evidence as to pH and
temperature of the chlorinated waters.
Inactivation of viruses in waste water by chlorination has been the subject of a number
of studies which considered the virucidal efficiency of chlorine in both water (Lothrop
and Sproul 1969, Weidenkopf 1958, Kelly and Sanderson 1958, Kelly and Sanderson
1960) and waste water (Burns and Sproul 1967, Shuval et al 1967, Marais et al 1967,
Warriner 1967, Lund 1964, 1965). However, none of these studies refers to the
concentration of HOC1, which is the disinfecting agent associated with chlorine necessary
to achieve inactivation of virus. It has been shown that the amount of hypochlorous acid
present in a solution of free residual chlorine is a function of pH. As the pH increased,
HOC1 is increasingly ionized to hypochlorite ion (OCX) which is at best a slow virucide.
Moreover HOC1 reacts rapidly with ammonia to form chloramines and with organic
compounds to form organic chloramines. In most waters little HOC1 occurs. Usually
ammonia and organic nitrogenous compounds are present in more than sufficient
quantities to react with all the HOC1 present. It therefore stands to reason, that, in order
to ensure virus inactivation, break-point chlorination should be applied to guarantee a
free available residual chlorine, subject always to the reservation of a low enough pH to
provide the required hypochlorous acid concentration. It must be emphasized that
irrespective of the amount of combined available chlorine present, the free chlorine
(HOC1 + OCP) will always be entirely responsible for the inactivation rate obtained where
both forms are actually present, because free residual chlorine gives the highest oxidation
potential (Lund 1965). According to Hart (1971), in many studies correlation between
free chlorine or even total chlorine and virus inactivation is attempted without serious
consideration of the temperature and pH; naturally, a wide variation of free chlorine
residuals for virus inactivation is reported. In order to compare results of some of these
studies free chlorine figures were recalculated by Hart to hypochlorous acid where pH
values were available (Table 1). The concentration range for inactivation of viruses
immediately narrows considerably and it can be seen that a concentration of 0.5 mg/1
HOC1 would inactivate viruses within 30 min.
"The latest European Standards of drinking water", issued by WHO Regional office
for Europe, recommends a concentration of 0.5 mg/1 free chlorine for 1 hr for the
inactivation of virus. No pH is specified; however, it is mentioned that a redox potential
of 650 mV will cause almost instantaneous inactivation of even high concentrations of
virus. Lund (1964, 1965) and Warriner (1967) have reported that a fair correlation
existed between the oxidation-reduction potential (ORP) of a wastewater being
chlorinated and enteric virus inactivation and there seems to be a correlation between
ORP and HOC1 concentration.
In the Windhoek waste water reclamation plant break-point chlorination with a
resultant free residual chlorine of 0.5 mg/1 is therefore the standard requirement for
disinfection.
To ensure disinfection, therefore, not only the required free available chlorine residual
must be obtained but vigilance over pH levels and turbidity must be maintained.
Epidemiology
No innovation such as wastewater reclamation for drinking purposes can omit to take
137
The resistance of the infectious hepatitis (HI) virus appears to be quite high, if the free
residual chlorine of 0.4 ppm for 30 min at room temperature (Neefe et al 1947) is used as
a criterion of satisfactory disinfection, but this is not really appreciably higher than the
case of other hardy viruses, especially in the light of lack of evidence as to pH and
temperature of the chlorinated waters.
Inactivation of viruses in waste water by chlorination has been the subject of a number
of studies which considered the virucidal efficiency of chlorine in both water (Lothrop
and Sproul 1969, Weidenkopf 1958, Kelly and Sanderson 1958, Kelly and Sanderson
1960) and waste water (Burns and Sproul 1967, Shuval et al 1967, Marais et al 1967,
Warriner 1967, Lund 1964, 1965). However, none of these studies refers to the
concentration of HOC1, which is the disinfecting agent associated with chlorine necessary
to achieve inactivation of virus. It has been shown that the amount of hypochlorous acid
present in a solution of free residual chlorine is a function of pH. As the pH increased,
HOC1 is increasingly ionized to hypochlorite ion (OCX) which is at best a slow virucide.
Moreover HOC1 reacts rapidly with ammonia to form chloramines and with organic
compounds to form organic chloramines. In most waters little HOC1 occurs. Usually
ammonia and organic nitrogenous compounds are present in more than sufficient
quantities to react with all the HOC1 present. It therefore stands to reason, that, in order
to ensure virus inactivation, break-point chlorination should be applied to guarantee a
free available residual chlorine, subject always to the reservation of a low enough pH to
provide the required hypochlorous acid concentration. It must be emphasized that
irrespective of the amount of combined available chlorine present, the free chlorine
(HOC1 + OCP) will always be entirely responsible for the inactivation rate obtained where
both forms are actually present, because free residual chlorine gives the highest oxidation
potential (Lund 1965). According to Hart (1971), in many studies correlation between
free chlorine or even total chlorine and virus inactivation is attempted without serious
consideration of the temperature and pH; naturally, a wide variation of free chlorine
residuals for virus inactivation is reported. In order to compare results of some of these
studies free chlorine figures were recalculated by Hart to hypochlorous acid where pH
values were available (Table 1). The concentration range for inactivation of viruses
immediately narrows considerably and it can be seen that a concentration of 0.5 mg/1
HOC1 would inactivate viruses within 30 min.
"The latest European Standards of drinking water", issued by WHO Regional office
for Europe, recommends a concentration of 0.5 mg/1 free chlorine for 1 hr for the
inactivation of virus. No pH is specified; however, it is mentioned that a redox potential
of 650 mV will cause almost instantaneous inactivation of even high concentrations of
virus. Lund (1964, 1965) and Warriner (1967) have reported that a fair correlation
existed between the oxidation-reduction potential (ORP) of a wastewater being
chlorinated and enteric virus inactivation and there seems to be a correlation between
ORP and HOC1 concentration.
In the Windhoek waste water reclamation plant break-point chlorination with a
resultant free residual chlorine of 0.5 mg/1 is therefore the standard requirement for
disinfection.
To ensure disinfection, therefore, not only the required free available chlorine residual
must be obtained but vigilance over pH levels and turbidity must be maintained.
Epidemiology
No innovation such as wastewater reclamation for drinking purposes can omit to take