Getting Technical
CHARLES NICOLSON
Charles Nicolson has a physics and chemistry degree from Natal University which he subsequently put to good use by applying speciality chemicals in mining and industrial processes where water is a major factor . This created an enduring interest in water technology , a passion that expanded to the HVAC industry in 1984 when he joined BHT Water Treatment . Since then , water technology in HVAC water circuits has continued to be an abiding interest .
VIRUSES IN WATER
By Charles Nicolson
As most of our world continues to live under the pandemic viral cloud of Covid-19 , the three previous articles in ‘ Getting Technical ’ have looked at what viruses are , some of their properties and the effects viruses can cause when they use human body cells as hosts .
Information was also given on how these ultra-tiny entities can survive for periods suspended in the atmosphere or in contact with various surfaces for several hours or even several days . This final section is concerned with viruses in water volumes and water flows .
In common with all other organisms , micro - , macro – or larger , viruses do not dissolve in water but remain in suspension although they will gradually sink downwards by gravity in static water volumes . There are also many variables that affect the survival of viruses in water such as temperature , light , pH , salinity , organic matter content , other suspended solids or sediments and air – water interfaces .
Temperature has a direct effect on how long viruses can survive in water . In general , survival time is longer at lower temperatures , since lower temperatures are the key to continuing virus survival . Research data published in 2018 showed that it takes up to a year for amounts of viruses such as poliovirus in water volumes to reduce by one 5log unit at a temperature of 4 ° C ,
whereas at normal human body temperature of 37 ° C the same reduction occurs within a few days .
Ultraviolet light such as UV in sunlight can deactivate viruses . Many viruses in water are exterminated in the presence of sunlight . The combination of higher temperatures and increased UV in summer time corresponds to shorter viral survival in summer compared to winter . Visible light can also affect virus survival by a process called photodynamic inactivation . The rate of inactivation depends on the duration of exposure to and intensity of the light .
The pH of most natural water is between 5 – 9 . Enteric viruses are stable in these conditions although some of these viruses are more stable at pH 3-5 than at pH 9 and 12 . Enteroviruses can also survive at high pH levels of 11 – 11.5 as well as low values of 1 – 2 , but for only short periods . Adenoviruses and rotaviruses are sensitive to a pH of 10 or greater which results in progressive de-activation .
In general , viruses do not survive in water containing high concentrations of dissolved salts . Therefore , viruses tend to live longer in freshwater habitats than water bodies with high salt concentrations . Also , certain heavy metals dissolved or dispersed in water are toxic to viruses . Another effect which is often seen is aggregation which is the one of the most common methods used for survival by viruses . In a liquid environment , viruses tend to clump together forming aggregations . These aggregations usually result in a reduced rate of virus inactivation indicating that viral particles that do not aggregate are more easily destroyed . Aggregation may form spontaneously or may result by nucleation on particles in water .
Figure 1 : Example of viral aggregation
Water that is intended for drinking should go through some treatment to reduce pathogenic viral and bacterial concentrations to levels which have proven to be generally safe . Reducing the amounts of viruses in drinking water is accomplished by various treatments that are typically part of drinking water treatment systems in developed countries .
In recent years there has been considerable progress in the quality of drinking water which is ensured through a framework
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