HEALTH AND SANITATION
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SILVER DISINFECTION Nawaz et al . ( 2012 ) studied the efficacy of silver ( AgNO 3
) in the removal of P . aeruginosa and E . coli in rooftopharvested rainwater supplies . The efficiency of silver disinfection was evaluated at concentrations , ranging from 0.01 to 0.1mg / l ; the safe limit approved by WHO . AgNO 3 in crystal form was dissolved in distilled ( non-ionized ) water to a stock concentration of 100ppm of silver ions and then 0.1 – 1ml volume of this stock solution was added to 1l of the test rainwater samples to obtain the final concentrations of 0.01 – 0.1mg / l of silver .
Prior to disinfection , samples were found to contain between 350 and 440cfu / 100ml P . aeruginosa and 740 – 920cfu / 100ml E . coli . The disinfection rate and residual effect of silver was determined using final silver concentrations between 10 and 100μg / l over a period of up to 168 hours . Samples were taken for microbial analysis every two hours for 14 hours after the application of silver and then daily for one week , to examine regrowth . At higher concentrations ( 80 – 100μg / l ), complete inactivation of both microorganisms was seen in 10 hours , with no regrowth of E . coli seen after 168 hours .
Inactivation was slower at lower concentrations ( 95 – 99 % inactivation for silver concentrations between 10 and 40μg / l after 14 hours ) and regrowth was also observed ( e . g . 7.5 % survival of P . aeruginosa exposed to 10μg / l silver for 168 hours compared to approximately 4.5 % survival at 14 hours ), thus , at the lower concentrations , silver only delayed bacterial reproduction and did not cause permanent damage . Adler et al . ( 2013 ) also researched the effectiveness of silver disinfection as part of rainwater harvesting treatment .
Ten rainwater harvesting systems in Mexico , equipped with silver electrodes were evaluated for a number of water quality parameters . The silver electrodes were located in line with the filtering system ( after a mesh filter , designed to remove large particles , and before an activated carbon filter ). On average , the ionisers reduced the level of total coliforms by approximately 1 log and E . coli by approximately 0.4 log and resulted in a silver concentration of approximately 0.01mg / l in the final water . The systems , as a whole , delivered water containing zero E . coli and less than 10 / 100ml CFU total coliforms .
BOILING Enteric bacteria , protozoa and viruses in water are sensitive to inactivation at temperatures of about 60 ° C ( WHO , 2013 ). Boiling is one of the oldest methods used in household water treatment ( Conant , 2005 ). The World Health Organisation ( WHO ) recommends bringing water to a rolling boil to indicate that a disinfection temperature is reached ( WHO , 2008 ).
Howard and Bartman ( 2003 ) reported that bringing water to a rolling boil for two minutes showed a 97 % reduction of heterotrophic bacteria and complete elimination of coliforms while five and ten minutes showed complete elimination of all bacterial contaminants . Heating water to 55 ° C inactivates most pathogens such as bacteria , viruses , helminths and protozoa ( Feachem et al ., 1983 ).
Other studies also reported on boiling as a water treatment option ( Sobsey and Leland , 2001 ; Conant , 2005 ). A major disadvantage of boiling is its consumption of energy , cost and sustainability of fuel . In areas of the world where wood and other biomass fuels or fossil fuels are in limited supply and must be purchased , the costs of boiling water are excessive ( Sobsey , 2002 ).
The use of wood and wood-derived fuels is also a concern because it contributes to the loss of woodlands and the accompanying ecological damage caused by deforestation ( Sobsey and Leland , 2001 ). Thus , boiling is highly efficacious , killing human pathogens even in turbid water and at high altitude . It however does not provide any residual protection .
OZONATION Ozonation disinfects by introducing ozone gas to the water . It is produced by passing an electrical current through air or oxygen . It has a very short half-life in water ( few seconds to minutes ) and therefore must be efficiently introduced into the water . It is a colourless gas that disinfects , oxidizes , deodorizes , and decolorizes .
Ozone gas is toxic and installation and maintenance of this type of system must be done by a licensed professional . Ozone systems can be positioned to treat the rainwater in the tank by recycling the water through an ozone injection system or by continuously bubbling the ozone in to the storage tank .
Ozone is a broad-spectrum biocide that treats all of the water in the tank as well as preventing the formation of biofilms on the tank surfaces . In addition , ozone can remove colour and odours from water that allow the water to be used in a wider array of applications . Ozonation is also effective against parasites , viruses and chemicals ( organic and inorganic ). Disadvantages include no residual disinfection and may result in ozonation by-products . PA
www . plumbingafrica . co . za September 2018 Volume 24 I Number 7