Household rainwater disinfection options

Household rainwater disinfection options

By Water Research Commission

The selection of appropriate household water treatment (HWT) systems requires prior knowledge of the quality of the water, as well as how various HWT options work and their effectiveness against different contaminants.

Disinfection methods

A traditional approach to treating water at household level is to kill or inactivate pathogens through disinfection (WHO, 2013). The most common methods used by households around the world to disinfect their drinking water are: chlorine disinfection, solar disinfection (SODIS), ultraviolet (UV) disinfection and boiling. These disinfection methods can be effectively applied at household level (Jordan et al., 2008).

However, the common disadvantage associated with most disinfection methods is reduced treatment efficiency on turbid water (Sobsey et al., 2002). Filtration is often required when using disinfection methods to reduce turbidity which can shield certain microorganisms, thereby resulting in treatment inefficiency (Qualls et al., 1983). The volume of water that can be treated during solar disinfection is also of concern as it can only treat small volumes of water (Jordan et al., 2008).

Three disinfection methods common to RWH systems are chlorination, UV light, and ozonation. Treatment of harvested rainwater with disinfection has proved to be successful as illustrated in a number of studies (Despins et al., 2009; Mendez et al., 2011; Ahmed et al., 2012).

Boil waterSolar disinfection (SODIS) has been shown to be an effective treatment method at household level. In SODIS treatment, water is exposed to sunlight for about six to eight hours and pathogens are inactivated by the synergistic effect of both temperature and sunlight radiations (Sichel et al., 2007; Ubomba-Jaswa et al., 2009; Dayem et al., 2011).

Ahammed and Meera (2008) studied the effectiveness of SODIS in the treatment of roof-harvested rainwater and reported that complete inactivation of total coliforms was observed after six hours when solar radiation exceeded 500W/m2. Limitations of SODIS include inefficiency in treatment of large volumes of water, its ineffectiveness during cloudy or rainy days and it is recommended the method not be used on turbid water (> 30 NTU) (EAWAG, 2012).


Whenever in doubt, boil water before use. Credit: Pixabay


Amin and Han (2009) investigated the benefits of solar collector disinfection (SOCO-DIS) as a potential treatment system for harvested rainwater for small-scale water supply. SOCO-DIS was compared to SODIS with the aim of overcoming the limitations of SODIS. They reported that in the SOCO-DIS system, disinfection improved by 20–30% compared with the SODIS system and that rainwater was fully disinfected even under average weather conditions due to the effects of concentrated sunlight radiation and the synergistic effects of thermal and optical inactivation.

An advantage of using SODIS on low pH waters include increased inactivation rates due to the depletion of Adenosine triphosphate, the main energy storage and transfer molecule in the cells (Amin and Han, 2009). Dowbrosky et al. (2015b) investigated the efficiency of a closed-coupled solar pasteurisation system in reducing the microbiological load in harvested rainwater and to determine the change in chemical components after pasteurisation.

Cations analysed were within drinking water guidelines, with the exception of iron, aluminium, lead, and nickel, which were detected at levels above the respective guidelines in the pasteurised tank water samples. Indicator bacteria including, heterotrophic bacteria, E. coli and total coliforms were reduced to below the detection limit at pasteurisation temperatures of 72°C and above. However, with the use of molecular techniques Yersinia spp., legionella spp. and Pseudomonas spp. were detected in tank water samples pasteurised at temperatures greater than 72°C.

Ultraviolet light

Disinfection using UV radiation is defined as a physical method where water is exposed to a lamp producing light at a wavelength of nearly 250nm. The wavelength is located in the middle of the germicidal band and is responsible for damaging the DNA of microorganisms (Bolton and Colton, 2008). Ultraviolet light treatment method often requires filtration as a pre-treatment step since it is not effective on turbid water (Qualls et al., 1983; Macomber, 2010). Several studies have reported on the effectiveness of UV as a disinfection method for harvested rainwater (Jordan et al., 2008; Ahmed et al., 2012). Kim et al. (2005) reported that the number of total coliform present in rainwater were reduced by 50% even at low exposure to UV.

Advantages of using UV radiation in treating harvested rainwater include its high efficiency in the removal of microbes from water and the fact that it does not introduce chemicals or produce harmful disinfection by-products (Vilhunen et al., 2009). Despite its positive attributes in the treatment of harvested rainwater, UV treatment has disadvantages which include: (i) lack of disinfectant residual to protect the water from recontamination or microbial regrowth after treatment, (ii) turbidity and certain dissolved constituents can interfere with or reduce its disinfection efficiency and (iii) high electricity usage is required to power the UV lamps (Kowalski et al., 2000).

Chlorination

Among common point-of-use interventions, household chlorination is the most cost-effective when resources are limited (Clasen et al., 2007). Chlorination requires that the Waterpurificationappropriate dosage be administered. Chlorination is known to be effective against bacteria, viruses and protozoa. Several studies reported on chlorination as an effective intervention strategy to prevent diarrhoeal diseases (Semenza et al., 1998; Quick et al., 1999 and Quick et al., 2002).


Water purification is essential. Credit: Pixabay


Free chlorine inactivates more than 99.99% of enteric pathogens except cryptosporidium and mycobacterium species (WHO, 2002). One of the disadvantages of water chlorination process is the formation of disinfection by-products which may pose a health risk to consumers (Baker et al., 2002). However, when compared to the other disinfection method, it has residual disinfection. Nath et al. (2006) reported that chlorination is less effective in turbid water of >30 NTU and that microbial contaminants may be protected by particulates in the water.

Silver disinfection

Nawaz et al. (2012) studied the efficacy of silver (AgNO3) in the removal of P. aeruginosa and E. coli in rooftop-harvested 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. AgNO3 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.

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