This article is targeted to create general public awareness that the installation of portable filtration systems at home should be based on the quality of incoming raw water. Such information is required to make the best possible outcomes from the expensive purchase of a variety of household filtration options. The group of experts contributing to the article also uses non-technical language aiming to provide insights into the underlying operation of disinfection systems in drinking water treatment plants in the Bhutanese context.

Bhutan Living Standard Survey 2017, reports 99.5 percent of households have improved access to the water supply. One of the Key Performance Indicators (KPI17.1) of the 12 Five Year Plan also targets that both urban and rural households will have access to 24 hours supply of “safe/piped” drinking water in the dwelling. In our opinion, the piped water supply from a natural source to the dwelling may not always mean access to safe drinking water.

There are numerous dimensions to ensuring a safe drinking water supply. This article discusses the context of disinfection processes in our drinking water supply chain. The water supply systems in Bhutan are mostly storage tanks connected to a natural water source and the distribution line. In some cases, additional structures were added to remove the inflow of debris and sediments. In most cases, disinfection agents such as Chlorine, are expected to be added to kill pathogenic organisms while the use of ozone and UV systems are also observed in some cases.

Water Treatment Challenges

One of the key missing points in our water treatment system is the management of disinfection by-products formed during the disinfection process. The use of disinfection agents like chlorine and chlorine-related chemicals results in by-products such as Trihalomethanes and Haloacetic acids, while the use of ozone results in by-products such as bromate. The disinfection by-products at higher concentrations have the potential to cause cancer and genetic disorders. However, the actual process of formation of those harmful substances during water treatment is still an ongoing active research area.

The applied dose of disinfection agents (e.g. Chlorine) must be accurate – it has to be as high as necessary to eliminate pathogens and as low as possible not to overdose. Overdose can be harmful to customers via the formation of those by-products while it is expensive to the water supply managers. On the other hand, underdose can cause the growth of slime and the build-up of contaminants in the distribution line. The efficacy of disinfection is determined by raw water quality parameters such as turbidity, pH, water temperature, nature of pathogens present, and also other factors like disinfection dosage and contact time. How far our water supply managers consider those raw water quality parameters during the ad-hoc chlorination in our water tanks will significantly affect the quality of our drinking water.

Most of our water sources are from surface runoff or subsurface flow with high content of organic matter as a result of seepage through forest floors. Even larger streams like Sherichhu along Mongar-Trashigang highway has dark tea-like colour indicating high organic matter content. The water sources with high organic matter content when treated with Chlorine significantly increase the production of those harmful by-products. Thus, our water supply managers should have in-depth long-term raw water quality information to determine disinfection options. There are no default values or reference ‘magic number’ to be used for disinfection given the fact that raw water quality is significantly influenced by land use, elevation, hydrology, etc.

Proposed Solutions 

The first step is to characterize raw water quality. Based on the characteristics a choice of disinfection agent should be made, and accordingly, the design of storage tanks needs to be built to ensure enough contact time for the disinfection agent to eliminate all possible pathogens. In many water treatment plants, plug-flow systems are built to ensure enough contact time, which is not a common sight in Bhutan. The main parameters of concern for disinfection should include total organic carbon, bromide, turbidity, pH, and other pathogens such as bacteria and viruses. The quarterly bulletin of the Royal Centre for Disease Control (RCDC) published a chlorination report (from only 6 health centers) covering July to September 2021 and found that more than 80% of chlorination is under the dosage requirement. This further highlights the need to better document those raw water quality parameters including the organic matter contents.

Organic matter load in raw water is one of the key concerns when water is sourced from surface runoff. Simple and easy measurements like UV absorbance using a benchtop spectrophotometer would serve the purpose to indicate the organic matter load. However, quality results can be achieved by measuring SUVA254. Such measurements could also be automated where DOCeq and UV absorbance gets measured at higher frequency using an in-situ spectrophotometer. The equipment comes with high cost but it would still be cost-effective considering high-quality data and reduced cost for sampling and laboratory processing needed when using conventional methods. Similar hi-tech IoT-based sensors are also available which measures water pH, temperature, UV absorbance, and then calculate the required dose of disinfection.

In worst-case scenarios, if possible holding the water in the storage tank and applying Chlorine when the water is less turbid could be the last resort. Loads of incoming external or terrestrial organic matter gets reduced by biological activity. The new class of organic matter generated in the storage tanks by biological activity is reported to be producing fewer harmful by-products than external organic matter in raw water.

In summary, the quality of incoming raw water would determine the effectiveness of any household filtration system. If a UV system is installed the incoming water has to be free of turbidity while the UV lamp has to be slime and dirt free. If ozone disinfection systems are installed, make sure there is low bromide in incoming water to reduce the risk of the formation of harmful bromate by-products. The complications of using chlorine and chlorine-related disinfections areas discussed above. However, the authors of the article acknowledge the fact that there are numerous other technical and financial factors that are beyond the scope of this article. 

The article is published based on personal experiences and observations by a group of water researchers from Bhutan. The group can be contacted via