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The Potability of Harvested Rainwater

SmartPaani’s Team has been working on a blog series to answer questions around sustainable water management, covid-19 and other topics during the lockdown. This is the second blog written by Environmental Engineer Amshu Chitrakar.

Freshwater is a scarce resource all over the world, only 2.5% of the world’s water is fresh. Even less of this is accessible by current technology, hence the pursuit to find freshwater is prevalent across the world (National Geographic, 2020). While developed countries are experimenting with technologies like desalination of saltwater and recycling sewage water on a large scale, these technologies are currently too expensive for developing countries or not possible for landlocked countries like Nepal (Desalination).

In the case of Kathmandu, the Metropolitan City has been unable to provide freshwater to its inhabitants mainly due to the delayed completion of the Melamchi Water Supply Project combined with the population growth of 4.7 percent yearly. This prompts the question “What after Melamchi?” as when it arrives it already won’t be enough.

The total water demand in Kathmandu is about 400 Million Liters per Day (MLD) (Thapa, Ishidara, Pandey, Bhandari, & Shakya, 2018). The Kathmandu Upatyaka Khane-Paani Limited (KUKL) is able to supply about 120 MLD while fully functional. The Melamchi Water Supply Project in its first phase will provide about 170 MLD (Project Implementation Directorate (PID)). After the completion of the project, there is still a need to offset the 110 MLD, keeping in mind the rapidly growing population.

Although there is groundwater available in Kathmandu, the filtering required to convert it to potable water is extensive due to the abundance of chemical contaminants like iron, ammonia and nitrate. It is also a depleting resource; the groundwater table is decreasing at a rate of ~80 centimeters per year (Sonia Awale, 2017) due to the lack of groundwater recharged into the ground because of the increase in impervious land (buildings, roads, pavement). This comes with consequences – it directly affects the soil bearing capacity in an already earthquake prone city (Padmanabhan).

There is however, an alternative source of freshwater in Kathmandu which has not been utilized to its maximum potential. Rainwater! It is an important source of freshwater for communities where surface water and groundwater are contaminated, scarce or unavailable. Many communities across the world rely on rainwater for domestic usage, as well as drinking. (Vikaskumar, Shah, Hugh, P.M., P.C., & Tony, 2007)

Kathmandu received around 1666 millimeters of rain annually the last three years (Department of Hydrology and Meteorology, 2017). If we assume that 20% of the rainfall is wasted, a roof size of 1 aana (31.79 sq. m.) can potentially capture about 42 thousand liters per year. A typical house of 3.5 anna can capture almost 120-160,000 liters based on historical rainfall averages from the last 3 decades. Because of the increasing population, vehicles and industries, there is a question that pops into everyone’s head when talking about harvesting and drinking rainwater – “Can we drink it?”

To answer this, we must first answer two more questions – “What are the contaminants in rainwater?” and “How was it harvested?”

To answer the first question – rainwater is fresh water; the physiochemical contaminants that get in the rainwater are anthropogenic. The main contaminants are nitrogen oxide and sulphur-dioxide which cause acid rain, dust and debris and biological contaminants, mainly E. coli. (Sodhi, 2005) (Atsuko, Yoko, Midori, Noriko, & Tadashi, 1990)

In a research done to assess the quality of rainwater in over 94 samples taken across Kathmandu valley, Shrestha et al, found that untreated rainwater had a pH value between 6.53 and 7.11. Comparing this with Nepal’s Drinking Water Quality Standards (NDWQS), the acceptable pH for drinking water is 6.5 to 8.5. It also showed that chemical contaminants like sodium, chloride, fluoride, sulphate and calcium were present in the water far below the permissible limits, in insignificant amounts. (Shrestha, Pandey, Yoneyama, Shrestha, & Kazama) This means that it is chemically safe to drink.

Dust and debris are larger physical contaminants that can easily be filtered out.

E. coli are biological contaminants that cause water borne diseases like diarrhea, dysentery and typhoid. In a study done to assess the potability of rainwater in Kathmandu, it was found that almost 100 percent of the samples taken had some amount of E. coli in them (Bhandari & Shrestha). Various filtration techniques can be used to filter or kill these bacteria present in the water which will be discussed in the next section.

“How is rainwater harvested?”

Rainwater harvesting (RWH) is an age-old process. While people have collected water from their roofs in small vessels, the Romans made large cisterns as large as 2 lakh liters to collect and distribute water in their cities. Traditionally, small ponds were also built with levees to collect runoff from hills mainly for irrigation. (Makato, 1999)

SmartPaani’s New Rapid Sand Filter for Rainwater Harvesting

However, to harvest rainfall for domestic purposes and make it potable, a modern approach should be taken. SmartPaani’s approach to RWH can be explained in the following steps:

  1. Capturing the Rainwater: Your roof acts as the catchment area. The amount of rain you get is determined by the size of your roof. The larger the roof, the more rainwater you are able to capture. It is governed by a simple formula:
    1. Potential for collecting rainwater (Liters) = Total Annual Rainfall (in meters) * Total Roof Area (in square meters) * Runoff Coefficient * 1000 (liters/ cubic meters)

Runoff Coefficient is a dimensionless coefficient that relates the amount of water received with total precipitation – a flat roof has an 80% coefficient, while a corrugated iron sloped roof can be taken at 90% (but with a good first flush this may reduce to 80%)

1000 Liters/ cubic meters is a conversion factor used to convert the volume from cubic meters to liters

  • Routing: Routing the rainwater involves a series of pipelines that help direct the rainfall to your desired storage. Routes are chosen carefully so as to not hinder the aesthetics of the infrastructure.
  • First Flush: The first rain event carries most of the pollutants present in the lower atmosphere. Hence, modern RWH systems are designed to discard the first rain events to ensure the quality of the captured rainwater. SmartPaani RWH systems are designed to flush a certain amount of the initial rainfall; the amount discarded is based on the size of the roof.
  • Primary Filtration: The first flush system also acts as a sedimentation unit, where larger debris is settled out. The smaller particles that make their way past the first flush system need to be filtered. This is done by a Rapid Sand Filter (RSF) which filters water at the rate it comes in. It is a filter that used various grades of coarse sand to filter physical contaminants. It is used to filter physical contaminants such as dust and debris.
  • Storage: After the primary filtration process, the water collected is sent to the storage. This water can be used for domestic purposes such as cooking, cleaning, bathing etc. However, this water is not yet potable.
  • Secondary Filtration: After the RSF, there are biological contaminants in the water that can cause various waterborne diseases. These contaminants can be removed easily with filters or chemicals (chlorine) found easily in the market. This filtration process can be as simple as boiling or ceramic filters to complex filters life Reverse Osmosis (RO) and Ultra Filtration (UF). The most common filter found easily in the Nepali market are candle filters while the most common chemical disinfectant is chlorine commonly sold as Piyush. SmartPaani Tripti filters work efficiently as the second level of filtration.
SmartPaani’s Tripti Filter an Excellent Solution to Make Rainwater Drinkable

RWH is simple and reliable. Due to the situation with municipal water supply in Kathmandu and with jar and tanker water suppliers, rainwater can also be economically beneficial. Evidence leads us to believe that rainwater can also be used as drinking water with appropriate treatment measures. As demand for water increases with the population, more and more groundwater is being pumped causing the groundwater table to sink. Less dependency in groundwater pumping automatically contributes towards preserving the groundwater table. This has many socio-economic and environmental benefits and helps replenish the stone water spouts present across the valley. It ensures safe soil bearing capacity for a city where infrastructures are being built at a rapid pace.

As more are more land is being paved, the chances of urban flooding also rapidly increase in the city due to a lack of impervious surfaces; this is seen every year in Kathmandu Valley during the heavy monsoon rains. Collecting, using and recharging excess rainwater back into the ground reduces this while providing households, schools and institutions better water that can be used for all purposes and financial savings. Application of appropriate technology can expand the utilization of rainwater as the primary source of freshwater (Oklo & Asemave).  


Atsuko, A., Yoko, A., Midori, O., Noriko, S., & Tadashi, K. (1990). Effect of Air Pollution Chemical Components on the Acidity of Rainwater in Japan. Bull. Environ. Contam. Toxicol.

Bhandari, B., & Shrestha, J. (n.d.). Rainwater quality and it’s theoretical potential for Household level harvesting in Kathmandu Valley. Environment and Public Health Organization.

Department of Hydrology and Meteorology. (2017). Observed Climate Trend Analysis of Nepal (1971-2014). Government of Nepal, Ministry of Population and Environment.

Makato, M. (1999). Creating Rainwater utilization base society for sustainable development, proceeding of the international symposium for water use in Urban Area UNEP. Int. Environ. Tech. Centre. Osaka, Japan.

Oklo, D., & Asemave, K. (n.d.). Potability of harvested rainwater in some local government areas of Benue state of Nigeria.

Padmanabhan, G. (n.d.). Effect of Water Table on Safe Bearing Capacity of Soil. Retrieved May 22, 2020, from The Constructor:

Project Implementation Directorate (PID). (n.d.). Retrieved from Government of Nepal Melamchi Sub-Project 2:

Shrestha, S., Pandey, V., Yoneyama, Y., Shrestha, S., & Kazama, F. (n.d.). An evaluation of rainwater quality in Kathmandu Valley, Nepal.

Sodhi, G. (2005). Fundamental Concept of Environmental Chemistry 2md ed.

Sonia Awale. (2017, December 28). Put back what you pump out. Retrieved May 22, 2020, from Nepali Times:,4100

Vikaskumar, G., Shah, R., Hugh, D., P.M., G., P.C., T., & Tony, R. (2007). Comparisons of water quality parameters from diverse catchments during dry periods and following rain events Water Research.

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