Quality Test in Rainwater Harvesting- H2S Strip Test

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Water quality tests must be conducted on rainwater if the main source of drinking water is from rainwater harvesting. Hydrogen Sulphide (H₂S) strip test is one of the easy test that can be conducted by the occupants who are dependent on rainwater as the major source of drinking water. The need for testing rainwater quality and the H2S testing method is explained in this section.

Need for Water Quality Test in Rainwater Harvesting

Proper maintenance of roof,the gutter, the first rain separator and the filter in a rainwater harvesting system will give crystal clear rainwater. If the water is however dirty in colour or it smells bad, then it means that the system is not kept clean. Clear water even if does not smell bad had chances to show presence of micro-biological contamination.In such situations, daily checking must be performed first month after which weekly checking is followed. The rainwater even though falls from clouds, they absorb large amount of dust and dirt from the atmosphere. This hence increase the need to check their water quality before it is used for consumption. Once it is established that the rainwater is not micro-biologically contaminated it can then be consumed directly.

Hydrogen Sulphide (H2S)Strip Test

H₂S Strip test is a water quality test conducted to determine the bacterial contamination of water. The testing sample is a H₂S strip that is available from the market. This is the initial test conducted to determine the level of bacteria in the rainwater harvested. The test involves the following steps: 1. As shown in figure-1, the water is collected in the test bottle till the fill line. The cap is replaced once the filling is done. 2. Based on the temperature of the sample, it is kept at incubation period normally at a temperature of 250 to 370 degree Celsius. The incubation periods for different temperature ranges are tabulated below.

Table.1: The incubation period for H2S strip test sample

Temperature Range (0C)	Incubation Period (Hours)
30-350	24
25-300	24-36
20-250	36-48
15-200	48-60

3. After the incubation period, the change in color of the sample medium is observed 4. If the water turns black it needs to be treated. If it changes to any other color other than black, the sample is free from bacterial contamination

Treatment of Harvested Rainwater

If the H2S strip test suggests that water has bacteria in it, it must then be treated before it can be used for drinking. The two common methods easily followed for treating contaminated rainwater are:

1. Chlorination
2. Solar Disinfection of Water (SODIS)

1. Chlorination

Chlorination is one common method for treating bacteria in the water. Liquid chlorine or chlorine tablets are available for the treatment of water. Depending on the volume of the rainwater in the tank, chlorine needs to be added to disinfect the water. Chlorination should be carried out every time there is rain and there is a fresh infusion of water into the tank. Using a chloroscope,  residual chlorine of 0.20 mg/ liter should be established before the water is used for drinking.

2. SODIS

Another common method employed to deactivate bacteria and making water fit for consumption is called SODIS – Solar disinfection of water. In this method, rainwater is kept in a PET bottle or a glass bottle in the sun for 6 hours. One side of the bottle is painted black. The black surface is kept on the ground. With a combination of UV disinfection and infra-red heat sterilization, the water becomes fit for consumption. IThe bottles must be kept under the sun for long if its a cloudy weather.