Rainwater harvesting and combating slightly acidic rainwaterSeptember 7, 2010
Simple methods to clean rainwater and make it potable
Rain is the single biggest source of water in India. Snow, ice, fog and desalinated sea water would make up the rest. As rainwater harvesting picks up all across the country it is important to focus on the cleanliness of rain itself.
Inevitably the first question is of ‘acid rain’. What is acid rain? pH is a measure of the acidic or basic nature of a substance . This is a measure used scientifically to understand the concentration of Hydrogen ions contained.
Rainwater with a pH below 5.6 would be acidic and would be called acid rain. Typically a pH-balanced water would have a pH value of 7. Rainwater being naturally acidic, coming in contact with the Carbon Dioxide in the air, would have a pH of 6.0 to 7.
Acid rain occurs when rainwater picks up weak sulphuric acid and weak nitric acid from the atmosphere. These acids are a result of the polluting sulphur and nitric oxide emissions from ehicles, thermal power plants, oil refineries and other gas-emitting industries. Acid rains are known to have had pH values of 4 and less.
What harm can acid rain do? Acidity in water may in itself be not harmful to health. However this water can leach lead or copper if present in pipes and this can be harmful when ingested. Acid rain can also destroy life in water bodies and vegetation by its corrosive effect.
BIS standards for drinking water in India specify that the pH value of potable water be between 6.5 and 8.5. It is therefore important that all waters we drink measure up to this standard though many carbonated drinks and even lime juice would have far less pH. How to tackle the impacts of stored rainwater if it is acidic? The ancient rainwater harvesters of Rajasthan and Gujarat had shown remarkable wisdom when it came to combating the natural acidity of rainwater. They would immerse a perforated clay pot with limestone or marble pieces in the storage tank. The limestone would be gradually released and would increase the pH and neutralise the acidic rainwater. This is a good method still if rainwater is stored and used for consumption purpose as in many fluoride-affected areas of the country.
If harvested rainwater is allowed to recharge the groundwater table, the buffering effect of soil with its minerals tends to neutralise the acidity in the rain.
Naturally present limestone and carbonate-rich rocks and a contact time which is long tend to make the water better.
Testing for pH is very simple and can also be done by children using the sensitive litmus paper or solution test. It is always good to check stored rainwater or the water from wells and borewells for their pH value. If the pH is not within the permitted range it can easily be balanced using limestone or caustic soda if the pH is higher than 7. Water wisdom is in understanding the interconnectedness of nature. Humankind cannot afford to pollute the air and believe its water will be unaffected.
Source control, lesser use of vehicles, and stabilising thermal power air pollutants are all measures equally necessary to maintain the sustainability of water resources.
Rain is the greatest distillation made available to mankind by the forces of nature. To preserve its pristine state is our responsibility.