Praise be to Allah.
Maintaining one’s health and that of one’s children and family is one of the most important of priorities.
We cannot say that exposing one’s health to danger or destruction is a lesser evil and less harmful than wasting an amount of water equal to the amount that is drunk.
Man is enjoined to preserve his health, for preservation of life is one of the five fundamental objectives of Islam, which are: preservation of religious commitment, life, honour (lineage), intellect and wealth. So long as a person needs to purify his water, and there is no other means that is as effective as this method and wastes less water, then there is nothing wrong with using this water purifier, and that is not regarded as extravagance, because extravagance means wasting a great deal of wealth on an insignificant purpose.
And it was said that extravagance means spending more than is appropriate on something that is not appropriate.
At-Ta‘reefaat by al-Jarjaani (p. 38)
Water purification is not an insignificant purpose; rather it is an important purpose that deserves to have wealth spent on it, because polluted water is one of the main causes of disease which may destroys people’s health and exhaust their wealth to an extent many times greater than what will be spent on purifying water.
In an article by Prof. Dr. ‘Ali ‘Abd ar-Rahmaan ‘Ali, former president of the Arab Union for Sustainable Development and Environment, entitled Athaar Talawwuth al-Miyaah al-‘Adhbah ‘ala Sihhat al-Insaan (Effects of Freshwater Pollution on Human Health), he begins his article by saying:
The least that can be said is that it destroys human health by causing internal diseases such as cholera, malaria, typhoid, bilharzia, hepatitis, dysentery of all types, and cases of poisoning. End quote.
But what is usually the case with regard to people who use water purification methods is that they use them only for water that will be used for drinking and cooking, and not for other domestic uses of water, such as cleaning and the like. In this way one may avoid extravagance and greatly reduce the level of waste.
And Allah knows best.