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According to the Shāfiʿī school, making water physically clean does not necessarily make it ritually purifying. The ruling differs between mustaʿmal water, which is pure but has been used in obligatory purification, and mutanajjis water, which has become impure through contamination.

1. Filtering water previously used for wuḍūʾ

A small quantity of water that has separated from the limbs after being used for obligatory purification is ṭāhir—pure in itself—but not ṭahūr, meaning it cannot be reused for wuḍūʾ or obligatory ghusl. This restriction results from its previous use in purification, rather than necessarily from dirt or pollutants that a filter can remove.

Therefore, merely passing such water through an activated-carbon filter, ceramic filter or reverse-osmosis system does not, by itself, establish that it has regained purifying status.

However, the school provides a clear means of restoring its purifying status: collecting it until it reaches two qullahs, provided no other disqualifying condition is present. Imām al-Nawawī states:

«فإن جمع قلتين فطهور في الأصح»

“If collected to reach two qullahs, it is purifying according to the stronger position.”

Ibn Ḥajar al-Haytamī explains that it remains purifying even if subsequently divided into smaller quantities. Water drawn from such a collection may therefore be used for wuḍūʾ. See Tuḥfat al-Muḥtāj, 1:80.

2. Treating sewage or other impure wastewater

Impure water can become ṭahūr again under the Shāfiʿī rules; its previous contamination does not permanently disqualify it. The principal distinction concerns its quantity:

• Below two qullahs: Water generally becomes impure through contact with impurity, even without a noticeable change. Simply removing an unpleasant smell, discolouration or visible dirt is therefore insufficient. It can regain purity when brought to two qullahs, provided no impurity-caused change remains.

• Two qullahs or more: Water becomes impure when contamination changes its colour, taste or smell. It can regain purity when that change genuinely disappears through a recognised means, including the addition of water or the disappearance of the change by itself.

Al-Nawawī explains these rules in Minhāj al-Ṭālibīn. Ibn Ḥajar further distinguishes genuinely removing the change from merely concealing it with substances such as musk or saffron. In discussing treatment with earth, he states that the water becomes pure once it clears and no impurity-caused change remains. See Tuḥfat al-Muḥtāj, 1:85–87 and the discussion of water clearing after the addition of earth.

Applying these principles to modern treatment, water processed in sufficient quantity can regain ṭahūr status when treatment genuinely removes the impurity and its effects, and the resulting water retains the character of unrestricted water (māʾ muṭlaq). It may then be used for wuḍūʾ and ghusl.

However, the labels “filtered,” “disinfected” or “treated” are insufficient on their own. The assessment must establish actual purification, fulfilment of the relevant quantity conditions, and freedom from subsequent contamination. Killing microorganisms alone does not establish that ritual impurity has been removed.

Other juristic opinions

Other schools differ on some of these conditions:

• Ḥanafī school: The relied-upon position regards mustaʿmal water as pure but unsuitable for lifting ritual impurity. However, mixing it with a greater quantity of fresh purifying water permits purification with the mixture. The school also has distinct rules for flowing water and does not adopt the Shāfiʿī two-qullah threshold. See Ibn ʿĀbidīn, Radd al-Muḥtār, 1:182 and 1:198–201.

• Mālikī school: Reaching two qullahs is not a requirement. Al-Dasūqī explains that water can regain purity when an impurity-caused change is removed by adding water or treating it with substances such as earth, subject to the condition of the resulting water. See Ḥāshiyat al-Dasūqī, 1:46–47.

Two qullahs are a traditional measure of water, described by Shāfiʿī jurists as approximately 500 Iraqi raṭls. Al-Nawawī also records a volume equivalent to a cube measuring one and a quarter cubits in length, width and depth. Using a cubit of approximately 54 cm, this amounts to about 300–307 litres. Modern estimates differ according to the conversion adopted for these traditional units. See al-Māwardī, al-Ḥāwī al-Kabīr, 1:335; al-Nawawī, al-Majmūʿ, 1:175.

Allah knows best.