Foreword                                             

Prayer is the most important act of worship. It is the pillar of religion. The Law-giver has ordained that the person saying prayer must be tahir of khabath (material uncleanliness ’najasah’), such as blood, urine, excrement, dead bodies... etc. and hadath(immaterial uncleanliness) that requires wudhu or ghusl, such as janabah.
Khabath could be removed by washing with pure(tahir) water and other means like soil or dust which should restore taharah to the najis object.
Hadath could be eliminated and taharah restored by wudhu or ghusl with pure water or by tayamum with dust. 
Discussion of all aspects relating to this area shall, God willing ,ensue.
Since tahir water is the main purifying agent, we have to talk about it and its types. Matters of discussion shall revolve around whether water can be deemed tahir, and retain its characteristics in order to be used for purifying and cleansing najis objects.

Types of Water                                 

1. Water can be classified into two types, pure and mixed, each of which has its own set of rules.
Pure water is that type of water mentioned in the Qur’anic ayah(verse or sentence of the Holy Qur’an

(, "..and We have made of water everything living, will they not then believe?").(03/12). 
It is the running water that flows through the water grid to our homes, hotels, public baths... etc. It is that water we and the animals drink, with which plants and trees grow and get nourished; it is that which we use to wash our bodies and clothes. Among this type of water is seawater, the water that results from the thawing of ice, snow, and hail, and mineral water.
Mixed Water is pure water laced with some other object, causing it to lose its properties, to the extent that it could no longer be described as water. Examples of such mixed water are tea and rose water. It could also be water extracted or pressed from certain types of fruit, e.g. melon water and lemon juice.

2. Both hypes of water, pure and mixed, are tahir. You may use them for drinking or any other purpose. The rules governing them are as follows:
a. Pure water can be used for cleansing and purifying najis objects, such as utensils, clothes; it could be used for washing our bodies. Mixed water cannot be used as such.
b. Pure water can be used for wudhu or ghusl. You cannot use mixed water for these purposes.
Hence the juristic technical term: Pure water is pure in itself and can be used in restoring taharah after occurrences of hadath or khabath; although mixed water is pure in itself, it cannot remove hadath or do away with khabath.
c. Pure water is not affected when it comes into contact with najasah, except in certain circumstances discussed in paragraphs(8) and(9) of this section.
Mixed water is prone to najasah and becomes najis on contact. Any source of najasah that meets with solid things turns them najis, irrespective of the amount of mixed water.
d. If pure water became najis, taharah could be restored to it once it is replenished with abundant water, such as rain water, as shall be discussed later. As for mixed water, it will not become tahir again, if it got najis irrespective of its mass. 

3. Liquids, which do not fit the description of water, such as oil and milk fall under the category of mixed water. Thus, they cannot be used to clean things from khabath or hadath; they become najis, if they meet with najasah. Taharah can never be restored to them, should they become najis.

4. Suppose you poured water from a jug full of rose water on a najis floor, only that part of the rose water, which came into contact with the floor, becomes najis. That is, najasah will not spread to the contents of the jug. As a general rule, in every situation that involves liquid movement in the direction of a najis object, what becomes najis is that part which comes into direct contact with the najis object, not the rest that remained intact.
The same rule applies to pure water. Please refer to paragraph(9) of this section.

5. Pure water has its own properties, taste, and color. Its taste or color may alter by adding some salt or dye to it. In this case, it is called altered water; nevertheless, it will not lose its properties as pure water. It should therefore be governed by the same rules. However, if the change was drastic, in that strange characteristics would emerge, it might change into mixed water. Thus, it would fall beyond the pale of the rules regulating pure water.

6. Pure water may change in character in this way, yet you cannot tell the extent of change. In such a case, you should resolve the matter by assuming that it still is pure water, until proved otherwise.
Pure water could change so much so that it might turn into mixed one. However, this change may gradually diminish, if we, for example, can manage to minimize the level of coloring by adding more pure water to it in such a manner that it should eventually restore much of its original color and taste. Nevertheless, we could still not be quite sure as to the level of the remaining change. In this case, we should conclude that it still is mixed water until the contrary is proven. 
Abundant and Little Water

7. Water can be divided into two categories:
a. Abundant water:
a. 1. Any water connected to a source supplying it with a continuous flow can be called abundant, such as that of wells, springs - whether flowing or not, streams, rivers, whether their supply comes from springs, underground water, or molten snow. This is so, irrespective of the level of abundance or scarcity beheld by the naked eye, because abundance here is judged by the level of the source of supply. a. 2. For rainfall to be judged abundant, it has to be flowing on a hard surface, albeit in a little quantity. It will still qualify for the title of "abundant water" even after it forms some sort of pool or puddle; it remains thus so long as rain continues falling.
a.3. stagnant water, which has no source of supply, can be called abundant, if its quantity amounts to a kurr (a unit of volume equivalent to 384 litres (or more.
Kurr will be discussed in paragraph)(01).
b. Little Water is that which is not abundant, i.e. it has no source of supply, does not amount to kurr, and not rainwater.

Rules Governing Abundance and Scarcity                         

8. Whether it is abundant or little, water is tahir and can render things tahir from hadath or khabath. There are, though, some differences as to the manner of cleansing with abundant and little water, as will be discussed under the section dealing with mutahirat(purifying agents). 
However, they are different as to their susceptibility to najasah. Because abundant water is really abundant and has a source of supply, it is not affected by najasah when it finds its way to it. So, if, say, urine or blood found their way into it, it can still retain its taharah. That is why abundant water is described as immunized (mu’tasim), for its abundance makes it impervious to najasah.
When the water is little, it is liable to become najis as soon as it meets with anything najis, such as urine and blood, or liquids that have become najis as a result of contacting najasah, like water and milk. The same goes for solid things, on the premise of ihtiyat wujubi; for example, a spoon may get contaminated with blood. If you wipe the blood off and place the spoon in a bowl containing little water, the water becomes najis.

9. If the najasah occurs in any part of the water, that is little, all the water becomes najis.
However, if the najis object was in a lower place and little water was poured on it from above, the najasah will affect the point of contact only. Let us imagine the opposite, in that the little water was pointed to the source of najasah from below, the najasah would impregnate the upper reaches of the column of water not the rest of the column.
The same rule applies when you point the water in a horizontal line to the source of najasah, i.e. only the contact point becomes najis.
To sum up, if the water was in fast flow in any direction and hit a najis object, only the contact point becomes najis, not the rest of the water.

10. In paragraph (7) we mentioned that kurr water is one of the types of abundant water. It is the water that weighs approximately 377 kg.
11. If kurr water was confined to one or more places, it would still retain its properties as abundant and immunized water, so long as the two compartments or more that hold it are connected with one another. Thus, it will not become najis on contact.

12. Water may be flowing or in some sort of movement, such as the water in the storage tanks in our homes. These tanks may be feeding smaller tanks, such as cisterns that are situated below them. Here, we may imagine two situations:
a. The quantity of water in the tank could amount to a kurr or more.
b. The quantity of water therein is less than a kurr. But, if the water that is trapped in the pipe network and in the smaller tank is taken into consideration, it could amount to a kurr.
In the first case, the water in the main tank, as well as that in the smaller one, is immunized water so long as the water flows from the bigger tank to the smaller one freely. Thus, if, for example, blood falls in either tank, neither will become najis. The water in the smaller tank could not be immunized, if it is not connected to a flowing source; so, should najasah meet with it, it turns najis.
In the second case, the water in the smaller tank is immunized, i.e. it does not become najis on contact as long as it remained connected to the main tank, whereas the main one should. So, if najasah comes its way, it becomes najis.

13. Any water, no matter how little it was, connected to a source of abundant water is deemed immunized. To give an example, suppose there was a brook holding little water. This water was being replenished with either rainfall, or another tributary with continuous water supply; it might as well be fed with water through a pipe, connected to a big reservoir. The water in the brook is judged immunized, as long as the water supply remained constant.
It is worth noting that the float in the cistern in a bathroom regulates the flow of water in it. If the cistern does not hold enough water that could amount to a kurr, the contents can be described as little water. However, it ceases to be deemed little as soon as you flush the water, thus allowing the cistern valve to open and refill the container. There and then the water in the container becomes abundant and immunized. 
The water running from a shower can be considered abundant, if the flow of water through its holes was fast and unbroken. It should be deemed little, if the water through the shower holes was dripping, even though the gaps between the chain of drops were negligible. 

14. One may come across a small storage tank and becomes doubtful as to its capacity, i.e. whether it constitutes a kurr or not. One should resort to considering its contents as little and apply the rules thereof, in that the water it contains becomes najis as soon as it is soiled with najasah.

15. Also, one may come across water below the quantity of a kurr flowing on the face of the earth and become doubtful as to whether or not it is connected to an abundant source of water. The rule applicable here is the one concerning little water, unless the doubt was based on prior knowledge that this flowing water was connected to an abundant source of supply; in this case, it should be considered tahir, if it comes into contact with najasah, provided that its properties did not change.

16. Suppose there was a tank, whose capacity is more than a kurr, full of water; some of the water was used up. We then became doubtful about the remaining quantity of water, i.e. whether it amounts to a kurr. How should we treat it? 
A. It should be deemed as though it is kurr water.

17. In paragraph (7) it was mentioned that rainwater is considered among the types of abundant water. However, it should be so abundant that it flows on any hard surface. This means that it does not become najis if it contacts a source of najasah. 
Suppose that a drop of this rain water fell on a najis object, such as dead body, that drop of water would not become najis, whether it remained attached to the dead body or fell of it, so long as the rainfall was continuous.
If rainfall formed a puddle and a najis object fell in it, it does not become najis so long as the rainfall continued. The same rule applies to the water coming down drainpipes, in that if it contacts najasah, it should not become najis because it is regarded as immunized water. Rainwater falling on tree leaves and streaming downwards to the ground is treated in the same way.
However, rainwater leaking from a roof should not qualify for abundant and immunized water. This is because the link between the leaked water and that of the rain was not sustained. Continuity of rainfall is immaterial.

 
18. In paragraph (7), it was mentioned that spring water is considered among the types of abundant water. It is noteworthy that there should be no difference between springs that provide water all year round and seasonal ones. The water of these springs is deemed abundant and immunized. However, the water of the seasonal ones is deemed as such, only so long as the water keeps flowing. 
How does abundant water become najis?

19. In paragraph(7), we have stressed that all types of water do not become najis on contact with najasah. However, we further said that those types of water become najis when the najasah causes an alteration of either color, odor, or taste. If the properties of such water change due to a fourth element, such as that it becomes heavier or lighter, yet retains its original attributes; it does not become najis.

20. Of no effect is the changing of the properties of abundant water that got mixed with an originally tahir object that became soiled with najasah. Of course, if the properties of the water change as a result of the najasah found in the najis object, it will no doubt become najis.
For example, a quantity of water became najis and discolored as result of getting soiled with blood; this contaminated water was poured in, or found its way to, a storage tank containing water to the tune of a kurr or more; as a result, the latter became yellowish in color. In this case, the water in the storage tank becomes najis

21. It is unimportant too if the properties of abundant water were altered without being soiled with najasah. 
For example, if the smell of a nearby source of najasah spread to the water, it does not become najis.

22. The change in the properties of water discussed in the aforesaid paragraphs does not necessarily mean that it becomes complete, i.e. acquiring the exact attributes of the najis object. It suffices for the water to be deemed najis, if discoloration, different smell, or taste takes place. 
For example, if the water turns into yellow as a result of blood falling into it, it should become najis.

23. Suppose that a najis object was dropped in the water. The effect of this was non-existent due to either the object, the water or something else not related to either of them. Had it not been for this, the effect would have been tangible. Would the water be judged as najis?
A. There is a number of issues to be considered here:
a. If no change in the water properties can be detected, this could be attributed to the fact that the najis object which soiled the water has, for example, neither colour nor smell to give to the water. In such a case, the water is deemed tahir.
b. The untraceable change could be attributed to the colour of the water before it was soiled with it, in that it had the same color. 
For example, the color of the water could be red due to a dye, which was added to it; if blood was dropped into such water, it should leave no trace, in that it acquired the same hue of the water. On such an assumption, the water becomes najis.
c. The najis object may have its own characteristics, which are different from those of the water. However, the untraceable effect on the water could be tracked back to something else beyond both the water and the najasah. For example, the cold weather could contribute to the untraceable effect of smell left in the water of a rotting carcass. On this premise, the water should retain its taharah.

 
24. If the water is of the type that is abundant and some of it gets polluted with najasah, does the najasah spread to all the water, or it should remain confined to the contaminated part?
A. Should the remaining water qualify as abundant water, it will still be deemed immunized water and shall therefore not become najis.
This could be illustrated in the following examples:
a. Suppose you have a big pond and blood seeped into one of its sides, turning the color of the water into yellow. Now, would the water on the opposite side become najis before the color of the blood permeates it? The answer is no, so long as the quantity of the water in the non-affected part amounts to a kurr.
b. Suppose there was a stream containing flowing water, the quantity of which was less than a kurr; it was connected to a main supply of water. Assume najasah soiled the middle of the stream, causing discoloration of the water in that part. Should the entire water become najis? 
The answer is that part of water falling between the position of najasah and the source of supply should not be affected by the najasah. As for the part of water falling down stream, if there was still a trickle of clean water running from the source through the affected area, and forming a link between the water upstream and that of downstream, the najasah will be confined to the affected parts only. If all changes, that part of water falling down stream will become najis.
How should one go about restoring taharah
to water that was rendered najis?

 
25. If water, that is little, becomes najis, taharah can be restored to it by connecting it to a source of abundant and immunized water. For example, if the water in a bowl becomes najis, you can make it tahir again by placing it under running water of a tap. Thus, as soon as the tap water blends with the water in the bowl, it becomes tahir again, i.e. without waiting for the tap water to spread to all parts of the bowl. Another example could be rainwater, in that it restores taharah to najis water, provided that the rainfall is continuous.
Indeed, the water discussed in both the examples becomes immunized; so long it remains connected to the tap water and rainwater. Furthermore, the bowl itself becomes tahir as well at the point of contact.
Should abundant water become najis as a result of getting mixed with a najis object, its taharah can be restored if two things were present.
a. The change that occurred in the properties of the water must be removed, irrespective of whether this is done gradually, i.e. by passage of time, or by adding more clean water to it.
b. The water can be replenished, while still unchanged, with abundant and immunized water amounting to a kurr, or by rainwater.
Both the objectives can be achieved in one go. We can turn, say, tap water on the water that got affected by najasah, allowing it to permeate the entirety of the najis water, through continuous supply, until the najasah is removed and the water becomes tahir again. 
To further illustrate this, consider the following examples:
a. Suppose a water storage tank turns smelly. You may leave it for some time, to allow the bad smell to disappear. You can then add clean water to it, thus returning it tahir again.
b. Suppose the water in that tank turned yellow as a result of getting polluted with blood. You may add fresh water to it, using a utensil of a kind until the coloration is diluted and eventually disappears. Then the tap should be turned on or the water exposed to rainfall. There and then its taharah is restored.

Evaporation of Najis Water                             

26. If the najis water evaporated and by way of condensation turned into water again, the latter is tahir. The same goes for all other kinds of fluids apart from pure water, such as rose water and milk, and even urine which passes through the same process.
How would the water used
for cleaning be treated? 

27. Would the water used for removing najasah, by way of ghusl, or wudhu still be treated as tahir or deemed najis? And can it be reused in restoring taharah to najis objects?
A. It should not become najis, unless it comes into contact with the najasah itself, and it was little, or it gets altered by any of the attributes of the najasah as has already been discussed. If it becomes najis, it cannot be reused for restoring taharah. If it did not, i.e. retaining its original properties, it can be used to remove najasah, and perform wudhu or ghusl.
What to do when in doubt?

28. If a person has doubts as to the taharah or otherwise of the water, he should resolve the matter by considering it tahir. That is unless he had prior knowledge that it was najis and he was not in a position to tell whether it had become tahir or remained najis. According to this hypothesis, the water should be deemed najis, unless the contrary is established.