Unlike other raw materials, the raw water supply varies significantly in quality both from one geographical region to another and from season to season. Water derived from an upland surface source, for instance, usually has a low TDS (Total Dissolved Solids) and is relatively soft, but has a high concentration of organic contamination, much of it colloidal. By contrast, water from an underground source generally has a high TDS and hardness level but a low organic content.
Water impurities generally include minerals, viruses, bacteria and other organic material. Sewer water from homes and industries contain a large variety of such water impurities. Currently, sewer water is going through some initial treatment to remove some impurities and then it is discharged in rivers and oceans.
If water impurities are not harmful to the plants, part of such water can be used for irrigation and watering gardens.
Water is critical for greenhouse production because a great deal of water is required to grow greenhouse crops. Since a large amount of water is needed to produce the crop, the quality of the water is important. The term quality in this case is related to what compounds are in the water. If a large number of impurities are in the water, than the quality of that water would be considered poor; however, if there were few impurities in the water the quality would be considered good or excellent.
One class of impurities that is extremely important is that of salts. How does the salt content affect plants? Normally water moves to the root by a concentration gradient, and into the root by osmosis. The water content in the soil after watering is high and the water content in the roots is lower so water moves from the soil into the root. If the salt content of the water is very high the salt will compete with the roots for the water and the water may not move into the roots because the salts are holding on to the water. If the salt content of the water gets too high, water could even be pulled out of the roots. It is important to mention that fertilizers are also salts so will add to the natural salt content of the water.