Current drinking water treatment techniques, while effective, is increasingly unable to cope with the larger set of trace contaminants in treated water found to impact on human health. For example, in some areas of the world, treated water contains trace concentration of lead (~ 3 parts per billion, ppb level), arsenic (~ 2 ppb), boron, perchlorate, trihalomethanes (from overdose of chlorine during the final disinfection phase of water treatment) etc. Hence, treated drinking water remains a health risk in some parts of the world, given the presence of trace contaminants known to affect human health such as lead and arsenic.
Thus, what is the problem with current drinking water treatment methods in removing trace contaminants? Specifically, the techniques useful for removing drinking water contaminants at the low concentration level are coagulation and slow sand filtration. In coagulation, a chemical, aluminium sulphate, is added to non-potable water to induce the coalescing of small particles into larger ones, which could be removed by the subsequent stage of gravity sedimentation, where the large particles settle to the bottom of the treatment tank and thus removed. Low concentration contaminants could be removed by this method given that they may be adsorbed on the small particles, which adhere to each other to form larger particles.
On the other hand, slow sand filtration is designed to remove small particles that could not be removed by the sedimentation tank after the coagulation stage. In allowing the water to slowly percolate through the dense bed of gravel and fine sand particles, a combination of adsorption and filtration helps remove the small particles. However, slow sand filtration is unable to remove ions and other contaminants down to the parts per billion level.
Therefore, reverse osmosis, which is a pressure filtration technique forcing water to pass through a non-porous membrane where only water molecules could permeate, is the only technique able to remove trace contaminants in water such as ions. But, the process is energy intensive and thus costly, which explains why it is not widely utilized in drinking water treatment. Nevertheless, given the increasing awareness of the roles of trace contaminants such as parts per billion level of lead and arsenic, in impacting human health, reverse osmosis may, in future, become a standard technique in the drinking water treatment train for processing raw water into drinking water. Doing so will raise the price of water, but it is an option that society needs to decide: on the cost it can expend to protect drinking water quality and human health.
Collectively, trace contaminants such as lead and arsenic remains in potable water after conventional drinking water treatment processes of sedimentation, coagulation, sedimentation, fast sand filtration, slow sand filtration, and chlorine and ultraviolet disinfection. Present at the parts per billion level and known to impact on human health, these trace contaminants could only be removed by the pressure filtration technique, reverse osmosis, which uses high pressure to force water through a non-porous membrane that water molecules are only able to permeate. While reverse osmosis is energy intensive and costly, it may gain more widespread use in the drinking water treatment process given that society is increasingly aware of the human health impacts of drinking potable water containing trace levels of heavy metals, organic compounds and disinfection byproducts.
Category: water treatment, environmental engineering, chemistry, physics,
Tags: trace contaminants, lead, arsenic, reverse osmosis, non-porous membrane, disinfection byproducts, coagulation, slow sand filtration, sedimentation,