It is much weaker than the other alternatives cited, more expensive, and leaves a objectionable pink or brown color. Still, some underdeveloped countries rely on it, especially in home-use applications. If it must be used, 1 gram per liter would probably be sufficient against bacteria and viruses (no data is available on it effectiveness against protozoan cysts). Hydrogen Peroxide can be used to purify water if nothing else is available. Studies have shown of 99 percent inactivation of poliovirus in 6 hr with 0.3 percent hydrogen peroxide and a 99% in-activation of rhinovirus with a 1.5% solution in 24 minutes. Hydrogen Peroxide is more effective against bacteria, though Fe+2 or Cu+2 needs to be present as a catalyst to get a reasonable concentration-time product.
While flocculation doesn't kill pathogens, it will reduce their levels along with removing particles that could shield the pathogens from chemical or thermal destruction, and organic matter that could tie up chlorine added for purification. 6098% of coliform bacteria, 65-99% of viruses, and 60-90% of giardia will be removed from the water, along with organic matter and heavy metals.
Some of the advantages of coagulation/flocculation can be obtained by allowing the particles to settle out of the water with time (sedimentation), but it will take a while for them to do so. Adding coagulation chemicals, such as alum, will increase the rate at which the suspended particles settle out by combining many smaller particles into larger floe, which will settle out faster. The usual dose for alum is 10-30 mg/liter of water. This dose must be rapidly mixed with the water, then the water must be agitated for 5 minutes to encourage the particles to form floes. After this at least 30 minutes of settling time is need for the floes to fall to the bottom, and them the clear water above the floes may be poured off.
Most of the flocculation agent is removed with the floe, nevertheless, some question the safety of using alum due to the toxicity of the aluminum in it. There is little to no scientific evidence to back this up. Virtually all municipal plants in the US dose the water with alum. In bulk water treatment, the alum dose can be varied until the idea dose is found. The needed dose varies with the pH of the water and the size of the particles. Increase turbidity makes the floes easier to produce not harder, due to the increased number of collisions between particles.
Was this article helpful?