The rotten egg smell in well water comes from hydrogen sulfide, a gas produced by certain bacteria in the groundwater. Hydrogen sulfide can be fatal in high doses, and it's flammable, but because it has such a distinctive odor, it's usually detected well before it becomes dangerous. It's corrosive, though, and can damage plumbing pipes, pumps and other metal parts associated with water distribution. Moreover, hydrogen sulfide promotes the growth of other bacteria -- such as iron bacteria -- and the resultant slime can clog the plumbing system. Filtration systems can remove hydrogen sulfide and its odor from the water.
Because hydrogen sulfide occurs naturally in groundwater, it's impractical to attempt to purify the well itself. A more practical approach is to install a filter at the point of use, which is the point at which the water enters the house.
Activated Carbon Filtration
If the concentration of hydrogen sulfide is low -- less than 1 milligram per liter -- an activated carbon filter may be the only purification method needed. A carbon filter traps hydrogen sulfide gas until it becomes saturated and needs to be replaced, but because it also traps other impurities, it's impossible to predict how long it will last. It could be years, weeks or even days, depending on the water.
A second option, which can handle hydrogen sulfide concentrations up to 6 milligrams per liter, entails an oxidizing filter using manganese greensand, which is sand coated with manganese oxide. The filter changes hydrogen sulfide gas into sulfur, but it must be recharged periodically by permeating it with potassium permanganate, a complex procedure that mitigates against the practicality of this filtration method.
Oxidizing Filtration Systems
The injection of an oxidizing agent, such as chlorine bleach or hydrogen peroxide, can augment carbon filtration to purify water with higher concentrations of hydrogen sulfide -- up to and exceeding 6 milligrams per liter. The oxidation reaction produces inert sulfur particles that can be removed from the water by the carbon filter. This type of system requires extensive maintenance to replenish the oxidizing chemical and to backwash or change the filter, which may become fouled by other impurities in the water.
Air oxidizes hydrogen sulfide and iron considerably faster than other chemicals, including chlorine bleach. A home aeration filter consists of a closed tube containing a pocket of compressed air. Water enters the tube as a spray and is aerated even more in the oxygen-rich water before it exits the bottom of the tube. The system includes a vent to provide a changeover of air and to release hydrogen sulfide, and it also includes a pump to compress the air.