veness, considerable expense, or maintenance problems. There are three chemicals most favored for water disinfection: bromine, iodine, and chlorine. For wastewater disinfection, chlorine is most commonly used. The chlorine is put into water as free avail- able chlorine and it reacts with organic material in the water. Organ- isms present in the wastewater are attacked and "oxidized." Nonliv- ing material is attacked as well, and these reactions often form chloramines and related compounds. This is called "combined chlo- rine." This combined chlorine can still disinfect, but is much less effective than the "free chlorine." Also, the combined chlorine com- pounds are attacked by free chlorine, using up much of the disin- fecting power otherwise available. Therefore, sewage can use up considerable amounts of chlorine (chlorine demand) as compared to swimming pool water. In treating sewage, however, there is rarely a need to add enough chlorine to produce a free chlorine residual. In practice, actual levels of combined chlorine residual to be main- tained are determined by looking at their effect on the bacteria present. The bacteriocidal action of the disinfectant is dependent on sev- eral factors: the concentration of disinfectant in the liquid, contact between the bacteria and the chlorine, the time that the disinfectant has to work on the bacteria, and the amount of particulate material in the liquid. There are, of course, other factors which are important too, such as temperature, pH, and the type of disinfectant used. The two relevant disinfectants are chlorine and bromine. Chlorine is the disinfectant of choice because it is relatively inexpensive, readily available, and reasonably effective. The first three factors, disinfec- tant concentration, contact time, and particulate material, are the most problematic for home sewage disinfection. The presence of particulate material in the effluent is the most confounding factor. Bacteria collect on the surface and in the crev- ices of the particles, and the disinfec