Wastewater: Even after primary and secondary treatment, disease-causing organisms may remain in the treated wastewater. To disinfect and kill harmful organisms, the wastewater spends a minimum of 15-20 minutes in chlorine-contact tanks mixing with sodium hypochlorite, the same chemical found in common household bleach. The treated wastewater, or effluent, is then released into local waterways.
A system that incorporates coagulation-flocculation followed by chlorination has been developed as a point of use technology, especially for developing countries.
It uses a small packet of powdered ferrous sulfate (a common flocculent) and calcium hypochlorite (a common disinfectant). A user opens the packet, adds the contents to an open bucket containing about ten liters of water, stirs for five minutes, lets the solids settle to the bottom, strains the water through cotton cloth into another container, and waits 20 minutes for the chlorine to disinfect the water.
The combination of particle removal and disinfection appears to produce high removal rates of bacteria, viruses, and protozoa, even in highly turbid waters. There is considerable evidence that the system has reduced diarrheal disease significantly in various locations. There is also evidence that the flocculation process helps remove arsenic; however, these systems are not an adequate substitute for high-quality centralized treatment when it can be made available.