Chemicals used in water treatment include disinfectants, coagulants, pH adjusters and specialist products for softening, corrosion control and equipment protection. Drinking-water, industrial and wastewater applications have different requirements; there is no universal mixture. Chlorine gas, sodium hypochlorite, calcium hypochlorite and on-site generation are disinfection approaches described by WHO. Performance depends on contact time, concentration, pH, temperature and water conditions. Manage by-products while maintaining microbial protection. Monochloramine, chlorine dioxide and ozone have distinct properties and limitations; ozone does not provide a lasting distribution residual. Aluminium sulphate, ferric sulphate, ferric chloride and polyaluminium chloride support coagulation, followed by separation of solids. Suitable polymers can aid flocculation or sludge processing. PAC can mean polyaluminium chloride or powdered activated carbon, so identify products clearly. Lime, sodium hydroxide, sodium carbonate, carbon dioxide or suitable acids can be used in defined pH or alkalinity management processes. Activated carbon is an adsorbent with finite capacity, not a universal disinfectant. Sodium chloride regenerates sodium-based ion-exchange softeners; softening is not disinfection. Orthophosphate can support drinking-water corrosion control under appropriate conditions. Industrial boilers may require oxygen control, with sodium sulphite discussed in Spirax Sarco guidance. Industrial formulations are not automatically suitable for drinking water. Wastewater chemistry depends on the stream and discharge objectives, with sludge and residues requiring management. DWI Regulation 31 guidance concerns products used by public suppliers in England and Wales up to delivery at the consumer's building; other applications and jurisdictions have different arrangements. Confirm relevant product requirements, safety documentation and competent operation. Never mix hypochlorite with acids or ammonia. Use appropriate analysis, monitoring and records to support selection. Reviewed 14 October 2026 using WHO, DWI and Spirax Sarco guidance. This is not a chemical dosing recipe.