The downsides of water treatment include installation and running costs, energy use, wastewater, residues and ongoing maintenance. Some technologies change water chemistry or produce unwanted by-products. The impacts differ between drinking-water, industrial and wastewater applications. These trade-offs are reasons to select and manage treatment carefully, not abandon necessary protection. Compare whole-life costs, demand, consumables, waste and maintenance. Reverse osmosis produces a concentrate stream, some filters require backwashing and salt-regenerated softeners generate regeneration wastewater. EPA WaterSense guidance explains that typical point-of-use RO systems may produce five or more volumes of reject water per volume of treated water; WaterSense-labelled units must send no more than 2.3 volumes to drain. These figures apply to that US product category, not every installation. Sludge, spent media and concentrates need appropriate management, and sewage sludge differs from drinking-water residues. WHO explains that chlorine can react with natural organic matter to form disinfection by-products and treatment optimisation can reduce formation. Alternative disinfectants also have limitations; do not remove essential disinfection without competent assessment. Filters, membranes, adsorbents and instruments need maintenance and monitoring. Chemicals introduce handling and operational obligations, while effective disinfection depends on water conditions, concentration and contact time. Sodium-based softening changes ion composition and is not a general disinfection process. Chemistry and corrosion considerations require a complete system assessment. Each technology has limits: particle filtration is not universal chemical removal, activated carbon has finite capacity, and UV does not remove all chemical contaminants. Additional equipment creates maintenance and installation demands. Avoid unnecessary treatment and select proportionate, reliable processes supported by relevant evidence. Reviewed 14 October 2026 using EPA, WHO and DWI resources.