Treatment processes
Municipal and household water treatment, with what each process removes and, more usefully, what it does not.
A dense semi-permeable membrane that rejects almost everything dissolved. Reverse osmosis is the most comprehensive treatment available at household scale, and the basis of nearly all mode…
The intervention most responsible for the collapse of waterborne disease in the twentieth century. Chlorine kills most pathogens quickly and, uniquely among common disinfectants, leaves a …
Highly porous carbon with an enormous internal surface area — around 1,000 square metres per gram — onto which organic molecules adsorb. It is the workhorse for taste, odour, pesticides an…
Not a treatment process but a designed relief point. In combined sewers that carry both foul sewage and rainfall runoff, overflows discharge diluted sewage to a watercourse during heavy ra…
Exchanges calcium and magnesium for sodium on a resin bed. The most common household hardness treatment, and the one whose trade-off is most often overlooked: hardness is not removed from …
Ultraviolet light at around 254 nm damages microbial DNA so that organisms cannot replicate. Its decisive advantage is that it is highly effective against Cryptosporidium, the pathogen chl…
The first chemical step in conventional surface water treatment. A coagulant neutralises the electrical charge that keeps fine particles suspended, allowing them to collide and grow into s…
The dominant secondary wastewater treatment process worldwide. A suspended community of micro-organisms is aerated so that it consumes the organic pollution in sewage, and is then settled …
Sand or multimedia filtration after clarification, and the barrier that matters most against chlorine-resistant pathogens such as Cryptosporidium.
Chlorine combined with ammonia to form monochloramine — a weaker but far more persistent disinfectant, used to maintain a residual across large distribution networks with less by-product f…
Boiling water and condensing the vapour. The oldest purification method and among the most complete, but energy intensive enough that it is rarely the right answer at scale.
A powerful oxidant generated on site, used both for disinfection and for breaking down taste, odour and micropollutant compounds.
The oldest engineered water treatment process still in wide use, and the one that first demonstrated that filtration prevents cholera. Its effectiveness depends on a living biological laye…
A looser membrane than reverse osmosis that rejects divalent ions and larger organics while passing much of the monovalent salt — useful where the goal is softening and organics removal ra…
Thermal desalination in which heated seawater flashes to steam in a sequence of chambers at successively lower pressure, recovering the latent heat at each stage.
Activated sludge combined with membrane filtration in place of a settlement tank, producing a much higher quality effluent in a much smaller footprint.
Bringing water into contact with air. A simple, low-energy step that oxidises iron and manganese, strips volatile compounds and dissolved gases, and raises pH by removing carbon dioxide.
A porous ceramic element with a controlled pore size, widely used in household and humanitarian settings because it is robust, cleanable and needs no power.
Uses an electric field to pull ions through selective membranes, rather than pushing water through a membrane. Most efficient on brackish water of low to moderate salinity.