How Water Filters Work

How It Works

Activated carbon has an enormous surface area (1 gram can have the surface area of a football pitch). Contaminant molecules adsorb — chemically bond to — the carbon surface as water passes through. Carbon block filters are denser and more effective than granular activated carbon (GAC), especially for organic compounds and some disinfection byproducts.

Removes

• Chlorine and chloramines (taste & odour)
• VOCs (volatile organic compounds)
• Pesticides and herbicides
• Some disinfection byproducts (THMs)

Limitations

• Does not remove heavy metals, nitrates, or PFAS
• Does not remove bacteria or viruses
• Filter replacement critical — exhausted carbon can release contaminants

How It Works

Water is forced under pressure through a semi-permeable membrane with pores approximately 0.0001 microns — small enough to block dissolved salts, metals, and most organic molecules. Modern under-sink RO systems use 4–7 stages: sediment pre-filter, carbon pre-filter, RO membrane, carbon post-filter, and sometimes remineralisation or UV stages. Tankless RO systems (like Waterdrop G3P800) deliver filtered water on demand without a storage tank.

Removes

• Heavy metals (lead, copper, arsenic)
• PFAS / forever chemicals
• Nitrates and nitrites
• Microplastics
• Bacteria and viruses
• Dissolved solids (TDS reduction 90–98%)

Trade-offs

• Produces wastewater (typically 2–4× the filtered volume)
• Removes beneficial minerals — remineralisation stage recommended
• Slow flow rate without pressurised tank

How It Works

A UV-C lamp (typically 254nm wavelength) irradiates water as it flows through a chamber. UV light damages the DNA of bacteria, viruses, and protozoa, preventing them from reproducing. UV is effective against chlorine-resistant pathogens like Cryptosporidium and Giardia. It is always paired with mechanical filtration, as suspended particles can shield microorganisms from UV exposure.

Removes / Inactivates

• Bacteria (E. coli, coliform, Legionella)
• Viruses (norovirus, rotavirus)
• Protozoa (Cryptosporidium, Giardia)

Limitations

• No effect on chemical contaminants, heavy metals, PFAS
• Requires clear water — turbidity reduces effectiveness
• No residual protection after treatment point

How It Works

Ceramic filters use a porous clay matrix with pore sizes of 0.2–0.5 microns. Hollow-fibre ultrafiltration uses bundles of hollow polymer fibres with similar pore sizes. Water passes through; anything larger than the pore size (bacteria, cysts, sediment) is mechanically blocked. Many ceramic filters also incorporate silver to prevent bacterial growth within the filter medium. Ceramic filters can be cleaned and reused multiple times.

Removes

• Sediment, turbidity, rust
• Bacteria and cysts (0.2µm pore size)
• Microplastics

Limitations

• Does not remove dissolved chemicals, chlorine, heavy metals
• Does not remove viruses (too small at standard pore sizes)
• Slow flow rate without pressure

How It Works

Ion exchange resins carry a charge that attracts specific ions from water and releases a different ion in exchange. Salt-based water softeners swap calcium and magnesium ions (hardness) for sodium ions. Specialised resins can target heavy metals, nitrates, or arsenic. KDF (kinetic degradation fluxion) media uses a redox process to remove chlorine and heavy metals. Ion exchange resins require periodic regeneration (salt for softeners, acid/base for others).

Removes

• Water hardness (calcium, magnesium)
• Some heavy metals (varies by resin type)
• Nitrates (specialised anion resin)

Limitations

• Softeners add sodium — not suitable for low-sodium diets
• Does not remove bacteria, viruses, or organic chemicals
• Resin requires regular regeneration or replacement