Information and insights into water treatment plant (WTP) and wastewater treatment plant (WWTP) technologies: what each stage does, and where it fits.
Last reviewed 27 October 2026
A water treatment plant turns raw water from a well, river, lake or municipal supply into water fit for a process, a boiler, a product or people. The stages used depend on the source water and the quality required, but most plants follow the same order.
Chemicals are added to destabilise fine particles so they clump into larger flocs, which then settle out in a clarifier or float off in dissolved air flotation. This is the main way surface waters lose their turbidity, colour and much of their organic load.
Multimedia (sand and anthracite) filters catch the particles left after clarification. Activated carbon filters remove chlorine, taste, odour and organics, and protect downstream membranes and resins from oxidation. Greensand and similar media remove iron and manganese.
Calcium and magnesium cause scale in boilers, heat exchangers and RO membranes. Sodium-form cation resin swaps them for sodium; the resin is regenerated with brine. Lime softening is used at larger scale.
Pressure forces water through a semi-permeable membrane, leaving dissolved salts and contaminants behind in a concentrate stream. RO typically removes over 95% of dissolved solids; NF removes hardness and larger molecules while letting more monovalent salts through.
Two-bed and mixed-bed demineralisers remove remaining ions to produce high-purity water. Electrodeionisation (EDI) does the polishing step after RO continuously, using an electric field and ion exchange membranes instead of chemical regeneration.
Chlorine and chloramine leave a residual that keeps protecting the water in storage and distribution. UV inactivates micro-organisms without chemicals or by-products. Ozone is a strong oxidant that also improves taste, odour and colour.
Pressure-driven membranes are grouped by how small a thing they can hold back. Tighter membranes remove more, but need more pressure and more careful pretreatment.
| Membrane | Approx. pore size | Typical pressure | Removes | Common use |
|---|---|---|---|---|
| Microfiltration (MF) | 0.1 to 10 µm | Under 30 psi (2 bar) | Suspended solids, bacteria | Pretreatment, clarification |
| Ultrafiltration (UF) | 0.01 to 0.1 µm | Under 45 psi (3 bar) | Colloids, bacteria, viruses, large organics | RO pretreatment, drinking water, MBR |
| Nanofiltration (NF) | About 0.001 µm | 70 to 220 psi (5 to 15 bar) | Hardness, colour, larger organics | Softening, organics removal |
| Reverse osmosis (RO) | Non-porous | 150 to 1,200 psi (10 to 80 bar) | Dissolved salts, most contaminants | Desalination, process and boiler water, reuse |
Need to size an RO system? Try the membrane and pressure vessel calculator, or see our RO membranes and pressure vessels.
A wastewater treatment plant removes solids, organic matter, nutrients and pathogens from sewage or industrial effluent so it can be discharged safely or reused. Treatment is described in levels, each one cleaner than the last.
Screens remove rags, plastics and large debris, and grit chambers settle out sand and gravel. This protects pumps and downstream equipment. Industrial plants often add an equalisation tank to even out flow and load.
Primary clarifiers let settleable solids sink and fats and oils float, removing a large share of suspended solids and some organic load before biological treatment. Industrial effluents may use oil separators, neutralisation or chemical precipitation here.
Micro-organisms consume the dissolved and fine organic matter, measured as BOD and COD. Aeration supplies the oxygen they need. The main process options are compared below.
Further polishing to meet tighter limits or prepare water for reuse: sand or disc filtration, ultrafiltration, activated carbon, and chemical or biological removal of nitrogen and phosphorus.
Chlorination (often followed by dechlorination before discharge), UV or ozone reduce pathogens to the level the discharge permit or reuse standard requires.
The solids removed along the way are thickened, stabilised (for example by digestion) and dewatered with a filter press, belt press or centrifuge, cutting the volume that has to be hauled away.
All of these use bacteria to break down organic matter. They differ in how the biomass is held in the reactor and how it is separated from the treated water.
| Process | How it works | Strengths | Watch-outs |
|---|---|---|---|
| Activated sludge (ASP) | Suspended biomass in an aeration tank, settled in a clarifier and partly returned | Proven, flexible, suits large flows | Large footprint; sludge settling must be managed |
| Extended aeration | Activated sludge with a long retention time | Stable, lower sludge production | Larger tanks, higher aeration energy |
| Sequencing batch reactor (SBR) | Fill, react, settle and decant happen in turn in one tank | No separate clarifier; good nutrient removal | Needs reliable automation and storage for batch flow |
| Moving bed biofilm reactor (MBBR) | Biofilm grows on free-moving plastic carriers in the tank | Compact, handles load swings, easy to upgrade | Needs a downstream clarifier or filter |
| Fluidised aerobic bioreactor (FAB) | Similar to MBBR: biofilm media kept in suspension by aeration | Compact, low sludge, simple operation | Media retention and aeration must be maintained |
| Membrane bioreactor (MBR) | Activated sludge separated by submerged MF/UF membranes instead of a clarifier | Very high effluent quality, smallest footprint, reuse-ready | Membrane cleaning and replacement, higher energy |
SBR, MBBR and FAB are common in package treatment plants because they are compact and run automatically.
Recycle and reuse plants take treated wastewater further, usually with ultrafiltration and reverse osmosis, so it can go back into cooling towers, boilers, process lines, irrigation or toilet flushing. Reuse cuts both freshwater demand and discharge volume.
Zero liquid discharge (ZLD) goes as far as it can: RO recovers most of the water, then the remaining brine is concentrated in evaporators and crystallisers until only solid salts remain. It is used where discharge is restricted or water is scarce, and it is energy-intensive, so it is designed carefully around the specific effluent.
Grey water treatment treats lightly used water from sinks, showers and laundry for non-drinking reuse on site.
Short answers to the questions we hear most.
Ask our teamA water treatment plant (WTP) turns raw water from a well, river or supply into water fit for drinking, a process or a boiler. A wastewater treatment plant (WWTP) cleans sewage or industrial effluent so it can be discharged safely or reused.
Both are pressure-driven membranes. Reverse osmosis removes almost all dissolved salts and needs higher pressure. Nanofiltration removes hardness, colour and larger molecules but lets more single-charge salts, such as sodium chloride, pass, so it runs at lower pressure.
An MBBR (moving bed biofilm reactor) grows bacteria on plastic carriers that move freely in a continuously fed tank, and needs a clarifier or filter after it. An SBR (sequencing batch reactor) fills, treats, settles and decants in turns in a single tank, so it needs no separate clarifier.
Zero liquid discharge is a treatment approach where no liquid wastewater leaves the site. RO recovers most of the water, then evaporators and crystallisers concentrate the remaining brine until only solid salts are left.
Electrodeionisation (EDI) polishes RO permeate to high-purity water using an electric field and ion exchange membranes. It runs continuously and, unlike mixed-bed ion exchange, needs no acid or caustic regeneration.
Pretreatment protects the membranes. Filtration removes particles, softening or antiscalant prevents scale, and activated carbon removes chlorine, which damages polyamide RO membranes.
Tell us about it. We can help with the components, consumables and packaged plants it needs.
