Wastewater treatment overview, WWTP, SWTP, STP, WRRF

Respect the environment, purify and reuse as much as possible

Water often becomes polluted after human or industrial usage. In households people only use a very little bit for consumption. The rest is used and flushed through toilet and sink, polluted! In industries water might be used for all kind of production processes. Often resulting in a changed and polluted water quality as well. All resulting in a water that needs some kind of wastewater treatment to become purified again for discharge (returned to the environment) or reuse.

Wastewater treatment is a process used to remove contaminants from wastewater or sewage and convert it into an effluent that can be returned to the environment, or even to be reused.

For us it’s often about a multi-step approach in purification techniques. Simplified in figure below:

  • primary treatment.
    • To remove ‘particles’ for protection of main (secondary) treatment.
    • To equalise quality and flow. For more stable performance of downflow equipment.

  • secondary treatment.
    • To remove ‘that what must be removed’. Often it’s at least about carbon, nitrogen and phosphorus removal.

  • tertiary treatment.
    • To polish the effluent, if it would otherwise (just) not meet the discharge limits.
    • To further purify the effluent for any kind of reuse application. Read more in water treatment.
    • To aim for a zero liquid discharge (ZLD) approach.

  • sludge treatment
    • An approach in valorisation and disposal of the ‘by-products’ from wastewater treatment.

Wastewater treatment removes contaminants and undesirable components. It reduces such concentrations low enough. Such treatment is crucial to keep nature clean and healthy, to protect (life on) our planet.

In de definition of such treatment plants we can furthermore use names like:

  • wastewater treatment plant (WWTP)
  • sewage treatment plant (STP, SWTP)
  • water resources recovery facility (WRRF)
  • Biological nutrient removal (BNR)
  • Enhanced biological phosphorus removal (EBPR)

Below you can find some information about different steps in the overall wastewater treatment process. A possible treatment solution is described briefly as well.

Preliminary treatment

Here we have our focus on removing inorganics, sand/grint and rubbish. To protect downstream equipment like pumps. For the inorganics it is also because we are not interested to have it in the (primary or secondary) sludge otherwise.

rack or screen

based on size difference big particles and rubbish will be separated.

sand trap

based on gravity/settling heavy particles will sink. to remove sand, grind and other settleable inorganic substances.

Primary treatment (or pre-treatment)

With our focus on biological wastewater treatment we aim for a proper preliminary, primary or pre-treatment. Ideally all those compounds should be removed from the water, that could otherwise negatively impact the performance off downstream bioreactor(s), instruments, membranes, etc.

In this context it’s often about removal of particles (TSS) and fat, oil, grease (FOG). Furthermore about neutralising of pH (acidity) and mixing/equalisation.

On the other hand, it would be good to have a constant water quality and flow to the downstream treatment steps. To have the treatment plant operating continuously 24/7. With as minimum fluctuations as possible.

Within this application many of our solutions will be based on technologies based on/for and named like:

size differences

(coarse) screen


microscreen MCF

settleability differences

settler, settling tank


flotation differences

fat separator


other aeration induced flotation techniques

quality differences (over time)

equalisation tank

buffer tank





pH adjustment

possible odor issues



odor treatment like biofilter

Secondary treatment

Secondary treatment is what we describe as the main treatment to be done. Basically it is done in a biological pathway.

Read more in our technology biological treatment.

Tertiary treatment

Effluent polishing

Sometimes the purified wastewater (from secondary treatment) is not of good enough quality for discharge. In such cases additional purification might be needed. Some examples:

  • Phosphate removal. Often with coagulation and filtration solutions.
  • Solids or TSS reduction. Often with filtration solutions.
  • Disinfection. Possibly with ozonation, chlorination or UV solutions.
  • Carbon filtration solutions. For removal of remaining contaminants and impurities (like drug or medicine residues) by adsorption.

Effluent reuse

The main question in wastewater treatment is about disposal or recycling! Or even better (direct) reuse!

In any case additional purification technology is needed to further improve the water quality. For example, the final effluent of the WWTP is further treated till irrigation water or a high quality process water. Or even drinking water?

Reusing water has the potential for cost reduction in water supply and to reduce pressure on water resources. We know about different solutions for direct water reuse as well as additional (waste)water treatment before recycling.

Sludge treatment

From the primary treatment (pre-treatment) and secondary treatment (bio-treatment) a sludge will be separated. That sludge contains also carbon, nitrogen and phosphorus. By removing the sludge from the WWTP also those components are removed (from the water line). Nevertheless, it is not just like that. In sludge treatment it is often about some needed equipment or technology as well:

thickening, dewatering and/or drying

To minimise the sludge volume and weight to be disposed.

Separated water to be returned to the water line.


To produce biogas from the carbon/COD containing sludge. The resulting digestate (digested sludge) to be thickened, dewatered and/or dried as well.

Separated centrate (water) to be returned to the water line.

nutrients removal and/or recovery

To remove and/or extract nutrients like phosphorus and nitrogen from the centrate, as fertiliser.

Treated centrate (water) to be returned to the water line.

Read more in our application sludge treatment.