Fourth-stage wastewater treatment: Why the process chain matters more than a single unit

A current practical example from North Rhine-Westphalia

In October 2026, the Erftverband inaugurated the new treatment stage at the Rheinbach-Flerzheim group wastewater treatment plant. According to the operator, wastewater now undergoes further treatment using membrane filtration and powdered activated carbon before entering the Swistbach. The plant combines biological treatment, membrane bioreactor treatment and adsorption in a coordinated process chain.

The example is technically interesting because, during dry summer months, the plant's effluent can make up a large share of the Swistbach's flow. Under these conditions, the quality of the treated wastewater has a particularly pronounced effect on the receiving water body.

However, fourth-stage treatment is not a single standard piece of equipment. Depending on target substances, wastewater characteristics and site conditions, other processes or combinations may be required. What matters is the actual task performed by each stage.

Membrane bioreactor treatment combines biology and solids retention

A membrane bioreactor, or MBR, combines biological degradation of suitable wastewater constituents with membrane separation. The membrane retains biomass and fine solids. This can achieve an effluent with a very low solids content. Many microorganisms and particle-bound substances are also reliably separated.

This performance must not be equated with the across-the-board removal of all dissolved micropollutants. Many pharmaceutical residues, household chemicals and other organic micropollutants are present in dissolved form. Their removal depends on their substance properties, biodegradation and additional treatment stages.

This distinction is important for planning. The membrane replaces conventional secondary clarification and produces a particle-free effluent. However, it is not automatically the stage that adequately removes every dissolved target substance.

Activated carbon performs a different task

Powdered activated carbon has a large internal surface area onto which suitable dissolved substances can be adsorbed. At the Flerzheim group wastewater treatment plant, it specifically complements membrane technology in micropollutant removal. The loaded carbon is subsequently withdrawn from the process together with solids.

Activated carbon is not a universal solution for every substance either. Adsorbability, competition from other organic compounds, dosage, contact time and loading determine its effect. The Hessian Agency for Nature Conservation, Environment and Geology points out that the removal performance of fourth-stage treatment varies depending on the substance and process.

Performance assessment must therefore be based on clearly defined target parameters. Stating that a plant has fourth-stage treatment is no substitute for selecting relevant indicator substances or implementing appropriate monitoring.

Good pretreatment protects the membrane stage

The Flerzheim process diagram also shows that the membrane is not at the beginning of the chain. Screens, grit removal, primary clarification and sieving remove coarse or membrane-damaging constituents. Biological treatment, membrane bioreactor treatment and activated carbon follow only afterwards.

This sequence is not a minor detail. Hair, fibres, sand, fats and unsuitable solids can cause hydraulic bottlenecks or put membranes under strain. Appropriate pretreatment therefore improves not only effluent quality but also availability, cleanability and the maintenance requirements of downstream stages.

For a new plant, the entire chain can be planned together. For a retrofit, it is also necessary to assess which existing tanks, pumps, pipelines and automation components can continue to be used. Space requirements, construction sequencing and maintaining ongoing operation thus become key engineering questions.

Four planning questions for a robust process chain

1. Which substances and effects are decisive?

The starting point is not the equipment available, but the treatment objectives. Should solids, pathogens, selected organic micropollutants or several substance groups be reduced? Which legal requirements and receiving-water requirements apply at the site?

2. What task does each stage perform?

Biodegradation, solids retention, adsorption and oxidation rely on different mechanisms. Clear allocation of functions prevents expectations being placed on a stage for which it was not designed.

3. How are residuals managed?

Adsorbed substances remain in loaded activated carbon or other residues. Sludge and rinse streams must also be considered. The treatment concept therefore needs a traceable route for all material streams withdrawn from the process.

4. How is operation demonstrated?

Measurement points, sampling and operating data are needed to make the condition of individual stages visible. Pressure, flow, turbidity and other process values support operational control. Appropriate laboratory analyses are required to assess micropollutant removal.

Not every plant needs the same solution

The Flerzheim example demonstrates the benefit of a specific combination at that location. It does not prove that membrane bioreactor treatment plus powdered activated carbon is the optimum solution everywhere. HLNUG also lists granular activated carbon and oxidative processes such as ozonation. Biologically active post-treatment may be necessary after ozonation because transformation products can form.

Selection is therefore based on wastewater characteristics, target substances, receiving-water conditions, space, energy demand, chemical consumption, residuals and costs over the operating life. Piloting or trials can improve the data basis where suitability under real conditions has not yet been sufficiently demonstrated.

The ROTREAT connection

ROTREAT designs MBR systems and modular wastewater treatment plants for municipal, decentralised and industrial applications. The focus is on a maintainable process chain that matches the actual water composition and treatment objective. This includes pretreatment, the membrane stage, automation, cleaning strategy and accessible plant construction.

The most important lesson from current projects is therefore not that a single technology solves every task. Good treatment performance results when each stage fulfils a clearly defined function and the interfaces are controlled in operation.

Next steps

Are you planning an MBR, a retrofit or a modular treatment stage? Talk to ROTREAT about feed data, target values, available space and integration into ongoing operation.