Engineer Better
Biological Treatment.

Secondary wastewater treatment is the biological stage that removes dissolved and colloidal organic matter left after primary (mechanical) treatment. Microbial communities break down biodegradable organics and, where required, nutrients such as ammonia and nitrogen. It is the core requirement of the EU Urban Wastewater Treatment Directive for agglomerations above the defined size thresholds.

Primary treatment physically removes settleable solids. Secondary treatment uses biological processes (such as activated sludge, MBBR or MBR) to remove dissolved organic pollution and typically achieves the BOD, COD and TSS reductions required by the directive. Tertiary and quaternary treatment add further steps for nutrient polishing, disinfection or micropollutant removal.

Typical triggers are effluent limit exceedances (for example ammonia or BOD), hydraulic or biological overload, poor sludge settling, tightening discharge requirements, or the expanded scope of Directive (EU) 2024/3019. A structured engineering assessment identifies whether the limitation is hydraulic, biological or clarifier-related before selecting an upgrade pathway.

Common pathways include optimising the existing activated sludge process (aeration, SRT, RAS/WAS control), adding attached-growth capacity with MBBR or IFAS, converting to a membrane bioreactor (MBR) for a smaller footprint, or improving secondary clarification. The right option depends on the specific bottleneck, available footprint and target effluent quality.