The project investigated a novel two‑stage membrane bioreactor concept for municipal wastewater treatment, with a particular focus on the removal of carbonaceous suspended solids (CSB) and the optimisation of nitrogen balances. In the first operational phase the pilot plant was operated as an A‑stage, where the influent CSB concentration was measured at roughly 1.6 kg CSB m⁻³ d⁻¹ and the sludge loading at about 1.2 kg CSB kg TS⁻¹ d⁻¹. These values are considerably lower than the typical loads reported for high‑load stages, which often exceed 2 kg CSB m⁻³ d⁻¹. The reactor achieved a CSB reduction of 47–60 % for total CSB and 60 % for dissolved CSB, surpassing the literature target of 50–70 % for high‑load stages. The high dissolved‑CSB removal is attributed to an extended hydraulic residence time of nearly 10 h and a sludge age of 4.6 days, both far above the usual 0.5–1.5 h and 0.1–0.5 days, respectively. This prolonged contact time led to additional oxidation of CSB, which, while beneficial for solids removal, reduces the organic fraction available for subsequent biogas production.
Nitrogen removal in the A‑stage was modest, with only about 10 % of the ammonium load being transformed. Ammonium concentrations in influent and effluent were nearly identical, and no detectable nitrate or nitrite was found in the effluent, indicating that nitrification and denitrification were not active under the operating conditions. Consequently, the C:N ratio in the effluent was favourable for the downstream PN‑A stages, which rely on a balanced carbon‑to‑nitrogen ratio for optimal performance.
During the second operational phase the plant was subjected to a series of optimisation interventions, including adjustments to aeration patterns and membrane module configurations. These measures were evaluated through detailed monitoring of CSB and nitrogen species, as well as through modelling of the plant’s carbon and nitrogen budgets. The results confirmed that the optimisation steps improved CSB removal efficiency while maintaining the desired C:N balance for the downstream processes.
The research was carried out in close collaboration between the Technical University of Darmstadt (TUDa) and the Institute for Sustainable Aquatic Systems (ISAH). TUDa led the experimental design, data acquisition, and analysis of the pilot plant performance, while ISAH contributed expertise in process modelling and the development of the denitrification strategy for the main stream. The project spanned two distinct operational phases, each lasting several months, and was supported by national research funding bodies that prioritise sustainable wastewater treatment technologies.
In summary, the pilot plant demonstrated that a membrane‑based A‑stage can achieve substantial CSB removal while preserving a favourable C:N ratio for subsequent biological nutrient removal stages. The extended hydraulic residence time and sludge age proved critical for the observed performance, offering insights for the design of future full‑scale systems. The collaborative effort between academia and research institutes provided a comprehensive assessment of both experimental and modelling aspects, laying the groundwork for further optimisation and scale‑up of the technology.