Rotating biological contactors, also known as rotating biological reactors, have been an established biological wastewater treatment process for decades. In Germany, this technology is particularly common at small and medium-sized wastewater treatment plants.
Many of these systems have now been operating for 25 years or more. At the same time, treatment requirements continue to increase. Population growth, stricter discharge limits and changing levels of infiltration and inflow place growing pressure on the existing equipment.
Operators therefore face a crucial decision: Does the plant require complete replacement, or will a targeted upgrade of the existing infrastructure provide the better solution?
Why Many Rotating Biological Contactor Systems Reach Their Capacity Limits
Rotating biological contactors provide reliable and energy-efficient treatment. However, hydraulic shock loads and fluctuating influent conditions often reduce their performance. The technology also offers limited scope for operational adjustments while the plant is running.
Common challenges include:
- Shaft bearings and drive components deteriorate after many years of operation, increasing the risk of complete mechanical failure.
- Changes in influent concentrations overload the plant hydraulically or organically.
- Shock loads detach parts of the biofilm and destabilise biological treatment performance.
- Population growth or new commercial dischargers increase the overall wastewater load.
- Stricter discharge limits exceed the performance of the existing treatment stage.
- Extensive repairs to ageing equipment result in high costs and considerable coordination effort.
Several of these factors often occur simultaneously. Individual repairs are then no longer sufficient. Operators need a solution that addresses both the current condition of the plant and its future treatment requirements.
Overview of Available Upgrade Options
Operators have several options for modernising an ageing rotating biological contactor system. The most suitable approach depends on the structural condition of the plant, the actual wastewater load and the required discharge limits.
Replacing the Disc Packs and Drive System
Where the civil structures remain in good condition, the operator replaces worn disc packs, bearings and drive components. This option retains the existing treatment process.
However, it does not resolve fundamental capacity limitations. The restricted options for process control also remain unchanged.
Adding an Additional Final Settlement Tank
An additional final settlement tank improves solids separation. This approach is particularly suitable when the clarification stage limits the plant’s overall performance.
However, an additional tank does not compensate for an overloaded biological treatment stage.
Adding a Further Biological Treatment Stage
A parallel or downstream treatment stage increases the plant’s biological capacity. A fixed-bed reactor is particularly well suited to this type of upgrade.
The fixed-bed reactor reduces the load on the existing plant and stabilises effluent quality. At the same time, the operator continues to use the existing tanks and infrastructure.
Completely Replacing the Biological Treatment Stage
Where the existing equipment is severely deteriorated or the treatment requirements have increased substantially, a new biological stage replaces the rotating biological contactors entirely.
This option provides a long-term solution. However, it requires careful planning to ensure that wastewater treatment continues reliably throughout the construction phase.
Why a Fixed-Bed Reactor Is an Efficient Retrofit Solution
A containerised fixed-bed reactor adds biological treatment capacity to an existing wastewater treatment plant with limited civil works. Where the existing structures remain intact, integration is generally faster and more economical than constructing a completely new plant.
The process offers several advantages over simply refurbishing the existing rotating biological contactors.
Flexible and Scalable Expansion
Where the existing plant remains fundamentally operational, the fixed-bed reactor supplies only the additional biological capacity required. Its compact footprint minimises space requirements.
For more extensive modernisation projects, FBR technology assumes responsibility for the entire biological treatment process. Installation takes place alongside the existing plant, allowing wastewater treatment to continue throughout the transition.
Standardised Design and Compact Construction
PPU manufactures the treatment stage in standardised modules. This reduces engineering requirements and shortens delivery and installation times.
The containerised design provides clearly defined interfaces. Operators therefore benefit from a high degree of cost certainty and reliable project planning.
Reliable Performance Under Fluctuating Loads
The fixed-bed reactor handles variations in influent flow, infiltration and inflow, and hydraulic shock loads with a high degree of stability. Its hydraulic design specifically addresses changing operating conditions.
A stable biofilm develops on the fixed-bed media. The large protected surface area supports high biological activity within a compact treatment volume.
Operational Adjustments During Ongoing Treatment
Operators adjust the process to match the actual wastewater load. Key parameters include the air supply, recirculation rate and configuration of the individual treatment stages.
The control strategy also accounts for seasonal changes. During winter, for example, the operating team extends aeration periods to maintain stable biological treatment performance.
Construction and Installation During Ongoing Operation
The new treatment stage is installed next to the existing plant. The rotating biological contactors remain operational throughout the construction and installation phase.
Following commissioning, the project team fully integrates the fixed-bed reactor into the treatment process. This approach significantly reduces the risk of prolonged operational interruptions.
Modular and Turnkey Implementation
The modular design simplifies future plant extensions. Population growth or new discharge requirements therefore do not immediately require complete plant replacement.
Operators receive a flexible, long-term treatment solution. The turnkey design also reduces the coordination effort required on site.
The Typical Upgrade Process
A successful modernisation project starts with a thorough assessment of the existing plant. Technical design, coordination with the relevant authorities and project implementation follow this initial analysis.
1. Assessment of the Existing Plant
The project team records influent flow rates, infiltration and inflow levels, and existing effluent values. It also evaluates the structural and technical condition of the plant.
The assessment includes seasonal variations and potential future expansion of the catchment area.
2. System Sizing
The engineering team sizes the FBR module according to the hydraulic and pollutant loads. It also considers the remaining treatment capacity of the existing plant.
This process results in a system that is neither undersized nor unnecessarily large.
3. Coordination with Authorities
PPU supports the operator in discussions with authorities and consulting engineers. This includes process descriptions, technical documentation and permit planning.
Early coordination establishes clear requirements and prevents unnecessary delays.
4. Manufacturing and Site Preparation
Once PPU receives the order, it manufactures the required module. At the same time, the operator prepares the installation area and the necessary connections.
The standardised design limits on-site construction work to a small number of clearly defined tasks.
5. Installation and Commissioning
The installation team positions the container next to the existing wastewater treatment plant. It then connects the hydraulic, electrical and process-related interfaces.
The existing plant remains operational throughout this work. Once installation is complete, the team begins the technical and biological commissioning process.
6. Optimisation During Operation
Following start-up, the project team evaluates the operating data. It then adjusts the air supply, recirculation and operating cycles to match the actual wastewater load.
This fine-tuning stabilises effluent quality and reduces energy consumption.
The result is a high-performance wastewater treatment plant that meets current technical standards. At the same time, the system retains the flexibility required for future treatment demands.
Conclusion
An ageing rotating biological contactor system does not automatically require complete replacement. In many cases, a targeted upgrade provides the faster and more economical solution.
A fixed-bed reactor increases biological treatment capacity and stabilises effluent quality. Its containerised design reduces civil engineering requirements and supports implementation while the existing plant remains operational.
The modular configuration also provides additional capacity for future requirements. Operators secure the long-term performance of their wastewater treatment plant while continuing to use large parts of the existing infrastructure.
Contact us to discuss your project. Together, we assess the technical condition of your plant and develop a tailored upgrade concept.
