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The managing water utility on the island of Bornholm is currently implementing a strategy to reduce water infiltration in sewers and at the same time minimise flood risks. Based on the result of a simulation study using DHI’s modelling software, WEST, the utility gained valuable information in planning of significant investments in new separate sewer systems. DHI’s solution helped the utility to quantify the benefits of the reduced hydraulic loading to Rønne WWTP in terms of improved effluent quality and lower operational costs.


Rønne WWTP is challenged with excess inflow during the winter period and wants to achieve higher treatment capacity and improve effluent quality following an investment in separate sewer infrastructure. To implement the new infrastructure, the water utility needed to quantify the positive impacts of the project and evaluate potential compensation for the new sewer system investment.


A model of Rønne WWTP was developed and calibrated using WEST, DHI’s dynamic modelling and simulation software. The model was used to evaluate improvements in effluent quality and cost savings in different scenarios by simulating progressive reduction of excess inflow.


Model-based scenario analysis

Evaluation of WWTP performance under different hydraulic loading

Improved efficiency

Up to 35% reduction in effluent load and 20% reduction in energy consumption

Yearly savings

Up to DKK 350,000 in lower operational costs per year

The full story


High hydraulic loading impacts overall WWTP efficiency
Rønne WWTP is the main wastewater treatment facility on the island of Bornholm, Denmark. The WWTP receives wastewater from a combined sewer system and comprises biological nutrient removal with post-precipitation treatment. In addition, waste sludge undergoes stabilisation through dewatering and dosing of lime and polymers.

The WWTP has traditionally been challenged by significant monthly variations in influent flow (Figure 1), mainly as a result of the contribution of stormwater and infiltration. High hydraulic loading is expected to have a negative impact on both treatment performance by affecting biological processes as well as operational costs due to increased energy consumption.

Figure 1. Variation of monthly average influent flow to Rønne WWTP (year 2018).

Reducing hydraulic loading: Investments and cost savings
As part of a strategy to reduce sewer infiltration and minimise flood risks, Bornholms Energi & Forsyning (BEOF) has planned the implementation of separate sewer infrastructure and replacement of aged pipes. This solution is expected to reduce the hydraulic loading to Rønne WWTP, resulting in improved efficiency and potentially significant cost savings that can compensate for the investment on new infrastructure. However, an exact quantification of these impacts requires appropriate modelling tools that accurately describe the present and future operation of Rønne WWTP.


The effects of hydraulic loading reduction in terms of improved treatment performance, energy efficiency and operational cost savings were quantified using DHI’s modelling software WEST. The initiative is part of the EU funded Interreg STEP (Sludge Technological Ecological Progress) project, which aims at improving resource recovery and energy efficiency in small to medium-sized WWTPs in the Baltic region.

Assessing the current performance
In order to get actionable insights on the performance of Rønne WWTP, a model of the WWTP was implemented in WEST. The model included two separate lines for biological treatment (enhanced biological phosphorus removal with intermittent aeration) and subsequent chemical dosing. Influent characterisation was performed based on one-year measured data provided by operators and verified with independent measurements.

The model was first calibrated against influent and effluent concentrations of COD, N and P fractions, mixed liquor suspended solids in activated sludge reactors and overall energy consumption. The model was subsequently verified with specific measured data from one month during which the WWTP operated under reduced hydraulic loading.

Figure 2. WEST layout implementation of Rønne WWTP and results of model calibration (average year 2018). © DHI

Performing scenario simulations to quantify the benefits of hydraulic loading reduction
The WEST model was subsequently used to evaluate improvements in Rønne WWTP’s efficiency under varying influent flows. The maximum excess flow (stormwater, infiltration), which in the future will be reduced through implementation of separate sewers, was estimated to be 3800 m3/d. Several scenarios were simulated, each representing different degrees of interception of excess inflow:

  • Baseline scenario with no excess inflow reduction
  • Progressive excess inflow reduction simulating 20%, 40%, 60%, 80% excess inflow reduction
  • Complete excess inflow reduction


Significant improvements in effluent quality and operational costs achieved
The results from the different simulations are presented for effluent quality, sludge production, energy consumption (pumping and aeration) and costs. Figure 3 shows a radar plot visualising changes in WWTP performance while considering multiple objectives.

A complete reduction of excess inflow was estimated to provide for considerable improvements in terms of effluent quality (-12% to -50% effluent load), energy consumption (-418 kWh/d) and operational costs (-34%).

Figure 3. Results of scenario analysis with increasing reduction of hydraulic loading to Rønne WWTP. © DHI

Considering an excess hydraulic loading of up to 3800 m3/d (accounting for 40% of average daily influent flow), it is estimated that every 20% step reduction of this excess flow will correspond to:

  • 2%, 9% and 10% reduction in effluent BOD, N and P, respectively
  • 4% reduction in energy consumption (pumping and aeration)
  • 7% reduction in costs (effluent taxes, energy)

Upon complete reduction of the hydraulic overloading, savings in operational costs have been estimated to be DKK 350,000 per year, which can be further reinvested in the implementation of the strategy for infiltration and flood minimisation.

About the client

Bornholms Energi & Forsyning (BEOF) is the managing utility company operating on the island of Bornholm, Denmark. It provides integrated water and energy services and is responsible for the operation and maintenance of sewer systems and eight wastewater treatment plants.

Software used

WEST - The state-of-the-art software for efficient dynamic modelling of wastewater treatment plants.

Case story - Highlights flyer

Reducing hydraulic loading to maximise efficiency

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