Aarhus Water, the managing water utility in the city of Aarhus, Denmark, has planned a capacity expansion of Viby wastewater treatment plant (WWTP) with minimum investments in new infrastructure. DHI partnered with other consultants and technology providers to identify a cost-efficient solution that relies on optimisation of existing processes. Interventions to achieve increased capacity were identified by using DHI’s modelling software, WEST. The proposed solution is now implemented in full-scale operation at the WWTP with considerable improvements in operational stability.

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Aarhus Water has planned to increase the capacity of Viby WWTP in order to ensure stable and efficient treatment performance. Due to the WWTP’s expected short lifetime (10–15 years), a cost-efficient capacity expansion should be achieved with minimum infrastructure interventions.


In partnership with NIRAS A/S, TechRas Miljø ApS and EssDe GmbH, DHI provided a solution relying on the optimisation of existing processes to achieve the desired increased capacity while minimising capital expenditures. A new optimal operational strategy was identified and tested using WEST. The strategy involved conversion of existing tanks and implementation of real-time control combined with innovative granular sludge technology.


33% capacity expansion

achieved through process optimisation and minimum infrastructure changes

Improved process stability

under high influent loading conditions

Less than EUR 2 million invested

in implementation of the capacity expansion

‘The expansion of Viby WWTP during 2019-2020 has ensured improvements in effluent quality during peak loads and in the sludge quality. These outcomes have been achieved through a very constructive collaboration between the partners who provided the right solutions to achieve the desired end result.’

Flemming B. Møller
Project Manager
Aarhus Water

arhusvand logo

The full story


Insufficient capacity at Viby WWTP
Aarhus Water has been facing several challenges in the operation of Viby WWTP. The WWTP (90,000 PE) has experienced sub-optimal nitrogen removal performance and solids handling, resulting from insufficient biological treatment capacity and hydraulic capacity under heavy rain events. The impact of insufficient capacity is expected to deteriorate due to forecasts of increased loading in the coming years.

Looking for cost-efficient solutions while considering short WWTP lifetime
To ensure stable performance, Aarhus Water has planned a 33% capacity expansion of Viby WWTP from 90,000 PE to 120,000 PE. Operation with increased capacity is expected to continue for the next 10–15 years when a shutdown of Viby WWTP and other WWTPs in Aarhus is planned, at which time the entire urban area will be served by a single, centralised WWTP.

Therefore, the short lifetime of Viby WWTP calls for cost-efficient solutions to be implemented, and the required expansion should be achieved by optimising existing processes and avoiding infrastructure investments with high capital expenditures.


DHI partnered with NIRAS A/S, TechRas Miljø ApS and EssDe GmbH to provide a solution for the desired capacity increase of Viby WWTP through revamping and optimisation of existing processes. The solution minimised interventions for new infrastructure and ensured the cost efficiency required by the client.

Using process expertise to identify interventions
A number of options for increasing the WWTP capacity were considered, namely:

  1. An increase in the aerated bioreactor volume
  2. Improved denitrification performance via external carbon dosing
  3. Control rules to regulate aeration, chemical dosing, internal and return sludge flows
  4. Improved solids handling

Using process modelling for time-efficient evaluation of alternatives
The combination of different options resulted in 22 possible WWTP configurations that were simulated using WEST. The performance of each configuration was evaluated under typical and extreme conditions (dry and wet weather, low temperature). Key performance indicators such as effluent quality, energy consumption and chemical usage were used to compare the different configurations and identify optimal operation.

Identifying a new operational strategy
The optimal WWTP configuration that would enable achieving the desired increased capacity was identified and included (Figure 1):

  1. Conversion of the hydrolysis tank into a pre-denitrifying tank
  2. Conversion of three pre-denitrifying tanks into intermittently aerated tanks
  3. External carbon dosing in the first intermittently aerated tank and in the last aerated tank (to be considered as post-denitrification tank)
  4. Implementation of FeCl3 dosing for chemical phosphorus removal
  5. Implementation of advanced control strategies for intermittent aeration, carbon dosing, recirculation of mixed liquor and settled sludge combined with the installation of additional online monitoring (ammonium, oxygen in process tanks, effluent nitrate)
  6. Improved solids settleability and retention using granular sludge technology
Figure 1. Comparison between currently operating configuration (90,000 PE) and optimised configuration with increased capacity (120,000 PE) of Viby WWTP. © DHI


Improved treatment performance
Model simulations of Viby WWTP operation with optimised process configuration showed significant improvements when compared to both current operation and other configurations tested, particularly:

  1. High effluent quality even under increased loading conditions (nitrate < 6 mg/L, total phosphorus < 0.25 mg/L)
  2. Reduced chemical consumption (< 200 kg/d for both external carbon and FeCl3)

Up to 47% reduction in effluent taxes was estimated to be achievable through the optimised operation, leaving considerable room to overcome costs for chemical usage and equipment installation.

Full-scale application of new operational strategy
At the end of 2019, the new operational strategy was implemented in Viby WWTP. A comparison of the treatment performance before (March 2019) and after the capacity expansion (March 2020) is presented in Figure 2. Despite significantly higher influent flow, the reported measurements indicate similar, if not improved, effluent nitrate and ammonia levels in 2020. Specifically, more efficient denitrification was achieved by implementing the advanced control strategy and increasing sludge retention in the system, as shown by improved settling characteristics (Figure 3).

Overall, the proposed solution for capacity expansion has provided tangible improvements in the performance and operational stability of Viby WWTP while proving to be cost-efficient and reducing the need of preliminary full-scale testing.

Figure 2. Influent flow, effluent nitrate and ammonium concentrations in Viby WWTPs before (2019) and after (2020) capacity expansion. Time series for nitrate and ammonium are indicated by arrows. © DHI
Figure 3. Comparison of sludge settling velocities before (2019) and after (2020) capacity expansion. © DHI

About the client

Aarhus Water (Aarhus Vand A/S) is the managing water utility in Aarhus, Denmark’s second largest city. The company is the local water supplier, storm- and wastewater service provider, and responsible for managing 10 wastewater treatment plants that receive 35 million m3 of wastewater per year.

Software used

WEST – The state-of-the-art software for efficient, dynamic modelling of wastewater treatment plants and virtual testing of a wide range of control strategies.

DIMS.CORE – The software platform has been deployed in the implementation of real-time control, data assimilation, automated anomaly detection, and automated reporting for process monitoring and optimisation.

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