Wastewater treatment plant

Cost-efficient capacity expansion of Viby wastewater treatment plant

Using the power of advanced control to minimise investment in new infrastructure


Aarhus Vand, the managing water utility in Aarhus, Denmark, needed to plan for a capacity expansion of Viby wastewater treatment plant (WWTP) to ensure stable and efficient treatment performance. Due to the plant’s expected short lifespan of approximately 10 years, they were looking for a solution requiring minimal investment in new infrastructure. By optimising plant processes through the use of existing infrastructure, DHI and partners identified a cost-efficient plan to achieve increased capacity. The proposed solution is now implemented on full-scale at the WWTP, with considerable improvements in operational stability.



Aarhus Vand was facing several challenges in the operation of Viby WWTP, including sub-optimal nitrogen removal and solids handling performance due to insufficient biological treatment capacity and hydraulic capacity during heavy rain. The impact of insufficient capacity was expected to intensify due to forecasts of increased loading in the coming years.


To ensure stable performance, Aarhus Vand wanted to expand the capacity of Viby WWTP by 33%, from 90,000 PECOD to 120,000 PECOD. The plant is expected to operate with increased capacity for the next 10 years, until the planned shutdown of Viby WWTP and other WWTPs in Aarhus – at which time the entire urban area will be served by a single, centralised WWTP.


The short lifespan of Viby WWTP meant that the expansion should be cost-efficient; achieved by optimising existing processes and avoiding infrastructure investments with high capital expenditures.



In partnership with NIRAS A/S, TechRas Miljø ApS and EssDe GmbH, DHI provided a solution that optimises existing processes to achieve increased capacity while minimising capital expenditures.


Using process expertise to identify interventions


The following options for increasing the WWTP capacity were implemented:

  • Increasing the aerated bioreactor volume, by introducing more aeration in existing tanks
  • Improving denitrification performance via external carbon dosing
  • Implementing control rules to regulate aeration, chemical dosing, internal and return sludge flows
  • Improving 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, DHI’s process modelling software that created a virtual replica of Viby WWTP. WEST enabled the team to test alternative operational strategies and different maintenance scenarios prior to physical implementation. 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 team identified the optimal strategy that would increase the capacity of Viby WWTP. This could be achieved through the use of the operational service DIMS.Core for real-time control and monitoring of the WWTPs, which is part of the TwinPlant solution. Specific interventions included:

  • Conversion of the hydrolysis tank into a pre-denitrifying tank
  • Conversion of three pre-denitrifying tanks into intermittently aerated tanks
  • External carbon dosing in the first intermittently aerated tank and in the last aerated tank (to be considered as post-denitrification tank)
  • 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 (e.g., ammonium, oxygen in process tanks )
  • Improved solids settleability and retention using granular sludge technology


Aarhus Vand A/S



Related SDGs:

SDG 6: Ensure availability and sustainable management of water and sanitation for all


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Model simulations of Viby WWTP using the optimised process showed significant improvements, i.e., high effluent quality even under increased loading conditions as well as reduced chemical consumption. The optimised operation could potentially achieve up to 47% reduction in effluent taxes, giving more leeway in costs for chemical usage and equipment installation.

At the end of 2019, the new operational strategy was implemented in Viby WWTP. Measurements before and after the implementation indicate reduced effluent nitrate and ammonia levels and improved capacity to accommodate high inflows. 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 for preliminary full-scale testing.


  • 33% capacity expansion achieved through process optimisation and minimum infrastructure changes
  • Improved process stability under high influent loading conditions
  • Less than EUR 2 million was invested in 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 constructive collaboration between the partners who provided the right solutions to achieve the desired end result.’

Flemming B. Møller, Project Manager
Aarhus Vand A/S

About our client

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 four wastewater treatment plants that receive 30-35 million m3 of wastewater per year.


In 2023, DHI and Aarhus Vand jointly established Utilizero, a new company that will support wastewater utilities in reducing greenhouse gas emissions and energy consumption based on solid experiences and solutions for automated process control. Learn more about Utilizero: https://www.utilizero.com/

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