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.

Challenge

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 PE_COD to 120,000 PE_COD. 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.

Solution

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