Hydraulic Behaviour in an Activated Sludge Tank : From Tracer Test through Hydraulic Modelling to Full-Scale Implementation

Hydraulic behaviour in a large denitrifying activated sludge tank, located at the Rya Wastewater treatment plant (WWTP) in Goteborg Sweden, was investigated and optimized by using fullscale tracer test and hydraulic modelling. The Rya WWTP is a high loaded plant with a small footprint. Expansion of the site is limited due to several restrictions. An efficient use of already existing facilities is therefore of great importance in order to meet the demands of decreased effluent limits, set by authorities. The presence of a short circuiting stream was identified with full-scale tracer tests and the hydraulic situation was successfully quantified using the Martin model, a compartment model based on curve fitting to the residence time distribution curve (RTD-curve) from the full-scale tracer test. Computational fluid dynamics (CFD) 3D modelling was used for virtual prototyping of different corrective measures near the inlet to break the previously identified short circuiting stream. From an operational point of view, inlet baffles were chosen as the preferable alternative. After implementation, improved tank hydraulics, were verified with another full-scale tracer test. At a normal flow (3.6 m3/s), the space time in the tank was found to be 16 minutes. In the original tank 30 % of the water had a residence time of less than 8 minutes, 49 % of the water had a residence time between 8 and 24 minutes and 21 % of the water had a residence time of more than 24 minutes. After implementation of corrective measures, the tank with baffles had 6 % of the water with a residence time of less than 8 minutes. 77 % of the water had a residence time between 8 and 24 minutes and 17 % of the water had a residence time of more than 24 minutes. Also, the mixing characteristics in the tank achieved a more plug-flow like character. Full-scale tracer tests are very informative when investigating hydraulic situations in activated sludge tanks. Mathematical modelling using black box approach can help to quantifying the hydraulics in a tank. For example can use of the Martin model help to quantify short circuiting streams and dead volumes. CFD modelling is a useful and informative tool for analyzing problematic hydraulics at wastewater treatment plants and for the design of theoretical corrective measures. Being a useful tool for virtual prototyping, this type of modelling also deserves to have a more central role when designing large reactors and other hydraulic systems.