Aeration tank settling and real time control as a tool to improve the hydraulic capacity and treatment efficiency during wet weather: results from 7 years' full-scale operational data

This paper investigates the aeration tank settling (ATS) operation in combination with real time control (RTC) as a tool for increasing the hydraulic capacity and improving the treatment efficiency of a wastewater treatment plant (WWTP) during wet weather flows. Results from 7 years' full-scale operational data at the Avedøre WWTP, Denmark, show that ATS operation in combination with RTC increases the hydraulic capacity of the treatment plant with up to 150 and 67% of the design capacity during winter and summer respectively. Compared to the conventional wet weather operation, the ATS in combination with RTC operation resulted in lower effluent concentrations for total phosphate (40–50%), suspended solids (30–60%) and chemical oxygen demand (30–50%), whereas no significant effect was observed on total nitrogen. Apart from the reduced effluent concentrations, the RTC resulted in economic savings in the form of reduced costs for electricity and green taxes. However, in very few cases the ATS operation in combination with RTC was not able to handle design capacity, and some overflows occurred at flows below the design capacity. The frequency of these overflows may increase in the future due to increased rain intensity resulting in shorter prediction time available for ATS.

Computational fluid dynamics modelling of hydraulics and sedimentation in process reactors during aeration tank settling

Aeration tank settling is a control method allowing settling in the process tank during high hydraulic load. The control method is patented. Aeration tank settling has been applied in several waste water treatment plants using the present design of the process tanks. Some process tank designs have shown to be more effective than others. To improve the design of less effective plants, computational fluid dynamics (CFD) modelling of hydraulics and sedimentation has been applied. This paper discusses the results at one particular plant experiencing problems with partly short-circuiting of the inlet and outlet causing a disruption of the sludge blanket at the outlet and thereby reducing the retention of sludge in the process tank. The model has allowed us to establish a clear picture of the problems arising at the plant during aeration tank settling. Secondly, several process tank design changes have been suggested and tested by means of computational fluid dynamics modelling. The most promising design changes have been found and reported.

Modelling and test of aeration tank settling (ATS)

The use of aeration tank settling during high hydraulic loads on large wastewater treatment plants has previously been demonstrated as a reliable technique and proven valuable. The paper proposes a simplified deterministic model to predict the efficiency of the method. It is shown that a qualitatively correct model can be established. The simplicity of the model allows for on-line identification of the necessary parameters, so that no maintenance is needed to use of the on-line model for control. The practical implementation on three plants indicates that implementation of STAR with ATS control gives 50% increase of plant capacity for 3% extra cost.