AQUASIM is described as 'In order to support environmental scientists in finding an «adequate» model of the system they are investigating, a computer program is necessary, which allows its users to perform simulations using different models, to assess the identifiability and to estimate the'. There are five alternatives to AQUASIM for Windows. The best alternative is BioWin. It's not free, so if you're looking for a free alternative, you could try STOAT. Other great apps like AQUASIM are SIMBA# (Paid), SIMBA (Paid) and WEST (Paid).
BioWin is a Microsoft Windows-based simulator used world-wide in the analysis and design of wastewater treatment plants. The figure below shows an example of a nutrient removal system configuration set up in BioWin.
SIMBA# is the newest member of the SIMBA™ simulator family and provides a fully integrated simulation platform for run-off, sewer systems, WWTPs and rivers. SIMBA# is a stand-alone simulator which takes urban wastewater system design and optimisation to another level.
WEST is a powerful and user-friendly tool for dynamic modelling and simulation of wastewater treatment plants (WWTP). The extensive state-of-the art model and process library of WEST enables you to model and evaluate almost any kind of modern WWTP.