Papermills manufacturing recycled board usually face severe calcium carbonate scaling problems. We present a mass flow balance simulation of this papermaking process (PS2000, G2-based), coupled with a chemical reaction engine (IPhreeqc). The simulation allows predicting the pH and calcite saturation index throughout the process. Developed chemistry modules can simulate anaerobic microbial activity in the process waters, together with local contact with the atmosphere. Also, chemistry models of the integrated wastewater treatment (with anaerobic and aerobic treatment steps) are developed. The coupled simulation accurately describes the scaling tendency of process waters. This makes it possible to study curative solutions to the scaling problems in recycled board mills. The effect of a biocide treatment is simulated and discussed. This is predicted to largely reduce scaling in the wastewater treatment and limit the amount of generated sludge.