Researchers at Aalto University have developed a simple method for reducing the amount of phosphorus in the wastewater of a pulp mill. The method is called simultaneous precipitation using iron sulphate. A separate treatment stage is not required, as the precipitation takes place simultaneously with the actual biological wastewater treatment.
Iron sulphate is added to the wastewater prior to the biological wastewater treatment process, and the phosphorus dissolved into the water is precipitated with the biomass at the treatment plant. Finally, the phosphorus is removed from the plant with the sludge. In Finland, sludge is generally burned, in which case the phosphorus would end up in the ashes and would thus be reusable in the form of fertilizers, for example.
Simultaneous precipitation creates savings in wastewater treatment costs
Public authorities are calling for the lowering of phosphorus emissions. For this reason, many factories have adopted an additional post-treatment precipitation stage, which is usually implemented using aluminium.
Simultaneous precipitation does not require additional wastewater treatment units, so there is no need for additional energy in the treatment of the water, either. An additional benefit of the method is that iron sulphate is an inexpensive chemical. A municipal wastewater treatment plant in Saint Petersburg constructed a few years ago employs simultaneous precipitation, among other methods. The industry, on the other hand, is only just waking up to the possibility of applying simultaneous precipitation. Wastewater from the forestry industry contains less phosphorus than municipal wastewater, so the dosage of the iron chemical remains within reasonable limits.
Experiments at the pulp mill
Initially, the research was carried out in the laboratories of Otaniemi and at the plant using pilot equipment. The results were so promising that, later on; iron precipitation was also successfully tested at the wastewater treatment section of a pulp mill.
The research has been published in the Water Science & Technology journal.