Drying of Activated Sludge Under Partial Vacuum Conditions—An Experimental Study

Abstract

A typical wastewater treatment system in a pulp and paper mill in Finland treats wastewater both mechanically and biologically. Sludges resulting from these processes have to be disposed of. One possible way of doing this is to incinerate them with solid fuel in the power plant of the mill. To minimize the amount of sludge and to make the use of the sludge energy efficient, it has to be dried before incineration. Mechanical drying of the sludge from biological wastewater treatment is difficult to carry out. Using secondary energies may provide a competitive way of arranging drying: a method of doing this by using partial vacuum evaporation to utilize the low temperature secondary heat in sludge drying is under development. A laboratory study to examine the behavior of activated sludge under partial vacuum evaporation conditions was carried out using a laboratory rotating evaporator to analyze the drying of activated sludge from three mills at 40–80°C boiling temperatures. This article presents the results from the tests. These are promising; it was possible to reach high dry solids content. Also, no boiling point temperature rise was detected, fouling of the evaporator seemed low and easily avoidable, condensate from the evaporator weas relatively clean and returnable to the processes of the mill.

Keywords:

• Evaporation
• Laboratory tests
• Pulp and paper industry

Acknowledgments

The study has been financed by the National Technology Agency in Finland (Tekes), Vapo Oy, Stora Enso Oyj and Pohjolan Voima Oy. The authors gratefully acknowledge all those, who have made a contribution to this work.