The Use of Factorial Designs to Identify Factors Affecting Particulate Removal in a Full Scale Tertiary Precipitation Plant with Flotation

The Sjölunda wastewater treatment plant comprises mechanical, biological and chemical treatment. The chemical treatment step includes flocculation and flotation. The concentration of suspended solids has been higher than expected when the plant was built. In this study factorial designs were used in combination with laboratory tests in order to identify factors of significance for the removal of suspended solids. The primary aim of the project was to reduce the suspended solids to about 10 mg/l.The investigation showed that the type of water, secondary effluent from trickling filters or an activated sludge process, and the flocculation energy were important for the process performance. The dosage of alum and polymer and the dosage point of the polymer were also significant. A high flocculation energy, i.e. a case with relatively small flocs, improved the overall removal. It also seems possible to decrease the concentration in the tertiary effluent to below 10 mg/l.