A pilot study was performed to investigate advanced wastewater treatment by pre-precipitation in combination with biological nitrogen removal supported by biological sludge hydrolysis. The influent wastewater was pretreated by addition of a pre-polymerized aluminium salt, followed by flocculation and sedimentation. Chemical pretreatment resulted in 60% COD-reduction and 75% phosphorus reduction. The chemically precipitated primary sludge was exposed to anaerobic sludge hydrolysis at retention times of 1 and 2 days at temperatures in the range of 15-30-degrees-C. At a retention time of two days at 20-degrees-C, resulting solubilization was 10-13% of the suspended COD. The liquid phase of the hydrolyzed sludge, the hydrolysate, was separated from the suspended fraction by centrifugation and added to the biological nitrogen removal stage to support denitrification. The hydrolysate COD consisted mainly of volatile fatty acids, resulting in high denitrification rates. Nitrogen reduction was performed based on the Bio-Denitro principle in an activated sludge system. Nitrogen was reduced from 45 mg/l to 9 mg/l and phosphorus was reduced from 11 mg/l to 0.5 mg/l. The sludge yield was low, approx. 0.3-0.4 gCOD/gCOD removed. This in combination with the efficient pretreatment emphasizes the concept as well suited for compact advanced treatment including nutrient removal.
