Optimisation of sludge line management to enhance phosphorus recovery in WWTP

The management of the sludge treatment line can be optimized to reduce uncontrolled phosphorus precipitation in the anaerobic digester and to enhance phosphorus recovery in WWTP. In this paper, four operational strategies, which are based on the handling of the prefermented primary sludge and the secondary sludge from an EBPR process, have been tested in a pilot plant. The separated or mixed sludge thickening, the use of a stirred contact tank and the elutriation of the thickened sludge are the main strategies studied. Both the reduction of phosphorus precipitation in the digester and the supernatant suitability for a struvite crystallization process were assessed in each configuration. The mixed sludge thickening combined with a high flowrate elutriation stream reduced the phosphorus precipitation in the digester by 46%, with respect to the separate sludge thickening configuration (common practice in WWTP). Moreover, in this configuration, 68% of the soluble phosphorus in the system is available for a possible phosphorus recovery process by crystallization (not studied in this work). However, a high Ca/P molar ratio was detected in the resultant supernatant which is pointed out as a problem for the efficiency of struvite crystallization.