Reduction of the content of organic micropollutants in digested sludge by a post-aeration process - a full-scale demonstration

Surplus wastewater sludge reused for agricultural purposes must observe cer tain limit values with respect to different xenobiotic substances. The late st revision of the statutory order in Denmark includes for the first time a list of limit Values on organic micropollutants in sludge. Four groups of micropollutants are included in the list (LAS, PAH, NPE, and DEHP). The lim it values will be revised in June 2000, at which time up to 50% of the slud ge used for agricultural purposes,will not comply with the standards. It ha s been observed that the level of organic micropollutants is much higher in anaerobically digested sludge than in aerobically stabilised sludge. This indicates that the organic micropollutants in question can be partly or ful ly degraded under aerobic conditions but not under anaerobic conditions. Th e observations have formed the basis of the development of a post-aeration process for biological degradation of organic micropollutants in anaerobica lly digested sludge with the aim of enabling continued reuse of the sludge for agricultural purposes. The process is presented in this paper together with a description of a full scale demonstration experience on a Dan ish wa stewater treatment plant.