Assessment of sludge characteristics from a Biological Trickling Filter (BTF) system

Abstract

This study investigates settling and drainage properties of Biological Trickling Filter (BTF) sludge for dewatering and mineralisation potentially using natural Sludge Treatment Wetlands (STWs). Sludge settling experiments were conducted to determine solid removal efficiency using 1-L Imhoff cones (after 1 hour) and its effect on the removal of particulate components such as BOD5, TN, and TP was investigated. Drainage experiments were also conducted in the laboratory to measure the Specific Resistance to Drainage (SRD, describing sludge dewaterability) and sludge settling velocity to determine gravitational drainage of the BTF sludge. Excellent solid removals (68–95%) were achieved after 1 hour settling of BTF samples, reducing BOD5, TN and TP concentrations by 72%, 48% and 55% respectively. Dewatering experiments showed that while the settling velocity of the sludge (6.2 × 10−4 m/s) was ∼15–50 times faster than that of typical activated sludge (in the order of 10−5 m/s), the SRD of the sludge (∼6 × 1010 m/kg) was ∼2–15 times higher than that of activated sludge. Anaerobic conditions in the sludge and vigorous mixing greatly increased the SRD (up to 3.4 × 1011 m/kg), increasing the time for dewatering. This result suggests that while the sludge can be easily removed from BTF effluent by simple gravity sedimentation, dewatering of the sludge may be much slower than for typical activated sludge. Sludge Treatment Wetlands (STWs) combining sludge drying beds with vertical flow constructed wetlands could be applicable for dewatering and mineralisation of the BTF sludge. However, the STWs may require a larger land area than one treating activated sludge due to the lower dewaterability.