AN EVALUATION OF SEDIMENT FORMATION IN THE ANAEROBIC DIGESTERS AT ROPEC

The Robert O. Pickard Environmental Centre in Ottawa, Ontario operates conventional mesophilic anaerobic digesters to stabilize a mixture of primary and thickened waste activated sludge. The digesters at ROPEC have been found to rapidly fill with sediment and hence require frequent cleaning. On average approximately 2 m of sediment accumulates every 4 years. This study was performed to characterize the solids entering and leaving the digester and present in the sediments with the primary objective of identifying the major contributors to sediment formation. In this regard the streams entering and leaving the digester and the digester sediment were characterized. The sediments were characterized with respect to their physical, chemical and mineralogical composition. Physical characterization involved a sieve analysis and measurement of the fixed and volatile fractions of the various particle size ranges. Visual observations were also employed to obtain further information on the nature of the particles that were present in the different size fractions.Chemical characterization was performed to identify the major elements making up the solids. A whole rock analysis that is normally employed for mineral characterization was employed to determine the composition of the solids. Three mineralogical techniques including Scanning electron microscopy, powdered X-Ray diffraction and an electron microprobe method were employed to characterize the sediment.These techniques provided information on the morphological properties as well as the elemental composition of individual particles in the sediment. By analyzing a cross-section of the sediment particles it was possible to obtain a semi-quantitative estimate of the mineral composition of the sediment. The results of this study indicated that the sediments present in the digester had a high fixed solids content. This was attributed to differential sedimentation of fixed solids and on-going destruction of organic solids in the sediment. A majority of the particles were found be smaller than 250 microns. Particles that were larger than 850 microns had a high organic fraction and visual inspection indicated that this material consisted of fibrous organic matter such as seeds, wood chips, hair etc. The sediment particles observed were found to consist primarily of vivianite and silicate based minerals. Vivianite is believed to have been formed in the digester due to the high concentrations of iron and phosphorous entering the digester and the highly reducing conditions in the digester. The silica minerals were similar to those found in grit. The results of this study suggest that the primary causes of sediment formation are the formation of vivianite in the digester and the deposition of grit particles that pass through the grit chamber and enter the digesters with the primary sludges.