Hydrocyclone as a cleaning device for anaerobic sludge digesters in a wastewater treatment plant

Anaerobic digestion is one of the oldest and preferred processes for the stabilization of waste-activated sludge produced from treating urban and industrial wastewater. Although there is no clear agreement on when to clean the anaerobic digester tanks, regular cleaning has proved to enhance the efficiency of the system, to reduce mechanical problems, and to avoid a decrease in their useful volumes. This paper describes the use of the hydrocyclone technology as an alternative for desludging. The coarse fraction recovered before the hydrocyclone plant contained potentially biodegradable matter that could be used as soil amendment, with low concentrations of heavy metals. This fraction had the highest mass loss in the thermogravimetric analysis (78%), thus showing the presence of calcium stearate as the major component. This was further demonstrated by Fourier transform infrared spectroscopy analysis. The hydrocyclone system showed time-dependent separation ability for all types of solids, with concentration factors that were higher for fixed (12 +/- 1 g/g) than for total (6 +/- 1 g/g) or volatile (2 +/- 1 gig) solids, being always higher for the warm than for the cold season. The X-ray diffraction analysis showed the presence of quartz in fine and underflow fractions. The formation of struvite was observed with the increase in phosphorus and magnesium especially in the fine fraction, with higher levels of chromium than the influent sludge, albeit without ever exceeding the maximum limited values admitted by most European countries. (C) 2014 Elsevier Ltd. All rights reserved.