Operation of a slow rate anaerobic digester treating municipal secondary sludge

• Published in: Electronic Journal of Biotechnology Vol. 5; no. 3
• Main Authors: Benatti, Cláudia Telles, Tavares, Célia Regina Granhen, Filho, Benedito Prado Dias, Moitinho, Maria da Luz Ribeiro
• Format: Journal Article
• Language: English
• Published: Universidad Católica de Valparaíso 24-03-2003
• Subjects: anaerobic digestion, enteric bacteria, enteric protozoa, sewage sludge, stabilisation
• ISSN: 0717-3458; 0717-3458

This study was designed to evaluate the performance of a slow rate
anaerobic digester in treating secondary sewage sludge received from
one local municipal wastewater treatment plant. The digester was fed by
secondary sewage sludge without any previous thickening. A series of
three independent batch experiments was investigated at an operation
time of 60 days. The total solids (TS) in the influent sludge contained
a percentage of organic matter of 59, 63 and 54%, a concentration of
volatile suspended solids (VSS) of 23.7, 29.2 and 27.8 g L-1 and a
chemical oxygen demand (COD) of 51.8, 32.9 and 65.7 g L-1 for the three
experiments, respectively. The operation of anaerobic digestion was
stable, with no noticeable scum or foaming problems. The COD reduction
in each experiment reached 29, 21 and 45% in the sludge and 95, 85 and
82% in the supernatant. The microbial indicators were surveyed by
sampling the sludge throughout the digester operation and counting the
number of bacteria in the sampled sludge. Counted bacteria included the
total culturable, the total and fecal coliform groups, Pseudomonas
aeruginosa and fecal streptococci. The percentage removal of the
indicator bacteria was higher for fecal streptococci (99.9%) than for
coliform bacteria (96.3%), which in turn was higher than for P.
aeruginosa (95.6%). Parasitological analysis was also performed on
multiple sludge samples by determination of protozoa and helminth eggs.
Protozoa ( Eimeria and Entamoeba ), helminth eggs ( Ascaris ,
Trichuris , Toxocara , Hymenolepis ) and mites were detected in the
influent sludge, and particularly among the helminth eggs, only
Trichuris was detected in the effluent sludge.