Sludge deterioration in a full scale UASB reactor after a pH drop working under low loading conditions

Abstract

A full scale UASB reactor treating the effluent of a malting plant was operated during nearly two years. During 37 weeks of operation the reactor worked with a COD removal efficiency of 80% and a biogas production of nearly 300 m(3)/d with a methane content of 77%. After the start up and during these months of operation the volumetric organic load was 4 kgCOD/m(3).d and the specific organic load was between 0.2-0.4 kgCOD/kgVSS.d. The sludge SMA in this period was around 0.25 kgCOD/kg VSS.d. On week 37 as a result of a problem at the industrial process the pH in the reactor dropped to a value of 4.8. After pH recovering, the reactor worked with fluctuating COD values in the exit and showed a downward trend in the COD removal efficiency. On week 81 the presence of filaments in the granules was detected. High proportion of Chloroflexi filaments were detected by FISH in the sludge. Changes in the microbial population caused by the low pH probably destabilize the reactor performance. The presence of filamentous granules in the sludge and its further growing could be encouraged by the pH drop and the low specific organic load applied to the reactor. The low specific organic load was a consequence of the high VSS content in the UASB reactor, due to the lack of purges. The length of the filaments attached to the granules grew throughout time. In order to eliminate the sludge with poor settlement properties a recycle was applied to the reactor. As a consequence, low amount of granular sludge stayed in the reactor. At the end, COD concentration in the influent reached higher values than in normal operation; at the same time a complete sludge wash out occurred. On the other hand, using the same sludge (after the recycle implementation) in a bench scale reactor the good properties of the sludge were completely recovered.

Preliminary analysis ofChloroflexipopulations in full-scale UASB methanogenic reactors

“…In reactor MO, our group detected sedimentation problems due to overgrowth of Chloroflexi filaments (Borzacconi et al . ). The inocula for reactors AL, COA, COB and SR were taken from the same anaerobic lagoon treating slaughterhouse wastewater but at different time points.…”

“…Reactor MO was previously studied by Borzacconi et al . (, ). Reactor AL and SR were sampled during the start‐up period.…”

“…But, in a previous study on the performance of an UASB reactor installed in our country, the overgrowth of filamentous Chloroflexi was related to an unusual anaerobic bulking process that caused sedimentation problems (Borzacconi et al . ). This bulking process seriously affects the performance of UASB reactors as it produces sudden washout of the granular sludge biomass.…”

Strategy to identify the causes and to solve a sludge granulation problem in methanogenic reactors: application to a full-scale plant treating cheese wastewater

“…"Degranulation/lack of granulation" is a problem that occurs sometimes in this type of anaerobic systems resulting often in heavy loss of biomass and poor treatment efficiencies or even complete reactor failure (e.g. Clark, 1988;Maat and Gorur 1990;Grootaerd et al 1999;Ahn et al, 2001;McHugh et al 2006;Borzacconi et al 2008;Li et al 2008;Sekiguchi et al 2015).…”

Release of Extracellular Polymeric Substance and Disintegration of Anaerobic Granular Sludge under Reduced Sulfur Compounds-Rich Conditions

“…Chou and Huang [14] observed that smaller granules were formed in the phenol-fed UASB reactor than in the sucrose-fed reactor. Borzacconi et al [15] demonstrated that formation of filamentous granule with poor settling velocity caused washout of the biomass in a full-scale UASB reactor treating malting plant effluent. Ismail et al [16] reported that granules developed under higher salinity condition contained lower EPS concentration and showed lower strength.…”