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High rate anaerobic treatment of LCFA-containing wastewater at low temperature

Research output: Book/ReportDoctoral thesisCollection of Articles

Details

Original languageEnglish
PublisherTampere University
Number of pages139
Publication statusPublished - 11 Dec 2019
Publication typeG5 Doctoral dissertation (article)

Publication series

NameTampere University Dissertations

Abstract

Fats, oil and grease (FOG) are a significant constituent in numerous wastewaters such as those in dairy industry. The hydrolysis of FOG result in the production of long chain fatty acids (LCFA) which destabilize the anaerobic treatment process due to their physico-chemical and microbial toxicity effects. Harnessing the high methanogenic potential of FOG necessitates effective treatment of high LCFA loads, wherein the feasibility of LCFA treatment at low temperatures has been not investigated up to now. The aim of this thesis was to study the feasibility of high-rate anaerobic treatment of LCFA-rich wastewaters at low ambient temperatures using dairy wastewater.

The screening of mesophilic inocula for treatment of mixed LCFA containing synthetic dairy wastewater (SDW) in batch studies showed that granular sludge inoculum achieved faster and higher methane yields (76-82% of theoretical yield) than the two municipal digestates (1-72%) at both 20 and 10°C. The LCFA
degradation capacity in the granular sludge inoculum was attributed to the presence of β-oxidizing bacteria from the family Syntrophaceae (Syntrophus and uncultured taxa), the acetotrophic activity of Methanosaeta and the putative syntrophic acetate oxidizing bacteria (SAOB).

Continuous high-rate treatment of SDW was found to be feasible in expanded granular sludge bed (EGSB) reactors at 20°C (hydraulic retention time (HRT) 24 h, LCFA loading rate (OLR) 670 mgCOD-LCFA/L·d) with a soluble COD (sCOD) removal of 84–91% and methane yield of 44–51%. SDW feeding for longer than two months resulted in LCFA accumulation, which led to granular sludge flotation (36-57%) and disintegration (reduction in d50 of 24–33% and 75–84% in settled and washed-out granules, respectively). To counter the LCFA induced granular sludge disintegration and flotation, a novel reactor type, dynamic sludge chamber-fixed film (DSC-FF), was designed and achieved sCOD removal of 87-98% at HRTs from 12-72 h (LCFA loading rate 220-1333 mgCOD-LCFA/L·d) at 20°C. Moreover, even at the 12 h HRT, the unsaturated LCFAs
(linoleate and oleate) were treated and only part of saturated LCFAs (stearate, palmitate) remained after treatment in the DSC-FF reactors. An increased methanogenic activity was established in the reactor sludges during reactor runs, which was evidenced by a higher acetotrophic activity in the granular sludge (from DSC), and a higher hydrogenotrophic activity in the biofilm (from FF) indicating development of distinct metabolic capabilities in the different reactor compartments.

High throughput 16S rRNA sequencing showed that the relative abundance of the acetoclastic methanogen, Methanosaeta, increased in EGSB reactors and in the active microbiomes of granules (from DSC) and biofilm (from FF) when fed with increasing LCFA concentrations. This suggested acetoclastic methanogenesis as the predominant methanogenesis pathway for SDW and presumably, LCFA degradation at 20°C. Relative abundances of the taxa known to have β-oxidizing and methanogenic activity were high in the active microbiomes during SDW treatment in DSC-FF reactors at 20°C. The biofilm
microbiome (from FF) had a prominent presence of the β-oxidizing bacteria Syntrophus and of the hydrogenotrophic methanogen Methanospirillum in comparison to the presence of the acetogenic bacteria, Syntrophobacter, Desulfobulbus, and Geobacter, and of the acetoclastic methanogen in the
granular sludge microbiome, suggesting a role of these different taxa during LCFA degradation.

In summary, this work demonstrated successful inoculum selection at low temperatures (10 and 20°C), and high-rate anaerobic LCFA degradation at 20°C using novel reactor design (here, DSC-FF). The key bacterial and archaeal taxa involved in the anaerobic conversion of LCFA to methane at 20°C were also
deduced.

Field of science, Statistics Finland