Foam formation in a downstream digester of a cascade running full-scale biogas plant: Influence of fat, oil and grease addition and abundance of the filamentous bacterium Microthrix parvicella

Foaming mechanisms and control strategies during the anaerobic digestion of organic waste: A critical review

Foaming is a problem that affects the efficient and stable operation of the anaerobic digestion process. Characterizing foaming mechanisms and developing early warning and foaming control methods is thus critically important. This review summarizes the correlation of process parameters, state parameters, and microbial communities with foaming in anaerobic digesters; discusses the applicability of the above-mentioned multi-scale parameters and foaming potential evaluation methods for the prediction of foaming risk; and introduces the principles and practical applications of antifoaming and defoaming methods. Multiple causes of foaming in anaerobic digestion systems have been identified, but a generalizable foaming mechanism has yet to be described. Further study of the correlation between extracellular polymeric substances and soluble microbial products and foaming may provide new insights into foaming mechanisms. Monitoring the foaming potential (including the volume expansion potential) is an effective approach for estimating the risk of foaming. An in-situ monitoring system for determining the foaming potential in anaerobic digestion sites could provide an early warning of foaming risk. Antifoaming methods based on operating parameter management and process regulation help prevent foaming from the source, and biological defoaming methods are highly targeted and efficient, which is a promising research direction. Clarifying foaming mechanisms will aid the development of active antifoaming methods and efficient biological defoaming methods.

Identification of microorganisms responsible for foam formation in mesophilic anaerobic digesters treating surplus activated sludge

Foaming is a common operational problem in anaerobic digestion (AD) systems, where hydrophobic filamentous microorganisms are usually considered to be the major cause. However, little is known about the identity of foam-stabilising microorganisms in AD systems, and control measures are lacking. This study identified putative foam forming microorganisms in 13 full-scale mesophilic digesters located at 11 wastewater treatment plants in Denmark, using 16S rRNA gene amplicon sequencing with species-level resolution and fluorescence in situ hybridization (FISH) for visualization. A foaming potential aeration test was applied to classify the digester sludges according to their foaming propensity. A high foaming potential for sludges was linked to the abundance of species from the genus Candidatus Microthrix, immigrating with the feed stream (surplus activated sludge), but also to several novel phylotypes potentially growing in the digester. These species were classified to the genera Ca. Brevefilum (Ca. B. fermentans) and Tetrasphaera (midas_s_5), the families ST-12K33 (midas_s_22), and Rikenellaceae (midas_s_141), and the archaeal genus Methanospirillum (midas_s_2576). Application of FISH showed that these potential foam-forming organisms all had a filamentous morphology. Additionally, it was shown that concentrations of ammonium and total nitrogen correlated strongly to the presence of foam-formers. This study provided new insight into the identity of putative foam-forming microorganisms in mesophilic AD systems, allowing for the subsequent surveillance of their abundances and studies of their ecology. Such information will importantly inform the development of control measures for these problematic microorganisms.

Performance of an anaerobic plug-flow reactor treating agro-industrial wastes supplemented with lipids at high organic loading rate

This study evaluated the performance of a plug-flow reactor (PFR) for high-rate anaerobic co-digestion of complex agro-industrial wastes and used cooking oil or animal fat. The PFR was successfully operated up to an organic loading rate (OLR) of 21 g L^-1 d^-1, yielding biogas at 0.35 L g^-1 chemical oxygen demand (COD) influent. During the study period, supernatant COD at the PFR effluent remained between 4 and 7 g L^-1, with negligible volatile fatty acids' concentrations (<500 mg L^-1) and no presence of foaming incidents. The biomass concentration inside the PFR, expressed as total suspended solids, remained between 30 and 60 g L^-1. Moreover, the above-mentioned anaerobic digestion technology has been currently scaled-up at 50 m³ PFR, while a full-scale facility of 240 kW-el is under construction in the region of north-eastern Greece.

A novel dicationic Quinoline-Carzole fluorescent probe: preparation and labelling of Microthrix parvicella

A novel dicationic Quinoline-Carbazole fluorescent probe with hydrophobic long-chain alkane was designed and synthesized based on the property of Microthrix parvicella (M. parvicella) in situ utilizing long-chain fatty acids (LCFA) in the activated sludge system. ¹H NMR spectrum, ultraviolet-visible (UV-Vis) absorption spectra, and fluorescent spectra analysis demonstrated that the probe was successfully obtained. The probe had a large stokes-shift ranging from 102 to 144 nm in different solvents, which were benefit for the fluorescent labelling properties. The labelling experiment indicated that the prepared probe could absorb onto the surface of M. parvicella through hydrophobic bond. Much stronger yellow fluorescence of M. parvicella was observed at the concentration of 1.0 × 10^-5mol/L when compared with the zooglea, which makes it easy to distinguish M. parvicella from the zooglea. In addition, the photostability of the probe was also investigated, and the result showed that the probe was quite stable in a long period of time. All the results indicated that the prepared probe was suitable for the labelling of M. parvicella.

Impact of the acetate/oleic acid ratio on the performance, quorum sensing, and microbial community of sequencing batch reactor system

This study comprehensively investigated the impact of acetate/oleic acid ratios (80%/20%, 60%/40%, 40%/60%, and 20%/80%) on sequencing batch reactor (SBR) with respect to the variations in performance, quorum sensing (QS), and microbial community. Results showed that NH⁺₄-N removal was not affected by the acetate/oleic acid ratios, while the COD, total nitrogen (TN), and PO^3-₄-P removal was considerably affected. The increasing oleic acid ratios led to severe sludge bulking, which was significantly positively correlated with proteins/polysaccharides (p < 0.001). The correlation of QS with the performance and sludge properties was also observed. High-throughput sequencing demonstrated that microbial compositions considerably shifted with varying acetate/oleic acid ratios. Moreover, the potential correlation of bacterial genera with the SBR performance and QS was proposed. This study elucidated the effect of acetate/oleic acid ratios on SBR from microbial viewpoint, which provided insights into fully understanding the essential roles of carbon source on wastewater treatment.

Elucidating the microbial community associated with the protein preference of sludge-degrading worms

Sludge predation by aquatic worms results in an increased sludge reduction rate, which is mainly due to the specific removal of a protein fraction from the sludge. As microorganisms play an essential role in sludge hydrolysis a better understanding of the microbial community involved in the worm predation process will provide more insight into the relations between the aquatic worms, their associated microbiome and the efficient sludge reduction. In this study, the microbial community associated with predation by the Tubifex tubifex was investigated. The microbial diversity in the samples of the worm faeces (WF), predated activated sludge and protein-rich substrates were compared. The results indicated that predation on sludge resulted in a microbial change from Actinobacteria (44%) in the sludge, to Proteobacteria (64%) and Bacteriodites (36%) in the WF. Interestingly, the faecal microbial community was more related to the community in (predated) protein-rich substrates than to the community in predated or endogenously respirated activated sludge samples. This similar microbial community could be due to microbial utilisation of protein hydrolysis products. Alternatively, conditions in the worm gut could facilitate a protein hydrolysing community which assists in protein hydrolysis. The genera Burkholderiales, Chryseobacterium and Flavobacterium were found to be associated with predation by T. tubifex.

Anaerobic digestion of thermal pre-treated emulsified slaughterhouse wastes (TESW): Effect of trace element limitation on process efficiency and sludge metabolic properties

Slaughterhouse solid wastes, characterized by a high lipid content, are considered a valuable resource for energy production by means of anaerobic digestion technologies. Aim of this study was to examine the effect of trace element limitation on the mesophilic anaerobic digestion of thermally pre-treated emulsified slaughterhouse wastes (TESW). Under two distinct experimental periods (Period I - low and Period II - high trace element dosage respectively) a CSTR with sludge recirculation was operated at increasing organic loading rate (OLR) from 1.5 to 10 g L^-1 d^-1. Under optimum conditions, COD removal was higher than 96%, biogas yield equal to 0.53 L g^-1 COD feed and the biogas methane content 77%. Trace element limitation however, resulted in a dramatic decline in process efficiency, with VFA accumulation and events of extreme sludge flotation, despite that the soluble concentration of Ni, Co and Mo were between 12 and 28 μg L^-1. This is indicative of mass transfer limitations caused by lipids adsorption onto the anaerobic biomass.

Investigation of foaming causes in three mesophilic food waste digesters: reactor performance and microbial analysis

Foaming negatively affects anaerobic digestion of food waste (FW). To identify the causes of foaming, reactor performance and microbial community dynamics were investigated in three mesophilic digesters treating FW. The digesters were operated under different modes, and foaming was induced with several methods. Proliferation of specific bacteria and accumulation of surface active materials may be the main causes of foaming. Volatile fatty acids (VFAs) and total ammonia nitrogen (TAN) accumulated in these reactors before foaming, which may have contributed to foam formation by decreasing the surface tension of sludge and increasing foam stability. The relative abundance of acid-producing bacteria (Petrimonas, Fastidiosipila, etc.) and ammonia producers (Proteiniphilum, Gelria, Aminobacterium, etc.) significantly increased after foaming, which explained the rapid accumulation of VFAs and NH4+ after foaming. In addition, the proportions of microbial genera known to contribute to foam formation and stabilization significantly increased in foaming samples, including bacteria containing mycolic acid in cell walls (Actinomyces, Corynebacterium, etc.) and those capable of producing biosurfactants (Corynebacterium, Lactobacillus, 060F05-B-SD-P93, etc.). These findings improve the understanding of foaming mechanisms in FW digesters and provide a theoretical basis for further research on effective suppression and early warning of foaming.

Bacterial community analysis of an industrial wastewater treatment plant in Colombia with screening for lipid-degrading microorganisms

The operation of wastewater treatment technologies depends on a combination of physical, chemical and biological factors. Microorganisms present in wastewater treatment plants play essential roles in the degradation and removal of organic waste and xenobiotic pollutants. Several microorganisms have been used in complementary treatments to process effluents rich in fats and oils. Microbial lipases have received significant industrial attention because of their stability, broad substrate specificity, high yields, and regular supply, as well as the fact that the microorganisms producing them grow rapidly on inexpensive media. In Colombia, bacterial community studies have focused on populations of cultivable nitrifying, heterotrophic and nitrogen-fixing bacteria present in constructed wetlands. In this study, culture-dependent methods, culture-independent methods (TTGE, RISA) and enzymatic methods were used to estimate bacterial diversity, to monitor temporal and spatial changes in bacterial communities, and to screen microorganisms that presented lipolytic activity. The dominant microorganisms in the Wastewater Treatment Plant (WWTP) examined in this study belonged to the phyla Firmicutes, Proteobacteria and Bacteroidetes. The enzymatic studies performed indicated that five bacterial isolates and three fungal isolates possessed the ability to degrade lipids; additionally, the Serratia, Kosakonia and Mucor genera presented lipase-mediated transesterification activity. The implications of these findings in regard to possible applications are discussed later in this paper. Our results indicate that there is a wide diversity of aerobic Gram-negative bacteria inhabiting the different sections of the WWTP, which could indicate its ecological condition, functioning and general efficiency.

Evaluating the influence of wastewater composition on the growth of Microthrix parvicella by GCxGC/qMS and real-time PCR

This study underlines the significance of long chain fatty acid (LCFA) content in wastewater influents as an influencing factor promoting the growth of Candidatus 'Microthrix parvicella' (M. parvicella), the most common filamentous bacteria causing foam in activated sludge systems worldwide. Quantification of M. parvicella by real-time polymerase chain reaction (real-time PCR) and analysis of LCFAs by means of two-dimensional gas chromatography coupled with mass spectrometry (GCxGC/qMS), involving solid phase micro-extraction (SPME) to enhance sensitivity, were combined for the first time as a monitoring tool. The results indicate a highly significant correlation between the abundance of M. parvicella and the total LCFA loading (r = 0.96) and linolenic acid C18:3 (r = 0.98) in particular. Additionally, comparison of slope values for the direct correlations of all significant LCFAs found in the analyses showed that the influence of LCFAs on M. parvicella growth increases with an increasing degree of unsaturation of carbon chains. These findings suggest that by removing lipid compounds from the incoming waters, substrate availability would be limited for M. parvicella.

Innovative test method for the estimation of the foaming tendency of substrates for biogas plants

Excessive foaming in anaerobic digestion occurs at many biogas plants and can cause problems including plugged gas pipes. Unfortunately, the majority of biogas plant operators are unable to identify the causes of foaming in their biogas reactor. The occurrence of foaming is often related to the chemical composition of substrates fed to the reactor. The consistency of the digestate itself is also a crucial part of the foam formation process. Thus, no specific recommendations concerning substrates can be given in order to prevent foam formation in biogas plants. The safest way to avoid foaming is to test the foaming tendency of substrates on-site. A possible solution is offered by an innovative foaming test. With the help of this tool, biogas plant operators can evaluate the foaming disposition of new substrates prior to use in order to adjust the composition of substrate mixes.

Effects of feedstock ratio and organic loading rate on the anaerobic mesophilic co-digestion of rice straw and pig manure

In order to investigate the effects of feedstock ratio and organic loading rate (OLR) on the anaerobic mesophilic co-digestion of rice straw (RS) and pig manure (PM), batch bottle tests (2.5L) were carried out at volatile solid (VS) ratios of 0:1, 1:2, 1:1, 2:1, and 1:0 (RS/PM), and continuous bench experiments (40L) were carried out at OLRs of 3.0, 3.6, 4.2, 4.8, 6.0, 8.0, and 12.0kg VS/(m(3)d) with optimal VS ratio. The results showed that the optimal ratio was 1:1 in terms of biogas yield. Stable biogas production with an average specific biogas production of 413L/kg VS was obtained at an OLR of 3-8kg VS/(m(3)d). Anaerobic co-digestion was severely inhibited by the accumulation of volatile fatty acids when the OLR was 12kg VS/(m(3)d). Further, light and serious foaming were observed at OLR of 8 and 12kg VS/(m(3)d), respectively.

Foam formation in biogas plants caused by anaerobic digestion of sugar beet

The use of sugar beet in anaerobic digestion (AD) during biogas production can lead to process upsets such as excessive foaming in fermenters. In the present study, foam formation in sugar beet-fed digestates was studied in foaming tests. The increasing disintegration grade of sugar beet was observed to have a promoting effect on foaming in the digestate but did not affect the biogas yield. Chemical analysis of foam and digestate from sugar beet silage AD showed high concentrations of pectin, other carbohydrates and N-containing substances in the foam. Both pectin and sucrose showed little foaming in AD. Nevertheless, sucrose and calcium chloride had a promoting effect on foaming for pectin AD. Salts of divalent ions also enhanced the foam intensity in the case of sugar beet silage AD, whereas ammonium chloride and urea had a lessening effect on sugar beet-based foaming.

Diagnosis of the anaerobic reject water effects on WWTP operational characteristics as a precursor of bulking and foaming

This study investigates the effects observed on operational parameters in a large and full-scale wastewater treatment plant subjected to anaerobic reject water (ARW) diversion off the main line for a 3-month period and further monitoring for a 2-year period. The plant's secondary unit consists of a two-stage plug-flow-modified Bardenpho process receiving wastewater from both municipal and industrial origins. As a result, ARW was found to have a direct effect on bulking in secondary clarifiers and foaming in anaerobic digesters (AD) despite its relatively small flow rate. During the cut-off period a highly stable sludge volume index at 80 mL g(-1) level was obtained in the secondary clarifiers, effluent suspended solids concentration was reduced and continuous feeding to AD was recovered. Sludge density increased in the thickeners during hot season. Secondary clarifiers showed good and stable settleability despite low dissolved oxygen, food/microorganism ratio and high sludge retention time and ammonium levels in the biological unit. The bulking and foaming effect was presented on the plant's internal flow balance. ARW needs serious consideration for elimination by appropriate technologies because of its high potential as a multi-dimensional pollutant source, not only as a carrier of nutrients but also as a possible carrier of filamentous bacteria, which might promote chronic seeding and retention in the system.

Effects of feedstock ratio and organic loading rate on the anaerobic mesophilic co-digestion of rice straw and cow manure

In order to investigate the effects of feedstock ratio and organic loading rate (OLR) on the anaerobic mesophilic co-digestion of rice straw (RS) and cow manure (CM), batch tests (2.5L) were carried out at volatile solid (VS) ratios of 0:1, 1:2, 1:1, 2:1, and 1:0 (RS/CM), and continuous bench experiments (40 L) were carried out at OLRs of 3.0, 3.6, 4.2, 4.8, 6.0, 8.0, and 12.0 kg VS/(m(3) d) with optimal VS ratio. The optimal VS ratio was found to be 1:1. Stable and efficient co-digestion with average specific biogas production of 383.5L/kg VS and volumetric biogas production rate of 2.30 m(3)/(m(3) d) was obtained at an OLR of 6 kg VS/(m(3) d). Anaerobic co-digestion was severely inhibited by the accumulation of volatile fatty acids instead of ammonia when the OLR was 12 kg VS/(m(3) d). Further, significant foaming was observed at OLR ⩾ 8 kg VS/(m(3) d).

Moderate temperature increase leads to disintegration of floating sludge and lower abundance of the filamentous bacterium Microthrix parvicella in anaerobic digesters

Filamentous bacteria such as Microthrix parvicella can cause serious foaming and floating sludge problems in anaerobic digesters fed with sewage sludge. The sewage sludge and oil co-fermenting laboratory-scale biogas digesters in this study were fed with substrates from a foaming-prone full-scale biogas plant containing the filamentous bacterium M. parvicella. At 37 °C, in both pneumatically mixed digesters a highly viscous and approximately 3 cm thick floating sludge was observed. A gradual increase of the temperature from 37 °C to 56 °C led to a significant decrease in the floating sludge thickness, which correlated with a strong decrease in the abundance of M. parvicella in the digestate. Furthermore, the stepwise temperature increase allowed for an adaption of the microbial community and prevented process failure. The study indicates that already a moderate temperature increase from 37 °C to 41 °C might help to control the M. parvicella abundance in full-scale biogas plants.