Effect of potassium inhibition on the thermophilic anaerobic digestion of swine waste

Impact of Polyethylene-Glycol-Induced Water Potential on Methane Yield and Microbial Consortium Dynamics in the Anaerobic Degradation of Glucose

This study investigated the relationship between water potential (Ψ) and the cation-induced inhibition of methane production in anaerobic digesters. The Ψ around methanogens was manipulated using polyethylene glycol (PEG) in a batch anaerobic reactor, ranging from -0.92 to -5.10 MPa. The ultimate methane potential (Bu) decreased significantly from 0.293 to 0.002 Nm3 kg-1-VSadded as Ψ decreased. When Ψ lowered from -0.92 MPa to -1.48 MPa, the community distribution of acetoclastic Methanosarcina decreased from 59.62% to 40.44%, while those of hydrogenotrophic Methanoculleus and Methanobacterium increased from 17.70% and 1.30% to 36.30% and 18.07%, respectively. These results mirrored changes observed in methanogenic communities affected by cation inhibition with KCl. Our findings strongly indicate that the inhibitory effect of cations on methane production may stem more from the water stress induced by cations than from their direct toxic effects. This study highlights the importance of considering Ψ dynamics in understanding cation-mediated inhibition in anaerobic digesters, providing insights into optimizing microbial processes for enhanced methane production from organic substrates.

Effect of Ecuadorian natural zeolite on the performance of anaerobic digestion of swine waste in semicontinuous regime

This study explores the potential of zeolite as an amendment to mitigate ammonium inhibition in the anaerobic digestion of swine waste. Two 50 L reactors, one with and one without zeolite amendment were operated at an OLR of 3.0 g VS L-1d-1 for 130 days, and fed with swine waste from a full-scale pig farm. Under these conditions, zeolite doses of 4 g L-1 allowed total ammonia nitrogen (TAN) concentrations to be kept below 1000 mgNH3-N L-1. The zeolite-amended reactor not only showed an average increase of 8% in methane production under stable conditions but also exhibited 34% reduction in H2S concentrations in the biogas, compared to the reactor without zeolite. The community of archaea originating from the inoculum was conserved in the reactor with zeolite amendment, particularly the acetoclastic methanogens of the genus Methanosaeta. On the other hand, in the reactor without zeolite addition, the microbial community went from being dominated by the acetoclastic methanogen Methanosaeta to having a high relative abundance of hydrogenotrophic methanogens. The zeolite addition also favoured the reactor stability, prevented foaming, and produced an enriched natural zeolite with N, P and K. However, additional studies on the potential of enriched zeolite as a fertilizer are required, which could make the use of zeolite in Anaerobic Digestion of swine waste not only energetically favourable but also economically feasible.

Effects of Water Potential on Anaerobic Methane Production and a Microbial Consortium

This study probed the effect of the water potential (Ψ) on anaerobic methane production and a microbial consortium. The Ψ level of the investigated anaerobic digester (n = 20) was in the range from −0.10 to −2.09 MPa with a mean value of −1.23 MPa, and the Ψ level of the anaerobic digester was significantly correlated with the SCOD, TKN, NH4+-N, alkalinity, salinity (SPS), NH4+, Na+, K+, Cl−, NO3−, and PO43− (p < 0.001). The maximum methane production rate (Rm) of the Control (−0.40 MPa) was 8.11 mL day−1 and decreased to 1.70 mL day−1 at −3.91 MPa (K5), and the lag growth phase time (λ) was delayed to 35.96 and 25.34 days at −2.85 MPa (K4) and −3.91 MPa (K5), respectively. The ultimate methane potential (Bu) was 0.264 Nm3 kg−1-VSadded for the Control, and when Ψ was adjusted, Bu increased to 0.278 Nm3 kg−1-VSadded at −1.49 MPa (K3) but decreased to 0.203 and 0.172 Nm3 kg−1-VSadded at −2.85 MPa (K4) and −3.91 MPa (K5), respectively. Therefore, the methane yield was inhibited due to the decrease in Ψ, and the methane yield is predicted to be inhibited from about −1.65 MPa. In the genus-level taxonomic classification of the microbial community, the relative abundance of Methanosarcina decreased significantly to 36.76% at −3.91 MPa (K5) compared to 58.15% for the Control; however, the relative abundance of Methanoculleus significantly increased to 35.16% at −3.91 MPa (K5) compared to 14.85% for the Control.

Potassium inhibition during sludge and biopulp co-digestion; experimental and model-based approaches

Process instability with consecutive low methane production are common challenges of the anaerobic digestion (AD) of municipal wastes. In the present study, the co-digestion of sewage sludge and municipal biopulp was investigated at batch and continuously fed digesters. At batch tests, the highest methane yield for co-digestion (467 ± 17 mLCH₄/gVS) was achieved when biopulp contributed to 80% of organic matter content and sludge the remaining 20%. At continuous mode operation, co-digestion achieved 0.91 ± 0.11 L/(L·d) methane productivity, while mono-digestion of sludge achieved 0.62 ± 0.05 L/(L·d). Potassium inhibition was investigated at the most efficient co-digestion scenario and was found that the half maximal inhibitory concentration (IC₅₀) occurred at 8 g-K⁺/L. Subsequently, the effect of K⁺ was investigated at different scenarios at continuous operation. Simulations based on BioModel described the inhibitory effect of K⁺ by introducing non-competitive inhibition of methanogens. Simulation results confirmed the strongly inhibitory effect of potassium to the AD process.

Cation exchange membrane behaviour of extracellular polymeric substances (EPS) in salt adapted granular sludge

This paper aims to elucidate the role of extracellular polymeric substances (EPS) in regulating anion and cation concentrations and toxicity towards microorganisms in anaerobic granular sludges adapted to low (0.22 M of Na⁺) and high salinity (0.87 M of Na⁺). The ion exchange properties of EPS were studied with a novel approach, where EPS were entangled with an inert binder (PVDF-HFP) to form a membrane and characterized in an electrodialysis cell. With a mixture of NaCl and KCl salts the EPS membrane was shown to act as a cation exchange membrane (CEM) with a current efficiency of ∼80%, meaning that EPS do not behave as ideal CEM. Surprisingly, the membrane had selectivity for transport of K⁺ compared to Na⁺ with a separation factor ( [Formula: see text] ) of 1.3. These properties were compared to a layer prepared from a model compound of EPS (alginate) and a commercial CEM. The alginate layer had a similar current efficiency (∼80%.), but even higher [Formula: see text] of 1.9, while the commercial CEM did not show selectivity towards K⁺ or Na⁺, but exhibited the highest current efficiency of 92%. The selectivity of EPS and alginate towards K⁺ transport has interesting potential applications for ion separation from water streams and should be further investigated. The anion repelling and cation binding properties of EPS in hydrated and dehydrated granules were further confirmed with microscopy (SEM-EDX, epifluorescence) and ion chromatography (ICP-OES, IC) techniques. Results of specific methanogenic activity (SMA) tests conducted with 0.22 and 0.87 M Na⁺ adapted granular sludges and with various monovalent salts suggested that ions which are preferentially transported by EPS are also more toxic towards methanogenic cells.

Characterising and modelling free ammonia and ammonium inhibition in anaerobic systems

Inhibition by ammoniacal nitrogen, consisting of free ammonia (NH₃) and ammonium ion (NH₄⁺), has been widely investigated for anaerobic digestion. However, despite the large amount of research on the subject, ammoniacal nitrogen inhibition still threatens many anaerobic digesters. This paper presents (i) a method to reliably characterise ammoniacal nitrogen inhibition and (ii) a robust inhibition modelling approach. Results showed that NH₃ and NH₄⁺ inhibition need to be jointly determined, which can only be done by performing inhibition tests at various total ammoniacal nitrogen (TAN) concentrations and pH values. These test conditions were reliably achieved using the salts NH₄HCO₃ and NH₄Cl without pH adjustment, rather than by using NH₄Cl with pH adjustment. The use of only salts showed a lower pH change during the inhibition test (∼1.5 days), thereby decreasing the uncertainty in TAN speciation and strengthening the test and model outputs. A threshold inhibition function satisfactorily described (R² > 0.99) the joint inhibition of NH₃ and NH₄⁺ on three distinct inocula, and provided a better description of the inhibition testing results than a non-competitive inhibition function (R² ∼0.70). The key advantage of the proposed threshold inhibition function is its capacity to identify the inhibition lower limit (concentration where inhibition starts; KI_min) and upper limit (concentration where inhibition is complete; KI_max). The threshold inhibition function also identifies the 50% inhibition concentration (KI₅₀) at the midpoint of KI_min and KI_max. Finally, experimental and model results show that at pH 7.3-7.7 and TAN concentrations above 2000 mgN·L^-1, both NH₃ and NH₄⁺ contribute significantly to overall inhibition.

Metal fractionation in sludge from sewage UASB treatment

This study evaluates the trace metal composition and fractionation in sludge samples from anaerobic sewage treatment plants from six cities in Brazil. Ten metals were evaluated: Ni, Mn, Se, Co, Fe, Zn, K, Cu, Pb and Cr. Specific methanogenic activity of the sludge was also evaluated using acetic acid as the substrate. Among the essential trace metals for anaerobic digestion, Se, Zn, Ni and Fe were found at a high percentage in the organic matter/sulfide fraction in all sludge samples analyzed. These metals are less available for microorganisms than other metals, i.e., Co and K, which were present in significant amounts in the exchangeable and carbonate fractions. Cu is not typically reported as an essential metal but as a possible inhibitor. One of the samples showed a total Cu concentration close to the maximal amount allowed for reuse as fertilizer. Among the non-essential trace metals, Pb was present in all sludge samples at similar low concentrations and was primarily present in the residual fraction, demonstrating very low availability. Cr was found at low concentrations in all sludge samples, except for the sludge from STP5; interestingly, this sludge presented the lowest specific methanogenic activity, indicating possible Cr toxicity.

Mesophilic anaerobic digestion of several types of spent livestock bedding in a batch leach-bed reactor: substrate characterization and process performance

Spent animal bedding is a valuable resource for green energy production in rural areas. The properties of six types of spent bedding collected from deep-litter stables, housing either sheeps, goats, horses or cows, were compared and their anaerobic digestion in a batch Leach-Bed Reactor (LBR) was assessed. Spent horse bedding, when compared to all the other types, appeared to differ the most due to a greater amount of straw added to the litter and a more frequent litter change. Total solids content appeared to vary significantly from one bedding type to another, with consequent impact on the methane produced from the raw substrate. However, all the types of spent bedding had similar VS/TS (82.3-88.9)%, a C/N well-suited to anaerobic digestion (20-28, except that of the horse, 42) and their BMPs were in a narrow range (192-239NmLCH₄/gVS). The anaerobic digestion in each LBR was stable and the pH always remained higher than 6.6 regardless of the type of bedding. In contrast to all the other substrates, spent goat bedding showed a stronger acidification resulting in a methane production lag phase. Finally, spent bedding of different origins reached, on average, (89±11)% of their BMP after 60days of operation. This means that this waste is well-suited for treatment in LBRs and that this is a promising process to recover energy from dry agricultural waste.

Development of appropriate technology for treatment of molasses-based wastewater

In this study, the performance of a proposed treatment system consisting of an anaerobic process (acidification, methane fermentation) and an aerobic process (trickling filter) was evaluated for treating high concentrations of molasses-based wastewater (43-120 gCOD/L) by a continuous flow experiment. An anaerobic up-flow staged sludge bed (USSB) reactor, equipped with multiple gas solid separators, was used as the main treatment/methane recovery process. The USSB showed good efficiency of both COD removal (80-87%) and methane recovery (70-80%) at an organic loading rate of 11-43 kgCOD/m(3) day. As the influent COD concentration was increased, the organic loading rate for stable operation of the USSB was reduced due to cation inhibition. However, the COD removal efficiency of the whole treatment system (including the aerobic post-treatment process) was 96% even at an influent COD concentration of 120 gCOD/L. Use of the treated wastewater as a fertilizer and/or irrigation-water for sugarcane was evaluated by a field cultivation test. Both growth of sugarcane and emission of greenhouse gases from the field soil were measured. A relatively high methane flux (352 μgCH4/m(2) h) was observed when the treated wastewater from day 0 was used. By day 3, however, this value was reduced to the same level as the control. In addition, growth of sugarcane was satisfactory when the treated wastewater was used. The treated wastewater was found to be useful for cultivation of sugarcane in terms of both a low risk of greenhouse gas emission from the field soil and effectiveness for growth of sugarcane.

Enhanced anaerobic digestion of piggery wastewater by ammonia stripping: effects of alkali types

Air stripping at alkaline pH was carried out to remove ammonia from the piggery wastewater, and its effects on subsequent anaerobic digestion were investigated in semi-continuous experiments. In ammonia stripping process, three alkalis (NaOH, KOH and CaO) were used for pH adjustment. When using NaOH and KOH, the methane production rate increased more than two folds as compared to the control (no ammonia stripped), but cation toxicity exerted by sodium and potassium ions was observed. When using lime, on the contrary, it was found that volumetric methane production rates (1040-1130 mL CH(4)/L day) and yields (262.3-258.9 mL CH(4)/g of COD(added)) were significantly higher than others. In addition, the organic removal efficiencies (54.2-59.5% of volatile solid, 59.6-64.0% of total COD, 72.1-81.9% of soluble COD and 89.3-98.9% of volatile fatty acid) were also high. Batch toxicity test results confirmed that cations of Na(+), K(+) were strong methanogenic inhibitors as compared to Ca(2+). From these observations, it was concluded that ammonia stripping at alkaline pH is important for anaerobic digestion of piggery wastewater and the alkali types should be chosen cautiously to avoid cation toxicity.