This study investigates the potential of poultry manure (PM) for volatile fatty acids (VFAs) production through alkaline fermentation, offering a sustainable alternative to methane production. The study explores the effects of organic loading rates (OLR) and food-to-inoculum (F:I) ratios on VFAs yield, optimizing conditions for enhanced process efficiency. Batch fermentation assays indicated that a low F:I ratio of 0.3 produced the highest VFAs yield, particularly under alkaline conditions (pH 9.5). In semicontinuous fermentation, a stepwise increase in OLR demonstrated that an OLR of 8 g VS L⁻¹ d⁻¹ was optimal for maximizing VFAs production. Microbial analysis revealed an increase in sulfate-reducing bacteria, particularly Desulfosporosinus , at high OLRs, while hydrolytic genera such as Bacteroides and Sphaerochaeta adapted to OLR stress. In contrast, syntrophic bacteria such as Sedimentibacter and Syntrophomonas decreased under these conditions. These shifts highlight the complex microbial interactions that influence fermentation performance. The findings indicate that PM is a viable substrate for VFAs production, and alkaline fermentation, coupled with optimal OLR and microbial community management, can enhance process sustainability. These results contribute to the growing body of research on valorizing livestock waste for bio-based chemical production.

This study investigated the impact of applying a pH shock on the anaerobic digestion (AD) performance, via digestate activity batch assays and microbial community dynamics in long-term continuous AD experiments. Sudden pH increases to 9.5 were applied, and the duration of the perturbation increased after every application (from 1 to 7 days - and up to 14 days in the batch assays). The potential for alkaline adaptation in AD processes was demonstrated, since the microbial communities under scrutiny exhibited an increasingly shorter recovery period at increasing pH shock duration, i.e., from 75 days to 39 days, and methane production was not irreversibly inhibited. Microbial analysis revealed a dynamic response that evolves from acidogenic dominance (e.g., Parabacteroides, Sphaerochaeta) to a more balanced community with a greater presence of hydrolytic bacteria (Clostridium and Ruminococcus). The research highlighted the resilience of microbial communities to pH shocks and suggested that extending pH shock durations could further reduce recovery times and facilitate gradual alkaline adaptation.

This paper examines the adaptive responses of microbial communities to gradual shifts in pH toward the mild alkaline range in anaerobic digestion (AD) systems. The results indicate that a pH of 8.0 serves as a critical upper limit for stable AD operation, beyond which microbial efficiency declines, underscoring the importance of microbial resilience against elevated pH stress. Specifically, hydrolysis genera, e.g. Eubacterium and Anaerobacterium, and syntrophic bacteria were crucial for reactor stability. Fibrobacter had also been shown to play a key role in the accumulation of propionate, thus leading to its dominance in the volatile fatty acid profile throughout the experimental phases. Overall, this investigation revealed the potential adaptability of microbial communities in AD systems to mild alkaline pH shifts, emphasizing the hydrolysis bacteria and syntrophic bacteria as key factors for maintaining metabolic function in elevated pH conditions.

Anaerobic digestion (AD), a well-known technology for converting organic wastes into renewable energy, has garnered extensive attention in both research and practical applications. Livestock manure, for example, can be processed via classic AD, although its complex nature and composition poses challenges for an effective and thorough valorization. This study provides a comprehensive examination of the alkaline hydrolysis (AH) of livestock manure during AD. In doing so, it addresses the concomitant challenges and opportunities inherent in this topic, revealing a lack of such reviews on alkaline anaerobic digestion in the current literature. The first part summarizes the complexities in livestock manure digestion and explores available strategies. In this regard, alkaline anaerobic digestion is put forward as a successful method for treating livestock manure, hereby dis-cussing advantages and drawbacks, and its underlying mechanism. The second part reviews the current state of research in this field, both in-situ and ex-situ. Ex-situ alkaline processes, including alkaline pre-treatment and post-treatment, offer better controllability and fewer negative effects on microorganisms compared to in situ, with a slight advantage for livestock manure pre-treatment. However, in-situ alkaline AD shows versatility for applications like biogas upgrading and volatile fatty acids (VFA) fermentation, leading to increased research in this area. This study further evaluates the influence of alkaline pH on microbial populations. By compiling a comprehensive survey of alkaline tolerance profiles within archaea and recently identified alkaliphilic bacteria, it unveils the intricate microbial constituents that underpin alkaline anaerobic digestion processes. Besides, the narrative underscores the discernible knowledge gaps in understanding alkaline acclimation, the functional roles of anaerobic microorganisms, and the imperative for rigorous economic analyses. Overall, this study provides a comprehensive exploration of alkaline AD, shedding light on its potential and future research directions.

The effects of temperature, pH, and gas-to-liquid-volume-ratio on ammonia recovery via gas–liquid stripping have been widely studied. However, there is a lack of a structured approach towards characterising the stripping process. Furthermore, limited information is available on the effect of the composition of the stripping gas on ammonia recovery. This study includes the application of a factorial design of experiments to ammonia stripping. The outcome is a mathematical relationship for ammonia recovery as a function of process conditions. The temperature was found to have the highest influence on ammonia recovery. With respect to the influence of the stripping gas, similar ammonia recoveries were reported when using air, CH4, and N2 (96, 92, and 95%, respectively). This was attributed to their similar influences on the pH of the digestate, and subsequently, on the free ammonia equilibrium. In addition, the presence of CO2 in the stripping gas had a critical effect on ammonia recovery due to its influence on the total ammonia equilibrium in the digestate. These results showed the possibility of using different stripping gases interchangeably to obtain similar ammonia recoveries, with a critical emphasis on their CO2 content.

This study investigated the selective production of volatile fatty acids (VFAs) during anaerobic mixed-culture fermentation. The experiment used chicken manure (CM) as a potential substrate to produce high added-value propionic acid and butyric acid under an alkaline environment. The conversion of CM into selective VFAs de-pends highly on operational conditions such as pH and redox balance. Therefore, the current experiment is designed to employ amino acid addition and develop a redox balance control method to control the final VFA profile. This study showed that 0.2-5.0 % valine and threonine addition successfully enhanced propionic acid and butyric acid production during alkaline fermentation and hence decreased the proportion of acetic acid from 83 % to approximately 47 %. The oxidation-reduction potential (ORP) and redox cofactor ratio (NADH/NAD+) were measured to support the selective VFA production mechanism. The results obtained in this study bring extra value to the valorization of CM within the circular economy concept for selective value-added VFA production.

Ammonia recovery from anaerobic digesters via side-stream stripping is a technique to recover nitrogen from manure wastes. This study demonstrated a novel approach to determining ammonia recovery to maintain total ammonia concentrations in the digester in the range of 1.7-2.1 gN/L. Increasing the pH during stripping from 8, 8.5 to 9.5 did not affect the stability of the digester. Methane yields of 60-80 mL/(gVS.d) and volatile fatty acid concentrations of 0-500 mg/L were reported throughout its operation. The low solubilisation increase upon recirculation of the digestate explained the lack of change in methane yields due to side-stream stripping. Increasing the pH during stripping also did not affect the digester's operating pH, which was attributed to the neutralising effect of biogas as stripping gas. Therefore, total ammonia concentrations in the digester can be controlled by determining the extent of ammonia recovery, and the pH during stripping can be increased without compromising the digester's stability.

•Dairy manure and digestate were assessed on their bioaccessibility and complexity.•Ultrasonication of 29 days old digestate increased sCOD by 55% and BMP by 32%.•Ultrasonication of 43 days old digestate did not increase BMP significantly.•The bioaccessibility of 29 days old digestate improved after ultrasonication.

Substantial insight into the effect of ultrasound disintegration on the changes in biochemical parameters of manure digestate and digestate age is needed to understand the potential of digestate treatment. To address this knowledge gap, in this study, the effect of digestate age on the efficiency of ultrasound (US) disintegration was investigated. In this scope, dairy manure digestate samples were incubated in an oven at 37 °C for a predetermined amount of time to obtain simulated digestate ages of 15, 22, 29, 36 and 43 days. The results showed that US disintegration efficiency significantly affected the initial biochemical characteristics of digestate and that the digestate age had a significant effect on the US disintegration efficiency. This effect diminished when the applied specific energy (SE) was higher than 3000 kJ/kg total solids (TS). A numerical partial least squares (PLS) model was constructed to investigate the relative influences of the initial biochemical parameters on the soluble chemical oxygen demand (sCOD) and soluble carbohydrates (sCARB) solubilization. The results of the high-quality (R2 = 0.8) model indicated that the most influential parameters for the efficiency of US disintegration were the SE, the initial sCARB0, the TS, the initial sCOD0 and the volatile solids (VS).

Anaerobic digestion is a well-established technology that valorises organic wastes to produce renewable energy as biogas. The ammonia released during this process can be inhibitory if it exceeds a threshold concentration. Additionally, ammonia is a harmful environmental pollutant and also a renewable source of energy, thus establishing a need for the recovery of ammonia. Gas-liquid stripping is one such widely researched technology to recover ammonia from wastewater. However, there have not been any reviews focusing solely on the application of stripping to recover ammonia from the anaerobic digestate of organic wastes. This study reviews the current state of research in this field, focusing on the factors affecting the ammonia recovery, stripping column designs and linking of an anaerobic digester to a stripping column. An overview of the work done on the modelling of the ammonia stripping from digestate is also included. This review offers a direction for further research in this field; i.e., experimental and modelling studies on i) the effect of digestate composition on ammonia recovery and ii) the effect of stripping gas on ammonia removal.

Digestate treatment techniques have recently been proposed as a strategy to increase the ultimate biogas yield from dairy manure and to improve the digestate quality as an organic fertilizer. These studies however rarely take the trace elements (TE) and nutrient partitioning into account. This study focusses on ozone treatment (5-40 g O-3 kg(-1) Total Solids (TS)) as a digestate treatment technique to control the concentration of TE and nutrients in the liquid phase of the digestate. Controlling the TE and nutrient concentrations in the liquid and solid digestate can improve the agronomic value of dairy manure digestate. The ozone concentration of the gas stream entering reactor was 48.53 g O-3/Nm(3) or 3.4% w/w O-3 in O-2 -gas. The experiments were repeated using pure oxygen gas to investigate its influence. The results from ozonation and oxygenation of the dairy manure digestates revealed that O-3 treatment up to 40 g O-3 kg(-1) TS did not have a more pronounced effect on the biochemical parameters compared to supplementation of pure O-2. Ozonation of the digestate and the supernatant showed that the TE concentration in the liquid phase followed a parabolic profile. The observed initial increase in this parabolic profile was explained by the release of TE from the organic matter to the supernatant causing an increase in TE concentration, followed by a decrease due to precipitation of TE as hydroxides and sulfides, due to the increasing pH and sulphur concentrations. (C) 2020 Elsevier Ltd. All rights reserved.

In this study, ultrasound (US) disintegration was evaluated as a digestate treatment strategy. Digestate treatment is the physical, chemical or biological treatment and recirculation AD of the effluents of the anaerobic digesters (i.e., digestate) to the digester as supplementary feed to increase their utilization potential for energy conversion or green chemical production in post-valorization pathways. In this scope, semi-continuous small pilot scale manure digesters were operated in parallel with various operational settings. One of the digesters was fed by adding disintegrated digestate to an equal volume of fresh manure feed (recycle ratio of 1). The obtained results showed that US-assisted digestate treatment at 1500 kJ/kg TS specific energy input with 30 days of hydraulic retention time increased the methane production rate by 18%. The increased methane production rate was found to be related to the applied specific energy and organic loading rate. A basic cost-benefit analysis showed that the energy demand of the US disintegration at lab scale was higher than the energy that can be recovered from the additional biogas produced. (C) 2020 Elsevier Ltd. All rights reserved.

Recently, digestate disintegration gained interest as an alternative strategy to feedstock pretreatment for anaerobic digestion. This study evaluated the effect of three different digestate disintegration methods (hydrogen peroxidation, ozone treatment and ultrasound) on manure digestate, potato waste digestate and mixed organic waste digestate. Lab-scale anaerobic digestion experiments were carried out by adding disintegrated digestate to the related substrate and inoculum with simulated recycle ratios of 0.2 and 0.5. Ultrasound disintegration of potato waste digestate yields 22.5% increase in biogas production. An increase in biogas production was linked to the treated digestate amount and the treatment dosage. First order model was used to investigate the effect of digestate disintegration on the first order reaction rate constant (k). The decrease in k and increase in biogas production were linearly correlated. This correlation was explained by the increased bioavailability of the organic matter and possible negative effects of digestate disintegration on the microorganisms.

The impact of humic acid (HA) on methanogenic activity was investigated. Methanogenic crushed granular sludge and pure cultures of mesophilic methanogens were incubated in batch cultures with HA. Initial methane production rates and substrate consumption rates were quantified. In the presence of 1 kg m(-3) HA, the methane production rate of all hydrogenotrophic methanogens was inhibited by more than 75%, except Methanospirillum hungatei that was not inhibited up to 5 kg m-3 HA. The acetoclastic Methanosarcina barkeri was completely inhibited by HA >= 1 kg m(-3). However. Methanosaeta concilii was only slightly affected by HA up to 3 kg m(-3). When methanogenic granular sludge was incubated with HA, the specific methanogenic activity (SMA) tests showed less inhibition, when compared to the pure cultures of methanogens. The SMA test with H-2/CO2, formate and acetate showed reduced initial methane production rate of 42%, 23% and 40%, respectively. Differences in HA susceptibility were explained by differences in cell wall structure. (C) 2017 Published by Elsevier Ltd.

Inhibition effect of humic acid (HA) on anaerobic digestion of cellulose and xylan and the mitigation potential of the inhibition were evaluated in controlled fed batch reactors at 30 °C and a hydraulic retention time (HRT) of 20 days. Reactor performances were evaluated by biogas production and metabolite measurements for 220 days. Microbial population dynamics of the reactors were monitored with next-generation 16S rRNA gene sequencing at nine different sampling times. Our results showed that increasing levels of HA inhibited the hydrolysis efficiency of the digestion by 40% and concomitantly reduced the methane yield. Addition of hydrolytic enzymes helped to reverse the negative effects of HA, whereas calcium addition did not reverse HA inhibition. Microbiological analyses showed that the relative abundance of hydrolytic/fermentative bacterial groups such as Clostridiales, Bacteroidales and Anaerolineales was significantly lowered by the presence of HA. HA also affected the archaeal populations. Mostly hydrogenotrophic methanogens were negatively affected by HA. The relative abundance of Methanobacteriaceae, Methanomicrobiales-WCHA208 and Unassigned Thermoplasmata WCHA1-57 were negatively affected by the presence of HA, whereas Methanosaetacea was not affected.

In recent years, biogas production from complex biomass has received great interest. Therefore, many studies have been conducted to understand the anaerobic digestion process and to characterize responsible microbes for the biochemical conversions. Although the knowledge about biogas production in general is rapidly increasing, less information is available about hydrolytic microbes within anaerobic bioreactors. Here, the authors pinpoint the urgent need for solid fundamental knowledge about hydrolytic bacteria within biogas plants. In this review, current knowledge about anaerobic hydrolytic microbes is presented, including their abundance in biogas plants, and the factors impacting their activity.

Humic compounds are inhibitory to the anaerobic hydrolysis of cellulosic biomass. In this study, the impact of salt addition to mitigate the inhibitory effects of humic compounds was investigated. The experiment was conducted using batch tests to monitor the anaerobic hydrolysis of cellulose in the presence of humic acid. Sodium, potassium, calcium, magnesium and iron salts were tested separately for their efficiency to mitigate humic acid inhibition. All experiments were done under mesophilic conditions (30 °C) and at pH 7. Methane production was monitored online, using the Automatic Methane Potential Test System. Methane production, soluble chemical oxygen demand and volatile fatty acid content of the samples were measured to calculate the hydrolysis efficiencies. Addition of magnesium, calcium and iron salts clearly mitigated the inhibitory effects of humic acid and hydrolysis efficiencies reached up to 75%, 65% and 72%, respectively, which were similar to control experiments. Conversely, potassium and sodium salts addition did not mitigate the inhibition and hydrolysis efficiencies were found to be less than 40%. Mitigation of humic acid inhibition via salt addition was also validated by inductively coupled plasma atomic emission spectroscopy analyses, which showed the binding capacity of different cations to humic acid.

Five hundred tons of antibiotics are consumed yearly in the world. In this study, the biodegradation characteristics of tetracycline (TET) under nitrate-reducing, sulfate-reducing, and methanogenic conditions were determined by batch tests. Also, effects of TET on mixed microbial cultures were revealed by microbiological analysis. In this scope, gas generation and composition, dissolved organic carbon, and electron acceptor concentrations were monitored during 120 days. Additionally, changes on quantities of specific microbial groups were determined by Q-PCR. TET showed non-biodegradable behavior under nitrate- and sulfate-reducing conditions, whereas slightly biodegradable behavior under methanogenic conditions approximately 46 % degradation. The effects of TET on the abundance of mixed culture varied according to taxonomic units. Sulfate-reducing bacteria were inhibited by TET, while archaeal, bacterial, and methanogenic populations were not affected significantly.

During last decades, concentration of human and veterinarian antibiotics in the environment, natural and engineered systems have been increased because of high amount production and consumption. This situation has aroused great concern due to the possibility of harmful effects on human, animals and plants [1,2]. Occurrence and fate of these compounds are one of the main issues because of their unknown potential risks and their effects on the environment. Approximately 500 tonnes of them are produced and consumed every year in the worldwide. Antibiotics are resistant to conventional biological treatment process and the wastewaters including these compounds are directly discharged to the receiving water bodies without efficient treatment. Hospitals and pharmaceutical industries are the main sources of high antibiotic concentration release to the environment [3]. Also sewage systems can transport these molecules and/or their metabolites since metabolization of them by humans and animals cannot be achieved completely [4]. During the transportation of antibiotics throughout treatment plants, elimination of these compounds can occur via biodegradation, photolysis and sorption to sludge but ultimate degradation of these compounds cannot be achieved in conventional treatment plants [4, 5, 6]. As a result of the introduction of metabolized and/or active antibiotics to the receiving water bodies caused an increase in the ratio of multiantibacterial resistant pathogens [7].