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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 depends 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

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.

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.

ispartof: FUEL vol:291 status: published

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 O3 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 O3/Nm³ or 3.4% w/w O3 in O2-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 O3 treatment up to 40 g O3 kg−1 TS did not have a more pronounced effect on the biochemical parameters compared to supplementation of pure O2. 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. ispartof: Chemosphere vol:252 ispartof: location:England status: Published online

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. ispartof: BIORESOURCE TECHNOLOGY vol:268 pages:568-576 ispartof: location:England status: published

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 /CO 2 , 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.

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.

Research on the hydrolysis step of the AD became more important with the increased use of recalcitrant waste products such as manure, sewage sludge and agricultural biomass for biogas production. Hydrolysis is often the rate limiting step of the overall AD. Hydrolysis enhancement is one of the required steps to optimise biogas production. Despite the progress to overcome the limitations of hydrolysis, inhibition of hydrolysis is still poorly researched. Humic acid-like molecules (HA) are one of the inhibitors of the anaerobic hydrolysis and their effect on the overall AD process is generally overlooked. In this thesis, the HA inhibition on anaerobic digestion of cellulosic material and mitigation strategies, using cation and enzyme addition, to overcome the inhibition were investigated. In addition, the microbial community dynamics during AD in the presence and absence of HA were examined. In this scope, in Chapter 2, we reviewed the literature and pinpointed the urgent need for comprehensive studies on the role of hydrolytic microorganisms and environmental factors that effects their abundance within biogas plants. Consequently, the hydrolysis mechanism and involved hydrolytic enzymes were discussed. The overall discussion showed that a holistic approach, including microbiological and engineering studies should be chosen to disclose the role of hydrolytic microbes within biogas reactors. In Chapter 3 and, Chapter 4 the effect of HA on anaerobic cellulose hydrolysis and methanogenesis, in batch wise incubations is reported, respectively. Our results showed that pulse addition of 5 g L-1 HA caused a 50 % decrease in hydrolysis rate of anaerobic cellulose degradation (Chapter 3). Moreover, VFA accumulation was observed in the presence of HA during the anaerobic cellulose degradation, which indicated the possible inhibition of HA on methanogenesis. Based on the results of Chapter 3, pure cultures of methanogens and a mixed culture were tested to study the vulnerability of methanogenesis to HA inhibition. Hydrogenotrophic methanogenesis in pure cultures was inhibited by more than 75% in the presence of 1 g L-1 HA whereas, acetoclastic methanogenesis by Methanosaeta concilii was only slightly affected by HA up to 3 g L-1. When methanogenic granular sludge was incubated with HA, the specific methanogenic activity tests showed less inhibition, when compared to the pure cultures of methanogens. HA inhibition was also observed during long-term CSTR operation at an HRT of 20 days, 35°C and a mixture of cellulose and xylan as a subtrate (Chapter 6). 8 g L-1 HA inhibited the hydrolysis efficiency of the cellulose and xylan digestion by 40 % and concomitantly reduced the methane yields. Mitigation of the HA inhibition is required to increase the hydrolysis efficiency and methane yields of cellulosic biomass digestion. Therefore, two different strategies were tested for their potential use as mitigation agents, viz. addition of cations such as, calcium magnesium and iron (Chapter 3 and Chapter 6) and addition of hydrolytic enzymes (Chapter 6). Addition of magnesium, calcium and iron salts mitigated the HA inhibition and hydrolysis efficiencies reached up to 75, 65 and 72%, respectively, compared to the control groups in the batch wise incubations (Chapter 3). However, in long term CSTR operations, calcium addition did not show a positive effect on hydrolysis inhibition. On the other hand, enzyme addition helped to reverse the negative effect of HA. The microbial communities involved in AD were also studied. Chapter 5 and Chapter 6 dealt with microbial community analyses with 16S rRNA next generation sequencing. In Chapter 5, five replicate reactors were monitored during the start-up period. Transient feeding strategy was used to acclimatise anaerobic sludge to efficient cellulose and xylan degradation. During the experiment, Bacteriodales, Clostridiales and Anaerolineales became dominant bacterial populations while, Methanobacteriaceae and Methanospirillaceae were the dominant archaeal populations within the reactors. In Chapter 6, the microbial population dynamics in the presence and absence of HA were monitored. Microbiological analyses showed that the 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. In conclusion, this thesis confirms that HA inhibit the hydrolysis and methanogenesis within both batch incubations and CSTR systems. Microbial populations were also affected by HA. Therefore, hydrolytic enzyme addition can be an option to mitigate HA inhibition and enhance hydrolysis and methanogenesis during conversion of biomass to biogas. In deze thesis is het remmende effect van HZ (humus zuren) op de anaerobe vergisting van (hemi)cellulose onderzocht. Ook is er gekeken naar het effect van HZ op hydrolyse bevorderende strategieën als toevoeging van kationen of enzymen. Daarnaast is het effect van HZ op de microbiologische populatie dynamica onderzocht.

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. ispartof: Bioengineering vol:2 issue:2 pages:54-65 ispartof: location:Switzerland status: published

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].