Semi-aerobic fermentation as a novel pre-treatment to obtain VFA and increase methane yield from primary sludge

Valorization of protein-rich waste and its application

Energy shortages present significant challenges with the rising population and dramatic urbanization development. The effective utilization of high-value products generated from massive protein-rich waste has emerged as an excellent solution for mitigating the growing energy crisis. However, the traditional disposal and treatment of protein-rich waste, have been proven to be ineffective in resource utilization, which led to high chemical oxygen demand and water eutrophication. To effectively address this issue, hydrolysate and bioconversion products from protein-rich waste have been widely investigated. Herein, we aim to provide an overview of the valorization of protein-rich waste based on a comprehensive analysis of publicly available literature. Firstly, the sources of protein-rich waste with various quantities and qualities are systematically summarized. Then, we scrutinize and analyze the hydrolysis approaches of protein-rich waste and the versatile applications of hydrolyzed products. Moreover, the main factors influencing protein biotransformation and the applications of bioconversion products are covered and extensively discussed. Finally, the potential prospects and future directions for the valorization of protein-rich waste are proposed pertinently.

The Measurement, Application and Effect of Oxygen in Microbial Fermentations: Focusing on Methane and Carboxylate Production

Oxygen is considered detrimental to anaerobic fermentation processes by many practitioners. However, deliberate oxygen sparging has been used successfully for decades to remove H2S in anaerobic digestion (AD) systems. Moreover, microaeration techniques during AD have shown that small doses of oxygen may enhance process performance and promote the in situ degradation of recalcitrant compounds. However, existing oxygen dosing techniques are imprecise, which has led to inconsistent results between studies. At the same time, real-time oxygen fluxes cannot be reliably quantified due to the complexity of most bioreactor systems. Thus, there is a pressing need for robust monitoring and process control in applications where oxygen serves as an operating parameter or an experimental variable. This review summarizes and evaluates the available methodologies for oxygen measurement and dosing as they pertain to anaerobic microbiomes. The historical use of (micro-)aeration in anaerobic digestion and its potential role in other anaerobic fermentation processes are critiqued in detail. This critique also provides insights into the effects of oxygen on these microbiomes. Our assessment suggests that oxygen dosing, when implemented in a controlled and quantifiable manner, could serve as an effective tool for bioprocess engineers to further manipulate anaerobic microbiomes for either bioenergy or biochemical production.

Potential of anaerobic co-fermentation in wastewater treatments plants: A review

Fermentation (not anaerobic digestion) is an emerging biotechnology to transform waste into easily assimilable organic compounds such as volatile fatty acids, lactic acid and alcohols. Co-fermentation, the simultaneous fermentation of two or more waste, is an opportunity for wastewater treatment plants (WWTPs) to increase the yields of sludge mono-fermentation. Most publications have studied waste activated sludge co-fermentation with food waste or agri-industrial waste. Mixing ratio, pH and temperature are the most studied variables. The highest fermentation yields have been generally achieved in mixtures dominated by the most biodegradable substrate at circumneutral pH and mesophilic conditions. Nonetheless, most experiments have been performed in batch assays which results are driven by the capabilities of the starting microbial community and do not allow evaluating the microbial acclimation that occurs under continuous conditions. Temperature, pH, hydraulic retention time and organic load are variables that can be controlled to optimise the performance of continuous co-fermenters (i.e., favour waste hydrolysis and fermentation and limit the proliferation of methanogens). This review also discusses the integration of co-fermentation with other biotechnologies in WWTPs. Overall, this review presents a comprehensive and critical review of the achievements on co-fermentation research and lays the foundation for future research.

Improving the efficiency of anaerobic digestion: Domesticated paddy soil microbes enhance the hydrolytic acidification of rice straw and pig manure

Improving the efficiency of hydrolytic acidification is critical for methane production from agricultural waste. This study is the first to apply domesticated paddy soil microbes to (DPSM) enhance the hydrolytic acidification of rice straw (RS) and pig manure (PM) to obtain acidizing fluid for anaerobic digestion (AD). At a substrate concentration of 20%, the inoculation of an RS-PM mixture (1:3) with 35% DPSM degraded the volatile solids by 48.1% and yielded 6.8 g/L of volatile fatty acids and 4.7 g/L of acetic acid after seven days of hydrolytic acidification. After 10 days of subsequent AD, the cumulative methane production of the acidizing fluid was 304.96 mL/g COD, similar (P ＞ 0.05) to the control (318.27 mL/g COD). However, the methane production time decreased by 43.4% (from 30 to 17 days), thereby improving the AD efficiency. Inoculation with DPSM is therefore an effective pre-treatment for agricultural waste for methane production.

Resource recovery from sugarcane vinasse by anaerobic digestion - A review

The increase in biofuel production by 2030, driven by the targets set at the 21st United Nations Framework Convention on Climate Change (COP21), will promote an increase in ethanol production, and consequently more vinasse generation. Sugarcane vinasse, despite having a high polluting potential due to its high concentration of organic matter and nutrients, has the potential to produce value-added resources such as volatile fatty acids (VFA), biohydrogen (bioH₂) and biomethane (bioCH₄) from anaerobic digestion. The objective of this paper is to present a critical review on the vinasse treatment by anaerobic digestion focusing on the final products. Effects of operational parameters on production and recovery of these resources, such as pH, temperature, retention time and type of inoculum were addressed. Given the importance of treating sugarcane vinasse due to its complex composition and high volume generated in the ethanol production process, this is the first review that evaluates the production of VFAs, bioH₂ and bioCH₄ in the treatment of this organic residue. Also, the challenges of the simultaneous production of VFA, bioH₂ and bioCH₄ and resources recovery in the wastewater streams generated in flex-fuel plants, using sugarcane and corn as raw material in ethanol production, are presented. The installation of flex-fuel plants was briefly discussed, with the main impacts on the treatment process of these effluents either jointly or simultaneously, depending on the harvest season.

Intermittent air mixing system for anaerobic digestion of animal wastewater: Operating conditions and full-scale validation

An air mixing system for anaerobic digestion has been proved to be beneficial for methane production. The aim of the present study was to further investigate the appropriate conditions for air mixing. The effective methane production time (EMPT) was defined to determine the air mixing time in the article. The results indicated that the appropriate aeration intensity was 66.7 mL air per volume of reactor per min and mixing time was 1.5 min. When air mixing time exceeded 3 min on each occasion, total CH₄ production was less than that achieved under the no mixing condition due to a decrease in the EMPT. In addition, the possibility of air mixing was evaluated in an anaerobic full-scale plant comprising a continuous stirred tank reactor. One year of operating data validated the feasibility of air mixing during the anaerobic digestion of swine wastewater.

Statistical optimization of methane production from brewery spent grain: Interaction effects of temperature and substrate concentration

This study evaluated the effects of thermal pretreatment of brewery spent grain (BSG) (by autoclave 121 °C, 1.45 atm for 30 min) on methane production (CH₄). Operation temperature (31-59 °C) and substrate concentration (8.3-19.7 g BSG.L^-1) factors were investigated by Response Surface Methodology (RSM) and Central Composite Design (CCD). Values ranging from 81.1 ± 2.0 to 290.1 ± 3.5 mL CH₄.g^-1 TVS were obtained according to operation temperature and substrate concentration variation. The most adverse condition for methanogenesis (81.1 ± 2.0 mL CH₄.g^-1 TVS) was at 59 °C and 14 g BSG.L^-1, in which there was increase in the organic matter concentration from 173.6 ± 4.94 to 3036 ± 7.78 mg.L^-1) result of a higher final concentration of volatile fatty acids (VFA, 2662.7 mg.L^-1). On the other hand, the optimum condition predicted by the statistical model was at 35 °C and 18 g BSG.L^-1 (289.1 mL CH₄.g^-1 TVS), which showed decrease in the organic matter concentration of 78.6% and a lower final concentration of VFA (533.2 mg.L^-1). Hydrogenospora and Methanosaeta were identified in this optimum CH₄ production condition, where acetoclastic methanogenic pathway prevailed. The CH₄ production enhancement was concomitant to acetic acid concentration decrease (from 578.9 to 135.7 mg.L^-1).

Effect of microaerobic microbial pretreatment on anaerobic digestion of a lignocellulosic substrate under controlled pH conditions

The effects of various microaeration strategies and process parameters on anaerobic digestion (AD) of lignocellulosic substrates have received increased attention; however, different results have been reported. To determine optimal conditions and clarify the mechanisms influencing this process, the effect of pretreatment of microaerobic microbial on corn stover decomposition and AD was investigated with real-time pH control. Fresh cow manure was chosen as the inoculum, as it has the strongest cellulose hydrolysis capacity under microaeration conditions. Microaeration microbial pretreatment effectively promoted the hydrolysis and acidogenesis of corn stover, and pH considerably affected total solid reduction, volatile fatty acid (VFA), and accumulation of soluble chemical oxygen demand (sCOD) patterns by shifting microbial communities. Different pH levels and pretreatment times led to positive and negative effects on methane yield. A 12-h pretreatment of substrate at pH 8 prior to AD increased the methane yield by 16.6% in comparison with the un-pretreated sample.

Anaerobic bioconversion efficiency of rice straw in continuously stirred tank reactor systems applying longer hydraulic retention time and higher load: One-stage vs. Two-stage

This study investigated the anaerobic bioconversion efficiency of rice straw in continuously stirred tank reactor (CSTR) applying longer hydraulic retention time (HRT) and higher load. Two HRT distributions and two loads were studied and compared for one-stage and two-stage CSTR systems. The results indicated that the two system with longer HRT (60d) and higher load (160g TS·L^-1) obtained 11.06% and 14.28%, 15.24% and 19.38%, more biogas and methane productions than those of one-stage system with HRT (50d) and load (140g TS·L^-1), respectively, while maintained stable operation at higher loads. It was also found that the microbial richness, diversity, and bacterial and archaeal community compositions showed some differences between two systems with different HRTs and loads, which was thought to be one of reasons leading to the differences in bioconversion efficiencies. The study indicated that two-stage system applying longer HRT and higher load could be one of the effective methods for more bioenergy recovery from rice straw.

Using a strong chemical oxidant, potassium ferrate (K2FeO4), in waste activated sludge treatment: A review

The ever-increasing production of waste activated sludge (WAS) has become a widespread problem to sewage treatment plants around the world. Among the multitudinous sludge treatment methods, chemical oxidation is considered as an excellent technology with both high efficiency and low investment cost. As an eco-friendly oxidant, potassium ferrate (PF) has attracted great attention in sludge treatment over the past decade. The applications of PF have demonstrated advantages in: (1) sludge dewatering; (2) minimization; (3) anaerobic fermentation; (4) removal of pollutants. This review summarizes recent work on the effects of PF on these four aspects of facilitating sludge disposal. Meanwhile, the underlying mechanisms for the diverse applications of PF on sludge treatment are analyzed. Furthermore, the shortages and knowledge gaps on current PF oxidizing methods are discussed, and directions for further research to simultaneously enhance treatment efficiency and reduce processing cost are suggested as well.

Degradation of rice straw at low temperature using a novel microbial consortium LTF-27 with efficient ability

In this study, a novel psychrotrophic lignocelluloses degrading microbial consortium LTF-27 was successfully obtained from cold perennial forest soil by successive enrichment culture under facultative anaerobic static conditions. The microbial consortium showed efficient degradation of rice straw, which cellulose, hemicelluloses and lignin lost 71.7%, 65.6% and 12.5% of its weigh, respectively, in 20 days at 15 °C. The predominant liquid products were acetic acid and butyric acid during degrading lignocellulose in anaerobic digestion (AD) process inoculated with the LTF-27. The consortium mainly composed of Parabacteroides, Alcaligenes, Lysinibacillus, Sphingobacterium, and Clostridium, along with some unclassified uncultured bacteria, indicating powerful synergistic interaction in AD process. A multi-species lignocellulolytic enzyme system working cooperatingly on lignocelluolse degradation was revealed by proteomics analysis of cellulose bound fraction of the crude extracellular enzyme, which provides key theoretical base for further exploration and application of LTF-27.

Municipal wastewater treatment by purple phototropic bacteria at low infrared irradiances using a photo-anaerobic membrane bioreactor

Light energy is one of the major costs for phototrophic systems. This study evaluated the photoreactor efficiency of purple phototropic bacteria anaerobic membrane bioreactor (PAnMBR) at low irradiance for the treatment of municipal wastewater. Infrared irradiance levels of 3.0 and 1.4 W/m² produced by an infrared (IR) lamp emitting in the 800-900 nm wavelength range were investigated, with the ultimate goal of optimizing the irradiance energy demand. Experimental and modeling results demonstrated the ability of PPB to grow and treat raw municipal wastewater at the applied low irradiances, with effluent quality below target limits of TCOD˂50 mg/L, TN˂10 mg/L, and TP˂1 mg/L. While Monod kinetic parameters, k_m and Y, were determined to be lower than previous high-energy studies (1.9 mgCOD/mgVSS-d and 0.38 mgVSS/mgCOD, respectively), the photobioreactor performance were consistently maintained, indicating that energy cost associated with IR illumination can be reduced by up to 97%. To determine whether the treatment process could approach energy neutrality, subsequent anaerobic digestion experiments of the residual PPB biomass proved a potential for biogas recovery of up to 240 NmLCH₄/gVSS_added, and a moderate biomass biodegradability of 41%. As a result, the net energy consumption of the process was estimated at 0.5 kWh/m³ of treated municipal wastewater, considering an energy demand for illumination of 0.67 kWh/m³ and an energy recovery attributed to the anaerobic digestion of 0.17 kWh/m³ from the excess PPB biomass wasted from PAnMBR.

Review on microaeration-based anaerobic digestion: State of the art, challenges, and prospectives

Microaeration (dosing small quantities of air or oxygen) is an effective approach to facilitate anaerobic digestion (AD) process and has gained increased attention in recent years. The underlying mechanisms of the facilitation effect of microaeration on AD process were reviewed in terms of accelerating hydrolysis, scavenging hydrogen sulfide, and affecting microbial diversity. Process parameters and control strategies were summarized to reveal considerable factors in implementing microaeration-based AD process. In addition, current applications, including lab-, pilot- and full-scale level cases, were summarized to provide guidance for further improvement in large-scale applications. The challenges and future perspectives were also highlighted to promote the development of AD process associated with microaeration.

Exploring the potential of co-fermenting sewage sludge and lipids in a resource recovery scenario

In this study, co-fermentation of primary sludge (PS) or waste activated sludge (WAS) with lipids was explored to improve volatile fatty acid production. PS and WAS were used as base substrate to facilitate lipid fermentation at 20 °C under semi-aerobic conditions. Mono-fermentation tests showed higher VFA yields for PS (32-89 mgCOD gVS^-1) than for WAS (20-41 mgCOD gVS^-1) where propionate production was favoured. The principal component analysis showed that the base substrate had a notable influence on co-fermentation yields and profile. Co-fermentation with WAS resulted in a greater extent of oleic acid degradation (up to 4.7%) and evidence of chain elongation producing valerate. The occurrence of chain elongation suggests that co-fermentation can be engineered to favour medium-chain fatty acids without the addition of external commodity chemicals. BMP tests showed that neither mono-fermentation nor co-fermentation had an impact on downstream anaerobic digestion.

Nitrite and free nitrous acid sludge pre-treatments to enhance methane production in continuous anaerobic digestion: Comparing process performance and associated costs

Secondary sludge pre-treatment with free nitrous acid (FNA) has been proven to enhance methane production during anaerobic digestion. However, it is still unclear if the same enhancement can be achieved only using nitrite, without sludge acidification. In this paper, secondary sludge was pre-treated during 5 h with nitrite within the range of 50-250 mg NO₂^--N/L at neutral pH (6.7). Results obtained from biochemical methane potential tests (BMPs) indicated that sludge pre-treatment at 150 mg NO₂^--N/L presented the best enhancement of methane production (24% as compared to the control). These conditions were used to pre-treat sludge added in a continuous lab-scale anaerobic digester that operated in parallel to another digester receiving sludge pre-treated with FNA (250 mg NO₂^--N/L at pH 5.5). Results showed a very similar performance in terms of methane enhancement in both reactors, indicating that sludge acidification is not needed to improve methane yield. A preliminary economic assessment also highlights the need for assessing real chemical costs and national power prices before the implementation of these pre-treatment steps as the associated benefits can significantly change depending on the country where the wastewater treatment plant is located.

Optimization of urban waste fermentation for volatile fatty acids production

The problem of waste disposal has recently focused on practices for waste recycling and bio-resources valorization. Organic waste produced in urban context together with biological sludge produced in wastewater treatment plants (WWTPs) can be used as renewable feedstock for the production of building blocks of different products, from biopolymers to methyl esters. This paper deals with the optimization of the fermentation process in order to transform urban organic waste (a mixture of pre-treated food waste and biological sludge) into added-value volatile fatty acid (VFA) rich stream, useful for biological processes within a biorefinery technology chain. Different temperatures, pH, hydraulic retention times (HRTs) and organic loading rates (OLRs) were tested both in batch and continuous trials. Batch tests showed the best working conditions at 37 °C and pH 9, using the bio-waste feedstock thermally pre-treated (76 h at 72 °C). These conditions were applied in continuous process, where higher HRT (6.0 d) and lower OLR [7.7 kg VS/(m³ d)] gave the best performances in terms of process yield and maximum VFA level achieved: 0.77 COD_VFA/VS₍₀₎ and 39 g COD_VFA/L. An optimized fermentation process is crucial in a biorefinery perspective since it has to give a final stream of constant composition or tailored products suitable for further applications.

Optimization of hydraulic retention time and organic loading rate for volatile fatty acid production from low strength wastewater in an anaerobic membrane bioreactor

This study aims to investigate the production of volatile fatty acids (VFAs) from low strength wastewater at various hydraulic retention time (HRT) and organic loading rate (OLR) in a continuous anaerobic membrane bioreactor (AnMBR) using glucose as carbon source. This experiment was performed without any selective inhibition of methanogens and the reactor pH was maintained at 7.0 ± 0.1. 48, 24, 18, 12, 8 and 6 h-HRTs were applied and the highest VFA concentration was recorded at 8 h with an overall VFA yield of 48.20 ± 1.21 mg VFA/100 mg COD_feed. Three different ORLs were applied (350, 550 and 715 mg COD_feed) at the optimum 8 h-HRT. The acetic and propanoic acid concentration maximums were (1.1845 ± 0.0165 and 0.5160 ± 0.0141 mili-mole/l respectively) at 550 mg COD_feed. The isobutyric acid concentration was highest (0.3580 ± 0.0407 mili-mole/l) at 715 mg COD_feed indicating butyric-type fermentation at higher organic loading rate.

Stimulation of methane yield rate from food waste by aerobic pre-treatment

Aerobic pre-treatment (AP) was applied to enhance methane yield from food waste through anaerobic digestion. Different AP durations (i.e. 2, 5 and 8 days) prior to anaerobic digestion were tested. The results indicated that AP of food waste led to no significant differences (p > 0.05) in methane yield potential (ca. 418 mL/g-VS). However, a suitable AP duration (5 days) increased methane yield rates (ca. 18 mL/d/g-VS; 22.0% higher than the control) by anticipating methane generation and shortening the methanogenic phase via volatile fatty acid reduction and pH increase. Although AP induced chemical oxygen demand loss to some extent (i.e. by 2.6%-9.9%) in the AP stage via aerobic degradation, the methane yield potential could be recovered by enhancing organic matter hydrolysis. Therefore, maximisation of hydrolysis should be used as a basis for determining a suitable AP duration for various types of organic matter.

Anaerobic treatment of N, N-dimethylformamide-containing wastewater by co-culturing two sources of inoculum

The complete methanogenic degradation of N, N-dimethylformamide (DMF) was achieved in this study. Initially, DMF was found to be feebly degradable by a lab-scale submerged anaerobic membrane bioreactor (SAnMBR) using normal anaerobic digestion sludge (ADS) even after 120-day's culturing. However, aerobic DMF-degrading activated sludge (AS) was rapidly cultivated in a continuous aeration reactor (CAR). A specially designed anaerobic co-cultured sludge (ACS) made by artificially mixing AS with ADS was successfully domesticated by a long term repeated batch experiment. The results demonstrated that ACS could effectively degrade over 5000 mg L^-1 DMF for methane recovery. The metabolic pathway and stoichiometric equation of DMF methanogenic degradation were also revealed and verified in detail. It was confirmed that under the anaerobic condition, with the help of enzyme, DMF converts into dimethylamine and formic acid, and the intermediates are effectively fermented through methylotrophic/hydrogenotrophic methanogenesis. Analysis of the microbial community suggested that some facultatively anaerobic bacteria played the key roles in methanogenic degradation due to their DMF-hydrolyzing ability. By co-culturing two sources of inoculum under the anaerobic condition, the symbiosis of facultatively anaerobic DMF-hydrolyzing bacteria and methylotrophic/hydrogenotrophic methanogens makes methanogenic degradation of DMF available. This study also provides a novel sludge cultivation method for anaerobic treatment of degradation-resistant organics.

Improved methane yield from wastewater grown algal biomass

Methane production from the algal biomass cultivated in a laboratory scale continuous photobioreactor (PBR) using sewage was evaluated in the present work. During the preliminary experiments, algal biomass reached up to 1.69 ± 0.35 g L^-1 in 12 days' growth period. Besides, 65 to 100% removal in concentrations of total dissolved phosphorus (TDP), nitrate nitrogen (NO₃-N), total ammoniacal nitrogen (TAN) and soluble chemical oxygen demand (sCOD) was also recorded. The sCOD removal in the reactor was 100%, whereas removal of TDP, NO₃-N and TAN were up to 75, 40 and 92%, respectively. Upon anaerobic digestion, the fresh algal biomass showed methane yield of 180 mL g^-1 VS_fed. Further, algal biomass was stored under natural conditions in open containers (aerobic conditions) in darkness at room temperature (27-30 °C) for 72 h. Interestingly, >48% COD solubilization from algal biomass was observed during storage. Pretreatment through natural storage was further confirmed with qualitative observations including scanning electron and fluorescence microscopic analysis. Moreover, higher methane yield (284.38 mL g^-1 VS_fed) was observed from the samples stored for 60 h. Thus, natural storage for a designated period may be recommended as a prerequisite stage in the process of methane production from wastewater-grown algal biomass.

Biogas production and microbial community properties during anaerobic digestion of corn stover at different temperatures

Temperature has different effects on anaerobic digestion (AD) of various biomasses, which could bring out changes in microbial communities. The relationship between microbial community and methane production at 35 °C (R35), 38 °C (R38), 41 °C (R41), and 44 °C (R44) was analyzed during AD of corn stover (CS). The results showed that the daily biogas and methane production from R44 were 16.6%-42.4% and 16.2%-40.6% higher than yields from R35, R38 and R41, respectively. The abundance of Bacteroidetes in R35, R38 and R41 was relatively close (30.70%-39.36%), which was low in R44 (16.00%). The abundance of Firmicutes in R35 was 32.30%, however, Firmicutes was the most dominant phylum at R44 (66.58%). The abundance of Miscellaneous_Crenarchaeotic_Group and Euryarchaeota were 54.63 ± 6.47% and 44.43 ± 6.73% across all digesters. This research demonstrated that among all temperatures studied, 44 °C could enhance the conversion efficiency of the substrates to methane and be recommended for better conversion of CS in AD process.

Effect of total solid content and pretreatment on the production of lactic acid from mixed culture dark fermentation of food waste

Food waste landfilling causes environmental degradation, and this work assesses a sustainable food valorization technique. In this study, food waste is converted into lactic acid in a batch assembly by dark fermentation without pH control and without the addition of external inoculum at 37 °C. The effect of total solid (TS), enzymatic and aeration pretreatment was investigated on liquid products concentration and product yield. The maximum possible TS content was 34% of enzymatic pretreated waste, and showed the highest lactic acid concentration of 52 g/L, with a lactic acid selectivity of 0.6 g_lactic/g_totalacids. The results indicated that aeration pretreatment does not significantly improve product concentration or yield. Non-pretreated waste in a 29% TS system showed a lactic acid concentration of 31 g/L. The results showed that enzymatic pretreated waste at TS of 34% results in the highest production of lactic acid.

A comprehensive review on food waste anaerobic digestion: Research updates and tendencies

Anaerobic digestion has been practically applied in agricultural and industrial waste treatment and recognized as an economical-effective way for food waste disposal. This paper presented an overview on the researches about anaerobic digestion of food waste. Technologies (e.g., pretreatment, co-digestion, inhibition and mitigation, anaerobic digestion systems, etc.) were introduced and evaluated on the basis of bibliometric analysis. Results indicated that ethanol and aerobic prefermentation were novel approaches to enhance substrates hydrolysis and methane yield. With the promotion of resource recovery, more attention should be paid to biorefinery technologies which can produce more useful products toward zero emissions. Furthermore, a technological route for food waste conversion based on anaerobic digestion was proposed.

Biochemical methane potential (BMP) tests: Reducing test time by early parameter estimation

Biochemical methane potential (BMP) test is a key analytical technique to assess the implementation and optimisation of anaerobic biotechnologies. However, this technique is characterised by long testing times (from 20 to >100days), which is not suitable for waste utilities, consulting companies or plants operators whose decision-making processes cannot be held for such a long time. This study develops a statistically robust mathematical strategy using sensitivity functions for early prediction of BMP first-order model parameters, i.e. methane yield (B₀) and kinetic constant rate (k). The minimum testing time for early parameter estimation showed a potential correlation with the k value, where (i) slowly biodegradable substrates (k≤0.1d^-1) have a minimum testing times of ≥15days, (ii) moderately biodegradable substrates (0.1<k<0.2d^-1) have a minimum testing times between 8 and 15 days, and (iii) rapidly biodegradable substrates (k≥0.2d^-1) have testing times lower than 7days.

High-efficiency removal of lead from wastewater by biochar derived from anaerobic digestion sludge

The properties of biochar derived from waste activated sludge and anaerobic digestion sludge under pyrolysis temperature varying from 400°C to 800°C were investigated. The heavy metals adsorption efficiency of the sludge-derived biochar was also examined. Among the biochar samples tested, ADSBC600 possessing highly porous structure, special surface chemical behaviors and high thermal stability was found to remove Pb²⁺ from aqueous solutions efficiently with an adsorption capacity of 51.20mg/g. The Pb²⁺ adsorption kinetics and isotherm for ADSBC600 can be described using the pseudo second-order model and Langmuir isotherm, respectively. Analysis of the characteristics of biochar before and after metal treatment suggests that electrostatic attraction, precipitation, surface complexation and ion exchange are the possible Pb²⁺ removal mechanisms. This study demonstrates a successful example of waste refinery by converting anaerobic digestion sludge to feasible heavy metal adsorbents to implement the concept of circular economy.

Seeking to enhance the bioenergy of municipal sludge: Effect of alkali pre-treatment and soluble organic matter supplementation

The aim of this research is to enhance the mesophilic anaerobic digestion of municipal sludge from Cadiz-San Fernando (Spain) wastewater treatment plant at 20days hydraulic retention time (HRT). Two different strategies were tested to improve the process: co-digestion with the addition of soluble organic matter (1% v/v); and alkali sludge pre-treatment (NaOH) prior to co-digestion with glycerine (1% v/v). Methane production (MP) was substantially enhanced (from 0.36±0.09 L CH₄ l/d to 0.85±0.16 L CH₄ l/d), as was specific methane production (SMP) (from 0.20±0.05 L CH₄/g VS to 0.49±0.09 L CH₄/g VS) when glycerine was added. The addition of glycerine does not seem to affect sludge stability, the quality of the effluent in terms of pH and organic matter content, i.e. volatile fatty acids (VFA), soluble organic matter and total volatile solid, or process stability (VFA/Alkalinity ratio<0.4). Alkali pre-treatment prior to co-digestion resulted in a high increase in soluble organic loading rates (more than 20%) and acidification yield (more than 50%). At 20days HRT, however, it led to overload of the system and total destabilization of the mesophilic anaerobic co-digestion of sewage sludge and glycerine.

Pre-aeration of food waste to augment acidogenic process at higher organic load: Valorizing biohydrogen, volatile fatty acids and biohythane

Application of pre-aeration (AS) to waste prior to feeding was evaluated on acidogenic process in a semi-pilot scale biosystem for the production of biobased products (biohydrogen, volatile fatty acids (VFA) and biohythane) from food waste. Oxygen assisted in pre-hydrolysis of waste along with the suppression of methanogenic activity resulting in enhanced acidogenic product formation. AS operation resulted in 97% improvement in hydrogen conversion efficiency (HCE) and 10% more VFA production than the control. Increasing the organic load (OL) of food waste in association with AS application improved the productivity. The application of AS also influenced concentration and composition of fatty acid. Highest fraction of acetic (5.3g/l), butyric (0.7g/l) and propionic acid (0.84g/l) was achieved at higher OL (100g COD/l) with good degree of acidification (DOA). AS strategy showed positive influence on biofuel (biohydrogen and biohythane) production along with the biosynthesis of short chain fatty acids functioning as a low-cost pretreatment strategy in a single stage bioprocess.

Effect of free nitrous acid pre-treatment on primary sludge at low exposure times

The present study was undertaken to investigate the effect of different free nitrous acid (FNA) concentrations at low pre-treatment times (PTs) (1, 2 and 5h) and without pH control with mild agitation on primary sludge (PS) biodegradability and methane production (MP). Increasing PTs resulted in an increase in the solubility of the organic matter (around 25%), but not on cell-mortality (>75% in all the cases with FNA) and neither on methane generation. FNA pre-treatment at low PTs improve MP (around 16% at PT of 1h and 650mg N-NO₂^-/L). However, a similar improvement was found with mild agitation of PS without FNA at 2 and 5h. Taking into account the potential costs associated with the FNA pre-treatment, a mild agitation without FNA would be preferred to enhance MP in PS.

Effect of micro-aeration and inoculum type on the biodegradation of lignocellulosic substrate

The effect of various micro-aeration strategies on the anaerobic digestion (AD) of wheat straw was thoroughly examined using a mixture of inocula, containing compost and well digested sludge from biogas plant. The aim was to determine the most efficient oxygen load, pulse repetition and treatment duration, resulting in the highest methane production. The oxygen load had the largest impact on the biodegradability of straw, among the examined variables. More specifically, a micro-aeration intensity of 10mLO₂/gVS was identified as the critical threshold above which the AD performance was more susceptible to instability. The highest enhancement in biogas production was achieved by injecting 5mLO₂/gVS for a consecutive 3-day treatment period, presenting a 7.2% increase compared to the untreated wheat straw. Nevertheless, the results from optimisation case study indicated a higher increase of 9% by injecting 7.3mLO₂/gVS, distributed in 2 pulses during a slightly shorter treatment period (i.e. 47h).

Resource Recovery from Wastewater by Biological Technologies: Opportunities, Challenges, and Prospects

Limits in resource availability are driving a change in current societal production systems, changing the focus from residues treatment, such as wastewater treatment, toward resource recovery. Biotechnological processes offer an economic and versatile way to concentrate and transform resources from waste/wastewater into valuable products, which is a prerequisite for the technological development of a cradle-to-cradle bio-based economy. This review identifies emerging technologies that enable resource recovery across the wastewater treatment cycle. As such, bioenergy in the form of biohydrogen (by photo and dark fermentation processes) and biogas (during anaerobic digestion processes) have been classic targets, whereby, direct transformation of lipidic biomass into biodiesel also gained attention. This concept is similar to previous biofuel concepts, but more sustainable, as third generation biofuels and other resources can be produced from waste biomass. The production of high value biopolymers (e.g., for bioplastics manufacturing) from organic acids, hydrogen, and methane is another option for carbon recovery. The recovery of carbon and nutrients can be achieved by organic fertilizer production, or single cell protein generation (depending on the source) which may be utilized as feed, feed additives, next generation fertilizers, or even as probiotics. Additionlly, chemical oxidation-reduction and bioelectrochemical systems can recover inorganics or synthesize organic products beyond the natural microbial metabolism. Anticipating the next generation of wastewater treatment plants driven by biological recovery technologies, this review is focused on the generation and re-synthesis of energetic resources and key resources to be recycled as raw materials in a cradle-to-cradle economy concept.

Effect of aerobic pre-treatment on hydrogen and methane production in a two-stage anaerobic digestion process using food waste with different compositions

Aerobic pre-treatment was applied prior to two-stage anaerobic digestion process. Three different food wastes samples, namely carbohydrate rich, protein rich and lipid rich, were prepared as substrates. Effect of aerobic pre-treatment on hydrogen and methane production was studied. Pre-aeration of substrates showed no positive impact on hydrogen production in the first stage. All three categories of pre-aerated food wastes produced less hydrogen compared to samples without pre-aeration. In the second stage, methane production increased for aerated protein rich and carbohydrate rich samples. In addition, the lag phase for carbohydrate rich substrate was shorter for aerated samples. Aerated protein rich substrate yielded the best results among substrates for methane production, with a cumulative production of approximately 351ml/gVS. With regard to non-aerated substrates, lipid rich was the best substrate for CH₄ production (263ml/gVS). Pre-aerated P substrate was the best in terms of total energy generation which amounted to 9.64kJ/gVS. This study revealed aerobic pre-treatment to be a promising option for use in achieving enhanced substrate conversion efficiencies and CH₄ production in a two-stage AD process, particularly when the substrate contains high amounts of proteins.

Influence of temperature and hydraulic retention on the production of volatile fatty acids during anaerobic fermentation of cow manure and maize silage

The aim of this study was to verify the efficiency of a separate hydrolysis step by testing different working temperatures (37-55°C) and hydraulic retention times (two, four and six days) and by evaluating readily biodegradable carbon production. The fermentation products included primarily acetic, propionic and butyric acids. These acids can be easily converted into biogas or can be recovered in a biorefinery approach, for example, to produce polyhydroxyalkanoates. The optimal condition was found by applying an organic loading rate of 17.9gTVSm^-3 with a four-day retention time at 37°C for an acidification yield of 183.2gCOD_VFAkgVS_fed^-1.

Optimization of process parameters for production of volatile fatty acid, biohydrogen and methane from anaerobic digestion

The anaerobic digestion process has been primarily utilized for methane containing biogas production over the past few years. However, the digestion process could also be optimized for producing volatile fatty acids (VFAs) and biohydrogen. This is the first review article that combines the optimization approaches for all three possible products from the anaerobic digestion. In this review study, the types and configurations of the bioreactor are discussed for each type of product. This is followed by a review on optimization of common process parameters (e.g. temperature, pH, retention time and organic loading rate) separately for the production of VFA, biohydrogen and methane. This review also includes additional parameters, treatment methods or special additives that wield a significant and positive effect on production rate and these products' yield.

Comparing the value of bioproducts from different stages of anaerobic membrane bioreactors

The anaerobic digestion process in anaerobic membrane bioreactors is an effective way for waste management, energy sustainability and pollution control in the environment. This digestion process basically involves the production of volatile fatty acids and biohydrogen as intermediate products and methane as a final product. This paper compares the value of bioproducts from different stages of anaerobic membrane bioreactors through a thorough assessment. The value was assessed in terms of technical feasibility, economic assessment, environmental impact and impact on society. Even though the current research objective is more inclined to optimize the production of methane, the intermediate products could also be considered as economically attractive and environment friendly options. Hence, this is the first review study to correlate the idea into an anaerobic membrane bioreactor which is expected to guide future research pathways regarding anaerobic process and its bioproducts.