Subsequent Anaerobic Digestion Research Articles

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, km 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/gVSSadded, 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.

Activation of nitrite by freezing process for anaerobic digestion enhancement of waste activated sludge: Performance and mechanisms

Nitrite-based pretreatment was demonstrated to effectively improve anaerobic digestion of waste activated sludge. It was found in this work that the freezing activated nitrite pretreatment could further enhance the performances. With the increase of nitrite addition from 0 to 600 mg NO2−-N/L during freezing process, the biochemical methane potential of pretreated-sludge gradually increased from 191.3 ± 8.0 to 233.2 ± 10.6 mL per gram volatile solid (VS), while only 178.6 ± 7.3 mL/g VS was obtained in the raw sludge. Mechanism explorations revealed that the freezing activated nitrite pretreatment remarkably facilitated the disintegration of sludge. Excitation emission matrix and fluorescence regional integration analyses further revealed that nitrite addition during freezing process promoted the proportion of biodegradable organics released, thereby providing more bio-available substrates for subsequent anaerobic digestion. Freezing condition induced reactive derivatives from nitrite (e.g., free nitrite acid, NO2, N2O3) were assumed to be the major contributors to the enhanced sludge disintegration and recalcitrant organics (e.g., humic acid-like substances) degradation. It was also found that 600 mg NO2−-N/L addition activated by freezing pretreatment produced an anaerobically digested sludge with an improved dewaterability, as indicated by the decrease of the specific resistance to filterability and moisture content of dewatered cake. Moreover, 600 mg NO2−-N/L addition activated by freezing pretreatment and subsequent anaerobic digestion largely inactivated the pathogens to the levels below Class A biosolids requirements. Considering that nitrite can be in-situ produced in wastewater treatment plants through nitritation of the digestion liquid, this nitrite-based freezing process for sludge pretreatment was environmental-friendly and economically attractive.

Assessment of fungal and thermo-alkaline post-treatments of solid digestate in a recirculation scheme to increase flexibility in feedstocks supply management of biogas plants

Agricultural biogas plants can suffer occasional feedstock shortages (poor harvest, storage …) and recirculation of solid digestate (SD) into digester has been identified as a simple way to offset methane production loss from these situations. Calculations show that recirculation of SD could offset for losses in plant methane production by up to 2.4%. In that context, two post-treatments were evaluated to enhance residual potential methane of agricultural SD. Effect of fungal solid state fermentation (SSF) of SD on subsequent methane production has never been explored before. It was hypothesized that: (i) ligninolytic fungi would be able to specifically use the complex fraction of SD for their growth and (ii) energy generation from the subsequent anaerobic digestion of the colonized SD will be enhanced. However, experiments showed that thermo-alkaline treatment of SD (used as alkalinization and sterilization process) and a high spawn level (20% w/w) were necessary to perform fungal SSF. Besides, the observed fungal activities on SD did not target specifically the most complex fractions. This led to uncontrolled organic matter losses and subsequent decreases of biodegradability and methane yield of SD (up to 50%). Therefore, fungal SSF of SD before its recirculation into biogas plants appeared not to be a viable option. Only thermo-alkaline treatment (CaO 2% w/w and 121 °C 30 min) enhanced methane yield of SD by 13% and decreased its complex fraction by 25%. Further studies on optimization of this post-treatment may enhance efficiency of SD recirculation strategy to offset plant methane production losses.

Maximizing energy recovery from wastewater via bioflocculation-enhanced primary treatment: a pilot scale study

ABSTRACT Anaerobic digestion of municipal sewage sludge is widely used for harvesting energy from wastewater organic content. The more organic carbon we can redirect into the primary sludge, the less energy is needed for aeration in secondary treatment and the more methane is produced in anaerobic digesters. Bioflocculation has been proposed as a promising separation technology to maximize carbon capture in primary sludge. Thus far, only limited data on bioflocculation are available under real conditions, i.e. from pilot-scale reactors treating raw sewage. Moreover, no study has discussed yet the influence of bioflocculation on denitrification potential of sewage. Therefore, we performed bioflocculation of raw sewage in high-rate contact stabilization process in pilot-scale to investigate maximal primary treatment efficiency. During 100 days of operation at sludge retention time of only 2 days, the average removal efficiencies of chemical oxygen demand (COD), suspended solids and total phosphorus were 75%, 87% and 51%, respectively, using no chemicals for precipitation. Up to 76% of incoming COD was captured in primary sludge and 46% for subsequent anaerobic digestion, where energy recovery potential achieved 0.33–0.37 g COD as CH4 per g COD of influent. This study showed in real conditions that this newly adapted separation process has significant benefits over chemically enhanced primary treatment, enabling sewage treatment process to overcome energy self-sufficiency.

Enhancing methane yield from crude glycerol anaerobic digestion by coupling with ultrasound or A. niger/E. coli biodegradation.

Anaerobic digestion of crude glycerol from biodiesel production is a feasible way for methane production. However, crude glycerol (CG) contains impurities, such as long-chain fatty acids (LCFA) that can inhibit methanogenic microorganisms. Ultrasound promotes the hydrolysis of LCFA and deagglomerates the microorganisms in biological flocs. Furthermore, Aspergillus niger and Escherichia coli produce lipases capable of degrading LCFA. This study aims at improving the methane yield from anaerobic digestion by coupling with ultrasound or E. coli/A. niger biodegradation. The effect of the different treatments was first assessed in a perfectly mixed batch reactor (PMBR), using diluted CG at concentrations of 0.2%, 1.7%, and 3.2% (v/v). Later, the best conditions were replicated in an upflow anaerobic sludge blanket (UASB) reactor to simulate full-scale practical applications. Experiments in the PMBR showed that ultrasound or A. niger biodegradation steps improved methane yield up to 11% for 0.2% CG and 99% for 1.7% CG, respectively. CG biodegradation by E. coli inhibited the subsequent anaerobic digestion for all concentrations tested. Using a UASB digester, ultrasonic treatment of CG led to an average increase of 29% in methane production. The application of ultrasound led to a lower accumulation of propionic acid in the digested material and increased biogas production. On the other hand, an average 77% increase in methane production was achieved using a preliminary CG biodegradation step by A. niger, when operated at a loading rate of 2.9 kg COD m-3 day-1. Under these conditions, an energy gain of 0.48 kWh day-1, with the production of the 0.434 m3 CH4 kg-1 CODremoval and 0.573 m3 CH4 kg-1 VS, and a biogas quality of 73% in methane were obtained. The digested material was analyzed for the detection and quantification of added-value by-products in order to obtain a broad assessment of the CG valorization through anaerobic digestion. In some experiments, propionic and oxalic acid were detected. However, the accumulation of propionic caused the inhibition of the acetogenic and methanogenic microorganisms.

Destruction of Staphylococcus aureus and the impact of chlortetracycline on biomethane production during anaerobic digestion of chicken manure

Research was undertaken to ascertain the effect on biogas potential during the anaerobic digestion of chicken manure containing Staphylococcus aureus and chlortetracycline (antibiotic) from infected chicken flocks. S. aureus is a pathogenic bacteria in chicken flocks that is usually treated with the broad-spectrum antibiotic, chlortetracycline. Veterinary antibiotics are often prescribed in the poultry sector for on-farm use at the flock level to control disease; consequently, significant quantities of antibiotics are excreted from the bird into the manure. Subsequent anaerobic digestion of this chicken manure could lead to pathogens and antibiotics affecting the digestion process. Anaerobic digestion biochemical methane potential assays were completed at 35°C for 39 days, with some assays receiving S. aureus and some receiving S. aureus and chlortetracycline. No viable S. aureus cells were detected after Day 0 of the experiment. A further experiment utilising an order of magnitude greater concentration of S. aureus demonstrated a significant reduction (>400 fold) in S. aureus within 24 h when inoculated into anaerobic digestate, with no viable S. aureus cells detected by the end of 3 days. Furthermore, the efficacy of chlortetracycline was significantly reduced when applied to anaerobic digestate compared to water alone. Total biogas yields from chicken manure were significantly lowered by the addition of S. aureus, with and without chlortetracycline. However, there was no significant difference in methane yields between treatments. The cellulose control assays showed a lag phase in methane production after receiving chlortetracycline. In comparison, the absence of a lag phase when the antibiotic were added to chicken manure may have been due to the relatively high nitrogen content of the feedstock reducing the inhibition of chlortetracycline on methanogens. Therefore, this study demonstrates that the addition of S. aureus and chlortetracycline does not have a commercially relevant effect on the digestion of chicken manure.

Sludge age impacted the distribution, occurrence state and structure of organic compounds in activated sludge and affected the anaerobic degradability

Long sludge age (10 ~ 30 d) is prevalent in the operation of wastewater treatment plants of China, leading to big challenge to subsequent anaerobic digestion of sewage sludge because of low degradability and poor methane production. In order to overcome the low sludge degradability and promote the directional enhancement, the effects of sludge age on anaerobic digestion performance, especially the corresponding influencing mechanism in terms of distribution, occurrence state and structure of organic compounds in sludge driven by different sludge age were investigated. It was found that when the sludge age during wastewater treatment extended from 5 d to 40 d, the VS reduction of sludge showed an “exponential” decrease, and the degradability of protein, polysaccharides and lipids in sludge all declined. Among them, the degradation of protein was the most affected (down-regulated by 35.8%), and the decrease in degradation of protein accounted for 80.4% of the decrease of VS conversion. Sludge age during wastewater treatment impact the distribution, occurrence state and structure of organic compounds in activated sludge. And it was clarified that instead of the changed amount and components of EPS, the increased molecular weight of EPS and more stable secondary structure of protein driven by α-helix and β-turn in EPS were mainly responsible for the declined degradability of protein and sludge.

Molecular composition of hydrothermal liquefaction wastewater from sewage sludge and its transformation during anaerobic digestion

Anaerobic digestion (AD) has shown potential to convert hydrothermal liquefaction wastewater (HTLWW) into biogas in previous studies. However, the identification of refractory components and further insights into the molecular transformations of organics in HTLWW are essential for developing more efficient AD processes. In this study, two HTLWWs were obtained from the temperature-derived hydrothermal liquefaction of sewage sludge at 170 ℃ and 320 ℃. Their molecular compositions, as well as their modifications in the subsequent AD process, were characterized using a suite of advanced molecular tools. The dissolved organic matter (DOM) in the high temperature-derived HTLWW was lower in molecular weight, less saturated, less oxidized, and enhanced in nitrogenous substances. During the AD process, most of the volatile compounds and low molecular weight (LMW) neutrals were removed, while biopolymers were the most refractory. Carboxylic-rich alicyclic molecules (CRAM), particularly those containing 3 to 5 N for low temperature-derived DOM and 1 to 3 N for high temperature-derived DOM, were resistant to anaerobic biodegradation. Meanwhile, compounds with fewer nitrogens and more carboxyl groups were preferentially produced. This molecular characterization of HTLWW-derived DOM and examination of its transformation during AD will contribute to the development of efficient methods for HTLWW treatment in the future.

Intensifying the biogas production from food waste using ultrasound: Understanding into effect of operating parameters.

The present work depicts the novel approach of using ultrasound (US) induced cavitation for feedstock pretreatment with an objective of improving the anaerobic digestion (AD) process for biogas production. Initially the critical analysis of literature based on US for improving the biogas production has been presented briefly followed by study on use of US in pretreatment of food waste (FW) as well as during AD to quantify the intensification in the biogas production. Effect of different operating parameters for pretreatment such as irradiation time (over the range 2-14 min), power density (0.2-1 W/mL), duty cycle (20-80%) and substrate loading (3-11%w/v) has been investigated. Highest increase in soluble chemical oxygen demand (sCOD) with final value as 18500 mg/L (±20) (increase of 61.5%) was obtained at optimum treatment conditions of 10 min as irradiation time, 0.4 W/mL as power density, 60% as duty cycle and 7% w/v as the substrate loading. Pretreated FW was further subjected to low intensity US assisted AD process. Parameters optimized for biogas production were power (over the range of 100-200 W), irradiation time (5-15 min) and duty cycle (10-60%). Increased biogas production (almost two times) with 80% sCOD removal after 15 days was obtained at best conditions of 200 W as the US power (0.04 W/mL as power density), 5 min as the US irradiation time and 10% of US duty cycle while AD process without US resulted in only 48% sCOD removal within same time period. The work demonstrated the effective application of US in the pretreatment of feedstock and subsequent AD process giving much higher yield of biogas in shorter time.

Use of organic waste for biohydrogen production and volatile fatty acids via dark fermentation and further processing to methane

Despite the suitability of organic waste for dark fermentation (DF), anaerobic digestion (AD) counteracts its large-scale use for biohydrogen production. Therefore, 12 types of organic waste obtained from sugar, textile, food, and milk industries are investigated in batch single-stage AD and compared energetically to batch two-stage DF with subsequent AD. From the viewpoint of DF, a parametric study of mesophilic and thermophilic conditions, different substrate concentrations, and mixed cultures, i.e., granular and digested sludge, is conducted. Hydrogen yields of 90–160 LN/kgoDM (mean) and maximum yields of 199–291 LN/kgoDM are achieved with starchy and sugary wastes. Concentrations of volatile fatty acids of 9.7–14.5 g/L (mean) show the possible material uses. Thermophilic conditions are more suitable than mesophilic ones. Furthermore granular sludge is applicable for DF. The energetic comparison of the procedures demonstrates a method for assessing the applicability of waste and allows preliminary economic estimations.

Electrochemical pretreatment of yard waste to improve biogas production: Understanding the mechanism of delignification, and energy balance

In the present study, electrochemical pretreatment with a pair of graphite electrode was conducted to pretreat yard waste prior to anaerobic digestion. The Response Surface Methodology was employed to optimize the pretreatment conditions. To determine the mechanism of delignification physical and chemical properties of untreated and pretreated yard waste were investigated. In the subsequent anaerobic digestion of pretreated yard waste, the ultimate biogas production of 446 mL/g VS was achieved in comparison to the untreated yard waste of 287 mL/g VS on 35th day of anaerobic digestion. A net energy gain of 4.75 kJ/g VS (Output energy of 5.73 kJ/g VS – Input energy of 0.98 kJ/g VS) and net profit of 518 rupees (US$ 7.4) per 1 ton of yard waste indicates the applicability of electrochemical pretreatment for pilot scale.

Cultivation of the microalga Eustigmatos magnus in different photobioreactor geometries and subsequent anaerobic digestion of pre-treated biomass

Microalgal biomass as a feedstock for biogas production is linked to the parameters biomass productivity and biogas yield. Besides an easy-to-use strain for anaerobic digestion, the photobioreactor (PBR) design is important. A microalgae strain selection revealed Eustigmatos magnus (SAG 36.89) as the most promising strain yielding an average of 100 mg total suspended solids (TSS) L−1 day−1. The strain was tested in cost-effective sleevebag-PBR-systems of 10 cm, 20 cm and 30 cm diameter facing the light from the front or laterally. Highest mean productivity on a volumetric basis was measured in PBRs with the lowest diameter (104 and 117 mg L−1 day−1. The highest productivity per m−2 was achieved in 10 cm PBRs with front light configuration (9.35 g TSS m−2 day−1). The lateral light configuration of 10 cm PBRs had positive aspects such as the lowest mean water demand to produce 1 kg TSS (481 L−1 kg−1) and the lowest mean energy demand for medium separation of 1 kg TSS (106 Wh). The concentrated microalgal biomass was then subjected to ultrasonication and thermal pre-treatment (90 °C and 120 °C) and tested in BMP tests. Mesophilic anaerobic mono-digestion of untreated microalgae biomass led to a methane (CH4) yield of 343 L−1 kg−1 volatile solids (VS). Thermal pre-treatment at 120 °C resulted in significantly increased CH4 yields of 430 L−1 kg−1 VS. As thermal pre-treatment can be easily installed nearby a biogas plant it could be an interesting option for AD of microalgal biomass with only little investment.

Cellulolytic Microflora Pretreatment Increases the Efficiency of Anaerobic Co-digestion of Rice Straw and Pig Manure

Agricultural wastes have severely polluted the environment and obstruct the sustainable development of modern agriculture due to a lack of effective disposal methods. The present study conducted batch experiments in which rice straw (RS) and pig manure (PM) mixtures were pretreated with a previously developed cellulolytic microflora prior to their mesophilic anaerobic co-digestion. Optimal anaerobic digestion (AD) performance of RS and PM could be achieved after biological pretreatment with this specific cellulolytic microflora for 30 h. Under this condition, the filter paper cellulase (FPase) and carboxymethyl cellulase (CMCase) activities in RS and PM degradation broths reached maxima of 2.25 and 2.58 IU/mL, respectively, and the weight loss ratio reached 39.4%. After the subsequent AD process, the methane yield of RS and PM mixtures reached 263.69 mL/g-VS, which was 47.6% higher than that of the control group (CK) without biological pretreatment (178.66 mL/g-VS). In addition, the daily methane production peak duration (3 day) of the anaerobic co-digestion of RS and PM after 30 h of biological pretreatment with this microflora was longer than that of CK (1 day). The above results further indicated that pretreatment of RS and PM mixtures with microflora greatly enhanced the methane yield and prolonged the peak period of methane production in the subsequent anaerobic co-digestion.

Thermal hydrolysis of sewage sludge partially removes organic micropollutants but does not enhance their anaerobic biotransformation

Pretreatment technologies prior to anaerobic digestion (AD) have been developed with the aim of enhancing biogas productivity and reducing the presence of pathogens in digested sludge. Among them, thermal hydrolysis (TH) appears as the most promising one. In wastewater treatment plants (WWTPs) sludge is the end point of many organic micropollutants (OMPs), which was proved to lead to important environmental and human risks since sludge is commonly used in agriculture. The objective of this work is to determine the fate OMPs in TH and subsequent AD. Sewage sludge was pretreated in a TH pilot plant at 170 °C for 20 min. Afterwards, two anaerobic digesters with a working volume of 14 L fed with fresh and pretreated sludge were operated in parallel in mesophilic conditions. TH proved to be an effective technology to partially or totally remove the dissolved fraction of OMPs as well as the fraction sorbed into those suspended solids that are solubilised after this pretreatment. However, it did not affect the OMPs sorbed concentration into solids that are not solubilised. Globally, the OMPs removal efficiency during TH appears to be linked to the solids solubilisation during this process. Afterwards, the OMPs biotransformation efficiency in AD of fresh and pretreated sludge was determined. Noticeable differences between the microbiome of both reactors was determined, but the anaerobic biotransformation was not substantially different for most of the OMPs. However, it affected musk fragrances, which presented considerably lower biotransformation efficiency in the reactor fed with pretreated sludge. Therefore, TH was proved effective in partially removing OMPs but not in enhancing their bioavailability and subsequent anaerobic biotransformation.

Effect of different co-treatments of waste activated sludge on biogas production and shaping microbial community in subsequent anaerobic digestion

Different pretreatment approaches enhance hydrolysis and biogas production of waste activated sludge (WAS). However, the best WAS pretreatment method may vary for different purposes. The aim of this study was to compare different combined treatments using potassium ferrate, alkali, and ultrasonication treatment in terms of hydrolysis, biogas production, and microbial community structure during subsequent anaerobic digestion. The soluble chemical oxygen demand (SCOD) increased sharply during sludge pretreatment. Co-treatment with potassium ferrate and ultrasonication (PF + ULT) provided the highest concentration of SCOD (10,206 mg/L) and volatile fatty acids (VFAs) (9160 mg/L). Maximum cumulative methane production, 752.6 mL during 21 days of subsequent anaerobic digestion, was achieved for alkali and ultrasonication (ALK + ULT) co-treated sludge, a 39.3% improvement compared with raw sludge. Our results indicate that PF + ULT and ALK + ULT co-treatments are suitable for methanogenesis in subsequent anaerobic digestion. Co-treatment typically improves biogas production and sludge hydrolysis relative to a single pretreatment. Illumina sequencing of 16S rRNA genes showed that sludge subjected to ALK pretreatment and ALK + ULT co-treatment contained the same predominant populations, and the community structure of PF was similar to PF + ULT. Principal coordinate analysis (PCoA) based on weighted UniFrac distance indicated differences in the microbial communities of the pretreated and raw sludges. The predominant archaeal populations were Methanomassiliicoccus and Methanobacterium in the ALK + ULT and ALK pretreated sludges, and the majority were Methanomassiliicoccus (76.82%) in PF + ULT co-treated sludge, indicating that different pretreatment methods could substantially shape archaeal communities of pretreated sludge. These results reveal that sludge pretreatment not only impacts sludge hydrolysis and biogas production, but also significantly influences the microbial community in subsequent anaerobic digestion.

Phosphorus complexation of sewage sludge during thermal hydrolysis with different reaction temperature and reaction time by P K-edge XANES and 31P NMR

Thermal hydrolysis pretreatment (THP) can improve sludge disintegration and biogas production. Phosphorus (P) is immobilized on hydrochars of sewage sludge (SS). It is critical to understand changes in P speciation in SS hydrochars under different reaction temperatures and reaction times during THP for reclamation and (re)cycling of P. This study combined sequential extraction, 31P liquid and solid-state nuclear magnetic resonance (NMR), and P K-edge X-ray absorption near edge structure (XANES) spectroscopy to systematically analyze variation in P speciation and related metals in hydrochars. The temperature of the THP has more influence on P extraction content than reaction time; the bioavailability of P from hydrochars declined with increasing temperature. A 31P liquid and solid-state NMR analysis demonstrated that orthophosphate was the most abundant phosphate present small deviations in the chemical shift were observed in different samples under different conditions. The results of XANES demonstrated that CaP was more stable than FeP and AlP. LCF results suggested OcataCa proportion raised under stable CaP ratio in samples when the temperature was above 155 °C and duration was 60 min. This study provides a theoretical basis that can meet the practical application of THP for sludge disposal and P reclamation on subsequent anaerobic digestion.

Distribution and removal of antibiotic resistance genes during anaerobic sludge digestion with alkaline, thermal hydrolysis and ultrasonic pretreatments

Sewage sludge in the wastewater treatment plants contains considerable amount of antibiotic resistance genes (ARGs). A few studies have reported that anaerobic digestion (AD) could successfully remove some ARGs from sewage sludge, but information on the fate of ARGs in sludge pretreatment-AD process is still very limited. In this study, three sludge pretreatment methods, including alkaline, thermal hydrolysis and ultrasonic pretreatments, were compared to investigate the distribution and removal of ARGs in the sludge pretreatment-AD process. Results showed that the ARGs removal efficiency of AD itself was approximately 50.77%, and if these three sludge pretreatments were applied, the total ARGs removal efficiency of the whole pretreatment-AD process could be improved up to 52.50%–75.07%. The ultrasonic pretreatment was more efficient than alkaline and thermal hydrolysis pretreatments. Although thermal hydrolysis reduced ARGs obviously, the total ARGs rebounded considerably after inoculation and were only removed slightly in the subsequent AD process. Furthermore, it was found that the total ARGs concentration significantly correlated with the amount of 16S rRNA gene during the pretreatment and AD processes, and the bacteria carrying ARGs could be mainly affiliated with Proteobacteria.

Enhancement of biogas production from agricultural wastes via pre-treatment with advanced oxidation processes

The effects of advanced oxidation processes (AOPs) including Fenton, ozone, and ozone combined with Fe(II) and H2O2 on disintegration/solubilization of three mixed agricultural solid wastes and subsequent anaerobic digestion were studied. Hydroxyl radicals (̇OH) generated from AOPs enhanced the oxidation of COD and solubilization of particulate COD (>0.45 µm). Upon treatment, substrates lost 15.2 to 29.5% of their solids, 33.6 to 37.5% of total chemical oxygen demand, 16.5 to 25% of total biopolymers (humic acids and protein). The soluble matter content increased by 3–6 folds facilitating metabolization rate of the treated substrate resulting in a 23–30% increase in the cumulative methane production and 11.2–25% increase in % digestion efficiency (%ηAD). Semi-batch tests revealed that mixing fresh and pre-treated substrate using a 50:50 ratio improved both the specific methane production (1.7 folds) and volatile sold reduction (79%) with a positive effect on sludge dewaterability.

Enhanced biogasification from ammonia-rich swine manure pretreated by ammonia fermentation and air stripping

Anaerobic digestion (AD) is widely applied for efficient energy recovery from livestock manure. However, due to its high ammonia concentration and the resultant inhibition to methanogens, dry AD of livestock manure is difficult to achieve high efficiency of energy recovery. In this study, ammonia fermentation and air stripping were attempted as pretreatments to remove and recover ammonia, targeting enhanced dry AD of ammonia-rich swine manure. The effect of manure pH on ammonia stripping was specifically investigated under high solids condition and then on the subsequent dry AD. In order to remove and recover more nitrogen from manure, ammonia fermentation was conducted to release ammonia before air stripping. Results indicated that 90% and 85% of ammonia removal from the ammonia fermented manure (with ammonia nitrogen content of 17–18 g for per kilogram of total solids (TS)) could be obtained after being stripped at pH10.2 and pH8.8, respectively. Moreover, the biogas production from the subsequent dry AD of ammonia-stripped swine manure was about 1.6–1.7 times higher than that from the raw manure (27.5 ml for per gram of volatile solids (VS)). This study implies that ammonia fermentation followed by air stripping is a prospective solution for ammonia removal and recovery from livestock manure and then enhanced dry AD.

Improving Anaerobic Digestion of Sewage Sludge by Hydrogen Addition: Analysis of Microbial Populations and Process Performance

The effect of hydrogen pulse addition on digestion performance of sewage sludge was evaluated as a means for studying the increase in efficiency of methane production. Microbial communities were also evaluated to get an insight of the changes caused by the operational modifications of the digester. An energy evaluation of this alternative was performed considering the theoretical process of coupling bioelectrochemical systems (BES) for the treatment of wastewater along with hydrogen production and the subsequent anaerobic digestion. The addition of hydrogen to sewage sludge digestion resulted in an increase of 12% in biogas production over the control (1353 mL CH4 d−1 at an injection flow rate of 1938 mL H2 d−1). The liquid phase of the sludge reactor and the H2 supplemented one did not show significant differences, thus indicating that the application of hydrogen as the co-substrate was not detrimental. High-throughput sequencing analysis showed slight changes in archaeal relative abundance after hydrogen addition, whereas eubacterial community structure and composition revealed noteworthy shifts. The mass and energy balance indicated that the amount of hydrogen obtained from a hypothetical BES can be assimilated in the sludge digester, improving biogas production, but this configuration was not capable of covering all energy needs under the proposed scenario.