Comparison of thermophilic bacteria and alkyl polyglucose pretreatment on two-stage anaerobic digestion with waste sludge: Biogas production potential and substrate metabolism process

Effects of anaerobic digestion on the dewaterability of sonicated biosludge

Studies on the impact of anaerobic digestion on biosludge dewatering are conflicting and mechanisms are poorly understood, with significant implications to both wastewater costs and environmental impact. To identify the mechanisms that improve dewaterability and to amplify the impacts of anaerobic digestion, biosludge was first sonicated to deteriorate dewaterability. Sonicated biosludge was next anaerobically digested under mesophilic conditions in three experiments, and the roles of extracellular polymeric substances (EPS) and supracolloidal particles on dewaterability were explored. Dewaterability, as measured by capillary suction time, improved with anaerobic digestion from 84.7 ± 11.3 s-L/g to 31.6 ± 3.2 s-L/g. The loosely-bound fraction of EPS saw only a minor decrease from 6.4 % ± 1.1 %-1.2 % ± 0.3 % and total EPS protein did not change significantly. On the other hand, in a separate experiment, significant removal of supracolloidal particles under 10 μm was observed within 6 days of anaerobic digestion, coinciding with an improvement in a capillary suction time from 116.0 ± 6.7 s-L/g to 69.8 ± 1.1 s-L/g and Crown Press filtrate total solids from 13.7 ± 0.2 g/L to 8.4 ± 0.5 g/L. In addition, in a third experiment, raw biosludge as well as sonicated at five different intensities, saw significantly faster capillary suction time and decreased Crown Press filtrate total solids which coincided with a decrease in the volume % of particles under 16 μm. The results suggest the net removal of small supracolloidal particles is a mechanism that occurs during anaerobic digestion to improve biosludge dewaterability.

Effect of hydraulic retention time on denitrification performance and microbial communities of solid-phase denitrifying reactors using polycaprolactone/corncob composite

This study investigated the effect of hydraulic retention time (HRT) on the denitrification performance and microbial composition of reactors, packed with composite polycaprolactone and corncob carbon sources, during the treatment mariculture wastewater. The optimal HRT was 3 h, and average nitrogen removal efficiency was 99.00 %, 99.07 %, and 98.98 % in the HRT =3, 5, and 7 h groups, respectively. However, the 3 h group (DOC 2.91 mg/L) was the only group with a lower DOC concentration than that of the influent group (3.31 mg/L). Moreover, species richness was lower at HRT =3 h, with a greater proportion of denitrification-dominant phyla, such as Proteobacteria. The abundance of the NarG, NirK, and NirS functional genes suggested that the HRT =3 h group had a significant advantage in the nitrate and nitrite reduction phases. Under a short HRT, the composite carbon source achieved a good denitrification effect.

Cation exchange resins enhance anaerobic digestion of sewage sludge: Roles in sequential recovery of hydrogen and methane

The recovery of renewable bioenergy from anaerobic digestion (AD) of sludge is a promising method to alleviate the energy problem. Although methane can be effectively recovered through sludge pretreatment by cation exchange resin (CER), the simultaneous enhancement of hydrogen and methane generation from AD using CER has not been extensively investigated. Herein, the effect of CER on the sequential recovery of hydrogen and methane and the corresponding mechanisms were investigated. When CER is introduced, the maximum increases for the hydrogen and methane production are 104.7 % and 35.3 %, respectively, confirming the sequential enhancement effects of CER on the hydrogen and methane production. Analyses of the variations in the main biochemical components with and without the effect of CER demonstrate that CER promotes sludge organic solubilisation, hydrolysis, and acidification in both hydrogen- and methane-production stages. Moreover, investigations of variations in the solid-liquid interfacial thermodynamics and removal rates of main multivalent metals of sludge reveal that the ion exchange reactions between the CER and sludge in the hydrogen-production stage provide the direct driving force of effective contact between bacteria and organic particulates. Additionally, the residual effect of the CER during methane production reduces the energy barrier for mass transfer and provides a driving force for this transfer. Further analyses of the microbial community structure and metagenomics indicate that CER directly drives the enrichment of hydrogen-producing bacteria (+ 15.1 %) and key genes encoding enzymes in the hydrogen-production stage. Moreover, CER indirectly induces the enrichment of methane-producing anaerobes (e.g. Methanosaeta: + 16.7 %, Methanosarcina: + 316.5 %); enhances the bioconversion of different substrates into methyl-coenzyme M; and promotes the metabolism pathway of acetoclastic process and CO2 reduction in the methane-production stage. This study can provide valuable insights for simultaneously enhancing the production of hydrogen and methane from AD through sequential recovery.

Characterizing molecular transformation of dissolved organic matter during high-solid anaerobic digestion of dewatered sludge using ESI FT-ICR MS

In this study, the effects of anaerobic digestion (AD) on molecular characteristics of dissolved organic matter (DOM) in the dewatered sludge has been described by advanced electrospray ionization combined with Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) technology. With the progress of AD, molecular amounts in DOM samples increased with the lowering in the carbon atom number of average molecular formula and average double bond equivalent (DBE). CHON and CHONS groups are the two main organic substances in sludge with their relative DOM proportions of 29.64% and 32.56%, respectively. The resistants (i.e., refractory organic matter) mainly consist of the proteins regions of CHO groups as well as the proteins/lignin regions of CHON groups. The contrasting temporal trends in protein contents (e.g., decrease (CHO and CHON) vs. increase (CHONS)) may imply differences in their degradation characteristics. Likewise, the multi-N (N₃, N₄) and S₂ organic groups in the sludge are converted to N₂ and S₁ molecules, while the relative abundance of O atoms (in Ox molecules) tends to increase. In addition, the resistants in sludge DOM contain high oxidizing C and low unsaturation. The overall results of this research are expected to provide the theoretical basis for further optimization of the sludge AD process.

Metagenomic insights into responses of microbial population and key functional genes to fulvic acid during partial nitritation

The long-term impact of fulvic acid (FA) on partial nitritation (PN) system was initially examined in this study. The obtained results revealed that the FA lower than 50 mg/L had negligible effect on the nitrite accumulation rate (NAR nearly 100%) and ammonium removal rate (ARR 56.85%), while FA over 50 mg/L decreased ARR from 56.85% to 0.7%. Sludge characteristics analysis found that appropriate FA (<50 mg/L) exposure promoted the settling performance and granulation of PN sludge by removing Bacteroidetes and accumulating Chloroflexi. The analysis of metagenomics suggested that the presence of limited FA (0-50 mg/L) stimulated the generation of NADH, which favors the denitrification and nitrite reduction. The negative impact of FA on the PN system could be divided into two stages. Initially, limited FA (50-120 mg/L) was decomposed by Anaerolineae to stimulate the growth and propagation of heterotrophic bacteria (Thauera). Increasing heterotrophs competed with AOB (Nitrosomonas) for dissolved oxygen, causing AOB to be eliminated and ARR to declined. Subsequently, when FA dosage was over 120 mg/L, Anaerolineae were inhibited and heterotrophic bacteria reduced, resulting in the abundance of AOB recovered. Nevertheless, the ammonium transformation pathway was suppressed because genes amoABC and hao were obviously reduced, leading to the deterioration of reactor performance. Overall, these results provide theoretical guidance for the practical application of PN for the treatment of FA-containing sewage.

Enhancing two-phase anaerobic digestion of mixture of primary and secondary sludge by adding granular activated carbon (GAC): Evaluating acidogenic and methanogenic efficiency

Although the granular activated carbon (GAC) has been proved to enhance conventional single-phase anaerobic digestion (AD), how it impacts on acidogenic and methanogenic fermentation is still unknown. In this study, GAC was introduced to elevate the efficiency of two-phase AD, with mixture of primary and secondary sludge as substrate. Five dosages: 0, 0.1, 0.3, 0.5 and 0.7 g GAC/g TSS (Total Suspended Solids) were investigated to determine influences of GAC. The variations of biogas (hydrogen and methane), volatile fatty acids (VFAs), organics degradation and transformation in extracellular polymeric substances (EPS) and dissolved organic matters (DOM) were analyzed. Modified Gompertz model and first-order reaction equation was applied to analyze the kinetics of biogas yield and VFAs utilization, respectively. Sludge reduction, electrical conductance and pH were also quantified to evaluate the system performance. The results showed that GAC could improve two-phase AD performance by enhancing methane production and organics conversion.

Effect of mixed primary and secondary sludge for two-stage anaerobic digestion (AD)

As an energy-efficient and eco-friendly sludge treatment process, two-stage anaerobic digestion (AD) is widely employed to recovery biomass energy from waste sludge. However, the effect of primary and secondary sludge for two-stage AD was not clear. In this study, two-stage AD of mixed sludge in different volume ratio was investigated. The maximum cumulative H₂ yield (100.5 ml) and CH₄ yield (2643.6 ml) were obtained in volume ratio of 1:3 (primary sludge: secondary sludge). In two-phase AD, mixed sludge could induce positive effect on both organics releasing in extracellular polymeric substances (EPS) and the utilization of volatile fatty acids (VFAs). By investigating the compositional characteristics of dissolved organic matters (DOM) through excitation-emission matrix (EEM) coupling with fluorescence regional integration (FRI), it revealed more degradable substances utilization in mixture of sludge. Results from this work suggest that two-phase AD with mixed sludge is efficient for renewable energy recovery.

Thermophilic bacteria combined with alkyl polyglucose pretreated mariculture solid wastes using as denitrification carbon source for marine recirculating aquaculture wastewater treatment

In marine recirculating aquaculture systems (RAS), efficient nitrogen removal is challenging due to the high NO₃^--N concentration, low organic matters content, and high salinity. In this study, mariculture solid wastes (MSW) acidogenic liquid pretreated by thermophilic bacteria (TB) combined with alkyl polyglucose (APG) was first used as carbon source for denitrification to remove NO₃^--N. TB + APG pretreatment could accelerate the hydrolysis of MSW, and the highest volatile fatty acids (VFAs) yield (40.3%) was obtained with TB + 0.2 g/g VSS APG pretreatment. MSW acidogenic liquid pretreated by TB + 0.2 g/g VSS APG was a reliable carbon source for denitrification, and the optimum COD/NO₃^--N ratio (C/N) was 8 with no residue of NO_x^--N. VFAs were more effectively utilized by denitrifiers than carbohydrate and protein. The high denitrification potential (P_DN) and denitrification rate (V_DN) indicated the higher denitrification ability at C/N of 8 using MSW acidogenic liquid as carbon source. The outcomes of this work could provide useful information for promoting technological innovation in marine RAS wastewater treatment.

Bacterial-algal coupling system for high strength mariculture wastewater treatment: Effect of temperature on nutrient recovery and microalgae cultivation

In the present study, bacterial-algal coupling system, an integration process of acidogenic fermentation and microalgae cultivation was used for high strength mariculture wastewater (HSMW) treatment, resource recovery and low-cost biomass production. The effect of temperature on Chlorella vulgaris (C. vulgaris) cultivation was investigated with culture medium of acidogenic liquid. The results showed that acidogenic liquid could be used as culture medium for C. vulgaris and higher biomass was obtained compared to control. The acidogenic liquid obtained at initial pH of 8 was the most suitable culture medium for C. vulgaris growth due to befitting C/N and considerable volatile fatty acids. Moreover, the optimum temperature for C. vulgaris cultivation was 25 °C and the removal efficiency of chemical oxygen demand (COD) and NH₄⁺-N from acidogenic liquid could reach 94.4% and 68.8%, respectively. The outcome could create an innovative value chain with environmental sustainability and economic feasibility in aquaculture industry.

Impact of storage duration and micro-aerobic conditions on lactic acid production from food waste

Food waste (FW) is an abundant resource with great potential for lactic acid (LA) production. In the present study, the effect of storage time on FW characteristics and its potential for LA production was investigated. The largest part of sugars was consumed during 7 to 15 days of FW storage and the sugar consumption reached 68.0% after 15 days. To enhance the LA production, micro-aerobic conditions (13 mL air/g VS) and addition of β-glucosidase were applied to improve polysaccharides hydrolysis, resulting to increase of monosaccharides content to 76.6%. Regarding fermentative LA production, the highest LA titer and yield of hydrolyzed FW was 32.1 ± 0.5 g/L and 0.76 ± 0.01 g/g_-sugar, respectively. Furthermore, L-LA isomer was higher than 70% when FW was stored for up to 7 days. However, attention should be paid on controlling the FW storage to approximately one week.

Integrating acidogenic fermentation and microalgae cultivation of bacterial-algal coupling system for mariculture wastewater treatment

In this study, Bacterial-Algal Coupling System, a method integrated acidogenic fermentation (AF) and microalgae cultivation, was applied to the mariculture wastewater (MW) treatment. The MW was acidogenic fermented at different initial pH (4.0-10.0), and different dilution rate (5%-20%) of AF effluent was used for Chlorella vulgaris cultivation. The results showed that the maximum biomass production (5.6 g/L) of microalgae was obtained with 10% AF effluent. Ammonium, phosphate and volatile fatty acids could be metabolized by microalgae. More specifically, acetic acid and propionic acid were utilized prior to butyric acid and valeric acid. To better understand the synergy of heterotrophic metabolism and photosynthesis, the activities of Rubisco and citrate synthase were revealed to provide additional insight for nutrients recovery from MW by mixotrophic cultivation of microalgae.

Alkaline aided thermophiles pretreatment of waste activated sludge to increase short chain fatty acids production: Microbial community evolution by alkaline on hydrolysis and fermentation

Adding alkaline into an anaerobic waste activated sludge (WAS) fermentation with thermophilic bacteria pretreatment could efficiently improve short-chain fatty acids (SCFAs) accumulation to 3550 ± 120 mg COD/L. The acidification rate in combined test was 21.2%, while that was 15.6% and 10.7% in sole thermophilic bacteria pretreatment and control tests respectively. Four distinct groups of microbes could be identified with noticeable shifts using the combined pretreatments, and tremendous effects were analyzed on organic content especially of the soluble proteins and SCFAs concentrations. Particularly, alkaline addition would significantly change the functional microbial structures, including the decrease of Caloramator with the function of thermophilic proteolytic and the increase of Acidobacteria TM7 and Petrimonas sp. The results above suggested that alkaline addition could decrease the hydrolytic substances consume by thermotolerance bacteria and final improve SCFAs accumulation in fermentation process.

Heterotrophic denitrification strategy for marine recirculating aquaculture wastewater treatment using mariculture solid wastes fermentation liquid as carbon source: Optimization of COD/NO3--N ratio and hydraulic retention time

Heterotrophic denitrification using mariculture solid wastes (MSW) fermentation liquid as carbon source is an economically and environmentally sustainable strategy for NO₃^--N removal in marine recycling aquaculture systems (RAS). The optimization of COD/NO₃^--N ratio (C/N) and hydraulic retention times (HRT) with respect to MSW fermentation liquid driven denitrification for marine RAS wastewater treatment was investigated. The optimum C/N of 8 and HRT of 6 h for heterotrophic denitrification was obtained with NO₃^--N removal efficiency of 97.8% and 94.2%, respectively. Using MSW fermentation liquid as carbon source, the utilization of VFAs was more effective than that of carbohydrates and proteins, and effluent COD concentration decreased with an increment in HRT from 4 to 8 h. The results of high-throughput sequencing analysis showed microbial communities were enriched selectively in the reactors by optimizing C/N and HRT, which obviously enhanced the nitrogen removal in respect to MSW fermentation liquid driven denitrification.

Effect of alkyl polyglycosides on the performance of thermophilic bacteria pretreatment for saline waste sludge hydrolysis

In this study, alkyl polyglycosides (APG) was used to further accelerate the hydrolysis of saline waste sludge with thermophilic bacteria (TB) pretreatment. In the presence of 0.4 g/g TSS APG, the concentrations of soluble chemical oxygen demand (SCOD), soluble carbohydrate and soluble protein in dissolved organic matters (DOM) were 0.4, 2.4 and 1.3 times of that without APG addition, respectively. Excitation emission matrix (EEM) fluorescence spectroscopy revealed that the addition of APG led to the increase of soluble microbial materials and the decrease of fulvic acid-like substances in DOM, which was beneficial for the subsequent process of anaerobic digestion. Using APG promoted the releasing of enzymes trapped in saline waste sludge and improved the activity of enzymes during hydrolysis. The activities of α-glucosidase and protease increased by 8.8% and 21.3% respectively in the presence of 0.4 g/g TSS APG comparing no APG addition.

Advances in enhanced volatile fatty acid production from anaerobic fermentation of waste activated sludge

Low acid production and acid-forming process instability are becoming the major issues to limit the popularization of anaerobic fermentation to produce volatile fatty acid. Considerable research efforts have been made to address these problems, from studying the microorganisms that are primarily responsible for or detrimental to this process, to determining their biochemical pathways and developing mathematical models that facilitate better prediction of process performance to identify the mechanism and optimization of process control. A limited understanding of the complex microbiology and biochemistry of anaerobic fermentation is the primary cause of acid production upset or failure. This review critically assesses the recent advances in enhanced volatile fatty acid production from anaerobic fermentation of waste activated sludge from micro to macro scale, particularly relating to the microbiology, biochemistry, impact factors, and enhancement methods. Previous results suggest that further studies are necessary to substantially promote the efficiency and stability of acid production. One of the promising directions appears to be integrating the existing and growing pretreatment technologies and fermentation processes to enhance metabolic pathways of acetogens but inhibit activities of methanogens, which this study hopes to partially achieve.

Accelerating waste sludge hydrolysis with alkyl polyglucose pretreatment coupled with biological process of thermophilic bacteria: Hydrolytic enzyme activity and organic matters transformation

A novel pretreatment method combining thermophilic bacteria (TB) with alkyl polyglucose (APG) was employed to pretreat waste sludge for enhancing the sludge hydrolysis. TB combined with APG pretreatment was effective in the releasing of soluble chemical oxygen demand (SCOD), protein and carbohydrate in extracellular polymeric substances (EPS) when the dosage of APG was below 0.1 g/g TSS. The enhancement of SCOD, carbohydrates and protein in dissolved organic matter (DOM) was promoted by the synthetic effect of APG and TB, which provides more carbon and energy source to the subsequent biochemical processes in sludge digestion. Excitation-emission matrix (EEM) fluorescence spectroscopy revealed that the combined pretreatment was beneficial for the decrease of non-biodegradable materials and the increase of biodegradable materials in DOM, resulting in the enhancement of the biodegradation of waste sludge. The combined use of TB and 0.4 g/g TSS APG achieved the maximal activities of protease (1.8) and α-glucosidase (1.9), and the activities of protease and α-glucosidase were positively correlated to the dosage of APG. The combined pretreatment was advantageous for the sludge reduction and sludge stabilization.

Enhancing the hydrolysis of saline waste sludge with thermophilic bacteria pretreatment: New insights through the evolution of extracellular polymeric substances and dissolved organic matters transformation

Recently, the treatment and utilization of saline waste sludge has drawn growing attention because large amounts of saline waste sludge were generated with the increase of saline wastewater discharge. In this study, thermophilic bacteria (TB) pretreatment was applied to accelerate the hydrolysis of saline waste sludge and the efficiency of hydrolysis at different salinities was evaluated. Compared with the group without salinity, the releasing of carbohydrate (up to a 67.0% decrease) in extracellular polymeric substances (EPS) was inhibited at the salinity ranging from 1.0% to 2.5%, and the releasing of protein (up to a 17.6% decrease) was inhibited under salinity conditions. Excess salinity (4.0%) caused the cell lysis, and the content of soluble chemical oxygen demand (SCOD), soluble carbohydrate and protein in dissolved organic matter (DOM) increased by 44.9%, 38.8% and 20.8% than that obtained without salinity, respectively. According to the excitation-emission matrix (EEM) fluorescence spectroscopy, the biodegradability of sludge was improved at 2.0% salinity. At 2.0% salinity, the maximum fluorescence intensity of soluble microbial byproduct substances (76,358.9 (au)) and the minimum fluorescence intensity of humic acid-like substances (173,424 (au)) were obtained. The increased salinity was beneficial for the sludge stabilization and was disadvantageous for the sludge reduction.

A Review of Sludge-to-Energy Recovery Methods

The increasing volume of sewage sludge from wastewater treatment facilities is becoming a prominent concern globally. The disposal of this sludge is particularly challenging and poses severe environmental hazards due to the high content of organic, toxic and heavy metal pollutants among its constituents. This study presents a simple review of four sewage to energy recovery routes (anaerobic digestion, combustion, pyrolysis and gasification) with emphasis on recent developments in research, as well as benefits and limitations of the technology for ensuring cost and environmentally viable sewage to energy pathway. This study focusses on the review of various commercially viable sludge conversion processes and technologies used for energy recovery from sewage sludge. This was done via in-depth process descriptions gathered from literatures and simplified schematic depiction of such energy recovery processes when utilised for sludge. Specifically, the impact of fuel properties and its effect on the recovery process were discussed to indicate the current challenges and recent scientific research undertaken to resolve these challenges and improve the operational, environmental and cost competitiveness of these technologies.