Stabilization of sewage sludge in the presence of nanoscale zero-valent iron (nZVI): abatement of odor and improvement of biogas production

The potential benefits of nanoscale zero-valent iron (nZVI) on sludge stabilization, either the abatement of odor or the improvement of biogas production, were investigated in this study. Two commercial-grade microscale iron powders were also utilized for comparison. Adding 0.10 wt% of nZVI in sludge during anaerobic incubation significantly reduced the concentration of H₂S in biogas by 98.0 % (96.2–98.9 %), probably attributed by reactions between sulfides and the neo-formed hydrous Fe(II)/Fe(III) oxides layer at the surface of ZVI nanoparticles. Meanwhile, the percentage of P in bioavailable fractions decreased from 76.8 to 52.5 %, possibly due to the formation of vivianite [Fe₃(PO₄)₂]. Furthermore, 0.10 wt% of nZVI in anaerobic digestion for 17 days enhanced the concentration of CH₄ in biogas by 5.1–13.2 % and improved the production of biogas and methane by 30.4 and 40.4 %, respectively. The amendment of iron nanoparticles during anaerobic digestion can not only effectively reduce H₂S in biogas, but also potentially boost methane production significantly.     

Co-digestion of grease trap sludge and sewage sludge

Redirection of organic waste, from landfilling or incineration, to biological treatment such as anaerobic digestion is of current interest in the Malmö-Copenhagen region. One type of waste that is expected to be suitable for anaerobic digestion is sludge from grease traps. Separate anaerobic digestion of this waste type and co-digestion with sewage sludge were evaluated. The methane potential was measured in batch laboratory tests, and the methane yield was determined in continuous pilot-scale digestion. Co-digestion of sludge from grease traps and sewage sludge was successfully performed both in laboratory batch and continuous pilot-scale digestion tests. The addition of grease trap sludge to sewage sludge digesters was seen to increase the methane yield of 9–27% when 10–30% of sludge from grease traps (on VS-basis) was added. It was also seen that the grease trap sludge increases the methane yield without increasing the sludge production. Single-substrate digestion of grease trap sludge gave high methane potentials in batch tests, but could not reach stable methane production in continuous digestion.     

Anaerobic co-digestion of the organic fraction of municipal solid waste with FOG waste from a sewage treatment plant: Recovering a wasted methane potential and enhancing the biogas yield

Anaerobic digestion is applied widely to treat the source collected organic fraction of municipal solid wastes (SC-OFMSW). Lipid-rich wastes are a valuable substrate for anaerobic digestion due to their high theoretical methane potential. Nevertheless, although fat, oil and grease waste from sewage treatment plants (STP-FOGW) are commonly disposed of in landfill, European legislation is aimed at encouraging more effective forms of treatment. Co-digestion of the above wastes may enhance valorisation of STP-FOGW and lead to a higher biogas yield throughout the anaerobic digestion process. In the present study, STP-FOGW was evaluated as a co-substrate in wet anaerobic digestion of SC-OFMSW under mesophilic conditions (37 °C). Batch experiments carried out at different co-digestion ratios showed an improvement in methane production related to STP-FOGW addition. A 1:7 (VS/VS) STP-FOGW:SC-OFMSW feed ratio was selected for use in performing further lab-scale studies in a 5 L continuous reactor. Biogas yield increased from 0.38 ± 0.02 L g VS_feed^?1 to 0.55 ± 0.05 L g VS_feed^?1 as a result of adding STP-FOGW to reactor feed. Both VS reduction values and biogas methane content were maintained and inhibition produced by long chain fatty acid (LCFA) accumulation was not observed. Recovery of a currently wasted methane potential from STP-FOGW was achieved in a co-digestion process with SC-OFMSW.     

BMP tests of source selected OFMSW to evaluate anaerobic codigestion with sewage sludge

The aim of this study is to characterize different types of source selected organic fraction of municipal solid waste (SS-OFMSW) in order to optimize the upgrade of a sewage sludge anaerobic digestion unit by codigestion. Various SS-OFMSW samples were collected from canteens, supermarkets, restaurants, households, fruit–vegetable markets and bakery shops. The substrates characterization was carried out getting traditional chemical–physical parameters, performing elemental analysis and measuring fundamental anaerobic digestion macromolecular compounds such as carbohydrates, proteins, lipids and volatile fatty acids. Biochemical methane potential (BMP) tests were conducted at mesophilic temperature both on single substrates and in codigestion regime with different substrates mixing ratios. The maximum methane yield was observed for restaurant (675 NmlCH₄/gVS) and canteens organic wastes (571 and 645 NmlCH₄/gVS). The best codigestion BMP test has highlighted an increase of 47% in methane production respect sewage sludge digestion.     

BMP estimation of landfilled municipal solid waste by multivariate statistical methods using specific waste parameters: case study of a sanitary landfill in Turkey

The main objective of this study was to determine whether methane potential of waste could be estimated more easily by a limited number of waste characterization variables. 36 samples were collected from 12 locations and 3 waste depths in order to represent almost all waste ages at the landfill. Actual remaining methane potential of all samples was determined by the biochemical methane potential (BMP) tests. The cumulative methane production of closed landfill (cLF) samples reached 75–125 mL at the end of experiment duration, while the samples from active landfill (aLF) produced in average 216–266 mL methane. The average experimental k and L ₀ values of cLF and aLF were determined by non-linear regression using BMP data with first-order kinetic equation as 0.0269 day^?1–30.38 mL/g dry MSW and 0.0125 day^?1–102.1 mL/g dry MSW, respectively. The principal component analysis (PCA) was applied to analyze the results for cLF and aLF along with BMP results. Three PCs for the data set were extracted explaining 72.34 % variability. The best MLR model for BMP prediction was determined for seven variables (pH–Cl–TKN–NH4–TOC–LOI–Ca). R ² and Adj. R ² values of this best model were determined as 80.4 and 75.3 %, respectively.     

High-solids anaerobic co-digestion of sewage sludge and food waste in comparison with mono digestions: Stability and performance

System stability and performance of high-solids anaerobic co-digestion of dewatered sludge (DS) and food waste (FW) in comparison with mono digestions were investigated. System stability was improved in co-digestion systems with co-substrate acting as a diluting agent to toxic chemicals like ammonia or Na⁺. For high-solids digestion of DS, the addition of FW not only improved system stability but also greatly enhanced volumetric biogas production. For high-solids digestion of FW, the addition of DS could reduce Na⁺ concentration and help maintain satisfactory stability during the conversion of FW into biogas. System performances of co-digestion systems were mainly determined by the mixing ratios of DS and FW. Biogas production and volatile solids (VSs) reduction in digestion of the co-mixture of DS and FW increased linearly with higher ratios of FW. A kinetic model, which aimed to forecast the performance of co-digestion and to assist reactor design, was developed from long-term semi-continuous experiments. Maximum VS reduction for DS and FW was estimated to be 44.3% and 90.3%, respectively, and first order constant k was found to be 0.17 d^?1 and 0.50 d^?1, respectively. Experimental data of co-digestion were in good conformity to the predictions of the model.     

Investigations on enhanced in situ bioxidation of methane from landfill gas (LFG) in a lab-scale model

The performance of an exogenous bacterium, Methylobacterium extorquens, in inducing bioxidation of methane from landfill gas (LFG) was assessed in a laboratory scale bioreactor. The study show that enhanced oxidation of methane is attained when the bacteria are introduced into the landfill soil. The maximum percentage reduction of methane fraction from LFG when the bioreactor was inoculated with the methanotrophic bacteria was 94.24 % in aerobic treatment process and 99.97 % in anaerobic process. In the experiments with only the indigenous microorganisms present in the landfill soil, the maximum percentage reduction of methane for the same flow rate of LFG was 59.67 % in aerobic treatment and 45 % in anaerobic treatment. The methane oxidation efficiency of this exogenous methanotrophic bacterium can be considered to be the optimum in anaerobic condition and at a flow rate of 0.6 L/m²/min when the removal percentage is 99.95 %. The results substantiate the use of exogenous microorganisms as potential remediation agents of methane in LFG.     

Effect of long chain fatty acids removal as a pretreatment on the anaerobic digestion of food waste

This study investigated the effect of long chain fatty acids (LCFAs) removal as a pretreatment prior to anaerobic digestion on the production of methane from food waste. The results showed that the anaerobic digestion of food waste containing 1.6 g COD/L of LCFAs was not inhibited (4 days lag-time, 78.3 % methane recovery in 35 days) compared to that of lipid free food waste (3 days lag time, 72.5 % methane recovery in 35 days); however, some unsaturated LCFAs, which are toxic to microorganism, were accumulated in the batch anaerobic digestion reactor. Meanwhile, in a methanogenic activity study, the activity of methanogens was observed to be linearly inhibited by the presence of more than 1 g COD/L of LCFAs. The possibility of the accumulation of unsaturated LCFAs in the reactor should be considered when operating a large-scale continuous system.     

Anaerobic co-digestion of tannery waste water and tannery solid waste using two-stage anaerobic sequencing batch reactor: focus on performances of methanogenic step

In this study, anaerobic co-digestion of the tannery waste water (TWW) and tannery solid waste (TSW) with four TWW to TSW mixing ratios (100:0, 75:25, 50:50 and 25:75) was carried out using semi-continuous two-phase anaerobic sequencing batch reactor system under mesophilic temperature (38?±?2 °C). During the experimental study, effluents resulted from previously optimized acidogenic reactors were used to feed subsequent methanogenic reactors and then operated at hydraulic retention time (HRT) of 20, 15 and 10 days and equivalent organic loading rate. The findings revealed that methanogenic reactor of 50:50 (TWW:TSW) treating the effluent from previously optimized acidogenic step exhibits best process performances in terms of daily biogas (415 ml/day), methane production (251 ml/day), methane content (60.5%) and COD removal efficiency (75%) when operated at HRT of 20 days. Process stability of methanogenic step also evaluated and the obtained results showed suitable pH (6.8), no VFA accumulation, i.e., VFA/Alkalinity (0.305), alkalinity (3210 mgCaCO₃/l) and ammonia (246 mg/l with in optimum operating range). In general, improved process stability as well as performance was achieved during anaerobic co-digestion of TWW with TSW compared to mono-digestion of TWW.     

污泥热水解技术研究进展

热水解可以改善污泥的生物降解性能和脱水性能,提高后续厌氧消化系统中有机物的去除率。本文阐述了热水解对污泥物理特性主要包括流变特性和脱水行为的影响;分析了热水解对污泥中氨氮浓度及对厌氧消化的作用;同时介绍了热水解技术对整个污泥处理系统能量需求的影响。指出：热水解改善了污泥的厌氧消化性能,提高了沼气等再生资源的产量,且厌氧消化产生的沼气量与热水解技术所需的能量基本平衡。     

Dry anaerobic digestion in batch mode: Design and operation of a laboratory-scale,completely mixed reactor

A laboratory-scale (40 l) reactor was designed to investigate dry anaerobic digestion. The reactor is equipped with an intermittent paddle mixer, enabling complete mixing in the reactor. Three consecutive batch dry digestion tests of municipal solid waste were performed under mesophilic conditions and compared to operation results obtained on a pilot-scale (21 m³) with the same feedstock. Biogas and methane production at the end of the tests were similar (around 200 m³ CH₄STP/tVS), and the dynamics of methane production and VFA accumulation concurred. However, the maximal levels of VFA transitory accumulation varied between reactors and between runs in a same reactor. Ammonia levels were similar in both reactors. These results show that the new reactor accurately imitates the conditions found in larger ones. Adaptation of micro-organisms to the waste and operating conditions was also pointed out along the consecutive batches.     

Batchwise mesophilic anaerobic co-digestion of secondary sludge from pulp and paper industry and municipal sewage sludge

Residues from forest-industry wastewater-treatment systems are treated as waste at many pulp and paper mills. These organic substances have previously been shown to have potential for production of large quantities of biogas. There is concern, however, that the process would require expensive equipment because of the slow degradation of these substances. Pure non-fibrous sludge from forest industry showed lower specific methane production during mesophilic digestion for 19 days, 53 ± 26 Nml/g of volatile solids as compared to municipal sewage sludge, 84 ± 24 Nml/g of volatile solids. This paper explores the possibility of using anaerobic co-digestion with municipal sewage sludge to enhance the potential of methane production from secondary sludge from a pulp and paper mill. It was seen in a batch anaerobic-digestion operation of 19 days that the specific methane production remained largely the same for municipal sewage sludge when up to 50% of the volatile solids were replaced with forest-industry secondary sludge. It was also shown that the solid residue from anaerobic digestion of the forest-industry sludge should be of suitable quality to use for improving soil quality on lands that are not used for food production.     

Pilot-scale anaerobic co-digestion of municipal biomass waste and waste activated sludge in China: Effect of organic loading rate

The effects of organic loading rate on the performance and stability of anaerobic co-digestion of municipal biomass waste (MBW) and waste activated sludge (WAS) were investigated on a pilot-scale reactor. The results showed that stable operation was achieved with organic loading rates (OLR) of 1.2–8.0 kg volatile solid (VS) (m³ d)^?1, with VS reduction rates of 61.7–69.9%, and volumetric biogas production of 0.89–5.28 m³ (m³ d)^?1. A maximum methane production rate of 2.94 m³ (m³ d)^?1 was achieved at OLR of 8.0 kg VS (m³ d)^?1 and hydraulic retention time of 15 days. With increasing OLRs, the anaerobic reactor showed a decrease in VS removal rate, average pH value and methane concentration, and a increase of volatile fatty acid concentration. By monitoring the biogas production rate (BPR), the anaerobic digestion system has a higher acidification risk under an OLR of 8.0 kg VS (m³ d)^?1. This result remarks the possibility of relating bioreactor performance with BPR in order to better understand and monitor anaerobic digestion process.     

Anaerobic digestion potential of urban organic waste: a case study in Malm?.

A study of existing organic waste types in Malm?, Sweden was performed. The purpose was to gather information about organic waste types in the city to be able to estimate the potential for anaerobic treatment in existing digesters at the wastewater treatment plan (WWTP). The urban organic waste types that could have a significant potential for anaerobic digestion amount to about 50 000 tonnes year(-1) (sludge excluded). Some of the waste types were further evaluated by methane potential tests and continuous pilot-scale digestion. Single-substrate digestion and co-digestion of pre-treated, source-sorted organic fraction of municipal solid waste, wastewater sludge, sludge from grease traps and fruit and vegetable waste were carried out. The experiments showed that codigestion of grease sludge and WWTP sludge was a better way of making use of the methane potential in the grease trap sludge than single-substrate digestion. Another way of increasing the methane production in sludge digesters is to add source-sorted organic fraction of municipal solid waste (SSOFMSW). Adding SSOFMSW (20% of the total volatile solids) gave a 10-15% higher yield than could be expected by comparison with separate digestion of sludge respective SSOFMSW. Co-digestion of sludge and organic waste is beneficial not just for increasing gas production but also for stabilizing the digestion process. This was seen when co-digesting fruit and vegetable waste and sludge. When co-digested with sludge, this waste gave a better result than the separate digestion of fruit and vegetable waste. Considering single-substrate digestion, SSOFMSW is the only waste in the study which makes up a sufficient quantity to be suitable as the base substrate in a full-scale digester that is separated from the sludge digestion. The two types of SSOFMSW tested in the pilot-scale digestion were operated successfully at mesophilic temperature. By adding SSOFMSW, grease trap sludge and fruit and vegetables waste to sludge digesters at the wastewater treatment plant, the yearly energy production from methane could be expected to increase from 24 to 43 GWh.     

The impact of pretreatment and inoculum to substrate ratio on methane potential of organic wastes from various origins

Biochemical methane potentials (BMP) of two different substrates from macroalgae (MA) and market place wastes (MPW) were investigated using anaerobic granulated sludge from food industry with different ratios of substrate to inoculum (S/X). The substrates were used as MA only, MPW only, MA–MPW mixture, pretreated MA, and pretreated MA–MPW mixture. Research involved investigation of the effects of parameters such as temperature (35, 45, and 55 °C), substrate to inoculum ratio (S/X = 0.5, 2.0, 4.0, and 6.0 as g VS_substrate/g VS_inoculum), and the type of pretreatment (by microwave, thermal, and ultrasonic) on BMP. BMP assays were performed for all substrates. The highest cumulative biogas production (and BMP) were obtained for MA only at an S/X ratio of 4.0 g VS/g VS as 357 L_biogas/kg VS (197 L CH₄/kg VS) and 33 L_biogas/kg VS (17 L CH₄/kg VS), respectively, at 35 and 55 °C. For pretreated substrates, the highest cumulative biogas production and BMP were observed as 287 L_biogas/kg VS and 146 L CH₄/kg VS using pretreated macroalgae at 35 °C. Results suggested that MA only and MA–MPW mixtures are suitable substrates for biogas production. It is also concluded that any type of pretreatment has adverse effects on biogas and methane productions.     

Performance evaluation of a completely stirred anaerobic reactor treating pig manure at a low range of mesophilic conditions

Many Chinese biogas plants run in the lower range of mesophilic conditions. This study evaluated the performance of a completely stirred anaerobic reactor treating pig manure at different temperatures (20, 28 and 38 °C). The start-up phase of the reactor at 20 °C was very long and extremely poor performance was observed with increasing organic loading rate (OLR). At an OLR of 4.3 g ODM L^?1 d^?1, methane production at 28 °C was comparable (3% less) with that at 38 °C, but the risk of acidification was high at 28 °C. At low OLR (1.3 g ODM L^?1 d^?1), the biogas process appeared stable at 28 °C and gave same methane yields as compared to the reactor operating at 38 °C. The estimated sludge yield at 28 °C was 0.065 g VSS g^?1 COD_removed, which was higher than that at 38 °C (0.016 g VSS g^?1 COD_removed).     

Stability of Aqueous Suspensions of Medium-Chain-Length Poly-3-Hydroxyalkanoate Particles

Although poly-3-hydroxyalkanoates (PHAs) and particularily medium-chain-length (mcl)-PHAs are likely to find industrial applications in a latex form, very few studies have examined their behavior in aqueous suspension and none have examined the dense suspensions required commercially. For this reason, the stability of mcl-PHA latexes containing saturated aliphatic (65 mol% 3-hydroxynonanoate, PHN), and for the first time, with vinyl (PHNU) or carboxylated side chains was examined. At 4 g L^?1 with no stabilizing agent, PHNU nanoparticles (199.4 ± 3.6 nm) were significantly smaller than those of PHN (211.5 ± 6.4 nm) while carboxylated PHN nanoparticles (76.1 ± 6.4 nm) were substantially smaller than those of either PHN or PHNU with particles stable for more than 110 days. Increasing the PHN concentration to 10 g L^?1 also resulted in stable latexes but with larger particles (410.8 ± 5.2 nm). Adjusting the pH of the suspending medium (water) before addition of the polymer (dissolved in acetone) resulted in much smaller PHN particles at pH = 11.3 (134 ± 2 nm) than at pH = 4.3 (312 ± 8 nm) at a 4 g L^?1 final polymer concentration. Zeta potentials of PHN suspensions decreased with pH, likely due to the carboxyl end groups. Above a pH of 4.0, adjusting the pH after particle formation had little effect. NaCl addition could be used to agglomerate and ultimately precipitate the particles. Stabilizers such as surfactants will likely be required to produce denser mcl-PHA latexes with suitable particle size for certain applications such as coatings and toner production.     

Effect of substrate feeding frequencies on the methane production and microbial communities of laboratory-scale anaerobic digestion reactors

Even though full-scale digesters have been designed based on laboratory-scale tests, the substrate feeding modes of laboratory-scale tests might be different from those of full-scale digesters. The effect of substrate feeding frequencies on the performance and microbial community of laboratory-scale anaerobic digestion reactors was investigated. Feeding frequencies of twice a day, once a day, and every two days were tested in three 2-L reactors with an organic loading rate of 0.5 g-glucose/L/day under mesophilic condition. According to the results of this study, all the reactors showed similar methane production rates and SCOD removal efficiencies after sufficient time of acclimation, but frequently feeding promoted more stable digestion. Although there was no significant difference in microbial diversities from pyrosequencing analyses, the changes of archaeal community composition were observed. The decrease in feeding frequency appeared to cause shifts from acetoclastic methanogens affiliated with Methanosaeta to H₂-utilizing methanogens. The increase of Methanosaeta at a frequently feeding might contribute to the stability of reactor operation. Since this study uses glucose as the substrate, there is still possibility of different results for more complex substrates, such as sludge, food waste, etc.     

Effect of pretreatment techniques on food waste solubilization and biogas production during thermophilic batch anaerobic digestion

The purpose of this study was to optimize the alkaline, ultrasonication, and thermal pretreatment in order to enhance the solubilization of food waste (FW) for the production of volatile fatty acids, hydrogen, and methane in thermophilic batch anaerobic digestion. Initially, the effect of pretreatment techniques in the acidogenic phase was studied, and the optimal combinations of different conditions were determined. It was found that each pretreatment technique affected food waste solubilization differently. Alkaline pretreatment increased hydrogen yield in the acidogenic sludge by four times over control. COD solubilization was increased by 47 % when FW pre-heated at 130 °C for 60 min. Ultrasonication at 20 kHz and 45 min reduced processing time to 38 h from the 60–80 h needed in normal operation. Response surface methodology (RSM) was used to optimize a combination of alkaline, ultrasonication, and thermal pretreatment. Optimized conditions were applied to methanogenic single-stage thermophilic AD process, and their impact on biogas production was monitored. Results showed that FW heated at 130 °C for 50 min geminates biogas production compared to control experiment. In conclusion, a short thermal pretreatment regime could significant affect biogas production in single-stage thermophilic AD.     

Potential of biogas production from mixed leaf and food waste in anaerobic reactors

Anaerobic digestion of mixed leaf (MLW) and food wastes (FW) was used to explore the potential use of MLW as an accelerator for FW digestion in two parts for biogas production and as a waste management option in a university community. The effects of the single substrate of FW, co-digestion, ratio of MLW and FW (3:2 and 2:3) and ratio of waste feed to inoculum: F/I (0.1 and 0.4), and feeding frequency (every other day and every 2 days) were evaluated in two neutralized anaerobic reactors. The results showed that different mixture ratios with the same F/I ratio were the major factor on biogas (39.87 m³/kg VS_added) and CH₄ yield (25.99 m³/kg VS_added), including %COD removal (84.50%). Co-digestion had the same effect as F/I on biogas production. Only FW provided the lowest biogas and CH₄ yield. The use of a MLW:FW 2:3, F/I 0.4 mixture with every 2 days feeding provided higher biogas production and %COD removal than with every other day feeding. Two neutralized anaerobic reactors were suitable for digestion with a high F/I, and a wider interval feeding. This finding affirms the possibility of biogas production using MLW as the co-substrate with FW, as opposed to using FW alone.