Combined thermophilic aerobic process and conventional anaerobic digestion: effect on sludge biodegradation and methane production

Enhanced anaerobic digestion of waste-activated sludge by thermal-alkali pretreatment: a pilot-scale study

The composition of waste-activated sludge (WAS) is complex, containing a large amount of harmful substances, which pose a threat to the environment and human health. The reduction and resource utilization of sludge has become a development demand in sludge treatment and disposal. Based on the technical bottlenecks in the practical application of direct anaerobic digestion technology, this study adopted two different thermal and thermal-alkali hydrolysis technologies to pretreat sludge. A pilot-scale experiment was conducted to investigate the experimental conditions, parameters, and effects of two hydrolysis technologies. This study showed that the optimal hydrolysis temperature was 70 °C, the hydrolysis effect and pH can reach equilibrium with the hydrolysis retention time was 4-8 h, and the optimal alkali concentration range was 0.0125-0.015 kg NaOH/kg dry-sludge. Thermal-alkali combination treatment greatly improved the performance of methane production, the addition of NaOH increased methane yield by 31.2% than that of 70 °C thermal hydrolysis. The average energy consumption is 75 kWh/m3 80% water-content sludge during the experiment. This study provides a better pretreatment strategy for exploring efficient anaerobic digestion treatment technologies suitable for southern characteristic sewage sludge.

Investigating the challenges of biogas provision in water limited environments through laboratory scale biodigesters

The potential for biogas provision through household-scale anaerobic digestion in rural sub-Saharan Africa is limited due to perceived water shortages. The most common substrate is animal dung diluted 1:1 with water. Two experimental methods tested the potential of reducing water demand. The first experiment compared the chemical oxygen demand (COD) and volatile solid removal of four cow dung dilutions ranging from 3.5-10.6% total solids. In the second experiment, bioslurry filtrate was recirculated back into the fresh substrate at different concentrations. The highest COD removal rate of 28.3% was obtained from mixing equal volumes of dung with filtrate (mean total solids 7.4%) while the highest methane production rate of 0.40 g/L/day, calculated from COD balance, was obtained from undiluted cow dung (total solids 10.6%). Results suggest the potential for a 75-100% reduction in water demand.

Determining Optimal Temperature Combination for Effective Pretreatment and Anaerobic Digestion of Corn Stalk

Temperature is one of the important factors affecting both chemical pretreatment and anaerobic digestion (AD) process of corn stalk (CS). In this work, the combined ways between pretreatment temperature (40 °C and 60 °C) and AD temperature (35 °C and 55 °C) were selected to investigate the AD performance for sodium hydroxide (NaOH) pretreated CS. Three organic loading rates (OLRs) of 1.6, 1.8 and 2.0 g·L⁻¹·d⁻¹ were studied within 255 days using continuously stirred tank reactors (CSTR). The results revealed that biogas yields of CS after pretreated were higher than that of untreated groups by 36.79–55.93% and 11.49–32.35%, respectively. When the temperature of NaOH pretreatment changed from 40 °C to 60 °C, there was no significant difference in enhancing the methane yields during the three OLRs. The mesophilic AD (MAD) of CS pretreated with 2% NaOH under 40 °C and 60 °C conditions produced 275 and 280 mL·g_vs⁻¹ methane yield at OLR of 1.6 g·L⁻¹·d⁻¹. However, as the OLR increased, the methane yield of CS under thermophilic AD (TAD) condition was further higher than under MAD condition. Furthermore, from the perspectives of energy balance and economic analysis, AD of 40 °C-treated CS recovered more energy and TAD is less expensive. Therefore, temperature of 40 °C was considered as an appropriate for pretreatment whether in mesophilic or thermophilic AD system. On the other hand, TAD was chosen as the optimal AD temperatures for higher OLRs.

Revealing the mechanisms for potassium ferrate affecting methane production from anaerobic digestion of waste activated sludge

This study investigated the issue of potassium ferrate (PF) affecting anaerobic methane generation from sludge by a set of experimental and model analyses. Experimental results indicated that the methane production was significantly promoted from 164.7 to 204.1 mL/g VSS (volatile suspended solids) with PF dosage enhanced from 0 to 0.05 g/g TSS (total suspended solids). Further enhancement of PF dosage reduced methane production, which even decreased to 135.4 mL/g VSS when PF dosage increased to 0.1 g/g TSS. Model-based analysis showed that except for methane production potential, the methane production rate was also promoted by PF treatment, which was sufficiently enhanced from 8.80 to 11.88 mL/g VSS/d when PF dosage was 0.05 g/g TSS. Mechanism studies indicated that PF not only promoted sludge disintegration, but also enhanced the proportion of biodegradable organics in sludge liquor, and the digestion potential of the non-biodegradable humus and lignocellulose were promoted.

Combining thermophilic aerobic reactor (TAR) with mesophilic anaerobic digestion (MAD) improves the degradation of pharmaceutical compounds

The removal efficiency of nine pharmaceutical compounds from primary sludge was evaluated in two different operating conditions: (i) in conventional Mesophilic Anaerobic Digestion (MAD) alone and (ii) in a co-treatment process combining Mesophilic Anaerobic Digestion and a Thermophilic Aerobic Reactor (MAD-TAR). The pilot scale reactors were fed with primary sludge obtained after decantation of urban wastewater. Concerning the biodegradation of organic matter, thermophilic aeration increased solubilization and hydrolysis yields of digestion, resulting in a further 26% supplementary removal of chemical oxygen demand (COD) in MAD-TAR process compared to the conventional mesophilic anaerobic digestion. The highest removal rate of target micropollutants were observed for caffeine (CAF) and sulfamethoxazole (SMX) (>89%) with no substantial differences between both processes. Furthermore, MAD-TAR process showed a significant increase of removal efficiency for oxazepam (OXA) (73%), propranolol (PRO) (61%) and ofloxacine (OFL) (41%) and a slight increase for diclofenac (DIC) (4%) and 2 hydroxy-ibuprofen (2OH-IBP) (5%). However, ibuprofen (IBP) and carbamazepine (CBZ) were not degraded during both processes. Anaerobic digestion affected the liquid-solid partition of most target compounds. Sorbed fraction of pharmaceutical compounds on the sludge tend to decrease after digestion, this tendency being more pronounced in the case of the MAD-TAR process due to much lower concentration of solids.

Micron-sized silica particles in wastewater influenced the distribution of organic matters in sludge and their anaerobic degradation

The influence of micron-sized silica particles on the distribution of organic matters in sludge and anaerobic degradation of macromolecular organic components (MOCs) in sludge was investigated. With the addition of micron-sized particles in the influent (VS/TS decreased gradually from 90.46 ± 0.21 % to 33.36 ± 0.17 %), the protein degradation percentage was significantly promoted while the polysaccharides degradation percentage was largely inhibited, resulting in the total MOCs degradation and methane production increasing firstly (with the promotion extent within 10 %) and then declining slightly, with the peak value at VS/TS of 56.03 ± 0.21 %. The shifted degradation percentage of protein and polysaccharides were caused by the significant changed distribution of organic matters in sludge. With the addition of micron-sized silica particles, the MW of EPS and secondary structure of protein in EPS changed little, which brought about little influence. While, the promoted extracellular protein content (so that the total protein content) and declined extracellular carbohydrates content (so that the total polysaccharides content), were found to be strongly correlated to the enhanced protein degradation and inhibited polysaccharides degradation. The results suggested that large amounts of grit in sludge might not be the main reason for lower degradability of sewage sludge in China.

Perspective on enhancing the anaerobic digestion of waste activated sludge

Anaerobic digestion (AD) of waste activated sludge (WAS) is an important bio-energy strategy that has been hindered by low conversion efficiency. This paper presents a comprehensive review of research on the sludge's property and enhancing AD of WAS, and proposes two perspectives of material structure and microbial activity on improving AD efficiency. In the first part of this review, the key principle problems for hindering AD efficiency are identified based on the concept of AD. Then, the possibility that the complex microstructure and composition of WAS are responsible for poor biodegradability is considered and main methods for enhancing AD are summarized. In the third part, according to the published works, the main knowledge gaps in research are recognized as the identification and specific activity adjustment of functional microbes, the understanding of key constituents of WAS and their interactions, the deciphering of complex structure of sludge organic substance, and the revealing of relationships between complex nature of WAS and biodegradability. Further discussions reveal that to enhance AD more studies should be centered on the sludge's structure and properties in future. However, this review is expected to provide the clear and accurate research directions for enhancing AD efficiency of WAS.

Biostimulation of sewage sludge solubilization and methanization by hyper-thermophilic pre-hydrolysis stage and the shifts of microbial structure profiles

This study evaluated the influence of hyper-thermophilic pre-hydrolysis stage (70 °C) on methane recovery of sewage sludge at 35 °C. In this configuration, the process performance in both temperatures were estimated and the microbial communities were characterized by full-length16S rRNA genes and/or microbial activities. In addition, the appropriate solubilization reaction time was assessed. The results revealed that the higher hydrolysis and acidogenesis activities were achieved with longer reaction time of pretreatment (5 days) and thus higher organic nitrogen conversion and alkalinity were attained. Under appropriate pretreatment reaction time, pretreated sludge was characterized by 65% higher organic matters solubilization and 1.4-fold higher volatile fatty acids (VFAs) concentration compared to raw sludge. The overall methane yield produced under this scenario was 179 L CH₄. KgVS_in, with 15% of the absolute yield was produced in hydrolysis reactor. 50% reduction in bacteria belong to Firmicurtes was observed at mesophilic reactor and meanwhile the relative abundance of Bacteroidetes and Cloacimonetes were enhanced. The predominant methanogens in both stages did not change implying adaptation of Methanothermobacter (>62%) to mesophilic condition. However, increasing acetoclastic methanogens up to 30% in mesophilic reactor indicating methane was produced from pretreated sludge mainly through H₂- mediated CO₂ reduction and partially from acetate cleavage. The results highlight the key role of hyper-thermophilic pre-hydrolysis stage for better stabilization of sewage sludge without further investments in current biogas plants.

Response of the microbial community to the methanogenic performance of biologically hydrolyzed sewage sludge with variable hydraulic retention times

Hyperthermophilic biological hydrolysis of sewage sludge was applied before long-term anaerobic digestion to investigate how shortening hydraulic retention times (HRT, 20-5d) affected methanogenic performances and microbial dynamics. Results indicated that although the three different HRTs provided a stable process with a steady-state of methane production, both methane yield (161 L kg-VS_in^-1_, 25% higher) and volatile solids removal (VS, 50%, 2-fold higher) increased during longer HRTs. Redundancy analysis results indicated that Sporosarcina and Methnosarcina positively correlated to VS removal and methane yield, and negatively correlated to volatile fatty acids (VFAs) accumulation. The relative abundance of Coprothermobacter (>60%), syntrophic acetate oxidation bacteria (SAOB), and Methanospirillum (8-15%), increased during shorter HRTs. A slight shift to two-stage acetate conversion was observed during shorter HRTs. The results demonstrated that HRTs played a key role in shaping microbial structure, leading to a new steady-state of microbial community profiles and process performances at variable HRTs.

Enhanced biogas production potential of microalgae and swine wastewater using co-digestion and alkaline pretreatment

Biogas yield obtained from anaerobic digestion of swine wastewater (SWW) needs to be increased to produce electrical energy. To enhance biogas and prevent pollution, use of mixed culture microalgae grown in wastewater (MWW) with SWW has attracted a lot of interest. This research was focused on the possibility of utilizing MWW. Six experiments using raw SWW and MWW, and their co-digestion were conducted on a laboratory scale in one-litre reactors with the ratio of inoculum and substrate of 70:30 under without and with alkaline pretreatment (using 3% NaOH for pH adjustment every 15 min at pH 11 for 3 h). The results showed that co-digestion had the major effect on increasing biogas and methane yields (0.735 and 0.326 m³/kg of volatile solids (VS) removed), and the highest chemical oxygen demand and VS removal (60.29% and 63.17%). For pretreatment, the effect of ammonia inhibition at a high pH of 11 had more influence on biodegradation than the effect of destruction of MWW's cell walls, resulting in a low biogas production of pretreated MWW and pretreated co-digestion. These findings affirm the potential of co-digestion, and the possibility of using both single and co-substrate MWW. Pretreatment could be improved at a lower alkaline pH condition. A pilot scale of co-digestion should be performed.

Integrating novel (thermophilic aerobic membrane reactor-TAMR) and conventional (conventional activated sludge-CAS) biological processes for the treatment of high strength aqueous wastes

A combination of thermophilic aerobic membrane reactor (TAMR) and conventional activated sludge (CAS) was studied by means of two pilot plants at semi-industrial scale in order to simulate the new configuration adopted in a full-scale facility for the treatment of high strength aqueous wastes. Aqueous wastes with high contents of organic pollutants were treated by means of the TAMR technology, progressively increasing the organic load (3-12 kg_COD m^-3 d^-1). A mixture of municipal wastewater and thermophilic permeate was fed to the CAS plant. The main results are the following: achievement of a high COD removal yield by both the TAMR (78%) and the CAS (85%) plants; ammonification of the organic nitrogen under thermophilic conditions and subsequent mesophilic nitrification; capacity of the downstream mesophilic process to complete the degradation of the organic matter partially obtained by the TAMR process and precipitation of phosphorus as vivianite and carbonatehydroxylapatite in the TAMR plant.

Inter-stage thermophilic aerobic digestion may increase organic matter removal from wastewater sludge without decreasing biogas production

Combining aerobic and anaerobic digestion in a two-stage system can improve the degradation of wastewater sludge over the use of either technology alone. But use of aerobic digestion as a pre-treatment before anaerobic digestion generally reduces methane production due to loss of substrate through oxidation. An inter-stage configuration may avoid this reduction in methane production. Here, we evaluated the use of thermophilic aerobic digestion (TAD) as an inter-stage treatment for wastewater sludge using laboratory-scale semi-continuous reactors. A single anaerobic digester was compared to an inter-stage system, where a thermophilic aerobic digester (55 °C) was used between two mesophilic anaerobic digesters (37 °C). Both systems had retention times of approximately 30 days, and the comparison was based on measurements made over 97 days. Results showed that the inter-stage system provided better sludge destruction (52% volatile solids (VS) removal vs. 40% for the single-stage system, 44% chemical oxygen demand (COD) removal vs. 34%) without a decrease in total biogas production (methane yield per g VS added was 0.22-0.24 L g^-1 for both systems).

Comparison of pre- and inter-stage aerobic treatment of wastewater sludge: Effects on biogas production and COD removal

The aim of this study was to investigate thermophilic (55°C) aerobic digestion (TAD) as pre- and inter-stage treatment of sludge anaerobic digestion and to analyse the change in organic matter accessibility and complexity. Pre-treatment decreased methane yield (up to -70%), due to oxidation losses whereas inter-stage treatment slightly improved overall methane yield (+2.6%) and total COD removal (+5%) compared to control. Anaerobic degradability and COD removal in the second anaerobic stage significantly increased, by 13-40%. Organic matter fractionation showed that TAD led to an increase in sludge organic matter accessibility in all cases. Organic matter complexity, measured by fluorimetry, increased after TAD pre-treatment whereas it remained constant after inter-stage treatment. TAD was shown to be more efficient if applied to a more recalcitrant substrate and should thus be used as inter-stage treatment to avoid decreasing methane production.

Sequential ultrasound and low-temperature thermal pretreatment: Process optimization and influence on sewage sludge solubilization, enzyme activity and anaerobic digestion

The influence of sequential ultrasound and low-temperature (55°C) thermal pretreatment on sewage sludge solubilization, enzyme activity and anaerobic digestion was assessed. The pretreatment led to significant increases of 427-1030% and 230-674% in the soluble concentrations of carbohydrates and proteins, respectively, and 1.6-4.3 times higher enzymatic activities in the soluble phase of the sludge. Optimal conditions for chemical oxygen demand solubilization were determined at 59.3kg/L total solids (TS) concentration, 30,500kJ/kg TS specific energy and 13h thermal treatment time using response surface methodology. The methane yield after pretreatment increased up to 50% compared with the raw sewage sludge, whereas the maximum methane production rate was 1.3-1.8 times higher. An energy assessment showed that the increased methane yield compensated for energy consumption only under conditions where 500kJ/kg TS specific energy was used for ultrasound, with up to 24% higher electricity recovery.

Review of feedstock pretreatment strategies for improved anaerobic digestion: From lab-scale research to full-scale application

When properly designed, pretreatments may enhance the methane potential and/or anaerobic digestion rate, improving digester performance. This paper aims at providing some guidelines on the most appropriate pretreatments for the main feedstocks of biogas plants. Waste activated sludge was firstly investigated and implemented at full-scale, its thermal pretreatment with steam explosion being most recommended as it increases the methane potential and digestion rate, ensures sludge sanitation and the heat needed is produced on-site. Regarding fatty residues, saponification is preferred for enhancing their solubilisation and bioavailability. In the case of animal by-products, this pretreatment can be optimised to ensure sterilisation, solubilisation and to reduce inhibition linked to long chain fatty acids. With regards to lignocellulosic biomass, the first goal should be delignification, followed by hemicellulose and cellulose hydrolysis, alkali or biological (fungi) pretreatments being most promising. As far as microalgae are concerned, thermal pretreatment seems the most promising technique so far.

Combined ultrasonication and thermal pre-treatment of sewage sludge for increasing methane production

This article focuses on the combination of ultrasonic and thermal treatment of sewage sludge (SS). The combination involved ultrasonicating a fraction of the sludge and thermal treatment at various temperatures and this resulted in solubilization of proteins and carbohydrates, and so contributing to increased COD solubilization. During the treatment, SCOD, soluble proteins and carbohydrates increased from 760 mg L(-1) to 10,200 mg L(-1), 110 mg L(-1) to 2,900 mg L(-1) and 60 mg L(-1) to 630 mg L(-1), respectively. It was found ultrasonication of only a fraction of the sludge (>20%) followed by thermal treatment led to significant improvement compared to thermal and ULS treatments applied on their own. At 65°C, the kinetic of solubilization was improved and the hyper-thermophilic treatment time could be reduced to a few hours when ultrasonication was used first. A linear correlation (R(2) = 95%) was found between the SCOD obtained after ultrasonication pre-treatment and anaerobic biodegradability. The combined treatment resulted in 20% increase in biogas production during the anaerobic digestion of the pre-treated sludge.

Enhanced anaerobic digestion of waste activated sludge of low organic content in a novel digester

A novel digester, termed an internal circulation anaerobic digester (ICAD), was developed to intensify sludge digestion. It consists of reaction zone, settling zone, thickening zone, riser and downcomer. Internal circulation in the digester is intensified by backflow biogas. The mesophilic ICAD treating thermal pretreated waste activated sludge with volatile suspended solids (VSS)/suspended solids (SS) of 0.45-0.49 was conducted in this study to reduce and stabilize the low organic content sludge. The results showed that the VSS removal rate and biogas rate reached 46.0% and 0.72 m(3)/kg VSS(fed) at hydraulic retention time (HRT) of 15 days. VSS/SS and soluble chemical oxygen demand (SCOD) of the effluent sludge ranged from 0.39 to 0.41 and 274 mg/L to 473 mg/L, respectively, under various HRTs from 10 to 27 days. The degradation ability of ICAD derived from the improved mass transfer by internal circulation and long solid retention time at short HRT is compared with continuous stirred tank reactor.

The thermophilic (55°C) microaerobic pretreatment of corn straw for anaerobic digestion

Microaerobic process has been proven to be an alternative pretreatment for the anaerobic digestion (AD) process in several studies. In this study, the effect of thermophilic microaerobic pretreatment (TMP) on the AD of corn straw was investigated. Results indicated that TMP process obviously improved the methane yield. The maximum methane yield was obtained at the oxygen loads of 5ml/g VSsubstrate, which was 16.24% higher than that of untreated group. The modified first order equation analysis showed the TMP process not only accelerated the hydrolysis rates but also reduced the lag-phase time of AD process. The structural characterization analysis showed cellulosic structures of corn straw were partly disrupted during TMP process. The crystallinity indexes were also decreased. In addition, large or destroyed pores and substantial structural disruption were observed after pretreatment. The results showed that TMP is an efficient pretreatment method for the AD of corn straw.

Sequential sludge digestion after diverse pre-treatment conditions: sludge removal, methane production and microbial community changes

A lab-scale sequential sludge digestion process which consists of a mesophilic anaerobic digester (MAD) and a thermophilic aerobic digester (TAD) was developed. Thermal, thermal-alkaline and long-term alkaline pre-treatments were applied to the feed sludge to examine their effects on sludge removal and methane production. Especially after thermal-alkaline pre-treatment, high COD removal was maintained; methane production rate was also drastically increased by improving the hydrolysis step of sludge degradation. Polymerase chain reaction-denaturing gel gradient electrophoresis indicated that bacterial communities were represented by three phyla (Firmicutes, Proteobacteria, Actinobacteria) and that Clostridium straminisolvens was the major bacterial species in MAD. Quantitative real-time PCR results indicated that Methanosaeta concilli was the major archaeal species in MAD, and that Ureibacillus sp. was the most abundant bacterial species in TAD.

Biochemical methane potential (BMP) of artichoke waste: the inoculum effect

The aim of this work was to investigate anaerobic digestibility of artichoke waste resulting from industrial transformation. A series of batch anaerobic digestion tests was performed in order to evaluate the biochemical methane potential of the matrix in respect of the process. A comparison of the different performances of the laboratory-scale reactors operating in mesophilic conditions and utilizing three different values of the inoculum/substrate ratio was carried out. The best performance was achieved with an inoculum/substrate ratio of 2. Artichoke-processing byproducts showed a classical organic waste decomposition behaviour: a fast start-up phase, an acclimation stage, and a final stabilization phase. Following this approach, artichoke waste reached chemical oxygen demand removal of about 90% in 40 days. The high methane yield (average 408.62 mL CH4 gvs (-1) voltatile solids), makes artichoke waste a good product to be utilized in anaerobic digestion plants for biogas production.

An innovative sewage sludge reduction by using a combined mesophilic anaerobic and thermophilic aerobic process with thermal-alkaline treatment and sludge recirculation

Lab-scale High Efficiency Digestion (HED) systems containing a Mesophilic Anaerobic Reactor (MAR), Thermophilic Aerobic Reactor (TAR), liquid/solid separation unit, and thermal-alkaline treatment were developed to evaluate the efficiencies of sludge reduction and methane production. The HED process was divided into three phases to examine the influence of sludge pretreatment and pretreated sludge recirculation using TCOD and VSS reduction, COD solubilization, and methane production. The VSS removal with a solid/liquid separation unit, sludge recirculation, and thermal-alkaline treatment drastically increased up to 95% compared to the feed concentration. In addition, the results of COD solubilization and VSS/TSS showed that the solubilization of cells and organic matters by the thermal-alkaline treatment was highly increased, which was also consistent with the SEM images. In particular, the methane production rate increased 24-fold when the feed sludge and recirculated sludge were pretreated together. Collectively, the HED experiments performed with sludge recirculation and thermal-alkaline treatment demonstrated that the HED systems can be successfully employed for highly efficient sewage sludge reduction and methane gas production.

The Effects of Alkaline Treatment on Excess Sludge Supernatant Characteristics

The effect of alkali treatment on excess sludge supernatant characteristics was studied in this experiment. 4 mol/L sodium hydroxide solution was utilized to adjust the initial pH value of excess sludge to 9.0, 11.0 and 13.0. Batch experiments were operated in 35 °C shaking bath for 12 h. The soluble chemical oxygen demand (SCOD), protein and polysaccharide concentration in excess sludge supernatant was measured every 3 h. The experimental results showed that maximum amount of protein, polysaccharide and SCOD were obtained when the initial pH value was 13.0.

Effect of heat-alkaline treatment as a pretreatment method on volatile fatty acid production and protein degradation in excess sludge, pure proteins and pure cultures

This study investigated the effect of heat-alkaline treatment (HAT) at pH 11 and 60 °C on volatile fatty acid (VFA) production and protein degradation in excess sludge, soluble and insoluble proteins, and pure cultures. In addition, quantification of bacteria present in the sludge was also examined. Experimental results showed that following acid fermentation under pH 7 and 37 °C, HAT enhanced VFA production in excess sludge, albumin, and Gram-negative bacteria, but not in casein or Gram-positive bacteria. Protein solubility was therefore found not to be the main criteria for VFA production. In the protein analysis, it was shown that the outer membrane protein (OmpC) of Escherichia coli K12 was resistant to chemical and enzymatic hydrolysis. Gram staining revealed that Gram-negative bacteria were predominant in the activated sludge used in this study. In addition, the bacteria present in the activated sludge comprised only 10% of mixed liquor suspended solids (MLSS) by quantitative PCR.

Promotion of polylactide degradation by ammonia under hyperthermophilic anaerobic conditions

The objective of this study was to evaluate the promotion effect of ammonia on the biodegradation of polylactide (PLA) under hyperthermophilic (80°C) and thermophilic (55°C) anaerobic condition. The results showed that PLA was transformed to lactic acid under hyperthermophilic conditions, but that the transformation ratio was negligible under thermophilic conditions. The hydrolysis process can be markedly increased with ammonia addition and microorganism activity. The maximum transformation ratios of the two kinds of PLA used in this study were 65.2% and 51.8%, respectively, with ammonia addition of 4 g N/L over 3 days treatment of anaerobic sludge. After the hyperthermophilic pretreatment, the hydrolysis products were converted to methane by methanogens under the thermophilic and anaerobic conditions. The final methane conversion ratios of the two kinds of PLA after 22 days treatment were 81.8% and 77.0%, respectively.

Escherichia coli inactivation kinetics in anaerobic digestion of dairy manure under moderate, mesophilic and thermophilic temperatures

Batch anaerobic digestion experiments using dairy manure as feedstocks were performed at moderate (25°C), mesophilic (37°C), and thermophilic (52.5°C) temperatures to understand E. coli, an indicator organism for pathogens, inactivation in dairy manure. Incubation periods at 25, 37, and 52.5°C, were 61, 41, and 28 days respectively. Results were used to develop models for predicting E. coli inactivation and survival in anaerobic digestion. For modeling we used the decay of E. coli at each temperature to calculate the first-order inactivation rate coefficients, and these rates were used to formulate the time - temperature - E. coli survival relationships. We found the inactivation rate coefficient at 52.5°C was 17 and 15 times larger than the inactivation rate coefficients at 25 and 37°C, respectively. Decimal reduction times (D10; time to achieve one log removal) at 25, 37, and 52.5°C, were 9 -10, 7 - 8 days, and < 1 day, respectively. The Arrhenius correlation between inactivation rate coefficients and temperatures over the range 25 -52.5°C was developed to understand the impacts of temperature on E. coli inactivation rate. Using this correlation, the time - temperature - E. coli survival relationships were derived. Besides E. coli inactivation, impacts of temperature on biogas production, methane content, pH change, ORP, and solid reduction were also studied. At higher temperatures, biogas production and methane content was greater than that at low temperatures. While at thermophilic temperature pH was increased, at mesophilic and moderate temperatures pH were reduced over the incubation period. These results can be used to understand pathogen inactivation during anaerobic digestion of dairy manure, and impacts of temperatures on performance of anaerobic digesters treating dairy manure.

Experimental study on a coupled process of production and anaerobic digestion of Chlorella vulgaris

The main goal of this present study is to investigate the feasibility of coupling algae production (Chlorella vulgaris) to an anaerobic digestion unit. An intermediate settling device was integrated in order to adapt the feed-flow concentration and the flow rate. Digestion of C. vulgaris was studied under 16 and 28 days hydraulic retention times (HRT), with a corresponding organic loading rate of 1g(COD)L(-1). Increasing the HRT achieved 51% COD removal with a methane production measured at 240 mL g(VSS)(-1). Performing different HRTs and dynamic monitoring during degradation highlighted differential hydrolysis of microalgae compartments. However, 50% of the biomass did not undergo anaerobic digestion, even under long retention times. This points out the interest for further studies on pre-treatment performances and more generally speaking on the need for intensifying microalgae biomass digestion.

Monitoring of bacterial communities during low temperature thermal treatment of activated sludge combining DNA phylochip and respirometry techniques

Sludge reduction is one of the major challenges in biological wastewater treatment. One approach is to increase the sludge degradation yield together with the biodegradation kinetics. Among the various sludge pretreatment strategies proposed, thermal pretreatment at around 65 °C was described as promising. The enhancement in the biodegradation activity due to the selection of thermophilic hydrolytic bacteria was proposed, but further experiments are needed to demonstrate the specific role of these bacteria. In this study, concentrated activated sludge grown at 20 °C was subjected to thermal treatment at 65 °C for different periods. The originality of the work relied on a polyphasic approach based on the correlation between kinetics (chemical oxygen demand, COD; mixed liquor suspended solids, MLSS), bacterial activity (respirometry) and bacterial community structure (phylochip monitoring) in order to characterize the mechanisms involved in the thermal reduction of sludge. The bacterial activity in the aeration basin decreased to a very low level when recycling sludge was treated at 65 °C from 13 to 60 h, but then, started to increase after 60 h. In parallel to these fluctuations in activity, a drastic shift occurred in the bacterial community structure with the selection of thermophilic bacteria (mainly related to genera Paenibacillus and Bacillus), which are known for their specific hydrolases.

Pretreatment methods to improve sludge anaerobic degradability: a review

This paper presents a review of the main sludge treatment techniques used as a pretreatment to anaerobic digestion. These processes include biological (largely thermal phased anaerobic), thermal hydrolysis, mechanical (such as ultrasound, high pressure and lysis), chemical with oxidation (mainly ozonation), and alkali treatments. The first three are the most widespread. Emphasis is put on their impact on the resulting sludge properties, on the potential biogas (renewable energy) production and on their application at industrial scale. Thermal biological provides a moderate performance increase over mesophilic digestion, with moderate energetic input. Mechanical treatment methods are comparable, and provide moderate performance improvements with moderate electrical input. Thermal hydrolysis provides substantial performance increases, with a substantial consumption of thermal energy. It is likely that low impact pretreatment methods such as mechanical and thermal phased improve speed of degradation, while high impact methods such as thermal hydrolysis or oxidation improve both speed and extent of degradation. While increased nutrient release can be a substantial cost in enhanced sludge destruction, it also offers opportunities to recover nutrients from a concentrated water stream as mineral fertiliser.