Ultrasonic sludge disintegration for enhanced methane production in anaerobic digestion: effects of sludge hydrolysis efficiency and hydraulic retention time

Hydrolysis of waste activated sludge (WAS) has been regarded as the rate limiting step of anaerobic sludge digestion. Therefore, in this study, the effect of ultrasound and hydraulic residence time during sludge hydrolysis was investigated with the goal of enhancing methane production from anaerobic digestion (AD). WAS was ultrasonically disintegrated for hydrolysis, and it was semi-continuously fed to an anaerobic digesters at various hydraulic retention times (HRTs). The results of these experiments showed that the solids and chemical oxygen demand (COD) removal efficiencies when using ultrasonically disintegrated sludge were higher during AD than the control sludge. The longer the HRT, the higher the removal efficiencies of solids and COD, while methane production increased with lower HRT. Sludge with 30% hydrolysis produced 7 × more methane production than the control sludge. The highest methane yields were 0.350 m(3)/kg volatile solids (VS)(add) and 0.301 m(3)/kg COD(con) for 16 and 30% hydrolyzed sludge, respectively. In addition, we found that excess ultrasound irradiation may inhibit AD since the 50% hydrolyzed sludge produced lower methane yields than 16 and 30% hydrolyzed sludge.     

Effect of bioaugmentation of activated sludge with white-rot fungi on olive mill wastewater detoxification

AIMS: To test the potential use of Phanerochaete chrysosporium and other white-rot fungi to detoxify olive mill wastewaters (OMW) in the presence of a complex activated sludge. To combine the aerobic with anaerobic treatment to optimize the conversion of OMW in biogas. METHODS AND RESULTS: A 25-l air lift reactor was used to pretreat OMW by white-rot fungi. Detoxification of the OMW was monitored by size exclusion HPLC analysis, chemical oxygen demand (COD)/biological oxygen demand (BOD(5)) ratio evolution, and bioluminescence toxicity test. Anaerobic treatment of OMW was performed in a 12-l anaerobic filter reactor. Efficiency of the treatment was evaluated by organic matter removal, and biogas production. By comparison with the pretreatment by activated sludge only, the bioaugmentation with Phanerochaete chrysosporium or Trametes versicolor led to high removal of organic matter, decreased the COD/BOD(5) ratio and the toxicity. The subsequent anaerobic digestion of the OMW pretreated with activated sludge-white-rot fungi showed higher biomethanization yields than that pretreated with activated sludge only. Higher loading rates (7 g COD l(-1) day(-1)) were reached without any acidification or inhibition of biomethanization. CONCLUSIONS: The use of white-rot fungi, even in the presence of complex biological consortia to detoxify OMW, proved to be possible and made the anaerobic digestion of OMW for methane production feasible. SIGNIFICANCE AND IMPACT OF THE STUDY: The use of fungi for OMW reuse and energy production could be adapted to industrial applications.     

The effect of acid pretreatment on the anaerobic digestion and dewatering of waste activated sludge

Waste activated sludge (WAS) is difficult to degrade in anaerobic digestion systems and pretreatments have been shown to speed up the hydrolysis stage. Here the effects of acid pretreatment (pH 6-1) using HCl on subsequent digestion and dewatering of WAS have been investigated. Optimisation of acid dosing was performed considering digestibility benefits and level of acid required. Pretreatment to pH 2 was concluded to be the most effective. In batch digestion this yielded the same biogas after 13 days as compared to untreated WAS at 21 days digestion. In semi-continuous digestion experiments (12 day hydraulic retention time at 35 °C) it resulted in a 14.3% increase in methane yield compared to untreated WAS, also Salmonella was eradicated in the digestate. Dewatering investigations suggested that the acid pretreated WAS required 40% less cationic polymer addition to achieve the same cake solid content. A cost analysis was also carried out.     

Degradation and characteristic changes of organic matter in sewage sludge using microbial fuel cell with ultrasound pretreatment

Microbial fuel cell (MFC) could be an efficient sludge treatment unit in regard of rates and extents of total chemical oxygen demand (TCOD) removal, particularly when ultrasound was applied to pretreat the sludge. This study characterized the organic matter in sludge before and after MFC treatment, with or without ultrasound as a pretreatment stage. The 5-d MFC tests with electric load significantly enhanced TCOD removal rate from 11.3% to 19.2% for raw sludge and from 25% to 57% for sludge pretreated with >0.6 W ml^?1 ultrasound, using conventional anaerobic digestion test (without electric load) as control. The aromatic proteins, soluble microbial byproduct-like fluorescent compounds and carboxylic components, aliphatic components (C–H related), hydrocarbon and carbohydrate materials were identified to be principally released by ultrasound pretreatment and the fuels in the present MFC study.     

Ultrasonic and Thermal Pretreatments on Anaerobic Digestion of Petrochemical Sludge: Dewaterability and Degradation of PAHs

Effects of different pretreatment methods on sludge dewaterability and polycyclic aromatic hydrocarbons (PAHs) degradation during petrochemical sludge anaerobic digestion were studied. Results showed that the total biogas production volume in the thermal pretreatment system was 4 and 5 times higher than that in the ultrasound pretreatment and in the control system, and the corresponding volatile solid removal efficiencies reached 28%, 15%, and 8%. Phenanthrene, paranaphthalene, fluoranthene, benzofluoranthene, and benzopyrene removal rates reached 43.3%, 55.5%, 30.6%, 42.9%, and 41.7%, respectively, in the thermal pretreatment system, which were much higher than those in the ultrasound pretreatment and in the control system. Moreover, capillary suction time (CST) of sludge increased after pretreatment, and then reduced after 20 days of anaerobic digestion, indicating that sludge dewaterability was greatly improved after anaerobic digestion. The decrease of protein and polysaccharide in the sludge could improve sludge dewaterability during petrochemical sludge anaerobic digestion. This study suggested that thermal pretreatment might be a promising enhancement method for petrochemical sludge solubilization, thus contributing to degradation of the PAHs, biogas production, and improvement of dewaterability during petrochemical sludge anaerobic digestion.     

添加厨余垃圾对剩余污泥厌氧消化产沼气过程的影响

为提高剩余污泥厌氧消化的沼气产量和甲烷含量,研究了厨余垃圾的不同添加量对剩余污泥厌氧消化性能的影响。结果表明,在35℃下,随着剩余污泥中厨余垃圾添加量的增加,厌氧消化系统中碳氮质量比（C／N）、胞外多聚物（EPS）等生理生化指标均有不同程度的改善。其中当剩余污泥与厨余垃圾质量比为2：1时,混合有机废弃物中沼气产量和甲烷含量均达到最大值,每克挥发性固体（VS）产生了156．56mL沼气,甲烷体积分数为67．52％,分别比剩余污泥单独厌氧消化时的产气量提高了5倍和1．5倍。     

Sequential anaerobic/aerobic digestion of waste activated sludge: analysis of the process performance and kinetic study

Sequential anaerobic-aerobic digestion was applied to waste activated sludge (WAS) of a full scale wastewater treatment plant. The study was performed with the objective of testing the sequential digestion process on WAS, which is characterized by worse digestibility in comparison with the mixed sludge. Process performance was evaluated in terms of biogas production, volatile solids (VS) and COD reduction, and patterns of biopolymers (proteins and polysaccharides) in the subsequent digestion stages. VS removal efficiency of 40%, in the anaerobic phase, and an additional removal of 26%, in the aerobic one, were observed. For total COD removal efficiencies of 35% and 25% for anaerobic and aerobic stage respectively, were obtained. Kinetics of VS degradation process was analyzed by assuming a first order equation with respect to VS concentration. Evaluated kinetic parameters were 0.44 ± 0.20 d(-1) and 0.25 ± 0.15 d(-1) for the anaerobic stage and aerobic stage, respectively. With regard to biopolymers, in the anaerobic phase the content of proteins and polysaccharides increased to 50% and 69%, respectively, whereas in the subsequent aerobic phase, a decrease of 71% for proteins and 67% for polysaccharides was observed. The average specific biogas production 0.74 m(3)/(kg VS destroyed), was in the range of values reported in the specialized literature for conventional anaerobic mesophilic WAS digestion.     

Pretreatment of municipal waste activated sludge for volatile sulfur compounds control in anaerobic digestion

The effect of combination of mechanical and chemical pretreatment of municipal waste activated sludge (WAS) prior to anaerobic digestion was studied using a laboratory scale system with an objective to decrease volatile sulfur compounds in biogas and digested sludge. Mechanical pretreatment was conducted using depressurization of WAS through a valve from a batch pretreatment reactor pressurized at 75 ± 1 psi, while combined pretreatments were conducted using six different dosages of hydrogen peroxide (H₂O₂) and ferrous chloride (FeCl₂) along with mechanical pretreatment. About 37-46% removal of H₂S in biogas occurred for different combined pretreatment conditions. Sludge solubilization achieved due to the mechanical pretreatment increased total cumulative methane production by 8-10% after 30 days during the biochemical methane potential (BMP) test. The pretreatment also improved dewaterability in terms of time to filter (TTF), and decreased methyl mercaptan generation potential of the digested sludge.     

Enhancement on biodegradation and anaerobic digestion efficiency of activated sludge using a dual irradiation process

A dual irradiation process involving aerobic thermophilic irradiation pretreatment (ATIP) and intermittent irradiation anaerobic digestion was developed to improve the digestion of waste-activated sludge. First, the effect of ATIP on further anaerobic digestion of activated sludge in batch mode was investigated. When exposed to ATIP for 24 h, the digestion reactor gave the highest methane yield, removed the most dissolved organic carbon (DOC) and showed the most effective reduction of VS compared to other irradiation times. This process was further enhanced by using an anaerobic fluidised-bed reactor packed with carbon felt in semi-continuous mode for digesting the pretreated activated sludge under intermittent irradiation conditions. Dual irradiation for 24 h followed by 60 min of anaerobic irradiation processing per day turned out to be optimal. This resulted in 65.3% of VS reduction, 83.9% of DOC removal ratio and 538 ml/g-VS of methane yield.     

Pretreatment of pulp mill secondary sludge for high-rate anaerobic conversion to biogas

Three pretreatment methods were compared based on their ability to increase the extent and rate of anaerobic bioconversion of pulp mill secondary sludge to biogas. The pretreatment technologies used in these experiments were: (i) thermal pretreatment performed at 170 °C; (ii) thermochemical (caustic) pretreatment performed at pH 12 and 140 °C; and (iii) sonication performed at 20 kHz and 1 W mL⁻¹. Sludge samples were obtained from a sulfite and a kraft pulp mill, and biochemical methane potential (BMP) assays were performed using microbial granules obtained from a high-rate anaerobic digester operating at a pulp mill. Biogas production from untreated sludge was 0.05 mL mg⁻¹ of measured chemical oxygen demand (COD) and 0.20 mL mg⁻¹ COD for kraft and sulfite sludge, respectively. Thermal pretreatment had the highest impact on sludge biodegradability. In this case, biogas yield and production rate from sulfite sludge increased by 50% and 10 times, respectively, while biogas yield and production rate from kraft sludge increased by 280% and 300 times, respectively. Biogas yield correlated to soluble carbohydrate content better than soluble COD.     

Pre-treatments to enhance the biodegradability of waste activated sludge: Elucidating the rate limiting step

Pre-treatments for waste activated sludge (WAS) are, in most cases, an attempt to increase the biodegradation and/or improve hydrolysis rate of WAS after anaerobic digestion. This review presents an extensive analysis of WAS pre-treatments effectiveness focusing on increasing the biodegradability. In the first part of the review, WAS is considered as a cluster of organic components: proteins, carbohydrates, humic substances and cells. Based on this breakdown into components, the effect of different pre-treatments on each component (and in combination) is described. Also, possible reasons for the contradictory results frequently found among different studies dealing with the same pre-treatment are included. In the second part, the review describes the effects on volatile solids removal by digestion after pre-treatment and on the dewaterability of the final digestate. The energy balance and potential limiting factors for each pre-treatment are also taken into account. From the published works it is concluded that some pre-treatment techniques, such as thermal hydrolysis, thermal phased anaerobic digestion and low-temperature pre-treatment are effective ways to increase energy production and to improve other sludge properties, such as dewatering. However, these techniques are very energy intensive and require a large capital outlay, so research on milder pre-treatment techniques is valuable.     

Autohydrolysis pretreatment of secondary sludge for anaerobic digestion

The increase in the number of wastewater treatment plants and the quality required for the residue produced makes it necessary to improve the efficiency of anaerobic digestion of sludge. Pretreatments of secondary sludge have shown important advantages in the elimination of volatile solids and pathogenic microorganisms from the sludge, and they have also had a positive effect on biogas production. However, such methods are associated with high operating costs. This paper shows the behavior of a autohydrolysis pretreatment, which consists of subjecting the secondary sludge to a temperature of 55 °C for 12–24 h with a limited amount of oxygen under batch operation. The pretreatment results in a high solubilization of organic matter, increasing the fluidity of the sludge and improving the biogas production. This study focuses on the evaluation of the influence of oxygen and the initial sludge concentration on the pretreatment behavior. The main results obtained showed that when autohydrolysis pretreatment was carried out for 12 h, with a high solid concentration and microaerobic conditions, the solubilization of organic matter was increased by 40%, the methane productivity was improved by 23%, and there was an overall improvement in sludge fluidity. Moreover, the energy assessment of the autohydrolysis pretreatment and anaerobic digestion system showed the energetic feasibility of this treatment method, since the increase in energy production compensates for the extra energy required to carry out the pretreatment.     

Effects of microwave and alkali induced pretreatment on sludge solubilization and subsequent aerobic digestion

Individual and combined effects of microwave (MW) and alkali pretreatments on sludge disintegration and subsequent aerobic digestion of waste activated sludge (WAS) were studied. Pretreatments with MW (600 W-85 °C-2 min), conventional heating (520 W-80 °C-12 min) and alkali (1.5 g NaOH/L - pH 12-30 min) achieved 8.5%, 7% and 18% COD solubilization, respectively. However, combined MW-alkali pretreatment synergistically enhanced sludge solubilization and achieved 46% COD solubilization, 20% greater than the additive value of MW alone and alkali alone (8.5 + 18% = 26.5%). Moreover, the results of the batch aerobic digestion study on MW-alkali pretreated sludge showed 93% and 63% reductions in SCOD and VSS concentrations, respectively, at 16 days of SRT. The VSS reduction was 20% higher than that of WAS without pretreatment.     

Sludge minimisation technologies

The treatment and disposal of excess sludge represents a bottleneck of wastewater treatment plants all over the world, due to environmental, economic, social and legal factors. There is therefore a growing interest in developing technologies to reduce the wastewater sludge generation. The goal of this paper is to present the state-of-the-art of current minimisation techniques for reducing sludge production in biological wastewater treatment processes. An overview of the main technologies is given considering three different strategies: The first option is to reduce the production of sludge by introducing in the wastewater treatment stage additional stages with a lower cellular yield coefficient compared to the one corresponding to the activated sludge process (lysis-cryptic growth, uncoupling and maintenance metabolism, predation on bacteria, anaerobic treatment). The second choice is to act on the sludge stage. As anaerobic digestion is the main process in sewage sludge treatment for reducing and stabilising the organic solids, two possibilities can be considered: introducing a pre-treatment process before the anaerobic reaction (physical, chemical or biological pre-treatments), or modifying the digestion configuration (two-stage and temperature-phased anaerobic digestion, anoxic gas flotation). And, finally, the last minimisation strategy is the removal of the sludge generated in the activated sludge plant (incineration, gasification, pyrolysis, wet air oxidation, supercritical water oxidation).     

Enhancement of biochemical methane potential from excess sludge with low organic content by mild thermal pretreatment

Excess sludge with low organic content always led to the failure of anaerobic digestion for methane production. Recently, the mild thermal pretreatment, which is usually operated at temperatures below 120 °C, has drawn much attention due to less energy consumption and no chemical addition. In this study the effect of mild thermal pretreatment (50–120 °C) on the solubilization and methane potential of excess sludge with a low concentration of organic matters was investigated. Experimental results showed that the concentration of soluble organic matters increased gradually with temperature during the mild thermal pretreatment of excess sludge. Biochemical methane potential experiments demonstrated that the potential of methane production from excess sludge was greatly enhanced by mild thermal pretreatment, and under the conditions of pretreatment temperature 100 °C and digestion time 20 d the methane yield was as high as 142.6 ± 2.5 mL/g of volatile solids. Mechanism investigation on the enhancement of methane production from excess sludge exhibited that the consumptions of sludge protein and carbohydrate, the adenosine 5′-triphosphate content of anaerobic microorganisms, the activities of key enzymes related to anaerobic digestion, and the amount of methanogens were all improved by mild thermal pretreatment, in correspondence with the production of methane.     

The use of ultrasound and gamma-irradiation as pre-treatments for the anaerobic digestion of waste activated sludge at mesophilic and thermophilic temperatures

The effect of ultrasound and gamma-irradiation used as pre-treatments for the anaerobic digestion of waste activated sludge at both mesophilic and thermophilic temperatures was examined. Untreated activated sludge was also subjected to anaerobic digestion at these temperatures as a control. The sonication time was 90 s using a Soniprep 150 (MSE Scientific Instruments) which operated at 23 kHz and had been adjusted to give an output of 47 W and the gamma-irradiation dose was 500 krad. The digesters were operated in a semi-continuous mode, being fed with fresh sludge every 24 h at hydraulic retention times (HRT) of 8, 10 and 12 days. Over the 24 h period the differences between the digesters, in terms of volatile solids (VS) reductions and biogas production, were not statistically significant for any particular set of conditions. Thermophilic digestion performed better than mesophilic digestion in terms of biogas production, VS reductions (except at HRT of 8 days) and specific methane yields and the optimum retention time was 10 days, at both temperatures. When gas production over the initial eight hours (probably the hydrolytic stage) was examined, it was found that the gas production rates for pre-treated sludges were higher than those for untreated sludges. This was most pronounced at thermophilic temperatures and a HRT of 10 days. Sonication did not affect the numbers of faecal coliforms in the sludge. However, gamma-radiation caused a 3-log reduction and, when coupled with mesophilic digestion, gave a product which contained < 100 g(-1) TS. Thermophilic anaerobic digestion produced sludges which contained < 1 g(-1) TS irrespective of any pre-treatment.     

Production and characterization of β-glucosidase from Aspergillus niger fermentation: Application for organic fraction of municipal solid waste hydrolysis and methane enhancement

The anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) is currently an attractive treatment process with energy production in the form of biogas. Hydrolysis is the rate-limiting step for the anaerobic digestion of solid wastes. Thus, in the present study fungal enzymatic pretreatment of OFMSW was applied to enhance biogas production. Two enzyme cocktails rich on β-glucosidase were produced from submerged fermentation of Aspergillus niger on basal medium using OFMSW as carbon source and urea (Urea cocktail) and Ulva rigida as nitrogen source (Ulva cocktail). Ulva cocktail displayed an important effect on OFMSW solubilization. Therefore, an increase of reducing sugar concentration about 60% was obtained which was in correlation with chemical oxygen demand (COD) increase. The performance of enzymatic pretreatment on anaerobic digestion of OFMSW was studied by conducting biochemical methane potential tests. Results showed that the enzymatic pretreatment improved methane yield of OFMSW even at high solid concentration. High methane yield about 500 ml/g total volatile solid was obtained, which corresponds up to 68% enhancement over the control.     

Ultrasonic pretreatment for thermophilic aerobic digestion in industrial waste activated sludge treatment

In order to enhance the degradation efficiency of waste activated sludge (WAS) by thermophilic aerobic digestion, an ultrasonic pretreatment was examined. It was observed that ultrasonic pretreatment increased the solubilization of organic matter in the WAS and that the solubilization ratio of the organics increased during the first 30 min but did not extensively increase thereafter. Therefore, a pretreatment time of 30 min was determined to be the economical pretreatment time from the experimental results. From the digestion experiments, which was conducted using the WAS collected from an oil refinery plant in Inchon, Korea, investigating the effects of an ultrasonic pretreatment on thermophilic aerobic digestion, it was confirmed that the proposed ultrasonic pretreatment was effective at enhancing the release of the cellular components in WAS and the degradation of released components in the thermophilic aerobic digestion.     

Experiences with the dual digestion of municipal sewage sludge

The dual digestion process was investigated using sludge samples collected at the WWTP of Tomaszow Mazowiecki (Poland). Mixed sludge was treated in a laboratory setup under batch and semi-continuous conditions. Dual digestion with a 1d SRT aerobic thermophilic pretreatment followed by an anaerobic step with 20 d of SRT turned out to be optimal, since a 44-46% VS reduction and a biogas yield of 480 dm(3)/kg VS fed were achieved. In the course of the process, the concentration of nitrogen in supernatant increased over 5 times and its major portion was converted into ammonia. Phosphorus also entered the supernatant, reaching over 200 g/m(3). The dual digestion noticeably deteriorated the sludge dewaterability. Following completion of the process, capillary suction time measurements averaged 64 s for the raw sludge, 400 s for aerobically pretreated sludge and 310-360 for the anaerobically digested sludge. Aerobic pretreatment consistently reduced Enterobacteriaceae content to below detectable limits.     

Petroleum coke supplementation for enhanced biogas production and phosphate removal under mesophilic conditions

The use of carbon-based conductive materials has been shown to lead to an increase in biogas and methane yields during anaerobic digestion (AD). The effect of these additives on AD using synthetic substrates has been extensively studied, yet their significance for wastewater sludge digestion has not been adequately investigated. Therefore, the aim of this research was to optimize the concentration of petroleum coke (PC) that is a waste by-product of oil refineries, for the anaerobic digestion of wastewater sludge and investigation of phosphate removal in the AD process in the mesophilic temperature range. According to the results of the experiments, supplementing reactors with PC could significantly improve biogas and methane production. Supplementation of reactors with 1.5 g/L PC led to 23.40 ± 0.26% and 42.55 ± 3.97% increase in biogas production and methane generation, respectively. Moreover, the average volatile solids (VS), phosphate, and chemical oxygen demand (COD) removals were 43.43 ± 0.73, 46.74 ± 0.77%, and 60.40 ± 0.38%, respectively.