Thermophilic anaerobic fermentation of olive pulp for hydrogen and methane production: modelling of the anaerobic digestion process.

The present study investigates the thermophilic biohydrogen and methane production from olive pulp, which is the semi-solid residue coming from the two-phase processing of olives. It focussed on: a) production of methane from the raw olive pulp; b) anaerobic bio-production of hydrogen from the olive pulp; c) subsequent anaerobic treatment of the hydrogen-effluent with the simultaneous production of methane; and d) development of a mathematical model able to describe the anaerobic digestion of the olive pulp and the effluent of hydrogen producing process. Both continuous and batch experiments were performed. The hydrogen potential of the olive pulp amounted to 1.6 mmole H2 per g TS. The methane potential of the raw olive pulp and hydrogen-effluent was as high as 19 mmole CH4 per g TS suggesting that: a) olive pulp is a suitable substrate for methane production; and b) biohydrogen production can be very efficiently coupled with a subsequent step for methane production.     

Comparison of different conditions, substrates and operation modes by dynamic simulation of a full-scale anaerobic SBR plant

Simulation studies for a full-scale anaerobic unit of a wastewater treatment plant (WWTP) were performed using the anaerobic digestion model no. 1 (ADM1). The anaerobic full-scale plant consists of one mesophilic and one thermophilic digester, operated in an anaerobic sequential batch reactor (ASBR) mode, and sludge enrichment reactors (SER) for each digester. The digesters are fed with a mixture of vegetable waste and process wastewater from the food factory. Characteristics such as COD(total), N(total) and NH(4)-N concentrations in the influent and effluent of the digester and SERs were measured and used for input fractionation. Parameters such as level, pH, biogas amount and composition in the digester were measured online and used for calibration. For simulation studies, different temperatures and operation modes with varying chemical oxygen demand (COD) input loads corresponding to feedstocks such as fruits, vegetables and grain were analysed and compared. Higher gas production and digestion efficiency in the thermophilic reactor and in shorter cycles were found and confirmed at full scale. Serial operation mode increased the gas production, but pH inhibition occurred earlier. Feeding only biosolids into digester I and the effluent of digester I together with process water into digester II further improved gas production in serial operation mode.     

Application of the IWA anaerobic digestion model (ADM1) for simulating full-scale anaerobic sewage sludge digestion.

A steady-state implementation of the IWA Anaerobic Digestion Model No. 1 (ADM1) has been applied to the anaerobic digesters in two wastewater treatment plants. The two plants have a wastewater treatment capacity of 76,000 and 820,000 m3/day, respectively, with approximately 12 and 205 dry metric tons sludge fed to digesters per day. The main purpose of this study is to compare the ADM1 model results with full-scale anaerobic digestion performance. For both plants, the prediction of the steady-state ADM1 implementation using the suggested physico-chemical and biochemical parameter values was able to reflect the results from the actual digester operations to a reasonable degree of accuracy on all parameters. The predicted total solids (TS) and volatile solids (VS) concentration in the digested biosolids, as well as the digester volatile solids destruction (VSD), biogas production and biogas yield are within 10% of the actual digester data. This study demonstrated that the ADM1 is a powerful tool for predicting the steady-state behaviour of anaerobic digesters treating sewage sludges. In addition, it showed that the use of a whole wastewater treatment plant simulator for fractionating the digester influent into the ADM1 input parameters was successful.     

Coupled anaerobic-aerobic treatment of whey wastewater in a sequencing batch reactor: proof of concept.

A proof of concept was performed in order to verify if the coupling of anaerobic and aerobic conditions inside the same digester could efficiently treat a reconstituted whey wastewater at 21 degrees C. The sequencing batch reactor (SBR) cycles combined initial anaerobic phase and final aerobic phase with reduced aeration. A series of 24 h cycles in 0.5 L digesters, with four different levels of oxygenation (none, 54, 108 and 182 mgO2 per gram of chemical oxygen demand (COD)), showed residual soluble chemical oxygen demand (sCOD) of 683 +/- 46, 720 +/- 33, 581 +/- 45, 1239 +/- 15 mg L(-1), respectively. Acetate and hydrogen specific activities were maintained for the anaerobic digester, but decreased by 10-25% for the acetate and by 20-50% for the hydrogen, in the coupled digesters. The experiment was repeated using 48 h cycles with limited aeration during 6 or 16 hours at 54 and 108 mgO2gCODinitial(-1), displaying residual sCOD of 177 +/- 43, 137 +/- 38, 104 +/- 22 and 112 +/- 9 mgL(-1) for the anaerobic and the coupled digesters, respectively. The coupled digesters recovered after a pH shock with residual sCOD as low as 132 mg L(-1) compared to 636 mg L(-1) for the anaerobic digester. With regard to the obtained results, the feasibility of the anaerobic-aerobic coupling in SBR digesters for the treatment of whey wastewater was demonstrated.     

Winery and distillery wastewater treatment by anaerobic digestion.

Anaerobic digestion is widely used for wastewater treatment, especially in the food industries. Generally after the anaerobic treatment there is an aerobic post-treatment in order to return the treated water to nature. Several technologies are applied for winery wastewater treatment. They are using free cells or flocs (anaerobic contact digesters, anaerobic sequencing batch reactors and anaerobic lagoons), anaerobic granules (Upflow Anaerobic Sludge Blanket--UASB), or biofilms on fixed support (anaerobic filter) or on mobile support as with the fluidised bed. Some technologies include two strategies, e.g. a sludge bed with anaerobic filter as in the hybrid digester. With winery wastewaters (as for vinasses from distilleries) the removal yield for anaerobic digestion is very high, up to 90-95% COD removal. The organic loads are between 5 and 15 kgCOD/m3 of digester/day. The biogas production is between 400 and 600 L per kg COD removed with 60 to 70% methane content. For anaerobic and aerobic post-treatment of vinasses in the Cognac region, REVICO company has 99.7% COD removal and the cost is 0.52 Euro/m3 of vinasses.     

Characterization of sludges for predicting anaerobic digester performance.

Batch anaerobic digestion tests of primary sludge and waste activated sludge were conducted for a duration of 123 days to determine the ultimate degradability of the sludges. For primary sludges the inert fraction of the particulate COD that was predicted by the wastewater models could be employed to predict their biodegradability under anaerobic conditions. The degradation of waste activated sludge was adequately characterized for the first 60 days of digestion using a model that assumed equivalent biodegradability of particulate COD components under aerobic and anaerobic conditions. However after 60 days of anaerobic digestion it appeared that decay of the endogenous products was occurring. This could be described with a first order decay function with a coefficient of 0.0075 d(-1). For continuous flow digesters operating at SRTs of 30-60 days, the predicted VSS destruction with the modified model was approximately 10% higher than that predicted on the basis of inert endogenous decay products.     

Minimisation of excess sludge production in a WWTP by coupling thermal hydrolysis and rapid anaerobic digestion.

In many cases, reducing sludge production could be the solution for wastewater treatment plants (WWTP) that here difficulty evacuating the residuals of wastewater treatment. The aim of this study was to test the possibility of minimising the excess sludge production by coupling a thermal hydrolysis stage and an anaerobic digestion with a very short HRT. The tests were carried out on a 2,500 p.e. pilot plant installed on a recycling loop between the clarifier and the actived sludge basin. The line equipped with the full scale pilot plant produced 38% TSS less than the control line during a 10 week period. Moreover, the rapid anaerobic digestion removed, on average, more than 50% of the total COD load with a hydraulic retention time (HRT) of 3 days. Lastly, the dryness of the remaining excess sludge, sanitised by the thermal hydrolysis, was more than 35% with an industrial centrifuge. This combination of thermal hydrolysis and rapid anaerobic digestion equally permits a significant gain of compactness compared to traditional anaerobic digesters.     

Anaerobic thermophilic digestion of sewage sludge with a thickened sludge recycle

The process of anaerobic thermophilic digestion of municipal wastewater sludge with a recycled part of thickened digested sludge, was studied in semi-continuous laboratory digesters. This modified recycling process resulted in increased solids retention time (SRT) with the same hydraulic retention time (HRT) as compared with traditional digestion without recycling. Increased SRT without increasing of HRT resulted in the enhancement of volatile substance reduction by up to 68% in the reactor with the recycling process compared with 34% in a control conventional reactor. Biogas production was intensified from 0.3 L/g of influent volatile solids (VS) in the control reactor up to 0.35 L/g VS. In addition, the recycling process improved the dewatering properties of digested sludge.     

SBR工艺动力学研究与工程应用

棉浆粕废水组分较为复杂,治理具有一定的技术难度,采用两相厌氧、序批式活性污泥法和物化处理组合工艺取得了较好的效果。生产运行结果表明:SBR工艺对COD,BOD的去除效率分别达到40％～57％和55％～70％。在水温自然变化情况下,测定了好氧SBR单元的生化动力学参数,并与已有文献报道的纸浆和造纸废水的测定结果进行了比较,表明棉浆粕废水经两相厌氧处理后的出水更易于生化处理,但仍属较难好氧生化处理废水。     

Integrated chemical, physical and biological processes modelling of anaerobic digestion of sewage sludge.

The biological kinetic processes for anaerobic digestion (AD) are integrated into a two phase subset of a three phase mixed weak acid/base chemistry kinetic model. The approach of characterising sewage sludge into carbohydrates, lipids and proteins, as is done in the International Water Association (IWA) AD model No 1 (ADM1), requires measurements that are not routinely available on sewage sludges. Instead, the sewage sludge is characterised with the COD, carbon, hydrogen, oxygen and nitrogen (CHON) composition and is formulated in mole units, based on conservation of C, N, O, H and COD. The model is calibrated and validated with data from laboratory mesophilic anaerobic digesters operating from 7 to 20 d sludge age and fed a sewage primary and humus sludge mixture. These digesters yielded COD mass balances between 107-109% and N mass balances between 91-99%, and hence the experimental data is accepted as reasonable. The sewage sludge COD is found to be 32-36% unbiodegradable (depending on the kinetic formulation selected for the hydrolysis process) and to have a C3.5H7O2N0.196 composition. For the selected hydrolysis kinetics of surface mediated reaction (Contois), with a single set of kinetic and stoichiometric constants, for all retention times good correlation is obtained between predicted and measured results for: (i) COD; (ii) free and saline ammonia (FSA); (iii) short chain fatty acids (SCFA); (iv) H2CO3 * alkalinity; (v) pH of the effluent stream; (vi) CO2; and (vii) CH4 gases in the gas stream. The measured composition of primary sludge from two local wastewater treatment plants ranged between C3.38H7O1.91 N0.21 and C3.91H7O2.04N0.16. The predicted composition based on mass balances is therefore within 5% of the average measured composition providing persuasive validation of the model.     

Application of Methanobrevibacter acididurans in anaerobic digestion.

To operate anaerobic digesters successfully under acidic conditions, hydrogen utilizing methanogens which can grow efficiently at low pH and tolerate high volatile fatty acids (VFA) are desirable. An acid tolerant hydrogenotrophic methanogen viz. Methanobrevibacter acididurans isolated from slurry of an anaerobic digester running on alcohol distillery wastewater has been described earlier by this lab. This organism could grow optimally at pH 6.0. In the experiments reported herein, M. acididurans showed better methanogenesis under acidic conditions with high VFA, particularly acetate, than Methanobacterium bryantii, a common hydrogenotrophic inhabitant of anaerobic digesters. Addition of M. acididurans culture to digesting slurry of acidogenic as well as methanogenic digesters running on distillery wastewater showed increase in methane production and decrease in accumulation of volatile fatty acids. The results proved the feasibility of application of M. acididurans in anaerobic digesters.     

Fate of nonylphenol polyethoxylates and nonylphenoxy acetic acids in an anaerobic digestion process for sewage sludge treatment.

Many environmental problems caused by endocrine disruptors (EDs) have been reported. It is reported that EDs flow into sewage treatment plants, and it has been pointed out that these may be shifted from the wastewater treatment process to the sludge treatment process. Little is known about the fate of EDs accumulated in sewage sludge, so we carried out a study to clarify the fate of EDs in sewage sludge treatment processes, especially in an anaerobic digestion process. In this study, nonylphenol (NP) was selected as a target ED. Nonylphenol ethoxylates (NPnEO) or nonylphenoxy acetic acids (NPnEC), which were the precursor of NP, were added to an anaerobic digestion process, and mass balance was investigated. The following results were obtained from the anaerobic digestion experiments. (1) NP1EO was injected to an anaerobic digestion testing apparatus that was operated at a retention time of approximately 28 d and a temperature of 35 degrees C with thickened sludge sampled from an actual wastewater treatment plant. Approximately 40% of the injected NP1EO was converted to NP. (2) NP1EC was injected to an anaerobic digestion testing apparatus with thickened sludge. As a result, almost all injected NP1EC was converted to NP. When NP2EC was injected, NP2EC was not converted to NP until the 20th day.     

ADM1 performance using SS-OFMSW with non-acclimated inoculums

This paper assesses the anaerobic digestion (AD) of the source-sorted organic fraction of municipal solid waste (SS-OFMSW). For this purpose, an experimental programme was implemented involving the operation and monitoring of two bench-scale anaerobic digesters, continuously fed with SS-OFMSW. The mathematical model (ADM1) was then applied to simulate the process of AD of SS-OFMSW. While start-up of the digesters was relatively slow, re-inoculation with cattle manure with effluent dilution reduced the acclimation period and achieved better stability, accommodating a feeding rate at an OLR = 2.39 kg TVS m(-3) day(-1). The high recorded methane gas production rate, reaching (0.1-2.5 m(3) CH(4)/m(3) reactor day), confirms the excellent biodegradability of the type of waste used (SS-OFMSW) and its suitability for AD. Satisfactory simulations of soluble chemical oxygen demand (COD), pH, and methane composition of biogas were obtained, whereas volatile fatty acid (VFA) concentrations in both reactors were over-predicted albeit capturing its general trend.     

Two-stage anaerobic digestion process for complete inactivation of enteric bacterial pathogens in human night soil.

Anaerobic digestion offers a good alternative for human waste treatment. However, the fate of enteric bacterial pathogens present in human night soil (HNS) remains a major concern for hygienic safety of the process. A two-stage anaerobic digestion process, consisting of separate acidogenic and methanogenic digesters, was designed and its efficacy in the inactivation of Salmonella typhi was compared to a single-stage digestion process. In a single-stage digestion, complete pathogen inactivation was achieved only in the digesters with high levels of volatile fatty acids (VFA approximately equal to 18,000 mg/l) and acidic pH (approximately equal to 6.0). These digesters, however, showed drastic reduction in methane yield. In the two-stage digestion process, S. typhi was completely inactivated in the acidogenic digester and the methanogenic digester was free from the pathogen even after receiving a daily dose of the pathogen. The process also achieved complete inactivation of other enteric pathogens, viz., Shigella dysenteriae and Vibrio cholerae. The two-stage process was efficient in biogas generation from HNS. Thus, the two-stage process ensures complete hygienic safety in anaerobic digestion of human night soil.     

Nutrient and trace element supply in anaerobic digestion plants and effect of trace element application

To investigate process behaviour and process disturbances in anaerobic digestion (AD) plants, samples of more than 1500 main digesters were taken and concentrations of macro- and micronutrients as well as volatile fatty acids were analyzed. Out of these samples, 600 digesters using energy crops only and 80 digesters using waste substrates were selected to compare the chemical composition as a result of different substrate feeding. High variations in element concentrations were found in both groups. In 60 plants with low micronutrient levels, trace elements were added and process changes were observed. A significant decrease of volatile fatty acids and an increase in digester performance were measured after trace element application. Furthermore, an increase of bacterial biomass could be detected.     

Potential for biohydrogen and methane production from olive pulp.

The present study investigates the potential for thermophilic biohydrogen and methane production from olive pulp, which is the semi-solid residue coming from the two-phase processing of olives. It focussed on: a) production of methane from the raw olive pulp, b) anaerobic bio-production of hydrogen from the olive pulp, and c) subsequent anaerobic treatment of the hydrogen-effluent with the simultaneous production of methane. Both continuous and batch experiments were performed. The hydrogen potential of the olive pulp amounted to 1.6 mmole H2 per g TS. The methane potential of the raw olive pulp and hydrogen-effluent was as high as 19 mmole CH4 per g TS. This suggests that olive pulp is an ideal substrate for methane production and it shows that biohydrogen production can be very efficiently coupled with a subsequent step for methane production.     

Performance of high-rate sludge digesters fed with sonicated sludge.

A major limitation of anaerobic sludge digestion is the long hydraulic retention time (HRT) required for satisfactory stabilization which results in large digester size. This study explored a possibility of operating digesters at shortened HRTs by sonication pretreatment of secondary sludges. Four identical digesters designated D1, D2, D3 and D4 were fed with untreated and sludge sonicated at densities of 0.18 W/ml, 0.33 W/ml and 0.52 W/ml, respectively. All digesters were operated at three HRTs of 8-day, 4-day and 2-day. Comparing with the control digester (D1), total solids removal efficiencies improved by 12-19%, 17-36% and 20-39% in digesters D2, D3 and D4, respectively. The volatile solids removal was also increased by 11-21%, 17-33% and 19-36% in the respective digesters. The improved solids degradation corresponded with increase in biogas production by 1.4-2.5, 1.9-3.0 and 1.6-3.1 times, respectively. Increase in methane composition by 2-17% was also noted in all digesters fed with sonicated sludge. An analysis indicated that sonication pretreatment could enhance degradation of carbon, nitrogen and sulfur substances in the digestion. The study suggested that sonication of sludge is a possible pretreatment to shorten the digester operating HRT with improvement in solids degradation, biogas production and methane content. It can be deduced that to maintain a consistent solids loading at a desire performance, sludge digester with smaller size can be designed.     

Thermal pre-treatment of primary and secondary sludge at 70 degrees C prior to anaerobic digestion.

In general, mesophilic anaerobic digestion of sewage sludge is more widely used compared to thermophilic digestion, mainly because of the lower energy requirements and higher stability of the process. However, the thermophilic anaerobic digestion process is usually characterised by accelerated biochemical reactions and higher growth rate of microorganisms resulting in an increased methanogenic potential at lower hydraulic retention times. Furthermore, thermal pre-treatment is suitable for the improvement of stabilization and could be realized at relatively low cost especially at low temperatures. The present study investigates the effect of the pre-treatment at 70 degrees C on thermophilic (55 degrees C) anaerobic digestion of primary and secondary sludge in continuously operated digesters. Thermal pre-treatment of primary and secondary sludge at 70 degrees C enhanced the removal of organic matter and the methane production during the subsequent anaerobic digestion step at 55 degrees C. It also greatly contributed to the destruction of pathogens present in primary sludge. Finally it results in enhanced microbial activities of the subsequent anaerobic step suggesting that the same efficiencies in organic matter removal and methane recovery could be obtained at lower HRTs.     

两相厌氧处理湿式氧化后剩余污泥的研究

针对工业废水剩余污泥可生化性差、处理费用高等问题,以石油化工废水剩余污泥为研究对象,进行了湿式氧化后两相厌氧处理的研究。即剩余污泥经温和的湿式氧化预处理后,有机物从微生物体内释放出来并转移到液相,上清液采用两相厌氧处理。系统研究了两相厌氧中 COD_(Cr)、产气量、BOD_5、N、SO_4~(2-)的变化,以及水力停留时间对处理效率的影响,对影响两相厌氧的主要因素讲行了分析。