Limitations of thermophilic anaerobic wastewater treatment and the consequences for process design

Thermophilic anaerobic digestion offers an attractive alternative for the treatment of medium- and high-strength wastewaters. However, literature reports reveal that thermophilic wastewater treatment systems are often more sensitive to environmental changes than the well-defined high-rate reactors at the mesophilic temperature range. Also, in many cases a poorer effluent quality is experienced while the carry over of suspended solids in the effluent is relatively high. In this paper recent achievements are discussed regarding the process stability of thermophilic anaerobic wastewater treatment systems. Laboratory experiments reveal a relatively low sensitivity to temperature changes if high-rate reactors with immobilized biomass are used. Other results show that if a staged process is applied, thermophilic reactors can be operated for prolonged periods of time under extreme loading conditions (80–100 kg chemical oxygen demand.m^-3.day^-1), while the concentrations of volatile fatty acids in the effluent remain at a low level.     

Mathematical Modelling of Anaerobic Reactors Treating Domestic Wastewater: Rational Criteria for Model Use

Anaerobic digestion modelling is an established method for assessing anaerobic wastewater treatment for design, systems analysis, operational analysis, and control. Anaerobic treatment of domestic wastewater is a relatively new, but rapidly maturing technology, especially in developing countries, where the combination of low cost, and moderate-good performance are particularly attractive. The key emerging technology is high-rate anaerobic treatment, particularly UASB reactors. Systems modelling can potentially offer a number of advantages to this field, and the key motivations for modelling have been identified as operational analysis, technology development, and model-based design. Design is particularly important, as it determines capital cost, a key motivation for implementers. Published modelling studies for anaerobic domestic sewage treatment are limited in number, but well directed at specific issues. Most have a low structural complexity, with first order kinetics, as compared to the more commonly used Monod kinetics. This review addresses the use of anaerobic models in general, application of models to domestic sewage systems, and evaluates future requirements for models that need to address the key motivations of operational analysis, technology development, and model-based design. For operational analysis and technology development, a complex model such as the ADM1 is recommended, with further extensions as required to address factors such as sulphate reduction. For design, the critical issSues are hydraulics and particles (i.e., biomass and solid substrate) modelling. Therefore, the kinetic structure should be relatively simple (at least two-step), but the hydraulic and particulate model should be relatively complex.     

Anaerobic dechlorination of pentachlorophenol in fixed-film and upflow anaerobic sludge blanket reactors using different inocula

Longterm performance and stability of two upflow anaerobic sludge blanket (UASB) reactors inoculated with granular sludge and treating a synthetic waste water containing pentachlorophenol (PCP) and phenol were studied. A similar system consisting of two fixed-film reactors inoculated with anaerobic digested sewage sludge were further studied. One reactor in each series received glucose in addition to the phenols. Dechlorination of PCP proceeded via two different dominating pathways in the respective reactor systems, suggesting that two distinct microbial populations were present, probably originating from the different inocula. Dechlorinating activity was maintained for more than 18 months in the UASB reactors and was generally higher than in the fixed-film reactors. In the fixed-film reactors, dechlorination of PCP suddenly decreased after 15.5 months of operation compared to earlier performance. Since no operational parameters had been changed, this indicated that the enriched culture was unstable on a longterm basis. Addition of yeast extract to the medium restored activity. General process stability in both reactor systems was clearly enhanced by the addition of glucose and was superior in the UASB/granular sludge system. The better performance and the higher stability in the UASB/granular sludge reactor highlights the importance of thorough screening of inocular prior to start-up of processes treating waste waters containing xenobiotic compounds.Abbreviations PCP pentachlorophenol - TeCP tetrachlorophenol - TCP trichlorophenol - DCP dichlorophenol - UASB upflow anaerobic sludge blanket - HRT hydraulic retention time     

厌氧反应器中颗粒污泥的培育及应用研究进展*

厌氧生物处理技术因其具有有机负荷高、污泥产量低、能耗低等优点被广泛应用于各种废水处理中。厌氧颗粒污泥具有沉降性能好、微生物浓度高、有机负荷高等优点,极大地提高了废水处理效率。尤其在处理含高氨氮废水中,厌氧颗粒污泥的形成对反应器的高效生物脱氮至关重要。但到目前为止,厌氧反应器中的颗粒污泥形成及废水处理效果还缺乏系统的认识。鉴于此,总结了厌氧反应器中颗粒污泥的形成机制,分析了影响厌氧反应器中颗粒污泥形成的因素,论述了厌氧反应器中厌氧颗粒污泥生长的模拟,最后介绍了厌氧颗粒污泥在国内外的主流应用。厌氧反应器中颗粒污泥的形成是综合因素影响的结果,对影响厌氧颗粒污泥形成的每个因素都需要认真对待,可为在厌氧反应器中颗粒污泥的培育和应用提供理论指导和技术支撑。     

Performance of staged and non-staged up-flow anaerobic sludge bed (USSB and UASB) reactors treating low strength complex wastewater

The use of anaerobic processes to treat low-strength wastewater has been increasing in recent years due to their favourable performance-costs balance. For optimal results, it is necessary to identify reactor configurations that are best suited for this kind of application. This paper reports on the comparative study carried out with two high-rate anaerobic reactor systems with the objective of evaluating their performances when used for the treatment of low-strength, complex wastewater. One of the systems is the commonly used up-flow anaerobic sludge blanket (UASB) reactor. The other is the up-flow staged sludge bed (USSB) system in which the reactor was divided longitudinally into 3, 5 and 7 compartments by the use of baffles. The reactors (9 l) were fed with a synthetic, soluble and colloidal waste (chemical oxygen demand (COD) < 1000 mg/l) and operated at 28°C and 24 h hydraulic retention time. Intermediate flow hydraulics, between plug-flow and completely-mixed, in the UASB and 7 stages USSB reactors allowed efficient degradation of substrates with minimum effluent concentrations. Low number of compartments in the USSB reactors increased the levels of short-circuiting thus reducing substrate removal efficiencies. All reactors showed high COD removal efficiencies (93–98%) and thus can be regarded as suitable for the treatment of low strength, complex wastewater. Staged anaerobic reactors can be a good alternative for this kind of application provided they are fitted with a large enough (≥7) number of compartments to fully take advantage of their strengths. Scale factors seem to have influenced importantly on the comparison between one and multi staged sludge-bed reactors and, therefore, observations made here could change at larger reactor volumes.     

Mathematical modeling of non-ideal mixing continuous flow reactors for anaerobic digestion of cattle manure

Most conventional digesters used for animal wastewater treatment include continuously stirred-tank reactors. While imperfect mixing patterns are more common than ideal ones in real reactors, anaerobic digestion models often assume complete mixing conditions. Therefore, their applicability appears to be limited. In this study, a mathematical model for anaerobic digestion of cattle manure was developed to describe the dynamic behavior of non-ideal mixing continuous flow reactors. The microbial kinetic model includes an enzymatic hydrolysis step and four microbial growth steps, together with the effects of substrate inhibition, pH and thermodynamic considerations. The biokinetic expressions were linked to a simple two-region liquid mixing model, which considered the reactor volume in two separate sections, the flow-through and the retention regions. Deviations from an ideal completely mixed regime were represented by changing the relative volume of the flow-through region (a) and the ratio of the internal exchange flow rate to the feed flow rate (b). The effects of the hydraulic retention time, the composition of feed, the initial conditions of the reactor and the degree of mixing on process performance can be evaluated by the dynamic model. The simulation results under different conditions showed that deviations from the ideal mixing regime decreased the methane yield and resulted in a reduced performance of the anaerobic reactors. The evaluation of the impact of the characteristic mixing parameters (a) and (b) on the anaerobic digestion of cattle manure showed that both liquid mixing parameters had significant effects on reactor performance.     

Enhancement of Anaerobic Digestion Using Particulate Growth Systems

Attached media reactors are used for enhancement of wastewater treatment processes including anaerobic condition. Selection of a suitable biofilm carrier is a compelling method to improve anaerobic digestion systems. This study investigates the performance of four fibrous biofilms installed in batch biogas reactors for treatment of cow manure. BioCords HS1, HS2, LS1, and LS2 are manufactured by Bishop Water Technologies, ON, Canada. Effluents and attached growth media were analyzed after batch experiment; methane production, methane yield, transfer efficiencies, organic and solid removal efficiencies, pH, and attached volatile suspended solid (VSS) were measured; VSS attached to biofilms mainly correlated with the specific surface area of each biofilm. Additionally, SEM (scanning electron microscopy) was used for further understanding of biofilm formation process for BioCords and the dissimilarity in their performance. The results indicated that BioCord LS2 had positive impact on achieving higher methane production and removal efficiencies compared to other support media utilized in batch reactors. It was also demonstrated from the experiment that BioCord LS2 potentially could generate higher methane production than conventional batch bioreactor.     

Low-dissolved-oxygen nitrifying systems exploit ammonia-oxidizing bacteria with unusually high yields

In wastewater treatment plants, nitrifying systems are usually operated with elevated levels of aeration to avoid nitrification failures. This approach contributes significantly to operational costs and the carbon footprint of nitrifying wastewater treatment processes. In this study, we tested the effect of aeration rate on nitrification by correlating ammonia oxidation rates with the structure of the ammonia-oxidizing bacterial (AOB) community and AOB abundance in four parallel continuous-flow reactors operated for 43 days. Two of the reactors were supplied with a constant airflow rate of 0.1 liter/min, while in the other two units the airflow rate was fixed at 4 liters/min. Complete nitrification was achieved in all configurations, though the dissolved oxygen (DO) concentration was only 0.5 ± 0.3 mg/liter in the low-aeration units. The data suggest that efficient performance in the low-DO units resulted from elevated AOB levels in the reactors and/or putative development of a mixotrophic AOB community. Denaturing gel electrophoresis and cloning of AOB 16S rRNA gene fragments followed by sequencing revealed that the AOB community in the low-DO systems was a subset of the community in the high-DO systems. However, in both configurations the dominant species belonged to the Nitrosomonas oligotropha lineage. Overall, the results demonstrated that complete nitrification can be achieved at low aeration in lab-scale reactors. If these findings could be extended to full-scale plants, it would be possible to minimize the operational costs and greenhouse gas emissions without risk of nitrification failure.     

Carbon balance of anaerobic granulation process: carbon credit

The concept of carbon credit arose out of increasing awareness of the need to reduce emissions of greenhouse gases to combat global warming which was formalized in the Kyoto protocol. In addition to contribution to sustainable development with energy recovery in the form of methane, carbon credits can be claimed by application of advanced anaerobic processes in wastewater treatment for reducing emissions of greenhouse gases. As anaerobic granular systems are capable of handling high organic loadings concomitant with high strength wastewater and short hydraulic retention time, they could render much more carbon credits than other conventional anaerobic systems. This study investigated the potential carbon credit derived from laboratory-scale upflow anaerobic sludge blanket (UASB) reactors based on a carbon balance analysis. Methane emission reduction could be calculated by calculating the difference of UASB reactors and open lagoon treatment systems. Based on the 2.5l bench-scale reactor, the total CH(4) emissions reduction was calculated as 29 kg CO(2)/year. On scaling up to a typical full-scale anaerobic digester, the total CH(4) emissions reduction could achieve 46,420 tons CO(2) reduction/year. The estimated carbon credits would amount to 278,500 US$ per year by assuming a carbon price of 6 US$ per metric ton CO(2) reduction. The analysis postulated that it is financially viable to invest in advanced anaerobic granular treatment system from the revenue generated from carbon credits.     

Anaerobic granulation technology for wastewater treatment

Anaerobic wastewater treatment using granular sludge reactors is a developing technology, in which granular sludge is the core component. So far, around 900 anaerobic granular sludge units have been operated worldwide. Although intensive research attention has been given to anaerobic granules in the past 20 years, the mechanisms responsible for anaerobic granulation and the strategy of how to expedite substantially the formation of granular sludge have not yet been completely clear. This paper reviews the mode of anaerobic granulation, including the mechanisms and models for anaerobic granulation, major factors influencing anaerobic granulation, characteristics of anaerobic granules, anaerobic granulation in other types of reactors, industrial application of anaerobic granulation technology and neural fuzzy model-based control strategy developed for anaerobic systems. Some approaches for future research are outlined.     

Effect of the feed frequency on the performance of anaerobic filters

Anaerobic filter technology is suitable for the treatment of biodegradable organic wastes: for example, wastewaters from food industries. These wastewaters are not produced in a continuous mode since they are of seasonal nature and hence their production varies considerably during the year. In this work, a study of the performance of anaerobic filters with changing feeding systems was undertaken. A comparison was made between continuous feeding and different semicontinuous modes of feeding (in which the overall volume to be added into the reactors was divided into several doses and each of them were added at a constant interval of time). Filter performance was characterized by determining the different operational variables: depurative efficiency, methane production, biogas composition and volatile acids. From the obtained results, we conclude that the optimum feed frequency range is 24 doses/day or more. The continuously fed system has both greater stability and degradation efficiency.     

Anaerobic Reactor Design Concepts for the Treatment of Domestic Wastewater

Since the earlier anaerobic treatment systems, the design concepts were improved from classic reactors like septic tanks and anaerobic ponds, to modern high rate reactor configurations like anaerobic filters, UASB, EGSB, fixed film fluidized bed and expanded bed reactors, and others. In this paper, anaerobic reactors are evaluated considering the historical evolution and types of wastewaters. The emphasis is on the potential for application in domestic sewage treatment, particularly in regions with a hot climate. Proper design and operation can result in a high capacity and efficiency of organic matter removal using single anaerobic reactors. Performance comparison of anaerobic treatment systems is presented based mostly on a single but practical parameter, the hydraulic retention time. Combined anaerobic reactor systems as well as combined anaerobic and non-anaerobic systems are also presented.     

Partial characterization of the polluting load of swine wastewater treated with an integrated biodigestion system

The stabilization of swine wastewaters from swine confined housing by the combination of a upflow anaerobic sludge blanket (UASB) reactor and waste stabilization ponds is a viable alternative to minimize the environmental impact caused by inadequate disposal of swine wastewaters. In the present study, the polluting load of pre-decanted swine wastewater treated with a series of two 0.705 m(3) UASB reactors and then in parallel in aerated and non-aerated stabilization tanks was investigated from January to July, 2000. Physicochemical and microbiological analyses were made adopting standard methods (Standard Methods for Examination of Water and Wastewater, 19th ed., American Public Health Association, Washington, DC, 1995). COD values decreased as the wastewater ran through the integrated biodigestion system dropping from about 3492+/-511-4094 mgl(-1)+/-481 to 124+/-52-490 mgl(-1)+/-230, while nitrate and nitrite levels increased in stabilization tanks, ranging respectively from 4+/-0 to 20 mgl(-1)+/-3 and 3+/-1 to 11 mgl(-1)+/-24. Although the removal of Escherichia coli was more than 97%+/-6, the effluents of the treatment system still contained unacceptable levels of E. coli (1.6 x 10(3)-1.2 x 10(6) 100 ml(-1)) according to WHO guidelines for use of wastewater in agriculture and aquaculture. These results indicate the necessity of changes on operational characteristics of the treatment system such as an increase of the hydraulic retention time in UASB reactors or in stabilization tanks.     

Anaerobic Granular Sludge Bioreactor Technology

Anaerobic digestion is a mature wastewater treatment technology, with worldwide application. The predominantly applied bioreactor designs, such as the upflow anaerobic sludge blanket and expanded granular sludge bed, are based on the spontaneous formation of granular sludge. Despite the exploitation of granular reactors at full-scale for more than two decades, the mechanisms of granulation are not completely understood and numerous theories have been put forward to describe the process from a biological, ecological and engineering point of view. New technological opportunities are emerging for anaerobic digestion, aided by an improved understanding of microbiological and environmental factors affecting the formation and activity of anaerobic granular sludge.     

Treatment of low strength soluble wastewaters in UASB reactors

Low strength wastewaters can be those with chemical oxygen demand (COD) below 2,000 mg/l. The anaerobic treatment of such wastewaters has not been fully explored so far. The suboptimal reaction rates with low substrate concentrations, and the presence of dissolved oxygen in the influent are regarded as possible constraints. In this study, the treatment of low strength soluble wastewaters containing ethanol or whey was studied in lab-scale upflow anaerobic sludged bed (UASB) reactors at 30°C. The high treatment performance obtained demonstrates that UASB reactors are viable for treating both types of wastewaters at low COD concentrations. The treatment of the ethanol containing wastewater resulted in COD removal efficiencies exceeding 95% at organic loading rates (OLR) between 0.3 to 6.8 g COD/l-d with influent concentrations in the range of 422 to 943 mg COD/l. In the case of the more complex whey containing wastewater, COD removal efficiencies exceeded 86% at OLRs up to 3.9 g COD/l·, as long as the COD influent was above 630 mg/l. Lowering the COD influent resulted in decreased efficiency with sharper decrease at values below 200 mg/l. Acidification instead of methanogenesis was found to be the rate limiting step in the COD removal at low concentrations, which was not the case when treating ethanol. The effect of dissolved oxygen in the influent as a potential danger in anaerobic treatment was investigated in reactors fed with and without dissolved oxygen. Compared with the control reactor, the reactor receiving oxygen showed no detrimental effects in the treatment performance. Thus, the presence of dissolved oxygen in dilute wastewaters is expected to be of minor importance in practice.     

Enhancement of microbiome management by machine learning for biological wastewater treatment

Here, we propose to develop microbiome-based machine learning models to predict the response of biological wastewater treatment systems to environmental or operational disturbances or to design specific microbiomes to achieve a desired system function. These machine learning models can be used to enhance the stability of microbiome-based biological systems and warn against the failure of these systems.     

Assessment of microbial community structure changes by amplified ribosomal DNA restriction analysis (ARDRA).

Amplified ribosomal DNA restriction analysis (ARDRA) is a simple method based on restriction endonuclease digestion of the amplified bacterial 16S rDNA. In this study we have evaluated the suitability of this method to detect differences in activated sludge bacterial communities fed on domestic or industrial wastewater, and subject to different operational conditions. The ability of ARDRA to detect these differences has been tested in modified Ludzack-Ettinger (MLE) configurations. Samples from three activated sludge wastewater treatment plants (WWTPs) with the MLE configuration were collected for both oxic and anoxic reactors, and ARDRA patterns using double enzyme digestions AluI+MspI were obtained. A matrix of Dice similarity coefficients was calculated and used to compare these restriction patterns. Differences in the community structure due to influent characteristics and temperature could be observed, but not between the oxic and anoxic reactors of each of the three MLE configurations. Other possible applications of ARDRA for detecting and monitoring changes in activated sludge systems are also discussed.     

Anaerobic co-digestion of table olive debittering & washing effluent, cattle manure and pig manure in batch and high volume laboratory anaerobic digesters: effect of temperature

The prospective of table olive debittering & washing Effluent (DWE) as feed stock wastewater for anaerobic digestion (AD) systems was investigated in batch and continuous systems together with cattle and pig manures. While DWE considered unsuitable for biological treatment methods due to its unbalanced nature, the co-digestion of the wastewaters resulted in a 50% increase in the methane production/gram volatile solids_added (CH₄/gVS_added), accompanied by 30% phenol reduction and 80% total organic carbon removal (TOC). pH increase during the co-digestion period was not identified as an inhibitory factor and all reactors were able to withstand this operational condition change. Moreover, no volatile fatty acid (VFA) accumulation was observed, indicating that the reactors were not operating under stress-overloading state. Under thermophilic conditions a 7% increase on the TOC removal efficiency was achieved when compared to the mesophilic systems while, under mesophilic conditions phenolic compounds reduction was 10% higher compared to the thermophilic systems.     

Anaerobic treatment of winery wastewater in fixed bed reactors

The treatment of winery wastewater in three upflow anaerobic fixed-bed reactors (S9, S30 and S40) with low density floating supports of varying size and specific surface area was investigated. A maximum OLR of 42 g/l day with 80 ± 0.5% removal efficiency was attained in S9, which had supports with the highest specific surface area. It was found that the efficiency of the reactors increased with decrease in size and increase in specific surface area of the support media. Total biomass accumulation in the reactors was also found to vary as a function of specific surface area and size of the support medium. The Stover–Kincannon kinetic model predicted satisfactorily the performance of the reactors. The maximum removal rate constant (U _max) was 161.3, 99.0 and 77.5 g/l day and the saturation value constant (K _B) was 162.0, 99.5 and 78.0 g/l day for S9, S30 and S40, respectively. Due to their higher biomass retention potential, the supports used in this study offer great promise as media in anaerobic fixed bed reactors. Anaerobic fixed-bed reactors with these supports can be applied as high-rate systems for the treatment of large volumes of wastewaters typically containing readily biodegradable organics, such as the winery wastewater.     

Controlled shear affinity filtration (CSAF): a new technology for integration of cell separation and protein isolation from mammalian cell cultures

Up-flow anaerobic sludge blanket (UASB) reactors are being used with increasing regularity all over the world, especially in India, for a variety of wastewater treatment operations. Consequently, there is a need to develop methodologies enabling one to determine UASB reactor performance, not only for designing more efficient UASB reactors but also for predicting the performance of existing reactors under various conditions of influent wastewater flows and characteristics. This work explores the feasibility of application of an artificial neural network-based model for simulating the performance of an existing UASB reactor. Accordingly, a neural network model was designed and trained to predict the steady-state performance of a UASB reactor treating high-strength (unrefined sugar based) wastewater. The model inputs were organic loading rate, hydraulic retention time, and influent bicarbonate alkalinity. The output variables were one or more of the following, effluent substrate concentration (Se), reactor bicarbonate alkalinity, reactor pH, reactor volatile fatty acid concentration, average gas production rate, and percent methane content of the gas. Training of the neural network model was achieved using a large amount of experimentally obtained reactor performance data from the reactor mentioned above as the training set. Training was followed by validation using independent sets of performance data obtained from the same UASB reactor. Subsequently, simulations were performed using the validated neural network model to determine the impact of changes in parameters like influent chemical oxygen demand (COD) concentration and hydraulic retention time on the reactor performance. Simulation results thus obtained were carefully analyzed based on qualitative understanding of UASB process and were found to provide important insights into key variables that were responsible for influencing the working of the UASB reactor under varying input conditions.