Fault detection filter design for an anaerobic digestion process

In this paper, a Fault Detection and Isolation observer based method has been applied to biological wastewater treatment process. The method is designed with a dynamic model and the observer is determined using the eigenstructure assignment approach. The efficiency of the method is demonstrated for both detection and isolation of an actuator and a sensor failure using experimental data from a pilot scale anaerobic digestion process for the treatment of an industrial wine distillery vinasses.     

Hydrolysis,acidification and methanogenesis during low-temperature anaerobic digestion of dilute dairy wastewater in an inverted fluidised bioreactor

The application of low-temperature (10 °C) anaerobic digestion (LtAD) for the treatment of complex dairy-based wastewater in an inverted fluidised bed (IFB) reactor was investigated. Inadequate mixing intensity provoked poor hydrolysis of the substrate (mostly protein), which resulted in low chemical oxygen demand (COD) removal efficiency throughout the trial, averaging ~69 % at the best operational period. Overgrowth of the attached biomass to the support particles (Extendospheres) induced bed stratification by provoking agglutination of the particles and supporting their washout by sedimentation, which contributed to unstable bioprocess performance at the organic loading rates (OLRs) between 0.5 and 5 kg COD m^?3 day^?1. An applied OLR above 2 kg COD m^?3 day^?1 additionally promoted acidification and strongly influenced the microbial composition and dynamics. Hydrogenotrophic methanogens appeared to be the mostly affected group by the Extendospheres particle washout as a decrease in their abundance was observed by quantitative PCR analysis towards the end of the trial, although the specific methanogenic activity and maximum substrate utilisation rate on H₂/CO₂ indicated high metabolic activity and preference towards hydrogenotrophic methanogenesis of the reactor biomass at this stage. The bacterial community in the bioreactor monitored via denaturing gradient gel electrophoresis (DGGE) also suggested an influence of OLR stress on bacterial community structure and population dynamics. The data presented in this work can provide useful information in future optimisation of fluidised reactors intended for digestion of complex industrial wastewaters during LtAD.     

Thermodynamic equilibrium model in anaerobic digestion process

Catabolic reactions provide the chemical energy necessary for the maintenance of living microorganisms. The catabolic reactions in anaerobic digestion process may progress close to the equilibrium state (ΔG = 0) depending strongly on the microorganisms in the digester. The thermodynamic equilibrium of catabolic reactions in the anaerobic digestion process was modelled under isothermal and isobaric conditions. Three thermodynamic models were considered; the ideal, the Debye-Hückel–Praunitz, and the Pitzer–Praunitz. The models in this paper concentrate on the methanogenic equilibrium of the anaerobic digestion process. The thermodynamic equilibrium model shows that the methanogenesis step requires thermal energy and electrons, so that anaerobic digestion may achieve high substrate degradation and high conversion to methane. Some thermodynamic recommendations are suggested for the future development of the methanogenic phase of anaerobic digestion.     

Effects of solids retention time on methanogenesis in anaerobic digestion of thickened mixed sludge

When a bench-scale digester fed thickened mixed sludge was operated over an SRT range of 4-20 days, removal efficiencies for total chemical oxygen demand and volatile suspended solids declined with decreasing SRT (especially <10 days), but methanogenesis was stable for SRT as low as 5 days. Quantitative PCR analyses showed that methanogens declined steadily for SRT<10 days, with the acetate-cleaving Methanosaetaceae becoming more dominant. Clone-library analyses indicated significant shifts in bacterial population from 20 to 4 day SRT: declining Chloroflexi (28 to 4.5%) and Syntrophomonas (9 to 0%), but increasing Bacteroidetes (12.5 to 20%) and two acetogenic genera belonging to the phyla Firmicutes and Spirochaetales (6.3 to 12%). Thus, the decrease in the apparent hydrolysis constant (khyd-app) with higher SRT and the process limiting size of Methanosaetaceae with the lower SRT are proactive signs for defining rate limitation in anaerobic digestion.     

Advanced dynamical risk analysis for monitoring anaerobic digestion process

Methanogenic fermentation involves a natural ecosystem that can be used for waste water treatment. This anaerobic process can have two locally stable steady‐states and an unstable one making the process hard to handle. The aim of this work is to propose analytical criteria to detect hazardous working modes, namely situations where the system evolves towards the acidification of the plant. We first introduce a commonly used simplified model and recall its main properties. To assess the evolution of the system we study the phase plane and split it into nineteen zones according to some qualitative traits. Then a methodology is introduced to monitor in real‐time the trajectory of the system across these zones and determine its position in the plane. It leads to a dynamical risk index based on the analysis of the transitions from one zone to another, and generates a classification of the zones according to their dangerousness. Finally the proposed strategy is applied to a virtual process based on model ADM1. It is worth noting that the proposed approach do not rely on the value of the parameters and is thus very robust. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009     

Measurement of dissolved hydrogen in an anaerobic digestion process by a membrane-covered electrode

Dissolved hydrogen in an anaerobic digestion process was continuously measured by a voltammetric membrane electrode which consisted of a Pt-Pt black and Ag-AgCl covered FEP membrane with 0.1 M KCl and 0.1 M HCl. This sensor showed high reliability and sensitivity (i.e., detection limit: 50 nM) in distilled water. The sensor was not affected by several compounds in the anaerobic digestion media (e.g., inorganic salts, acetate, and propionate) except for sulfide. The indication in a sample containing 1.56 mM sulfide corresponded to that of 0.26 μM dissolved hydrogen. The sensor was also applied to measure the dissolved hydrogen in a laboratory-scale anaerobic reactor, and the dissolved hydrogen was continuously monitored for 565-h. The sensor was calibrated every 120-h, and the output signal was very stable during this period. The dissolved hydrogen concentration ranged from 0.5 to 3 μM, and H₂ partial pressure from 2 to 7 Pa in the gas phase. A good correlation (r = 0.85) between theoretical values calculated with H₂ partial pressure and the output signals was recognized. The actual dissolved hydrogen concentration was about 60-fold higher than the theoretical values calculated with H₂ partial pressure.     

Inhibition of anaerobic digestion process: a review

Anaerobic digestion is an attractive waste treatment practice in which both pollution control and energy recovery can be achieved. Many agricultural and industrial wastes are ideal candidates for anaerobic digestion because they contain high levels of easily biodegradable materials. Problems such as low methane yield and process instability are often encountered in anaerobic digestion, preventing this technique from being widely applied. A wide variety of inhibitory substances are the primary cause of anaerobic digester upset or failure since they are present in substantial concentrations in wastes. Considerable research efforts have been made to identify the mechanism and the controlling factors of inhibition. This review provides a detailed summary of the research conducted on the inhibition of anaerobic processes. The inhibitors commonly present in anaerobic digesters include ammonia, sulfide, light metal ions, heavy metals, and organics. Due to the difference in anaerobic inocula, waste composition, and experimental methods and conditions, literature results on inhibition caused by specific toxicants vary widely. Co-digestion with other waste, adaptation of microorganisms to inhibitory substances, and incorporation of methods to remove or counteract toxicants before anaerobic digestion can significantly improve the waste treatment efficiency.     

Hydrogen-dependent control of the continuous anaerobic digestion process

Summary Membrane inlet mass spectrometry was used to directly measure the concentrations of CH₄ and H₂ in a mesophilic (37°C) completely mixed, laboratory scale, anaerobic digester, continuously fed at a retention time of 7 days with a glucose (50 mM) mineral salts medium. When the digester was overloaded by an increase in the influent substrate concentration, equivalent to 15.5 kg (COD) m^-3 (digester) day^-1 the concentrations of H₂ and short chain fatty acids increased with a concomitant decline in the pH: following an initial stimulation methanogenesis was inhibited. Regulation of the H₂ signal from the mass spectrometer in a closed feedback loop by controlled addition of carbon source under a potential overload condition, enabled the H₂ concentration to the controlled around 1M and a high steady state rate of methanogenesis of 42 M min^-1 to be maintained; this is equivalent to 1.4 volumes of CH₄ per culture volume per day. The hydrogen-dependent control system was also used to prevent inhibition of methanogenesis when the digester was subject to volumetric overloading potentially equivalent to a retention time of 1 day.     

Adaptive identification of anaerobic digestion process for biogas production management systems

Bioprocess and Biosystems Engineering - To achieve the goals of sustainable development, supplies of renewable energy must be increased and methods of stable production developed. This study...     

Biodegradability and toxicity of styrene in the anaerobic digestion process

Start-up and operation of an Upflow Anaerobic Sludge Blanket (UASB) reactor fed with an industrial effluent from a polymer synthesis plant containing 6 mg styrene l^–1 was unstable. In batch assays with 200 mg styrene l^–1, 74% of styrene was degraded at a rate of 7 ml methane g^–1 volatile suspended solids.day, without a lag phase. The toxicity limit (IC₅₀) of styrene was 1.4 mM for the acetoclastic activity, 0.45 and 1.6 mM for the methanogenic activity in the presence of 30 mM of propionate and ethanol respectively. Instability of UASB operation was attributed to other compounds such as acrylates or detergents present in the industrial effluent.     

Fluorimetric monitoring of methanogenesis in anaerobic digestors

A new parameter, Q_CH4(F₄₂₀), is proposed to determine the potential methanogenic activity in the mixed microbial communities of anaerobic digestors. It is based on the particular fluorimetric properties of F420, a coenzyme common to and specific for methanogenic bacteria.     

The action of antibiotics on the anaerobic digestion process

Antibiotics can disturb the production of biogas during anaerobic digestion. This study shows a systematic approach to understanding how the different bacterial populations involved in the final conversion of organic matter into methane are inhibited by 15 antimicrobial agents with different specificities and modes of action. The results obtained show the following trends: (i) some inhibitors, such as the macrolide erythromycin, lack any inhibitory effect on biogas production; (ii) some antibiotics, with different specificities, have partial inhibitory effects on anaerobic digestion and decrease methane production by interfering with the activity of propionic-acid- and butyric-acid-degrading bacteria,␣(e.g. antibiotics that interfere with cell wall synthesis, RNA polymerase activity and protein synthesis, especially the aminoglycosides); (iii) the protein synthesis inhibitors chlortetracycline (IC₅₀ 40 mg l⁻¹) and chloramphenicol (IC₅₀ 15–20 mg l⁻¹) are very powerful inhibitors of anaerobic digestion. The majority of the antibiotics tested lacked activity against acetoclastic methanogens, being active only on the acetogenic bacteria. However, chloramphenicol and chlortetracycline could cause the complete inhibition of the acetoclastic methanogenic archaea. Received: 6 February 1996 / Received revision: 24 July 1996 / Accepted: 5 August 1996     

Zero valent iron as an electron-donor for methanogenesis and sulfate reduction in anaerobic sludge

Zero valent iron (ZVI) is a reactive media commonly utilized in permeable reactive barriers (PRBs). Sulfate reducing bacteria are being considered for the immobilization of heavy metals in PRBs. The purpose of this study was to evaluate the potential of ZVI as an electron donor for sulfate reduction in natural mixed anaerobic cultures. The ability of methanogens to utilize ZVI as an electron-donor was also explored since these microorganisms often compete with sulfate reducers for common substrates. Four grades of ZVI of different particle sizes (1.120, 0.149, 0.044, and 0.010 mm diameter) were compared as electron donor in batch bioassays inoculated with anaerobic bioreactor sludge. Methanogenesis was evaluated in mineral media lacking sulfate. Sulfate reduction was evaluated in mineral media containing sulfate and the specific methanogenic inhibitor, 2-bromoethane sulfonate. ZVI contributed to significant increases in methane production and sulfate reduction-compared to endogenous substrate controls. The rates of methane formation or sulfate reduction were positively correlated with the surface area of ZVI. The highest rates of 0.310 mmol CH4 formed/mol Fe0.day and 0.804 mmol SO4(2-) reduced/mol Fe0.day were obtained with the finest grade of ZVI (0.01 mm). The results demonstrate that ZVI is readily utilized as a slow-release electron donor for methanogenesis and sulfate reduction in anaerobic sludge; and therefore, has a promising potential in bioremediation applications.     

Semi-continuous anaerobic digestion of a food industry wastewater in an anaerobic filter

The experimental results of semi-continuous tests of anaerobic digestion of confectionery industry wastewater, carried out at different residence times and organic loads in an upflow anaerobic filter, are presented and discussed. Giving COD removals higher than 80% under the whole range of conditions tested, the anaerobic filter demonstrated not only a great ability of biomass to adapt itself to a new carbon source but also an excellent capability to deal with organic load fluctuations and to utilise dilute feeds. Sampling at different filter heights demonstrated that the biogas development ensured mixing within the filter and that most of the organic substances were utilised at the bottom of the reactor, especially when very dilute wastewater was fed. The results of this work could be taken as a starting basis for scaling-up the process to the industrial scale.     

Microbial fuel cell based biosensor for in situ monitoring of anaerobic digestion process

A wall-jet microbial fuel cell (MFC) was developed for the monitoring of anaerobic digestion (AD). This biofilm based MFC biosensor had a character of being portable, short hydraulic retention time (HRT) for sample flow through and convenient for continuous operation. The MFC was installed in the recirculation loop of an upflow anaerobic fixed-bed (UAFB) reactor in bench-scale where pH of the fermentation broth and biogas flow were monitored in real time. External disturbances to the AD were added on purpose by changing feedstock concentration, as well as process configuration. MFC signals had good correlations with online measurements (i.e. pH, gas flow rate) and offline analysis (i.e. COD) over 6-month operation. These results suggest that the MFC signal can reflect the dynamic variation of AD and can potentially be a valuable tool for monitoring and control of bioprocess.     

Dichloromethane utilized by an anaerobic mixed culture: acetogenesis and methanogenesis

Dichloromethane (8.9 mg/l) was eliminated from industrially polluted, anaerobic groundwater in a fixed-bed reactor (43 m³) which was packed with activated charcoal and operated continuously for over three years. The elimination of dichloromethane over this period was some ten-fold in excess of the sorptive capacity of the charcoal, and the elimination (3.7 mg/h·[kg of charcoal]: residence time, 49 h) was tentatively attributed to dehalogenative microorganisms immobilized on the charcoal. Anaerobic enrichment cultures, with dichloromethane as the sole added source of carbon and energy, were inoculated with material from the reactor. Reproducibly complete substrate disappearance in subcultures was observed when traces of groundwater (1%) or yeast extract (0.01%) were supplied. Fed-batch experiments under an atmosphere of CO₂ plus N₂ led to the conversion in 11 days of 11 mM dichloromethane to 3 mM acetate and 2 mM methane, with a growth yield of 0.4 g of protein/mol of dichloromethane; insignificant amounts (<1 M) of chloromethane accumulated. Methanogenesis could be inhibited by 50 mM 2-bromoethane sulfonate without any effect on the dehalogenation rate. The maximum dehalogenation rate was 0.13 mmol dichloromethane/h·l (2.6 mkat/kg of protein).Abbreviation DCM dichloromethane     

Inhibition of the anaerobic digestion process by linear alkylbenzene sulfonates

Linear Alkylbenzene Sulfonates (LAS) are the most widely used synthetic anionicsurfactants. They are anthropogenic, toxic compounds and are found in the primarysludge generated in municipal wastewater treatment plants. Primary sludge is usuallystabilized anaerobically and therefore it is important to investigate the effect of thesexenobiotic compounds on an anaerobic environment. The inhibitory effect of LinearAlkylbenzene Sulfonates (LAS) on the acetogenic and methanogenic step of theanaerobic digestion process was studied. LAS inhibit both acetogenesis from propionate and methanogenesis from acetate and hydrogen and it is shown that the propionate-utilising bacteria are more sensitive to the presence of LAS than the acetoclastic methanogens. It has been proven that the inhibition intensity depends on the solids concentration and thus the term ``biomass specific LAS concentration' has been introduced in order to describe the phenomenon better. Conclusively, it is believed that the inhibitory effect of LAS is the main reason that anaerobic microbial enrichments on LAS have not been succeeded yet. Also, the inhibition caused by LAS on the acetogenic and methanogenic step of the anaerobic digestion process should be seriously taken into consideration when wastewater from a surfactant producing industry is to be treated biologically or enter a municipal wastewater treatment plant that employs anaerobic technology. The upper allowable biomass specific LAS concentration should be 14 mg LAS (gVSS)^-1.     

Mixing in anaerobic digestion

The mixing of the anaerobic digester contents significantly influences the efficiency of this operation; in particular, hydraulic dead zones are extremely detrimental to the reaction kinetics involved in anaerobic digestion. An analysis of the relative importance of thermal fluid movement in the digester to those caused by fluid inflow and outflow is presented. As an example, these principles are applied to a digester at the South Bend Wastewater Treatment Plant. Experimental measurements, which have general applicability for the measurement of digester mixing volume, confirm the theoretical conjectures. Various types of optimizations can be attempted on this mixing operation. One such optimization applied to gas lift mixers, as employed in the South Bend Treatment Plant, is illustrated.     

Low temperature thermo-chemical pretreatment of dairy waste activated sludge for anaerobic digestion process

An investigation into the influence of low temperature thermo-chemical pretreatment on sludge reduction in a semi-continuous anaerobic reactor was performed. Firstly, effect of sludge pretreatment was evaluated by COD solubilization, suspended solids reduction and biogas production. At optimized condition (60 °C with pH 12), COD solubilization, suspended solids, reduction and biogas production was 23%, 22% and 51% higher than the control, respectively. Secondly, semi-continuous process performance was studied in a lab-scale semi-continuous anaerobic reactor (5 L), with 4 L working volume. With three operated SRTs, the SRT of 15 days was found to be most appropriate for economic operation of the reactor. Combining pretreatment with anaerobic digestion led to 80.5%, 117% and 90.4% of TS, SS and VS reduction respectively, with an improvement of 103% in biogas production. Thus, low temperature thermo-chemical can play an important role in reducing sludge production.