Digester Failure Research Articles

Combined amperometric/field-effect sensor for the detection of dissolved hydrogen

Real-time and reliable monitoring of the biogas process is crucial for a stable and efficient operation of biogas production in order to avoid digester breakdowns. The concentration of dissolved hydrogen (H2) represents one of the key parameters for biogas process control. In this work, a one-chip integrated combined amperometric/field-effect sensor for monitoring the dissolved H2 concentration has been developed for biogas applications. The combination of two different transducer principles might allow a more accurate and reliable measurement of dissolved H2 as an early warning indicator of digester failures. The feasibility of the approach has been demonstrated by simultaneous amperometric/field-effect measurements of dissolved H2 concentrations in electrolyte solutions. Both, the amperometric and the field-effect transducer show a linear response behaviour in the H2 concentration range from 0.1 to 3% (v/v) with a slope of 198.4±13.7nA/% (v/v) and 14.9±0.5mV/% (v/v), respectively.

Automatic Carbon Dioxide-Methane Gas Sensor Based on the Solubility of Gases in Water

Biogas methane content is a relevant variable in anaerobic digestion processing where knowledge of process kinetics or an early indicator of digester failure is needed. The contribution of this work is the development of a novel, simple and low cost automatic carbon dioxide-methane gas sensor based on the solubility of gases in water as the precursor of a sensor for biogas quality monitoring. The device described in this work was used for determining the composition of binary mixtures, such as carbon dioxide-methane, in the range of 0–100%. The design and implementation of a digital signal processor and control system into a low-cost Field Programmable Gate Array (FPGA) platform has permitted the successful application of data acquisition, data distribution and digital data processing, making the construction of a standalone carbon dioxide-methane gas sensor possible.

On the Nature of Tintinnid Loricae (Ciliophora: Spirotricha: Tintinnina): a Histochemical, Enzymatic, EDX, and High-resolution TEM Study.

Tintinnids (Ciliophora: Spirotricha: Tintinnina) are occasionally the dominant ciliates in the marine plankton. The tintinnid loricae are minute artworks fascinating scientists for more than 230 years, but their chemical composition remained unclear, viz., chitinous or proteinaceous substances were discussed. Since sedimenting loricae contribute to the flux of elements and organic compounds in the oceans, knowledge about their nature is necessary in assessing their ecological role. Previous techniques and new methods, e.g. enzymatic digestion and high-resolution transmission electron microscopy, are applied in the present study. A chitinous nature of the loricae is rejected by the Van-Wisselingh test and failure of chitinase digestion. Only proteins might show a resistance against strong hot bases (KOH at 160°C for ~ 40 min. in tintinnid loricae) similar to that of chitin. Actually, the presence of nitrogen in the EDX analyses and the digestion of at least some loricae by proteinase K strongly indicate a proteinaceous nature. Furthermore, the crystal lattice revealed by high-resolution TEM in Eutintinnus loricae is similar to the proteinaceous surface layer (S-layer) of archaea, and the striation recognizable in transverse sections of Eutintinnus loricae has a periodicity resembling that of the crystalline proteins in the extruded trichocysts of Paramecium and Frontonia. The proteolytic resistance of some loricae does not reject the idea of a proteinaceous nature, as proteins in S-layers of some archaea and in most naturally occurring prions show comparable reactions. The data from the present study and the literature indicate proteins in the loricae of thirteen genera. Differences in the proteolytic resistance and staining properties between genera and congeners are probably due to deviations in the protein composition and the additional substances, e.g. lipids, carbohydrates. At the present state of knowledge, correlations between lorica structure, wall texture, ultrastructure of the lorica forming granules, and the histochemical and enzymatic findings are not evident. Therefore, further studies are required to estimate the taxonomic significance of these features and the ecological role of sedimenting loricae.

Long-term anaerobic digestion of food waste stabilized by trace elements

The purpose of this study was to examine if long-term anaerobic digestion of food waste in a semi-continuous single-stage reactor could be stabilized by supplementing trace elements. Contrary to the failure of anaerobic digestion of food waste alone, stable anaerobic digestion of food waste was achieved for 368days by supplementing trace elements. Under the conditions of OLR (organic loading rates) of 2.19–6.64g VS (volatile solid)/Lday and 20–30days of HRT (hydraulic retention time), a high methane yield (352–450mLCH4/gVSadded) was obtained, and no significant accumulation of volatile fatty acids was observed. The subsequent investigation on effects of individual trace elements (Co, Fe, Mo and Ni) showed that iron was essential for maintaining stable methane production. These results proved that the food waste used in this study was deficient in trace elements.

Essential Role of Trace Elements in Continuous Anaerobic Digestion of Food Waste

The purpose of this study is to explore the possibility of long-term sustainable anaerobic digestion of food waste in semi-continuous single-stage reactors by supplementing trace elements. Compared with the failure of anaerobic digestion of food waste after prolonged operation, a clearly enhancement of process performance was observed by supplementing a model trace element solution. Although the sustainable continuous anaerobic digestion of food waste could not be achieved by supplementing a model trace element solution, the correlation of process performance and trace element profile during continuous operating period revealed that the declining performance was highly likely due to the decreasing trace element concentrations, especially Co, Mo, Ni and Fe. This finding was expected to provide a promising strategy for sustainable anaerobic digestion of food waste.

Anaerobic digestion modeling: from one to several bacterial populations

Los sistemas de digestión anaeróbica son procesos complejos que desafortunadamente a menudo sufren de inestabilidad. A fin de diseñar, optimizar y operar eficientemente estos sistemas es necesario desarrollar estrategias apropiadas de control. Estas estrategias requieren, en general del desarrollo de modelos matemáticos. El proceso de digestión anaerobia comprende una red compleja de reacciones secuenciales y paralelas de naturaleza bioquímica y fisicoquímica. Generalmente, estas reacciones contienen un paso en particular, denominado reacción-limitante el cual siendo el más lento, limita la tasa de reacción del proceso global. Los primeros intentos del modelado de la digestión anaerobia condujeron a modelos que describen este paso limitante. Sin embargo, en una amplia gama de condiciones de funcionamiento, el paso limitante no siempre es el mismo. Este puede depender de las características de las aguas residuales, de la carga hidráulica, de la temperatura, etc. Es evidente que la "hipótesis del paso limitante" conduce a modelos simples y fácilmente utilizables. Sin embargo, estos modelos no describen muy bien el comportamiento del digestor, especialmente en condiciones transitorias de funcionamiento. Este trabajo revisa el estado del arte en modelización de la digestión anaerobia. Damos una breve descripción de los modelos clave de digestión anaerobia que se han desarrollado hasta el momento para describir sistemas con crecimiento de biomasa, incluyendo el modelo de digestión anaerobia No 1 (ADM1) de la Asociación Internacional del Agua (IWA) e identificamos las áreas que requieren esfuerzos futuros de investigación.

Ammonia stripping for enhanced biomethanization of piggery wastewater

In this study, the effects of ammonia removal by air stripping as a pretreatment on the anaerobic digestion of piggery wastewater were investigated. Ammonia stripping results indicated that ammonia removal was strongly dependent on pH and aeration rate, and the ammonia removal rate followed the pseudo-first-order kinetics. A significant enhancement of biomethanization was observed for wastewaters of which ammonia was air-stripped at pH 9.5 and pH 10.0. The methane productivity increased from 0.23 ± 0.08 L CH 4/L d of the control (raw piggery wastewater) to 0.75 ± 0.11 L CH 4/L d (ammonia-stripped at pH 9.5) and 0.57 ± 0.04 L CH 4/L d (ammonia-stripped at pH 10.0). However, the improvement of methane production from the piggery wastewater pretreated at pH 11.0 was negligible compared to the control, which was thought to be due to the high concentration of sodium ions supplied from sodium hydroxide for pH adjustment. From these results, it was concluded that ammonia removal through air stripping at the alkaline pH could be a viable option for preventing the failure of anaerobic digestion of the raw piggery wastewater. Additionally, it was also found that a high concentration of sodium ion originated from sodium hydroxide for pH adjustment inhibited methane production.

Response of lab-scale methanogenic reactors inoculated from different sources to organic loading rate shocks

Anaerobic digester failure due to a pH drop may be overcome with the use of an acidotolerant methanogenic community. To test this, lab-scale reactors were inoculated from acidic bog sediments, a municipal sludge digester, or a combination of these inocula and challenged with glucose pulses without pH control. Only the bog reactor survived the first glucose shock, and the methanogen community was dominated by members of the acidic Fen Cluster. After restarting the digester and hybrid reactors, two subsequent glucose shocks were applied. Methanogenic communities converged in all reactors and were dominated by Methanosarcina and Methanobacteriaceae. The Fen Cluster was eventually nondetectable in bog and hybrid reactors, presumably due to periods of circumneutral pH with only intermittent periods of low pH following glucose shocks. Although the resultant communities required base addition, an increase in Methanosarcina numbers after glucose pulses resulted in decreased acetate and increased reactor pH and methane production.

Methane conversion efficiency as a simple control parameter for an anaerobic digester at high loading rates

The anaerobic digestion process is globally applied to the treatment of highly concentrated wastes such as industrial and rural effluents, and sewage sludge. However, it is known to be relatively unstable. When loaded with high concentrations of organic material, unwanted volatile fatty acids (VFA) are often produced rather than methane (CH4) gas which can lead to digester acidification and failure. This study investigated digester behaviour under high loading rates, testing the usefulness of stoichiometric methane conversion efficiency as a digester control parameter at high loading rates. Our results show that, in general, the CH4 production rate was proportional to the feed rate (loading rate). However, at very high loading rates, the CH4 production rate was not proportional to the increase in the feeding rate. Consequently, VFA accumulated and the H2 partial pressure increased. The proportionality of the loading rate and gas production rate is stoichiometrically expressed as the conversion efficiency. We found that conversion efficiency was a useful indicator as an early warning of digester imbalance. The digester remained stable at conversion efficiencies above 75%. Dropping below 70% signified the onset of digester failure. As loading rate and methane production data are readily available on-line in most anaerobic digestion plants, the conversion efficiency can be monitored on-line and used as an efficient control technique to maintain safe operation of anaerobic digesters at high loading rates.

Co-Digestion of Hog Manure with Glycerol to Boost Biogas and Methane Production

The use of off-farm materials as amendments in anaerobic digestion of manure is an option that is being explored more extensively due to the benefits of boosting methane production and making the process more economical for the farmer. The addition of varying amounts of glycerol, which is a by-product in the biodiesel industry, was used as an amendment to anaerobic digestion of hog manure in bench-scale tests. The use of 2% glycerol produced the greatest amount of methane and biogas, while the addition of 4% glycerol resulted in an overloading of chemical oxygen demand (COD) and digester failure. The addition of 1% glycerol doubled the methane and biogas production, and there appeared to be no detrimental effects of using crude, industrial-based glycerol compared to pure, chemical-grade glycerol.

Automatic volumetric gas flow meter for monitoring biogas production from laboratory-scale anaerobic digester

Biogas rate production is a relevant variable in anaerobic digestion processing, where, an indicator of volatile solids conversion rate or an earlier indicator of digester failure is needed. Reconfigurable devices like Field Programmable Gate Arrays (FPGAs) have advantages over microprocessors and DSPs with similar logic complexity because their open architecture makes them suitable for system on-a-chip applications. The contribution of this work is the development of an improved water displacement gas flow meter, intended to be used in laboratory-scale anaerobic digestion reactors. Results show automatic gas flow meter functionality, where its calibration stability promotes reliable biogas rate production readings. The synthesis of its digital processing and control system into a low-cost FPGA platform had permitted the successful application of the digital data acquisition and processing in a standalone water displacement gas flow meter.

Population dynamics at digester overload conditions

Two different case studies concerning potential overload situations of anaerobic digesters were investigated and mathematically modelled by means of the Anaerobic Digestion Model No. 1 (ADM1). The first scenario included a digester failure at a municipal WWTP which occurred during revision works of the upstream digester within a two-step digestion system when the sludge was directly by-passed to the 2nd-step reactor. Secondly, the non-occurrence of a highly expected upset situation in a lab-scale digester fed with cattle manure was investigated. ADM1 was utilized to derive indicators which were used to investigate the relationship between digester stability and biomass population dynamics. Conventional design parameters such as the organic loading rate appeared unsuitable for process description under dynamic conditions. Indicators reflecting the biokinetic state (e.g. F net/ M net or the VFA/alkalinity ratio) are more adequate for the assessment of the stability of reactors in transient situations.

An evaluation of a mesophilic reactor for treating wastewater from a Zimbabwean potato-processing plant

An evaluation of anaerobic treatment of potato-processing wastewater using an up flow Anaerobic Sludge Bed (UASB) reactor at 37°C was conducted. Wastewater from a potato-processing plant in Harare, with an average of 6.8 g COD/l, (COD = chemical oxygen demand) a high concentration of total solids (up to 6725 mg/l) and low pH of 4 - 4.9 was subjected to anaerobic treatment in an UASB reactor. The start up period, which was indicated by gas production and stabilization of COD reduction was 21 days. The organic loading rates (OLRs) investigated varied between 1.3 g COD/l/d and 13.1 g COD/l/d, the maximum methane yield was 0.3 l/g CODremoved at an organic loading rate of 6.6 g COD/l/d. An increase in OLRs to values above 13.1 g COD/ l/d resulted in digester failure. The maximum treatment efficiency (TE) in terms of COD reduction achieved was 90% at a hydraulic retention time (HRT) of 2.1 d and OLR of 3.3 g COD/l/d. The TE was reduced at higher reactor influent velocities, with a minimum TE (71%) recorded at HRT of 0.5 d. Total solids (TS) removal was inconsistent with an average reduction around 50% and phosphorus removal was poor and erratic. pH from the effluent oscillated around 7 with a notable decrease from 7.8 to 6.2 at HRT of 0.5 d. Based on these observations, the UASB process has potential to treat potato-processing wastewater as a pretreatment step before discharge into municipal sewerage system of Harare. Key words: UASB reactor, potato wastewater, anaerobic digestion, methane yield, organic loading rate.

Treatability and kinetic analysis of anaerobic moving bed biofilm reactor treating high strength milk permeate

High strength milk permeate wastewater with chemical oxygen demand (COD) of 50–77 g·l–1 from cheese making process was treated respectively under sub-mesophilic (20–25°C) and mesophilic (35–40°C) conditions using bench-scale anaerobic moving bed biofilm reactors (AMBBR). Organic loading rates (OLR) were gradually increased from 0.5 to 6.2 g total COD (TCOD)·l–1·d–1 and from 1.5 to 11 g TCOD·l–1·d–1, achieving maximum stable OLRs at 5.5 and 9.5 g TCOD·l–1·d–1 with corresponding hydraulic retention times (HRT) as 14 and 6.8 days, respectively, under sub-mesophilic and mesophilic condition. Within the maximum stable OLR levels, ratios of VFA (volatile fatty acids)/TA (total alkalinity) were well maintained under suggested limit of digester failure in most cases. Therefore, TCOD and soluble COD (SCOD) removal efficiencies were found to be 82% and 92% in sub-mesophilic treatment and 84% and 96% in mesophilic one. Kinetic analysis found the maximum substrate utilization rates (µmax) were 46.1 and 95.2 g SCOD·l–1·d–1 along with maximum specific methane yield (Ymax) values being 0.301 and 0.313 l CH4·g–1 SCOD removed, respectively, in sub-mesophilic and mesophilic treatments.

Treatment of wastewater from red and tropical fruit wine production by zeolite anaerobic fluidized bed reactor

A study of the anaerobic treatment of wastewaters derived from red (RWWW) and tropical fruit wine (TFWWW) production was carried out in four laboratory-scale fluidized bed reactors with natural zeolite as bacterial support. These reactors operated at mesophilic temperature (35°C). Reactors R1 and R2 contained Chilean natural zeolite, while reactors R3 and R4 used Cuban natural zeolite as microorganism support. In addition, reactors R1 and R3 processed RWWW, while reactors R2 and R4 used TFWWW as substrate. The biomass concentration attached to zeolites in the four reactors studied was found to be in the range of 44–46 g volatile solids (VS)/L after 90 days of operation time. Both types of zeolites can be used indistinctly in the fluidized bed reactors achieving more than 80%–86% chemical oxygen demand (COD) removals for organic loading rates (OLR) of up to at least 20 g COD/L d. pH values remained within the optimal range for anaerobic microorganisms for OLR values of up to 20 and 22 g COD/L d for RWWW and TFWWW, respectively. Toxicity and inhibition levels were observed at an OLR of 20 g COD/L d in reactors R1 and R3 while processing RWWW, whereas the aforementioned inhibitory phenomena were not observed at an OLR of 24 g COD/L d in R2 and R4, treating TFWWW as a consequence of the lower phenolic compound content present in this substrate. The volatile fatty acid (VFA) levels were always lower in reactors processing TFWWW (R2 and R4) and these values (< 400 mg/L, as acetic acid) were lower than the suggested limits for digester failure. The specific methanogenic activity (SMA) was twice as high in reactors R2 and R4 than in R1 and R3 after 120 days of operation when all reactors operated at an OLR of 20 g COD/L d.

Retooling the Ethanol Industry: Thermophilic Anaerobic Digestion of Thin Stillage for Methane Production and Pollution Prevention

Anaerobic digestion of corn ethanol thin stillage was tested at thermophilic temperature (55 degrees C) with two completely stirred tank reactors. The thin stillage wastestream was organically concentrated with 100 g/L total chemical oxygen demand and 60 g/L volatiles solids and a low pH of approximately 4.0. Steady-state was achieved at 30-, 20-, and 15-day hydraulic retention times (HRTs) and digester failure at a 12-day HRT. Significant reduction of volatile solids was achieved, with a maximum reduction (89.8%) at the 20-day HRT. Methane yield ranged from 0.6 to 0.7 L methane/g volatile solids removed during steady-state operation. Effluent volatile fatty acids below 200 mg/L as acetic acid were achieved at 20- and 30-day HRTs. Ultrasonic pretreatment was used for one digester, although no significant improvement was observed. Ethanol plant natural gas consumption could be reduced 43 to 59% with the methane produced, while saving an estimated $7 to $17 million ($10 million likely) for a facility producing 360 million L ethanol/y.

Real evidence about zeolite as microorganisms immobilizer in anaerobic fluidized bed reactors

Using the scanning electronic microscopy, it was observed that natural zeolite possesses excellent physical characteristics as a support medium in anaerobic fluidized bed reactors (AFBR). Samples for biomass analysis were taken from two identical laboratory-scale AFBR (R-1 and R-2), which were operated with 25% of fluidization. These reactors treated distillery wastewaters (vinasses) at mesophilic temperature (30 ± 2 °C). The experiments were carried out with 0.25–0.50 and 0.50–0.80 mm zeolite particle diameter in reactors R-1 and R-2, respectively. The biomass concentration attached to zeolite in both reactors was found to be in the range of 40–45 g volatile solids/l. COD removal efficiencies as high as 90% were achieved at organic loading rate (OLRs) of up to 20 g COD/l day. The volatile fatty acid (VFA) levels were always lower that the suggested limits for digester failure. The yield coefficient of methane production was 0.29 l CH 4(at STP)/g COD consumed and was virtually independent of the OLR applied. A hybridization technique (fluorescence in situ hybridization, FISH) helped determine the predominant anaerobic microorganisms that colonized the natural zeolite, which were found to be Methanosaeta and Methanosarcinaceae, observing a reduced number of sulphate reducing bacteria. The results obtained for reactors R-1 and R-2 were very similar, showing that the particle size did not significantly influence the microbial community immobilized on zeolite.

Anaerobic digestion of solid material: Multidimensional modeling of continuous‐flow reactor with non‐uniform influent concentration distributions

A new multidimensional (3 and 2D) anaerobic digestion model for cylindrical reactor with non-uniform influent concentration distributions was developed to study the way in which mixing intensity affects the efficiency of continuous-flow anaerobic digestion. Batch experiments reported and simulated earlier by Vavilin and Angelidaki (2005) were used to modernize a kinetic scheme and to obtain the corresponding kinetic coefficients. In the new models, hydrolytic microorganisms were included using Contois kinetics for the hydrolysis/acidogenesis degradation of municipal solid waste (MSW). Monod kinetics was applied for description of methanogenesis. Both hydrolytic and methanogenic microorganisms were assumed to be inhibited by high volatile fatty acids (VFA) concentration. According to the new distributed models, the mixing level reduction expressed by increasing dimensionless Peclet number may improve the continuous flow reactor performance at the relatively low influent methanogenic biomass concentration. In the continuously stirred tank reactor (CSTR) there are two steady states with and without methane production at slightly different values of initial methanogenic biomass concentration. In the system, the threshold methanogenic biomass concentration existed because of inhibition by high VFA concentration. High methanogenic biomass concentration is required for efficient anaerobic digestion of MSW in order to avoid possible inhibition due to high VFA build-up. Thus, CSTR configuration might have unstable dynamics at high organic loading as shown in earlier experiments carried out by Stroot et al. (2001). A gradual increase of organic loading during the start up of a completely mixed digester causing an accumulation of methanogenic biomass is a solution to prevent a probable digester failure. According to the distributed models a plug-flow reactor with non-uniform influent concentration distributions where methanogenic and hydrolytic microorganisms are separated has significant methane production and solids removal at the relatively low influent methanogenic biomass concentration.

Prise en charge thérapeutique de la péritonite primitive à Streptococcus pneumoniae de l'adulte : étude d'un cas et revue de la littérature

Introduction Streptococcus pneumoniae primary peritonitis is rare. The diagnosis is uneasy and the treatment is not standardised. Case report We report a single case of S. pneumoniae primary peritonitis needing surgical treatment. Discussion S. pneumoniae primary peritonitis can be medically treated. Surgery is needed in case of sepsis, associated digestive injuries or failure of medical treatment.

Enzyme treatment to decrease solids and improve digestion of primary sewage sludge

The aim of anaerobic digestion of primary sewage sludge is to convert the carbonaceous material contained in the solids into methane and carbon dioxide. The products of digestion are therefore gases, stabilised sludge solids which are subsequently dewatered and disposed of, and sludge liquor which is generally further treated. This investigation assessed the impact of addition of hydrolytic enzymes to anaerobic digesters. Cellulase and pronase E were added singly and in combination, and it was found that the mixture of the two enzymes resulted in an 80% reduction in solids (cf. 20% in the control), 93% removal of particulate chemical oxygen demand (COD) (59% in the control) and 97% total COD removal (vs. 63%). The total suspended solids (TSS) concentration was reduced by 80%, from 25 g/l to 5 g/l. Single enzymes had little or no impact on sludge solubilisation, and final COD and TSS, but all of the enzyme additions were seen to decrease the production of volatile fatty acids (VFAs). Since accumulation of VFAs can lead to digester failure, it was concluded that the enzyme additives enhanced digester performance in terms of degradation of COD, reduction in sludge solids remaining after digestion and improved digester stability owing to the stable prevailing pH. The results indicate that enzyme addition at full scale could be expected to lead to greater methane yields, lower strength sludge liquors and a significant reduction in the requirements for and costs of digested sludge dewatering and disposal.