Precipitation, chelation, and the availability of metals as nutrients in anaerobic digestion. II. Applications

The relative importance of the individual effects of precipitation and chelation of metal ions in anaerobic digestion is assessed. Experimentally determined soluble metal ion levels are compared with predicted levels obtained by using a previously described methodology.(1) It is found that soluble metal complexes may increase the level of soluble metals in the presence of CO(3) (2-) and S(2-) by a factor of up to 10(4). The formation of a soluble complex may increase or decrease the availability of the metal ion in question for microbial uptake. Two case studies are presented, one using a defined medium and one a complex medium. It is possible, in the case of the defined medium, to accurately predict the free metal ion concentration using the methodology previously developed.(1) While the identification of the presence of natural chelating compounds in a complex medium is not routinely possible, the significant discrepancy between the measured level of the soluble metal ion Fe(2+) and the calculated level in the case studies presented indicates that natural chelating compounds may play a vital role in providing available metal ions to the microorganisms of an anaerobic digester.     

Transport of heavy metals during anaerobic digestion of water hyacinth

Rates of adsorbtion or chelation of Pb, Cd, Zn and Cu by anaerobic sludges were measured during a two phase digestion of water hyacinth leaf extract. It was found that absorption or chelation was higher in the methanogenic aludge than in the acidogenic sludge. Transport of metals was dependent upon total organic carbon, volatile and total suspended solids content of both anaerobic sludges.     

On-field study of anaerobic digestion full-scale plants (part I): an on-field methodology to determine mass, carbon and nutrients balance

The mass balance (input/output mass flows) of full-scale anaerobic digestion (AD) processes should be known for a series of purposes, e.g. to understand carbon and nutrients balances, to evaluate the contribution of AD processes to elemental cycles, especially when digestates are applied to agricultural land and to measure the biodegradation yields and the process efficiency. In this paper, three alternative methods were studied, to determine the mass balance in full-scale processes, discussing their reliability and applicability. Through a 1-year survey on three full-scale AD plants and through 38 laboratory-scale batch digesters, the congruency of the considered methods was demonstrated and a linear equation was provided that allows calculating the wet weight losses (WL) from the methane produced (MP) by the plant (WL = 41.949 * MP + 20.853, R² = 0.950, p < 0.01). Additionally, this new tool was used to calculate carbon, nitrogen, phosphorous and potassium balances of the three observed AD plants.     

Lactate and ethanol as intermediates in two-phase anaerobic digestion

The thermodynamics of the various anaerobic digestion patterns of hexose to methane are compared. It appears that by directing the hexose-hydrolysis phase towards ethanol and lactic acid production, methanogenesis can be enhanced because the syntrophic bacteria are allocated more potentially available energy. This hypothesis was confirmed in a series of laboratory test runs. They revealed that lactic acid and ethanol as intermediates, in comparison to lower volatile fatty acids, give rise to a considerably higher effluent quality and a slightly larger biogas production.     

The fate of crop nutrients during digestion of swine manure in psychrophilic anaerobic sequencing batch reactors

The objectives of the study were to measure the levels of manure nutrients retained in psychrophilic anaerobic sequencing batch reactors (PASBRs) digesting swine manure, and to determine the distribution of nutrients in the sludge and supernatant zones of settled bioreactor effluent. Anaerobic digestion reduced the total solids (TS) concentration and the soluble chemical oxygen demand (SCOD) of manure by 71.4% and 79.9%, respectively. The nitrogen, potassium, and sodium fed with the manure to the PASBRs were recovered in the effluent. The bioreactors retained on average 25.5% of the P, 8.7% of the Ca, 41.5% of the Cu, 18.4% of the Zn, and 67.7% of the S fed to the PASBRs. The natural settling of bioreactor effluent allowed further nutrient separation. The supernatant fraction, which represented 71.4% of effluent volume, contained 61.8% of the total N, 67.1% of the NH4-N, and 73.3% of the Na. The settled sludge fraction, which represented 28.6% of the volume, contained 57.6% of the solids, 62.3% of the P, 71.6% of the Ca, 89.6% of the Mg, 76.1% of the Al, 90.0% of the Cu, 74.2% of the Zn, and 52.2% of the S. The N/P ratio was increased from 3.9 in the raw manure to 5.2 in the bioreactor effluent and 9.2 in the supernatant fraction of the settled effluent. The PASBR technology will then substantially decrease the manure management costs of swine operations producing excess phosphorus, by reducing the volume of manure to export outside the farm. The separation of nutrients will also allow land spreading strategies that increase the agronomic value of manure by matching more closely the crop nutrient requirements.     

Characterization of food waste as feedstock for anaerobic digestion

Food waste collected in the City of San Francisco, California, was characterized for its potential for use as a feedstock for anaerobic digestion processes. The daily and weekly variations of food waste composition over a two-month period were measured. The anaerobic digestibility and biogas and methane yields of the food waste were evaluated using batch anaerobic digestion tests performed at 50 degrees C. The daily average moisture content (MC) and the ratio of volatile solids to total solids (VS/TS) determined from a week-long sampling were 70% and 83%, respectively, while the weekly average MC and VS/TS were 74% and 87%, respectively. The nutrient content analysis showed that the food waste contained well balanced nutrients for anaerobic microorganisms. The methane yield was determined to be 348 and 435 mL/gVS, respectively, after 10 and 28 days of digestion. The average methane content of biogas was 73%. The average VS destruction was 81% at the end of the 28-day digestion test. The results of this study indicate that the food waste is a highly desirable substrate for anaerobic digesters with regards to its high biodegradability and methane yield.     

Soil microbes and the availability of soil nutrients

It is likely to provide plants with their necessary nutrients using chemical and biological fertilization. Although chemical fertilization is a quick method, it is not recommendable economically and environmentally, especially if overused. Biological fertilization is the use of soil microbes including arbuscular mycorrhizal fungi and plant growth promoting rhizobacteria to inoculate plants. It has been proved that biological fertilization is an efficient method to supply plants with their necessary nutrients. It is economically and environmentally recommendable, because it results in sustainability. In this article, some of the most important details including the mechanisms and processes regarding the effects of soil microbes on the availability and hence uptake of nutrients by plant are reviewed. Such details can be important for the selection and hence production of microbial inoculums, which are appropriate for biological fertilization.     

Biological nutrients removal from the supernatant originating from the anaerobic digestion of the organic fraction of municipal solid waste

Abstract

This study critically evaluates the biological processes and techniques applied to remove nitrogen and phosphorus from the anaerobic supernatant produced from the treatment of the organic fraction of municipal solid waste (OFMSW) and from its co-digestion with other biodegradable organic waste (BOW) streams. The wide application of anaerobic digestion for the treatment of several organic waste streams results in the production of high quantities of anaerobic effluents. Such effluents are characterized by high nutrient content, because organic and particulate nitrogen and phosphorus are hydrolyzed in the anaerobic digestion process. Consequently, adequate post-treatment is required in order to comply with the existing land application and discharge legislation in the European Union countries. This may include physicochemical and biological processes, with the latter being more advantageous due to their lower cost. Nitrogen removal is accomplished through the conventional nitrification/denitrification, nitritation/denitritation and the complete autotrophic nitrogen removal process; the latter is accomplished by nitritation coupled with the anoxic ammonium oxidation process. As anaerobic digestion effluents are characterized by low COD/TKN ratio, conventional denitrification/nitrification is not an attractive option; short-cut nitrogen removal processes are more promising. Both suspended and attached growth processes have been employed to treat the anaerobic supernatant. Specifically, the sequencing batch reactor, the membrane bioreactor, the conventional activated sludge and the moving bed biofilm reactor processes have been investigated. Physicochemical phosphorus removal via struvite precipitation has been extensively examined. Enhanced biological phosphorus removal from the anaerobic supernatant can take place through the sequencing anaerobic/aerobic process. More recently, denitrifying phosphorus removal via nitrite or nitrate has been explored. The removal of phosphorus from the anaerobic supernatant of OFMSW is an interesting research topic that has not yet been explored. At the moment, standardization in the design of facilities that treat anaerobic supernatant produced from the treatment of OFMSW is still under development. To move toward this direction, it is first necessary to assess the performance of alternative treatment options. It study concentrates existing data regarding the characteristics of the anaerobic supernatant produced from the treatment of OFMSW and from their co-digestion with other BOW. This provides data documenting the effect of the anaerobic digestion operating conditions on the supernatant quality and critically evaluates alternative options for the post-treatment of the liquid fraction produced from the anaerobic digestion process.     

Precipitation and the transfer of water, nutrients and pollutants in tree canopies

The functioning of a forest community depends upon the routes, rates and transformations of material flowing through it. The canopy's interaction with precipitation can result in the interception of rainfall and the scavenging of water from clouds and fog. But this movement of water through the canopy also mediates the transfer of mineral nutrients and the deposition of some atmospheric pollutants. The past decade has seen the development of increasingly complex models of transfer processes in the canopy and new methodologies for studying them. One impulse for these advances has been the need to understand not only the movement of water, nutrients and pollutants but also their reciprocal effects. Another has been the need to evaluate the nature of pollutant-induced effects with respect to the costs and effectiveness of possible remedies.     

Cadmium retention in rice roots is influenced by cadmium availability, chelation and translocation

Analysis of rice plants exposed to a broad range of relatively low and environmentally realistic Cd concentrations showed that the root capacity to retain Cd ions rose from 49 to 79%, corresponding to increases in the external Cd2+ concentration in the 0.01-1 μM range. Fractioning of Cd ions retained by roots revealed that different events along the metal sequestration pathway (i.e. chelation by thiols, vacuolar compartmentalization, adsorption) contributed to Cd immobilization in the roots. However, large amounts of Cd ions (around 24% of the total amount) predictable as potentially mobile were still found in all conditions, while the amount of Cd ions loaded in the xylem seemed to have already reached saturation at 0.1 μM Cd2+, suggesting that Cd translocation may also play an indirect role in determining Cd root retention, especially at the highest external concentrations. In silico search and preliminary analyses in yeast suggest OsHMA2 as a good candidate for the control of Cd xylem loading in rice. Taken as a whole, data indicate Cd chelation, compartmentalization, adsorption and translocation processes as components of a complex 'firewall system' which acts in limiting Cd translocation from the root to the shoot and which reaches different equilibrium positions depending on Cd external concentration.     

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.     

Methanosarcina as the dominant aceticlastic methanogens during mesophilic anaerobic digestion of putrescible waste

Taking into account isotope ¹³C value a mathematical model was developed to describe the dynamics of methanogenic population during mesophilic anaerobic digestion of putrescible solid waste and waste imitating Chinese municipal solid waste. Three groups of methanogens were considered in the model including unified hydrogenotrophic methanogens and two aceticlastic methanogens Methanosaeta sp. and Methanosarcina sp. It was assumed that Methanosaeta sp. and Methanosarcina sp. are inhibited by high volatile fatty acids concentration. The total organic and inorganic carbon concentrations, methane production, methane and carbon dioxide partial pressures as well as the isotope ¹³C incorporation in PSW and CMSW were used for the model calibration and validation. The model showed that in spite of the high initial biomass concentration of Methanosaeta sp. Methanosarcina sp. became the dominant aceticlastic methanogens in the system. This prediction was confirmed by FISH. It is concluded that Methanosarcina sp. forming multicellular aggregates may resist to inhibition by volatile fatty acids (VFAs) because a slow diffusion rate of the acids limits the VFA concentrations inside the Methanosarcina sp. aggregates.     

Changes in proportions of acetate and carbon dioxide used as methane precursors during the anaerobic digestion of bovine waste.

In an anaerobic digestor which was fed daily with bovine waste, during the early stages after feeding (4 to 7 h) acetate (via the methyl group) accounted for almost 90% of the methane produced. As time after feeding increased, acetate declined as a precursor so that in the 12- to 14-h and 21- to 23-h periods, after feeding the methyl group accounted for 80 and 73% of the methane produced, respectively. Measurements of methane production from CO2 reduction showed that in the 2- to 12-h period after feeding, CO2 accounted for 14% of the methane produced, whereas in the 12- to 24-h period it accounted for 27-5%. These results show that the percentages of methane accounted for by acetate and CO2 vary with time after feeding the digestor.     

Evolution of microorganisms in thermophilic-dry anaerobic digestion

Microbial population dynamics were studied during the start-up and stabilization periods in thermophilic-dry anaerobic digestion at lab-scale. The experimental protocol was defined to quantify Eubacteria and Archaea using Fluorescent in situ hybridization (FISH) in a continuously stirred tank reactor (CSTR), without recycling solids. The reactor was subjected to a programme of steady-state operation over a range of the retention times from 40 to 25 days, with an organic loading rate between 4.42 and 7.50 kg volatile solid/m3/day. Changes in microbial concentrations were linked to traditional performance parameters such as biogas production and VS removal. The relations of Eubacteria:Archaea and H2-utilising methanogens:acetate-utilising methanogens were 88:12 and 11:1, respectively, during start-up stage. Hydrogenotrophic methanogens, although important in the initial phase of the reactor start-up, were displaced by acetoclastic methanogens at steady-state, thus their relation were 7:32, respectively. The methane yield coefficient, the methane content in the biogas and VS removal were stabilized around 0.30 LCH4/gCOD, 50% and 80%, respectively. Methanogenic population correlated well with performance measurements.     

Kinetics of two subgroups of propionate-using organisms in anaerobic digestion.

A novel method for measuring the kinetics of a subgroup of organisms growing in mixed culture was used to measure the kinetics of propionate-using organisms in a mixed-population anaerobic digester. It was shown that there are at least two subgroups of propionate-using organisms with distinctly different growth kinetics. Both subgroups could grow without reducing sulfate. The slower-growing subgroup had a Ks of 11 mg/liter and a mumax of 0.0054 h-1 which is similar to the mumax reported for Syntrophobacter wolinii. The faster-growing group had a mumax of 0.050 h-1 and a Ks of 330 mg/liter. The slower-growing group was inhibited by a pH shock from 7.0 to 6.0, whereas the faster-growing group was less sensitive to the pH shock.     

Parametric sensitivity in modeling of anaerobic digestion

Summary A mathematical model for the anaerobic digestion process was analyzed by the parametric sensitivity method to determine the influence of individual model parameters on the selected output variables. The relative parametric sensitivity value indicated the most influential parameters in the process being yield coefficients and rate constants.     

Biomass stabilization in the anaerobic digestion of wastewater sludges

Sludge stabilization processes include both volatile solid destruction and biomass stabilization. Traditionally, both processes have been considered together, in such a way that, when volatile solid destruction is achieved, the biomass is considered stabilized. In this study, volatile solids reduction and biomass stabilization in the anaerobic digestion of primary, secondary and mixed sludges from municipal wastewater treatment plants were researched in batch cultures by measurements of suspended solids and suspended lipid-phosphate. The estimated kinetic constants were higher in all sludge types tested for the biomass stabilization process, indicating that volatile solids destruction and biomass stabilization are not parallel processes, since the latter one is reached before the former.     

嗜酸产甲烷菌及其在厌氧处理中的应用

产甲烷菌在自然界碳素循环过程中发挥着重要作用.酸性泥炭沼泽环境中存在着多种未知的产甲烷古菌,其中嗜酸产甲烷菌因其特殊的生长代谢特征近年来引起学者的广泛关注.若将嗜酸产甲烷菌应用于高浓度有机废物或废水的厌氧消化过程中,可从本质上克服因酸积累造成的产甲烷抑制,减少运行成本,扩展厌氧消化处理技术的应用范围.本文综述了嗜酸产甲烷菌的富集分离培养方法、生理生化特性、代谢特征及相关分子生物学研究等内容,并对其在厌氧处理中的应用前景进行了分析和展望,提出了未来研究的方向.