David P. Chynoweth

Renewable methane from anaerobic digestion of biomass

Production of methane via anaerobic digestion of energy crops and organic wastes would benefit society by providing a clean fuel from renewable feedstocks. This would replace fossil fuel-derived energy and reduce environmental impacts including global warming and acid rain. Although biomass energy is more costly than fossil fuel-derived energy, trends to limit carbon dioxide and other emissions through emission regulations, carbon taxes, and subsidies of biomass energy would make it cost competitive. Methane derived from anaerobic digestion is competitive in efficiencies and costs to other biomass energy forms including heat, synthesis gases, and ethanol.

Biochemical methane potential of biomass and waste feedstocks

Abstract The biochemical methane potential (BMP)) assay was evaluated in terms of inoculum (rumen versus primary sludge digester), inoculum-to-feed ratio, and particle size for analysis of extent and rate of conversion of biomass and waste feedstocks to methane. The rumen and sludge inocula exhibited similar solubilization of particulate matter. An inoculum-to-feed ratio of 2:1 was shown to give maximum conversion rates. Particle size did not influence rate in the range of 1–8 mm. An extensive data base on the biochemical methane potential of several biomass and waste feedstocks is presented, including freshwater, marine, herbaceous, and woody feedstocks and municipal wastes; data for plant parts are also included. In addition, the influence of several parameters on the BMP of feedstocks are presented, including growth and harvest conditions, and ensiling.

Stable performance of anaerobic digestion in the presence of a high concentration of propionic acid

An automatically controlled, glucose-fed, anaerobic digester was deliberately inhibited by addition of phenol. To overcome the phenol inhibition the feed dilution rate was lowered in such a way that the methane yield from glucose was kept the same as that under normal conditions. The concentrations of acetic and butyric acids remained below 100 mg/l, however, propionic acid accumulated to 2,750 mg/l. Phenol apparently inhibited all tropic groups of organisms and it was shown that the propionic acid was formed from the metabolism of phenol. From the nature of the operating strategy, it was deduced that the digester continued to convert all the glucose that was supplied to methane showing that propionic acid accumulation did not inhibit conversion of glucose to methane. Therefore, propionic acid accumulation may be an effect and not a cause of inhibition of the anaerobic digestion process.

Influence of phase separation, leachate recycle and aeration on treatment of municipal solid waste in simulated landfill cells

Decomposition in landfills is erratic and influenced primarily by moisture, inoculum, oxygen and accumulation of inhibitory fermentation products. These parameters may be optimized for decomposition by use of controlled landfill cells or bioreactors. This work evaluated the effect of combined-phase and two-phase anaerobic digestion of municipal solid waste (MSW) in laboratory leachbed (LB) cells. Phase separation resulted in increased rates of decomposition and greater process stability. Leachate recycle with an upflow sludge anaerobic reactor resulted in improvement of performance in a LB reactor which was attributed to removal of inhibitory fermentation products and buffering of acids in the leachate. The effects of aeration and leachate flooding of the LB were also studied. Intermittent aeration of the LB enhanced phase separation but inhibition of methanogenesis was not sustainable beyond six days. Conversion of organic matter was similar in the anaerobic and aerated LBs.

Degradation of unsorted municipal solid waste by a leach-bed process

In current landfills breakdown of municipal solid waste (MSW) occurs slowly and the landfill leaves a legacy of care, management, monitoring and potential catastrophic failure over several generations. Social concern over these long term issues, with their legislative and economic implementation, increasingly favour practices which promote short stabilisation times and minimise environmental impact. This paper describes experiments carried out on mixed and unsorted municipal solid waste (MSW) in which 75% of the rapidly biodegradable fraction was degraded in about 2 months with an average yield of 0.18 m(3) CH4/kg volatile solids at s.t.p. The experiments served to demonstrate that with proper leachate management very rapid decomposition of waste can be accomplished by taking the waste through a series of controlled degradation stages

Effects of marine algal proximate composition on methane yields

The suitability of differentGracilaria spp. and twoSargassum species for bioconversion to methane was determined through bioassays of methane yield.Gracilaria species and strains were excellent feedstocks for high methane yields, ranging from 0.28 to 0.40 m3 kg−1 volatile solids added. These yields ranged from 58 to 95% of theoretical stoichiometric yields. Methane yields were highly correlated with acid soluble carbohydrate components of theGracilaria spp. BothSargassum fluitans andS. pteropleuron were poor feedstocks, with methane yields ranging from 0.12 to 0.19 m3 kg−1 volatile solids added, 27 to 46% of theoretical stoichiometric yields, respectively. The various tissue types of theseSargassum species were also poor feedstocks for anaerobic digestion to methane. While there is no clear explanation for the low methane yields, the twoSargassum spp. appear to contain a high proportion of an insoluble, non-extractable component which may not be available as a substrate for bioconversion to methane.

Methane fermentation of woody biomass

Abstract Woody biomass has been previously considered to be highly refractile to anaerobic digestion without extensive pretreatment. However, this study has demonstrated that high rates and ultimate methane yields may be obtained in batch assays without pretreatment, other than some particle size reduction. In a survey of 32 woody samples (chiefly willow and poplar species grown in a wood-grass system) significant yields of methane (≥ 0·25 liters g−1 volatile solids added) were obtained from 19 of the samples, while the yield from 3 of the samples (Salix eriocephala, S. exigua and Platanus occidentalis) exceeded 0·30 liters g−1 volatile solids added. The commercial production of energy through the biological gasification of woody biomass may therefore represent a viable alternative for energy production in the United States. An interesting feature of this study was the biphasic nature of methane production from two-thirds of the samples tested.

Ecology and microbiology of biogasification

Abstract The biodegradation of organic matter to form methane and carbon dioxide requires the interactions of diverse populations of bacteria. The roles of each of these organisms in the process and how they interact with each other is understood only in a rudimentary way. This paper describes the investigation of the microbial ecology of the anaerobic degradation of biomass feedstocks.

Expert system for control of anaerobic digesters

Continuous anaerobic digesters are systems that present challenging control problems including the possibility that an unmeasured disturbance can change the sign of the steady-state process gain. An expert system is developed that recognizes changes in the sign of process gain and implements appropriate control laws. The sole on-line measured variable is the methane production rate, and the manipulated input is the dilution rate. The expert system changes the dilution rate according to one of four possible strategies: a constrained conventional set-point control law, a constant yield control law (CYCL) that is nearly optimal for the most common cause of change in the sign of the process gain, batch operation, or constant dilution rate. The algorithm uses a t test for determining when to switch to the CYCL and returns to the conventional set-point control law with bumpless transfer. The expert system has proved successful in several experimental tests: severe overload; mild, moderate, and severe underload; and addition of phenol in low and high levels. Phenol is an inhibitor that in high concentrations changes the sign of the process gain.

Hydraulic conductivity of compacted municipal solid waste

Abstract Dry municipal solid waste (MSW) samples were compacted in Plexiglas columns to densities of 160, 320 and 480 kg m−3. The columns were set-up as constant head permeameters. Water flowed continuously through the columns under hydraulic gradients of 2–4·0 m m−1. Hydraulic conductivities were calculated according to Darcys equation. Hydraulic conductivity was found to be time-dependent. The temporal variation was attributed to varying degrees of saturation due to gas formation and relative movement of fine particles in the columns. The average hydraulic conductivities at 160, 320 and 480 kg m−3 were 9·6 × 10−2, 7·3 × 10−4 and 4·7 × 10−5 cm s−1, respectively.