The microbiological basis of process control in methanogenic fermentation of soluble wastes

Biomass waste-to-energy valorisation technologies: a review case for banana processing in Uganda

BACKGROUND

Uganda's banana industry is heavily impeded by the lack of cheap, reliable and sustainable energy mainly needed for processing of banana fruit into pulp and subsequent drying into chips before milling into banana flour that has several uses in the bakery industry, among others. Uganda has one of the lowest electricity access levels, estimated at only 2-3% in rural areas where most of the banana growing is located. In addition, most banana farmers have limited financial capacity to access modern solar energy technologies that can generate sufficient energy for industrial processing. Besides energy scarcity and unreliability, banana production, marketing and industrial processing generate large quantities of organic wastes that are disposed of majorly by unregulated dumping in places such as swamps, thereby forming huge putrefying biomass that emit green house gases (methane and carbon dioxide). On the other hand, the energy content of banana waste, if harnessed through appropriate waste-to-energy technologies, would not only solve the energy requirement for processing of banana pulp, but would also offer an additional benefit of avoiding fossil fuels through the use of renewable energy.

MAIN BODY

The potential waste-to-energy technologies that can be used in valorisation of banana waste can be grouped into three: Thermal (Direct combustion and Incineration), Thermo-chemical (Torrefaction, Plasma treatment, Gasification and Pyrolysis) and Biochemical (Composting, Ethanol fermentation and Anaerobic Digestion). However, due to high moisture content of banana waste, direct application of either thermal or thermo-chemical waste-to-energy technologies is challenging. Although, supercritical water gasification does not require drying of feedstock beforehand and can be a promising thermo-chemical technology for gasification of wet biomass such as banana waste, it is an expensive technology that may not be adopted by banana farmers in Uganda. Biochemical conversion technologies are reported to be more eco-friendly and appropriate for waste biomass with high moisture content such as banana waste.

CONCLUSION

Uganda's banana industrialisation is rural based with limited technical knowledge and economic capability to setup modern solar technologies and thermo-conversions for drying banana fruit pulp. This review explored the advantages of various waste-to-energy technologies as well as their shortfalls. Anaerobic digestion stands out as the most feasible and appropriate waste-to-energy technology for solving the energy scarcity and waste burden in banana industry. Finally, potential options for the enhancement of anaerobic digestion of banana waste were also elucidated.

Applications of the anaerobic digestion process

At the start of the new millennium waste management has become a political priority in many countries. One of the main problems today is to cope with an increasing amount of primary waste in an environmentally acceptable way. Biowastes, i.e., municipal, agricultural or industrial organic waste, as well as contaminated soils etc., have traditionally been deposited in landfills or even dumped into the sea or lakes without much environmental concern. In recent times, environmental standards of waste incineration and controlled land filling have gradually improved, and new methods of waste sorting and resource/energy recovery have been developed. Treatment of biowastes by anaerobic digestion processes is in many cases the optimal way to convert organic waste into useful products such as energy (in the form of biogas) and a fertilizer product. Other waste management options, such as land filling and incineration of organic waste has become less desirable, and legislation, both in Europe and elsewhere, tends to favor biological treatment as a way of recycling minerals and nutrients of organic wastes from society back to the food production and supply chain. Removing the relatively wet organic waste from the general waste streams also results in a better calorific value of the remainder for incineration, and a more stable fraction for land filling.

Biomethanation under psychrophilic conditions: a review

Anaerobic digestion of animal manure, sewage and other agricultural wastes at psychrophilic temperatures has not been explored as extensively as either mesophilic or thermophilic digestion, probably due to little anticipation of the development of economically attractive systems using this technology. This review article discusses psychrophilic anaerobic digestion studies reported by various researchers using different substrates. The effect of operational parameters such as type of substrate, size of inoculum, concentration of volatile fatty acids, hydraulic retention time and loading rate, on reduction of TS/VS, BOD/COD and biogas yield is discussed in detail.

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.

Modelling of the biodegradation of organic matter in municipal landfills

A model is presented to simulate the biodegradation of easily and slowly hydrolyzable organic matter, as well as the generation of biogas and heat release. The model is based on fundamental relationships among physical/chemical, thermodynamical and microbial processes occurring in municipal landfills. Local, microbially-mediated degradation processes occurring in municipal landfills, are simulated in terms of the hydrolysis of solid organic matter, the formation of glucose and acetate as intermediary carbon substrates and the generation of the biogases CH4 and CO2. Thus, the overall decomposition of the organic matter has been assumed to follow four sequential biochemical reactions: hydrolysis, acidogenesis, acetogenesis and methanogenesis. In order to study the impact of environmental factors on the biological decomposition processes, pH, temperature and hydrogen changes have been integrated into the degradation model as inhibition terms.

Anaerobic waste stabilization

The present knowledge of the microbiology, physiology and regulation of anaerobic digestion in conventional or advanced processes is reviewed. In all systems the carbon flow from biopolymers to biogas is determined by syntrophic interactions of fermentative or acetogenic bacteria with methanogens at the level of interspecies hydrogen transfer. Inhibitors or heavy metal ions may interfere at different levels. The stabilization of waste at mesophilic and thermophilic temperatures is compared and the process stability as well as the inactivation of pathogens is discussed. Characteristics of conventional digestion systems and of recently developed advanced processes with solids and liquids uncoupling are compared and selection criteria with respect to the type of sludge are outlined. Areas of future research for a better understanding of the biochemistry, the physiology and the regulation of the degradation of pollutants are suggested.