Strategies for stable anaerobic digestion of vegetable waste

Abstract

International trade and the market demand for pre-prepared agricultural produce is not only increasing the total quantity of waste agricultural biomass but also centralising its availability, making it potentially useful for energy production. The current work considers the suitability of vegetable trimmings and rejects from high-value produce air-freighted between Africa and Europe as a feedstock for anaerobic digestion. The physical and chemical characteristics of a typical mixed vegetable waste of this type were determined and the theoretical energy yield predicted and compared to experimentally-determined calorific values, and to the energy recovered through a batch biochemical methane potential test. A semi-continuous digestion trial was then carried out with daily feed additions at different organic loading rates (OLR). At an OLR of 2 g VS L -1 day -1 the substrate gave a methane yield of 0.345 L g -1 VS added with VS destruction 81.3%, and showed that 76.2% of the measured calorific value of the waste could be reclaimed as methane. This was in good agreement with the estimated energy recovery of 68.6% based on reaction stoichiometry, and was 99% of the biochemical methane potential (BMP). Higher loading rates reduced the specific methane yield and energy conversion efficiency, and led to a drop in digester pH which could not be effectively controlled by alkali additions. To maintain digester stability it was necessary to supplement with additional trace elements including tungsten, which allowed loading rates up to 4 g VS L -1 day -1 to be achieved. Stability was also improved by addition of yeast extract (YE), but the higher gas yield obtained was as a result of the contribution made by the YE and no synergy was shown. Co-digestion using card packaging and cattle slurry as co-substrates also proved to be an effective means of restoring and maintaining stable operating conditions.

Exploring the Potential of Organic Waste as a Source of Methane Gas for Electricity Generation in Nigeria

“…Hence, the medium becomes more acidic, this acidification of the process has an adverse effect on the micro-organisms responsible for the breakdown of organic components. These findings agree with other research on anaerobic digestion (Jiang et al, 2012;Raynal et al, 1998). We can therefore infer that, if pH value falls significantly lower than the recommended pH value for anaerobic digestion, very little or no methane gas will be produced.…”

Potencial de biogás de los residuos agroindustriales generados en el departamento de Cundinamarca

“…Sin embargo, el rendimiento de la producción de metano se puede ver afectado por la presencia de un alto porcentaje de proteínas ya que podrían ser responsables de la baja biodegradabilidad del residuo debido a los productos intermedios de su degradación [27]. En cuanto a los residuos vegetales de algunos cultivos, los elevados valores de C/N y la escasez de elementos traza pueden afectar el correcto desarrollo del proceso por la producción de ácidos grasos y el decrecimiento del pH del sistema [32].…”

Anaerobic Digestion

“…A study by Jiang [33] showed that vegetable waste was not suitable for use as a sole substrate for AD without the addition of trace elements due to the rapid build-up of volatile acids, which can result in a significant decrease in pH within the reactor which can in turn stress and inhibit the activity of methanogenic bacteria [32].…”