No abstract

Most of the so called developing countries are located in tropical and/or sub-tropical regions whilst in contrast, most of the developed countries are in the temperate climate zones. It is expected that a huge increase in the future global energy consumption will be caused by the demand of the developing countries. Caused by the favourable climate conditions in tropical and/or sub-tropical regions the average productivity of biomass is 4 – 5 times higher than that of biomass grown in the temperate regions. Many of the developing countries today are agricultural and agro-industrial countries producing huge amounts of agricultural residues and wastes that can be used as source for energy generation. It is estimated that if only all process-based agricultural residues alone would be used, they could contribute between 25 % and 40 % of the total primary energy demand in such regions [1]. Until now, the huge amount of agricultural waste generated each year in developing countries is a headache for farmers, who are obliged to get rid of it. Open field burning and improper disposal are omnipresent in many regions and pollute the environment. Converting such waste into bioenergy such as biogas by using anaerobic digestion technologies represents an alternative treatment with a promising potential. Such treatment prevents pollution, is producing valuable and climate friendly energy and will contribute to nutrient recovery by using the digestate as fertilizer. The paper will introduce some of the most promising agricultural residues in tropical and subtropical regions which can be used as substrates or co-substrates for biogas production. Results of a research project carried out in Costa Rica will be used as a case study to show the potential of two of the major agricultural residues (pineapple and banana residues) as sources for biogas production. The opportunities and prospects for the dissemination and implementation of new and more developed technologies to improve the efficiency of the technologies will be shown.

No abstract

The annual phosphate (PO43−) utilization has increased, leading to a depletion of existing sources of phosphorus (P). To overcome this, digestate as a source to recover P is being investigated. Due to the abundance of nutrients, the digestate from an agricultural biogas plant is used as fertilizer for crops. The separation of solids and liquids from the digestate by a screw press is the simplest form of concentrating, therefore, recovering PO43−. This is the most commonly employed method in existing biogas plants. However, the separation is not very efficient as only 20–30% of P is recovered in the solid phase. The goal of this study is to increase the separation efficiency and recover more P into the solid phase, in order to improve the transportability. For this, separation trials at a laboratory scale were performed for five experimental groups, with biochar and straw flour as non-reactive additives and kieserite as a reactive additive. In addition, untreated digestate was studied as a control. The control and the treatment with biochar and straw flour were carried out at 25 °C, while the treatment with kieserite was performed at 25 °C and 50 °C. The separation trails were performed at treatment times of 0 h, 1 h, 2 h, 8 h, and 20 h. The results showed that the treatment with additives had a beneficial effect on the recovery of P. It was noted that kieserite treatment at 25 °C and 50 °C bound about 61% of the total P present in the digestate to the solid phase. A sequential extraction was performed to study the effect of additives on the recovery of different P species. The results concluded that, compared to biochar and straw flour, kieserite was efficient in recovering the non-labile fractions (NaOH-P and HCl-P) of P, which act as slow-release fertilizers. This study shows that the use of additives, especially kieserite, has a positive influence on recovering P from digestate, and further research to optimize the recovery process would be beneficial.