In order to reduce dependence on dwindling oil resources and increasing carbon dioxide emissions at the same time, the use of renewable energies represents a sustainable alternative. Here, the use of plant biomass for the production of bioenergy – electricity, heat or fuel – plays a crucial role. Nevertheless, to date the potential of waste biomass to produce biogas as well as its use as fuel has been underexploited so far.
Organic waste materials with very high proportion of water and low content of lignin and lignocellulose, for example waste from the food industry, wholesale market waste or algae residues, are perfectly suited for digestion.
Coordinated by Fraunhofer IGB, a project consortium has thus set itself the objective to obtain maximum energy generation by completely converting easily digestable, low-lignocellulosic wet biomass into biogas using an adapted high-load digestion process and closing all material cycles at the same time. In the project EtaMax the consortium is focusing in particular on cost-effective biowaste and algal biomass, both of which present no competition to food production. Local production and utilization of renewable methane from biogas represent the core of the project. Purified biomethane was used as a fuel to power CNG (Compressed Natural Gas) vehicles. Liquid, nutrient-rich digestates accruing during digestion were used to cultivate microalgae, since the residues contain a sufficient quantity of inorganic nutrients required for the growth of algae.
For the first time, expired fruit and vegetable waste from a wholesale market (Stuttgart wholesale market) was very efficiently converted into biogas using a two-stage process in two gas-lift reactors each with a capacity of 3.2 m3. The high-load digestion process for this was developed at Fraunhofer IGB and has been technically realized for sewage sludge several times since 1994; the process was expanded and adapted for this substrate.
With an adjusted hydraulic retention time of 17 days per stage, the system could also be run by changing fruit and vegetable waste in a permanent and stable manner. Degradation levels of up to 95 percent could be reached with the largest part of degradation in stage 1. The biogas yield was between 840 – 920 norm liters of biogas per kilogram TVS (total volatile solids) added; the methane content was 55 – 60 percent.
or the energetic use of algal ingredients, Fraunhofer IGB developed a two-stage, fully automated process for the outdoor production of lipid-rich algal biomass in flat-panel airlift reactors (FPA) and transferred the process to pilot scale.
Closing the material cycles for nitrogen and phosphate between biogas production and algae cultivation was carried out using liquid digestate from biogas reactors as the medium component for the production of algal biomass. A mixed algae culture specifically adapted to this liquid digestate has been successfully cultivated with liquid digestate from the fruit and vegetable waste digestion in 180 liter FPA reactors over a four month period. The ammonium concentration of approximately 800 mg / L contained in the liquid digestate was fully depleted. The biomass concentration in the FPA reactor was 2.5 g / L to 5.5 g / L between the times of harvest. The volumetric biomass productivity fluctuated between 0.1 and 0.35 g L–1 d–1 depending on the weather conditions.
For the first time, digestion of fruit and vegetable waste in changing composition could be carried out in a long-term operation under continuous conditions with a retention time of 17 days and in a stable manner with high degradation level and high biogas yield. The material cycles were closed by utilization of the biogas (not shown) and utilization of the digestate. The use of liquid digestate as a medium component for algae cultivation is one step towards decreased production costs for algal biomass. But the results also show a way to reduce the nitrogen and phosphate loads of liquid digestate and to produce phototrophic biomasses that can be used for material and energy recovery. This result is linked to a reduction of costs in both algae production and in wastewater treatment of biogas processes.
We would like to thank the German Federal Ministry of Education and Research for funding the project “EtaMax – Mehr Biogas aus lignocellulosearmen Abfall- und Mikroalgenreststoffen durch kombinierte Bio- / Hydrothermalvergasung”, promotional reference 03SF0350A, in the “Bio-Energie 2021” program.
Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB