Detailed monitoring of two biogas plants and mechanical solid-liquid separation of fermentation residues

The Austrian “green electricity act” (Ökostromgesetz) has led to an increase in biogas power plant size and consequently to an increased use of biomass. A biogas power plant with a generating capacity of 500kWel consumes up to 38,000kg of biomass per day. 260ha of cropland is required to produce this mass. The high water content of biomass necessitates a high transport volume for energy crops and fermentation residues. The transport and application of fermentation residues to farmland is the last step in this logistic chain. The use of fermentation residues as fertilizer closes the nutrient cycle and is a central element in the efficient use of biomass for power production. Treatment of fermentation residues by separation into liquid and solid phases may be a solution to the transport problem. This paper presents detailed results from the monitoring of two biogas plants and from the analysis of the separation of fermentation residues. Furthermore, two different separator technologies for the separation of fermentation residues of biogas plants were analyzed. The examined biogas plants correspond to the current technological state of the art and have designs developed specifically for the utilization of energy crops. The hydraulic retention time ranged between 45.0 and 83.7 days. The specific methane yields were 0.40-0.43m³ N CH₄ per kg VS. The volume loads ranged between 3.69 and 4.00kg VS/m³. The degree of degradation was between 77.3% and 82.14%. The screw extractor separator was better suited for biogas slurry separation than the rotary screen separator. The screw extractor separator exhibited a high throughput and good separation efficiency. The efficiency of slurry separation depended on the dry matter content of the fermentation residue. The higher the dry matter content, the higher the proportion of solid phase after separation. In this project, we found that the fermentation residues could be divided into 79.2% fluid phase with a dry matter content of 4.5% and 20.8% solid phase with a dry matter content of 19.3%. Dry matter, volatile solids and carbon, raw ash and phosphate - in relation to the mass - accumulated strongly in the solid phase. Nitrogen and ammonia nitrogen were slightly enriched in the solid phase. Only the potassium content decreased slightly in the solid phase.