Determining the appropriate mixing ratio in a multi-substrate anaerobic digestion of organic solid wastes employing Taguchi method.

The current research proposed a method for optimally combining feed input ratios in order to improve the quantity and quality of daily biogas production through optimizing the variable response level in the Taguchi method. The anaerobic digestion (AD) process of an existing plant in Iran was simulated through a set of two-stage pilot reactors under mesophilic temperature conditions in order to achieve optimal operational performance. Three common substrates (organic fraction of municipal solid wastes, fruit and vegetable wastes, and horse manure) along with two recirculated materials; the post-digestion sludge and the secondary digester slurry, were investigated in 16 experimental runs based on four different pre-surface hypotheses. Comparison of the results of daily biogas energy (J/d/g-VS) in Run#9, to which the actual yield of hydrogen sulfide was minimal in parallel to a methane yield above 100 mL/g-VS, with the result of the optimal run with the ratios provided by the model, showed that the daily biogas energy was improved by 50% comparing to the control Run that had similar conditions to which was being applied in the full-scale existing AD plant.

Energy performance evaluation of ultrasonic pretreatment of organic solid waste in a pilot-scale digester.

It has been proven that ultrasonic pretreatment (UP) has positive effect on biogas generation from previous lab-scale studies. However, that is not always the case in larger scale processes. The purpose of this study was to evaluate the effectiveness of UP to biogas generation in terms of anaerobic digestion process and energy efficiency. Parameters including total solids (TS) and ultrasonic treatment operational parameters of organic solid waste (OSW) resulted from our past lab scale UP studies were applied in this study. OSW with 6-10% TS was treated using a lab-scale ultrasonic processor using various power densities (0.2-0.6 W/mL) at different time periods up to 30 min. Results of lab scale confirmed that OSW with 6% TS sonicated with 0.2 W/mL power density in 30 min gave the best outcome for the pilot scale experiment. To simulate the condition of an actual scale, in addition to energy analysis, two different organic loading rates (OLR), namely 500 and 1500 gVS/m3day were examined. The pilot digester was fed with OSW with or without the pretreatment based on the aforementioned specifications. The results showed that UP effectively improves biogas generation in terms of quantity and quality (CH4/CO2). Furthermore, it decreases the time to reach the maximum cumulative biogas volume comparing to the untreated feed. The key achievement of this research has confirmed that although the relative increase in the energy gain by the influence of UP was more remarkable under the 500 gVS/m3day OLR, energy analysis showed a better energy gain and energy benefit as well as jumping in biogas yield up to 80% for UP treated OSW under 1500 gVS/m3day OLR.

Comparative evaluation of aeration methods for municipal solid waste composting from the perspective of resource management: A practical case study in Tehran, Iran.

During four months of practical composting examination, common aeration techniques including forced aeration static pile, pile turning, natural ventilation static pile and a combination of pile turning and natural ventilation static pile were investigated to determine the most appropriate method for a full-scale composting procedure using the organic fraction of Tehran's municipal solid wastes. The results of measured parameters such as temperature, pH, electrical conductivity (EC), C/N, and main nutrients including nitrogen, phosphorus and potassium suggested that both forced aeration and pile turning have efficacy in terms of final compost quality although pile turning showed better results for agricultural applications nevertheless significant energy consumption and pollutant emissions were associated with them. The combination of pile turning and natural ventilation could solve the problem of long degradation time and concurrently guarantee the acceptable quality of finished compost for agricultural purposes. Furthermore, this combinative method showed a specific energy consumption as low as 0.218 MJ per kg-dry and had a potential to save 288.8 kg-CO2/ha by applying the achieved compost on the farm in order to replace the chemical fertilizers.