• Energy Research

The paper reports the findings of a two-year pilot scale experimental trial for the mesophilic (35°C), thermophilic (55°C) and temperature phased (65+55°C) anaerobic digestion of waste activated sludge. During the mesophilic and thermophilic runs, the reactor operated at an organic loading rate of 2.2 kgVS/m(3)d and a hydraulic retention time of 20 days. In the temperature phased run, the first reactor operated at an organic loading rate of 15 kgVS/m(3)d and a hydraulic retention time of 2 days while the second reactor operated at an organic loading rate of 2.2 kgVS/m(3)d and a hydraulic retention time of 18 days (20 days for the whole temperature phased system). The performance of the reactor improved with increases in temperature. The COD removal increased from 35% in mesophilic conditions, to 45% in thermophilic conditions, and 55% in the two stage temperature phased system. As a consequence, the specific biogas production increased from 0.33 to 0.45 and to 0.49 m(3)/kgVS(fed) at 35, 55, and 65+55°C, respectively. The extreme thermophilic reactor working at 65°C showed a high hydrolytic capability and a specific yield of 0.33 g COD (soluble) per gVS(fed). The effluent of the extreme thermophilic reactor showed an average concentration of soluble COD and volatile fatty acids of 20 and 9 g/l, respectively. Acetic and propionic acids were the main compounds found in the acids mixture. Because of the improved digestion efficiency, organic nitrogen and phosphorus were solubilised in the bulk. Their concentration, however, did not increase as expected because of the formation of salts of hydroxyapatite and struvite inside the reactor.

AbstractThis work investigates an innovative process to valorise agricultural digestate by the exploitation of solar energy. Digestate has been located in a lab-scale greenhouse to evaporate the liquid phase. Digestate vapours, rich in ammonia, are sent in a Drechsler trap, filled with 38%w/wsulfuric acid solution, through three solar air fans. A concentration of about 2 M of ammonium sulphate solution was recovered. The remaining dried solid phase, rich in phosphorous compounds, was evaluated as alternative to the commercial fertilizers (46% P2O5) in the growth of maize plants. Equal amount of P was applied to each pot (25 mg/kg soil). The plants were evaluated along the 8 weeks in a greenhouse monitoring the growth parameters and leaf SPAD index, micro-, macronutrients and non-essential heavy metals. The results evidenced that the dried solid phase of digestate can be used as an alternative source of P.Graphic Abstract

Abstract Six million tons of Coffee Spent Grounds (SCG) are produced every year all around the world. Their physical and chemical characterization, rich in high value molecules and organic compounds, make SCG ideal for the recovery of bioactive molecules and bioenergy production according to the “cascade biorefinery approach”. This work investigates the effects of the implementation of sequential processes for the recovery of the coffee oil, rich in valuable molecules, and the productions of bioethanol and biogas from SCG. The use of a 50:50 (v/v) ethanol-iso-propanol mixture allowed a coffee oil recovery of about 16% w/w. In particular, the extracted coffee oil was rich in tocopherols (about 15 mg/100 g SCG). On the other hand, n-hexane showed a lower performance in terms of coffee oil extraction (10% w/w) but allowed for the extraction of different molecules: linoleic and palmitic acids (38% and 34% w/w of the coffee oil, respectively) and of Cafestol (383 mg/100g SCG) and Kahweol (194 mg/100g SCG). The extracted SCG underwent to an acid-enzymatic hydrolysis process followed by a solid/liquid separation. The liquid fraction was then used for the bioethanol production which reached the final concentration of 50 g/L, while the solid fraction was used for biogas production by Anaerobic Digestion. The final methane yield resulted in a production of about 250 NLCH4/kgVS. A complete train of operations, composing a biorefinery approach, can be therefore adopted to fully valorise this particular organic waste.

Abstract In this paper are presented the results of the investigation on optimal process operational conditions of thermophilic dark fermentation and anaerobic digestion of food waste, testing a long-term run, applying an organic loading rate of 16.3 kgTVS/m3d in the first phase and 4.8 kgTVS/m3d in the second phase. The hydraulic retention times (HRTs) were maintained at 3.3 days and 12.6 days, respectively, for the first and second phase. Recirculation of anaerobic digested sludge, after a mild solid separation, was applied to the dark fermentation reactor in order to control the pH in the optimal hydrogen production range of 5–6. It was confirmed the possibility to obtain a stable hydrogen production, without using external chemicals for pH control, in a long-term test, with a specific hydrogen production of 66.7 l per kg of total volatile solid (TVS) fed and a specific biogas production in the second phase of 0.72 m3 per kgTVS fed; the produced biogas presented a typical composition with a stable presence of hydrogen and methane in the biogas mixture around 6 and 58%, respectively, carbon dioxide being the rest.

Abstract The “European Strategy for plastics” approved by the European Union aims at drastically reducing the use of plastic materials derived from fossil resources, especially single use plastic (SUP). As a consequence, the adoption of biodegradable plastics is forced by different national regulations, especially in France and Italy which banned the usage of single use plastics. Being classified as biodegradable and compostable, the major part of these materials is often collected with the Organic Fraction of Municipal Solid Wastes (OFMSW), basically due by food waste, and sent to biological treatment plants, namely composting and anaerobic digestion for bioenergy recovery or their combination. This study tested the specific methane production and the relative kinetics of the most common single use biodegradable items (carrier bag, cutlery and plates), available on the market. It was demonstrated that sugar cane cellulosic pulp materials have good methane production of 390 L CH4 /kg TVS and a kinetic which is consistent with the anaerobic digestion’s residence time typically applied for OFMSW. On the contrary, starch-based bioplastic and PLA materials remained almost undegraded after 250 days and showed low specific methane production yields in the range 100–200​ L CH4 /kg TVS . The adoption of acidic and basic pretreatments improved the anaerobic digestion performances of starch-based bioplastic and PLA samples. Materials made of poly-hydroxy-alkanoates (PHA) showed higher methane production rates, up to 402 L CH4 /kg TVS in short residence times (around 10 days), which make them adequate to be treated together with food waste in anaerobic digestion plants.

The performance of mesophilic anaerobic digesters of four large Italian wastewater treatment plants without primary sedimentation were studied. Only the waste activated sludge is stabilised by means of the mesophilic (35‐37 8C) anaerobic digestion process. The anaerobic digesters generally worked with a hydraulic retention time in a range of 20‐40 days and an organic loading rate of some 1 kg VS/m 3 day. The solids content of the sludge fed to the digesters was in the range 2.6‐3.9% and the gas produced per kilogram of volatile solids added was in the range 0.07‐0.18 m 3 /kg VSfed. The specific gas production per kilogram of volatile solids destroyed was in the range 0.5‐0.9 m 3 / kg VSdestroyed and the reduction of the volatile solids concentration was in the range 13‐27% (average 18%). These figures are particularly significant when designing anaerobic digesters for the treatment of waste activated sludge as single substrate. Moreover, it was observed that the higher the applied solid retention time in the activated sludge process for wastewater treatment, the lower the gas production. In particular, the specific gas production decreased from 0.18 to 0.07 m 3 /kg VSfed when increasing the solid retention time in the wastewater treatment line from 8 to 35 days. Finally, a mathematical model for the prediction of biogas production on the solid retention time applied in the wastewater treatment process was developed. # 2004 Elsevier Ltd. All rights reserved.

Spent coffee grounds are rich in high-value compounds, such as saturate and unsaturated fatty acids, and polysaccharides. Therefore, this work investigated a cascade biorefinery to produce: i) biodiesel from coffee oils, ii) cellulose- and hemicellulose-derived fermentable sugars and iii) biomethane from the residual solid fraction after sugars extraction. Transesterification reached the best performances of 86% w/w of fatty acid methyl esters using 1:8 coffee oil/methanol ratio and 2% w/w of KOH as catalyst. The use of glycerol for the pretreatment of spent coffee grounds allowed the internal circulation of a process leftover from transesterification; thus, avoiding the use of clean water. In the best conditions, the total released fermentable sugars were about 40-50% (w/w) on dry weight basis. The low content of easily degradable compounds led to a low methane production of 50 LCH4/kgVS, indicating the need to search for better performing alternatives to close the biorefinery loop.

Abstract The winemaking process generates many by-products besides wastewater, mainly grape marcs, grape stalks, and wine lees. Anaerobic digestion is particularly suitable to treat winery waste because of its high content of nutrient-rich organic matter and for its noticeable energetic potential. To date, only results from mesophilic tests have been extensively reported. In this study, potential methane production and kinetic constants were determined by batch trials under thermophilic conditions and compared with mesophilic values already reported in literature. Grape marcs and wine lees appeared to be the most promising substrates with an estimated potential of 0.34 and 0.37 Nm 3 CH 4 /kgVS fed , respectively, while grape stalks generated only 0.13 Nm 3 CH 4 /kgVS fed . In order to assess the feasibility of a continuous anaerobic digestion process, a lab-scale semi-continuous reactor was constructed. Because of the consumption of buffer capacity, the biological process was difficult to control. On the other hand, biogas was produced when working with a hydraulic retention time of 40 d and with previously fermented grape marcs; a specific biogas production of 0.29 Nm 3 /kgVS fed was observed. The results of the continuous tests were used to calculate the potential energy recovery from grape marcs produced in Italy (808 thousands of tons per year) in terms of heat and electricity; about 245 GWh of heat and 201 GWh of electricity per annum could be generated in Italian scenario.