• Energy Research

This paper describes anaerobic thermophilic sludge digestion (55 °C) in a continuously stirred tank reactor (CSTR) on a pilot-plant scale (150 L). The experimental protocol was defined to examine the effect of the increase in the organic loading rate on the efficiency of the digester and to report on its steady-state performance. The reactor was subjected to a programme of steady-state operation over a range of solids retention times (SRTs) of 75, 40, 27, 20 and 15 days and organic loading rates (OLR) in the range 0.4–2.2 kg VS/(m3 day). The digester was fed with raw sludge (containing approximately 35 kg/m3 volatile solids (VS)) once daily during the 75-day SRT period, twice daily during the 40-day SRT period and three times a day during the 27-, 20- and 15-day SRT periods. The reactor was initially operated with an organic loading rate of 0.4 kg VS/(m3 day) and an SRT of 75 days. The volatile solids removal efficiency in the reactor was found to be 73%, while the volumetric methane production rate produced in the digester reached 0.02 m3/(m3 day). Over a 338-day operating period, an OLR of 2.2 kg VS/(m3 day) was achieved with 49.1% VS removal efficiency in the pilot sludge digester, at which time the volumetric methane production rate content of biogas produced in the digester reached 0.4 m3/(m3 day). The chemical oxygen demand (COD) mass balance obtained indicated that COD used for methane generation increased when the SRT was decreased or when the influent organic loading rate was increased. This implies that the amount of COD used in the anabolism route decreased with SRT due the microbial population becoming adapted to new operational conditions and more COD being used to generate methane.

This work presents a new standard in the model, identification, and control of monitoring purposes over anaerobic reactors. One requirement that guarantees a normal controller operation is for the faculty to measure the data needed periodically. Due to its inability to easily obtain the concentrations of acidogenic bacteria and methanogenic archaea periodically using reliable and commercial sensors, this paper presents an algorithm composed of an asymptotic observer (considering the reaction rates are unknown), aiming to estimate these concentrations. This method represents a significant advantage because it is possible to perform a resource-saving strategy using standard measurements, such as pH or alkalinity, to calculate them analytically in natural environments. Additionally, two yield parameters were included in the original anaerobic model two (AM2) to unlock implementations for a wide range of organic substrates. The static parameter identification was improved using a new method called step-ahead optimization. It demonstrates significant improvements fitting the mathematical model to data until a 78.7% increase in efficiency (compared with the traditional optimization method genetic algorithm). After the period of convergence, the state observer evidences a small error with a maximum 2% deviation. Finally, numerical simulations demonstrate the structure’s strengths, which constitutes a significant step in paving the way further to implement feasible, cost-effective controls and monitoring systems in the industry.

10 páginas; 5 figuras; 5 tablas Biochemical methane potential assays were performed to assess the influence of ultrasonic pretreatment on the temperature-phased anaerobic digestion (TPAD) of waste-activated sludge (WAS). Ultrasound (specific energy = 3380 kJ kg−1 TS) was applied to the WAS before the thermophilic stage or to the effluent after the thermophilic stage. In addition, a control system without pretreatment was also carried out. No significant differences were found in the overall performance of the TPAD process, but different behaviours were observed between the thermophilic and mesophilic stages. Total methane production was enhanced by more than 50% and the volatile solid removal increased by 13% in comparison to the TPAD control process. Finally, a previously defined kinetic model was applied successfully to the experimental data and showed an excellent fit. The authors wish to express their gratitude to the Spanish Ministry for the Environment, Rural Affairs and Marine Policy (Project 148/PC08/3-04.3) for providing financial support and to the 2008–2011 Research Plan of the University of Cadiz for supporting the first author through the ‘Contrato Puente para Doctores’. Peer reviewed

A study of the anaerobic digestion of the solid waste generated in the extraction process of sunflower oil (sunflower oil cake, SuOC) was conducted at mesophilic temperature (35 °C) in batch mode. A laboratory-scale multi-reactor system was used to compare the volatile solids (VS) degradation and methane production (G) at inoculum–substrate ratios (ISRs) of 3.0, 2.0, 1.5, 1.0, 0.8 and 0.5 (expressed as VS basis). All tests were carried out against controls of inoculum without substrate. The stability and progress of the reaction from solid substrate to methane as an end product was monitored by measuring the pH, the soluble chemical oxygen demand, and the total volatile fatty acids-total alkalinity (TVFA/TA) ratio. The results obtained demonstrated that in the ISR range from 3.0 to 0.8, the pH ranged from 7.1 to 7.6 and this parameter was always stable during the anaerobic digestion process. In addition, within the above ISR range the TVFA/TA ratios were always lower than the failure limit values (0.3–0.4), which demonstrated the high stability of the anaerobic digestion process of this substrate at mesophilic temperature. Two kinetic models for substrate (VS) degradation and methane production were proposed and evaluated. The apparent kinetic constants for volatile solids degradation (K1) and methane production (K2) decreased from 0.54 ± 0.09 to 0.32 ± 0.03 d−1 and from 0.36 ± 0.04 to 0.16 ± 0.03 d−1, respectively, when the ISR decreased from 3.0 to 0.5, showing the occurrence of an inhibition phenomenon by substrate concentration. The kinetic equations obtained were used to simulate the anaerobic digestion process of SuOC and to obtain the theoretical VS and methane production values. The low deviations obtained (equal to or lower than 10%) between the theoretical and experimental values suggest that the proposed models predict the behaviour of the reactors very accurately. The authors wish to express their gratitude to the European Union (Cropgen project, reference: SES-CT-2004-502824- 6th Frame work programme, Sustainable Energy Systems) and to the “Comisión Interministerial de Ciencia y Tecnología, CICYT” of the Ministry of “Educación y Ciencia” of the Spanish Government (Project CTM 2005-01260/TECNO) for providing financial support. Peer reviewed

Co-hydrothermal carbonization (co-HTC) is a promising strategy to improve hydrothermal carbonization (HTC) of low-quality wastes. HTC of swine manure (SM), with high N (2.9 wt%), S (0.7 wt%) and ash (22.6 wt%) contents, as well as low C (35.6 wt%) and higher heating value (HHV; 14.3 MJ kg-1), resulted in a hydrochar with unsuitable characteristics as a solid fuel. Co-HTC of SM and garden and park waste (GPW) improved hydrochar properties (C content (43 - 48 wt%) and HHV (18 - 20 MJ kg-1), and decreased N (∼2 wt%), S (50 wt%) during co-HTC resulted in a hydrochar similar to that obtained from GPW. The co-HTC increased nutrient migration to the process water, which allowed the precipitation of salt with high P (7.8 wt%) and negligible heavy metal content. Anaerobic digestion of co-HTC process water allowed high organic matter removal (up to 65%), and methane production (315 - 325 mL CH4 g-1CODadded). Gross energy recovery by HTC and anaerobic digestion was 5 - 6-fold higher than anaerobic treatment of feedstocks. Therefore, co-HTC of SM and GPW with a ratio > 50% GPW proved to be a suitable approach to valorize and manage SM and obtain value-added products (hydrochar, mineral fertilizer and methane).

In this work, chicken meat and bones (C-MBM) waste is treated through a sequence of stages including hydrothermal treatment (HTT), nutrient recovery and anaerobic digestion, with the aim of evaluating their potential synergy as a circular economy approach. HTT was carried out at 170, 200 and 230 ◦C, under non-acidic and acidic conditions using 0.5 M HCl (HTT-A). Phosphorous from process water was recovered by chemical precipitation with the addition of a Mg salt, and the liquid effluent was anaerobically treated to degrade organic matter and produce a methane-rich biogas. Hydrochar obtained under non-acidic conditions presented poor combustion characteristics, while HTT-A yielded a bio-oil with high higher heating value (≈38 MJ/kg), good combustibility performance and high reactivity. More than 95% phosphorous (as phosphate) and almost 100% nitrogen (being 30% as NH4–N) content in C-MBM were solubilized in the process water upon HTT-A, while these nutrients were mainly retained in the hydrochar in non-acidic reactions. Chemical precipitation of P and NH4–N from HTT-A process water allowed recovering a crystalline solid identified as struvite and a struvite-apatite mixture, with negligible heavy metals content. High methane production (250–300 mL CH4/g CODadded) and organic matter removal (up to 75%) were achieved in the anaerobic tests. HTT proves to be a suitable treatment for material and energetic valorization of C-MBM, within a circular economy framework, which allows to obtain high value-added products (hydrochar/bio-oil, biofertilizers and biogas) Authors greatly appreciate funding from Spain’s MINECO (PID2019- 108445RB-I00, PDC2021-120755-I00), Madrid Regional Government (Project S2018/EMT-4344) and Grupo Kerbest Company. A. Sarrion wishes to thank the Spanish MICINN and ESF for a research grant (BES2017-081515). R.P. Ipiales acknowledges financial support from Community of Madrid (IND2019/AMB-17092) and Arquimea-Agrotech Company

Anaerobic digestion is considered a competitive source for the production of renewable energy as far as efficiency and cost are concerned. To evaluate the anaerobic biodegradability of an organic substrate such as feedstocks, a test known as biochemical methane potential (BMP) has been commonly used. Current worldwide interest in using different organic substrates for anaerobic bioconversion is growing but there is a lack of clear references and comparability as a result of multiple factors that affect BMP determination. Several batch methods have been used to determine the methane potential. However, these technical approaches vary significantly from one reported method to the next another. In this review, the research works on the influence of different parameters of BMP determination have been discussed for critical and comparative evaluation. In addition, the extensive literature previously published dealing with BMP assays has been compiled and summarized focusing on two main subjects: firstly, methane yields of substrates, and secondly, the description of the various experimental procedures used to achieve the reported data. © 2011 Elsevier Ltd. All rights reserved. The authors wish to express their gratitude to the Spanish Ministry of “Ciencia and Innovación” (Project CTM 2008-05772/Tecno) and Junta de Andalucia for providing financial support, to the Program “FPI” of the above-mentioned ministry for contracting graduates, as well as to the contract JAE-Doc from “Junta para la Ampliación de Estudios del CSIC” co-financed by the European Social Funds. Peer Reviewed

Hydrothermal carbonisation (HTC) is a relatively new alternative for the management of sewage sludge that allows obtaining a HTC char (hydrochar) with a high heating value (≈22 MJ/kg). The aim of this work has been to study the anaerobic digestion of the liquid fraction generated as by-product during HTC (LFHTC) of dewatered sewage sludge, to get more value to the overall process. For this purpose, three different inocula: granular biomass from industrial reactors treating brewery and sugar beet wastewaters and a flocculent biomass from a full-scale digester of municipal sewage sludge, at two initial inoculum concentrations (IC) (10 and 25 g COD/L) were tested. ANOVA test was applied to evaluate the ultimate methane yield for each IC. The effect was different for each inoculum studied: an increase from 10 to 25 g COD/L increased the methane yield by 23% for brewery waste, achieving the highest value obtained (177 ± 5 mL STP CH4/g CODadded), while declining to 99 ± 2 mL STP CH4/g CODadded for sugar beet; it is not affected by the municipal sludge, yielding around 135 mL STP CH4/g CODadded. Therefore, among the inocula tested, brewery waste was the most appropriate for the anaerobic digestion of the LFHTC of dewatered sewage sludge at high IC The authors wish to express their gratitude to the Spanish MINECO (CTM2016-76564-R) for providing financial support. M.A. de la Rubia acknowledges financial support from the Spanish Ministry of Economy and Competitiveness (RYC-2013-12549)