Burak Demirel

Production of Methane and Hydrogen from Biomass through Conventional and High-Rate Anaerobic Digestion Processes

Anaerobic digestion processes have often been applied for biological stabilization of solid and liquid wastes. These processes generate energy in the form of biogas. Recently, high-rate methane and hydrogen fermentation from renewable biomass has drawn much attention due to current environmental problems, particularly related to global warming. Therefore, laboratory-scale research on this topic has significantly accelerated. The primary aim of this review paper is to summarize the most recent research activities covering production of methane and hydrogen via both conventional single and high-rate two-phase anaerobic digestion processes of natural sources of biomass.

Application of a fuzzy logic control system for continuous anaerobic digestion of low buffered, acidic energy crops as mono-substrate

A fuzzy logic control (FLC) system was developed at the Hamburg University of Applied Sciences (HAW Hamburg) for operation of biogas reactors running on energy crops. Three commercially available measuring parameters, namely pH, the methane (CH4) content, and the specific gas production rate (spec. GPR = m3/kg VS/day) were included. The objective was to avoid stabilization of pH with use of buffering supplements, like lime or manure. The developed FLC system can cover most of all applications, such as a careful start‐up process and a gentle recovery strategy after a severe reactor failure, also enabling a process with a high organic loading rate (OLR) and a low hydraulic retention time (HRT), that is, a high throughput anaerobic digestion process with a stable pH and CH4 content. A precondition for a high load process was the concept of interval feeding, for example, with 8 h of interval. The FLC system was proved to be reliable during the long term fermentation studies over 3 years in one‐stage, completely stirred tank reactors (CSTR) with acidic beet silage as mono‐input (pH 3.3–3.4). During fermentation of the fodder beet silage (FBS), a stable HRT of 6.0 days with an OLR of up to 15 kg VS/m3/day and a volumetric GPR of 9 m3/m3/day could be reached. The FLC enabled an automatic recovery of the digester after two induced severe reactor failures. In another attempt to prove the feasibility of the FLC, substrate FBS was changed to sugar beet silage (SBS), which had a substantially lower buffering capacity than that of the FBS. With SBS, the FLC accomplished a stable fermentation at a pH level between 6.5 and 6.6, and a volatile fatty acid level (VFA) below 500 mg/L, but the FLC had to interact and to change the substrate dosage permanently. In a further experiment, the reactor temperature was increased from 41 to 50°C. Concomitantly, the specific GPR, pH and CH4 dropped down. Finally, the FLC automatically enabled a complete recovery in 16 days. Biotechnol. Bioeng. 2009; 102: 736–748.

The impact of Ni, Co and Mo supplementation on methane yield from anaerobic mono-digestion of maize silage

The objective of this experimental study was to demonstrate the impact of trace metal supplementation, namely nickel (Ni), cobalt (Co) and molybdenum (Mo), on the methane yields obtained from batch mesophilic anaerobic digestion of maize silage as mono-substrate. The maize silage used in this experimental work initially lacked Ni and Co. Trace metal concentration selected was 0.1 and 0.5 mg/L for Ni and Co, respectively, while it was 0.05 and 0.25 mg/L for Mo. The supplementation by Ni, Co and Mo, individually or in combination at different doses, seemed to improve the methane yields for mono-digestion of maize silage and particularly, the highest methane yield of 0.429 L CH4/g VSadded was obtained, when Ni, Co and Mo were supplemented together at concentrations of 0.5, 0.5 and 0.25 mg/L, respectively.

Determination of biogas generation potential as a renewable energy source from supermarket wastes

Fruit, vegetable, flower waste (FVFW), dairy products waste (DPW), meat waste (MW) and sugar waste (SW) obtained from a supermarket chain were anaerobically digested, in order to recover methane as a source of renewable energy. Batch mesophilic anaerobic reactors were run at total solids (TS) ratios of 5%, 8% and 10%. The highest methane yield of 0.44 L CH4/g VS(added) was obtained from anaerobic digestion of wastes (FVFW+DPW+MW+SW) at 10% TS, with 66.4% of methane (CH4) composition in biogas. Anaerobic digestion of mixed wastes at 5% and 8% TS provided slightly lower methane yields of 0.41 and 0.40 L CH4/g VS(added), respectively. When the wastes were digested alone without co-substrate addition, the highest methane yield of 0.40 L CH4/g VS(added) was obtained from FVFW at 5% TS. Generally, although the volatile solids (VS) conversion percentages seemed low during the experiments, higher methane yields could be obtained from anaerobic digestion of supermarket wastes. A suitable carbon/nitrogen (C/N) ratio, proper adjustment of the buffering capacity and the addition of essential trace nutrients (such as Ni) could improve VS conversion and biogas production yields significantly.

Recovery of biogas as a source of renewable energy from ice-cream production residues and wastewater

Proper management of waste streams and residues from agro-industry is very important to prevent environmental pollution. In particular, the anaerobic co-digestion process can be used as an important tool for safe disposal and energy recovery from agro-industry waste streams and residues. The primary objective of this laboratory-scale study was to determine whether it was possible to recover energy (biogas) from ice-cream production residues and wastewater, through a mesophilic anaerobic co-digestion process. A high methane yield of 0.338 L CH4/g CODremoved could be achieved from anaerobic digestion of ice-cream wastewater alone, with almost 70% of methane in biogas, while anaerobic digestion of ice-cream production residue alone did not seem feasible. When wastewater and ice-cream production residue were anaerobically co-digested at a ratio of 9:1 by weight, the highest methane yield of 0.131 L CH4/gCODremoved was observed. Buffering capacity seemed to be imperative in energy recovery from these substrates in the anaerobic digestion process.

Methods of ammonia removal in anaerobic digestion: a review

The anaerobic digestion of substrates with high ammonia content has always been a bottleneck in the methanisation process of biomasses. Since microbial communities in anaerobic digesters are sensitive to free ammonia at certain conditions, the digestion of nitrogen-rich substrates such as livestock wastes may result in inhibition/toxicity eventually leading to process failures, unless appropriate engineering precautions are taken. There are many different options reported in literature to remove ammonia from anaerobic digesters to achieve a safe and stable process so that along with high methane yields, a good quality of effluents can also be obtained. Conventional techniques to remove ammonia include physical/chemical methods, immobilization and adaptation of microorganisms, while novel methods include ultrasonication, microwave, hollow fiber membranes and microbial fuel cell applications. This paper discusses conventional and novel methods of ammonia removal from anaerobic digesters using nitrogen-rich substrates, with particular focus on recent literature available about this topic.

Impact of food waste fraction in municipal solid waste on sorption of heavy metals.

The presence of organic materials plays an important role in the fate of heavy metals that are co-disposed together with municipal solid wastes. As a part of an on-going research project, which aims to find out the most effective attenuation mechanism of heavy metal removal in landfills, sorption batch experiments were performed to assess the sorption behaviour of iron, copper, nickel and zinc on synthetic solid wastes containing 76% (W1) and 45% (W2) food waste percentages and waste-to-solution ratios ranging from 1:4 to 1:16. The analysis of sorption data suggested that the data fit a Freundlich equilibrium isotherm. The time required for reaching equilibrium conditions varied for each metal investigated, but all generally reached equilibrium conditions within 7 h. For both solid waste compositions, metal sorption increased with increase in waste-to-solution ratio, with the order of metal removal percentages consistently found to be Zn > Ni > Cu > Fe. The results also show that a large fraction of the heavy metals could be attenuated by sorption on the solid waste. The removal percentages for Zn and Ni were slightly higher for W2, whereas the removal percentages for Fe and Cu were approximately equal for both waste types. Overall, this study demonstrates that sorption is a viable process that can mitigate the potential adverse impacts of landfill leachate.