I. Díaz

Anaerobic membrane bioreactors: Are membranes really necessary?

Membranes themselves represent a significant cost for the full scale application of anaerobic membrane bioreactors (AnMBR). The possibility of operating an AnMBR with a self-forming dynamic membrane generated by the substances present in the reactor liquor would translate into an important saving. A self-forming dynamic membrane only requires a support material over which a cake layer is formed, which determines the rejection properties of the system. The present research studies the application of self-forming dynamic membranes in AnMBRs. An AnMBR was operated under thermophilic and mesophilic conditions, using woven and non woven materials as support for the dynamic membranes. Results showed that the formation of a cake layer over the support materials enables the retention of more than 99% of the solids present in the reactor. However, only low levels of flux were achieved, up to 3 L/m2 x h, and reactor operation was unstable, with sudden increases in filtration resistance, due to excessive cake layer formation. Further fine-tuning of the proposed technology involves looking for conditions that can control effectively cake layer formation

Effect of oxygen dosing point and mixing on the microaerobic removal of hydrogen sulphide in sludge digesters

Limited oxygen supply to anaerobic sludge digesters to remove hydrogen sulphide from biogas was studied. Micro-oxygenation showed competitive performance to reduce considerably the additional equipment necessary to perform biogas desulphurization. Two pilot-plant digesters with an HRT of ∼ 20 d were micro-oxygenated at a rate of 0.25 NL per L of feed sludge with a removal efficiency higher than 98%. The way of mixing (sludge or biogas recirculation) and the point of oxygen supply (headspace or liquid phase) played an important role on hydrogen sulphide oxidation. While micro-oxygenation with sludge recirculation removed only hydrogen sulphide from the biogas, dissolved sulphide was removed if micro-oxygenation was performed with biogas recirculation. Dosage in the headspace resulted in a more stable operation. The result of the hydrogen sulphide oxidation was mostly elemental sulphur, partially accumulated in the headspace of the digester, where different sulphide-oxidising bacteria were found.

Performance evaluation of oxygen, air and nitrate for the microaerobic removal of hydrogen sulphide in biogas from sludge digestion

The removal performance of hydrogen sulphide in severely polluted biogas produced during the anaerobic digestion of sludge was studied by employing pure oxygen, air and nitrate as oxidant reactives supplied to the biodigester. Research was performed in a 200-L digester with an hydraulic retention time (HRT) of ∼20 days under mesophilic conditions. The oxygen supply (0.25 N m³/m³ feed) to the bioreactor successfully reduced the hydrogen sulphide content from 15,811 mg/N m³ to less than 400 mg/N m³. The introduction of air (1.27 N m³/m³ feed) removed more than 99% of the hydrogen sulphide content, with a final concentration of ∼55 mg/N m³. COD removal, VS reduction and methane yield were not affected under microaerobic conditions; however, methane concentration in the biogas decreased when air was employed as a result of nitrogen dilution. The nitrate addition was not effective for hydrogen sulphide removal in the biogas.

Microaeration for hydrogen sulfide removal during anaerobic treatment: a review

High sulfide concentrations in biogas are a major problem associated with the anaerobic treatment of sulfate-rich substrates. It causes the corrosion of concrete and steel, compromises the functions of cogeneration units, produces the emissions of unpleasant odors, and is toxic to humans. Microaeration, i.e. the dosing of small amounts of air (oxygen) into an anaerobic digester, is a highly efficient, simple and economically feasible technique for hydrogen sulfide removal from biogas. Due to microaeration, sulfide is oxidized to elemental sulfur by the action of sulfide oxidizing bacteria. This process takes place directly in the digester. This paper reviews the most important aspects and recent developments of microaeration technology. It describes the basic principles (microbiology, chemistry) of microaeration and the key technological factors influencing microaeration. Other aspects such as process economy, mathematical modelling and control strategies are discussed as well. Besides its advantages, the limitations of microaeration such as partial oxidation of soluble substrate, clogging the walls and pipes with elemental sulfur or toxicity to methanogens are pointed out as well. An integrated mathematical model describing microaeration has not been developed so far and remains an important research gap.

A feasibility study on the bioconversion of CO2 and H2 to biomethane by gas sparging through polymeric membranes.

In this study, the potential of a pilot hollow-fiber membrane bioreactor for the conversion of H2 and CO2 to CH4 was evaluated. The system transformed 95% of H2 and CO2 fed at a maximum loading rate of 40.2 [Formula: see text] and produced 0.22m(3) of CH4 per m(3) of H2 fed at thermophilic conditions. H2 mass transfer to the liquid phase was identified as the limiting step for the conversion, and kLa values of 430h(-1) were reached in the bioreactor by sparging gas through the membrane module. A simulation showed that the bioreactor could upgrade biogas at a rate of 25m(3)/mR(3)d, increasing the CH4 concentration from 60 to 95%v. This proof-of-concept study verified that gas sparging through a membrane module can efficiently transfer H2 from gas to liquid phase and that the conversion of H2 and CO2 to biomethane is feasible on a pilot scale at noteworthy load rates.

Robustness of the microaerobic removal of hydrogen sulfide from biogas

Several disturbances presented in full-scale digesters can potentially affect the efficiency of the microaerobic removal process. This study evaluates the variation of the sulfur load and the performance of the system in situations of oxygen lack or excess and after normal rates are recovered. The process was shown to recover from oxygen lack or excess within 28 h when the original conditions were restored in a pilot-plant digester of 200 L treating sewage sludge with HRT of 20 days. The decrease of the sulfur load to the digester did not affect the biogas composition in the short-term and when oxygen rate was reduced to adjust to the lower hydrogen sulfide production, the removal proceeded normally with a lower unemployed oxygen amount. The digester opening to remove accumulated sulfur in the headspace did not alter process performance once the microaerobic removal was restarted.

Economic analysis of microaerobic removal of H2S from biogas in full-scale sludge digesters

The application of microaerobic conditions during sludge digestion has been proven to be an efficient method for H2S removal from biogas. In this study, three microaerobic treatments were considered as an alternative to the technique of biogas desulfurization applied (FeCl3 dosing to the digesters) in a WWTP comprising three full-scale anaerobic reactors treating sewage sludge, depending on the reactant: pure O2 from cryogenic tanks, concentrated O2 from PSA generators, and air. These alternatives were compared in terms of net present value (NPV) with a fourth scenario consisting in the utilization of iron-sponge-bed filter inoculated with thiobacteria. The analysis revealed that the most profitable alternative to FeCl3 addition was the injection of concentrated O2 (0.0019 €/m(3) biogas), and this scenario presented the highest robustness towards variations in the price of FeCl3, electricity, and in the H2S concentration.

The role of the headspace in hydrogen sulfide removal during microaerobic digestion of sludge.

The role of the headspace (HS) in the microaerobic removal of hydrogen sulfide from biogas produced during sludge digestion was studied. Research was carried out in a pilot reactor with a total volume of 265 L, under mesophilic conditions. Biogas was successfully desulfurized (99%) by introducing pure oxygen (0.46 NL/L(fed)) into the recirculation stream when the HS volume was both 50.0 and 9.5 L. The removal efficacy dropped sharply to ≈15% when the HS was reduced to 1.5 L. The system responded quickly to the operational changes imposed: micro-oxygenation stops and variations in supply, as well as HS volume reductions and increases. As the final result, the microaerobic process required a minimum surface into the gas space to occur, which along with the elemental sulfur deposition in this area indicated that the oxidation took place there. Additionally, the pattern of sulfur accumulation suggested that the removal occurred preferentially on certain materials, and pointed to a significant biological contribution.

Biogas Purification and Upgrading Technologies

The fact that most countries do not promote the use of biogas as energy vector via tax incentives entails the need for an optimization of biogas upgrading technologies in order to support a cost-competitive utilization of this renewable energy source. Nowadays, the contaminants present in biogas such as CO2, H2S, H2O, N2, O2, siloxanes, and halocarbons are removed through the implementation of costly and environmentally unfriendly upgrading processes. Conventional biogas upgrading is based on physical/chemical technologies leading to CH4 purities of 88–98% and removal efficiencies of higher than 99% for H2S, halocarbons, and siloxanes. Unfortunately, their high energy and chemical demands limit the environmental and economic sustainability of these conventional biogas upgrading technologies. In this sense, biological processes have emerged in the past decade as an economic and environmentally friendly alternative to conventional biogas upgrading technologies. Thus, biotechnologies such as microalgae-based CO2 fixation, H2-assisted litoautotrophic CO2 bioconversion to CH4, enzymatic CO2 dissolution or fermentative CO2 reduction have been consistently shown to result in CO2 removals of 80–100% with CH4 purities of 88–100%, while allowing the valorization of CO2 into bioproducts of commercial interest (therefore preventing its release to the atmosphere). Similarly, H2S removals > 99% are consistently achieved in aerobic and anoxic biotrickling filters, algal-bacterial photobioreactors, and digesters under microaerobic conditions. In addition, recent investigations have shown the potential biodegradability of siloxanes and halocarbons under both aerobic and anaerobic conditions. This chapter constitutes a state of the art comparison of physical/chemical and biological technologies for the removal of CO2, H2S, halocarbons, and siloxanes from biogas.

Evaluation of process performance, energy consumption and microbiota characterization in a ceramic membrane bioreactor for ex-situ biomethanation of H 2 and CO 2

The performance of a pilot ceramic membrane bioreactor for the bioconversion of H2 and CO2 to bioCH4 was evaluated in thermophilic conditions. The loading rate was between 10 and 30 m3 H2/m3reactor d and the system transformed 95% of H2 fed. The highest methane yield found was 0.22 m3 CH4/m3 H2, close to the maximum stoichiometric value (0.25 m3 CH4/m3 H2) thus indicating that archaeas employed almost all H2 transferred to produce CH4. kLa value of 268 h-1 was reached at 30 m3 H2/m3reactor d. DGGE and FISH revealed a remarkable archaeas increase related to the selection-effect of H2 on community composition over time. Methanothermobacter thermautotrophicus was the archaea found with high level of similarity. This study verified the successful application of membrane technology to efficiently transfer H2 from gas to the liquid phase, the development of a hydrogenotrophic community from a conventional thermophilic sludge and the technical feasibility of the bioconversion.