Effects of temperature and mixing modes on the performance of municipal solid waste anaerobic slurry digester

PURPOSE

Anaerobic digestion is a promising technology for simultaneous treatment of biodegradable organic matter of municipal solid waste (MSW) and production of renewable energy. Mixing modes and temperature have influences on biogas production in anaerobic digesters treating MSW. Therefore, in this study, digester was operated at different modes of mixing and temperatures to obtain design criteria.

METHODS

The experiments were carried out in a semi-continuous digester. In the first part of the investigation, temperature was set at 25, 28, 31 and 34 °C. During this step, digester content was mixed in an intermittent mode by mechanical mixers. In the second part of the study, mixing condition of the digester was set at various modes: continuous, intermittent (15 min on and 30 min off) and minimal (twice in a batch).

RESULTS

Digestion with a temperature in this range resulted in biogas yield of 0.23-0.33 m³ biogas/kg VS, with a methane content of 60.2-71.8% in biogas. The methane content and yield decreased with reduction of digestion temperature. However, this reduction was almost negligible from 34 to 31 °C. In addition, in comparison to intermittent mixing, continuous and minimal mixing modes reduced the biogas production by 40% and 50%, respectively. Therefore, in this digester greatest biogas yield of 0.33 ± 0.02 m³ biogas/kg VS were obtained at 34 °C and intermittent mixing mode.

CONCLUSIONS

Based on the data obtained from this study, temperature in the range of 31-34 °C and intermittent mixing is suggested as a base for design purposes.

Flux enhancement of thin-film composite membrane by graphene oxide incorporation

Reverse Osmosis (RO) is a rapid-developing desalination technology; however, it suffers from inefficient energy consumption. To reduce energy consumption, in this study, reverse osmosis thin-film composite membrane (TFC) module was prepared and composed of m-phenylenediamine (MPD), graphene oxide, and 1,3,5-benzenetricarbonyl chloride (TMC) by interfacial polymerization on the surface of a polysulfone substrate. The graphene oxide was embedded in the mentioned thin-film composite by adding it to MPD aqueous solution to enhance permeation flux and, thus, reduce energy consumption. This study assessed the performance of the membrane using a lab-scale RO setup and evaluated permeability and salt rejection. The chemical properties of TFC were also analyzed using ATR-FTIR. Incorporating various concentrations (0, 20, 40, 60, and 80 ppm) of graphene oxide into the TFC was shown to improve water flux. Flux improvement of 50% was achieved by using graphene (80 ppm), while 10% of salt rejection was lost. These flux increases resulted from the changes in surface charge, surface roughness, and hydrophilicity due to the embedment of GO nanosheets. The simplicity of the method, compatibility of GO with polyamide membrane, and quite short-time reaction are the highlights of this technique for developing novel TFC membranes for water treatment.

Oxidation of toluene in humid air by metal oxides supported on γ-alumina

Monometallic and bimetallic supported metal oxides catalysts on γ-alumina were prepared by heterogeneous deposition-precipitation. The γ-alumina used as a support was synthesized by the sol-gel and the co-precipitation methods. Supports and catalysts were characterized by Brunauer-Emmett-Teller (BET) surface area, X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The performance of the prepared catalysts was studied for total oxidation of toluene in air at different relative humidity and oxidation temperatures. Efficiency of bimetallic catalysts for deep oxidation of toluene was higher than copper oxide supported on γ-alumina. Although increasing the lanthanum, cobalt, and nickel loading on the support led to a modified catalyst surface and morphology, the catalytic activity of bimetallic catalysts decreased with increasing lanthanum, cobalt, and nickel content due to the reduced amount of copper oxide which has a higher activity for oxidation of volatile organic compounds. The γ-alumina prepared by the sol-gel method using ethanol as a solvent (AlSE) was the best support and La-Cu/AlSE had the best performance (toluene removal efficiency >90%). In addition, the presence of water vapor in the feed had a negative effect on toluene conversion.

Biodegradation kinetics and interactions of styrene and ethylbenzene as single and dual substrates for a mixed bacterial culture

This study examined biodegradation kinetics of styrene and ethylbenzene as representatives of alkenylbenzenes and mono-alkylbenzenes, respectively. The compounds were studied independently and as binary mixtures using a series of aerobic batch degradation experiments introduced by acclimatized mix culture. Initial concentration of styrene and ethylbenzene in the liquid phase vacillated from 0 to 220 mg/l. The Andrew model was applied for the biodegradation of individual substrates and the estimated constants of the equation for styrene and ethylbenzene were μmax = 0.1581, 0.2090 (1/h), KS =25.91, 37.77 (mg/L), KI =13.15, 62.62 (mg/L), respectively. The accomplished parameters from single substrate degradation tests were used to predict possible interaction factors achieved from dual substrate experiments. The Sum Kinetics with Interaction Parameters (SKIP) model and the purely competitive enzyme kinetics model were employed to evaluate any interactions. The SKIP model was found to accurately describe these interactions. Moreover, it was revealed that ethylbenzene plays an influential role on styrene consumption (e.g. IE,S = 1.64) compared to styrene which has insignificant inhibitory effect on ethylbenzene usage (e.g. IS,E =0.4) . The active site differences for styrene and ethylbenzene biodegradation and the pathway variations for biodegradation are among the major potential reasons for failure of the estimation that occurred in purely competitive kinetics model. This study is the first to calculate the interactions between styrene and ethylbenzene.

Rapid start-up and improvement of granulation in SBR

BACKGROUND

The aim of this study is to accelerate and improve aerobic granulation within a Sequencing Batch Reactor (SBR) by cationic polymer addition.

METHODS

To identify whether the polymer additive is capable of enhancing granule formation, two SBRs (R1 and R2, each 0.15 m in diameter and 2 m in height) are used by feeding synthetic wastewater. The cationic polymer with concentration of 30 to 2 ppm is added to R2, while no cationic polymer is added to R1.

RESULTS

Results show that the cationic polymer addition causes faster granule formation and consequently shorter reactor start-up period. The polymer-amended reactor contains higher concentration of biomass with better settling ability (23% reduction in SVI15) and larger and denser granules (112% increase of granular diameter). In addition, the results demonstrate that the cationic polymer improve the sludge granulation process by 31% increase in Extracellular Polymer Substance(EPS) concentration, 7% increase in Specific Oxygen Uptake Rate(SOUR), 18% increase in hydrophobicity, and 17% reduction in effluent Mixed Liquor Suspended Solid(MLSS) concentration.

CONCLUSIONS

Concludingly, it is found that using the cationic polymer to an aerobic granular system has the potential to enhance the sludge granulation process.

Supported copper and cobalt oxides on activated carbon for simultaneous oxidation of toluene and cyclohexane in air

Dispersed nano mixed copper–cobalt oxides bimetallic catalysts were prepared by deposition–precipitation method for simultaneous and complete oxidation of toluene and cyclohexane.

Anaerobic slurry co-digestion of poultry manure and straw: effect of organic loading and temperature

In order to obtain basic design criteria for anaerobic digestion of a mixture of poultry manure and wheat straw, the effects of different temperatures and organic loading rates on the biogas yield and methane contents were evaluated. Since poultry manure is a poor substrate, in term of the availability of the nutrients, external supplementation of carbon has to be regularly performed, in order to achieve a stable and efficient process. The complete-mix, pilot-scale digester with working volume of 70 L was used. The digestion operated at 25°C, 30°C and 35°C with organic loading rates of 1.0, 2.0, 2.5, 3.0, 3.5 and 4.0 kg Volatile solid/m3d and a HRT of 15 days. At a temperature of 35°C, the methane yield was increased by 43% compared to 25°C. Anaerobic co-digestion appeared feasible with a loading rate of 3.0 kg VS/m3d at 35°C. At this state, the specific methane yield was calculated about 0.12 m3/kg VS with a methane content of 53-70.2% in the biogas. The volatile solid (VS) removal was 72%. As a result of volatile fatty acid accumulation and decrease in pH, when the loading rate was less than 1 or greater than 4 kg VS/m3d, the process was inhibited or overloaded, respectively. Both the lower and higher loading rates resulted in a decline in the methane yield.

Upgrading activated sludge systems and reduction in excess sludge

Most of 200 Activated Sludge Plant in Iran are overloaded and as a result, their efficiency is low. In this work, a pilot plant is manufactured and put into operation in one of the wastewater treatment plants in the west of Tehran. Instead of conventional activated sludge, a membrane bioreactor and an upflow anaerobic sludge blanket reactor used as a pretreatment unit in this pilot. For the sake of data accuracy and precision, an enriched municipal wastewater was opted as an influent to the pilot. Based on the attained result, the optimum retention time in this system was 4h, and the overall COD removal efficiency was 98%. As a whole, the application of this retrofit would increase the plant's capacity by a factor of 5 and reducing the excess sludge by a factor of 10. The sludge volume index in the anaerobic reactor was about 12 after granulation occurred.