Multiphase CFD modeling to evaluate and to improve mixing in Chinese dome digester

Household or domestic biogas plants constitute a growing sub-sector of the anaerobic digestion industry worldwide, but had received low research attention for improvements. The Chinese dome digester (CDD), a major type of domestic biogas plant, is a naturally mixed, unheated and low tech system that is mainly used in rural areas. In this study, a multiphase computational fluid dynamics (CFD) model was applied to evaluate and subsequently improve mixing in a lab scale Chinese dome digester. The normal Chinese dome digester (CDD1) and two baffle configurations were investigated to improve the hydraulic mixing in the digester (CDD2 and CDD3 respectively) . 2-D time dependent numerical simulations were done with the three-phase, phase field model in COMSOL Multiphysics in a CDD geometry. Residence time distribution (RTD) curves were derived for all the configurations to evaluate and compare performances. In addition, three hydraulic indicators were also studied to evaluate mixing improvement. The Anaerobic digestion model No. 1 (ADM1) was used to evaluate biogas production. The effects of the addition of baffles to the CDDs did not significantly improve mixing, however about 16 % of dead zones was reduced in the two-baffle configuration

Mechanisms of arsenate removal and membrane fouling in ferric based coprecipitation-low pressure membrane filtration systems

Ferric based coprecipitation-low pressure membrane filtration is a promising arsenic (As) removal method, however, membrane fouling mechanisms are not fully understood. In this study we investigated the effect of feed water composition and membrane pore size on arsenate [As(V)] removal and membrane fouling. We observed that As removal efficiency was independent of the membrane pore size because the size of the Fe(III) particles was larger than the pore size of the membranes, attributed to a high calcium concentration in the feed water. Arsenic coprecipitation with Fe(III) (oxyhydr)oxides rapidly reached equilibrium before membrane filtration, within 1 min. Therefore, As removal efficiency was not improved by increasing residence time before membrane filtration. The removal of As(V) was strongly dependent on feed water composition. A higher Fe(III) dose was required to reduce As(V) to sub-mu g/L levels for feed water containing higher concentration of oxyanions such as phosphate and silicate, and lower concentration of cations such as calcium. Cake-layer formation was observed to be the predominant membrane fouling mechanism

Mechanisms of arsenate removal and membrane fouling in ferric based coprecipitation–low pressure membrane filtration systems

Ferric based coprecipitation–low pressure membrane filtration is a promising arsenic (As) removal method, however, membrane fouling mechanisms are not fully understood. In this study we investigated the effect of feed water composition and membrane pore size on arsenate [As(V)] removal and membrane fouling. We observed that As removal efficiency was independent of the membrane pore size because the size of the Fe(III) particles was larger than the pore size of the membranes, attributed to a high calcium concentration in the feed water. Arsenic coprecipitation with Fe(III) (oxyhydr)oxides rapidly reached equilibrium before membrane filtration, within 1 min. Therefore, As removal efficiency was not improved by increasing residence time before membrane filtration. The removal of As(V) was strongly dependent on feed water composition. A higher Fe(III) dose was required to reduce As(V) to sub-µg/L levels for feed water containing higher concentration of oxyanions such as phosphate and silicate, and lower concentration of cations such as calcium. Cake-layer formation was observed to be the predominant membrane fouling mechanism.</p

SEEC: Student Enrollment and Engagement through Connections (SEEC)

The Student Enrollment and Engagement through Connections (SEEC) is a five year project funded by the National Science Foundation’s STEM Talent Expansion Program (STEP). The goal of the SEEC project is to increase the number of engineering graduates at Iowa State University by approximately 100 per year. In addition, the percentage of women and minority graduates will approach 20% and 10%, respectively. The project is a collaborative partnership between Iowa State University (ISU) and Des Moines Area Community College (DMACC). Project objectives are designed within the areas of learning communities, curriculum, advising, networking, and evaluation. Activities are planned in each of these areas using a logic model approach that identifies resources, outputs, outcomes, and impact

Effect of mixing regimes on cow manure digestion in impeller mixed, unmixed and Chinese dome digesters

<p>This study examines the effect of mixing on the performance of anaerobic digestion of cow manure in Chinese dome digesters (CDDs) at ambient temperatures (27–32 ^◦C) in comparison with impeller mixed digesters (STRs) and unmixed digesters (UMDs) at the laboratory scale. The CDD is a type of household digester used in rural and pre-urban areas of developing countries for cooking. They are mixed by hydraulic variation during gas production and gas use. Six digesters (two of each type) were operated at two different influent total solids (TS) concentration, at a hydraulic retention time (HRT) of 30 days for 319 days. The STRs were mixed at 55 rpm, 10 min/hour; the unmixed digesters were not mixed, and the Chinese dome digesters were mixed once a day releasing the stored biogas under pressure. The reactors exhibited different specific biogas production and treatment efficiencies at steady state conditions. The STR 1 exhibited the highest methane (CH₄) production and treatment efficiency (volatile solid (VS) reduction), followed by STR 2. The CDDs performed better (10% more methane) than the UMDs, but less (approx. 8%) compared to STRs. The mixing regime via hydraulic variation in the CDD was limited despite a higher volumetric biogas rate and therefore requires optimization.</p

Location of the inlets and outlets of Chinese dome digesters to mitigate biogas emission

A model based on three equations was developed and validated for the design of the inlet and outlet of the Chinese dome digester (CDD) to prevent biogas emission or leakage. The model was implemented in MATLAB software and validated with results from a pilot study. Biogas and temperature data from the pilot experiment were used to validate the model and the results fitted well with the experimental data at low gas volume (G) and the slurry displacement in the expansion chamber, inlet pipe head (h) and head inside the digester (hG). The relationship between the gas and h was not linear at high gas volume (>20 mols). The model can be used to predict the optimal size of the CDD for daily biogas storage based on different reactor and expansion chamber sizes, in order to mitigate the emission of surplus biogas from the inlet and outlet pipes.</p

Effect of mixing regimes on cow manure digestion in impeller mixed, unmixed and Chinese dome digesters

This study examines the effect of mixing on the performance of anaerobic digestion of cow manure in Chinese dome digesters (CDDs) at ambient temperatures (27–32 ◦C) in comparison with impeller mixed digesters (STRs) and unmixed digesters (UMDs) at the laboratory scale. The CDD is a type of household digester used in rural and pre-urban areas of developing countries for cooking. They are mixed by hydraulic variation during gas production and gas use. Six digesters (two of each type) were operated at two different influent total solids (TS) concentration, at a hydraulic retention time (HRT) of 30 days for 319 days. The STRs were mixed at 55 rpm, 10 min/hour; the unmixed digesters were not mixed, and the Chinese dome digesters were mixed once a day releasing the stored biogas under pressure. The reactors exhibited different specific biogas production and treatment efficiencies at steady state conditions. The STR 1 exhibited the highest methane (CH4) production and treatment efficiency (volatile solid (VS) reduction), followed by STR 2. The CDDs performed better (10% more methane) than the UMDs, but less (approx. 8%) compared to STRs. The mixing regime via hydraulic variation in the CDD was limited despite a higher volumetric biogas rate and therefore requires optimization.</p

Development of an optimised Chinese dome digester enables smaller reactor volumes; pilot scale performance

Chinese dome digesters are usually operated at long hydraulic retention times (HRT) and low influent total solids (TS) concentration because of limited mixing. In this study, a newly optimised Chinese dome digester with a self-agitating mechanism was investigated at a pilot scale (digester volume = 500 L) and compared with a conventional Chinese dome digester (as blank) at 15% influent TS concentration at two retention times (30 and 40 days). The reactors were operated at ambient temperature: 27–33 ◦C. The average specific methane production, volatile fatty acids and percentage of volatile solids (VS) reduction are 0.16 ± 0.13 and 0.25 ± 0.05L CH4/g VS; 1 ± 0.5 and 0.7 ± 0.3 g/L; and 51 ± 14 and 57 ± 10% at 40 days HRT (day 52–136) for the blank and optimised digester, respectively. At 30 days HRT (day 137–309) the results are 0.19 ± 0.12 and 0.23 ± 0.04 L CH4/g VS; 1.2 ± 0.6 and 0.7 ± 0.3 g/L; and 51 ± 9 and 58 ± 11.6%. Overall, the optimised digester produced 40% more methane than the blank, despite the high loading rates applied. The optimised digester showed superior digestion treatment efficiency and was more stable in terms of VFA concentration than the blank digester, can be therefore operated at high influent TS (15%) concentration.</p

Lowering the fresh water footprint of cooling towers : A treatment-train for the reuse of discharged water consisting of constructed wetlands, nanofiltration, electrochemical oxidation and reverse osmosis

The reuse instead of discharge of cooling tower water can significantly lower the industrial fresh water footprint. The reuse of cooling tower water requires the removal of different chemical fractions from the cooling tower water, such as dissolved minerals, phosphate, organic carbon and industrial chemicals. However, there is no stand-alone water treatment technology that can remove all these fractions simultaneously. Therefore, the aim of this study was to assess the removal of these fractions with an innovative pilot-scale technology train that was fed with real cooling tower water consisting of green and grey technologies: constructed wetlands, nanofiltration, electrochemical oxidation and reverse osmosis. In addition, attention was paid to the fate of emerging industrial contaminant benzotriazole, the performance of the nanofiltration membranes and potential production of unwanted by-products by electrochemical oxidation. The experiments showed that 1) The treatment-train was capable of reaching the desired water quality; 2) Benzotriazole was completely removed by the constructed wetlands that acted as pre-treatment before nanofiltration as a result of biodegradation; 3) the production of inorganic carbon species in the constructed wetlands resulted in increased fouling of the subsequent nanofiltration membranes; 4) these membranes mainly retained divalent ions and did not retain monovalent ions, which resulted in a permeate stream whose EC was too high for reuse applications and therefore required further treatment by reverse osmosis; 5) electrochemical oxidation of the nanofiltration concentrate was more efficient than direct electrochemical oxidation of cooling tower water in terms of degradation of recalcitrant humic acids. In addition, less unwanted chlorinated by-products were produced as a result of different ions ratios due to retention of ions by the nanofiltration membranes. Before the full-scale application of the studied treatment train for cooling tower water treatment, it is recommended to identify strategies to improve the water recovery, to include innovative new nanofiltration membranes that can make the use of reverse osmosis redundant and to adopt new insights in preventing unwanted by-product formation during electrochemical oxidation

Sorption of micropollutants on selected constructed wetland support matrices

Micropollutants (MPs) are organic chemicals that are present in the environment at low concentrations (ng/L-μg/L), for example pharmaceuticals. A constructed wetland (CW) is a promising post-treatment technique to remove MPs from wastewater effluent. Selecting a suitable material for support matrix is important when designing such a CW. Nine materials were studied as potential support matrices: Light Expanded Clay Aggregates (LECA), compost, bark, granulated activated carbon (GAC), biochar, granulated cork, lava rock, sand and gravel. Batch experiments were conducted to study MP removal by nine materials in phosphate buffer with 5 or 50 μg/L MPs, or wastewater effluent with 50 μg/L of MPs. GAC and biochar removed almost all MPs in both phosphate buffer and wastewater effluent, followed by bark, compost, granulated cork. Sand, gravel, LECA and lava rock removed less than 30% of most MPs in both matrixes. Based on set criteria (e.g. removal efficiency), biochar, bark, compost, LECA and sand were selected, and used in combinations in column studies to test their overall performance. A combination of bark and biochar performed the best on MP removal, as 4 MPs were highly (70%–100%) removed, 4 MPs were moderately (30%–70%) removed while only 3 MPs were hardly removed. The main flow regime of this combination was both plug flow and dispersive flow. Moreover, we hypothesized to apply bark and biochar in a CW. Based on the assumptions and calculations, some benefits are expected, such as increasing MP removal and extending operation time