Higher Total Solids Concentrations Research Articles

Optimizing mixing efficiency of anaerobic digesters with high total solids concentrations using validated CFD simulations

In this article, viscosity dependent mixing in anaerobic digestion (AD) towers is analysed using finite volume based computational fluid dynamics (CFD) simulations validated with ultrasound particle image velocimetry (uPIV). The study focuses on improving mixing characteristics for enhanced biogas production resulting from increased organic loading. Mixing of fluids with viscosity ranges found in real-life AD is investigated using the multiple reference frame (MRF) method. The aim is to increase the energetic efficiency of AD by reducing the energy demand required for sufficient mixing such that microorganisms and organic substrates are homogeneously distributed. The distinguishing feature of this work is the implementation of less power-consuming helical ribbon stirring devices (HRSD) for enhanced mixing in different reactor designs. It is demonstrated that HRSD become progressively effective for increasing total solids concentrations (% TS) ranging from 2.5 % TS to 12.1 % TS, particularly in cylindrical reactors. In comparison with conventional propeller-based mixers, innovative HRSD operation reduces dead volume (DV) by up to 22 %. Additionally, HRSD cause less shear forces and require less power due to lower rotational speed. The most energy-efficient mode of operation reduces the power demand by 9.29 % at 12 rpm, while still providing decreased DV of 13.38 % for the most viscous fluid investigated. The outcome is a more efficient and cost-effective mixing approach, which holds strong potential for mixing applications involving highly viscous fluids.

Heat and Mass Limitations in an Anaerobic Digestion Process

In this study, heat and mass limitations in an anaerobic reactor containing domestic solids were researched in batch reactors. The dynamic and static anaerobic data for 365 days showed that the methane production for the dynamic digestion reactor was measured as 176.86 m3 which is extremely high for static anaerobic one (102.78 m3). As the heat transfer data increased with elevated temperature the methane productions also were highlighted. The external mass transfer was observed for easily degradable solids. In the calculation of external mass transfer during the degradation of organics dissolved with difficulty some semiempirical regressions were used. In the calculation of internal mass transfer the microorganisms in the solids were taken into consideration and the diffusion was defined with Fick's law. The diffusion coefficient D, was found to be constant. Generally, the diffusion coefficient of solids in water (Dw) was < 1.0. The effect of the total solid (TS) concentration in anaerobic batch reactors (TS between 12% and 39%) was investigated. The methane gas production decreased minorly when the TS levels elevated to 30%. At a TS percentage of 39%, the methane generation decreased significantly. At high TS, the mass transfer was inhibited and ended with lowered methane generations while the hydrolysis process did not affect significantly at high TS concentrations.

Restrictions in water treatment by conventional processes (coagulation, flocculation, and sand-filtration) following scenarios of dam failure

Dam failure events can result in a high suspended solids concentration in surface waters. To understand the impact of high suspended solids concentration on the treatment process of water treatment plants, this study investigated coagulation-flocculation stages using different coagulants (ferric chloride and a polymeric coagulant – CAT-FLOC 8799 Plus) and solids concentrations (2000–46,000 mg/L), and how they impacted on the mean diameter (d50), density (ρs), and settling velocity (vs) of the agglomerates formed. After the sedimentation process, the turbidity values varied between 201–26 NTU when FeCl3 was used as a coagulant, and 36–11 NTU when using the organic coagulant. The study reinforces the effect of particle diameter on sedimentation velocity, which is lower for cases involving smaller average diameters. Even so, solids removal was high regardless of the initial conditions and characteristics of the agglomerated particles, starting early in flocculators with removals >80 %. The results point to the need for frequent maintenance of these units to remove the sludge settled, which could compromise the operation of water treatment plants. In this sense, two alternatives were proposed as pre- and post-treatment. Under high total solids concentration (>20,000 mg/L), pre-treatment with thickeners becomes mandatory, and ultrafiltration comes as an additional barrier that will prevent high turbidity values in drinking water. In this study, the use of ultrafiltration membranes in partial or total replacement of sand filters showed stable performance (permeate flow: 130–136 L/m2h) with no tendency to fouling and guaranteeing a residual turbidity value <0.3 NTU. Future studies should prove the technical and operational feasibility of the strategies proposed, which until then showed promising results to guarantee the operation of water treatment plants in adverse scenarios.

Phosphorus recovery from high solid content liquid fraction of digestate using seawater bittern as the magnesium source

Phosphorus recovery from digestate is considered a challenge because the possible discharge can lead to eutrophication. This study focuses on phosphorus recovery as struvite from the liquid fraction of swine manure digestate at a high total solids concentration, by using a lab-scale crystallizer operated in continuous mode (7 L·d−1). A by-product of salt production (seawater bittern, SWB) was assessed as Mg source for the formation of struvite instead of a chemical dosage (MgCl2) within a circular economy approach. Different Mg/P (1.8:1; 2:1; 3:1) ratios and different TS contents (TS 3.5 and 4.5 %) were studied. The maximum P recovery of 85 % and N recovery of 52 % was obtained at 4.5 % of TS and Mg/P ratio of 2:1, corresponding to an overall P and N recovery on the raw digestate of 70 % and 46 %, respectively. The presence of struvite was confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM-EDS). Dried samples were then used as fertilizer in agronomic pot tests using Brassica rapa chinensis. Struvite obtained, showed comparable fertilizing properties in comparison with conventional fertilizers in terms of P (Mineral 5.6 ± 0.4; Poultry 5.7 ± 0.2; Struvite 5.9 ± 0.1 g kg−1), N and total biomass content such as chlorophylls ratio. The growth tests confirmed the possible use of struvite recovered as competitive alternative to conventional chemical phosphate fertilizers. The results showed that it can be possible to promote sustainable P recovery from high solids digestates by the combination of crystallizer reactor and Mg-salt byproducts.

Production of Biogas from Kitchen Wastes Generated from the Mess of NIT Agartala

A review has been done on the formulation of mathematical models for designing anaerobic batch reactor for the producing of biogas from solid wastes. Using material balance analysis, the design parameters, especially biokinetic behavior of solid wastes are studied. Hydrolysis constants and reaction orders at both low total solids concentrations and high total solids concentrations were thoroughly reviewed by initial rate method. Population growth model and first order hydrolysis model are also studied. It has been found that if sufficient reaction time is given, in terms of reaction kinetics, the hydrolysis process in kitchen waste fermentation can be described as a first order reaction. An attempt is done by utilizing the biodegradable kitchen waste collected from Aryabhatta hostel, NIT Agartala for generation of biogas and demonstrating the possibilities of energy recovery. A simple anaerobic digester has been fabricated to capture the generated biogas. To produce methane as a clean fuel the use of cow dung as substrate is studied to ferment the kitchen wastes.

Effect of feed slurry dilution and total solids on specific biogas production by anaerobic digestion in batch and semi-batch reactors

Biomass from various sources such as cow dung is a significant source of renewable energy (as biogas) in many regions globally, especially in India, Africa, Brazil, and China. However, biogas production from biomass such as cattle dung is a slow, inefficient biochemical process, and the specific biogas produced per kg of biomass is relatively small. The improvement of specific biogas production efficiency using various dilution ratios (and, hence, total solids [TS]) is investigated in this work. A wide range of feed dilution (FD) ratios of cow dung: water (CD: W) was tested in batch biogas digesters with total solids ranging from 1% to 12.5% and FD ratio ranging from 2:1 to 1:20. To further verify the results from the above batch experiments, semi-batch experiments representative of field-scale biodigesters were conducted. Semi-batch reactors have a steady-state process, unlike batch reactors, which have an unsteady state process. Our results suggested that specific biogas production (mL/g VS) increased continuously when the total solids decreased from 12.5% to 1% (or when dilution increased). Our experiments also indicate that the commonly used 1:1 feed dilution ratio (TS ~ 10% for cow dung) does not produce the maximum specific biogas production. The possible reason for this could be that anaerobic digestion at higher total solids is rate limited due to substrate inhibition, mass transfer limitations, and viscous mixing problems that arise at higher total solids concentration. Hence, a higher feed dilution ratio between 1:2 and 1:4 (TS between 4 and 6.7%) is recommended for a more efficient biomass utilization of cowdung. Empirical relationships were also developed for variation of specific biogas yield with the total solids content of the cow dung slurry.Graphic abstract

Alkaline and acid solubilisation of waste activated sludge.

The influence of acidic and alkaline conditions on the solubilisation process of waste activated sludge (WAS) was investigated using HCl and NaOH at pH 2, 10, 11 and 12. The rise in concentration of solubilised compounds, the influence of reaction time, and the influence of the concentration of total solids (TS) during the solubilisation process were determined. Physical and chemical tests demonstrated that pre-treatment provided a release of compounds from the sludge floc matrix into the soluble fraction, characterising the solubilisation process. The highest degree of WAS solubilisation was observed when a pH of 12 was applied. Although largest effects were already attained after 0.25 h, WAS solubilisation continued reaching an increase in total dissolved solids by a factor 10.4 after 720 hrs. Under these conditions, the dissolved organic carbon (DOC), proteins, and carbohydrates resulted in releases up to 15, 40 and 41 times, respectively; phosphorus increased 5.7 times. Results indicate that by applying alkaline pre-treatment, higher TS concentrations can be treated per reactor volume compared to non-pre-treated WAS. Aerobic and anaerobic biodegradability tests showed increased bioconversion potentials in full-scale treatment plants. The respirometry tests ratify the improvement in solubilisation, with O2 consumption rates increasing 1.4 times, concomitant with an additional 261 mg·L-1 of the COD used, which represents 90% bioconversion of waste activated sludge. Biomethanisation test indicated an increase of 3.6 times relative to the blank.

Utilising hen feather as an adsorbent on comparison with bentonite in distillery effluent using Fourier-transform infrared spectroscopy and thermogravimetric analyser

ABSTRACT Distillery effluent is a dark-coloured recalcitrant organic formed as a result of mailard’s reaction during fermentation of molasses. The high strength effluent is highly complicated to dispose due to its properties, such as acidic nature, low pH, high total solids concentration, high concentration of Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD). The disposal of effluent from the distilleries is attempted using hen feather as an organic adsorbent which is a biodegradable waste, and is compared with the bentonite as an inorganic adsorbent. The adsorption experiments were conducted in the real-time distillery effluent using hen feather and bentonite for a concentration range of 1000 to 3000 ppm. The effect of different adsorbents was studied using Fourier-transform infrared spectroscopy and Thermogravimetric analyser. The gravimetric analysis of the hen feather and bentonite after adsorption indicates the 72% and 49.96% of total dissolved solids could be removed, respectively. The TGA results showed that the residue obtained by distillery effluent solids in oxygen mode was 30% which is reduced to 9% for hen feather adsorbed distillery solid samples. From the experiments, the results show that the residue from the distillery effluent solids was reduced to 9% on comparing with the bentonite 40%. The amide group in the hen feather adsorbs higher as the distillery effluent is the product of sugar and amine fermentation. Hence, the hen feather is the better option to use for the adsorption and to reduce the solid waste.

Optimized Production of a Redox Metabolite (pyocyanin) by Pseudomonas aeruginosa NEJ01R Using a Maize By-Product.

Pseudomonas aeruginosa metabolizes pyocyanin, a redox molecule related to diverse biological activities. Culture conditions for the production of pyocyanin in a defined medium were optimized using a statistical design and response surface methodology. The obtained conditions were replicated using as substrate an alkaline residual liquid of cooked maize and its by-products. The untreated effluent (raw nejayote, RN) was processed to obtain a fraction without insoluble solids (clarified fraction, CL), then separated by a 30 kDa membrane where two fractions, namely, retentate (RE) and filtered (FI), were obtained. Optimal conditions in the defined medium were 29.6 °C, 223.7 rpm and pH = 6.92, which produced 2.21 μg mL−1 of pyocyanin, and by using the wastewater, it was possible to obtain 3.25 μg mL−1 of pyocyanin in the retentate fraction at 40 h. The retentate fraction presented the highest concentration of total solids related to the maximum concentration of pyocyanin (PYO) obtained. The pyocyanin redox behavior was analyzed using electrochemical techniques. In this way, valorization of lime-cooked maize wastewater (nejayote) used as a substrate was demonstrated in the production of a value-added compound, such as pyocyanin, a redox metabolite of Pseudomonas aeruginosa NEJ01R.

Modelization of anaerobic processes during co-digestion of slowly biodegradable substrates

The influence of the soluble substrates over the anaerobic processes has been extensively investigated, but little is known about the effects of particulate substrate. The biodegradation of these substrates starts with the hydrolytic step, this process is slower than the other ones involved in the biodegradation of particulate substrates and usually becomes the rate-limiting step. This study investigate the effect of the initial total solids (TS) concentration on the anaerobic co-digestion of two slowly biodegradable organic substrates. The wastes mixtures were prepared at different dilutions in the range from 10% to 28% TS. From these experiments it was observed that as TS concentration increased, the methane production decreased. These results were modelled and it was observed that neither hydrolysis nor fermentation stages controlled the methane production rate. Being a substrate inhibition event experienced at the methanogenic stage the responsible of the lower methane production when operating at high TS concentrations.

Potential Reuse of Treated Textile Effluent in Fly Ash Concrete

Abstract In a preliminary effort to deal with the present fresh water demand in the construction industry, this article examines the potential reuse of textile effluent as mixing water in making fly ash concrete. The effluents were collected from a textile industry at different stages of treatment, such as Collection Tank Effluent, Effluent after Anaerobic Treatment (EAT), Tertiary Treatment Effluent, and Reverse Osmosis Feed Effluent. The collected effluent samples have been analyzed to establish physicochemical characteristics and their effects on the properties of concrete, such as compressive strength, split tensile strength, and flexural strength. Higher salt content in effluent mix water significantly increased the final setting time of the binder. Furthermore, the workability of the concrete made with effluent mix water is relatively higher than that of the control concrete mix. Also, concrete mixes made with textile effluent mix water demonstrated greater strength than that of the control concrete. Furthermore, EAT mixes have exhibited maximum compressive strength compared with other mixes and also had superior durability compared with the control concrete mix. The higher strength can be attributed to the nucleation effect caused by the presence of a high concentration of total solids in the effluent. However, this concrete can be recommended for applications of plain cement concrete, such as pavements, industrial floors, and curbs/gutters.

Development of a Modified Plug-Flow Anaerobic Digester for Biogas Production from Animal Manures

Traditional plug-flow anaerobic reactors (PFRs) are characterized by lacking a mixing system and operating at high total solid concentrations, which limits their applicability for several kinds of manures. This paper studies the performance of a novel modified PFR for the treatment of pig manure, characterized by having an internal sludge mixing system by biogas recirculation in the range of 0.270–0.336 m3 m−3 h−1. The influence on the methane yield of four operating parameters (recirculation rate, hydraulic retention time, organic loading rate, and total solids) was evaluated by running four modified PFRs at the pilot scale in mesophilic conditions. While the previous biodegradability of organic matter by biochemical methane potential tests were between 31% and 47% with a methane yield between 125 and 184 LCH4 kgVS−1, the PFRs showed a suitable performance with organic matter degradation between 25% and 51% and a methane yield of up to 374 LCH4 kgVS−1. Operational problems such as solid stratification, foaming, or scum generation were avoided.

Modified Ludzack–Ettinger system role in efficient nitrogen removal from swine manure under high total suspended solids concentration

The current scenario requires treatment systems with high efficiency, easy operation and low cost. The solid–liquid separation process as a pre-treatment of swine manure is an efficient strategy improving the wastewater management and treatment. In this sense, the aim of this study was to establish for the first time a condition in which swine manure treatment could be successfully performed using a simple solid-liquid separation, combined with nitrification and denitrification process in a modified Ludzack–Ettinger system. The solids separation was done with a brush-roller screen (2 mm mesh) followed by a settling unit. The nitrogen removal unit was composed of a modified Ludzack–Ettinger process, operated in three phases, according to different sedimentation times in the solid–liquid separation process. The modified Ludzack–Ettinger system showed good performance for treatment of swine wastewater with total suspend solids concentration higher than 14 g L−1, reaching removal efficiency of 88.3% for nitrogen and 86.3% for total organic carbon removal. The results obtained in this study confirm that it is possible to operate the modified Ludzack–Ettinger system at high total solids concentration using a simple solid–liquid separation process keeping a high nitrogen removal efficiency.

Evaluation of Semi-dry Mesophilic Anaerobic Co-digestion of Corn Stover and Vegetable Waste by a Single-Phase Process

The performance of semi-dry anaerobic co-digestion of corn stover and vegetable waste was investigated using batch reactors and leaching bed reactors (LBRs). Six mixing ratios of corn stover and vegetable waste were employed to investigate optimal methane production using batch reactors, and process stability was assessed using the two LBRs. Kinetic assessment, methane production, volatile solid (VS) reduction, and digestate characteristics were used to evaluate the performance of anaerobic co-digestion. After 50 days of anaerobic co-digestion in the batch reactor, methane yields reached 314.5–323.4 mL/g VS, and increase in methane production rate indicated that synergistic effects occurred during anaerobic co-digestion. The corn stover and vegetable waste with a total solids (TS) ratio of 14:1 yielded the highest methane production efficiency and rate, as well as VS removal efficiency. However, serious acidification at the initial stage of digestion in the LBR experiments suggests that the scaling-up process from batch reactor to LBR was unstable, which may be attributed to high TS concentration and substrate characteristics.

Mixing non-Newtonian flows in anaerobic digesters by impellers and pumped recirculation

In this article, a finite volume method based CFD analysis of the mixing of Newtonian and non-Newtonian flows in anaerobic digesters is conducted. The multiple reference frame method is used to model the mixing which is induced by an impeller rotating within a mechanical draft tube. Moreover, the feeding of biomass is considered in the model formulation. Following the validation of the method against reference data, the mixing of cylindrical and egg-shaped digesters is investigated. The distinguishing feature of this article is that the theoretical findings are adopted for the operation of a real-life anaerobic digester. In the context of a case study slurry flows with high total solids concentrations are investigated.

Simultaneous ammonia stripping and anaerobic digestion for efficient thermophilic conversion of dairy manure at high solids concentration

A major challenge to take high rate advantage of thermophilic anaerobic digestion (AD) is overcoming ammonia inhibition without adding any significant cost to the process. A concept of thermophilic AD with simultaneous ammonia stripping was tested for treating dairy manure at high total solids concentration (TS%) as an attempt to address this challenge. The results showed that ammonia inhibition occurred at 1.8–2.4 g/L total ammonia nitrogen (TAN) concentration which corresponded to 10% TS as a threshold concentration. Thermophilic AD of dairy manure efficiency at the threshold TS% was significantly improved by simultaneously stripping ammonia with the optimum stripping rate of 1 L min−1. The required time for reaching stable state was 4 days shorter than control, and the highest methane content (56.5–75.5%) was obtained. The ammonia stripping strategy maintained TAN level below the inhibition limit of 1.5 g/L throughout AD process. The maximum cumulative methane production of 192.3 L/kg-volatile solids (VS) was obtained, which was 2.3-fold the control. Therefore, simultaneous ammonia stripping overcame the ammonia inhibition on thermophilic AD of dairy manure at the threshold TS%. The proposed concept simplified the system by combining ammonia stripping and thermophilic AD within the same digester.

Anaerobic digestion of tuna waste for the production of volatile fatty acids

Fish canning industries generate a significant amount of solid waste that can be digested anaerobically into volatile fatty acids (VFA). The aim of this research was to study the effect of various pHs, ranging from 5.0 to 10.0, and percentage of total solids on the anaerobic digestion of tuna waste into VFA, both in batch assays and continuous reactor. The production of VFA was affected by pH and was significantly higher under alkaline conditions. At pH 8.0, the VFA production reached 30,611mgCOD/L. The VFA mainly consisted of acetic, propionic, n-butyric and i-valeric acids. Acetic acid was the main product at all the pHs tested. In terms of total solids (TS) the best results were obtained with 2.5% total solids, reaching 0.73gCODVFA/gCODwaste. At higher TS concentrations (5 and 8% TS) lower yields were reached probably due to inhibition at high VFA concentration.

Spatial and Temporal Variation in Physicochemical Properties of Dairy Lagoons in South-Central Idaho

<abstract> Large quantities of wastewater are generated on dairies in south-central Idaho, which can be a source of valuable nutrients as well as contribute to air quality and climate change issues via ammonia (NH₃) and greenhouse gas (GHG) emissions. The objective of this study was to examine the range of lagoon water properties among dairies in the region and to determine how they varied spatially and temporally. Twenty-seven lagoons were sampled twice in a nutrient survey to determine physicochemical characteristics, while six lagoons were sampled (3 to 27 times) over a longer period to determine how these characteristics changed with space and time. Lagoon properties measured consisted of total solids (TS), volatile solids (VS), chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), total ammoniacal nitrogen (TAN), total phosphorus (P), total potassium (K), temperature, pH, dissolved oxygen (DO), and specific conductivity. Results indicate that all lagoon characteristics varied greatly between dairies and with sampling date. Seasonal trends indicated that N decreased from spring to fall, while specific conductivity, total P, total K, and in some instances TS and VS increased over the same period. There was an effect of housing type on these properties, with freestall dairies having higher concentrations of TS, VS, COD, TKN, TAN, and specific conductivity than dry-lot dairies. There was little effect of dairy size on the physicochemical characteristics measured. These results suggest that it is important to account for the nutrients applied with lagoon water in nutrient budgets in order to prevent over-application of N and K, which could lead to N leaching and forage quality issues. In addition, capturing the temporal variation in lagoon properties is important to accurately model seasonal variations in NH₃ and GHG emissions.

Application of the Initial Rate Method in Anaerobic Digestion of Kitchen Waste.

This article proposes a methane production approach through sequenced anaerobic digestion of kitchen waste, determines the hydrolysis constants and reaction orders at both low total solid (TS) concentrations and high TS concentrations using the initial rate method, and examines the population growth model and first-order hydrolysis model. The findings indicate that the first-order hydrolysis model better reflects the kinetic process of gas production. During the experiment, all the influential factors of anaerobic fermentation retained their optimal values. The hydrolysis constants and reaction orders at low TS concentrations are then employed to demonstrate that the first-order gas production model can describe the kinetics of the gas production process. At low TS concentrations, the hydrolysis constants and reaction orders demonstrated opposite trends, with both stabilizing after 24 days at 0.99 and 1.1252, respectively. At high TS concentrations, the hydrolysis constants and the reaction orders stabilized at 0.98 (after 18 days) and 0.3507 (after 14 days), respectively. Given sufficient reaction time, the hydrolysis involved in anaerobic fermentation of kitchen waste can be regarded as a first-order reaction in terms of reaction kinetics. This study serves as a good reference for future studies regarding the kinetics of anaerobic digestion of kitchen waste.

Effect of High Total Solids Concentration on Biogas Production from Cattle Dung

Effect of High Total Solids Concentration on Biogas Production from Cattle Dung