Kinetic parameter estimation model for anaerobic co-digestion of waste activated sludge and microalgae

Advances, challenges, and prospects in microalgal-bacterial symbiosis system treating heavy metal wastewater

Heavy metal (HM) pollution, particularly in its ionic form in water bodies, is a chronic issue threatening environmental security and human health. The microalgal-bacterial symbiosis (MABS) system, as the basis of water ecosystems, has the potential to treat HM wastewater in a sustainable manner, with the advantages of environmental friendliness and carbon sequestration. However, the differences between laboratory studies and engineering practices, including the complexity of pollutant compositions and extreme environmental conditions, limit the applications of the MABS system. Additionally, the biomass from the MABS system containing HMs requires further disposal or recycling. This review summarized the recent advances of the MABS system treating HM wastewater, including key mechanisms, influence factors related to HM removal, and the tolerance threshold values of the MABS system to HM toxicity. Furthermore, the challenges and prospects of the MABS system in treating actual HM wastewater are analyzed and discussed, and suggestions for biochar preparation from the MABS biomass containing HMs are provided. This review provides a reference point for the MABS system treating HM wastewater and the corresponding challenges faced by future engineering practices.

Anaerobic co-digestion of thermo-alkaline pretreated microalgae and sewage sludge: Methane potential and microbial community

To improve methane production from sewage sludge (SS), co-digestion of SS and microalgae (MA) was studied and the application of thermo-alkaline pretreatment to MA was evaluated. The results showed that thermo-alkaline pretreatment at 90°C for 120 min on MA was the optimum pretreatment condition. Furthermore, when the volatile solids (VS) ratio of SS and MA was 1:2, the methane yield reached maximum (368.94 mL/g VS). Fourier transform infrared (FT-IR) and thermogravimetric analysis confirmed the synergetic effects of thermo-alkaline pretreated MA on its co-digestion with SS. The analyses of microbial community indicated that Methanobacterium and Methanosarcina were the dominant methanogens during the co-digestion process. However, the relative abundance of Methanosarcina in thermo-alkaline pretreated groups was higher compared to unpretreated groups. The microbial community structure might be affected by thermo-alkaline pretreatment rather than by the MA dosage in the co-digestion.

Simultaneous sludge minimization and membrane fouling mitigation in membrane bioreactors by using a microaerobic - Settling pretreatment module

Membrane fouling is the major obstacle for membrane bioreactors operated at a long sludge retention time to reduce sludge production. In this study, a sludge process reduction (SPR) module, consisting of a microaerobic tank and a settler, was inserted before an anoxic/oxic MBR (AO-MBR) to achieve dual objectives of fouling alleviation and sludge reduction. Three SPR-MBRs were operated to investigate influences of sludge recirculation ratios from the SPR settler to the microaerobic tank on process performance. Compared to AO-MBR, the SPR-MBRs reduced sludge production by 43.1-56.4% by maintaining sludge retention times above 175 d, and decreased foulant layer resistance and pore clogging resistance. Inserting SPR reduced the accumulation of dissolved organic matters and extracellular polymeric substances, enlarged sludge flocs, and decreased sludge viscoelasticity. However, increasing R_SPR stimulated outward diffusion of extracellular polymeric substances and increased sludge viscosity. SPR-MBRs achieved effective sludge reduction by enriching hydrolytic (Trichococcus and Aeromonas) and fermentative genera (Lactococcus, Paludibacter, Macellibacteroides, and Acinetobacter) in the SPR, and alleviated membrane fouling by prohibiting the growth of extracellular polymeric substance-secreting bacteria and enriching filamentous bacteria to enlarge particle size. The results revealed that the SPR-MBR maximized sludge reduction with a very long sludge retention time, and alleviated membrane fouling synchronously.

Thermal treatment's enhancement on high solid anaerobic digestion: effects of temperature and reaction time

The methane production rate of high solid anaerobic digestion (HSAD) was poor although it was a promising technology with the advantages of small reactor, low energy consumption, and less digestate. In our research before, thermal treatment was proved to enhance HSAD's methane production rate via both batch experiments and continuous experiments of swine manure. However, the effect or investigation of thermal treatment's temperature-time combinations was not yet reported. In this study, swine manure was firstly thermally treated in 500-mL glass bottles with 400-mL work volume at 45-65 °C for 1-4 days. HSAD experiment of 10% solid content was then set up. The VS ratio of substrate to inoculum was 1:1. Thermal treatment at 45 °C (3 days), 55 °C (1 day), and 65 °C (3 days) could obtain the highest methane production rate, which was around 40% higher. Kinetics analysis suggested that the degradation of swine manure was quite different at different temperatures. Furthermore, energy assessment indicated that "thermal treatment + HSAD" had significant advantages in improving HSAD economic feasibility, because the improved methane production rate could compensate for the extra energy utilized for thermal treatment. Heat treatment at 45 °C (4 days) was preferred when the heating equipment was limited. Heat treatment at 55 °C (1 day) was preferred when the floor space and reactor volume were restricted. Heat treatment at 65°C (3 days) was preferred when the requirement of the digestate's sanitary condition is strict.

Prediction of the Long-Term Effect of Iron on Methane Yield in an Anaerobic Membrane Bioreactor Using Bayesian Network Meta-Analysis

A method for predicting the long-term effects of ferric on methane production was developed in an anaerobic membrane bioreactor treating food processing wastewater to provide management tools for maximizing methane recovery using ferric based on a batch test. The results demonstrated the accuracy of the predictions for both batch and long-term continuous operations using a Bayesian network meta-analysis based on the Gompertz model. The prediction bias of methane production for batch and continuous operations was minimized, from 11~19% to less than 0.5%. A biochemical methane potential-based Bayesian network meta-analysis suggested a maximum 2.55% ± 0.42% enhancement for Fe2.25. An anaerobic membrane bioreactor improved the methane yield by 2.27% and loading rate by 4.57% for Fe2.25, operating in the sequenced batch mode. The method allowed for a predictable methane yield enhancement based on the biochemical methane potential. Ferric enhanced the biochemical methane potential in batch tests and the methane yield in a continuously operated reactor by a maximum of 8.20% and 7.61% for Fe2.25, respectively. Copper demonstrated a higher methane (18.91%) and sludge yield (17.22%) in batch but faded in the continuous operation (0.32% of methane yield). The enhancement was primarily due to changing the kinetic patterns for the last period, i.e., increasing the second methane production peak (k₇₁), bringing forward the second peak (λ₇, λ₈), and prolonging the second period (k₆₂). The dual exponential function demonstrated a better fit in the last three stages (after the first peak), which implied that syntrophic methanogenesis with a ferric shuttle played a primary role in the last three methane production periods, in which long-term effects were sustained, as the Bayesian network meta-analysis predicted.

A practical approach for modelling the growth of microalgae with population balance equation

Microalgae are versatile microorganisms with applications in food, energy and fine chemicals, among others. Modelling the dynamics of microalgae inside a photobioreactor is a convenient and inexpensive way to determine the concentration of cells over time. Numerous models have been developed in the literature, but only a few are able to give relevant biological information. Such information can then be used to further improve the production process. The objective of this work was to develop a model for the determination of microalgal dynamics inside a photobioreactor as a function of the environmental conditions, to retrieve the size-specific growth and division rates as well as the number of daughter cells. The results demonstrate how to evaluate the time needed for microalgae to complete a full life-cycle. Inexpensive laboratory-based procedures and mathematical modelling are combined for the determination of relevant biological parameters.

Evaluation of biogas production potential of trace element-contaminated plants via anaerobic digestion

Within the domain of phytoremediation research, the proper disposal of harvestable plant parts, that remove pollutants from contaminated soil, has been attracted extensive attention. Here, the bioenergy generation capability of trace metals (Cu, Pb, Zn, Cd, Mn, and As) polluted plants was assessed. The biogas production potential of accumulators or hyperaccumulator plants, Elsholtzia haichowensis, Sedum alfredii, Solanum nigrum, Phytolacca americana and Pteris vittata were 259.2 ± 1.9, 238.7 ± 4.2, 135.9 ± 0.9, 129.5 ± 2.9 and 106.8 ± 2.1 mL/g, respectively. The presence of Cu (at approximately 1000 mg/kg) increased the cumulative biogas production, the daily methane production and the methane yield of E. haichowensis. For S. alfredii, the presence of Zn (≥500 mg/kg) showed a significant negative impact on the methane content in biogas, and the daily methane production, which decreased the biogas and methane yield. The biogas production potential increased when the content of Mn was at 5 000-10,000 mg/kg, subsequently, decreased when the value of Mn at 20,000 mg/kg. However, Cd (1-200 mg/kg), Pb (125-2000 mg/kg) and As (1250-10,000 mg/kg) showed no distinctive change in the cumulative biogas production of S. nigrum, S. alfredii and P. vittata, respectively. The methane yield showed a strong positive correlation (R² =0.9704) with cumulative biogas production, and the energy potential of the plant residues were at 415-985 kWh/ton. Thus, the anaerobic digestion has bright potential for the disposal of trace metal contaminated plants, and has promising prospects for the use in energy production.

Evaluation of biomethanization during co-digestion of thermally pretreated microalgae and waste activated sludge, and estimation of its kinetic parameters

The maximum methane yield that can be obtained from anaerobic co-digestion of microalgae and waste activated sludge (WAS) mixtures, after thermal pretreatment at 65 °C during 4 h, was investigated. Furthermore, the fitting of the experimental data by five kinetic models (first-order, second-order, modified Gompertz, Logistic, and two-substrate) was evaluated. Thermal pretreatment increased the methane yield of single microalgae and WAS digestion by ≈ 44 and by ≈ 52%, respectively. The results also showed that up to 60% of WAS can be co-digested with microalgae without impairing the methane yield, producing up to 338 mL_CH4 g_VS^-1. Data from digestion of non-pretreated microalgae and WAS were well described by all kinetic models, but digestion of thermally pretreated microalgae, WAS, and their co-digestion mixtures, was best fitted by means of a two-substrate model, indicating that after pretreatment it is necessary to take into account the contribution of both rapidly and slowly biodegradable fractions.

Electrochemical pretreatment of yard waste to improve biogas production: Understanding the mechanism of delignification, and energy balance

In the present study, electrochemical pretreatment with a pair of graphite electrode was conducted to pretreat yard waste prior to anaerobic digestion. The Response Surface Methodology was employed to optimize the pretreatment conditions. To determine the mechanism of delignification physical and chemical properties of untreated and pretreated yard waste were investigated. In the subsequent anaerobic digestion of pretreated yard waste, the ultimate biogas production of 446 mL/g VS was achieved in comparison to the untreated yard waste of 287 mL/g VS on 35th day of anaerobic digestion. A net energy gain of 4.75 kJ/g VS (Output energy of 5.73 kJ/g VS - Input energy of 0.98 kJ/g VS) and net profit of 518 rupees (US$ 7.4) per 1 ton of yard waste indicates the applicability of electrochemical pretreatment for pilot scale.

Nonlinear estimation and control schemes for a complex anaerobic digestion of microalgae with unknown kinetics and inputs

This work addresses estimation and advanced control schemes for a complex microalgae anaerobic digestion process, used for wastewater treatment as well as for converting organic waste and biomass into biogas. The control goal is to keep the inhibitory compounds at imposed low levels despite the influent pollutant concentration and load variations. The proposed innovative control schemes work in accurate operating conditions and can cope with harsh assumptions: unknown kinetic rates, unknown feed-in pollutant concentration, unmeasurable state variables. The adaptive and robust-adaptive control laws are based on nonlinear algorithms: a sliding mode observer for the unknown inputs, kinetics parameter estimators, asymptotic and interval state observers. The tests performed under realistic conditions showed that the closed-loop bioprocess is preserved at specific operating points such that the control goal is fulfilled. The state and parameter estimators provide enough on-line information such that the control schemes can handle all the input and kinetic uncertainties.

Mesophilic anaerobic co-digestion of the organic fraction of municipal solid waste with the liquid fraction from hydrothermal carbonization of sewage sludge

In the present study, the influence of substrate pre-treatment (grinding and sieving) on batch anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) was first assessed, then followed by co-digestion experiments with the liquid fraction from hydrothermal carbonization (LFHTC) of dewatered sewage sludge (DSS). The methane yield of batch anaerobic digestion after grinding and sieving (20 mm diameter) the OFMSW was considerably higher (453 mL CH₄ STP g^-1 VS_added) than that of untreated OFMSW (285 mL CH₄ STP g^-1 VS_added). The modified Gompertz model adequately predicted process performance. The maximum methane production rate, R_m, for ground and sieved OFMSW was 2.4 times higher than that of untreated OFMSW. The anaerobic co-digestion of different mixtures of OFMSW and LFHTC of DSS did not increase the methane yield above that of the anaerobic digestion of OFMSW alone, and no synergistic effects were observed. However, the co-digestion of both wastes at a ratio of 75% OFMSW-25% LFHTC provides a practical waste management option. The experimental results were adequately fitted to a first-order kinetic model showing a kinetic constant virtually independent of the percentage of LFHTC (0.52-0.56 d^-1) and decreasing slightly for 100% LFHTC (0.44 d^-1).

Sludge treatment: Current research trends

Sludge is produced during wastewater treatment as a residue containing most insoluble and adsorbed soluble impurities in wastewaters. This paper summarized the currently available review papers on sludge treatments and proposed the research trends based on the points raised therein. On partition aspect, sludge production rate and the reduction of production rate and the fate and transformation of involved emergent contaminants including endocrine disrupting chemicals and pharmaceuticals and personal care products are widely studied. On release aspect, development of thermal processes on sludge with migration and transformation of heavy metals in sludge during treatment is a research focus. The use of detailed fluid and biological reaction models and advanced instrumentation and control systems is studied to optimize treatment performances. On recovery part, co-digestion of sludge with co-substrates at mesophilic and hyperthermophilic conditions and the recovery of phosphorus at low costs are research highlights.