Anaerobic Digestion Of Microalgae Research Articles

Iron oxide nanoparticle-based pretreatment for simultaneous elevated hydrolysis efficiency and methanization augmentation of Chlorella vulgaris biomass

The impact of iron oxide nanoparticle (Fe3O4 NPs) on Chlorella vulgaris biomass solubilization and biogas production was investigated. The maximum hydrolysis efficiency (30 %) was achieved in the presence of 10 ppm of Fe3O4NPs, however, this degree decreased respectively, to 23.4 and 22 % at 15 and 20 ppm. Concurrently, a highest biogas yield (595 mL/g VSin) was attained in reactors fed with pretreated biomass at 10 ppm. A key threshold of 40 min for Fe3O4NPs pretreatment was established to increase organic materials solubilization in reactors. The effort to expand biomass loads over 16 g/L resulted in lower hydrolysis efficiency and a simultaneous drop in the augmentation of biogas production. All pretreated trials steadily displayed higher first-order kinetic constants as compared to control. Therefore, Fe3O4NPs application prior microalgae anaerobic digestion might be a potential strategy to reduce energy input needed for cell wall disruption with the added benefits of biogas production enhancement.

Microalgae and Cyanobacteria Biomass Pretreatment Methods: A Comparative Analysis of Chemical and Thermochemical Pretreatment Methods Aimed at Methane Production

Anaerobic digestion of microalgae and cyanobacteria was first proposed as a destination for algal biomass accumulated on stabilization ponds since it could not be disposed of directly in the environment. Now, the versatility of algal biomass makes them a suitable candidate to produce biofuels and other biomolecules in biorefineries. Anaerobic digestion of biomass is advantageous because it does not require the extraction of specific cellular constituents or drying of the biomass. Nevertheless, challenges remain regarding biomass concentration and their resistant cell walls, which are factors that could hamper methane yield. Many pretreatment methods, including chemical and thermochemical, have been proposed to break down the complex polymers present on the cell wall into smaller molecules. Unfortunately, the relationship between biomass solubilization and methane yield is not well defined. This article intends to review the anaerobic digestion of algal biomass and the role of chemical and thermochemical pretreatments in enhancing methane production. Several pretreatment conditions selected from the scientific literature were compared to verify which conditions actually improve methane yield. The severity of the selected pretreatments was also assessed using the combined severity factor. Results suggest that thermochemical pretreatment in less severe conditions is the most efficient, leading to a greater increase in methane yield. Only enzymatic pretreatments and some thermal pretreatments result in a positive energy balance. The large-scale implementation of pretreatment methods requires technological innovations to reduce energy consumption and its integration with other processes in wastewater treatment plants.

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.

Renewable energy: evaluation of low energy demand pre‐treatments to optimise methane production from microalgae

Anaerobic digestion (AD) of microalgae is a sustainable process to produce methane-rich biogas for bioenergy production. However, the rigid cell walls of microalgae protect them against the attack from hydrolytic bacteria, and consequently prevent efficient biodegradability and lower methane production. In this study, the effects of low energy demand enzymatic and low-temperature thermo-alkaline pre-treatments on microalgae AD were investigated in batch biochemical methane potential tests. The results found that methane yields were significantly enhanced by both pre-treatments. Microalgae Chlorella vulgaris (C. vulgaris) after enzymatic pre-treatment enhanced methane yields the most, by 22–162%, whilst C. vulgaris pre-treated by thermo-alkaline pre-treatment improved methane yields by 4–26%. In enzymatic pre-treatment, C. vulgaris pre-treated with mixed enzymes showed higher methane yields compared to single enzymes. In low-temperature thermo-alkaline pre-treatment, the level of enhancement in methane yields depended on the alkaline dosage and pre-treatment temperature, but the high alkaline dosages were associated with limitations such as a prolonged lag phase in the digestion process. From an energy viewpoint, both pre-treatments showed positive energy balances for the majority of experimental conditions, and therefore both pre-treatments are considered to be energetically efficient methods to pre-treat microalgae for methane production via AD.

Correlations between biochemical composition and biogas production during anaerobic digestion of microalgae and cyanobacteria isolated from different sources of Turkey

Twenty one species of microalgae and Cyanobacteria were isolated from different ecosystems in Turkey to investigate the relation between biochemical methane potential (BMP) and biochemical characterization. Since the highest dry weight (X), specific growth rate (µ) and maximum productivity (Pmax) were obtained from the five species, identification of species and BMP tests with the composition analyzes were examined. BMP values were determined 308, 293, 242, 229 and 230 mLCH4/gVS for Desertifilum tharense, Phormidium animale, Chlorella sp., Anabeana variabilis and Chlorophyta uncultured. The Pearson correlation and principal component analysis (PCA) were applied to extract and clarify the correlation between composition of species and their methane production. Pearson correlation shows that glucose, Kjeldahl nitrogen and chlorophyll are highly and positively correlated with BMP. PCA revealed that Chlorella sp., Chlorophyta uncultured and Desertifilum tharense were placed against Phormidium animale distinguished by its extreme and different profile because of Kjeldahl nitrogen and glucose content.

Strategies to Optimize Microalgae Conversion to Biogas: Co-Digestion, Pretreatment and Hydraulic Retention Time.

This study aims at optimizing the anaerobic digestion (AD) of biomass in microalgal-based wastewater treatment systems. It comprises the co-digestion of microalgae with primary sludge, the thermal pretreatment (75 °C for 10 h) of microalgae and the role of the hydraulic retention time (HRT) in anaerobic digesters. Initially, a batch test comparing different microalgae (untreated and pretreated) and primary sludge proportions showed how the co-digestion improved the AD kinetics. The highest methane yield was observed by adding 75% of primary sludge to pretreated microalgae (339 mL CH4/g VS). This condition was then investigated in mesophilic lab-scale reactors. The average methane yield was 0.46 L CH4/g VS, which represented a 2.9-fold increase compared to pretreated microalgae mono-digestion. Conversely, microalgae showed a low methane yield despite the thermal pretreatment (0.16 L CH4/g VS). Indeed, microscopic analysis confirmed the presence of microalgae species with resistant cell walls (i.e., Stigioclonium sp. and diatoms). In order to improve their anaerobic biodegradability, the HRT was increased from 20 to 30 days, which led to a 50% methane yield increase. Overall, microalgae AD was substantially improved by the co-digestion with primary sludge, even without pretreatment, and increasing the HRT enhanced the AD of microalgae with resistant cell walls.

Nanoparticles augmentation on biogas yield from microalgal biomass anaerobic digestion

This study evaluates the influence of metal and metal-oxide nanoparticles (NPs) on biogas production from green microalgae Enteromorpha. The concentration of metallic NPs (Ni, Co) was 1 mg/L and oxides NPs (Fe3O4, MgO) was 10 mg/L. An anaerobic digestion was carried out batch-wise with working volume, operating temperature, mixing rate and hydraulic retention time as 500 ml, 37 °C, 150 rpm and 170 h, respectively. The measurements of chemical oxygen demand (COD), volatile fatty acids (VFAs), reducing sugar and biogas production were observed to monitor effectivity of nanoparticles. The results showed that NPs has moderate positive influence in biogas production until 60 h of retention time but significantly improve afterward. The maximum total biogas yield of 624 ml was achieved by Fe3O4 NPs whereas highest biohydrogen, 51.42% (v/v) was achieved by Ni NPs. The cumulative increase in biogas production for Fe3O4, Ni, Co and MgO NPs was 28%, 26%, 9% and 8%, respectively. A modified Gompertz and Logistic function model were used to determine kinetic constants of the reaction. The logistic model has the better predicting ability for microalgae anaerobic digestion.

Effect of control parameters on biogas production during the anaerobic digestion of protein-rich substrates

This paper deals with the theoretical study of the anaerobic digestion of protein-rich substrates. Since the success of such digestion raises the issue of the possible release of ammonia, we have carried out a mathematical analysis of an Anaerobic Digestion model specifically developed for the treatment of Microalgae, termed MAD (Microalgae Anaerobic Digestion) in the literature. Our aim is to investigate the qualitative properties of the system via the calculation of its equilibria, their conditions of existence as well as their stability according to the operating parameters values. Simulation results highlight the key parameters acting on model behavior. In particular, it is shown that the control parameters can greatly affect biogas yield and, thus, process performance. The optimal conditions for process operation are then identified. To emphasize the effect of pH on biological reactions, we plot the operating diagram of the MAD model for different fixed pH values. We demonstrate that a rising pH favors the formation of the free ammonia, leading ultimately to methanogen inhibition. Surprisingly, our study highlights the insensitivity of the model to acidic environments, thus limiting its potential applications at low pH. Finally, we investigate the qualitative and quantitative properties of a modified ADM1 (Anaerobic Digestion Model no1) model in the light of those of the MAD model.

Hydrothermal post-treatment of digestate to maximize the methane yield from the anaerobic digestion of microalgae

As an alternative to applying the hydrothermal treatment to the raw algal feedstock before the anaerobic digestion (i.e. pre-treatment), one considered a post-treatment scenario where anaerobic digestion is directly used as the primary treatment while the hydrothermal treatment is thereafter applied to the digestate. Hydrothermal treatments such as wet oxidation (WetOx) and hydrothermal carbonization (HTC) were compared at a temperature of 200°C, for initial pressure of 0.1 and 0.82MPa, and no holding time after the process had reached the temperature setpoint. Both WetOx and HTC resulted in a substantial solids conversion (47–62% with HTC, 64–83% with WetOx, both at 0.82MPa) into soluble products, while some total chemical oxygen demand–based carbon loss from the solid-liquid phases was observed (20–39%). This generated high soluble products concentrations (from 6.2 to 10.9g soluble chemical oxygen demand/L). Biomethane potential tests showed that these hydrothermal treatments allowed for a 4-fold improvement of the digestate anaerobic biodegradability. The hydrothermal treatments increased the methane yield to about 200 LSTP CH4/kg volatile solids, when related to the untreated digestate, compared to 66 LSTP CH4/kg volatile solids, without treatment.

Enhancement of microalgae anaerobic digestion by thermo-alkaline pretreatment with lime (CaO)

The aim of this study was to evaluate for the first time the effect of a thermo-alkaline pretreatment with lime (CaO) on microalgae anaerobic digestion. The pretreatment was carried out by adding different CaO doses (4 and 10%) at different temperatures (room temperature (25°C), 55 and 72°C). The exposure time was 4days for pretreatments at 25°C, and 24h for pretreatments at 55 and 72°C. Following, a biochemical methane potential test was conducted with pretreated and untreated microalgae. According to the results, the pretreatment enhanced proteins solubilisation by 32.4% and carbohydrates solubilisation by 31.4% with the highest lime dose and temperature (10% CaO and 72°C). Furthermore, anaerobic digestion kinetics were improved in all cases (from 0.08 to 0.14day−1 for untreated and pretreated microalgae, respectively). The maximum biochemical methane potential increase (25%) was achieved with 10% CaO at 72°C, in accordance with the highest biomass solubilisation. Thus, lime pretreatment appears as a potential strategy to improve microalgae anaerobic digestion.

Assessing the agricultural reuse of the digestate from microalgae anaerobic digestion and co-digestion with sewage sludge

Microalgae anaerobic digestion produces biogas along with a digestate that may be reused in agriculture. However, the properties of this digestate for agricultural reuse have yet to be determined. The aim of this study was to characterise digestates from different microalgae anaerobic digestion processes (i.e. digestion of untreated microalgae, thermally pretreated microalgae and thermally pretreated microalgae in co-digestion with primary sludge). The main parameters evaluated were organic matter, macronutrients and heavy metals content, hygenisation, potential phytotoxicity and organic matter stabilisation. According to the results, all microalgae digestates presented suitable organic matter and macronutrients, especially organic and ammonium nitrogen, for agricultural soils amendment. However, the thermally pretreated microalgae digestate was the least stabilised digestate in comparison with untreated microalgae and co-digestion digestates. In vivo bioassays demonstrated that the digestates did not show residual phytotoxicity when properly diluted, being the co-digestion digestate the one which presented less phytotoxicity. Heavy metals contents resulted far below the threshold established by the European legislation on sludge spreading. Moreover, low presence of E. coli was observed in all digestates. Therefore, agricultural reuse of thermally pretreated microalgae and primary sludge co-digestate through irrigation emerges a suitable strategy to recycle nutrients from wastewater.

Enzymatic pretreatment of microalgae using fungal broth from Trametes versicolor and commercial laccase for improved biogas production

Coupling microalgae production to wastewater treatment can reduce the costs of microalgae production for non-food bioproducts and energy consumption for wastewater treatment. Furthermore, microalgae anaerobic digestion can be enhanced by applying pretreatment techniques. The aim of this study is to improve the biogas production from microalgal biomass grown in urban wastewater treatment systems by applying an enzymatic pretreatment with crude fungal broth and commercial laccase. To this end, the fungus Trametes versicolor was cultured, and the enzymatic activity of the culture broth analysed by measuring laccase concentration. The results showed that both the fungal broth and commercial laccase pretreatment (100UL−1) over an exposure time of 20min increased the methane yield in batch tests. Indeed, the fungal broth pretreatment increased the methane yield by 74%, while commercial laccase increased the methane yield by 20% as compared to non-pretreated microalgal biomass. In this manner, laccase addition enhanced microalgal biomass anaerobic biodegradability, and addition of T. versicolor broth further improved the results. This fact may be attributed to the presence of other molecules excreted by the fungus.

Comparing pretreatment methods for improving microalgae anaerobic digestion: Thermal, hydrothermal, microwave and ultrasound

The anaerobic digestion of microalgae is hindered by its complex cell wall structure and composition. Thus, several pretreatment methods have been used for increasing microalgae anaerobic biodegradability. Since the methane yield depends on biomass characteristics, pretreatments should be compared using the same microalgal biomass. In this study, physical pretreatments including thermal (95°C; 10h), hydrothermal (130°C; 15min), microwave irradiation (900W; 3min; 34.3MJ/kgTS) and ultrasonication (70W; 30min; 26.7MJ/kgTS) were evaluated in terms of microalgae solubilisation and methane yield increase in batch tests. Organic matter solubilisation was improved in all cases, with the highest increase on soluble proteins, followed by soluble carbohydrates and soluble lipids. This was attributed to the macromolecular and cell wall composition of the main microalgae species composing the biomass, i.e. Monoraphidium sp. and Stigeoclonium sp. Furthermore, the methane yield was increased by 72% for thermal, 28% for hydrothermal and 21% for microwave pretreatments, whereas no significant increase was found for ultrasonication as compared to control. Outstanding results of the thermal pretreatment should be validated in prospective pilot-scale studies in order to quantify the potential increase in biogas production upon continuous operation.

Microalgae production in wastewater treatment systems, anaerobic digestion and modelling using ADM1

An integrated microalgae-based system for urban wastewater treatment, microalgae production and bioenergy generation through anaerobic digestion was evaluated over a period of one year. The pilot HRAP was effective at removing COD (~80%) and ammonium (~95%) and robust, despite common variations in wastewater composition and weather conditions in the Mediterranean region. Biomass production showed a strong seasonality, reaching an annual average of 10gTSS/m2·day and the highest values in spring (23gTSS/m2·day). Conversely, the macromolecular composition was fairly constant (58% proteins, 22% carbohydrates and 20% lipids). Predominant microalgae species varied throughout the year, influencing biogas production. Indeed, the anaerobic biodegradability of harvested biomass was 20–25% in July–October 2012 and May–July 2013 and 25–38% in November 2012–April 2013. Adapting the content of particulate inert COD in Anaerobic Digestion Model No. 1 (ADM1) was crucial for model calibration. After adjustment, ADM1 was able to predict microalgae anaerobic digestion performance, which showed an average methane yield of 0.09LCH4/gCOD at 15days HRT and 0.16LCH4/gCOD at 20days HRT.

Continuous-Time Enclosures for Uncertain Implicit Differential Equations

The computation of enclosures for the reachable set of uncertain dynamic systems is a crucial component in a wide variety of applications, from global and robust dynamic optimization to safety verification and fault detection. Even though many systems in engineering are best modeled as implicit differential equations (IDEs) and differential algebraic equations (DAEs), methods for the construction of enclosures for these are not as well developed as they are for ordinary differential equations (ODEs). In this paper, we propose a continuous-time approach for the guaranteed over approximations of the reachable set for quasilinear IDEs. This approach builds on novel high-order inclusion techniques for the solution set of algebraic equations and state-of-the-art techniques for bounding the solution of nonlinear ODEs. We show how this approach can be used to bound the reachable set of uncertain semi-explicit DAEs by bounding the underlying IDEs. We demonstrate this approach on two case studies, a double pendulum where it proves superior with delayed break-down times compared to other methods, and anaerobic digestion of microalgae which has nine differential and two algebraic states.

Anaerobic digestion of microalgal biomass after ultrasound pretreatment

High rate algal ponds are an economic and sustainable alternative for wastewater treatment, where microalgae and bacteria grow in symbiosis removing organic matter and nutrients. Microalgal biomass produced in these systems can be valorised through anaerobic digestion. However, microalgae anaerobic biodegradability is limited by the complex cell wall structure and therefore a pretreatment step may be required to improve the methane yield. In this study, ultrasound pretreatment at a range of applied specific energy (16–67MJ/kgTS) was investigated prior to microalgae anaerobic digestion. Experiments showed how organic matter solubilisation (16–100%), hydrolysis rate (25–56%) and methane yield (6–33%) were improved as the pretreatment intensity increased. Mathematical modelling revealed that ultrasonication had a higher effect on the methane yield than on the hydrolysis rate. A preliminary energy assessment indicated that the methane yield increase was not high enough as to compensate the electricity requirement of ultrasonication without biomass dewatering (8% VS).

Methane production of thermally pretreated Chlorella vulgaris and Scenedesmus sp. biomass at increasing biomass loads

Anaerobic digestion of microalgae has been enhanced by several pretreatments; however the reported net energy ratio was negative. In order to cope with this issue, this investigation focused on thermal pretreatment (120°C for 40min) at increasing biomass loads of Chlorella vulgaris and Scenedesmus sp. During that thermal pretreatment, carbohydrates solubilisation prevailed over proteins for both strains. Regardless the biomass load pretreated, anaerobic biodegradability of C. vulgaris was enhanced by 50% and therefore, pretreatments of high biomass loads was suggested to counterbalance the energy input. On the other hand, thermally pretreated Scenedesmus sp. biomass supported an enhancement of 21–27%. The specific cell wall composition was suggested as a potential reason for the differences registered on their anaerobic biodegradabilities.

Integrating microalgae production with anaerobic digestion: a biorefinery approach

AbstractIn the energy and chemical sectors, alternative production chains should be considered in order to simultaneously reduce the dependence on oil and mitigate climate change. Biomass is probably the only viable alternative to fossil resources for production of liquid transportation fuels and chemicals since, besides fossils, it is one of the only available sources of carbon‐rich material on Earth. Over recent years, interest in microalgae biomass has grown in both fundamental and applied research fields. The biorefinery concept includes different technologies able to convert biomass into added‐value chemicals, products (food and feed) and biofuels (biodiesel, bioethanol, biohydrogen). As in oil refinery, a biorefinery aims at producing multiple products, maximizing the value derived from differences in biomass components, including microalgae. This paper provides an overview of the various microalgae‐derived products, focusing on anaerobic digestion for conversion of microalgal biomass into methane. Special attention is paid to the range of possible inputs for anaerobic digestion (microalgal biomass and microalgal residue after lipid extraction) and the outputs resulting from the process (e.g. biogas and digestate). The strong interest in microalgae anaerobic digestion lies in its ability to mineralize microalgae containing organic nitrogen and phosphorus, resulting in a flux of ammonium and phosphate that can then be used as substrate for growing microalgae or that can be further processed to produce fertilizers. At present, anaerobic digestion outputs can provide nutrients, CO2 and water to cultivate microalgae, which in turn, are used as substrate for methane and fertilizer generation. © 2014 Society of Chemical Industry and John Wiley & Sons, Ltd

High pressure thermal hydrolysis as pre-treatment to increase the methane yield during anaerobic digestion of microalgae

Anaerobic digestion of algal biomass will be an essential component of algal biofuel production systems, yet the methane yield from digestion of algae is typically much lower than the theoretical potential. In this work, high pressure thermal hydrolysis (HPTH) is shown to enhance methane yield during algae digestion. HPTH pre-treatment was applied to both raw algae and algal residue resulting from lipid extraction. HPTH and even the lipid extraction process itself increased methane yield, by 81% and 33% respectively; in combination they increased yield by 110% over that of the raw algae (18L CH4 gVS−1 substrate). HPTH had little effect on the rate of anaerobic digestion, however lipid extraction enhanced it by 33% over that for raw algae (0.21day−1). Digestion resulted in solubilisation of nitrogen (and phosphorous to a lesser degree) in all cases, showing that there is potential for nutrient recycling for algal growth.

Modeling anaerobic digestion of microalgae using ADM1

The coupling between a microalgal pond and an anaerobic digester is a promising alternative for sustainable energy production by transforming carbon dioxide into methane using solar energy. In this paper, we demonstrate the ability of the original ADM1 model and a modified version (based on Contois kinetics for the hydrolysis steps) to represent microalgae anaerobic digestion. Simulations were compared to experimental data of an anaerobic digester fed with Chlorella vulgaris. The modified ADM1 fits adequately the data for the considered 140 day experiment encompassing a variety of influent load and flow rates. It turns out to be a reliable predictive tool for optimising the coupling of microalgae with anaerobic digestion processes.