Characterization of Anaerobic Degradability and Kinetics of Harvested Submerged Aquatic Weeds Used for Nutrient Phytoremediation

A comprehensive review on anaerobic digestion with focus on potential feedstocks, limitations associated and recent advances for biogas production

With the escalating energy demand to accommodate the growing population and its needs along with the responsibility to mitigate climate change and its consequences, anaerobic digestion (AD) has become the potential approach to sustainably fulfil our demands and tackle environmental issues. Notably, a lot of attention has been drawn in recent years towards the production of biogas around the world in waste-to-energy perspective. Nevertheless, the progress of AD is hindered by several factors such as operating parameters, designing and the performance of AD reactors. Furthermore, the full potential of this approach is not fully realised yet due the dependence on people's acceptance and government policies. This article focuses on the different types of feedstocks and their biogas production potential. The feedstock selection is the basic and most important step for accessing the biogas yield. Furthermore, different stages of the AD process, design and the configuration of the biogas digester/reactors have been discussed to get better insight into process. The important aspect to talk about this process is its limitations associated which have been focused upon in detail. Biogas is considered to attain the sustainable development goals (SDG) proposed by United Nations. Therefore, the huge focus should be drawn towards its improvements to counter the limitation and makes it available to all the rural communities in developing countries and set-up the pilot scale AD plants in both developing and developed countries. In this regard, this article talks about the improvements and futures perspective related to the AD process and biogas enhancement.

Recovery and applications of ammoniacal nitrogen from nitrogen-loaded residual streams: A review

Total ammoniacal nitrogen (TAN) is considered to be a pollutant, but is also a versatile resource. This review presents an overview of the TAN recovery potentials from nitrogen (N)-loaded residual streams by discussing the sources, recovery technologies and potential applications. The first section of the review addresses the fate of TAN after its production. The second section describes the identification and categorisation of N-loaded (≥0.5 g L^-1 of reduced N) residual streams based on total suspended solids (TSS), chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), TAN, and TAN/TKN ratio. Category 1 represents streams with a low TAN/TKN ratio (<0.5) that need conversion of organic-N to TAN prior to TAN recovery, for example by anaerobic digestion (AD). Category 2 represents streams with a high TAN/TKN ratio (≥0.5) and high TSS (>1 g L^-1) that require a decrease of the TSS prior to TAN recovery, whereas category 3 represents streams with a high TAN/TKN ratio (≥0.5) and low TSS (≤1 g L^-1) that are suitable for direct TAN recovery. The third section focuses on the key processes and limitations of AD, which is identified as a suitable technology to increase the TAN/TKN ratio by converting organic-N to TAN. In the fourth section, TAN recovery technologies are evaluated in terms of the feed composition tolerance, the required inputs (energy, chemicals, etc.) and obtained outputs of TAN (chemical form, concentration, etc.). Finally, in the fifth section, the use of recovered TAN for three major potential applications (fertilizer, fuel, and resource for chemical and biochemical processes) is discussed. This review presents an overview of possible TAN recovery strategies based on the available technologies, but the choice of the recovery strategy shall ultimately depend on the product characteristics required by the application. The major challenges identified in this review are the lack of information on enhancing the conversion of organic-N into TAN by AD, the difficulties in comparing the performance and required input of the recovery technologies, and the deficiency of information on the required concentration and quality of the final TAN products for reuse.

Remediation Techniques for Cadmium-Contaminated Dredged River Sediments after Land Disposal

This paper examines the remediation techniques of cadmium (Cd)-contaminated dredged river sediments after land disposal in a city in East China. Three remediation techniques, including stabilization, soil leaching, and phytoremediation, are compared by analyzing the performance of the techniques for Cd-contaminated soil remediation. The experimental results showed that the stabilization technique reduced the leaching rate of soil Cd from 33.3% to 14.3%, thus effectively reducing the biological toxicity of environmental Cd, but the total amount of Cd in soil did not decrease. Leaching soil with citric acid and oxalic acid achieved Cd removal rates of 90.1% and 92.4%, respectively. Compared with these two remediation techniques, phytoremediation was more efficient and easier to implement and had less secondary pollution, but it took more time, usually several years. In this study, these three remediation techniques were analyzed and discussed from technical, economic, and environmental safety perspectives by comprehensively considering the current status and future plans of the study site. Soil leaching was found to be the best technique for timely treatment of Cd contamination in dredged river sediments after land disposal.

Novel wet-solid states serial anaerobic digestion process for enhancing methane recovery of aquatic plant biomass

Aquatic plant biomass is characterised by high moisture content and a lignocellulose structure. To apply the anaerobic digestion (AD) treatment to aquatic plants, the simultaneous achievement of high methane (CH₄) recovery per biomass volume and high biodegradability have been a challenge owing to these characteristics. Herein, we propose a novel two-stage serial wet- and solid-state AD (SS-AD) system that quickly digests the labile cytoplasm fraction in the first wet AD reactor in a short retention time while slowly digesting the lignocellulosic fraction in the later SS-AD with long retention time. In this study, the effect of this serial AD on CH₄ recovery and chemical oxygen demand (COD) balance from aquatic plant biomass was examined in a semi-continuous operation. Elodea nuttallii, which grows excessively in the southern basin of Lake Biwa, Japan, was used as the substrate. For comparison, single-stage AD with different hydraulic retention times (HRTs) (30 d and 15 d) was performed. The CH₄ conversion efficiency in single-stage AD deteriorated from 47.6 to 33.1% COD with shortened HRT, probably owing to the low degradability of slowly degrading fraction (i.e. lignocellulose) in the short retention time. In contrast, the serial AD under the same HRT (15 d) as a single-stage AD exhibited higher CH₄ conversion efficiency of 65.1% COD, mainly owing to the enhanced degradation of slowly degrading fraction because of the prolonged solid retention time (52.2 d) of the entire system. The CH₄ recovery from the wet AD alone in the serial AD system surpassed that from the 30 d-HRT of the single-stage AD, possibly due to the appropriate HRT for labile fraction and/or the microbial recirculation. The serial wet and SS-AD was suggested as a suitable technology for the treatment of aquatic plant biomass with recalcitrant cell walls and a labile cytoplasm.

Crop Characteristics of Aquatic Macrophytes for Use as a Substrate in Anaerobic Digestion Plants—A Study from Germany

Several thousand metric tonnes of aquatic biomass are removed from water bodies every year, so that these waters can continue to be used for ship and boat traffic and for leisure activities. The mowed material is generally disposed off without any further use. Therefore, the crop properties of samples from 39 weed control measures all over Germany were examined to assess the suitability of aquatic plant biomass as a substrate for anaerobic digestion systems. Analysis of the crop samples consisted of the identification of plant species and the evaluation of sediment contents and concentrations of macroelements. The methane yield was determined for selected aquatic plants. Analysis revealed a carbon/nitrogen ratio (C/N) of between 10 and 20 in 74% of samples. The concentrations of nitrogen and phosphorous in the samples were comparable to grass silages. With regard to heavy metal concentrations, the threshold values for biowaste for nickel, zinc, and cadmium were exceeded in three samples. There were no significant seasonal differences in substrate characteristics and qualities. The specific methane yields of individual aquatic plants were between 142 and 372 LCH4/kg volatile solids (VS). The results of this study showed that aquatic macrophytes can be used as substrates in anaerobic digestion plants without any restrictions.

Process analysis of anaerobic fermentation of Phragmites australis straw and cow dung exposing to elevated chromium (VI) concentrations

Anaerobic fermentation is considered as a cost-effective way of biomass waste disposal. Chromium (Cr) is one of the heavy metals that often been blamed for unsatisfactory operation or failure of anaerobic fermentation. The impact of Cr (added as K₂Cr₂O₇) on mesophilic anaerobic fermentation of Phragmites australis straw and cow dung was demonstrated by investigating the biogas properties, process stability, substrate degradation and enzyme activities during the fermentation process. The results showed that 30, 100 and 500 mg/L Cr⁶⁺ addition increased the cumulative biogas yields by up to 19.00%, 14.85% and 7.68% respectively, and brought forward the daily biogas yield peak. Meanwhile, the methane (CH₄) content in the 30 (52.47%) and 100 (40.57%) mg/L Cr⁶⁺-added groups were generally higher than the control group (37.70%). Higher pH values (close to pH 7) and lower oxidation-reduction potential (ORP) values in the Cr⁶⁺-added groups after the 15th day indicated the better process stability compared to the control group. Taking the whole fermentation process into account, the promoting effect of Cr⁶⁺ addition on biogas yields was mainly attributable to better process stability, the enhanced degradation of lignin and hemicellulose, the transformation of intermediates into VFA, the higher coenzyme F₄₂₀ activities and the efficient generation of CH₄. These results demonstrate that an appropriate addition of Cr⁶⁺ could enhance the anaerobic fermentation which support the regulations utilizing of the Cr⁶⁺ contaminated biowaste.

Characterization and prediction of organic nitrogen biodegradability during anaerobic digestion: A bioaccessibility approach

Prediction of organic nitrogen mineralization into ammonium during anaerobic digestion is required for optimizing substitution of mineral fertilizer by digestates. The aim of this study was to understand organic nitrogen biodegradability and to investigate how it can be predicted from carbon biodegradability, and nitrogen bioaccessibility, respectively. Bioaccessibility was assessed using fractionation methods based on sequential extractions. Results showed that organic nitrogen was present in fractions whose bioaccessibility levels differed. Organic nitrogen and carbon biodegradability were also determined and compared. Results highlighted two groups of substrates: the first with an initial NH₄⁺/TKN < 30%, whose carbon and nitrogen biodegradability are similar; the second with an initial NH₄⁺/TKN > 30%, whose carbon and nitrogen biodegradability differ significantly. To enable prediction on all substrates, partial least square (PLS) regressions were carried out to link organic nitrogen bioaccessibility indicators to biodegradability. The models successfully predicted organic nitrogen biodegradability with a maximum prediction error of 10%.

Biogas production from different lignocellulosic biomass sources: advances and perspectives

The present work summarizes different sources of biomass used as raw material for the production of biogas, focusing mainly on the use of plants that do not compete with the food supply. Biogas obtained from edible plants entails a developed technology and good yield of methane production; however, its use may not be sustainable. Biomass from agricultural waste is a cheap option, but in general, with lower methane yields than those obtained from edible plants. On the other hand, the use of algae or aquatic plants promises to be an efficient and sustainable option with high yields of methane produced, but it necessary to overcome the existing technological barriers. Moreover, these last raw materials have the additional advantage that they can be obtained from wastewater treatment and, therefore, they could be applied to the concept of biorefinery. An estimation of methane yield per hectare per year of the some types of biomass and operational conditions employed is presented as well. In addition, different strategies to improve the yield of biogas, such as physical, chemical, and biological pretreatments, are presented. Other alternatives for enhanced the biogas production such as bioaugmentation and biohythane are showed and finally perspectives are mentioned.

Marsilea spp.-A novel source of lignocellulosic biomass: Effect of solubilized lignin on anaerobic biodegradability and cost of energy products

The present study concerns the liquefying potential of an unusual source of lignocellulosic biomass (Marsilea spp., water clover, an aquatic fern) during combinative pretreatment. The focus was on how the pretreatment affects the biodegradability, methane production, and profitability of thermochemical dispersion disintegration (TCDD) based on liquefaction and soluble lignin. The TCDD process was effective at 12,000 rpm and 11 min under the optimized thermochemical conditions (80 °C and pH 11). The results from biodegradability tests imply that 30% liquefaction was sufficient to achieve enhanced biodegradability of about 0.280 g-COD/g-COD. When biodegradability was >30% inhibition was observed (0.267 and 0.264 g-COD/g-COD at 35-40% liquefaction) due to higher soluble lignin release (4.53-4.95 g/L). Scalable studies revealed that achievement of 30% liquefaction was beneficial in terms of the energy and cost benefit ratios (0.956 and 1.02), when compared to other choices.

The opportunities for obtaining of the biogas on methane fermentation from marine algae biomass and water plant biomass

The aim of the research was to try to obtain of the biogas on a laboratory scale from marine algae biomass and water plant biomass. The research was conducted in 2016 year and samples were taken from the Polish coast of the Baltic Sea. In laboratory work, algae and plant species were first identified. The next, in order to subject them to methane fermentation processes and to obtain biogas,partial mechanical treatment of the biomass was conducted. Dry matter content and dry organic matter content were also determined. The research has shown different production of the biogas depending on the various species of the algae and plants. The percentage composition of the biogas was also determined (% CO2and % CH4). In this research some kinds and species of algae and aquatic plants were distinguished:Scytosiphon cf. S. tortilis, Fucus vesiculosus, Cladophora, Audouinella, Potamogeton perfoliatus. Production of biogas from selected algae and water plants oscillated between 0.023 dm3·g-1and 0.303 dm3·g-1. The highest content of the methane in biogas was obtained from the mixture of Ectocarpus from spring and autumn harvest (values oscillated from 80.7 % to 81.2 %), while the highest percentage share of carbon dioxide in the biogas was characterized by the mixtureFucus vesiculosusandAudouinella(22 %). Due to a small amount of the research in this field, more research is needed.

Phase separation and microbial distribution in the hyperthermophilic-mesophilic-type temperature-phased anaerobic digestion (TPAD) of waste activated sludge (WAS)

In order to investigate the phase separation and microbial distribution in the TPAD, the conventional thermophilic-mesophilic type (TM-TPAD) and the hyperthermophilic-mesophilic type (HM-TPAD) were operated with a single-stage mesophilic anaerobic digestion (MAD) as control. HM-TPAD accomplished the volatile solids destruction 14.5% higher than MAD. Calculating conversion efficiencies distinguished the separation of acidogenic and methanogenic phases in HM-TPAD, which was not found in TM-TPAD. The differences on microbial distributions also reflected the phase separation in HM-TPAD. The protein degraders, Coprothermobacter had higher abundance in the first stage than the second stage of HM-TPAD but it had similar abundance between the two stages of TM-TPAD. Also, the archaeal communities from the two stages of HM-TPAD shared the least similarity but those from the two stages of TM-TPAD were closely similar.

Biomethane recovery from Egeria densa in a microbial electrolysis cell-assisted anaerobic system: Performance and stability assessment

Renewable energy recovery from submerged aquatic plants such as Egeria densa (E. densa) via continuous anaerobic digestion (AD) represents a bottleneck because of process instability. Here, a single-chamber membrane-free microbial electrolysis cell (MEC) equipped with a pair of Ti/RuO2 mesh electrodes (i.e. the combined MEC-AD system) was implemented at different applied voltages (0-1.0 V) to evaluate the potential effects of bioelectrochemical stimulation on methane production and process stability of E. densa fermentation. The application of MEC effectively stabilized E. densa fermentation and upgraded overall process performance, especially solid matters removal. E. densa AD process was operated steadily throughout bioelectrochemical process without any signs of imbalance. The solubilization-removal of solid matters and methane conversion efficiency gradually increased with increasing applied voltage, with an average methane yield of approximately 248.2 ± 21.0 mL L(-1) d(-1) at 1.0 V. Whereas, the stability of the process became worse immediately once the external power was removed, with weaken solid matters removal along with methane output, evidencing the favorable and indispensable role in maintaining process stability. The stabilizing effect was further quantitatively demonstrated by statistical analysis using standard deviation (SD), coefficient of variance (CV) and box-plots. The syntrophic and win-win interactions between fermenting bacteria and electroactive bacteria might have contributed to the improved process stability and bioenergy recovery.