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Biochar and hydrochar are carbonaceous materials with valuable applications. They can be synthesized from a wide range of organic wastes, including digestate. Digestate is the byproduct of anaerobic digestion (AD), which is performed for bioenergy (biogas) production from organic residues. Through a thermochemical process, such as pyrolysis, gasification, and hydrothermal carbonization - HTC, digestate can be converted into biochar or hydrochar. The addition of either biochar or hydrochar in AD has been reported to improve biochemical reactions and microbial growth, increasing the buffer capacity, and facilitating direct interspecies electrons transfer (DIET), resulting in higher methane (CH 4 ) yields. Both biochar and hydrochar can adsorb undesired compounds present in biogas, such as carbon dioxide (CO 2 ), hydrogen sulfide (H 2 S), ammonia (NH 3 ), and even siloxanes. However, an integrated understanding of biochar and hydrochar produced from digestate through their return to the AD process, as additives or as adsorbents for biogas purification, is yet to be attained to close the material flow loop in a circular economy model. Therefore, this overview aimed at addressing the integration of biochar and hydrochar production from digestate, their utilization as additives and effects on AD, and their potential to adsorb biogas contaminants. This integration is supported by life cycle assessment (LCA) studies, showing positive results when combining AD and the aforementioned thermochemical processes, although more LCA is still necessary. Techno-economic assessment (TEA) studies of the processes considered are also presented, and despite an expanding market of biochar and hydrochar, further TEA is required to verify the profitability of the proposed integration, given the specificities of each process design. Overall, the synthesis of biochar and hydrochar from digestate can contribute to improving the AD process, establishing a cyclic process that is in agreement with the circular economy concept., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Silver nanoparticles (AgNPs) are widely applied in several types of products since they act as a biocide. However, their high level of release into the environment can bring risks to ecosystems. Thus, the toxicity of AgNPs toward duckweed (Landoltia punctata) was investigated by monitoring the growth rate inhibition and the effect on the photosynthetic metabolism through morphological and ultrastructural analysis. The AgNPs were characterized by transmission electron microscopy and the effective diameter (dynamic light scattering) and zeta potential were determined. Plants were grown according to the environmental conditions recommended in ISO/DIS 20079 and then exposed to different concentrations of AgNPs. Inhibition of the growth rate was measured based on the EC 50 and changes in the morphology, cellular structures and photosynthetic pigments were evaluated along with the silver accumulation. Although the results showed low growth inhibition when compared to other studies, significant damage to the ultrastructure, decreases in the photosynthetic pigments and starch grains, an increase in the phenolic compounds and physiological changes, such as a loss of color, were observed. Moreover, the accumulation of silver ions was noted and this could lead to bioamplification in consumer organisms, since duckweed belongs to the first level of the food chain., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2019. Published by Elsevier B.V.)

The reduction of greenhouse gases (GHG) emissions is important challenge in the wastewater treatment plants. In this way, the present study aimed to evaluate the GHG emissions and carbon dioxide fixation by duckweed ponds (DWP) applied to treat municipal wastewater in a polishing stage. Two pilot DWP (3000 L) were operated in a series with real wastewater receiving a flow rate of 200 L d -1 and organic load rate of 39 g COD ha -1 d -1 . Beyond the standard physicochemical parameters for wastewater monitoring, the gases emissions from pond surface were measures by using a static flux chamber with infrared probes installed inside to detect CO 2 and CH 4 concentration. Operating the DWP with a load of 18.1 kg TN ha -1 d -1 and 2 kg TP ha -1 d -1 , across 425 days of monitoring, higher COD and nutrient removal efficiency was identified (79%, 93% and 84% for COD, TN and TP, respectively). The CO 2 emission rate ranged from 3048 to 6017 mg CO 2 m -2 d -1 and the fixation rate ranged from 19,592 to 42,052 mg CO 2 m -2 d -1 . Methane emission was not detected (less than 0.1%). Moreover, low abundance of archaeal community was identified in both DWP. The results showed that in presented conditions, under low COD loading rate DWP could fix at least three times more CO 2 than it emits, highlighting the sustainability of this natural technology., (Copyright © 2019 Elsevier B.V. All rights reserved.)