The problems linked to plastic wastes have led to the development of biodegradable plastics. More specifically, biodegradable bioplastics are the polymers that are mineralized into carbon dioxide, methane, water, inorganic compounds, or biomass through the enzymatic action of specific microorganisms. They could, therefore, be a suitable and environmentally friendly substitute to conventional petrochemical plastics. The physico-chemical structure of the biopolymers, the environmental conditions, as well as the microbial populations to which the bioplastics are exposed to are the most influential factors to biodegradation. This process can occur in both natural and industrial environments, in aerobic and anaerobic conditions, with the latter being the least researched. The examined aerobic environments include compost, soil, and some aquatic environments, whereas the anaerobic environments include anaerobic digestion plants and a few aquatic habitats. This review investigates both the extent and the biodegradation rates under different environments and explores the state-of-the-art knowledge of the environmental and biological factors involved in biodegradation. Moreover, the review demonstrates the need for more research on the long-term fate of bioplastics under natural and industrial (engineered) environments. However, bioplastics cannot be considered a panacea when dealing with the elimination of plastic pollution.
Opuntia ficus-indica (OFI) is an emerging biomass that has the potential to be used as substrate in anaerobic digestion. The goal of this work was to investigate the effect of three pretreatment techniques (thermal, alkaline, acidic) on the chemical composition and the methane yield of OFI biomass. A composite experimental design with three factors and two to three levels was implemented, and regression modelling was employed using a total of 10 biochemical methane potential (BMP) tests. The measured methane yields ranged from 289 to 604 NmL/gVS; according to the results, only the acidic pretreatment (HCl) was found to significantly increase methane generation. However, as the experimental values were quite high with regards to the theoretical methane yield of the substrate, this effect still needs to be confirmed via further research. The alkaline pretreatment (NaOH) did not noticeably affect methane yields (an average reduction of 8% was recorded), despite the fact that it did significantly reduce the lignin content. Thermal pretreatment had no effect on the methane yields or the chemical composition. Scanning electron microscopy images revealed changes in the chemical structure after the addition of NaOH and HCl. Modelling of the cumulated methane production by the Gompertz modified equation was successful and aided in understanding kinetic advantages linked to some of the pretreatments. For example, the alkaline treatment (at the 20% dosage) at room temperature resulted to a μ (maximum specific methane production rate [NmLCH/(gVS·d)]) equal to 36.3 against 18.6 for the control.
Citrus-processing industries produce large volumes of wastewater (CPWW). The large variability of these volumes coupled to physicochemical characteristics of CPWW determine severe constraints for their disposal due to both economic and environmental factors. To minimize the management costs and prevent the negative ecological impacts of CPWW, several systems have been proposed and adopted. However, all these treatment/valorization routes have many issues that are not yet thoroughly known by the scientific community and stakeholders of the citrus-processing chain. This paper reports an overview of the possible treatment/valorization opportunities for CPWW: intensive biological treatment, lagooning, direct land application, energy conversion, and biorefinery uses for the extraction of added-value compounds. Advantages and constraints are presented and discussed, and the following conclusions are achieved: (i) there is not a unique solution for CPWW treatment, since the best management system of CPWW must be chosen case by case, taking into account the quality/quantity of the effluent and the location of the transformation industry; (ii) the adoption of a biorefinery approach can increase the competitiveness and the further development of the whole citrus sector, but the cost of novel technologies (some of which have not been tested at real scale) still limits their development.
The valorization of orange peel waste (OPW) is sought worldwide mainly via anaerobic digestion. A common problem encountered during the biological treatment is the seasonality of its production and the presence of d-Limonene. The latter is a typical anti-microbial compound. This work aims to evaluate the effect of the use of granular activated carbon (GAC) combined with alkaline pretreatment to enhance methane generation during semi-continuous anaerobic digestion of OPW. The experimental design consisted of two groups of experiments, A and B. Experiment A was designed to verify the maximum OPW loading and to assess the effect of pH and nutrients on the process. Experiment B was designed to study the effect of alkaline pretreatment alone and of alkaline pretreatment aided by biochar addition to the process. Apart from the methane yields, the d-Limonene contents were measured in all experiments. The preliminary results showed that OPW alkaline pretreatment after the addition of a moderate amount of GAC can render anaerobic digestion of OPW sustainable as long as the organic loading does not exceed 2 gVS·L−1·day−1 and nutrients are supplemented. The experiment in which GAC was added after alkaline pretreatment resulted in the highest methane yield and reactor stability.
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