A new approach to the kinetic study of anaerobic degradation of the organic fraction of municipal solid waste

Anaerobic digestion of bio-waste: A mini-review focusing on territorial and environmental aspects

Scientific and industrial experiences, together with economical and policies changes of last 30 years, bring anaerobic digestion among the most environmental friendly and economically advantageous technologies for organic waste treatment and management in Europe. In this short review, the role of anaerobic digestion of organic wastes is discussed, considering the opportunity of a territorial friendly approach, without barriers, where different organic wastes are co-treated. This objective can be achieved through two proposed strategies: one is the anaerobic digestion applied as a service for the agricultural and farming sector; the other as a service for citizen (biowaste, diapers and wastewater treatment integration). The union of these two strategies is an environmental- and territorial-friendly process that aims to produce renewable energy and fertiliser material, with a low greenhouse gas emission and nutrients recovery. The advantage of forthcoming application of anaerobic digestion of organic wastes, even for added value bioproducts production and new energy carriers, are finally discussed. Among several advantages of anaerobic digestion, the role of the environmental controller was evaluated, considering the ability of minimising the impacts exploiting the biochemical equilibrium and sensitivity as a quality assurance for digestate.

Two-phase anaerobic digestion of the organic fraction of municipal solid waste: estimation of methane production

Energy generation from methane (CH(4)) is one of the primary targets of the anaerobic digestion process. Consequently, the focus of this study was to investigate the effect on CH(4) production of total solids (TS) loading (measured as % TS) and hydraulic residence time (HRT) during the treatment of the organic fraction of municipal solid waste (OFMSW). Laboratory-scale, two-phase anaerobic digestion systems were employed with each system consisting of an acidogenic reactor and a methanogenic reactor linked in series. The group A runs in the experiment explored the effect on digester performance of four variations in methanogenic HRT (15, 20, 25 and 30 days) at three different feed TS concentrations (8, 12 and 15%). The group B runs compared the actual methane yield (0.14 to 0.45 L g VSfeed−1)) to that predicted by the Chen-Hashimoto model. Results from the group A runs indicated that acidogenesis improved with an increase in % TS and a decrease in HRT; while, methanogenesis behaved inversely, achieving higher yields at the lower % TS and longer HRT values. In comparison with the group B runs, the Chen-Hashimoto model under-predicted (by an average of 16.5 ± 6.6%) the CH(4) yield obtained from the digestion of OFMSW.

Regression models of ultimate methane yields of fruits and vegetable solid wastes, sorghum and napiergrass on chemical composition

Several fractions of fruits and vegetable solid wastes (FVSW), sorghum and napiergrass were analyzed for total solids (TS), volatile solids (VS), total organic carbon, total kjeldahl nitrogen, total soluble carbohydrate, extractable protein, acid-detergent fiber (ADF), lignin, cellulose and ash contents. Their ultimate methane yields (B(o)) were determined using the biochemical methane potential (BMP) assay. A series of simple and multiple regression models relating the B(o) to the various substrate constituents were generated and evaluated using computer statistical software, Statistical Package for Social Sciences (SPSS). The results of simple regression analyses revealed that, only weak relationship existed between the individual components such as carbohydrate, protein, ADF, lignin and cellulose versus B(o). A regression of B(o) versus combination of two variables as a single independent variable such as carbohydrate/ADF and carbohydrate+protein/ADF also showed that the relationship is not strong. Thus it does not appear possible to relate the B(o) of FVSW, sorghum and napiergrass with single compositional characteristics. The results of multiple regression analyses showed promise and the relationship appeared to be good. When ADF and lignin/ADF were used as independent variables, the percentage of variation accounted for by the model is low for FVSW (r(2)=0.665) and sorghum and napiergrass (r(2)=0.746). Addition of nitrogen, ash and total soluble carbohydrate data to the model had a significantly higher effect on prediction of B(o) of these wastes with the r(2) values ranging from 0.9 to 0.99. More than 90% of variation in B(o) of FVSW could be accounted for by the models when the variables carbohydrate, lignin, lignin/ADF, nitrogen and ash (r(2)=0.904), carbohydrate, ADF, lignin/ADF, nitrogen and ash (r(2)=0.90) and carbohydrate/ADF, lignin/ADF, lignin and ash (r(2)=0.901) were used. All the models have low standard error values, which indicate the amount of spread is less. Thus, considering only the higher r(2) values, six models are proposed for predicting the B(o) based on FVSW data and sorghum and napiergrass data. It would be more convenient if B(o) could be predicted by analyzing the chemical composition of the substrate rather than performing the long-term batch fermentation. To test the validity of the regression models, chemical constituents of FVSW that were not included in the regression analyses were determined and their experimental B(o) were determined by BMP assay. All the six models were used to predict the B(o) from the chemical constituents of these FVSW. It was found that most of the predicted values were within 20% of the experimental B(o) in models 1, 3 and 6. Since models 3 and 6 used the same variables namely, total soluble carbohydrate, ADF, lignin/ADF, nitrogen and ash, B(o) can be predicted from these five chemical constituents which accounts for more than 90% of the variation in B(o) (r(2)>90).

Enzymatic hydrolysis of polysaccharide-rich particulate organic waste

Hydrolysis of organic particulates in rapid fermentative processes can be inhibited. The volatile fatty acids (VFA) released during fermentation reduce pH. Whether VFA or the drop in pH inhibits hydrolysis is unclear. The effects of pH and acetate on the enzymatic hydrolysis of a potato sample that contains both carbohydrate and protein were studied at fixed pH (5-9) in the presence/absence of 20 g/L of acetate. Experimental results showed that the effects of pH and acetate on the hydrolysis of carbohydrate differed from those on the hydrolysis of protein. Numerous kinetic models fitted the hydrolysis data obtained during the first 40 h of hydrolysis when inhibitory effects were insignificant. The Chen-Hashimoto model was used herein to fit the hydrolysis data obtained during 144 h of reaction. Also, the non-competitive inhibition model of three inhibitors (H(+), OH(-), total/undissociated/dissociated acetate) successfully described the inhibition of the hydrolysis of both carbohydrate and protein.