Evaluating the impact of substrate addition for anaerobic co-digestion on biogas production and digestate quality: The case of deinking sludge.

To reduce greenhouse gas emissions from organic waste, anaerobic digestion has created new opportunities for energy and nutrient recovery from these wastes. However, the use of certain organic wastes in anaerobic digestion is limited due to their atypical physicochemical characteristics (e.g. unbalanced carbon to nitrogen ratio, high ash concentration). Deinking sludge is a residue from the paper recycling industry and is one of such substrates. This study aims at evaluating the impact of deinking sludge (DS) addition into a conventional co-digestion mixture on methane production and digestate quality. To this end, an integrated method was proposed, combining the analysis of physicochemical and biodegradability characteristics with parsimonious modeling using the SYS-Metha tool. The measured characteristics of the deinking sludge showed that its potential use in mono-digestion conditions is very limited. When co-digested with food waste and municipal sludge, no significant synergies or antagonisms were found. Based on these experiments, model simulations were executed to determine the optimal conditions for co-digestion with food waste and municipal sludge. A maximum of 22% of deinking sludge on a fresh mass basis can be added into a co-digestion mixture to achieve proper wet anaerobic digestion conditions. Regarding digestate quality, the addition of DS reduced nutrient and contaminants concentrations, which have an impact on digestate management, particularly for land application. Overall, the proposed methodology in this study allows determining optimal co-digestion mixtures and highlighted the limits needing further investigation under pilot/real conditions.

Use of fermentation processes for improving the dissolution of phosphorus and its recovery from waste activated sludge.

Recycling phosphorus from waste activated sludge has attracted a lot of interest to tackle the problem of phosphorus stocks depletion and the increase in food demand. In this study, the use of fermentation processes was investigated to enhance phosphorus dissolution from waste activated sludge to improve its recycling. Two fermentation processes, bioacidification and dark fermentation, were used on two different sludges fermented with wheat starch syrup in continuous operating conditions. Hydrogen yield from the co-substrate fermentation with waste activated sludge reached 3.9 mmolH2.gCODcosubstrate-1 yield during dark fermentation process and was negligible during bioacidification. Dissolved phosphorus in the waste activated sludge increased by 68% during bioacidification and by 43% during dark fermentation. In both processes, phosphorus dissolution was accompanied by iron, calcium and magnesium dissolution. Results show that fermentation enhances phosphorus dissolution in waste activated sludge to improve its recovery along with hydrogen and organic acids.

A simple mass balance tool to predict carbon and nitrogen fluxes in anaerobic digestion systems.

The increase in anaerobic digestion systems has profoundly affected the waste management of territories, particularly for agricultural systems. Changes in cultural practices and imports of organic waste modify the carbon (C) and nitrogen (N) fluxes on territories where anaerobic digestion is implemented. Successful anaerobic digestion can increase the economic and ecological efficiency of the waste management system. Conversely, poor anaerobic digestion leads to low economic and environmental efficiency due to greenhouse gas emissions and nutrient loss. Modeling the impact of anaerobic digestion on the systems integrating anaerobic digestion can improve the efficiency of these practices. The aim of this study was to develop, analyze, and evaluate a simple mass balance tool able to predict carbon and nitrogen fluxes in anaerobic digestion systems. The tool is composed of an exhaustive substrate database used by three models: (i) an anaerobic digestion model that predicts C and N contents in biogas and digestate; (ii) a phase separation model that predicts C and N content in liquid and solid phase digestates; and (iii) a storage model that predicts C and N content in raw, liquid phase, and solid phase digestates, as well as C and N emissions during storage. Sensitivity analyses were performed on the tool to determine critical inputs. Sensitivity analysis showed that outputs were highly sensitive to their respective inputs and to total inputs of solids. Performance evaluation showed that the tool can provide good quality predictions with R2 correlations between observation and prediction varying from 0.72 to 0.99 with the best predictions obtained for raw digestate.

Modeling the fate of organic nitrogen during anaerobic digestion: Development of a bioaccessibility based ADM1.

Simulating the fate of nitrogen during anaerobic digestion is required to predict the characteristics of digestates and to improve their exploitation for agricultural uses. The aim of this study was to develop a modified ADM1 model that includes bioaccessibility-based fractionation to accurately simulate the fate of nitrogen during anaerobic digestion. To this end, two complementary approaches were used: (i) changes in the bioaccessibility of protein and non-protein compounds were assessed on eight substrates during anaerobic digestion in batch experiments using the "EPS" fractionation method; (ii) experimental results were used to develop a bio-kinetic model based on anaerobic digestion model n°1. This new model incorporates bioaccessibility-based fractionation in its input state variables. The model was successfully calibrated and model evaluation showed that predicted methane production, ammonium production and changes in protein and non-protein bioaccessibility during anaerobic digestion were accurate.

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%.