JOURNAL HIGHLIGHTS BY STEVE RANGER
Researchers have investigated the potential of turning waste from paper production into bioethanol and biogas.
Paper production creates paper sludge, a waste-stream largely made up of short cellulose fibre rejects, impurities, fillers and clay. As much as 500m tonnes of wet paper sludge is created annually in this way.
Now researchers at the University of Stellenbosch, South Africa, have investigated more sustainable uses for this paper sludge. This fibre-rich waste often ends up in landfill, leading not only to methane emissions, but also the loss of water due to its high moisture content.
‘The need for innovative approaches for resource recovery and/or reuse is therefore necessary for the sustainable development of the pulp and paper industry,’ their paper said.
Production of biofuels including biomethane, biohydrogen, and bioethanol from lignocellulosic biomass has attracted interest because of its potential as a sustainable alternative to fossil fuels.
The researchers said that, in this context, paper sludge has some advantages over other potential feedstocks including its minimal need for pretreatment before enzymatic hydrolysis, zero or negative biomass cost, and the possibility of direct integration into existing industrial infrastructures such as paper mills.
‘These attributes position this waste stream as a promising feedstock for second-generation biofuel production and an attractive option for promoting circular bioeconomy initiatives in the paper and pulp industry,’ they said in the paper published in the journal Biofuels, Bioproducts and Biorefining (Biofpr).
The researchers looked at the potential of three types of waste – corrugated recycled sludge (CR), tissue and printing paper recycled sludge (TPR), or virgin pulp (VP) sludge – for bioethanol and biomethane production by simultaneous saccharification and fermentation and anaerobic digestion on a pilot scale.
Johann Görgens, professor in the Department of Chemical Engineering at Stellenbosch University said that different kinds of fibre residues have different properties, which affect the process of their conversion into bioethanol.
Unique properties such as viscosity, enzymatic digestibility and presence of contaminating substances affect the fermentation process performance, he said. There is also a compromise required in terms of enzyme dosage: ‘Adding a larger quantity of enzymes will improve process performance, but enzymes become less efficient at higher dosages (amount of ethanol produced per unit of enzyme added), effectively increasing the cost of enzymes,’ he said.
‘Enzyme cost can be a financial limitation to viability of the process, balanced against the technical performance needed for an industrial process,’ he added.
The researchers said that their study revealed ‘substantial differences’ in the biochemical and physical properties of the various types of paper sludge. They found that, from their experimental runs, VP sludge exhibited the highest ethanol yield, which they said can be attributed to its high glucan content and efficient enzymatic hydrolysis.
Corrugated cardboard sludge, despite its higher lignin content, demonstrated superior biogas and methane production likely due to its elevated xylan levels and favorable bulk density. The TPR sludge was characterised by high ash content and showed lower performance for both bioenergy production pathways but displayed improved solid handling due to its higher bulk density and lower water-holding capacity.
‘Based on this study’s findings, the choice of the process to deploy to an industry should be guided by sludge characteristics. The results are also applicable to other regions with similar pulp and paper industries,’ the paper said.
Görgens said that for two years the researchers have been operating an industrial demonstration plant adjacent to a paper mill and using a similar fibre-rich residue as feedstock for ethanol production. This has given them access to a robust data set on variabilities in process performance due to on-going changes in the feedstock properties.
‘The plant is mobile and we plan to do similar demonstrations at other pulp and paper mills, eventually also applying similar concepts to textile wastes. We would be interested to talk to potential industry partners for the latter kind of developments,’ he said.
Valorization of paper sludge for bioethanol and biogas production
Anné Williams, John Edison Sempiira, Lalitha Devi Gottumukkala, Sampson Mamphweli, Eugene Van Rensburg, Johann Görgens
Biofuels, Bioproducts and Biorefining (Biofpr)
doi.org/10.1002/bbb.70111
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