Treatment and beneficiation of Kraft mill sludge to hydrogen and methane for bioenergy production.

Abstract

The key drivers behind research into renewable and sustainable practices are escalating global energy demands and financial instability linked to the depletion of fossil fuel-derived energy resources. Additional drivers include the environmental ramifications of continued fossil fuel usage and unsustainable industrial waste disposal methods. As the fifth-largest global coal producer, South Africa is heavily dependent on coal for its energy needs, accounting for approximately 80% of its greenhouse gas (GHG) emissions in 2015. Efforts to decarbonise the economy and promote low-carbon technologies have thus been on the rise. Examples include implementing a carbon tax, sectoral GHG emission targets, carbon budgets and phasing out inefficient fossil fuel incentives. The South African pulp and paper industry plays an essential role in the economy by contributing 4% and 25% to the country’s manufacturing and agricultural GDP, respectively. In 2019, 4.1 million tonnes of pulp and paper products were produced in South Africa. As a water-intensive industry, advanced water treatment and recycling technologies are implemented, generating significant solid waste volumes as paper mill sludge (PMS) of up to 100 tonnes per 550 tonnes of pulp produced. The industry relies heavily on landfilling as its primary waste disposal method, with up to 69% of the generated PMS being landfilled. Due to the environmental concerns of landfilling, such as GHG emissions, leaching of toxic compounds into the surrounding land and water and limited land space availability, environmental regulations have become stricter, impeding the future applicability of landfilling. Biorefineries have gained significant interest as an approach to utilise biomass such as industrial wastes efficiently. Fuels, chemicals, energy and heat can be produced from industrial waste streams, adding value to what many industries consider a financial and environmental burden. Therefore, the beneficiation of PMS can contribute to the circular economy by serving as a low-cost, abundantly available feedstock for sustainable energy generation. The bioproduction of hydrogen and methane from PMS as alternative energy carriers offers a sustainable approach to energy production. Hydrogen is a CO2-neutral energy source with a high energy content that can be converted to electricity in fuel cells, producing only water when combusted. In comparison, methane exhibits an octane rating higher than gasoline and produces less CO2 than fossil fuels upon combustion. Furthermore, the application of biohythane, an advanced fuel mixture of hydrogen and methane, shows great potential to reduce GHG emissions and improve engine combustion yield in the automotive industry. Unfortunately, the presence of process rejects, ash, and residual lignin in PMS waste streams further contributes to its recalcitrance to microbial degradation, which leads to low product yields and high process times, thus significantly hampering the applicability of its beneficiation. Therefore, this study aimed to develop economic beneficiation strategies to enhance the amenability of PMS to microbial digestion and improve subsequent process efficiency of hydrogen and methane production. A combined mixture and factorial Response Surface Methodology (RSM) design was used to develop a novel pretreatment protocol using green liquor dregs (GLD), a waste generated by the pulp and paper industry, to improve the enzymatic hydrolysis of PMS. Optimised process conditions of 56% GLD: 44% PMS, 4.5% Tween-80, 60 min heating time, and a solid-to-liquid ratio (S:L) of 9.5% gave a maximal reducing sugar release of 16.38 g/L. Elemental analysis of each phase of the pretreatment protocol also showed a significant reduction in the concentrations of heavy metals; aluminium (81.39%), chromium (74.05%), cobalt (83.66%), nickel (88.06%), cadmium (88.89%), tin (83.82%) and lead (75.24%), and the complete removal of mercury. This is the first account of the utilisation of GLD as a pretreatment agent for PMS. Thereafter, the use of pharmaceutical wastewater (PW) as a supplementary nitrogen source to balance the system’s carbon-to-nitrogen ratio (C/N) in the optimisation of simultaneous saccharification and fermentation (SSF) hydrogen production from pretreated PMS was assessed, using a Box-Behnken design. The investigated process parameters included nitrogen source, enzyme dosage, substrate concentration, and pH. Using PW as the nitrogen source gave a 2.26 and 39.38% increase in the optimised hydrogen yield, compared to yeast extract and ammonium nitrate, respectively. A maximum hydrogen yield of 33.56 mL/g volatile solids (VS)added was obtained. Kinetic studies using the modified Gompertz model produced comparable data between PW and yeast extract, displaying a 3.26 h difference in process lag time. Furthermore, the maximum potential hydrogen production rate (Rm) obtained using PW was 9.22 mL/h, exhibiting an 8.73% increase compared to ammonium nitrate. The feasibility of using the effluent generated from the optimised processes for a two-stage anaerobic digestion (TSAD) system for methane production was then determined and compared to conventional single-stage anaerobic digestion (SSAD) of pretreated PMS. A low methane yield of 4.79 mL/gVSadded was obtained using SSAD. A significant improvement in the methane yield was observed using TSAD, reaching 30.88 mL/gVSadded. However, re-adjustment of the effluent’s C/N to 25 increased the process duration from 10 days to 17 days and negatively impacted the methane yield, resulting in an 8.16% reduction. In addition, a significant variation in process kinetics was observed between PW and yeast extract in the SSAD system, illustrated by production rates (Rm) of 0.29 mL/h for PW and 1.29 mL/h for yeast extract. The TSAD system exhibited enhanced stability for both nitrogen sources, exhibited by PW and yeast extract supplemented processes giving maximum potential gas production volumes (Gm), maximum potential gas production rates (Rm) and lag times (tL) of 113.28 and 104.93 mL, 1.28 and 1.40 mL/h and 4.25 and 1.71 h, respectively. Although C/N adjustment showed higher Gm values of 119.70 and 135.89 mL for PW and yeast extract, respectively, production rates were significantly reduced (0.50 mL/h). Compositional analysis of the resultant digestate showed a high VS content and C/N ratio, and an approximate NPK ratio of 2.3:1:2 from SSAD, whereas TSAD digestate exhibited an NPK ratio of 4.8:1:2, presenting almost double the nitrogen content. A high electrical conductivity (EC) value of 18.50 mS/cm observed in the digestate of TSAD was indicative of high sodium levels from PW, illustrating that, if the effluent C/N is adjusted to 25, the digestate of TSAD would require dilution before use as a soil amendment. To the author’s best knowledge, this is the first study describing the optimal beneficiation of PMS from the South African pulp and paper industry by pretreatment and subsequent two-stage anaerobic digestion for the production of hydrogen and methane. This study demonstrated the potential of PMS as a feedstock for bioenergy production, as well as the effects of supplementing its nitrogen deficiency with a secondary waste stream such as PW on process efficiency. Incorporating industrial wastes such as GLD and PW as alternatives to reagents such as NaOH and yeast extract to supplement fermentation processes can contribute to the circular economy concept by recycling waste streams. This improves the economic feasibility of PMS beneficiation and creates a sustainable energy solution to the disposal of PMS. This study has generated; one paper that has been published in a high impact, peer-reviewed journal, two papers that have been submitted for publication, and one more paper that is currently being prepared for submission to a peer-reviewed journal.