Converting wastewater cellulose to valuable products: A techno-economic assessment
Graphical abstract
Introduction
The activated sludge process for treating wastewater is a widely used technology for reducing the environmental impact of wastewater discharge. However, wastewater contains a lot of useful carbon-based materials such as cellulose that could be recovered and used as a raw material to produce several bio-based products. This minimizes the net energy consumption of the sewage treatment plant (STP). The potential recovery of cellulose has not been exploited much in North-West Europe (NWE). Generally, cellulose is treated and used in anaerobic digestion to produce biogas. Currently, very few such plants are in operation such as the Aarle-Rixtel STP in the Netherlands (Cirtec, 2020). Utilizing this valuable material could reduce the use of natural resources and subsequent carbon dioxide (CO2) emissions helping in realizing a circular economy. Cellulose fibers are the major constituent of chemical oxygen demand (COD) due to the increased usage of toilet paper. In western Europe, the per capita annual toilet paper consumption was reported to be about 14 kg/capita (Ruiken et al., 2013). On average, the COD is generally about 121 g per population equivalent (PE) per day (Wupperverband, 2019). Out of which, cellulose fibers constitute 25 %–30% of the COD fraction of the wastewater (Ruiken et al., 2013).
In typical STPs, the cellulose is first hydrolyzed followed by metabolization. This depends on the temperature and sludge retention time which ultimately affects the oxygen demand, sludge production, and nutrient removal (Ruiken et al., 2013). The degradation efficiency of cellulose fibers ranged from 6.7% to 100% at contact time ranging from 3 to 20 days, on a lab scale (Ahmed et al., 2019). The cellulose fibers are mainly biodegraded in the secondary treatment of STPs and as a result, the aeration energy consumption of the plant increases. Alternatively, the cellulose fibers can be recovered from wastewater before the primary sludge stage by using a sieve. Fine mesh has been in use as a mechanical treatment option as an alternative to the primary treatment of wastewater (Rusten and Ødegaard, 2006). It has also been observed that the cellulose fibers are the major fraction of COD removed by the sieve and have a consistency similar to paper mache indicating the presence of toilet paper in the wastewater (Ruiken et al., 2013).
The recovered cellulose could be used in many applications such as in mortar mix in the building sector (Cipolletta et al., 2019). Mixing the cellulose fibers was found to increase the performance of mortar in terms of lightness, flexural strength, and hygrometric properties. However, since the recovered cellulose is a carbon-based element, it can also be used to produce bio-based products using processes such as pyrolysis. In the pyrolysis process, the carbon-based material is subjected to heat in an oxygen-deficient environment such that it removes the volatiles from the material as vapor and converts the rest to biochar. The volatiles can be condensed to separate the stream into liquid and pyrolysis gas. The liquid fraction from condensation constitutes bio-oil and pyroligneous acid and some other minor components. A fast pyrolysis process occurs in a temperature range of 300–1000 °C and has a gas phase residence time of fewer than 2 s (Miltcon Services Limited, 2018). This process is used if more bio-oil is desired in the products (Cha et al., 2016). On the other hand, a slow pyrolysis process occurs in a similar temperature range (100–1000 °C) but with a residence time ranging from minutes to hours (Miltcon Services Limited, 2018). In slow pyrolysis, biochar is the main product due to its higher yield. The volatiles after pyrolysis remains in the reactor for longer residence times at low temperatures for vapor-phase reactions to continue increasing biochar yield (Cha et al., 2016).
Biochar produced from pyrolysis has limited direct applications. It is mainly used as a solid fuel for combustion or to enhance the soil quality in agriculture. To increase the number of applications, biochar can be activated. Activation results in an increased surface area allowing numerous applications such as in water purification, wastewater treatment, as adsorbent, etc. Physical activation is performed by using steam or inert gas at 800–1000 °C to remove the volatile components and oxidize the solid carbon residues (Hagemann et al., 2018). The surface area is increased by removing the partially combusted compounds that have been formed in the pores during carbonization. This increases the available pores and their volume, consequently, increasing the adsorption capacity of the activated biochar (Mohammad-Khah and Ansari, 2009). Another activation method is chemical activation in which the biochar is permeated with a liquid activation agent such as ZnCl2, KOH, or H3PO4 (Hagemann et al., 2018). When the biochar is soaked in the inorganic liquid activation agent, the organic molecules are degraded and prevented from depositing in the pores. After soaking, the activation agent is flushed with water and recovered (Mohammad-Khah and Ansari, 2009). The chemical activation process generates a higher surface area and has a higher activation efficiency than the physical activation process. However, it imposes additional costs such as chemicals and their recovery and reduces the equipment's life by corrosion (Cha et al., 2016).
This study was carried out as part of the Interreg North-West Europe project titled “Wider business Opportunities for raw materials from Wastewater (WOW!) (Interreg, 2022). The objective of the project was to identify the possibilities to recover and utilize the carbon-based elements from the wastewater in North-West Europe. As such, a detailed review of the state-of-the-art processes and technologies (Wupperverband, 2019) and the market potential (Wupperverband, 2020) of the envisioned products were conducted in the project. In the current techno-economic assessment, the use of the recovered cellulose from wastewater to produce bio-based products via a fast pyrolysis process was evaluated. The data (mass flow, energy consumption, and cost) were obtained from the pilot plant developed by Pulsed Heat (Pulsed Heat, 2021) and CirTec (Cirtec, 2022). In the next section, the process flow diagrams and the mass and energy assumptions are presented followed by the economic assessment methodology. Lastly, the results obtained, the effect of key parameters on the economic indicators, product quality, environmental impact, and the strategies to optimize the process from an economic point of view are discussed.
Section snippets
Methodology
The following sections present the pyrolysis pilot plant, the biochar activation unit, and the methodology used in the economic assessment of the entire value chain.
Results and discussion
Results are presented and discussed in seven sections: technical and economic performance of the base case pyrolysis plant followed by physical and chemical activation of biochar, economic comparison of the three cases (base case, physical and chemical activations), sensitivity analysis, product quality, environmental impact, and future recommendations.
Conclusions
A techno-economic assessment was performed based on the pyrolysis pilot plant with recovered cellulose from wastewater as feedstock. The collected cellulose required extensive pretreatment before use in pyrolysis. However, the associated costs were mostly offset by the operating expenditure savings in STP. The MSP estimated for these products was lower than the market prices. The quality of activated char obtained from cellulose was low compared to a commercially available char. Higher ash
CRediT authorship contribution statement
Mohammed Nazeer Khan: Methodology, Validation, Formal analysis, Investigation, Writing – original draft, Visualization. Mark Lacroix: Conceptualization, Investigation, Resources, Writing – review & editing. Coos Wessels: Conceptualization, Investigation, Resources, Writing – review & editing. Miet Van Dael: Methodology, Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This study is a part of the North-West European Interreg project VB WOW (Wider business Opportunities for raw materials from Wastewater – NWE project number 619). The authors would like to acknowledge the support from all the partners involved in the project.
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