. Life Cycle Assessment .

Compared to direct incineration or landfilling of the waste paper, recycling can significantly reduce carbon emissions and it accounts for only 20-30% of incineration emissions and 70-80% of landfill emissions (1). We need a quantitative evaluation to further understand how much our project can contribute to reducing carbon emissions.

Life Cycle Assessment (LCA) is a comprehensive method used to evaluate the environmental impacts of a product or process from its inception toc disposal (from “cradle” to “grave”). In the context of waste paper recycling, LCA examines every stage, including raw material extraction, production, usage, and end-of-life treatment (2). By quantifying factors like energy consumption, greenhouse gas emissions, and waste generation, this assessment provide a holistic view of environmental impact. LCA shows that in the pulp and paper industry, production processes contribute to over half of the total carbon emissions (3) (Figure 1). Therefore, reducing emissions at this stage is critical. Using data from our experiments as well as relevant researches and databases, we calculated and compared the carbon emissions of enzymatic and chemical deinking methods in recycled paper production.

Figure 1 Distribution of carbon footprint in paper and pulp industry.

Our waste paper production process is divided into three parts: Pretreatment, Deinking, and Waste Treatment. The calculation boundaries include all the inputs and outputs in these three parts, including chemical reagent/enzymes production, electricity consumption, and post-deinking waste treatment. The calculation equations are as follows:

Where C_T is the total carbon emission (kg CO₂eq), C_P is the carbon emission (kg CO₂eq) in pretreatment process, C_D is the deinking carbon emission (kg CO₂eq) and C_W is the carbon emission (kg CO₂eq) in waste treatment. Carbon emission in each step (C_i) is calculated with input amount (I_ij) and its carbon emission factor (α_ij). Specific carbon emission factors are as follows:

Pretreatment

This part involves mixing waste paper with water to generate pulp, primarily consuming electrical energy. Based on our machine's power and the average carbon emission factor of the China's power grid (4), the CO₂ emissions were calculated and the result is 17.6 g CO₂eq/100 mL pulp.

Deinking

After pulp preparation, chemical reagents or enzymes are added to the pulp to separate ink from the pulp. Stirring was applied to facilitate better reaction. We calculated the carbon emission of chemical reagents' production (5), the respiration of enzyme-producing bacteria (6), and the corresponding electricity consumption. Results indicate that enzymatic deinking (22.6 g CO₂eq/100 mL pulp) reduces carbon emissions by 30.7% compared to chemical methods (32.6 g CO₂eq/100 mL pulp). The reduction is mainly due to the decreased chemical reagent usage and the high efficiency of enzymes.

Figure 2 Carbon emission in deinking process.

Waste Treatment

The deinking process generates large amount of wastewater (7). The waste treatment process deals with harmful substances in wastewater such as chemical oxygen demand (COD), biochemical oxygen demand (BOD), and suspended solids (SS). Enzymatic methods significantly reduce COD and BOD compared to chemical methods due to lower chemical reagent usage (8). Additionally, the alkaline condition of chemical methods leads to fiber breakage, reducing fiber recovery rate and increasing SS (9). These SS becomes deinking sludge during wastewater treatment, which, when landfilled or incinerated, also generates considerable amount of CO₂. Overall, wastewater treated with enzymatic methods results in 54.2% lower carbon emissions compared to chemical methods.

Figure 3 Post-deinking waste of chemical and enzymatic methods.

Conclusion

The enzymatic deinking method demonstrates a substantial reduction in carbon emissions (21.1%, compared to chemical methods) across all stages of recycled paper production.

Figure 4 Overall carbon emission and distribution.

In addition to carbon emission, we also calculated the economic costs of chemical and enzymatic methods. We assessed the costs of electricity and chemical reagents consumed throughout the entire production process. The results indicate that the use of enzymatic methods can reduce the production cost of recycled paper by 31.9%.

Figure 5 Economic cost of chemical and enzymatic methods

In conclusion, our enzymatic deinking method reduced both economic and emission cost, highlighting our method's potential as a more sustainable alternative to traditional chemical methods.

The Whole Life of Paper

When employing different deinking methods, the properties of recycled paper, such as brightness and fiber strength, exhibit notable variations (10). Our experimental results indicate that although both methods reduce the brightness of recycled paper compared to virgin pulp paper, the paper treated with the enzymatic method is approximately 5% brighter than that treated with the chemical method. Considering multiple recycling scenarios, the number of recycling cycles for paper can be described by the following equation:

Where β is the percentage of brightness reduction and e is the base of natural logarithm. For chemical deinking β is 0.27 and for enzymatic deinking β is 0.23. The calculation shows that recycling cycles for paper using chemical deinking is 3 and for enzymatic method the number is 4, leading to extra cost of raw material for chemical method.

References

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