Key Takeaways & Executive Findings
- •• • Optimal preparation conditions: PVA 8.64 g, SA 2.41 g, rice husk 3.82 g, corncob 4.47 g, reed straw 6.06 g, freezing time 19.11 h, crosslinking time 12 h, yielding a 7-day cumulative carbon release of 43.38 mg·g−1, which ensures stable long-term carbon supply for denitrification. • • The carbon release kinetics fit first-order, Higuchi, Ritger-Peppas, and Weibull models, confirming a dual-mechanism control (diffusion and skeleton erosion) that prevents rapid carbon depletion and supports sustained denitrification. • • Under static conditions, the composite achieved a maximum NO3−-N removal rate of 90.8% at an influent concentration of 50 mg·L−1, with a removal rate of 0.079 mg·(g·h)−1, demonstrating high efficiency in batch reactors. • • Under dynamic conditions with HRT = 3 h, the average NO3−-N removal rate remained at 88.1%, indicating robust performance in continuous-flow systems and suitability for practical wastewater treatment.
Abstract
To address the issues of insufficient carbon sources and low denitrification efficiency in rural domestic wastewater, this study developed and optimized a composite slow-release carbon source using corncob, rice husk, reed straw, polyvinyl alcohol (PVA), and sodium alginate (SA). The preparation conditions and raw material ratios were systematically optimized using Plackett-Burman (PB) design, response surface methodology (Box-Behnken design, BBD), and mixture-optimal design (MOD). The denitrification performance was evaluated through carbon release characteristics and denitrification experiments. The optimal preparation conditions were determined as PVA 8.64 g, SA 2.41 g, rice husk 3.82 g, corncob 4.47 g, reed straw 6.06 g, freezing time 19.11 h, and crosslinking time 12 h. The 7-day cumulative carbon release was (43.38 ± 1.3) mg·(g·h)−1. The release process followed first-order kinetics, Higuchi, Ritger-Peppas, and Weibull models, indicating that carbon release is controlled by multiple mechanisms including diffusion and skeleton erosion, ensuring stable slow-release characteristics. In denitrification experiments with influent NO3−-N concentration of 50 mg·L−1, the composite carbon source (RCR-PVA-SA) achieved a maximum NO3−-N removal rate of 90.8% after 10 days of operation, with a removal rate of 0.079 mg·(g·h)−1. Under dynamic conditions with hydraulic retention time (HRT) of 3 h, the average removal rate remained at 87.9%, demonstrating efficient and stable denitrification performance under both static and dynamic conditions. This research provides a reference for the preparation of natural slow-release carbon sources and the resource utilization of agricultural waste.
1. Introduction
Rural domestic wastewater in China frequently suffers from a low carbon-to-nitrogen ratio (C/N < 4), which severely limits biological denitrification due to insufficient electron donors. Conventional liquid carbon sources, such as sodium acetate, offer rapid reaction kinetics but suffer from high costs (up to 4,931 CNY per ton) and require precise dosing control, making them economically and operationally impractical for decentralized rural treatment facilities. Solid carbon sources, including synthetic polymers like polycaprolactone (PCL) and polylactic acid (PLA), provide a more controlled release but are expensive and exhibit poor microbial compatibility. Natural cellulosic materials like corncob, rice husk, and reed straw are abundant and inexpensive, yet their uncontrolled carbon release and susceptibility to hydraulic washout lead to unstable long-term denitrification performance.
To overcome these limitations, this study integrates natural cellulosic materials into a polyvinyl alcohol (PVA) and sodium alginate (SA) hydrogel matrix, creating a composite slow-release carbon source (RCR-PVA-SA). The PVA/SA network provides mechanical strength and structural stability, while the embedded natural materials serve as biodegradable carbon reservoirs. By systematically optimizing the preparation parameters using Plackett-Burman design, response surface methodology, and mixture-optimal design, the resulting composite achieves a balanced carbon release profile and high denitrification efficiency. This approach not only addresses the technical bottleneck of unstable carbon supply but also offers a sustainable strategy for agricultural waste valorization, aligning with circular economy principles.
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CHEN Xiaonan, GU Jiayan, HE Guofu, JU, GUO Linrui, ZHANG Ruirui (2026). Optimization of Preparation Conditions for a Novel Composite Slow-Release Carbon Source and Its Denitrification Performance. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202508036
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Frequently Asked Questions
What are the failure mechanisms of the composite carbon source under prolonged operation or hydraulic stress?
The composite carbon source is designed to release carbon via diffusion and skeleton erosion, as confirmed by kinetic modeling. Under hydraulic stress, the PVA/SA hydrogel matrix provides structural integrity, preventing rapid disintegration. However, over extended operation, gradual erosion of the natural cellulose components may reduce mechanical strength, potentially leading to increased carbon release or physical breakdown. The study did not report long-term stability beyond 10 days, so further investigation is needed to assess performance over months.
How does the cost of this composite carbon source compare with conventional liquid carbon sources like sodium acetate?
The raw materials (corncob, rice husk, reed straw) are agricultural by-products, which are inexpensive and locally available. PVA and SA are relatively low-cost polymers. The study does not provide a detailed cost analysis, but given the low cost of natural materials and the reduced need for continuous dosing compared to liquid carbon sources, the composite is likely to be more cost-effective. A full economic assessment is recommended to quantify savings.
What is the scalability potential of the preparation process for industrial production?
The preparation involves simple steps: mixing, heating, molding, freezing, and crosslinking. These are amenable to scale-up using standard industrial equipment. The optimal conditions (e.g., freezing time 19.11 h, crosslinking time 12 h) are feasible for batch production. However, the use of a mold and manual steps may limit throughput; automation and continuous processing could be developed to enhance scalability.
How does the composite perform under varying influent nitrate concentrations and temperatures?
The study tested a fixed influent NO3−-N concentration of 50 mg·L−1 and did not vary temperature. The denitrification efficiency was high (90.8% static, 88.1% dynamic at HRT 3 h), but performance under lower temperatures or higher nitrate loads is unknown. Future work should evaluate robustness across seasonal temperature variations and shock loads.
What is the environmental impact of the composite, particularly regarding secondary pollution from nitrogen or color release?
The study selected rice husk for its low nitrogen and color release, but quantitative data on nitrogen leaching or color generation were not provided. The PVA/SA matrix may also degrade over time, potentially releasing synthetic polymers. A comprehensive life-cycle assessment and leachate analysis are necessary to ensure no secondary pollution occurs.
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