Key Takeaways & Executive Findings
- •• • Sediment elutriation achieved average reductions of 5.11% for organic matter, 10.19% for total nitrogen (TN), and 8.71% for total phosphorus (TP) in the top 0–30 cm sediment layer, directly lowering the internal nutrient load that fuels eutrophication and black-odor events. • • Water quality improved concurrently: chemical oxygen demand (COD) and ammonia nitrogen (NH4+-N) removal rates averaged 35.67% and 22.65%, while transparency, dissolved oxygen (DO), and oxidation-reduction potential (ORP) increased by 102.46%, 11.07%, and 15.66%, respectively—critical for restoring aerobic conditions and suppressing anaerobic decomposition. • • The treatment shifted sediment particle size distribution: clay content decreased by 8.87%, sand content increased by 12.37%, and D50 and D90 increased by 32.39% and 159.97%, forming a stable coarse-grained layer that physically prevents sediment resuspension and pollutant release. • • Mechanical disturbance and particle size redistribution enhanced oxygen transfer at the sediment-water interface, increasing the abundance of facultative anaerobic phyla (e.g., Chloroflexi, Spirochaetes), which suppressed anaerobic fermentation and the generation of odorous gases (H2S, NH3), thereby reducing malodor and sediment upwelling.
Abstract
To support the construction of an ecologically clean small watershed in Harbin, a pilot-scale trial of sediment elutriation was conducted in the Hejia Ditch to evaluate its effectiveness in controlling endogenous pollution and to elucidate the underlying mechanisms. After treatment, sediment organic matter, total nitrogen (TN), and total phosphorus (TP) decreased by 5.11%, 10.19%, and 8.71%, respectively. Water transparency, dissolved oxygen (DO), and oxidation-reduction potential (ORP) increased by 102.46%, 11.07%, and 15.66%, while chemical oxygen demand (COD) and ammonia nitrogen (NH4+-N) removal rates reached 35.67% and 22.65%. The technology effectively removed surface suspended sediment, leaving a stable layer of coarse inorganic particles that formed a clear mud-water interface. Post-treatment, clay content decreased by 8.87%, sand content increased by 12.37%, and median (D50) and 90th percentile (D90) particle sizes increased by 32.39% and 159.97%, respectively. Mechanical disturbance and particle size redistribution enhanced oxygen transfer at the interface, increasing the abundance of facultative anaerobic phyla such as Chloroflexi and Spirochaetes, thereby suppressing the generation of odorous gases (H2S, NH3) and preventing sediment resuspension. Increased microbial diversity and richness improved ecosystem stability and self-purification capacity. These results demonstrate that sediment elutriation is an effective method for controlling endogenous pollution in Hejia Ditch, providing a scientific basis for ecological restoration and long-term management.
1. Introduction
Endogenous pollution from contaminated sediments remains a critical bottleneck in the remediation of urban small watersheds, particularly in black-odor water bodies. Conventional approaches such as dredging are costly, disruptive, and risk secondary contamination, while in-situ methods like aeration or chemical dosing often fail to address the root cause: the unstable, nutrient-rich surface sediment layer that readily resuspends and releases pollutants. The Hejia Ditch in Harbin, a tributary of the Songhua River, exemplifies this challenge, suffering from chronic internal loading that triggers seasonal black-odor episodes, threatening downstream water quality and public health.
This pilot trial introduces sediment elutriation as a targeted in-situ technology that mechanically separates and removes the fine, pollutant-laden surface sediment while leaving a stable coarse inorganic layer. By directly altering the physical and biological properties of the sediment-water interface, the method not only reduces immediate pollutant release but also fosters conditions for ecological recovery. The study provides quantitative evidence of its efficacy and elucidates the mechanisms—particle size redistribution and microbial community shifts—that underpin its long-term benefits, offering a scalable solution for similar urban waterways.
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WANG Huazhe, MENG Zhaohui, WANG Miao, GUO Wanqian (2026). Pilot Trial on Endogenous Pollution Control in Hejia Ditch, Harbin: Efficacy and Mechanisms of Sediment Elutriation. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202510056
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Frequently Asked Questions
What are the long-term stability and potential recontamination risks of the treated sediment layer under varying hydrodynamic conditions?
The pilot trial demonstrated that the treated sediment layer is more stable due to increased sand content (up by 12.37%) and larger particle sizes (D50 increased by 32.39%, D90 by 159.97%). This coarser matrix is less prone to resuspension under typical flow conditions. However, extreme flood events could still disturb the layer. Long-term monitoring is recommended to assess resilience, but the observed increase in microbial diversity and suppression of anaerobic pathways suggests enhanced self-purification capacity, which may mitigate recontamination.
How does the cost of sediment elutriation compare to conventional dredging or capping methods for similar scale projects?
The paper does not provide a direct cost comparison. However, sediment elutriation is an in-situ technology that avoids the high costs of dredging, transport, and disposal of contaminated sediments. It also reduces the need for external capping materials. The pilot covered 59,830 m² with mechanical elutriation on 55,812 m² and manual assistance on 4,018 m², indicating scalability. A full cost-benefit analysis would require site-specific data, but the reduction in secondary pollution and ecological recovery potential likely offer long-term economic advantages.
What are the operational parameters (e.g., treatment depth, residence time, energy input) that influence the removal efficiency of sediment elutriation?
The study treated the top 0–30 cm of sediment. The mechanical device generates controlled turbulence to selectively remove fine particles. Specific operational parameters such as rotation speed, forward velocity, and treatment duration are not detailed in the provided text, but the observed removal rates (organic matter 5.11%, TN 10.19%, TP 8.71%) indicate effective separation. Optimization would depend on sediment characteristics and desired outcomes, and further studies should correlate operational variables with performance.
How does the change in microbial community composition affect the long-term biogeochemical cycling of nutrients and pollutants?
The increase in facultative anaerobic phyla (e.g., Chloroflexi, Spirochaetes) suggests a shift from obligate anaerobic pathways that produce H2S and NH3 to more metabolically versatile processes. This reduces malodorous gas generation and may enhance aerobic degradation of organic matter. Increased microbial diversity and richness improve ecosystem resilience and self-purification, potentially leading to sustained nutrient cycling and reduced internal loading over time. However, the specific functional implications require metagenomic or metatranscriptomic analysis.
What are the scalability bottlenecks when applying this technology to larger river sections or different sediment types?
Scalability depends on sediment composition, water depth, flow velocity, and accessibility. The pilot covered 1.3 km with 8 treatment zones, demonstrating feasibility in a real urban setting. For larger areas, equipment deployment and operational efficiency become critical. Sediments with high clay content may require more intensive treatment, while sandy sediments might be less responsive. The technology's effectiveness on varied sediment types needs validation. Additionally, the disposal of the removed fine sediment slurry must be managed to avoid secondary pollution.
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