Biological Solutions for Chloride Excess in Outdoor Rivers

Chloride contamination in outdoor rivers often comes from industrial discharge, de-icing salt runoff, reclaimed water recharge, and the release of salts from bottom sediments. Unlike enclosed water bodies, rivers are open, dynamic, and subject to fluctuating flow, which makes dilution or chemical precipitation alone difficult to sustain. Biological methods—using salt-tolerant plants, microorganisms, and their combined systems—offer an environmentally friendly and cost-effective way to remove or immobilize chloride in situ.

Phytoremediation

Plant-based remediation is the foundation of biological chloride control. Salt-tolerant plants take up chloride through their roots and store it in their tissues. When the plants are harvested, the chloride is removed from the water body. Species such as reeds, cattails, sedges, iris, and canna have shown useful tolerance and uptake capacity. Floating treatment beds and constructed wetlands can be installed along riverbanks or in side channels, allowing river water to pass through the root zone.

The main limitations are biological and hydraulic. Plant uptake depends on growth rate and biomass, and removal is slow compared with the volume of flowing water. Rivers with high flow velocity may require weirs or bypass wetlands to extend contact time. Seasonal changes also matter: in cold seasons, plant metabolism slows, and removal efficiency drops. Regular harvesting is essential, because dead plant material can release chloride back into the water.

Microbial Remediation

Microorganisms provide another pathway. Certain halotolerant fungi and bacteria can adsorb chloride onto cell surfaces, accumulate it inside cells, or transform it through enzymatic processes. Fungi isolated from mangrove wetlands, for example, have demonstrated the ability to remove chloride from saline wastewater. Halotolerant bacteria such as Bacillus amyloliquefaciens can improve nutrient removal under salt stress and may also enhance the salt tolerance of plants when applied together.

Microbial treatment is attractive because it can be applied directly to river water or to biofilms on carrier media. However, in open rivers, maintaining a stable microbial community is difficult. Temperature, pH, dissolved oxygen, and hydraulic disturbance all affect performance. In winter, microbial activity declines sharply, and repeated inoculation may be needed to sustain results.

Combined Plant–Microbial Systems

The most promising approach is the integration of plants and microorganisms. Plants provide attachment surfaces and root exudates that support microbial growth. In return, microbes secrete extracellular polymers, produce plant hormones, and enhance antioxidant enzyme activity, helping plants tolerate salt stress. Combined systems—such as ecological ditches, constructed wetlands, and plant-based microbial fuel cells—have shown better chloride and nutrient removal than either plants or microbes alone.

These systems can be built alongside the river channel, using the natural flow to drive treatment. They also provide landscape and habitat benefits. The key design considerations are hydraulic retention time, salt concentration, and seasonal temperature. In cold periods, supplemental aeration or additional treatment may be necessary.