Lower Excessively High Oxidation-Reduction Potential in Water

Oxidation-reduction potential (ORP) is a critical indicator of the redox environment in water bodies. Excessively high ORP values indicate a strongly oxidizing condition, which can inhibit the activity of beneficial microorganisms, signal excessive dissolved oxygen levels, or suggest the accumulation of oxidative by-products. 

While chemical reduction methods exist, biological approaches offer a greener, more sustainable pathway for ORP regulation. This paper reviews the principal biological strategies for lowering high ORP in aquatic systems.

1. Enhancing Denitrification Processes

Denitrification is one of the most effective biological pathways for reducing ORP. Under anoxic conditions, denitrifying bacteria use nitrate as an electron acceptor and organic matter as an electron donor, sequentially reducing nitrate to nitrite, nitric oxide, nitrous oxide, and finally to nitrogen gas. This reduction reaction involves substantial electron transfer and consumption, significantly lowering the ORP of the water body.

Research indicates that incomplete denitrification typically maintains ORP above -200 mV, whereas complete denitrification requires ORP to drop below -150 mV, and can reach as low as -300 mV. In practical engineering applications, supplementing carbon sources—such as methanol, sodium acetate, or glucose—enhances the activity of denitrifying bacteria, promoting their consumption of dissolved oxygen and oxidizing substances, thereby driving ORP downward. 

In recirculating aquaculture systems, ORP-monitored denitrification bioreactors have been successfully developed to automatically control carbon source dosing, maintaining nitrate concentrations below 5 ppm while effectively preventing the formation of toxic by-products such as nitrite and hydrogen sulfide.

2. Utilizing Photosynthetic Bacteria

Photosynthetic bacteria, such as Rhodopseudomonas capsulatus, possess unique advantages in lowering water ORP due to their combined photosynthetic and heterotrophic metabolic capabilities. During denitrifying metabolism, R. capsulatus rapidly consumes dissolved oxygen, causing a significant decrease in ORP and a concurrent rise in pH. 

When introduced into water bodies in either free or immobilized forms, their metabolic activity systematically shifts the redox balance, gradually reducing ORP from elevated levels. Immobilized photosynthetic bacterial bead technologies have demonstrated promising nitrogen removal and ORP regulation effects at the laboratory scale.

3. Applying Biodegradable Organic Matter

Adding biodegradable organic matter to water bodies represents another practical biological approach to ORP reduction. Natural biomass materials such as rice bran, straw leachate, and molasses, when introduced into water, are decomposed by indigenous microorganisms. 

During aerobic degradation, microorganisms consume substantial amounts of dissolved oxygen, transitioning the system from aerobic to anoxic and eventually anaerobic conditions. As dissolved oxygen is depleted, microorganisms shift to fermentation or anaerobic respiration, producing abundant reducing metabolites—including organic acids and alcohols—that drive ORP progressively lower. 

Patent literature documents that rice bran addition in activated sludge systems can reduce ORP by approximately 10 to 200 mV. This method is operationally simple, cost-effective, and particularly suitable for ORP regulation in open water bodies such as landscape ponds and aquaculture systems.