Physical Methods for Reducing Ammonia Nitrogen in River Water

Ammonia nitrogen pollution in rivers, originating from industrial discharge, agricultural runoff, and domestic sewage, poses a significant threat to aquatic ecosystems and human health. Elevated ammonia levels can lead to eutrophication, oxygen depletion, and toxicity to aquatic life. While biological and chemical treatments are common, physical methods offer effective, often simpler, alternatives for mitigation. This article outlines key physical approaches for reducing ammonia nitrogen in河道 water.

One prominent physical technique is air stripping, specifically using an ammonia stripping tower. This method leverages the principle that under alkaline conditions (pH 10.5–11.5), ammonium ions (NH₄⁺) convert to free ammonia (NH₃), which is a gas. The process involves pumping water into the top of a tower where it is distributed over a packing material to create a large surface area. Air is then blown upwards from the bottom of the tower. As the water trickles down, the free ammonia is volatilized into the air stream and carried away. The de-ammoniated water is collected at the bottom for further treatment if necessary. This method is highly efficient (70–95% removal) for high-concentration ammonia wastewater.

Aeration is a fundamental physical process that indirectly aids in ammonia reduction by enhancing dissolved oxygen (DO) levels. While often a precursor to biological nitrification, the physical mixing and oxygenation itself can facilitate the stripping of volatile ammonia. Devices like solar-powered water-layer exchange devices or solar-powered surface agitators/aerators utilize renewable energy to create water flow and promote vertical mixing (water layer exchange). This exchange brings oxygen-depleted, potentially ammonia-rich bottom water to the surface where gas exchange with the atmosphere can occur, and distributes oxygenated water downwards. Studies have shown such solar-powered devices can significantly increase dissolved oxygen and reduce ammonia nitrogen levels (e.g., by over 60% in trials). Micro-bubble aeration systems, installed near river sediments, are particularly effective for both oxygenating the water and helping to control the release of ammonia from sediments.

Flow manipulation through structures like weirs can be a simple yet effective physical strategy. By slowing down water flow, weirs increase the hydraulic retention time, allowing more time for natural physical processes like volatilization of ammonia to occur. The turbulent flow over the weir also enhances air-water contact, promoting natural aeration and increasing dissolved oxygen levels, which is beneficial for subsequent biological processing of ammonia.

Each physical method has its advantages and considerations. Air stripping is highly effective for high concentrations but may require pH adjustment and air pollution control for the stripped ammonia. Aeration and flow manipulation are generally more environmentally friendly and low-cost, especially when using solar energy, but their efficiency can be influenced by water temperature, weather conditions, and initial pollutant concentrations. Often, integrating these physical methods with other approaches can yield the most sustainable and efficient results for restoring river health.

In conclusion, physical methods such as air stripping, various aeration techniques, and flow manipulation provide valuable tools for mitigating ammonia nitrogen pollution in rivers. The choice of method depends on specific factors like ammonia concentration, flow conditions, energy availability, and economic considerations.