Chromium-containing wastewater treatment encompasses multiple technical approaches, among which chemical reduction and physical adsorption are recognized as relatively practical solutions compared to alternative methods. Additionally, reverse osmosis and other technologies are employed by some enterprises. This paper, in collaboration with a manufacturer of Online Water Quality Monitoring Instruments, provides a concise overview of partial chromium treatment methods for wastewater, focusing on the following key aspects:

1. Hexavalent Chromium Reduction and Trivalent Chromium Precipitation

The primary step involves reducing highly toxic hexavalent chromium (Cr⁶⁺) to the less toxic trivalent chromium (Cr³⁺), followed by precipitation for removal. The core criterion for this process is the completeness of reduction achieved by the selected reducing agent.

Currently, sodium metabisulfite is widely utilized as a reducing agent for Cr⁶⁺. Sodium sulfite, which exists in various forms, is generally alkaline; however, the reduction of Cr⁶⁺ requires acidic conditions. Under such conditions, sodium sulfite undergoes significant decomposition, leading to air pollution. Sodium pyrosulfite, an acidic alternative, has been proposed to address this issue, but its application is limited by the large volume of sludge generated—a factor that deters many enterprises. Ferrous sulfate is another option: while it also produces waste sludge, it offers distinct advantages, including thorough reduction, low salt content in treated effluent, and minimal wastewater discharge. Enterprises should select an appropriate method based on their specific operational conditions.

2. Treatment of Coexisting Pollutants  

Subsequent treatment targets other metal cations, anions, and certain organic compounds in the wastewater. Notably, surfactant-based organic compounds from rinsing water do not require removal, as they extend the lifespan of active oxygen; sodium and chloride ions are also exempt from treatment. Furthermore, sulfuric acid or phosphoric acid is recommended for rust removal and activation processes, with hydrochloric acid avoided whenever possible—this is because sulfate and phosphate ions can be effectively removed via precipitation.

3. Neutralization, Precipitation, Flocculation, and Filtration-Adsorption  

Following neutralization-precipitation and flocculation-sedimentation, the wastewater proceeds to filtration and adsorption units. This sequence ensures compliance with the technical requirements stipulated by relevant regulatory authorities.

Note on Real-Time Monitoring  

Throughout the entire wastewater treatment process, the installation of an Online Total Chromium Analyzer is imperative to enable real-time monitoring of chromium removal efficiency and residual chromium levels.