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Pilot-Scale Testing of a Continuously Operated Catalytic Oxidation System

Date Published: December 12, 2025

Test Method and Process

The treatment units used in the pilot-scale catalytic oxidation system are shown in Figure 1. Wastewater from the ozone holding tank was introduced into the holding tank of the pilot system. Chloride ions (Cl^- in the wastewater were converted to hypochlorous acid through electrolysis. The wastewater was then conveyed to the catalytic oxidation unit, where high-molecular-weight organic compounds underwent catalytic oxidation. The treated wastewater was subsequently returned to the biological treatment unit of the existing system to reduce the operating load on that unit.

Pilot-scale testing began in 2024. After stable operation of each treatment unit was achieved, water quality analyses were to be conducted and the optimal operating parameters confirmed. The target was a chemical oxygen demand (COD) removal efficiency greater than 80%, in compliance with the applicable industrial park effluent standard.

Field verification of the catalytic denitrification process was conducted at a stainless steel plant. The nitrate-nitrogen concentration in the effluent from the plant's chemical treatment process was as high as approximately 350 mg/L. After treatment in the denitrifying bioball module at a hydraulic retention time of four days, the nitrate-nitrogen concentration met the regulatory limit. When the retention time was shortened to two days, the nitrate-nitrogen concentration decreased to 50-150 mg/L. A downstream reducing agent/activated carbon (AC) process or reducing agent/catalyst (DNC+) process was therefore required to achieve either a removal efficiency of 50% or a final concentration below 50 mg/L.

As shown in Figure 2, effluent from the denitrifying bioball module was first directed to a holding tank for water quality adjustment and then subjected to simple filtration. Continuous-flow tests were conducted at a hydraulic retention time (HRT) of 0.5 hour. Different reducing-agent dosages were evaluated to determine whether the treated nitrate-nitrogen concentration could be reduced to below 50 mg/L and whether a removal efficiency greater than 50% could be achieved.

Test Results

As shown in the table below, the influent, consisting of effluent from the denitrifying bioball module, had a pH of 7-9. Following continuous-flow treatment at an HRT of 0.5 hour, the effluent pH remained between 7 and 8. The nitrate-nitrogen results are summarized below.

1. At influent nitrate-nitrogen concentrations of 100-200 mg/L, dosage tests were conducted at NO_3^-N:reducing agent mass ratios of 1:10 and 1:8. The treated nitrate-nitrogen concentrations were below 30 mg/L and 30-50 mg/L, respectively. These results indicate that a ratio of 1:8 was sufficient to maintain the effluent NO_3^-N concentration below 50 mg/L.

2. At influent nitrate-nitrogen concentrations of 200-300 mg/L, dosage tests were conducted at NO_3^-N:reducing agent mass ratios of 1:10 and 1:8. The treated nitrate-nitrogen concentrations were below 40 mg/L and 40-70 mg/L, respectively. These results indicate that a ratio of 1:10 was required to maintain the effluent NO_3^-N concentration below 50 mg/L.

In summary, when the influent NO_3^-N concentration was 100-200 mg/L, a mass ratio of NO_3^-N:reducing agent = 1:8 was sufficient to maintain the effluent concentration below 50 mg/L. When the influent NO_3^-N concentration was 200-300 mg/L, a ratio of 1:10 was required. Reducing NO_3^-N discharge at the process source and maintaining the treatment performance of the biological system would therefore help minimize downstream reducing-agent consumption.