The town of Eerbeek in The Netherlands is the location of one of the first THIOPAQ ® units for the biological treatment of sulfide. Installed at Industry water Eerbeek BV, the THIOPAQ unit has been successfully converting sulfide in biogas from paper industry waste-water since 1993 (Figure 1). The THIOPAQ process for the biological conversion of sulfide in caustic streams was developed by Pacques BV, an environmental biotechnology company with headquarters in The Netherlands. Now, UOP, in Des Plaines, Illinois, has entered into a partnership agreement with Pacques that allows UOP to be the exclusive licensor of this process in the refining and petrochemical industries. As licensor, UOP will apply THIOPAQ technology wherever appropriate for the treatment of sulfide-containing streams.


Inside the THIOPAQ reactor, a fixed bed of naturally occurring, specially selected living Thiobacillus microorganisms continuously oxidizes sulfide to sulfur or to sulfate according to these reactions:

  • Equation (1) (Available in full paper)

  • Equation (1) (Available in full paper)

These reactions result in the virtual elimination of three of the most troublesome problems associated with sulfidic caustic:

  • Sulfide, a toxic component, is completely eliminated.

  • The complete conversion of sulfide to sulfate results in a significant reduction, up to 90%, in effluent biological oxygen demand (BOD) and chemical oxygen demand (COD)

  • The production of sulfate decreases the pH of the effluent stream to approximately 8.


The heart of the THIOPAQ process is its proprietary aerobic bioreactor. Here, the microorganisms are attached to a unique support media which ensures that they remain in the reactor. The microorganisms are arranged on the support medium to provide excellent contact with the sulfidic feed stream. The reactor contains only microorganisms that have been selected for maximum sulfide-conversion activity. The reactor's design allows high feed throughput and sulfide conversion. The design allows the reactor volume and plot size to be small compared to what would be required if conventional treating ponds were used.

Because the THIOPAQ process uses a living organism to facilitate the sulfide conversion, the process is capable of adapting itself to changes in feed stream composition. The microorganisms reproduce swiftly to maintain a stable colony. Additionally, the microorganisms exhibit resistance to potentially toxic hydrocarbons, such as benzene, toluene, and xylene. The adaptability of the system is a key feature and is responsible for some of the advantages of the THIOPAQ approach to sulfidic caustic treatment.

The Thiobacillus bacteria are chemo-autotrophic bacteria, which obtain their energy from the oxidation of sulfur compounds and use the carbon dioxide in the air as a carbon source. The microorganisms need a small supplemental nutrient supply to maintain optimal growth. The nutrients are nitrogen, phosphorus, and an inexpensive mix of other micronutrients.

The THIOPAQ process flow is shown in Figure 2. The sulfidic caustic is brought into a mix tank (typical retention time is about one hour). Here, the caustic is mixed with the nutrients and a small amount of acid.

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