Description

Rotating biological contactors (RBCs) aerobically treat aqueous waste streams, especially those containing alcohols, phenols, phthalates, cyanides, and ammonia. Primary treatment (e.g., clarifiers or screens) to remove materials that could settle in RBC tanks or plug the discs, is often essential for good operation. Influents containing high concentrations of floatables (e.g., grease) require treatment with a primary clarifier or an alternate removal system (EPA 1984; EPA 1992).

A typical RBC unit consists of 12-ft-dia plastic discs mounted along a 25-ft horizontal shaft. The disc surface is normally 100,000 sq ft for a standard unit and 150,000

sqft for a high density unit. Figure 11.17.2 details a typical RBC system.

As the discs rotate through leachate at 1.5 rpm, a mi-crobial slime forms on the discs. These microorganisms degrade the organic and nitrogenous contaminants present in the waste stream. During rotation, about 40% of the discs' surface area is in contact with the aqueous waste, while the remaining area is exposed to the atmosphere. The rotation of the media through the atmosphere causes oxygenation of the attached organisms. When operated properly, the shearing motion of the discs through the aqueous waste causes excess biomass to shear off at a steady rate. Suspended biological solids are carried through the successive stages before entering the secondary clarifier.

The RBC treatment process involves a number of steps as indicated in Figure 11.17.3. Typically, aqueous waste is transferred from a storage or equalization tank (1) to a mixing tank (2) where chemicals are added for metal precipitation, nutrient adjustment, and pH control. The waste stream then enters a clarifier (3) where solids are separated from the liquid. The clarifier effluent enters the RBC (4) where the organics and/or ammonia are converted to innocuous products. The treated waste is then pumped into a second clarifier (5) for removal of biological solids. After secondary clarification, the effluent enters a storage tank (6) where, depending upon remaining contamination, the waste may be stored pending additional treatment or discharged to a sewer system or surface stream. Throughout this process, offgases should be collected for treatment (7).

In addition to maximizing the system's efficiency, staging can improve the system's ability to handle shock loads by absorbing the impact in stages. Staging, which employs a number of RBCs in series, enhances biochemical kinetics and establishes selective biological cultures acclimated to successively decreasing organic loading. As the waste stream passes from stage to stage, progressively increasing levels of treatment occur.

Influent

Clarifier

Offgas Treatment

Offgas Treatment

Solids Sludge

Disposal Disposal

FIG. 11.17.2 Typical RBC plant schematic. (Reprinted, from U.S. Environmental Protection Agency (EPA), 1992, Engineering Bulletin: rotating biological contactors, [EPA 540-5-92-007, October].)

Solids Sludge

Disposal Disposal

FIG. 11.17.2 Typical RBC plant schematic. (Reprinted, from U.S. Environmental Protection Agency (EPA), 1992, Engineering Bulletin: rotating biological contactors, [EPA 540-5-92-007, October].)

Aqueous

Aqueous

FIG. 11.17.3 Block diagram of the RBC treatment process. (Reprinted, from U.S. EPA, 1992.)

Factors effecting the removal efficiency of RBC systems include the type and concentration of organics present, hydraulic residence time, rotational speed, media surface area exposed and submerged, and pre- and post-treatment activities. See Section 7.24 on the design of RBCs.

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