11.2 Rotating Biological Contactors & Hybrid Processes

11.2 Rotating Biological Contactors & Hybrid Processes

The rotating biological contactor (RBC) and the modern hybrid processes that followed all solve the same problem: get more biological treatment capacity into a given volume than suspended growth alone provides.

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RBC Construction and Operation

A series of large-diameter plastic media discs are mounted on a horizontal shaft and rotate slowly through a contour-bottomed tank of wastewater.

The 40 percent submergence figure is the design characteristic to remember. As each portion of the disc rotates:

1. It passes through the wastewater, where the biofilm adsorbs organic matter.
2. It emerges into the air, where the thin water film on the biofilm rapidly absorbs atmospheric oxygen.

The rotation is the aeration system — there are no blowers or diffusers in a conventional RBC, which is why energy consumption is low. Rotation also provides mixing and shears excess biomass off the discs.

Drives are either mechanical (motor and gear reducer) or air drive (cups on the disc periphery filled by diffused air, turning the shaft by buoyancy). Air drive allows speed adjustment and can be used to strip excess biofilm by temporarily increasing speed.

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Staging

RBCs are almost always arranged in stages in series, separated by baffles.

Staging works because each stage sees a different substrate concentration and develops the organism population suited to it. Nitrification occurs in the later stages only after carbonaceous BOD has been largely removed.

Removable baffles let the operator re-stage the unit, which is the primary tool for rebalancing a system where the first stage is overloaded.

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Loading

$\text{Hydraulic Loading (gpd/ft}^2\text{)} = \frac{\text{Flow (gpd)}}{\text{Media Surface Area (ft}^2\text{)}}$

$\text{Organic Loading (lb BOD/day/1,000 ft}^2\text{)} = \frac{\text{BOD (mg/L)} \times \text{Flow (MGD)} \times 8.34}{\text{Media Area (ft}^2\text{)} \div 1,000}$

Soluble BOD is the more meaningful loading basis for an RBC, because the biofilm responds to dissolved substrate.

Worked example. A first-stage shaft carries 100,000 ft² of media and receives 0.30 MGD at 110 mg/L soluble BOD.

$\text{BOD load} = 110 \times 0.30 \times 8.34 = 275\text{ lb/day}$ $\text{Organic loading} = \frac{275}{100} = 2.75\text{ lb SBOD/day/1,000 ft}^2$

Design guidance commonly limits first-stage soluble BOD loading to roughly 4 lb per day per 1,000 ft² to avoid overload, so this unit is within range.

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RBC Operating Problems

[!WARNING] Shaft overloading is a failure mode unique to RBCs and it is genuinely structural. A saturated biofilm on a fully loaded shaft weighs many tons, and shaft fatigue failures have occurred. This is why controlling biofilm thickness is not merely a process concern. White Beggiatoa growth on the first stage is the standard visual warning that the stage has gone anaerobic under excessive load.

RBCs must be covered — sunlight degrades the plastic media and promotes algae growth, and covers also contain odors and retain heat.

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Hybrid Attached-Growth Processes

Integrated Fixed-Film Activated Sludge (IFAS)

Carrier media is added into an existing activated sludge aeration basin, so treatment occurs on both suspended biomass and attached biomass simultaneously.

• Increases treatment capacity without enlarging the tank — the principal reason utilities choose it
• Adds nitrification capacity in particular, because nitrifiers attach and are retained regardless of the suspended solids sludge age
• Media may be fixed (rope, web, or sheet) or free-floating carriers retained by screens
• Total biomass rises without a corresponding rise in mixed liquor suspended solids, so clarifier solids loading does not increase proportionally

Moving Bed Biofilm Reactor (MBBR)

Free-floating plastic carriers are kept in suspension by aeration in an aerobic reactor or by a mixer in an anoxic one.

• No return activated sludge — biomass is retained on the carriers, so there is no sludge recycle and no sludge-age control in the conventional sense
• Effluent screens are mandatory to retain the carriers in the reactor
• Carrier fill fraction is typically 25 to 67 percent of reactor volume; too high and the carriers will not circulate
• Very compact, resistant to shock loads, and simple to operate
• Downstream solids separation is still required, by clarification, dissolved air flotation, or filtration

Comparing the Options