OL Yvr 100 mg BODg VSS x g VSSL

Design Example 4. A municipal conventional activated sludge treatment plant is planning to receive the potato processing wastewater given in design example 2, without pretreatment (in an aerobic lagoon). Determine what changes need to be made in the processing conditions of the plant to avoid filamentous bulking. Assume: T=20°C, d=0.55, 6=0.15/ day, X=0.6, Nb=1.5 lb O2/(hp.hour).

For the potato processing wastewater (example 2): BOD concentration=2400 mg/L, Flow=1150 gal/tonx150 t/day=172,500 gal/day or=4.35 m3/tonx150 t/day=652.5 m3/day.

Solution: The municipal activated sludge treatment plant before potato processing discharge has the following characteristics: Qbef=2.5 MG/day (9450 m3/day), Staf.= 300 mg/L, Se=10 mg/L, Srb=300-10=290 mg/L, tb=6 hours=0.25 day, Xvb= 3000 mg/L, (F/M)=0.3/day.

The dissolved oxygen required can be taken from reference (International water pollution control, Figs. 6-15): DOb=1.7 mg/L. The oxygen needed can be calculated by equation:

ORM=(a'SI,b+b'XXv,btb)Qb = (0.55x290+0.15x0.60x3000x0.25)mg/L x20.5 MGDx8.34(lb/MG)/(mg/L) =2733 lb/day (1241 kg/day) =113.9 lb/hour (51.71 kg/hour)

The power requirement is:

After the potato industry discharge in the municipal activated sludge plant, the following will apply. Assume for the MLVSS, the value Xv,a=4000 mg/L.

The BOD removed will be: Sr,a=435.5-10=425.5 mg/L

The new retention time will be:

The new F/M ratio can be computed using theequation:

From the reference mentioned above, the dissolved oxygen required is: DOa=3.6 mg/L. Assuming the same values for a, b' and X, the oxygen required can be computed: OR,a=(0.55x425.5+0.15x0.60x0.234)mg/L x2.6725 MGDx8.34(lb/MG)/(mg/L) =7093.7 lb/day (3220.5 kg/day) =295.6 lb/hour (134.2 kg/hour)

The oxygen saturation at 20°C is: Cs=9.2 mg/L. The new Na:

The oxygen saturation at 20°C is: Cs=9.2 mg/L. The new Na:

The power required is:

295.6 lb/hour L12!b/(hpJiouO

The additional power required is:

HPadd=HPa-HPb=264-76=188 HP (140 kW)

Remark: To avoid the filamentous bulking in the conventional activated sludge plant, the following modifications are needed:

• increasing the MLVSS from 3000 to 4000 mg/L;

• increasing the power required from 76 HP (57 kW) to 264 HP (197 kW), in addition to the necessity to control the bulking.

Rotating Biological Contactors. The rotating biological contactor (RBC) is an aerobic fixed-film biological treatment process. Media in the form of large, flat discs mounted on a horizontal shaft are rotated through specially contoured tanks in which wastewater flows on a continuous basis. The media consist of plastic sheets ranging from 2 to 4 m in diameter and up to 10 mm thick. Spacing between the flat discs is approximately 30-40 mm. Each shaft, full of medium, along with its tanks and rotating device, forms a reactor module. Several modules may be arranged in parallel and/or in series to meet the flow and treatment requirements (Fig. 14). The contactor or disc is slowly rotated by power supplied to the shaft, with about 40% of the surface area submerged in wastewater in the reactor.

A layer of 1-4 mm of slime biomass is developed on the media (equivalent to 250010,000 mg/L in a mixed system) [24], according to the wastewater strength and the rotational speed of the disc. The discs, which develop a slime layer over the entire wetted surface, rotate through the wastewater and contact the biomass with the organic matter in the waste stream and then with the atmosphere for absorption of oxygen. Excess biomass on the media is stripped off by rotational shear forces, and the stripped solids are held in suspension with the wastewater by the mixing action of the discs. The sloughed solids (excess biomass) are carried with the effluent to a clarifier, where they are settled and separated from the treated wastewater.

The RBC system is a relatively new process for wastewater treatment; thus full-scale applications are not widespread. This process appears to be well suited to both the treatment of industrial and municipal wastewater. In the treatment of industrial wastewaters with high BOD levels or low reactivity, more than four stages may be desirable. For high-strength wastewaters, the first stage can be enlarged to maintain aerobic conditions. An intermediate clarifier may be

Figure 14 Rotating biological contactor system. (a) Flow-sheet of typical staged rotating biological contactors (RBCs). (b) Schematic diagram of the RBCs.

employed where high solids are generated to avoid anaerobic conditions in the contactor basins. Currently used media consist of high-density polyethylene with a specific surface of 37 ft2/ft3 (121 m2/m3). One module or unit, 17 ft (3.7 m) in diameter by 25 ft (7.6 m) long, contains approximately 10,000 m2 of surface area for biofilm growth. This large amount of biomass permits a short contact time, maintains a stable system under variable loading, and should produce an effluent meeting secondary-treatment limits or standards.

Recirculating effluent through the reactor is not necessary. The sloughed solids (biomass) are relatively dense and settle well in the secondary clarifier. Low power requirement and simple operating procedure are additional advantages. A 40-kW motor is sufficient to turn the 3.7x7.6 m unit previously described [43]. Therefore, it can be clearly realized that the RBC can be applied successfully for treatment of potato processing effluents, in particular for values of BOD5 and COD concentrations not exceeding, in the main, 5000 to 6000 mg/L in the wastewater stream. Depending on these properties, the data taken from case studies for treating contaminated wastewater with BOD5 and COD concentrations close to those found in wastewater from potato processing, can be of much benefit. These data are based on the experience published by USEPA [44]. Table 8 summarizes the experience represented in design criteria and performance of the applied RBC for treating landfill leachate, which can be successfully applied to the potato processing industry within the range of pollutant concentrations mentioned above. However, an optimum design can be achieved by a pilot-plant study of the RBC.

Design Example 5. Design a rotating biological contactor (RBC). Determine the surface area required for an RBC system to treat preclarified potato processing wastewater with a flow of 150,000 gal/day (567 m3/day) and BOD concentration of 4000 mg/L, with a maximum system effluent of 20 mg BOD/L. Minimum temperature is expected to be 32°C (90°F). The selected plastic medium is manufactured in 8 m shaft lengths, with each shaft containing 1.2x104 m3 of surface area.

Solution: RBC performance:

No temperature correction in loading is needed, because the wastewater temperature is >55°F (13°C). Based on the hydraulic surface loading, the selected design value of Table 8 is: Hydraulic loading rate =1.2 gal/ft2-day (49 L/m2-day).

Table 8 Design Criteria and Performance of

Rotating Biological Contactors [44]



(a) Design criteria

MLSS (mg/L)


MLVSS (mg/L)


F/M (lb BOD/lb MLVSS/day)


Maximum BOD volumetric loading (lb


BOD/1000 ft3/day)

Maximum BOD surface loading (lb

0.05-0.7 (4-8 g BOD5/m2.day according to

BOD/1000 ft2/day)

German experience)

Number of stages per train


Hydraulic surface loading (gal/day/ft2)


HRT (days)



Influent (mg/L)

Removal (%)

(b) Performance



















Remark: These design and performance data are based on results of different references including EPA publications that handle landfill leachate treatment.

Remark: These design and performance data are based on results of different references including EPA publications that handle landfill leachate treatment.

Disc area is calculated directly in a simple form:

Disc area is calculated directly in a simple form:

Based on the organic surface loading, normally adopted in Germany, the selected design value of Table 8 is: Organic loading rate=4 g BOD/m2-day.

567 m3/day it 4WOmg/L

in lint m EOD loading -

Disc area is:

In comparing Ad and Ad, it is clear that the required disc area will be:

Modules number =

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