Lin S.D. Water and Wastewater Calculations Manual

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Wastewater Engineering 715

penetrate the liquid film through mixing and diffusion into the biofilm

for biological oxidation. Excess DO in the wastewater film is mixed with

bulk wastewater in the tank and results in aeration of the wastewater.

The rotating media are used for supporting growth of microorganisms

and for providing contact between the microorganisms, the substrates,

and DO.

A group of RBC units is usually separated by baffling into stages to

avoid short circuiting in the tank. There can be one shaft or more in a

stage. The hydraulic detention time in each stage is relatively short, on

Figure 6.31 Relative concentration of substrate and dis-

solved oxygen for a loading condition and RBC rotation

speed as a function of media location (source: US EPA 1984).

716 Chapter 6

the order of 20 min under normal loading. Each RBC stage tends to oper-

ate as a complete-mix reactor. The density and species of microbial pop-

ulation in each stage can vary significantly, depending on wastewater

loading conditions. If RBCs are designed for secondary treatment, heavy

microbial growth, shaggy and gray in color, develops. Good carbonaceous

removal usually occurs in the first and second stages. The succeeding

stages can be used for nitrification, if designed, which will exhibit nitri-

fier growth, brown in color.

The shearing process from rotation exerted on the biomass periodi-

cally sloughs off the excess biomass from the media into the waste-

water stream. This sloughing action prevents bridging and clogging

between adjacent media. The mixing action of the rotating media keeps

the sloughed biomass in suspension from settling to the RBC tank. The

sloughed-off solids flow from stage to stage and finally into the clarifier

following the RBC units. Intermediate clarification and sludge recycle

are not necessary for the RBC process.

In comparison with other biological treatment process, the RBC

process differs from the trickling filter process by having substantially

longer retention time and dynamic rather than stationary media; and

from the activated-sludge process by having attached (fixed) biomass

rather than a suspended culture and sludge recycle.

The patented Surfact process was created and developed by the

Philadelphia Wastewater Department (Nelson and Guarino, 1977). The

process uses air-driven RBCs that are partially submerged in the aer-

ation basins of an activated-sludge system. The RBCs provide fixed-

film media for biological growth that are present in the recycled

activated sludge in the aeration tanks. The results are more active bio-

logical coating on the fixed-film media than that which is found on such

media when used as a separate secondary treatment. Surfact combines

the advantages of both RBC (fixed-film growth) and activated-sludge

systems in a single tank, producing additional biological solids in the

system. The results can be either a higher treatment efficiency at the

same flow rate or the same level of treatment at a higher flow rate.

23.3 Advantages

Whether used in a small or a large municipal wastewater treatment

plant, the RBC process has provided 85% or more of BOD

and ammo-

nia nitrogen removal from sewage. In addition to high treatment capac-

ity, it gives good sludge separation. It has the advantages of smaller

operation and maintenance costs, and of simplicity in operation. It can

be retrofitted easily to existing plants.

The RBC process is similar to the trickling filter (fixed-film biolog-

ical reactor) and to the activated-sludge process (suspended culture in

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the mixed liquor). However, the RBC has advantages over the trick-

ling filter process, such as longer contact time (eight to ten times), rel-

atively low land requirement (40% less), and less excavation, more

surface area renewal for aeration, greater effectiveness for handling

shock loadings, effective sloughing off of the excessive biomass, and

without the nuisance of “filter flies.” The RBC system may use less

power than either the mechanical aeration (activated-sludge) or the

trickling filter system of an equivalent capacity. It is anticipated that

the RBC would exhibit a more consistent treatment efficiency during

the winter months.

Over 95% of biological solids in the RBC units are attached to the

media. These result in lower maintenance and power consumption of

the RBC process over the activated-sludge process. In comparison with

the activated-sludge process, there is no sludge or effluent recycle with

a minimum process control requirement. Less skilled personnel are

needed to operate the RBC process. A hydraulic surge or organic over-

loading will cause activated-sludge units to upset in process operation

and thereby cause sludge bulking. This is not the case for the RBC

process, which has a better process stability. Other advantages of the

RBC process over the activated-sludge process are relatively low land

requirement and less excavation, more flexibility for upgrading treat-

ment facilities, less expense for nitrification, and better sludge settling

without hindered settling.

23.4 Disadvantages

The problems of the first generation of RBC units are mainly caused by

the failure of hardware and equipment. Significant effort has been made

to correct these problems by the manufacturers. The second or third gen-

eration of RBC units may perform as designed.

Capital and installation costs for the RBC system, including an over-

head structure, will be higher than that for an activated-sludge system

of equal capacity. The land area requirement for the RBC process is

about 30% to 40% about that for the activated-sludge process.

If low dissolved oxygen is coupled with available sulfide, the nuisance

bacteria Beggiatoa may grow on the RBC media (Hitdlebaugh and

Miller, 1980). The white biomass phenomenon is caused by the Beggiatoa

propagation. These problems can be corrected by addition of hydrogen

peroxide. Some minor disadvantages related to the RBC process as well

as other biological treatment processes include that a large land area

may be required for a very large facility, additional cost for enclosures,

possible foul odor problem, shock loading recovery, extremes of waste-

water, pH, Thiotrix or Beggiatoa growth, overloading, and controversy

regarding the technological nature of RBC.

718 Chapter 6

23.5 Soluble BOD

It is accepted that soluble BOD

(SBOD) is the control parameter for

RBC performance. The SBOD can be determined by the BOD test using

filtrate from the wastewater samples. For RBC design purposes, it can

also be estimated from historical data on total BOD (TBOD) and

suspended solids. The SBOD is suspended BOD subtracted from TBOD.

Suspended BOD is directly correlated with total suspended solids (TSS).

Their relationships are expressed as

SBOD ⫽ TBOD ⫺ suspended BOD (6.144)

Suspended BOD ⫽ c (TSS) (6.145)

SBOD ⫽ TBOD ⫺ c (TSS) (6.146)

where c ⫽ a coefficient

⫽ 0.5 to 0.7 for domestic wastewater

⫽ 0.5 for raw domestic wastewater (TSS > TBOD)

⫽ 0.6 for raw wastewater (TSS > TBOD)

(municipal with commercial and industrial wastewaters)

⫽ 0.6 for primary effluents

⫽ 0.5 for secondary effluents

Example: Historical data of the primary effluent show that the average

TBOD is 145 mg/L and TSS is 130 mg/L. What is the influent SBOD con-

centration that can be used for the design of an RBC system? RBC is used as

the secondary treatment unit.

solution:

For the primary effluent (RBC influent)

c ⫽ 0.6

Estimate SBOD concentration of RBC influent using Eq. (6.146)

SBOD ⫽ TBOD ⫺ c (TSS)

⫽ 145 mg/L ⫺ 0.6 (130 mg/L)

⫽ 67 mg/L

23.6 RBC process design

Many studies employ either the Monod growth kinetics or Michaelis—

Menton enzyme kinetics for modeling organic and nitrogenous removals

by the RBC process. Under steady-state conditions and a complete-mix

chamber, based on the material mass balance expression for RBC stages,

zero-, half-, and first-order kinetics, nonlinear second-order differential

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equations, conceptional models, and empirical models have been proposed.

However, the usefulness of these models to predict RBC performance is

not well established.

Factors affecting the RBC performance include wastewater temper-

ature, influent substrate concentration, hydraulic retention time, tank

volume to media surface area ratio, media rotational speed, and dis-

solved oxygen (Poon et al., 1979).

Manufacturer’s empirical design approach. In the United States, the

design of an RBC system for most municipal wastewaters is normally

based on empirical curves developed by various manufacturers.

Unfortunately the empirical approach is often the least rational in its

methodologies and omits many important performance parameters. The

designers or the users have relied heavily on the manufacturers for

planning and design assistance.

The RBC design aspects for organic removal vary considerably among

the various manufacturers. For the design loading, Autotrol (Envirex)

(1979) and Clow (1980) use applied SBOD

while Lyco (1982) uses

applied TBOD

. All can predict the water quality at intermediate points

in the treatment process and at the effluent. However, the predicted per-

formances are quite different among manufacturers.

The manufacturer’s design curves are developed on the basis of

observed municipal RBC wastewater treatment performances. For

example, Figs. 6.32 and 6.33 show organic removal design curves. The

manufacturer’s empirical curves define effluent concentration (soluble

or total BOD

or NH

-N) or percent removal in terms of applied hydraulic

loading rate or organic loading rate in conjunction with influent sub-

strate concentration.

In design, starting with a desired effluent concentration or percent

removal (

①

in Figs. 6.32 and 6.33) and given influent concentration

➁

the hydraulic loading

➂

can be selected. The RBC total media-surface

area required is calculated by dividing the design flow rate by the

hydraulic loading derived.

Work by Antonie (1978) demonstrated that, for any kinetics order

higher than zero, overall BOD

removal for a given media surface area

is enhanced by increasing the number of stages. Table 6.15 gives guide-

lines for stages recommended by three manufacturers.

One to four stages are recommended in most design manuals. A large

number of stages are required to achieve higher removal efficiency. The

relationship between design curves and stages has not been established

and the reasons for recommendations are not generally given. The stage

selection is an integral part of the design procedure and should be used

intelligently. Based on substrate loading, the percent surface area for

the first stage can be determined from the manufacturer’s design curves

720 Chapter 6

Figure 6.33 RBC (mechanical drive) process design curves for total and soluble BOD

removal (T ⬎ 13°C, 55°F).

Figure 6.32 RBC process (mechanical drive) design curves for percent total BOD

removal (temperature ⬎ 13°C, 55°F).

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TABLE 6.15 RBC Stage Recommendations

Autotrol (1979) Lyco (1980)

Target effluent Min. number Target TBOD

Number of

SBOD

, mg/L of stages Clow (1980) reduction stages

⬎25 1 At least up to 40% 1

15–25 1 or 2 4 stages 35–65% 2

per train

10–15 2 or 3 60–85% 3

⬍10 3 or 4 80–95% 4

Minimum of four stages

recommended for combined BOD

and NH

-N removal

TABLE 6.16 Typical Design Parameters for Rotating Biological Contactors Used

for Various Treatment Levels

Parameter Secondary Combined nitrification Separate nitrification

Hydraulic loading

gal/(d ⭈ ft

) 2.0–40 0.75–2.0 1.0–2.5

/(m

⭈ d) 0.081–0.163 0.030–0.081 0.041–0.102

Soluble BOD

lb/(1000 ft

⭈ d) 0.75–2.0 0.5–1.5 0.1–0.3

Total (T) BOD

lb/(1000 ft

⭈ d) 2.0–3.5 1.5–3.0 0.2–0.6

Maximum loading on

first stage

lb SBOD/4–6 4–6

(1000 ft

⭈ d)

lb TBOD/ 8–12 8–12

(1000 ft

⭈ d)

-N loading

lb/(1000 ft

⭈ d) 0.15–0.3 0.2–0.4

Hydraulic 0.7–1.5 1.5–4 1.2–2.9

retention time u,h

Effluent 15–30 7–15 7–15

BOD

, mg/L

Effluent < 2 1–2

-N, mg/L

Note: lb/(1000 ft

⭈ d) ⫻ 0.00488 ⫽ kg/(m

⭈ d)

SOURCE: Metcalf and Eddy Inc. (1991)

(not included). Typical design information for RBC used for various

treatment levels is summarized in Table 6.16.

To avoid organic overloading and possible growth of nuisance organ-

isms, and shaft overloads, limitation of organic loadings has been rec-

ommended as follows (Autotrol, 1979):

722 Chapter 6

TABLE 6.17 Factors for Required Surface Area Correction to Temperature

Temperature

°F °C For soluble BOD

For ammonia-

removal nitrogen removal

64 17.8 1.0 0.71

62 16.7 1.0 0.77

60 15.5 1.0 0.82

58 14.4 1.0 0.89

56 13.3 1.0 0.98

55 12.8 1.0 1.00

54 12.2 1.03 1.02

52 11.1 1.09 1.14

50 10.0 1.15 1.28

48 8.9 1.21 1.40

46 7.8 1.27 1.63

45 7.2 1.31 1.75

44 6.7 1.34 1.85

42 5.6 1.42 2.32

40 4.4 1.50 –

Then media distribution and choice of configurations can be made with

engineering judgment or according to the manufacturer’s design manual.

Most RBC manufacturers have a standard ratio of tank volume to media

surface area at 1.2 gal/ft

(48.9 L/m

The manufacturers universally contend that wastewater tempera-

tures above 55⬚F (12.8⬚C) do not affect the organic removal rate. Below

55⬚F varying degrees of decreased biological activity rate are predicted

by the manufacturers. Surface area correction factors for wastewater

treatment below 55⬚F are shown in Table 6.17. Most manufacturers

require wastewater temperature not to exceed 90⬚F (32.2⬚C) which can

damage the RBC media. Fortunately, this is normally not the case for

municipal wastewaters.

In summary, an RBC system can be designed on the basis of infor-

mation on total required media surface area, temperature correction,

staging, and organic loading limits.

RBC performance histories relative to empirical design have been exam-

ined. Operational data indicated that the prediction results from manu-

facturer’s design curves were considerably different from actual results

Organic (SBOD

) loading, lb/(d ⭈ 1000 ft

)

Media Mechanical drive Air drive

Standard density 4.0 5.0

High density 1.7 2.5

Note: lb/(d ⭈ 1000 ft

) ⫻ 0.00488 ⫽ kg/(d ⭈ m

)

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(Opatken 1982, Brenner et al., 1984). They usually gave an overestimate

of attainable removals. The manufacturer’s design curves should be used

with caution whenever first-stage loading exceeds the recommended values.

Example 1: At the first stage of the RBC system, what is the maximum

hydraulic loading if the total BOD

for the influent and effluent are 150 and

60 mg/L, respectively?

solution:

Step 1. Calculate percent TBOD removal

Step 2. Determine hydraulic loading limit

From Fig. 6.32, starting with 60% TBOD removal at point 1, draw a horizontal

line to the intersect with the influent TBOD concentration of 150 mg/L at point

2. Next, starting at point 2, draw a vertical line to intersect the hydraulic load-

ing axis at point 3. Read the value at point 3 as 5.8 gal/(d ⭈ ft

). This is the answer.

Example 2: Compute the hydraulic and organic loading rates of an RBC

system and of the first stage. Given:

Primary effluent (influent) flow ⫽1.5 Mgal/d

Influent total BOD

⫽ 140 mg/L

Influent SBOD

⫽ 75 mg/L

Area of each RBC shaft ⫽ 100,000 ft

Number of RBC shafts ⫽ 6

Number of trains ⫽ 2 (3 stages for each train)

No recirculation

solution:

Step 1. Compute the overall system hydraulic loading (HL)

Surface area of total system ⫽ 6 ⫻ 100,000 ft

⫽ 600,000 ft

Note: It is in the range of 2 to 4 gal/(d ⭈ ft

5 2.5 gal/sd

System HL 5

1,500,000 gal/d

600,000 ft

5 60%

% removal 5

s150 2 60d mg/L 3 100%

150 mg/L

724 Chapter 6

Step 2. Compute the organic loadings of the overall RBC system

Note: Design range ⫽ 2 to 3.5 lb/1000 ft

Note: Design range ⫽ 0.75 to 2.0 lb/1000 ft

Step 3. Compute the organic loadings of the first stage

Two units are used as the first stage.

Example 3: A municipal primary effluent with total and soluble BOD

of 140

and 60 mg/L, respectively, is to be treated with an RBC system. The RBC efflu-

ent TBOD and SBOD is to be 24 and 12 mg/L, respectively, or less. The tem-

perature of wastewater is 60⬚F (above 55⬚F; no temperature correction

needed). The design plant flow is 4.0 Mgal/d (15,140 m

/d). The maximum

hourly flow is 10.0 Mgal/d (37,850 m

/d).

solution:

Step 1. Determine the allowable hydraulic loading rate

From Fig. 6.33, at effluent

SBOD ⫽ 12 mg/L with

Hydraulic loading ⫽ 3.44 gal/(d ⭈ ft

)

5 4.69 [lb/s1000ft

dd]

Soluble BOD loading 5

1.5 3 75 3 8.34

200 3 1000

5 8.76 [lb/s1000 ft

dd]

Total BOD loading 5

1.5 3 140 3 8.34

2 3 100,000

5 7.6 g/sm

5 1.56

lbs1000 ft

Soluble BOD loading 5

1.5 3 75 3 8.34

600 3 1000

5 14.2 g/sm

5 2.92/lbs1000 ft

Total BOD loading 5

1.5 Mgal/d 3 140 mg/L 3 8.34 lb/sMgal.mg/ Ld

600 3 1000 ft