Lin S.D. Water and Wastewater Calculations Manual

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Wastewater Engineering 695

Step 10. Calculate the ratio of oxygen required to BOD removal

Step 11. Determine the field transfer rate for the surface aerators

The example in the mechanical aerator shows that the oxygen transfer rate

is lower in summer (higher temperature). From that example, the altitude cor-

rection factor is 0.98 for an elevation of 210 m. Select the surface aerator

rating as 2.1 kg oxygen/kW ⭈ h (3.5 lb/hp ⭈ h). The solubility of tap water at

26.5 and 20.0⬚C is 7.95 and 9.02 mg/L, respectively. Let C

⫽ 2.0 g/L as

GLUMRB (1996) recommended. The power requirements under field condi-

tions can be estimated using Eq. (6.123).

Step 12. Determine the power requirements of the surface aerator

From Steps 9 and 11

Note: This is the power required for oxygen transfer.

Step 13. Determine the total energy required for mixing the lagoon

Assume power required for complete-mix flow is 0.015 kW/m

(0.57 hp/1000 ft

From Step 1

Volume of the lagoon ⫽ 15,000 m

Total power needed ⫽ 0.015 kW/m

⫻ 15,000 m

⫽ 225 kW

Use 8 of 30 kW (40 hp) surface aerators providing 240 kW.

5 13.6 kW

Power required 5

oxygen required

field transfer rate

440 kg/d

32.4 kg/kW

5 32.4 kg/kW

5 1.35 kg/kW

5 s2.1

kg/kW

hda

1 3 0.98 3 7.95 mg /L 2 2 mg /L

9.02 mg /L

bs1.024d

26.5220

s0.86d

N 5 N

2 C

s20

b1.024

T220

5 0.84

Oxygen

required

BOD removed

440 kg/d

522.9 kg/d

5 1153 lb/d

5 522.9 kg/d

BOD removal 5 3000 m

/d 3 s180 2 5.7d g/m

4 1000 g/kg

696 Chapter 6

Note: This is the power required for mixing and is commonly the control

factor in sizing the aerators for domestic wastewater treatment. For indus-

trial wastewater treatment, the control factor is usually reversed.

22 Trickling Filter

A trickling filter is actually a unit process for introducing primary efflu-

ent into contact with biological growth and is a biological oxidation bed.

The word “filter” does not mean any filtering or straining action; nev-

ertheless, it is popularly and universally used.

22.1 Process description

The trickling filter is the most commonly used unit of the fixed-growth

film-flow-type process. A trickling filter consists of: (1) a bed of coarse

material, such as stone slates or plastic media, over which wastewater

from primary effluent is sprayed; (2) an underdrain system; and (3) dis-

tributors. The underdrain is used to carry wastewater passing through

the biological filter and drain to the subsequent treatment units and to

provide ventilation of the filter and maintenance of the aerobic condi-

tion. Wastewater from the primary effluent is distributed to the surface

of the filter bed by fixed spray nozzles (first developed) or rotary dis-

tributors. Sloughs of biomass from the media are settled in the second-

ary sedimentation tank.

Biological slime occurs on the surface of the support media while

oxygen is supplied by air diffusion through the void spaces. It allows

wastewater to trickle (usually in an intermittent fashion) downward

through the bed media. Organic and inorganic nutrients are extracted

from the liquid film by the microorganisms in the slime. The biological

slime layer consists of aerobic, anaerobic, and facultative bacteria, algae,

fungi, and protozoans. Higher animals such as sludge worms, filter-fly

larvae, rotifers, and snails are also present. Facultative bacteria are the

predominant microorganisms in the trickling filter. Nitrifying bacteria

may occur in the lower part of a deep filter.

The biological activity of the trickling filter process can be described

as shown in Fig. 6.27. The microbial layer on the filter is aerobic usu-

ally to a depth of only 0.1 to 0.2 mm. Most of the depth of the microbial

film is anaerobic. As the wastewater flows over the slime layer, organic

matter (nutrient) and dissolved oxygen are transferred to the aerobic

zone by diffusion and extracted, and then metabolic end products such

as carbon dioxide are released to the water.

Dissolved oxygen in the liquid is replenished by adsorption from air

in the voids surrounding the support media. Microorganisms near the

surface of the filter bed are in a rapid growth rate due to plenteous food

Wastewater Engineering 697

supply, whereas microorganisms in the lower portion of the filter may

be in a state of starvation. Overall, a trickling filter operation is con-

sidered to be in the endogenous growth phase. When slime becomes

thicker and cells die and lyse, the slime layer will slough off and is sub-

sequently removed by secondary settling.

For over a century, the trickling filter has been used as a secondary

treatment unit. In a rock-fill conventional trickling filter, the rock size

is 25 to 100 mm (1 to 4 in). The depth of the rock bed varies from 0.9 to

2.5 m (3 to 8 ft) with an average of 1.8 m (6 ft). The Ten States Standards

recommends a minimum depth of trickling filter media of 1.8 m above the

underdrains and that a rock and/or slag filter shall not exceed 3 m (10 ft)

in depth (GLUMRB, 1996).

After declining in use in the late 1960s and early 1970s because of

the development of RBC, trickling filters regained popularity in the late

1970s and 1980s, primarily due to new synthetic media. The new high-

rate media were generally preferred over rocks because they are lighter,

increased the surface area for biological growth, and improved treatment

Figure 6.27 Schematic diagram of attached-growth process.

698 Chapter 6

efficiency. High-rate media minimizes many of the common problems

with rock media, such as uncontrolled sloughing, plugging, odors, and

filter flies. Consequently, almost all trickling filters constructed in the

late 1980s have been of high-rate media type (WPCF, 1988b).

Plastic media built in square, round, and other modules of corrugated

shape have become popular. The depths of these plastic media range

from 4 to 12 m (14 to 40 ft) (US EPA, 1980; Metcalf and Eddy, Inc. 1991).

These materials increase void ratios and air flow. The plastics are 30%

lighter than rock. A minimum clearance of 0.3 m (1 ft) between media

and distribution arms shall be provided (GLUMRB, 1996).

22.2 Filter classiﬁcation

Trickling filters are classified according to the applied hydraulic and organic

loading rates. The hydraulic loading rate is expressed as the quantity of

wastewater applied per day per unit area of bulk filter surface (m

/(d ⭈ m

gal/(d ⭈ ft

), or Mgal/(d ⭈ acre) or as depth of wastewater applied per unit

of time. Organic loading rate is expressed as mass of BOD

applied per day

per unit of bulk filter volume (kg/(m

⭈ d), lb/(1000 ft

⭈ d)). Common clas-

sifications, are low- or standard-rate, intermediate-rate, high-rate, super-

high-rate, and roughing. Two-stage filters are frequently used, in which two

trickling filters are connected in series. Various trickling filter classifica-

tions are summarized in Table 6.14.

22.3 Recirculation

Recirculation of a portion of the effluent to flow back through the filter

is generally practised in modern trickling filter plants. The ratio of the

return flow Q

, to the influent flow Q is called the recirculation ratio r.

Techniques of recirculation vary widely, with a variety of configura-

tions. The recirculation ratios range from 0 to 4 (Table 6.14) with usual

ratios being 0.5 to 3.0.

The advantages of recirculation include an increase in biological solids

in the system with continuous seeding of active biological material; elim-

ination of shock load by diluting strong influent; maintenance of more

uniform hydraulic and organic loads; an increase in the DO level of the

influent; thinning of the biological slime layer; an improvement of treat-

ment efficiency; reduction of filter clogging; and less nuisance problems.

22.4 Design formulas

Attempts have been made by numerous investigators to correlate oper-

ational data with the design parameters of trickling filters. The design

of trickling filter plants is based on empirical, semiempirical, and mass

balance concepts. Mathematical equations have been developed for

699

TABLE 6.14 Typical Design Information for Trickling Filter Categories

Low rate Super

Parameter (Standard) Intermediate rate High rate high rate Roughing Two stage

Filter media Rock, slag Rock, slag Rock Plastic Plastic, redwood Rock, plastic

Hydraulic loading

/(m

⭈ d) 1–3.7 3.7–9.4 9.4–37 14–86 47–187 9.4–37

gal/(d ⭈ ft

) 25–90 90–230 230–900 350–2100 1150–4600 230–900

Mgal/(d ⭈ acre) 1–4 4–10 10–39 15–91 50–200 10–39

BOD

kg/(m

⭈ d) 0.08–0.32 0.24–0.48 0.48–1.0 0.48–1.6 1.6–8.0 1.0–2.0

lb/(1000 ft

⭈ d) 5–20 15–30 30–60 30–100 100–500 60–120

Recirculation rate 0 0–1 1–2 1–2 1–4 0.5–2

Filter flies Many some few few or none few or none few or none

Sloughing intermittent intermittent continuous continuous continuous continuous

Depth, m 1.8–2.4 1.8–2.4 0.9–1.8 3–12 4.6–12 1.8–2.4

ft 6–8 6–8 3–6 10–40 15–40 6–8

BOD

removal % 80–90 50–70 65–85 65–80 40–65 85–95

(includes settling)

Nitrification well partial little little no well

Combined process

for secondary treatment no unlikely likely yes yes yes

for tertiary treatment yes yes yes yes yes yes

Type normally SC/TF SC/TF SC/TF AS/TF AS/TF TF/TF

used* ABF ABF RBC/TF AS/BF — —

* SC/TF⫽trickling filter after solid contactor

AS⫽activated sludge

ABF⫽activated biofilter

RBC⫽rotating biological contactor

BF⫽biofilter

SOURCES: US EPA (1974a), Metcalf and Eddy, Inc. (1991), WEF and ASCE (1991a, 1996a)

700 Chapter 6

calculating the BOD

removal efficiency of biological filters on the basis

of factors such as bed depth, types of media, recirculation, temperature,

and loading rates. The design formulations of trickling filters of major

interest include the NRC formula (1946), Velz formula (1948), Fairall

(1956), Schulze formula (1960), Eckenfelder formula (1963), Galler and

Gotaas formulas (1964, 1966), Germain formula (1966), Kincannon and

Stover formula (1982), Logan formula (Logan et al., 1987a, b), and British

multiple regression analysis equation (1988). These are summarized

elsewhere (WEF and ASCE, 1991a; McGhee, 1991).

NRC formula. The NRC (National Research Council) formula for

trickling-filter performance is an empirical expression developed by

the National Research Council from an extensive study of the oper-

ating data of trickling treatment plants at military bases within the

United States during World War II in the early 1940s (NRC, 1946).

It may be applied to single-stage and multistage rock filters with

varying recirculation ratios. Graphic expressions for BOD removal

efficiency are available. The equation for a single-stage or first-stage

rock filter is

(6.129)

(6.130)

where E

⫽ efficiency of BOD removal for first stage at 20⬚C

including recirculation and sedimentation, %

W ⫽ BOD loading to filter, kg/d or lb/d

⫽ flow times influent concentration

V ⫽ volume of filter media, m

or 1000 ft

F ⫽ recirculation factor

The recirculation factor is calculated by

(6.131)

where r ⫽ recirculation ratio, Q

⫽ recirculation flow, m

/d or Mgal/d

Q ⫽ wastewater flow, m

/d or Mgal/d

F 5

1 1 r

s1 1 0.1rd

100

1 1 0.561

2W/VF

sUS customary unitsd

100

1 1 0.532

2W/VF

sSI unitsd

Wastewater Engineering 701

The recirculation factor represents the average number of passes of the

influent organic matter through the trickling filter. For the second-

stage filter, the formula becomes

(SI units) (6.132)

(British system) (6.133)

where E

⫽ efficiency of BOD

removal for second-stage filter, %

W ⬘⫽BOD loading applied to second-stage filter, kg/d or lb/d

Other terms are as described previously. Overall BOD removal efficiency

of a two-stage filter system can be computed by

(6.134)

where the term 35 means that 35% of BOD of raw wastewater is

removed by primary settling.

BOD removal efficiency in biological treatment process is significantly

influenced by wastewater temperature. The effect of temperature can

be calculated as

⫽ E

1.035

T–20

(6.135)

Example 1: Estimate the BOD removal efficiency and effluent BOD

of a two-

stage trickling filter using the NRC formula with the following given conditions.

Wastewater temperature 20⬚C

Plant flow Q 2 Mgal/d (7570 m

)

BOD

in raw waste 300 mg/L

Volume of filter (each) 16,000 ft

(453 m

)

Depth of filter 7 ft (2.13 m)

Recirculation for filter 1 ⫽ 1.5 Q

Recirculation for filter 2 ⫽ 0.8 Q

solution:

Step 1. Estimate BOD loading at the first stage

Influent BOD C

⫽ 300 mg/L (1 – 0.35) ⫽ 195 mg/L

W ⫽ QC

⫽ 2 Mgal/d ⫻195 mg/L ⫻ 8.34 lb/(Mgal ⭈ mg/L)

⫽ 3252 lb/d

E 5 100 2 100a1 2

100

b a1 2

100

b a1 2

100

1 1

0.0561

1 2 E

W r

100

1 1

0.0532

1 2 E

W r

702 Chapter 6

Step 2. Calculate BOD removal efficiency of filter 1

Using Eqs. (6.131) and (6.130)

Step 3. Calculate effluent BOD concentration of filter 1

⫽ 195 mg/L (1 – 0.632)

⫽ 71.8 mg/L

Step 4. Calculate BOD removal efficiency of filter 2

Step 5. Calculate effluent BOD concentration of filter 2

⫽ 71.8 mg/L ⫻ (1 ⫺ 0.719)

⫽ 20.1 mg/L

Step 6. Calculate the overall efficiency

(a) Using Eq. (6.134)

E ⫽ 100 ⫺ 100[(1 ⫺ 0.35)(1 ⫺ 0.632)(1 ⫺ 0.719)]

⫽ 93.3%

(b)

E ⫽ (300 mg/L ⫺ 20.1 mg/L)/(300 mg/L)

⫽ 0.933

⫽ 93.3%

Note: At 20⬚C, no temperature correction is needed.

5 71.9%

100%

1 1 0.0561

21198/s16 3 1.54d

5 1198 slb/dd

Mass of influent BOD 5 2 3 71.8 3 8.34

F r 5

1 1 0.8

s1 1 0.1 3 0.8d

5 1.54

5 63.2s%d

100

1 1 0.0561 23252/s16 3 1.89d

100

1 1 0.0561 2W/VF

5 1.89

F 5

1 1 r

s1 1 0.1r

1 1 1.5

s1 1 0.1 3 1.5d

Wastewater Engineering 703

Example 2: Determine the size of a two-stage trickling filter using the NRC

equations. Assume both filters have the same efficiency of BOD

removal and

the same recirculation ratio. Other conditions are as follows.

Wastewater temperature 20⬚C

Wastewater flow Q 3785 m

/d (1 Mgal/d)

Influent BOD

195 mg/L

Design effluent BOD

20 mg/L

Depth of each filter 2 m (6.6 ft)

Recirculations for filters 1 and 2, r

⫽ r

1.8

solution:

Step 1. Determine E

and E

Overall efficiency ⫽ (195 – 20) mg/L ⫻ 100% /195 mg/L

⫽ 89.7%

+ E

(1 ⫺ E

) ⫽ 0.897

+ E

– E

⫽ 0.897

Since E

⫽ E

Thus

Step 2. Calculate the recirculation factor F

Step 3. Calculate mass BOD load to the first-stage filter, W

Influent BOD ⫽ 195 mg/L ⫽ 195 g/m

⫽ 0.195 kg/m

W ⫽ QC

⫽ 3785 m

/d ⫻ 0.195 kg/m

⫽ 738 kg/d

Step 4. Calculate the volume V of the first filter using Eq. (6.129)

100

1 1 0.532

2W/VF

5 2.01

F 5

1 1 r

s1 1 0.1rd

1 1 1.8

s1 1 0.1 3 1.8d

5 0.68 5 68%

2 6 24 2 4 3 0.897

2 2E

1 0.897 5 0

704 Chapter 6

From Step 1

Step 5. Calculate the diameter d of the first filter

Area ⫽ V/depth ⫽ 469 m

/2 m

⫽ 234.5 m

p/4 ⫽ 234.5 m

d ⫽ 17.3 m

Step 6. Calculate the mass BOD loading W⬘ to the second-stage filter

W⬘⫽W(1 ⫺ E) ⫽ 738 kg/d (1 ⫺ 0.68)

⫽ 236.2 kg/d

Step 7. Calculate the volume of the second filter using Eq. (6.132)

Step 8. Calculate the diameter of the second-stage filter

A ⫽ 1467 m

/2 m

⫽ 733.5 m

⫽ 733.5m

⫻ 4/p

d ⫽ 30.6 (m)

Step 9. Check the BOD loading rate to each filter

(a) First-stage filter, from Steps 3 and 4

BOD loading rate ⫽ W ⫼ V ⫽ 738 kg/d ⫼ 469 m

⫽ 1.57 kg/(m

⭈ d)

Note: The rates are between 1.0 and 2.0 kg/(m

⭈ d) (Table 6.14).

V 5 1467sm

68 1 1225 21/V

5 100

68 5

100

1 1

0.532

1 2 0.68

236.2

2.01V

100

1 1

0.532

1 2 E

W r

V 5 469sm

2367.2/V

5 0.885

68 5

100

1 1 0.532

2738/2.01V