Lin S.D. Water and Wastewater Calculations Manual

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Wastewater treatment facilities are designed to speed up the natural

purification process that occurs in natural waters and to remove con-

taminants in wastewater that might otherwise interfere with the nat-

ural process in the receiving waters.

Wastewater contains varying quantities of suspended and floating

solids, organic matter, and fragments of debris. Conventional waste-

water treatment systems are combinations of physical and biological

(sometimes with chemical) processes to remove its impurities.

The alternative methods for municipal wastewater treatment are

simply classified into three major categories: (1) primary (physical

process) treatment, (2) secondary (biological process) treatment, and (3)

tertiary (combination of physical, chemical, and biological process) or

advanced treatment. As can be seen in Fig. 6.3, each category should

include previous treatment devices (preliminary), disinfection, and sludge

management (treatment and disposal). The treatment devices shown in

the preliminary treatment are not necessarily to be included, depending

on the wastewater characteristics and regulatory requirements.

For over a century, environmental engineers and aquatic scientists

have developed wastewater treatment technologies. Many patented

treatment process and package treatment plants have been developed

and applied. The goal of wastewater treatment processes is to produce

clean effluents and to protect public health, natural resources, and the

ambient environment.

The Ten States Recommended Standards for Sewage Works, adopted

by the Great Lakes—Upper Mississippi River Board (GLUMRB), was

revised five times as a model for the design of wastewater treatment

plants and for the recommended standards for other regional and state

agencies. The original members of the ten states were Illinois, Indiana,

Iowa, Michigan, Minnesota, Missouri, New York, Ohio, Pennsylvania,

and Wisconsin. Recently, Ontario, Canada was added as a new member.

The new title of the standards is “Recommended Standards for

Wastewater Facilities—Policies for design, review, and approval of plans

and specifications for wastewater collection and treatment facilities,”

1996 edition, by Great Lakes—Upper Mississippi River Board of State

and Provincial Public Health and Environmental Managers.

13.1 Preliminary treatment systems

Preliminary systems are designed to physically remove or cut up the

larger suspended and floating materials, and to remove the heavy inor-

ganic solids and excessive amounts of oil and grease. The purpose of pre-

liminary treatment is to protect pumping equipment and the subsequent

treatment units. Preliminary systems consist of flow measurement devices

and regulators (flow equalization), racks and screens, comminuting

Wastewater Engineering 575

devices (grinders, cutters, and shredders), flow equalization, grit cham-

bers, preaeration tanks, and (possibly) chlorination. The quality of waste-

water is not substantially improved by preliminary treatment.

13.2 Primary treatment systems

The object of primary treatment is to reduce the flow velocity of the

wastewater sufficiently to permit suspended solids to settle, i.e. to

576 Chapter 6

Figure 6.3 Flow chart for wastewater treatment processes.

remove settleable materials. Floating materials are also removed by

skimming. Thus, a primary treatment device may be called a settling

tank (or basin). Due to variations in design and operation, settling tanks

can be divided into four groups: plain sedimentation with mechanical

sludge removal, two story tanks (Imhoff tank, and several patented

units), upflow clarifiers with mechanical sludge removal, and septic

tanks. When chemicals are applied, other auxiliary units are needed.

Auxiliary units such as chemical feeders, mixing devices, and floccula-

tors (New York State Department of Health, 1950) and sludge (biosolids)

management (treatment and dispose of) are required if there is no fur-

ther treatment.

The physical process of sedimentation in settling tanks removes

approximately 50% to 70% of total suspended solids from the wastewater.

The BOD

removal efficiency by primary system is 25% to 35%. When

certain coagulants are applied in settling tanks, much of the colloidal as

well as the settleable solids, or a total of 80% to 90% of TSS, is removed.

Approximately 10% of the phosphorus corresponding insoluble is nor-

mally removed by primary settling. During the primary treatment

process, biological activity in the wastewater is negligible.

Primary clarification is achieved commonly in large sedimentation

basins under relatively quiescent conditions. The settled solids are

then collected by mechanical scrapers into a hopper and pumped to a

sludge treatment unit. Fats, oils, greases and other floating matter

are skimmed off from the basin surface. The settling basin effluent is

discharged over weirs into a collection conduit for further treatment,

or to a discharging outfall.

In many cases, especially in developing countries, primary treatment

is adequate to permit the wastewater effluent discharge, due to proper

receiving water conditions or to the economic situation. Unfortunately,

many wastewaters are untreated and discharged in many countries. If

primary systems only are used, solids management and disinfection

processes should be included.

13.3 Secondary treatment systems

After primary treatment the wastewater still contains organic matter

in suspended, colloidal, and dissolved states. This matter should be

removed before discharging to receiving waters, to avoid interfering

with subsequent downstream users.

Secondary treatment is used to remove the soluble and colloidal

organic matter which remains after primary treatment. Although the

removal of those materials can be effected by physicochemical means

providing further removal of suspended solids, secondary treatment is

commonly referred to as the biological process.

Wastewater Engineering 577

Biological treatment consists of application of a controlled natural

process in which a very large number of microorganisms consume sol-

uble and colloidal organic matter from the wastewater in a relatively

small container over a reasonable time. It is comparable to biological

reactions that would occur in the zone of recovery during the self-

purification of a stream.

Secondary treatment devices may be divided into two groups: attached

and suspended growth processes. The attached (film) growth processes

are trickling filters, rotating biologic contactors (RBC) and intermit-

tent sand filters. The suspended growth processes include activated

sludge and its modifications, such as contact stabilization (aeration)

tanks, sequencing batch reactors, aerobic and anaerobic digestors,

anaerobic filters, stabilization ponds, and aerated lagoons. Secondary

treatment can also be achieved by physical–chemical or land applica-

tion systems.

Secondary treatment processes may remove more than 85% of BOD

and TSS. However, they are not effective for the removal of nutrients

(N and P), heavy metals, nonbiodegradable organic matter, bacteria,

viruses, and other microorganisms. Disinfection is needed to reduce

densities of microorganisms. In addition, a secondary clarifier is required

to remove solids from the secondary processes. Sludges generated from

the primary and secondary clarifiers need to undergo treatment and

proper disposal.

13.4 Advanced treatment systems

Advanced wastewater treatment is defined as the methods and processes

that remove more contaminants from wastewater than the conventional

treatment. The term advanced treatment may be applied to any system

that follows the secondary, or that modifies or replaces a step in the con-

ventional process. The term tertiary treatment is often used as a syn-

onym; however, the two are not synonymous. A tertiary system is the

third treatment step that is used after primary and secondary treatment

processes.

Since the early 1970s, the use of advanced wastewater treatment

facilities has increased significantly in the United States. Most of their

goals are to remove nitrogen, phosphorus, and suspended solids (includ-

ing BOD

) and to meet certain regulations for specific conditions. In some

areas where water supply sources are limited, reuse of waste-water is

becoming more important. Also, there are strict rules and regulations

regarding the removal of suspended solids, organic matter, nutrients,

specific toxic compounds and refractory organics that cannot be achieved

by conventional secondary treatment systems as well as some industrial

wastewater; thus, advanced wastewater treatment processes are needed.

578 Chapter 6

In the U.S. federal standards for secondary effluent are BOD 30 mg/L

and TSS 30 mg/L. In Illinois, the standards are more stringent: BOD

20 mg/L and TSS 25 mg/L for secondary effluent. In some areas, 10 to

12 (BOD ⫽ 10 mg/L and TSS ⫽ 12 mg/L) standards are implied. For

ammonia nitrogen standards, very complicated formulas depending on

the time of the year and local conditions are used.

In the European Community, the European Community Commission

for Environmental Protection has drafted the minimum effluent stan-

dards for large wastewater treatment plants. The standards include:

BOD

⬍25 mg/L, COD ⬍125 mg/L, suspended solids ⬍35 mg/L, total

nitrogen ⬍10 mg/L, and phosphorus ⬍1 mg/L. Stricter standards are

presented in various countries. The new regulations were expected to

be ratified in 1998 (Boehnke et al., 1997).

TSS concentrations less than 20 mg/L are difficult to achieve by sed-

imentation through the primary and secondary systems. The purpose

of advanced wastewater treatment techniques is specifically to reduce

TSS, TDS, BOD, organic nitrogen, ammonia nitrogen, total nitrogen, or

phosphorus. Biological nutrient removal processes can eliminate nitro-

gen or phosphorus, and any combination.

Advanced processes use some processes for the drinking water treat-

ment. These include chemical coagulation of wastewater, wedge-wire

screens, granular media filters, diatomaceous earth filters, micro-

screening, and ultrafiltration and nanofiltration, which are used to

remove colloidal and fine-size suspended solids.

For nitrogen control, techniques such as biological assimilation, nitri-

fication (conversion of ammonia to nitrogen and nitrate), and denitrifi-

cation, ion exchange, breakpoint chlorination, air stripping are used.

Soluble phosphorus may be removed from wastewater by chemical pre-

cipitation and biological (bacteria and algae) uptake for normal cell

growth in a control system. Filtration is required after chemical and bio-

logical processes. Physical processes such as reverse osmosis and ultra-

filtration also help to achieve phosphorus reduction, but these are

primarily for overall dissolved inorganic solids reduction. Oxidation

ditch, Bardenpho process, anaerobic/oxidation (A/O) process, and other

patented processes are available.

The use of lagoons, aerated lagoons, and natural and constructed

wetlands is an effective method for nutrients (N and P) removal.

Removal of some species of groups of toxic compounds and refractory

organics can be achieved by activated carbon adsorption, air stripping,

activated sludge powder, activated-carbon processes, and chemical oxi-

dation. Conventional coagulation–sedimentation–filtration and biolog-

ical treatment (trickling filter, RBC, and activated sludge) processes

are also used to remove the priority pollutants and some refractory

organic compounds.

Wastewater Engineering 579

13.5 Compliance with standards

The National Pollutant Discharge Elimination System (NPDES) has

promulgated discharge standards to protect and preserve beneficial

uses of receiving water bodies based on water quality criteria, or

technology-based limits, or both. The receiving water quality criteria are

typically established for a 7-day, 10-year, low-flow period.

Table 6.7 shows the national minimum performance standards for sec-

ondary treatment and its equivalency for public owned treatment works

580 Chapter 6

TABLE 6.7 Minimum National Performance Standards for Public Owned Treatment

Works (Secondary Treatment and Its Equivalency)

30-day average 7-day average

Parameter shall not exceed shall not exceed

Conventional secondary treatment processes

5-day biochemical oxygen demand,

∗

BOD

Effluent, mg/L 30 45

Percent removal

†

85 –

5-day carbonaceous biochemical oxygen

demand,

∗

CBOD

Effluent, mg/L 25 40

Percent removal

†

85 –

Suspended solids, SS

Effluent, mg/L 30 45

Percent removal

†

85 –

pH 6.0 to 9.0 at all –

times

Whole effluent toxicity Site specific –

Fecal coliform, MPN/100 mL 200 400

Stabilization ponds and other equivalent of secondary treatment

5-day biochemical oxygen demand,

∗

BOD

Effluent, mg/L 45 65

Percent removal

†

65 –

5-day carbonaceous biochemical oxygen

demand,

∗

CBOD

Effluent, mg/L 40 60

Percent removal

†

65 –

Suspended solids, SS

Effluents, mg/L 45 65

Percent removal

†

65 –

pH 6.0 to 9.0 at all –

times

Whole effluent toxicity Site specific –

Fecal coliform, MPN/100 mL 200 400

Notes:

∗

Chemical oxygen demand (COD) or total organic carbon (TOC) may be substituted for

BOD

when a long-term BOD

: COD or BOD

:TOC correlation has been demonstrated.

†

Percent removal may be waived on a case-by-case basis for combined sewer service areas and

for separated sewer areas not subject to excessive inflow and infiltration (I/I) where the base

flow plus infiltration is ⱕ 120 gpd/capita and the base flow plus I/I is ⱕ 275 gpd/capita.

SOURCE: Federal Register, 1991 (40 CFR 133; 49FR 37006, September 20, 1984; revised

through July 1, 1991. http/www.access.gpo.gov/nara/cfr/index.htm/, 1999)

(POTW) (Federal Register, 1991). The secondary treatment regulation was

established from 40 Code of Regulation, Part 133, 49 Federal Register

37006, on September 20, 1984; and revised through July 1, 1991.

The values in Table 6.7 are still relevant as far as national standards

go; however, they really do not have much applicability in Illinois and

many other states because state standards for most facilities are more

stringent than the national standards. For example, in Illinois, most

POTWs must meet monthly averages of 10/12 (10 mg/L of BOD

and

12 mg/L of SS) effluent standards if the dilution factor is less than 5:1.

The daily maximum effluent concentrations are 20/24. For lagoon efflu-

ents, the standards are 30/30 if the dilution factor is greater than 5:1.

The NPDES applies to each facility of interest because most have some

water quality based effluent limits, especially for ammonia. The Illinois

state effluent standards are in Title 35, Subtile C, Chapter II, Part 370

(IEPA, 1997) which can be found on the Illinois Pollution Control Board’s

web site at http:/www.state.il.us/title35/35conten.htm.

14 Screening Devices

The wastewater from the sewer system either flows by gravity or is

pumped into the treatment plant. Screening is usually the first unit

operation at wastewater treatment plants. The screening units include

racks, coarse screens, and fine screens. The racks and screens used in

preliminary treatment are to remove large objects such as rags, plastics,

paper, metals, dead animals, and the like. The purpose is to protect

pumps and to prevent solids from fouling subsequent treatment facilities.

14.1 Racks and screens

Coarse screens are classified as either bar racks (trash racks) or bar

screens, depending on the spacing between the bars. Bar racks have

clear spacing of 5.08 to 10.16 cm (2.0 to 4.0 in), whereas bar screens typ-

ically have clear spacing of 0.64 to 5.08 cm (0.25 to 2.0 in). Both consist

of a vertical arrangement of equally spaced parallel bars designed to trap

coarse debris. The debris captured on the bar screen depends on the bar

spacing and the amount of debris caught on the screen (WEF, 1996a).

Clear openings for manually cleaned screens between bars should be

from 25 to 44 mm (1 to ). Manually cleaned screens should be

placed on a slope of 30 to 45 degrees to the horizontal. For manually or

mechanically raked bar screens, the maximum velocities during peak

flow periods should not exceed 0.76 m/s (2.5 ft/s) (Ten States Standards

(GLUMRB) 1996; Illinois EPA, 1998).

Hydraulic losses through bar racks are a function of approach

(upstream) velocity, and the velocity through the bars (downstream),

Wastewater Engineering 581

with a discharge coefficient. Referring to Fig. 6.4, Bernoulli’s equation

can be used to estimate the headloss through bar racks:

(6.28)

and

(6.29)

where h

⫽ upstream depth of water, m or ft

⫽ downstream depth of water, m or ft

h ⫽ headloss, m or ft

V ⫽ flow velocity through the bar rack, m/s or ft/s

v ⫽ approach velocity in upstream channel, m/s or ft/s

g ⫽ acceleration due to gravity, 9.81 m/s

or 32.2 ft/s

The headloss is usually incorporated into a discharge coefficient C; a typ-

ical value of C ⫽ 0.84, thus C

⫽ 0.7. Equation (6.29) becomes (for bar racks)

(6.30)

Kirschmer (1926) proposed the following equation to describe the head-

loss through racks:

(6.31)

where H ⫽ headloss, m

w ⫽ maximum width of the bar facing the flow, m

b ⫽ minimum clear spacing of bars, m

v ⫽ velocity of flow approaching the rack, m/s

H 5 Ba

4/3

sin u

h 5

0.7

2 v

h 5 h

2 h

2 v

2gC

5 h

1 ⌬h

582 Chapter 6

Figure 6.4 Profile for wastewater flowing through a bar screen.

g ⫽ gravitational acceleration, 9.81 m/s

u ⫽ angle of the rack to the horizontal

B ⫽ bar shape factor, as follows

The maximum allowable headloss for a rack is about 0.60 to 0.70 m.

Racks should be cleaned when headloss is more than the allowable

values.

Example 1: Compute the velocity through a rack when the approach veloc-

ity is 0.60 m/s (2 ft/s) and the measured headloss is 38 mm (0.15 in)

solution: Using Eq. (6.30)

Example 2: Design a coarse screen and calculate the headloss through the

rack, using the following information:

Peak design wet weather flow ⫽ 0.631 m

/s (10,000 gal/min)

Velocity through rack at peak wet weather flow ⫽ 0.90 m/s (3 ft/s)

Velocity through rack at maximum design dry weather flow ⫽ 0.6 m/s (2 ft/s)

u ⫽ 60⬚, with a mechanical cleaning device

Upstream depth of wastewater ⫽ 1.12 m (3.67 ft)

solution:

Step 1. Calculate bar spacing and dimensions

5 3.08

ft/s

V 5 0.94 m/s

5 0.882

0.038 m 5

2 s0.6 m/sd

0.7s2 3 9.81 m/sd

h 5

2 v

0.7s2gd

Bar type B

Sharp-edged rectangular 2.42

Rectangular with semicircular face 1.83

Circular 1.79

Rectangular with semicircular upstream and

downstream faces 1.67

Tear shape 0.76

Wastewater Engineering 583

(a) Determine total clear area (A) through the rack

(b) Calculate total width of the opening at the rack, w

w ⫽ A/d ⫽ 0.70 m

/l.12 m

⫽ 0.625 m

(d) Calculate number of opening, n

n ⫽ w/opening ⫽ 0.625 m/0.025 m

⫽ 25

Note: Use 24 bars with 10 mm (0.01 m) width and 50 mm thick.

(e) Calculate the width (W ) of the chamber

width ⫽ 0.625 m ⫹ 0.01 m ⫻ 24

⫽ 0.865 m

(f) Calculate the height of the rack

height ⫽ 1.12 m/sin 60⬚⫽1.12 m/0.866

⫽ 1.29 m

Allowing at least 0.6 m of freeboard, a 2-m height is selected.

(g) Determine the efficiency coefficient, EC

Note: The efficiency coefficient is available from the manufacturer.

Step 2. Determine headloss of the rack by Eq. (6.31)

Select rectangular bars with semicircular upstream face, thus

B ⫽ 1.83

w/b ⫽ 1

sinu ⫽ sin 60⬚⫽0.866

5 0.72

5 0.625 m/0.865 m

EC 5

clear opening

width of the chamber

5 0.70 m

0.631 m

0.90 m/s

A 5

peak flow

584 Chapter 6