Lin S.D. Water and Wastewater Calculations Manual

Подождите немного. Документ загружается.

Wastewater Engineering 745

Step 2. Calculate the solution feed rate

Example 3: A wastewater treatment plant having an average flow of 28,400

/d (7.5 Mgal/d) needs an average chlorine dosage of 8 mg/L. Chlorine is

applied daily via 10% NaOCl solution. The time of shipment from the vendor

to the plant is 2 days. A minimum 10-day supply is required for reserve pur-

poses. Estimate the storage tank capacity required for NaOCl solution with

a 0.03% per day decay rate.

solution:

Step 1. Determine daily chlorine requirement

Step 2. Calculate daily volume of NaOCl solution used

10% solution ⫽ 100 g/L ⫽100 kg/m

Step 3. Calculate the storage tank for NaOCl solution

Tank volume ⫽ (2.27 m

/d)(2 days ⫹ 10 days)

⫽ 27.24 m

Step 4. Calculate the tank volume with NaOCl decay correction.

Correction factor ⫽ (0.03%/day) ⫻ 12 days

⫽ 0.36%

Example 4: Calculate the residual chlorine concentration that must be

maintained to achieve a fecal coliform (FC) density equal or less than 200/

100 mL in the chlorinated secondary effluent. The influent of chlorine con-

tact tank contains 4 3 106 FC/100 mL. The contact time is 30 min.

5 28.3 m

The volume requirement 5

27.24 m

3 10%

10% 2 0.36%

5 2.27 m

Volume 5

227.2 kg/d

100 kg/m

5 227.2 kg/d

Chlorine 5

28,000 m

/d 3 8 g/m

1000 g/kg

5 0.88 gal/min

5 3.333 L/min

Feeding

rate 5

480,000 g/d

100 g/L 3 1440 min/d

746 Chapter 6

solution:

Step 1. Calculate the residual chlorine needed during the average flow using

equation (White, 1986) modified from Eq. (5.180a)

N/N

⫽ (1 ⫹ 0.23C

)

⫺3

where C

is chlorine residual at time t in mg/L

200/(4 ⫻ 10

) ⫽ (1 ⫹ 0.23 C

⫺3

5 ⫻ 10

⫺5

⫽ (1 ⫹ 0.23 C

⫺3

1 ⫹ 0.23 C

t ⫽ (0.2 ⫻ 10

)

1/3

⫽ 27.14

t ⫽ 113.7

When t ⫽ 30 min, C

⫽ 113.7/30 ⫽ 3.8 (mg/L)

Step 2. Calculate the residual chlorine needed during the peak hourly flow

Assume the ratio of the peak hourly flow to the average flow is 3.0

⫽ 3.8 mg/L ⫻ 3 ⫽ 11.4 mg/L

27.2 Dechlorination

There have been concerns that wastewater disinfection may do more

harm than good due to the toxicity of disinfection by-products. Many

states have adopted seasonal chlorination (warm weather periods) in

addition to dechlorination. Dechlorination is important in cases where

chlorinated wastewater comes into contact with fish and other aquatic

animals. Sulfur compounds, activated carbon, hydrogen peroxide, and

ammonia can be used to reduce the residual chlorine in a disinfected

wastewater prior to discharge. The first two are the most widely used.

Sulfur compounds include sulfur dioxide (SO

), sodium metabisulfite

(NaS

), sodium bisulfite (NaHSO

), and sodium sulfite (Na

Dechlorination reactions with those compounds are shown below.

Free chlorine:

⫹ 2H

O ⫹ Cl

S H

⫹ 2HCl (6.152)

⫹ H

O ⫹ HOCl S 3H

⫹

⫹ Cl

⫺

⫹ (6.153)

⫹ 2Cl

⫹ 3H

O S 2NaHSO

⫹ 4HCl (6.154)

NaHSO

⫹ H

O ⫹ Cl

S NaHSO

⫹ 2HCl (6.155)

Chloramine:

⫹ 2H

O ⫹ NH

Cl S ⫹ 2H

⫹

⫹ Cl

⫺

⫹ (6.156)

3Na

⫹ 9H

O ⫹ 2NH

⫹ 6Cl

S 6NaHSO

⫹ 10HCl ⫹ 2NH

(6.157)

3NaHSO

⫹ 3H

O ⫹ NH

⫹ 3Cl

S 3NaHSO

⫹ 5HCl ⫹ NH

(6.158)

Wastewater Engineering 747

∗

⫹ 2H

O ⫹ 2Cl

S 4HCl ⫹ C

∗

(6.159)

∗

⫹ H

O ⫹ NH

Cl S ⫹ Cl

⫺

⫹ C

∗

O (6.160)

∗

O ⫹ 2NH

Cl S N

⫹ 2HCl ⫹ H

O ⫹ C

∗

(6.161)

∗

⫹ H

O ⫹ 2NHCl

S N

⫹ 4HCl ⫹ C

∗

O (6.162)

The most common dechlorination chemicals are sulfur compounds, par-

ticularly sulfur dioxide gas, or aqueous solutions of bisulfite or sulfite.

A pellet dechlorination system can be used for small plants. Liquid

sulfur dioxide gas cylinders are in 50 gal (190 L) drums.

Sulfur dioxide is a deadly gas that affects the central nervous system.

It is colorless and nonflammable. When sulfur dioxide dissolves in water,

it forms sulfurous acid which is a strong reducing chemical. As sulfur

dioxide is introduced to chlorinated effluent, it reduces all forms of chlo-

rine to chloride and converts sulfur to sulfate. Both by-products, chlo-

rides and sulfates, occur commonly in natural waters.

Sodium metabisulfite is a cream-colored powder readily soluble in

water at various strengths. Sodium bisulfite is a white powder, or in

granular form, and is available in solution with strengths up to 44%. Both

sodium metabisulfite and sodium bisulfite are safe chemicals. These are

most commonly used in small plants and in a few larger plants.

The dosage of dechlorination chemical depends on the chlorine resid-

ual in the effluent, the final residual chlorine standards, and the type of

dechlorinating chemical used. Theoretical requirements to neutralize

1 mg/L of chlorine for sulfur dioxide (gas), sodium bisulfite, and sodium

metabisulfite are 0.90, 1.46, and 1.34 mg/L, respectively. Theoretical

values may be employed for initial estimation for sizing dechlorinating

equipment under good mixing conditions. An extra 10% of dechlorination

is designed above theoretical values. However, excess sulfur dioxide may

consume dissolved oxygen at a maximum of 0.25 mg DO for every 1 mg

of sulfur dioxide. Since the sulfur dioxide reacts very rapidly with the

chlorine residual, no additional contact time is necessary. Dechlorination

may remove all chlorine-induced toxicity from the effluents.

27.3 Process design

The design of an ozonation or ultraviolet irradiation system can usually

be provided by the manufacturers. Only chlorination is discussed in

this section.

Traditional design. The disinfection efficiency of effluent chlorination

depends on chlorine dosage, contact time, temperature, pH, and char-

acteristics of wastewater such as TSS, nitrogen concentrations, and type

and density of organisms. The first two factors can be designed for the

best performance for chlorination of water and wastewater effluent. In

748 Chapter 6

general, residual chlorine at 0.5 mg/L after 20 to 30 min of contact time

is required. The State of California requires a minimum of 30 min con-

tact time. The chlorine contact tank can be sized on the basis of contact

time and plant flow. However, many engineers disagree with this method.

Contact time is related to the configuration of the chlorine contact

tank. The drag on the sides of a deep, narrow tank causes relatively poor

dispersion characteristics. Sepp (1977) observed that the dispersion

number usually decreases with increasing length-to-width (L/W ) ratios.

The depth-to-width (D/W ) ratio should be 1.0 or less in chlorine tanks.

Adequate baffling, reduction of side drag, and elimination of dead spaces

can provide a reasonably long tank with adequate plug-flow hydraulics

(Sepp, 1981). Marske and Boyle (1973) claimed that adequate plug-flow

tanks can be achieved by L/W ratios of 40–70 to 1. In design, in addi-

tion to L/W ratios, the usual dispersion parameters and geometric con-

figurations must be considered.

Example: Determine the size of a chlorine contact tank for a design aver-

age flow of 0.131 m

/s (3 Mgal/d) and a peak flow of 0.329 m

/s (7.5 Mgal/d).

solution:

Step 1. Calculate the tank volume required at peak design flow. Select con-

tact time of 20 min at the peak flow

Volume ⫽ 0.329 m

/s ⫻ 60 s/min ⫻ 20 min

⫽ 395 m

Step 2. Calculate the volume required at the average flow

Use contact time of 30 min at the design flow

Volume ⫽ 0.131 m

/s ⫻ 60 s/min ⫻ 30 min

⫽ 236 m

Peak flow is the control factor.

Step 3. Determine chlorine contact tank configuration and dimensions

Provide one basin with three-pass-around-the-end baffled arrangement. Select

channel width W of 2.2 m (6.1 ft) and water depth D of 1.8 m (5.9 ft) with

0.6 m (2.0 ft) of freeboard; thus

D/W ⫽ 1.8 m/2.2 m

⫽ 0.82 (OK if ⬍ 1.0)

Cross-sectional area ⫽ 1.8 m ⫻ 2.2 m

⫽ 3.96 m

Total length L ⫽ 395 m

/3.96 m

⫽ 99.7 m ( say 100 m)

Length of each pass ⫽ 100 m/3 ⫽ 33.3 m

Wastewater Engineering 749

Use each pass ⫽ 33.5 m (110 ft)

Check L/W ⫽ 100 m/2.2 m

⫽ 45.5 (OK if in the range of 40 to 70)

Tank size: D ⫻ W ⫻ L ⫽ (1.8 m ⫹ 0.6 m) ⫻ (2.2 m ⫻ 3) ⫻ 33.5 m

Actual liquid volume ⫽ 1.8 m ⫻ 2.2 m ⫻ 100 m

⫽ 396 m

Step 4. Check the contact time t at peak flow

Collins–Selleck model. Traditionally, chlorination has been considered

as a first-order reaction that follows the theoretical Chick or

Chick–Watson model. However, in practice, it has been observed not to

be the case for wastewater chlorination. Collins et al. (1971) found that

initial mixing of the chlorine solution and wastewater has a profound

effect on coliform bacteria reduction. The efficiency of wastewater chlo-

rination, as measured by coliform reduction, was related to the residence

time. It was recommended that the chlorine contact tank be designed

to approach plug-flow reactors with rapid initial mixing. Collins et al.

(1971) suggested that amperometric chlorine residuals and contact time

could be used to predict the effluent coliform density.

Collins and Selleck (1972) found that the coliform bacteria survival

ratio often produces an initial log period and a declining rate of inacti-

vation as a plot of the log survival ratio versus time (Fig. 6.35a). After

the initial log period, there is a straight-line relationship between log

survival and log of the product of chlorine residual concentration C and

contact time t (Fig. 6.35b).

The rate equation of the Collins and Selleck model is

(6.163)

where N ⫽ bacteria density at time t

t ⫽ time

k ⫽ rate constant characteristic of type of disinfectant,

microorganism, and water quality

⫽ 0 for Ct ⬍ b

⫽ K for Ct ⫽ b

⫽ K/b(Ct) for Ct ⬎ b

b ⫽ log time

52kN

5 20.1 min

t 5

396 m

0.329 m

/s 3 60 s/ min

750 Chapter 6

Figure 6.35 Form of plots after Collins–Selleck model: (a) log S versus time and (b) log S

versus log Ct.

After integration and application of the boundary conditions, the rate

equation for Collins and Selleck becomes

for Ct ⬍ b (6.164)

for Ct ⬎ b (6.165)

5 a

5 1

Wastewater Engineering 751

where N ⫽ bacteria density at time t after chlorine contact

⫽ bacteria density at time zero in unchlorinated effluent

C ⫽ chlorine residual after contact, mg/L

t ⫽ theoretical contact time, min

b ⫽ an intercept when N/N

⫽ 1.0, min ⭈ mg/L

⫽ a product of concentration and time

n ⫽ slope of the curve

Equation (6.164) applies to the lag period. Equation (6.165) plots as a

straight line on log–log paper and expresses the declining rate. The

values of b and n, obtained from several researchers for various types

of chlorine applied to various stages of wastewaters with different bac-

teria species, are summarized by J.M. Montgomery Engineers (1985).

Wide variations of those values were observed. Collins and Selleck

(1972) found that the slope n ⫽⫺3 for ideal plug flow, n ⫽ ⫺1.5 for

complete-mix flow, and b was about 4 in a primary effluent.

Robert et al. (1980) stated that the Collins–Selleck model is a useful

empirical design tool although it has no rational mechanistic basis in

describing chemical disinfection. Sepp (1977) recommended that the

design of chlorine disinfection systems should incorporate rapid initial

mixing, reliable automatic chlorine residual control, and adequate chlo-

rine contact in well-designed contact tanks.

Example: A pilot study was conducted to determine the efficiency of combined

chlorine to disinfect a secondary effluent. The results show that when b ⫽ 2.87

min ⭈ mg/L and n ⫽ –3.38, the geometric mean density of fecal coliform (FC)

in the effluent is 14,000 FC/100 mL. Estimate FC density in the chlorinated

effluent when contact times are 10, 20, and 30 min with the same chlorine

residual of 1.0 mg/L.

solution:

Step 1. Determine mean FC survival ratios

Ct ⫽ 1.0 mg/L ⫻ 10 min

⫽ 10 min ⭈ mg/L (⬎b ⫽ 2.87)

Using Eq. (6.165)

(1) At t ⫽ 10 min

5 0.0147

5 a

1.0 mg /L 3 10 min

2.87 min

mg/L

23.38

752 Chapter 6

(2) At t ⫽ 20 min

(3) At t ⫽ 30 min

Step 2. Calculate FC densities in chlorinated effluent

(1) At t ⫽ 10 min

N ⫽ 0.0147 ⫻ N

⫽ 0.0147 ⫻ 14,000 FC/100 mL

⫽ 210 FC/100 mL

(2) At t ⫽ 20 min

N ⫽ 0.00141 ⫻ 14,000

⫽ 20(FC/100 mL)

(3) At t ⫽ 30 min

N ⫽ 0.000359 ⫻ 14,000

⫽ 5(FC/100 mL)

28 Advanced Wastewater Treatment

Advanced wastewater treatment (AWT) refers to those additional treat-

ment techniques needed to further reduce suspended and dissolved sub-

stances remaining after secondary treatment. It is also called tertiary

treatment. Secondary treatment removes 85% to 95% of BOD and TSS and

minor portions of nitrogen, phosphorus, and heavy metals. The purposes

of AWT are to improve the effluent quality to meet stringent effluent stan-

dards and to reclaim waste-water for reuse as a valuable water resource.

The targets for removal by the AWT process include suspended solids,

organic matter, nutrients, dissolved solids, refractory organics, and spe-

cific toxic compounds. More specifically, the common AWT processes are

suspended solids removal, nitrogen control, and phosphorus removal.

Suspended solids are removed by chemical coagulation and filtration.

Phosphorus removal is done to reduce eutrophication of receiving waters

from wastewater discharge. Ammonia is oxidized to nitrate to reduce its

toxicity to aquatic life and nitrogenous oxygen demand in the receiving

water bodies.

5 0.000359

5 a

1.0 3 30

2.87

23.38

5 0.00141

5 a

1.0 3 20

2.87

23.38

Wastewater Engineering 753

28.1 Suspended solids removal

Treatment techniques for the reduction of suspended solids in second-

ary effluents include chemical coagulation followed by gravity sedi-

mentation or dissolved air flotation, and physical straining or filtration

such as wedge-wire screens, microscreens, other screening devices,

diatomaceous earth filters, ultrafiltration, and granular media filters.

These treatment processes are discussed in Chapter 5. More detailed

descriptions and practical applications for SS removal are given in the

US EPA’s manual (1975a). Chemical coagulants used for SS removal

include aluminum compounds, iron compounds, soda ash, caustic soda,

carbon dioxide, and polymers. Some of the above processes are described

in Chapter 5 and in other textbooks.

The use of a filtration process similar to that employed in drinking

water treatment plant can remove the residual SS, BOD, and microor-

ganisms from secondary effluent. Conventional sand filters or

granular-media filters for water treatment may serve as advanced

wastewater treatment and are discussed by Metcalf and Eddy, Inc.

(1991). However, the filters may require more frequent backwashing.

Design surface loading rates for wastewater filtration usually range

from 2.0 to 2.7 L/(m

⭈ s) (3 to 4 gal/(min ⭈ ft

)). Filtration lasts about

24 h, and the effluent quality expected is 5 to 10 mg/L of suspended

solids.

28.2 Phosphorus removal

The typical forms of phosphorus found in wastewater include the

orthophosphates, polyphosphates (molecularly dehydrated phosphates),

and organic phosphates. Orthophosphates such as , , ,

are available for biological uptake without further breakdown.

The polyphosphates undergo hydrolysis in aqueous solutions and revert

to the orthophosphate forms. This hydrolysis is a very slow process.

The organic phosphorus is an important constituent of industrial wastes

and less important in most domestic wastewaters.

The total domestic phosphorus contribution to wastewater is about

1.6 kg per person per year (3.5 lb per capita per year). The average total

phosphorus concentration in domestic raw wastewater is about 10 mg/L,

expressed as elemental phosphorus, P (US EPA, 1976). Approximately

30% to 50% of the phosphorus is from sanitary wastes, while the remain-

ing 70% to 50% is from phosphate builders in detergents.

Phosphorus is one of major contributors to eutrophication of receiv-

ing waters. Removal of phosphorus is a necessary part of pollution pre-

vention to reduce eutrophication. In most cases, the effluent standards

range from 0.1 to 2.0 mg/L as P, with many established at 1.0 mg/L.

Percentage reduction requirements range from 80% to 95%.

HPO

754 Chapter 6

Chemical precipitation. Phosphorus removal can be achieved by chemi-

cal precipitation or a biological method. In practice, phosphorus in waste-

water is removed by chemical precipitation with metal salt (aluminum

or iron) or/and polymer, or lime. The chemical can be dosed before pri-

mary sedimentation, in the trickling filter, in the activated sludge

process, and in the secondary classifier. Aluminum ions can flocculate

phosphate ions to form aluminum phosphate which then precipitates:

3⫹

⫹ : AlPO

p ⫹ nH

⫹

(6.166)

Alum, Al

(SO

)

⭈ 14H

O, is most commonly used as a source of alu-

minum. The precipitation reactions for phosphorus removal by three

chemical additions are as follows:

Alum:

(SO

)

⭈ 14H

O ⫹

(6.167)

Sodium aluminate:

O ⭈ Al

⫹ 2HPO

⫺

⫹ 4H

O S 2AlPO

T ⫹ 2NaOH ⫹ 6OH

⫺

⫹2H

⫹

(6.168)

Ferric chloride, FeCl

⭈ 6H

FeCl

⫹ HPO

⫺

S FePO

T ⫹ H

⫹

⫹ 3Cl

⫺

(6.169)

Lime CaO:

CaO ⫹ H

O S Ca(OH)

(6.170)

5Ca(OH)

⫹ 3HPO

⫺

S Ca

(PO

)

OH T ⫹ 3H

O ⫹ 6OH

⫺

(6.171)

Alum and ferric chloride decrease the pH while lime increases pH. The

optimum pH for alum and ferric chloride is between 5.5 and 6.0. The

effective pH for lime is above 10.0. Theoretically, 9.6 g of alum is required

to remove 1 g of phosphorus. However, there is an excess alum require-

ment due to the competing reactions with natural alkalinity. The alka-

linity reaction is

(SO

)

⭈ 18H

O ⫹ 6HCO

⫺

S 2Al(OH)

T ⫹ 3SO

⫺

⫹ 6CO

⫹ 18H

(6.172)

As a result, the requirement of alum to remove phosphorus demands

weight ratios of approximately 13:1, 16:1, and 22:1 for 75%, 85%, and

95% phosphorus removal, respectively (from Eqs. (6.167) and (6.172)).

2HPO

2AlPO

T 1 2H

1 3SO

1 14H

32,n1