Wastewater - Secondary

7/27/2019 Wastewater - Secondary

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CIV3264:URBAN WATER AND WASTEWATER SYSTEMSTopic 4: Wastewater Secondary, Tertiary Treatment and Sludge Processing

Department of Civil Engineering, Monash University

CIV3264 Revised 5/2012

0

Topic 4: Secondary & Tertiary Treatment andSludge Processing

TABLE OF CONTENTS

Page No

4.1 SECONDARY TREATMENT 1

4.1.1 Microorganisms and pollutant decompositions 1

4.1.2 Activated sludge treatment 2

4.1.3 Trickling filters 12

4.1.4 Secondary clarifiers 16

4.1.5 Rotating biological contactors 18

4.1.6 Oxidation ponds 19

4.1.7 Disinfection 20

4.2 TERTIARY TREATMENT AND EFFLUENT DISPOSAL 21

4.2.1 Nutrient removals 21

4.2.2 Filtration 25

4.2.3 Carbon adsorption 26

4.2.4 Effluent disposal 26

4.2.5 Alternative treatment processes 31

4.3 SLUDGE TREATMENT AND DISPOSAL 37

4.3.1 Thickening 37

4.3.2 Digestion 384.3.3 Conditioning 39

4.3.4 Dewatering 40

4.3.5 Reduction 40

4.3.6 Sludge disposal 41

CHAPTER REVIEW 42

CONTENTS


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4.1 SECONDARY TREATMENT

Learning Outcome: (1) knowing the role of microorganisms in secondary wastewater treatment and the

concept of aerobic and anaerobic decomposition; (2) being able to carry out basic calculations for the designof an activated sludge facility for the treatment of typical domestic wastewaters; (3) understanding the

working principles of trickling filters, rotating biological contactors and oxidation ponds, and the concept of

effluent disinfection.

Secondary treatment usually consists of biological treatment and a secondary clarifier

(Figure 4.1). Secondary treatment is often referred to as biological treatment because it

uses microorganisms to remove organic materials/pollutants (represented by BOD) from

wastewater.

The basic conditions needed for the biological treatment are: (1) the availability ofmicroorganisms; (2) sufficient contact between microorganisms and organic materials in

the wastewater; (3) the availability of oxygen; and (4) the maintenance of favourable

environmental conditions (appropriate temperature, sufficient time for microorganisms to

function, etc).

Figure 4.1: Typical Secondary treatment process

A number of biological wastewater treatment techniques are available. The most common

types are:

1. Activated sludge2. Trickling filters3. Rotation biological contractor (RBC)4. Oxidation ponds (or lagoons)

These treatment facilities will be introduced in this chapter.

Primary

clarifier

Secondary

clarifierSecondary treatment unit

Sludge

Sludge


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4.1.1 MICROORGANISMS AND POLLUTANT DECOMPOSITIONS

The most basic ingredient needed for conventional aerobic biological treatment is the

availability of sufficient amounts of microorganisms. The decomposition of organic

pollutants is accomplished as the microorganisms convert colloidal and dissolved organicmatter into gases, water and other smaller molecules.

Most microorganisms operate primarily under either aerobic or anaerobic condition (but

some types of microorganisms have metabolism pathways under both aerobic and

anaerobic conditions). Accordingly, in a wastewater treatment facility, the decompositions

of organic pollutants, which results in BOD reduction, can be achieved via two routes:

aerobic or anaerobic.

Aerobic decomposition. The basic equation of aerobic decomposition is:

Complex Organics + O2 CO2 + H2O + Stable Products (4.1)

Carbon dioxide and water are always two of the end products of aerobic decomposition,

while the other products can be organic materials for the cells of microorganisms, and

stable sulphur, phosphorus and nitrogen products.

Anaerobic decomposition. The second route of degradation is anaerobic, performed by a

different set of microorganisms, to which oxygen is often toxic. The basic expression of

anaerobic decomposition is:

Complex Organics CO2 + CH4 + Partially Stable Products (4.2)

Exercise 4.1 (2 minu tes)Understanding the role of microorganisms

(1) Under what type of condition (aerobic or anaerobic) are most secondary

treatment facilities operated?

(2) Where could the microorganisms be located in a secondary wastewater

treatment facility?

Box 4.1: Microorganisms involved in wastewater treatment

Some microorganisms of interest in wastewater treatment include: bacteria, fungi, algae and protozoa.The dominance of any groups of microbes in a treatment facility depends on the characteristics of the

wastewater and operating condition of the facility. No particular species can be considered as the best.

The microorganisms can be classified in different ways. Further information, such as the classification

and pollutant bio-removal pathways, will be introduced in a Level 4 Module, CIV4268 Water Resource

Management.


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4.1.2 ACTIVATED SLUDGE TREATMENT

Process Description

The activated sludge process is a biological wastewater treatment technique in which a

mixture of wastewater and microorganisms (biological sludge) is agitated and aerated. Asthe microorganisms grow, the individual organisms clump together (flocculate) to form an

active mass of microbes (flocs), called activated sludge.

The basic compounds of an activated sludge facility are: (i) aeration tank, (ii) air supply,

(iii) secondary clarifier, and (iv) sludge recirculation. These components are presented in

Figure 4.2. In practice, wastewater flows continuously into an aerated tank (Figure 4.2),

where air is injected to mix the activated sludge with the wastewater and to supply the

oxygen needed to break down the organic matter. As wastewater flows continuously into

the aeration tank, compressed air is injected through diffusers in the bottom of the tank, or

mechanical aerators entrapping atmospheric air. The air mixes with activated sludge in the

wastewater and supplies the oxygen needed for the organisms to break down the organics.The mixture of wastewater and activated sludge is called mixed liquor.

(a)

(b)

Figure 4.2: A typical activated sludge wastewater treatment process

The mixed liquor flows from the aeration tank to secondary clarifier where the activated

sludge is settled out (where microorganisms are starving). Most of the activated sludge

is returned to the aeration tank and hence is called return activated sludge (RAS) to

maintain high population of microbes that permits rapid breakdown of the organic matter,but a portion of the sludge is diverted to disposal units for treatment and disposal.

Secondary

clarifier

An aerated tank - plan view

Wastewater feed

Sludge

An aerated tank - plan view

Returned activated sludge


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Usually, more activated sludge is produced in the aerated tank than needed, and the

activated sludge treatment is controlled by wasting a portion of the microorganisms each

day in order to maintain the proper amount of microorganisms to efficiently degrade BOD.

Wasting means a portion of the microorganisms is discarded from the process. The

discarded microorganisms are called waste activated sludge (WAS).

In conventional activated sludge systems, wastewater is typically aerated for 6-8 hours in

long, rectangular aeration basins. Sufficient air is provided to keep the sludge in

suspension. About 8 m3 of air is provided for each cubic meter of wastewater treated.

Figure 4.3 shows the schematic diagram of a conventionally activated sludge treatment

plant, and Figure 4.4 gives the photo of an operating aeration tank.

Figure 4.3: Process diagram of a typical activated wastewater treatment plant

Figure 4.4: A typical aeration tank


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Process Calculation

The activated sludge process was first used in Manchester, England. It is now perhaps the

most widely used process in secondary wastewater treatment. A number of activated

sludge processes and design configurations have evolved since its early conception (dated

back to the early 1880s). In present process calculation for a completely mixed aerationtank, the main parameters involved are shown in Figure 4.5.

Parameters of Aeration Tank L oadings

Figure 4.5: Parameters of aeration tank loadings (sludge can be wasted from either the

tank or the return sludge line).

Descriptions of Basic Parameters

Hydraulic detention time (aeration time), [day] represents the average time thewastewater being aerated in the aeration tank:

Q

V (4.3)Vvolume of the aeration tank [m3]

Qwastewater inflow into the tank [m3

/day]

The value of varies between 3-30 h (4-8 h recommended for conventional systems)

BOD loading (or BOD load) [g/(m3 day)] is mass of BOD applied per day per unit

volume of liquid in the aeration tank:

V

QSloadBOD 0 (4.4)

S0concentration of BOD5 in the wastewater inflow (into the tank) [g/m3]

BOD load is typically in the range of0.3 to 3 kg BOD5, per m3 per day.

AERATION TANK

Vvolume [m3]

Xvolatile SS in tank [g/m3]

Sdissolved BOD5 in tank [g/m3]

MLSSmixed liquor SS [g/m3]

Qinflow rate[m3/day]

S0BOD5 in

inflow [g/m

3

]

X0volatile SS in

the influent [g/m3]

SETTLING

TANK

Qrreturn sludge flow [m /day]Sdissolved BOD5 in effluent [g/m

3]

Xrvolatile SS in return sludge [g/m3]

SSrSS in return sludge [g/m3]

Q + Qr

Qw- excess sludge

wasted [m3/day]

Q-Qweffluent

rate [m3/day]

Sdissolved

BOD5 in the

effluent [g/m3]

Xevolatile SSin effluent [g/m

3]

SSeSS in

effluent [g/m3]


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The food to microorganisms ratio, F/M [day-1], is a ratio between BOD5 (food)

input and the mass of microorganisms (represented by volatile SS) in the system:

X

S

VX

QSM/F 00 (4.5)

X [g/m3]concentration of volatile suspended solids in the aeration tank

F/M is a commonly used parameter in regulating the performance of the activated

sludge treatment process. Typical value for F/M = 0.05~1 day-1.

Exercise 4.2 (5 minu tes)Calculation of F/M ratio

An activated sludge tank has an influent BOD5 concentration of 140 mg/L,influent flow rate 18900 m3/d, and 16100 kg of suspended solids (SS) under

aeration in the tank. Assuming 80% of the SS are volatile suspended solids,

what is the value of F/M in this tank?

Answer: d-1

Specific Utilisation Ratio, U [day-1], is the percentage of F/M that has been utilised

in the tank:

100

EM/FU

E [%]is the tank efficiency in terms of BOD removal: 100S

SSE

0

0

X

r

X

SS

S

SS

X

S

100

EM/FU su0

0

00 (4.6)S [g/m3] - concentration of BOD in effluent in the tank (and leaving the tank)

rsu [g/(m3 day)] - BOD utilisation rate:

SSr 0su

Sludge age, c[day], (ormean cell residence time, orsolids retention time - SRT)

is mean residence time of microorganisms in the system (due to sludge recirculation

it is higher than ). It s a measure of the average detention of the organisms in the

system, typically ranging from 6 to 15 days.

ewrw

cXQQXQ

XV(4.7)

X [g/m3]volatile suspended solids in the tank

Xe [g/m3]volatile suspended solids in effluent

Qw [m3/day]flow rate of the sludge wasted from the tank


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In a very well designed and operated settling unit (secondary clarifier), the mass of

microorganisms (activated sludge) flowing out with the effluent, Xe(Q-Qw), should be

very small and could be neglected. Then, Equation 4.7 becomes:

rwc XQ

XV

(4.8a)

Xr[g/m3]volatile suspended solids in return activated sludge

If sludge age ( c) is known, the amount of sludge wastewater can be calculated as:

rcw

X

XVQ (4.8b)

Net rate of bacteria growth in the tank, rg[g/m3 day], represents the increase in

the mass of microorganisms (i.e. volatile SS) in the aeration tank, as a result of food(BOD in the wastewater) and oxygen supplies for microbial activities.

XkYrr dsug (4.9)rsu [g/(m

3 day)]substrate (BOD) utilisation rate

Y [g/g]maximum yield coefficient in bacterial growth

kd [day-1]endogenous decay coefficient for the microorganisms

Sludge Return, Qr[m3/day]. The purpose of sludge return is to maintain a sufficient

population of microorganisms in the aeration tank. The pumping rate for sludgereturn can be determined from a mass balance around the settling tank in Figure 4.5;

assuming that the amount of sludge in the settling tank is constant (steady-state

conditions) and that suspended solids in the effluent are negligible (SSe=0):

rwrrr SSQSSQMLSSQQ0 (4.10a)

Solving for the return sludge flow from the mass balance we obtain:

MLSSSS

SSQMLSSQQ

r

rwr (4.10b)

Microorganism Mass Balance

A mass balance for microorganisms in the entire activated sludge system (including tank

and clarifier) can be written as:

Rate of

microorganism

accumulation in

the system

= Rate of

inflow of

organisms

- Rate of outflow of

microorganisms

+ Rate of net growth

of microorganisms

in aeration tank

V(dX/dt) = QX0 - QwXr+ (Q-Qw)Xe + rgV


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Expressed in mathematical form:

ewrwg0 XQQXQVrQXdt

dXV (4.11)

dX/dt [g/(m

3

day)]rate of change of microorganism concentration in the systemX0 [g/m

3]microorganism (i.e. volatile SS) concentration in the influent

Under steady-state operating conditions (dX/dt=0) and assuming no microorganisms in the

influent (X0=0), Equation 4.11 is combined with Equation 4.9 to give:

dsuewrw kX

rY

XV

XQQXQ(4.12)

The left-hand side of Equation 4.12 is the inverse of the mean cell residence time (or

sludge age) as defined in Equation 4.7. Therefore:

dsu

c

kX

rY

1(4.13a)

or dc

kYU1

(4.13b)

Process Design

Combining Equation 4.13 and the definition of rsu, the following equation is developed for

the calculation of aeration tank volume:

cd

0c

k1X

SSQYV (4.14)

In a design situation, parameters Q (daily sewage flow), S0 (wastewater BOD5 value), S

(treatment standard) are usually provided with the design problem. Kinetic coefficients

(i.e. Y, kd, etc) vary from site to site, depending on factors such as wastewater

characteristics, climate, etc. Typical kinetic coefficients for activated sludge facilities in

domestic sewage treatment are given in Table 4.1.

Table 4.1: Typical kinetic coefficients for domestic wastewater treatment in an activated sludge facility

Coefficients Range Typical value

Kd, day-1

0.025-0.075 0.06

Y, g VSS/g BOD5 0.4-0.8 0.6

The values of X and c are related to other parameters, such as the food-to-microorganism

ratio; appropriate values of these parameters should be determined by designers (civil and

environmental engineers), as a function of the type of activated sludge process selected

and economic factors. A summery of loadings and operation parameters for different

activated sludge processes is presented in Table 4.2:


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Table 4.2: Typical loadings and operation parameters in different aerated sludge processes

Process BOD load

[g/(m3

day)]

F/M

[day-1

]

Sludge age

c

[day]

Aeration

time,

[hour]

Return

Sludge Rate,

Qr/Q [%]

BOD removal

efficiency, E

[%]

Conventional 300-700 0.2-0.6 5-15 6-7.5 20-40 80-90

Step aeration 480-800 0.2-0.5 5-15 5-7 30-50 80-90

Contact

stabilisation

480-800 0.2-0.5 5-15 6-9 50-100 75-90

High rate >1280 0.5-2 3-10 1.5-3.5 50-100 70-85

High purity

oxygen

>1920 0.6-1.5 3-10 1-3 30-50 80-90

Extended

aeration

160-320 0.05-0.2 >20 20-30 50-100 85-95

A process is selected according to required removal efficiency of BOD (E). One of the

parameters is picked, and others are then calculated. All parameters should be in the given

ranges. Note:The process is usually designed based on Average Dry Weather Flow.

Example 4.1

Calculation of aeration tank for an activated sludge process

A domestic sewage after primary treatment has an inflow rate, Q=50000 m3/day and a

BOD5 concentration of S0 = 140 mg/l. Design the aeration tank and estimate the sludgeage and return sludge flow for a step aeration activated sludge process, provided that

there are negligible amounts of microorganisms in the influent and the required

treatment standard is: BOD5 concentration in the effluent S=20 mg/l.

It is known that: (1) SS in return sludge SS r = 10000 mg/l (80 % is volatile); (2) mixed

liquor suspended solids MLSS = 3000 mg/l (80 % is volatile); and (3) the typical values

of kinetic coefficients in Table 4.1 provide appropriate kinetic constants for the

calculation.

Solution:

Tank efficiency: %86100140

20140100

S

SSE

0

0

Volatile SS (microorganisms) in the tank: L/mg240030008.0X

Select: F/M = 0.4 day-1

]m/g[2400V

]day/m[50000]m/g[140]day[4.0:Thus

XV

QSM/F

3

3310

Tank volume: V = 7292 m3


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Aeration time: hr5.3d1458.050000

7292

Q

V 1 This is too short in comparison with the recommended values in Table 4.2. Try another

F/M!

Select: F/M = 0.25 day-1

V = 11667 m3

= 5.6 hr this is fine

BOD loading: dmg600V

QSloadBOD 30 this is fine

Sludge agecan be calculated from Equation 4.12 as well as typical kinetic coefficients of

Y=0.6 gVSS/gBOD5 and Kd = 0.06 d-1:

%5.21100

EM/FU

069.006.06.0215.0kYU1

dc

c = 14.5 days fine

Wasted sludge flow: dm241100005.14

300011667

SS

MLSSV

X

XV

Q3

rcrcw

Return sludge flow: dm21084300010000

10000241300050000

MLSSSS

SSQMLSSQQ 3

r

rwr

%4250000

21084

Q

Qr fine for step aeration

* Note that a volume of 11667 m3 is too big for a typical aeration tank. Three tanks could

be designed, each having a volume of 3900 m3 (Depth = 4 m, Width =18 m, Length =54

m).

Sludge Production

The amount of sludge (bacterial cell materials will eventually become sludge) produced in

an aerated sludge treatment facility needs to be calculated because it is directly related to

the sludge handling and disposal. The amount of sludge that must be wasted each day is

the difference between the amount of increase in sludge mass and solid loss in the

wastewater. Net sludge produced per day in a treatment facility, Px [kg/day], is:

SSQYP 0obsx (4.15)


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Px [kg/day]net waste activated sludge produced each day in terms of VSS

Yobs [kg/kg]observed yield (kg VSS / kg BOD5 removed)

Yobs is related to other parameters in that:

cd

obsK1YY (4.16)

c[day]mean cell residence time

kd [day-1]endogenous decay coefficient for the microorganisms

Y [g/g]yield coefficient in bacterial growth (dismal fraction of food mass converted to

biomass)

Oxygen Requi rements and Transfer

Oxygen is consumed by aerobic microorganisms for the decomposition of carbonaceous

pollutants (represented by BOD5). The amount of oxygen requirement in an aeration tankcan be estimated by:

x

30

2OP42.1

f

10SSQM (4.17)

MO2[kg/day]daily oxygen consumption rate

Px [kg/day]net waste activated sludge produced each day in terms of VSS

fa conversion factor from BOD5 to ultimate BODL (typical value = 0.68)

The conversion factor (f = BOD5/BODL) can be calculated from the following equation ifthe reaction constant of BOD decomposition (k, unit day-1) is known.

k5e1f (4.18)

When calculating the air supply requirement, students should be aware of the composition

of air, as well as the transfer efficiency of oxygen from air to wastewater (see below).

Other factors to be considered in process design

Transfer effi ciency

To calculate the total amount of air that should be pumped into the tank, the following

should be taken into account:

1 m3 of air contains 0.279 kg of O2.

The transfer of O2 from air to water has a typical efficiency of about 10%.

To meet peak organic loadings it is often recommended that the aeration equipmentis designed with a safety factor (for example, a safety factor of 2.0).

Nutri ent requi rements

For a biological wastewater treatment system to function properly, nutrients (for the

microorganisms) must be available in adequate amounts. Principal nutrients are nitrogen

and phosphorous. Other nutrients include: sodium, potassium, calcium, magnesium and

chlorine. Trace quantities of iron, copper, manganese, zinc, and cobalt are also necessary.


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Envir onmental r equir ements

The two most important factors are temperature and pH. Temperature affects biological

reaction rates and therefore alters the volume of the aeration tank required. The control of

pH is often required, i.e. typically greater than 6.2 and preferably in the range 6.6 to 7.6. Inaddition, the alkalinity of wastewater is reduced in some biological reactions (e.g.

nitrification of ammonia into nitrite and nitrate) which needs to be controlled.

The character istics of tr eated effl uent

Organic content is a major parameter of effluent quality. The organic content of effluent

from biological treatment is usually composed of the following three constituents:

(i) soluble biodegradable organics (escaped treatment; cell depth, etc)

(ii) suspended organic material (colloidal organic solids)

(iii) non-biodegradable organic matters (originally present in the wastewater, by-products of biological degradation)

In a well-designed and operated activated sludge plant treating domestic wastewater, the

soluble carbonaceous BOD5 in the effluent typically ranges between 2-10 mg/l.

4.1.3 TRICKLING FILTERS

Trickling filters (as shown in Figures 4.6 and 4.7) can be used either as a complete

secondary treatment facility or as a means of reducing the biological load on some other

treatment processes.

A trickling filter consists of a bed of coarse material (stones, slats, or plastic media) over

which wastewater is applied (usually by a rotating arm), as shown in Figure 4.6. As the

wastewater trickles through the bed, microbial communities are being established in the

form of biofilms on the surface of the stone (media grains). The passing of wastewater

provides contact between the microorganisms and organic pollutants in the wastewater,

and biological degradations allow the pollutants to be removed from the wastewater.

Figure 4.6: Cutaway view of a stone-media trickling filter


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Figure 4.7: Photo of an operating trickling filter

Trickling filters do not just provide a filtering process. The bed provides a large amount of

surface area where microorganisms cling and grow in slimes on rocks and feed on organic

the matter contained in the wastewater. The effluent from trickling filters contains high

level of SS (washed from the bed) and should be clarified in a secondary sedimentation. A

complete trickling filter plant flow diagram is shown in Figure 4.8.

Figure 4.8: Schematic layout of a trickling filter wastewater treatment plant

Disinfectant(chlorine)

Treated

effluent

Fine bar

screens

Primary

sedimentation

tank

Raw

Grit chamber

sewage

disposal

screenings grit scum

Primary sludge

Primary treatment

Anaerobic

digestion

Sludgethickening

Belt press or

sludge lagoon

Sludge disposal

Sludge treatment

Secondary

sedimentation

tank

Secondary treatment

High rate

trickling filter

Recycled flow

Q, So

Qr

Q, S


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In an operating trickling filter, the recirculation of a portion of the effluent is often

practised in stone filters for the following reasons: (i) to increase contact efficiency, (ii)

to dampen variations in loading, (iii) to raise the DO value in the effluent, (iv) to improve

distribution over the filter surface, and (v) to prevent biofilms to dry.

The main filter characteristics are:

daym

m

A

QQ

areaFilter

volumesewegeDaily=rate)loading(surfaceloadingHydraulic

2

3r

Q [m3/d]wastewater inflow

Qr[m3/d]recalculated flow

A [m2]the horizontal cross-section area of the filter

Recirculation ratio,

Q

QR r

daym

kg

V

SQ

volumeFilter

loadBODDaily=loadingOrganic

3

0

S0 [kg/m3]BOD5 concentration in influent (primary effluent)

V [m3]superficial volume of the filter medium

Trickling filters are classified according to applied hydraulic and organic load, which are

considered in system design, as shown in Table 4.3.

Table 4.3: Design guidance for trickling filter

Design

characteristics

Low or

standard rate

Intermediate

rate

High rate

(stone media)

Super rate

(plastic media)

Hydraulic loading[m/d]

1 to 4 4 to 10 10 to 40 40-200

Organic loading[kg BOD5/m

3d]

0.08 to 0.32 0.24 to 0.48 0.32 to 1.0 0.8 to 6.0

Depth, [m] 1.5 to 3 1.5 to 2.5 1 to 2 4.5-12

Recirculation ratio 0 to 1 0 to 1 1 to 3 0 to 4

Filter media Rock, slag Rock, slag Rock, slag,synthetic material

Synthetic materialredwoods

Power requirements[kW/10

3m

3]

2-4 2-8 6-10 10-20

Filter flies many Varies Few Few or none

Effluent quality Well nitrified Somenitrification

Nitrified at lowloadings

Nitrified at lowloadings

BOD5 removal, % 80 to 85 50 to 70 65 to 80 65 to 85


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An ideal bed material should have:

(i) large surface area on which life can grow

(ii) large enough voids to allow growth without clogging

(iii) a structure which distributes the sewage over the whole bed to facilitate oxygen

transfer

(iv) The materials in current use are: rocks 25-100 mm in diameter (clogging may

occur), modules of corrugated plastic sheets and plastic rings (large surface and

voids; light)

Table 4.4 gives the physical properties of several filter media commonly used in trickling

filters.

Table 4.4: Physical properties of several filter media

Media Nominal size[mm]

Mass/unitvolume

[kg/m3]

Specificsurface area

[m2/m

3]

Void space[%]

River rock

Small 25-65 125-1450 55-70 40-50

Large 100-120 800-1000 40-50 50-60

Blast furnace slag

Small 50-80 900-1200 55-70 40-50

Large 75-125 800-1000 45-60 50-60

PlasticConventional 600 600 1200 30-100 80-100 94-97

High-specific

surface600 600 1200 30-100 100-200 94-97

Redwood 1200 1200 1200 150-175 40-50 70-80

Design Formul as

The design of trickling filters is based on the reaction kinetics involved (instead using only

the loads described in Table 4.3). Unfortunately, no universal equation is available. The

most widely consulted literature source for system design is Metcalf and Eddy series of

Wastewater Engineering. Several editions of this book are available in the Universitylibrary.

Exercise 4.3 (5 minu tes)Understanding trickling filters

(1) Where are microorganisms located in trickling filters?

(2) How does the size of medium material affect the treatment of wastewater

in a trickling filter?


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4.1.4 SECONDARY CLARIFIERS

Secondary clarifiers or sedimentation tanks (Figure 4.9) settle out the treated effluent

(from activated sludge tanks or trickling filters). At this stage, BOD can be further reduced

and most suspended solids are removed from the effluent.

Figure 4.9: Secondary sedimentation tank

Most secondary sedimentation tanks are circular, but they can also be in rectangular or

square shape. Figure 4.10 demonstrates a typical design.

Figure 4.10: Schematic diagram of a secondary sedimentation tank

Secondary clarifiers are designed using the same procedure as for primary clarifiersthat have been explained in a previous section. Some design guidelines used in the USA

are outlined in Table 4.5.


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Table 4.5: Typical Design Guidelines for Circular Secondary Clarifiers

Parameter Value

Detention time

For peak flow 1.5 and 2.5 hoursSurface loading rate

For average dry weather flow

For peak flow conditions

16 - 29 m3/m2/day

41 - 65 m3/m2/day

Sidewater depth 3.0 - 5.5 m

Floor slope Nearly flat to 1:12

Maximum diameter 46 m

Example 4.2

Determine the size of two identical circular final clarifiers, operating in parallel for an

activated sludge system, with a design flow of 20,000 m3/day, and a peak hourly flow of

32,000 m3/day. The maximum surface loading rate is 33 m3/m2/day at design flow, and

65 m3/m2/day at peak flow. The minimum requirement for detention time at design flow

is 2 hours.

Solution

At design flow, surface area required for each tank 2

23

3

m303/day/mm332

/daym20,000

Check peak surface loading rate for each tank:

3032

000,32/day/mm53 23 /day/mm65 23 (this is fine)

Tank diameter: 3034

D2

21

4303D m6.19

Detention time :rateFlow

volumeTank hours2

rateFlow

DepthArea

24303

2

20,0002

>Depth m75.2

Make the depth 3.5 m (recommended for tank diameter > 15m, the depth is typically

around 3.4 m); this will also give a reasonable detention time at peak flow rate.


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4.1.5 ROTATING BIOLOGICAL CONTRACTOR

Rotating biological contactors (RBC) came to be used commercially in wastewater

treatment in the 1960s. An RBC, as shown in Figure 4.11, is constructed of a series of

closely spaced circular disks (typically 3 to 3.5m in diameter) attached to a shaft, forminga cylinder of media. The disks are submerged in wastewater and rotated through it.

Figure 4.11 Photo of a rotating biological contactor

In operation, microorganisms grow on the surface of the disks and eventually form a layer

of biofilm. The rotation alternately contacts the biofilm with pollutants in the wastewater,

and then exposes the surface of the disks to the atmosphere; this affects the oxygen

transport and maintains the biofilm in an aerobic condition, enabling aerobic

decomposition of the pollutants to take place. The rotation is also the mechanism for

removing excess solids from the disks by the shearing forces it creates. Rotating biological

contactors are good for small groups of houses (camp-site). Typical efficiencies of the

systems are:

BOD removal rate = 90% or higher

SS removal rate = 85% or higher

Running cost 1/2 of activated sludge method

Figure 4.12: RBC with plastic discs Figure 4.13: A RBC treatment plant


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4.1.6 OXIDATION PONDS

A photo of a typical wastewater pond is presented in Figure 4.14. Screened and de-gritted

wastewater is passed through a series of ponds with total retention in the range of 10-50

days. Microorganisms in the ponds oxidise pollutants and work symbiotically with algaethat provide oxygen to the microorganisms through photosynthesis. In return, the algae

obtain CO2 for cell synthesis.

Figure 4.14: A photo of treatment ponds

The term oxidation pond is used as a collective term for the following types of ponds:

1. Aerobic ponds: Shallow ponds, less than 1 m in depth, where DO is maintained

throughout the entire depth, mainly by the action of photosynthesis (during the day)

and by wind mixing (surface re-aeration). The decomposition of organic pollutants is

accomplished mainly through the action of aerobic bacteria. Aerated lagoonsare types

of aerobic ponds, which are oxygenated through the action of surface or diffused air

aeration.

2. Anaerobic ponds:Deep ponds that receive high organic loading such that anaerobic

conditions prevail throughout the entire pond depth: applied BOD load exceeds

oxygen production from photosynthesis. Proper design of these ponds must result in

environmental conditions favourable to methane fermentation. They are used primarily

as a pre-treatment process, and are particularly suited for the treatment of high-

temperature, high-strength wastewater.

3. Facultati ve ponds:These are ponds that are 1 to 2.5 m deep and have an anaerobic

lower zone, a facultative middle zone, and an aerobic upper zone maintained by

photosynthesis and re-aeration. The treatment mechanism is outlined in Figure 4.15.

They are the most common type, selected as wastewater treatment system for small

communities.


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These systems have long retention times to facilitate the management of large flow

fluctuations; their operating and maintenance costs are usually less than that of other

biological systems. US criteria for the design of facultative lagoons are: (i) BOD5

loading rate should not exceed 22 kg/ha.day, and (ii) the detention time in the lagoon

must be in the order of six months.

Figure 4.15: Treatment processes in facultative ponds

4.1.7 DISINFECTION

In some secondary wastewater treatment plants, the last treatment step is the addition of a

disinfectant to treated wastewater, in order to eliminate pathogens. As an example, the

addition ofchlorine gas or some other form of chlorine is the disinfection process most

commonly used in the USA. Chlorine is injected into the wastewater by automatedfeeding systems; wastewater then flows into a basin where it is held for around 15 minutes

to allow the chlorine to react with pathogens.

There are arguments regarding whether disinfection by chlorine is necessary or, indeed,

effective in killing the pathogens. Other processes, such as ultraviolet light radiation could

be used in place of chlorine injection.


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4.2 TERTIARY TREATMENT AND EFFLUENTDISPOSAL

Learning Outcome: (1) understanding the needs for tertiary treatment, the common route of nitrogen andphosphorus removal from wastewater, and the roles of filtration and carbon adsorption in tertiary treatment;

(2) knowing the common disposal route for treated wastewater; (3) knowing the basic working principles

and components of five alternative wastewater treatment systems.

After the secondary treatment, where the bulk organic matter is removed from the

wastewater, nutrients (e.g. nitrogen and phosphorus) and other substances (e.g. heavy

metals, and endocrine-disrupting chemicals - EDCs) remaining in the wastewater can still

cause damage to the receiving watercourse. If these pollutants are of major concern, a

tertiary treatment, also called advanced wastewater treatment (AWT), must be carried

out before discharge to the natural water ways. In some circumstances, the quality ofeffluent from tertiary treatment is adequate for reuse purposes.

4.2.1 NUTRIENT REMOVALS

Nitrogen and phosphorous are the key nutrients that should be removed from wastewater

because they often cause a particular type of pollution known as eutrophication, indicated

by excessive growth of algae in lakes and bays.

Ni trogen Removal

Nitrogen is a water pollutant, mainly in four of its oxidation states that are all part of the N

cycle in nature:

Organic N ammoniacal N nitrite N nitrate N

Total nitrogen, TN, in water is the sum of all forms of nitrogen. In many wastewaters,

there are only negligible amounts of nitrite N and nitrate N so that the value of TN is close

to the Total Kjeldahl Nitrogen (TKN) the sum of organic nitrogen and ammoniacal-

nitrogen:

TKN = Organic N + Ammoniacal N

It is often desirable to reduce TN in wastewater to below 10 mg/l; this process can be done

by oxidising Kjeldahl N into nitrate N, and then transforming into nitrogen gas (N2) that is

inoffensive. This is achieved in two steps:

1. Nitrification : Ammoniacal N nitrite N nitrate N

2. Denitrification: Nitrate N N2 (gas)

Nitrification is done by nitrifying bacteria, while denitrification is a result of a complex

consortium of microorganisms. Nitrification and denitrification require different

environments, but they could take place in the same tank throughout different zones and

operation times. The conditions are characterised mainly by: (1) dissolved oxygen (DO)


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levels in the wastewater, (2) organic loading rate (mass of BOD inflow per m 3 of

wastewater). Students should know that:

1. Nitrification takes place primarily in aerobic environment

2. Denitrification takes place primarily in anoxic environment

Box 4.2: Nitrification

Transformation byNitroso-bacteria:

OH2H4NO2O3NH2 2224 (4.19)

Transformation byNitro-bacteria:

322 NO2ONO2 (4.20)

The nitrification process is very oxygen demanding. Approximately 3.4 g of oxygen are

needed to oxidise 1 g NH4-N into NO2-N, and 4.5 g are needed to oxidise 1 g NH4-N into

NO3-N. Thus nitrification can only occur in the aerobic regions of a wastewater treatment

system; the availability of oxygen is often the limiting factor for the efficiency of NH4-N

removal. In the nitrification process, a large amount of alkalinity is also consumed (8.64

g 3HCO per g NH4-N oxidised into NO3-N). Inorganic carbon is used for the synthesis of

cell materials of nitrifying bacteria.

Very limited amounts of biomass are produced in terms of either NH4-N or NO2-N

removed during the process of nitrification. In the oxidation of 1 g NH4-N only 0.15 g (dry

weight) of bacterial biomass in the form ofNitrosomonas is produced, and 0.02 g (dry

weight) of the cell material ofNitrobacter when NO2-N is further oxidised to NO3-N.Therefore, wastewater treatment facilities for nitrification purposes usually produce less

biomass and consequently a smaller amount of sludge. When a small amount of nitrogen

for cell synthesis is subtracted, the following equation can be used to estimate the oxygen

consumption (g) for NH4-N removal by nitrification.

Oxygen consumption = 3.23 increase in NO2-N + 4.35 increase in NO3-N (4.21)

Because nitrifying bacteria are autotrophic microorganisms, they multiply relatively

slowly (in comparison with heterotrophic microorganisms); it is estimated that nitrifying

bacteria have a doubling time of 2-6 days.

Box 4.3: Denitrification

Denitrification is a series of reduction reactions where electrons are added to nitrate or

nitrite nitrogen, resulting in the production of nitrogen gas, nitrous oxide (N 2O), or nitric

oxide (NO). Nitrate is gradually reduced to nitrogen gas, by various groups of chemo-

heterotrophic bacteria; they are capable of replacing O2 with 3NO as the terminal electron

acceptor for the chemical oxidation-reduction reactions, by which they obtain energy for

cell synthesis and reproduction. Their respiration proceeds with the reduction of nitrate to

nitrite, nitric oxide, nitrous oxide and nitrogen gas. This process is shown in simple form

in the following equation.


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3NO 2NO NO N2O N2 (4.22)

Box 4.3 Denitrification Process (continued)

Alkalinity is produced during the conversion of nitrate to nitrogen gas, resulting in anincrease in pH of the wastewater. The observed rate of bicarbonate production by this

process is about 3.0 g of CaCO3 per gram of reduced NO3-N. Temperature affects the

removal of nitrate, because the denitrification microorganisms are sensitive to it.

Denitrification can only proceed at a very low rate, if at all, at a temperature below 5 0C.

The overall stoichiometric nitrate dissimilation reaction, based on methanol (CH3OH) as a

carbon source, is summarized as follows by the US EPA (1993):

OHOH167.1CO833.0N5.0OHCH833.0NO22233

(4.23)

From the stoichiometry of Equation 4.23, 1.9 g of methanol (or another equivalent carbon

source) is required to support the denitrification of 1 g of NO3-N. In the absence of this

carbon source, conventional denitrification is inhibited.

Much detail on the routes of nitrogen and phosphorus removals will be introduced in the

fourth year in CIV4268 Water Resources Management.

It is desirable to have nitrification and denitrification happening in a logical order. Figure

4.16 presents a system that is designed to remove nitrogen biologically. Wastewater, rich

in ammonia but also nitrate, enters a tank that is anoxic (no aeration or mixing to maintainlow O2 level). Here, denitrification takes place. The effluent then goes into a fully aerobic

tank where DO > 2 mg/L (this is achieved by mixing and air injection). An essential return

cycle of nitrified effluent is returned to an anaerobic tank. Only fully oxidised effluent is

sent to a secondary clarifier. As such, nitrogen removal can be improved by linking

several anaerobic and aerobic tanks in a row.

aerobicanoxic


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Figure 4.16: Nitrification-denitrification system

Phosphorous Removal

It would be highly damaging if a large amount of phosphorus in wastewater were to be

discharged into lakes, bays and slow moving rivers, because phosphorus is a majorsource for eutrophication. Phosphorus exists in nature in different organic and inorganic

forms, and it can be dissolved in water but also particle bound. Total Phosphorus, TP, is

the sum of all P forms. In domestic wastewater treatment, phosphorus is often required to

be reduced to below 2 mg/l.

Biological phosphorus removal is based on forcing the micro-organisms to accumulate

more P than required for cell growth (a typical P content of microbial solids is about 1.5-

2% of dry matter). It was observed that if wastewater is exposed to aerobic conditions

followed by anaerobic conditions, some microorganisms are capable of taking up P at

levels above average to thrive. In this environment the biomass accumulates P to levels of

4-12 % of microbial solids. When the biomass (sludge) is wasted, 2.5-4 times more P

removal occurs than in conventional systems. In other words, P levels in wastewater could

be reduced by 70-80%, from typically 10 mg/l down to 2-3 mg/l.

If further reductions are necessary they can be achieved by chemical precipitation.

Traditionally, phosphorus has been removed by chemical precipitation using one of three

compounds:

Ferric chloride - FeCl3

Alum - Al2(SO4)3

Lime - CaO

For example, in an activated sludge treatment facility,FeCl3 can be added into the aeration

tank or into the wastewater before primary sedimentation, and then removed by

sedimentation in the clarifier (i.e. phosphorus removal occurs during secondary treatment

in the first case and primary treatment in the second case).

Exercise 4.4 (5 minu tes)Chemical precipitation for phosphorus removal

If a wastewater has a soluble orthophosphate concentration of 4.0 mg/L as P,

what theoretical amount of ferric chloride will be required to remove the

phosphorus completely from the wastewater?

Note: The molar mass of FeCl3 is 162.21 g/mol, and the molar mass of P is

30.97 g/mol.

Answer: mg/L


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Combined Biological Ni trogen and Phosphorous Removal

This is still an area of intensive research. A number of systems are being developed and

tested. Figure 4.17 (Bardenpho process) gives an example of combined removal.

Figure 4.17: Modified Bardenpho process for combined P and N removal

4.2.2 FILTRATION

Filtration, especially filtration by sand filters, is

a unit operation that is commonly used in

tertiary treatment. Filtration processes, similar

to that used in drinking water treatment plants,

can further remove the residual suspendedsolids in the effluent from secondary treatment.

If designed and operated effectively, filtration

processes can reduce suspended solids to

undetectable levels. Figure 4.18 shows the

photo of a typical wastewater filter.

In sand filterswastewater passes through graded

sand and gravel filters as shown in Figure 4.19.

After a certain period of operation, backwash is

needed to remove trapped solids in the filters.

Traditionally, sand filtration is one of the

principle unit operations used in the treatment of

potable water. Nowadays, the filtration of

effluent from secondary wastewater treatment

processes is becoming more common because

of the requirement for higher effluent quality

and the need for wastewater reclamation and

reuse.

Figure 4.19: General feature of a sand filter: (a) flow

during filtration cycle, and (b) flow during backwashcycle

Figure 4.18: A photo of wastewater filter


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4.2.3 CARBON ADSORPTION

After secondary treatment, soluble organic materials that are resistant to biological

breakdown may persist in the effluent. The persistent materials are often referred to as

refractory organic compounds, which can be detected in the effluent as soluble COD.After the secondary treatment of domestic sewage, the soluble COD in the effluent is often

in the range of 30-60 mg/L. These soluble organic materials (represented by COD) can be

removed by adsorbing on activated carbon prepared from organic materials, such as

woods, bones, coal, etc. A photo of an activated carbon adsorption facility for wastewater

treatment is shown in Figure 4.20.

Figure 4.20: Activated carbon adsorbers in tertiary wastewater treatment

Further detail on a number of tertiary treatment processes will be introduced in the fourth

year in the wastewater part of CIV4261 Integrated Urban Water Management.

4.2.4 EFFLUENT DISPOSAL

Once wastewater has been sufficiently treated, it has to be returned back to the

environment, i.e. disposed into a receiving natural water body. The single most important

element of effluent disposal is the associated environmental impact. There is a regulatory

framework which deals with issues such as the selection of discharge locations, the

selection of outfall structures, and the level of treatment required. Thus, sewage treatment

and disposal are linked and cannot be considered in isolation.

Disposal Options

Treated sewage is either re-used or disposed ofinto the water environment. Disposal is

by far the most common methodology and since this is a re-entry into the hydrological

cycle, it can also be seen as the first step in indirect and long-term re-use. The most


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common method of disposal is discharge into river systems or larger open water

bodies such as oceans.

It should be noted that another means of disposal is by discharge on land where part of the

treated sewage seeps into the ground and recharges underlying groundwater aquifers and

the other part of the sewage evaporates (in desert areas, the evaporated fraction can be

substantial). Note that land applications are not covered in this section.

River Outfall

Many existing effluent discharges into rivers are poorly designed. They often comprise

open-ended pipes which achieve minimal initial mixing. In shallow streams, open-ended

discharges on the bank may fall directly onto the water surface, creating the potential for

foaming problems. Such problems can often be minimized by utilising a submerged

discharge point located farther out within the stream. Where rivers are navigable,

however, the outfall design requires special attention and is likely to be closely regulated.

Rapid initial mixing of effluent discharge into a river can be achieved with a multi-port

diffuser. Such a structure discharges the effluent through a series of holes or ports

along a pipe extending into the river. For shallow rivers, very rapid vertical mixing is

achieved over the full river depth. Turbulent entrainment then draws river water into the

effluent plume, promoting rapid dilution. This situation is shown schematically in Figure

4.21, which also shows a typical elevation of a riser.

Figure 4.21: Plan and elevation view of a typical river diffuser


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Figure 4.22 shows a typical river diffuser arrangement. The port spacing adopted is

usually of the same order as the water depth. At the outboard end of the diffuser, a large

cleanout port is provided to facilitate flushing. The primary purpose of a multi-port

diffuser is to distribute the flow evenly along the entire length of the structure. For this

reason, the discharge of wastewater per port should be as uniform as possible along the

length of the diffuser. This is achieved by decreasing the diameter of the diffuser pipe in

steps as shown in Figure 4.22.

Figure 4.22: Schematic diagram of a typical diffuser outfall

The initial dilution, S, achieved in the near field, defined as being within approximately

one diffuser length, is given by:

LHU

cosUQ211

Q2

UHLS

2

DD

D

(4.23)

Where: U - the river flow velocity (m/s)

H - the river depth (m)

L - the diffuser length (m)

QD - effluent discharge requirement (m3/s)

UD - the discharge velocity through each port (m/s)

- the orientation (angle) of the ports above the horizontal

The diffuser length, L, is often the most important parameter as it largely determines the

cost of the structure. Equation 4.23 can be used to determine the length of diffuser (L)

required to achieve a prescribed level of dilution (S). The equation is applicable to shore-

attached as well as mid-river diffusers.


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In practice, the port velocity (UD) should not exceed 3 m/s, although this may be

exceeded where circumstances warrant it and especially during infrequent high-flow

events. The use of Equation 4.23 in practice is illustrated with the following example.

Example 4.3

Effluent disposal into a river

Determine the length and number of discharge ports for a multi-port diffuser that will

provide a dilution rate of 10 when discharging a maximum flow of 1.5 m3/s of

wastewater effluent into a river. Under low flow conditions, the river water depth is 1.2

m and the current speed is 0.6 m/s.

For the shallow water conditions prevalent under low river flow conditions, the

maximum discharge velocity, UD, is suggested to be around the value of 2 m/s and lower

than 3 m/s to reduce the risk of bottom erosion and hazards to boat traffic. Because of

the shallow depth, the ports will discharge horizontally in the same direction as the river

flow.

Solution:

Calculate required diffuser length using Equation 4.23:

LHU

cosUQ211

Q2

UHLS

2

DD

D

2.1L6.0

125.1211

5.12

L2.16.010

2

Solve by trial: L=18m

Determine the required number of ports:

Port spacing water depth

No. of ports = 18/1.2 +1 = 16

Determine port diameter:

24

D14.361U

4

DportsofNo.5.1Q

20

D

20

m244.0216

5.14D

21

0

Nearest standard pipe size = 0.25m


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Check required number of ports:

3.1525.02

415

4

DU

QN

220

Select 15 ports

Port velocity UD

22 25.015

45.1

4

DN

Q

= 2.04 m/s (OK)

Dilution rate:

2.1186.0

104.25.1211

5.12

182.16.0S

2= 10.1 (OK)

Diffuser length: 18 m

Number of ports: 15

Port diameter: 250 mm

Port spacing: 1.29 m

Port velocity: 2.04 m/s

Ocean Disposal

Oceans and large lakes are used for effluent disposal by many communities. Provided that

the outfall structure is appropriately designed, water bodies like oceans and large lakes

provide extensive assimilation capacities.

Sewage effluent is typically carried to an offshore discharge point by a pipe or tunnel. Theactual discharge may be through a single port or multi-port diffuser. The characteristics of

the effluent plume are complicated by the density difference that exists between the lighter

effluent and the denser sea water.

In the initial mixing region between the effluent and ocean, also known as the discharge

near field the effluent is strongly buoyant and rises rapidly in the water column. At

some point during the rise of the plume, its density may become equal to that of the

surrounding water and the plume will rise no further. Beyond the initial mixing region is

the so-called far field where the effluent travels on ocean currents and is further diluted

by turbulent diffusion. It is clear that the dilution mechanisms acting in the near field and

the far field are very different and, for this reason, they are treated separately. The details


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of the dilution mechanisms are outside the scope of this manuscript. The configuration of

a typical effluent plume in the ocean is shown in Figure 4.23.

Figure 4.23: Schematic diagram of the plume of effluent discharge in an ocean

4.2.5 ALTERNATIVE TREATMENT PROCESSES

Land TreatmentLand treatment is the application of effluent (usually secondary treated) on the land by

certain conventional irrigation method. It is an alternative to AWT processes for

producing high-quality effluent. Treatment is provided by natural processes as the effluent

moves through the natural filter provided by soil and plants. In semiarid areas it is a very

useful irrigation method. In some cases, land treatment can be considered not only as a

disposal technique but also a form of wastewater reuse.

There are three basic approaches:

(i) Slow rate ir ri gation


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Slow rate irrigation involves the application of wastewater to vegetated land to provide

treatment and meet the growth needs of the vegetation; the wastewater provides both

water and nutrients to enhance plant growth. The applied water is either consumed through

evapotranspiration or percolates vertically and horizontally through the soil profile, as

shown in Figure 4.24. Any surface runoff is usually collected and reapplied to the system.

Treatment occurs as the applied water percolates through the soil profile. In most cases,

the percolate will reach the underlying groundwater, but in some cases, the percolate may

be intercepted by natural surface water or recovered by under-drains or recovery wells.

Figure 4.24: Slow-rate irrigation for wastewater treatment: (a) hydraulic pathway, (b)

surface distribution, and (c) sprinkler distribution.


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In a slow-rate system, wastewater can be applied to crops or vegetation by various

sprinkling methods (Figure 4.24c) or by some surface techniques. Intermittent application

cycles, typically every 4-10 days, are used. Irrigation rates are typically 3.5-15 mm/day

(mm of water per day per m2 of land per day), depending on the crop type. It can not be

used to irrigate vegetables that are eaten raw. The seasonal nature of irrigation water

needs, possible transport of pathogens, and the high cost of irrigation distribution systems

are the main disadvantages of this techniques.

Two major new wastewater irrigation projects are being developed in Melbourne for

Melbourne's Eastern and Western regions:

Werribee Irrigation District Recycled Water Project - This significant project

delivers up to 8,500 million litres of recycled water a year to about 90 farmers in

the Werribee area (http://www.water.vic.gov.au/initiatives/recycling/werribee).

Eastern Irrigation Scheme - In total 60km of pipeline have been built to carry

recycled water from the Eastern Treatment Plant, at Bangholme, to Five Ways,south of Cranbourne. During the initial stages of the project, the equivalent of

3,200 Olympic swimming pools of recycled water have been made available to

customers each year (http://www.topaq.com.au/).

(ii) Overland flow

This is a biological treatment process (used instead of conventional secondary treatment),

as shown in Figure 4.25, in which water is applied over upper reaches of sloped terraces

and allowed to flow across the vegetated surface to runoff collectors. As a side effect,

nutrients and BOD removal is achieved. A typical application rate is 18 mm/day or higher.

Figure 4.25: Overland flow treatment

(iii ) Rapid inf il tration

In rapid-infiltration systems, wastewater is applied on an intermittent schedule usually to

shallow infiltration or spreading basin, as shown schematically in Figure 4.26. Application

of wastewater by high-rate sprinkling is also practised. Vegetation is usually not provided

in infiltration basins but is necessary for sprinkler application. Because the loading rate of

wastewater is relatively high, the evaporative loss is only a small fraction of the appliedwater, and most applied wastewater percolates through the soil where treatment occurs.


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Figure 4.26: Rapid infiltration hydraulic pathways: (a) hydraulic pathway; (b) recovery

pathway using under-drains; and (c) recovery pathway using wells.

Ecological Treatment Methods

Ecological treatment systems are generally decentralised wastewater treatment systems.

Most of them make use of natural purification processes that take place through the

interaction of wastewater, soil and vegetation. Currently, the predominant ecological

treatment system is the constructed wetland.


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(i ) Constructed wetland

Figure 4.27 shows a constructed wetland that is used in the tertiary treatment of domestic

sewage. Such wetlands consist of a wetland matrix that is filled with medium materials

(such as gravel) and planted with aquatic plants (macrophytes). At the upstream end of thewetland, an inlet structure distributes the inflow either over or into the top section of the

bed. At the downstream end, an outlet structure collects the treated wastewater.

Figure 4.27: Wetland for domestic wastewater treatment

(ii) L iving machines

These are relatively new systems for the treatment of domestic wastewater. Their layoutconsists of a preliminary septic tank where sedimentation takes place. The water then

enters, where required, a greenhouse, where the first phase of aerobic treatment takes

place in a closed tank. The

water then flows into open

aerobic reactors. Floating

on plant racks, macrophytes

grow with their roots

submerged in the

wastewater. In this phase,

BOD and TSS are reduced

and ammonia is nitrified.Oxygen is constantly

bubbled into the reactors.

Clarifiers then settle solids

left in the treated effluent

from the aerobic reactors.

Denitrification takes place

in a subsequent stage - the

ecological fluidised beds.

Live fish in the final

effluent ensure an effective

indication of quality control Figure 4.28: Living machine


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of wastewater treatment.

(iii) Aquaculture

Aquaculture consists of large ponds in which different plants and fish live. The plants feed

on the nutrients from either raw, screened sewage or secondary treated effluent. The fishin the pond feed on the plants and normally grow to full size in one vegetative season.

They are then harvested and sold. Plants for human consumption can also be reared in

aquacultures.

Aquaculture treatment is a good measure to directly recycle human waste. However, after

being used for centuries, it was abandoned in Europe towards the end of the 20th century.

This was due to the spatial requirements, the need to employ fish-keepers and their

locations on the fringes of expanding cities where land prices rose significantly, rendering

them economically unattractive. The only remaining large aquaculture in Europe is the

basin in Munich that serves as a polishing treatment stage for the refurbished treatment

works.


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4.3 SLUDGE TREATMENT AND DISPOSAL

Learning Outcome: (1) knowing the common unit operations for sludge treatment, and understanding the

concept of thickening, digestion, conditioning, dewatering and reduction; (2) knowing the common routesfor the disposal of treated wastewater sludge.

In the process of wastewater treatment, anothermajorproblem is created: sludge.

Wastewater sludge is made of materials settled from the raw wastewater and of solids

generated in the wastewater treatment processes (in all stages of the treatment). It still has

very high moisture content (93-97% water).

The following are common types of sludge that are different to some extend:

Primary or raw sludge from primary clarifiers; it has 3-8 % solid content,consisting of approximately 70% organics.

Secondary sludge from secondary clarifiers; 2-5 % solids, of which 90% are

organics.

Tertiary sludgefrom advanced treatment processes; the sludge can be similar to

the sludge from the secondary treatment, but if chemical treatment is used, it can

be of very different nature.

Sludge can be treated in many different ways. In Figures 4.3 and 4.8 two possible sludge

treatment processes have already been indicated. The basic processes that can be used for

sludge treatment are:

(1) Thickening: separating as much water as possible by gravity or flotation.

(2) Digestion (stabilisation): converting the organic solids to more refractory (inert) forms

so that they can be used without causing a nuisance.

(3) Conditioning: the sludge could be treated by heat or chemicals so that the water can

be readily separated.

(4) Dewatering: separating water by subjecting the sludge to vacuum, pressure, or drying.

(5) Reduction: convening dewatered sludge to a stable form by incineration.

4.3.1 THICKENING

Gravity th ickeners(Figure 4.29) are similar in design and operation to the primary

clarifiers, based on gravity sedimentation. Purely primary sludge can be thickened from 1-

3% to 10% solids. This thickening method is less effective on secondary sludge.

Floatation thickening process. Air is injected into the sludge under pressure. The air

bubbles attach themselves to sludge solids particles and float the solids to the surface. The

floating layer at the top of the tank is removed by skimming mechanisms. This thickeningprocess is fairly effective on activated sludge.


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Figure 4.29: Photo of a sludge

thickener

Exercise 4.5 (5 minu tes)Calculation in relation to sludge thickening process

A wastewater treatment plant produces sludge at a daily rate of 100 m3/d.

Through a thickening process, the solid concentration is reduced from 4 to 7

percent. Estimate the percentage volume reduction achieved for the sludge

during the thickening process.

Answer: %

4.3.2 DIGESTION (STABILISATION)

The principle purposes of sludge

stabilisation are to bio-chemically

breakdown organics, so that they are

more stable, and to reduce the mass of

sludge. The most common means of

sludge stabilisation is biological

degradation. There are two basic

processes in use: anaerobic digestion

and aerobic digestion

Anaerobic digestionis by far the most

common process for dealing with

primary wastewater sludge. There are

two types of anaerobic digestion tanks:

(1) standard-rate anaerobic digester,

and (2) high-rate anaerobic digester.

The standard-rate anaerobic digester

(Fig. 4.30) is typically heated at 35 0C

with a retention time ranging from 30 to

60 days. The mixing of sludge is not

Stabilized

Solids

Activated

Layer

Supernatant

Scum Layer

Gas

Gas Relief

Influent Effluent

Solids Removal

Figure 4.30: A standard-rate anaerobic digester


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employed, and the contents are stratified into different zones, as illustrated in Figure 4.30.

A high-rate system (Figure 4.31) contains two separate units: (1) mixed and heated tank,

and (2) stratified tank.

Figure 4.31: The high-rate anaerobic sludge digestion system

Gas produced in sludge digestion (CH4 and CO2) can be used for electricity generation. In

some large treatment plants a substantial percentage of energy used in the plant could be

generated in this way.

Aerobic digestionis effectively a continuation ofthe activated sludge process.

Aerobic sludge digestion is accomplished by

aerating the wastewater sludge in a tank

resembling an activated aeration tank. The

aerobic digestor must be followed by a settling

tank unless the sludge is to be disposed of on

land in liquid form. The effluent (supernatant)

from the clarifier is recycled back to the head of

the plant.

4.3.3 CONDITIONING

Chemical conditioningis accomplished by addition of coagulants (ferric chloride, lime,

aluminium sulphate). Polymers are also popular. When chemical coagulants are added into

the sludge, the coagulants act to clump the solids together so that the solids are more

easily separated from the water.

Heat treatmentis done by heating the sludge at high temperature (175 to 2300C) and

pressure (1000 to 2000 kPa). Under such conditions, much like those of a pressure cooker,

water bounded up in the solids is released, improving the dewatering characteristics of the

Figure 4.32: Egg-shape sludge digesters


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sludge. Heat conditioning has the advantage of producing a sludge that dewaters better

than chemically conditioned sludge. This process has the disadvantages of (1) relatively

complex operation and maintenance, and (2) the creation of highly polluted cooking

liquors which, when recycled in a wastewater treatment plant, impose a significant added

treatment burden.

4.3.4 DEWATERING

Sludge drying bedsare the traditional and the most popular dewatering method. The

operational procedure involves the following steps: (1) 0.2-0.3 m of stabilised liquid

sludge is pumped onto the drying bed surface; (2) chemical conditioners are added and the

sludge is dried for 2 weeks in summer and up to 2 months in winter; and (3) the removal

of the dewatered sludge.

Sand drying beds are the oldest, most commonly used type of sludge drying beds; they areparticularly popular in small treatment plants because of their simplicity in operation and

maintenance. However, when space is limited and climate conditions make the sand

drying beds prohibitively expensive, vacuum filters may be used for sludge dewatering.

Vacuum fi ltrationis accomplished by a vacuum filter that consists of a cylindrical drum

covered with a filtering material or fabric, which rotates partially submerged in

conditioned sludge (Figure 4.33). A vacuum is applied inside the drum to extract water,

leaving solids on the filter medium. As the drum completes its rotation cycle, a blade

scrapes the solids from the filter, before the cycle starts again.

sludge

vacuum

par tially drysludge

blade

Figure 4.33: Vacuum filter for sludge dewatering

4.3.5 REDUCTION

If the use of sludge as a fertilizer or soil conditioner is not practical, or if a site is not

available for landfill using dewatered sludge, city authorities may turn to the alternative of

sludge reduction, i.e. incineration. Incineration completely evaporates the moisture in the

sludge and combusts organic solids to sterile ash. To minimise the amount of fuel used,

the sludge must be dewatered as completely as possible before incineration. The exhaust

gas from an incinerator must be treated carefully to avoid air pollution.


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4.3.6 SLUDGE DISPOSAL

Ultimately, sludge needs to be disposed of on the land, in the ocean, or in the air (by

incineration that in practice is often not the preferred disposal route).

Land Spreading

The practice of spreading treated wastewater sludge (biosolids) for the purpose of

recovering nutrients, water, or reclaiming despoiled land (such as a strip of mine spoils) is

called land spreading. If applicable, land spreading can be the most beneficial route of

sludge disposal. Land spreading is a land-intensive process, and the application rate of

sludge on the land is governed by the character of soil and the ability of the crops or

forests, on which the sludge is spread, to accommodate the sludge.

Landfill ing

This is effectively the planned burial of wastewater solids at a designated site. The solids

are placed into a prepared site or trench and covered with a layer of soil on designated

landfill sites.

Dedicated Land Di sposal

Dedicated land disposal refers to the application of heavy sludge loadings to some finite

land area that has limited public access and has been dedicated for all time to the disposal

of wastewater sludge. Dedicated land disposal does not mean in-place use of the sludge.

No crops may be grown. Dedicated sites typically receive liquid sludge.

Utilisation

The sludge is utilised for agriculture purposes or for other purposes such as compositing to

produce garden fertilizers. Sludge transport could be expensive. The demand is seasonal

and, sometimes, the waste itself maybe unsuitable for specific utilisations.

Further detail on the disposal of biosolids will be introduced in a fourth year module: the

wastewater part of CIV4261 Integrated Urban Water Management.


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CHAPTER REVIEW

After students have completed the study of this topic, they should be able to demonstrate

the following knowledge and ability:

1. Describe the role of microorganisms in secondary wastewater treatment and the basic

conditions needed for biological wastewater treatment.

2. List four common biological wastewater treatment systems.

3. Describe the basic components and operation of activated sludge treatment system.

4. Know how to carry out various calculations related to the activated sludge treatment

process.

5. Briefly describe the working principles and operations of secondary clarifier, trickling

filter, rotating biological contactor, and oxidation pond.

6. Explain how nitrogen and phosphorus can be removed from wastewater in a tertiary

treatment process.7. Explain the basic working principles of filtration and carbon adsorption in tertiary

wastewater treatment.

8. Describe where and how treated wastewater effluents are commonly disposed.

9. List four alternative treatment systems and briefly explain how these systems work.

10. Capable of carrying out simple calculations, based on mass balance, on volume

reduction in sludge thickening process.

11. Describe four basic processes in the treatment of wastewater sludge (i.e. thickening,

digestion, conditioning, and dewatering).

12. List four common routes of sludge disposal.

FURTHER READING

American Public Health Association, 1995. Standard Methods for the Examination of Water and

Wastewater. 19th Ed., Washington DC, USA.

Environmental Protection Agency, 1993. Nitrogen Control Manual, Office of Research and Development

EPA Report 625-R-93-010, USA.

Forster, C. F., 2003. Wastewater Treatment and Technology, Thomas Telford Publishing, London, UK.

Gray, N. F., 2004.Biology of Wastewater Treatment, 2nd Ed., Imperial College Press, London, UK.

Lin, S., 2001. Water and Wastewater Calculations Manual. McGraw-Hill, New York, USA.

Metcalf and Eddy, revised by Tchobanoglous, G., Burton, F. L., Stensel, H. D., 2003. Wastewater

Engineering: Treatment and Reuse. 4th Ed., McGraw-Hill, New York, USA.

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