7.

SEWAGE TREATMENT

1. Which of the following is not true for a septic tank?  A septic tank mainly operates under anaerobic conditions
(RPSC Lecturer Paper 2 2021) where bacteria break down organic matter. The growth of
(a) It is combined sedimentation and digestion tank bacteria occurs attached to the surfaces inside the tank
(attached growth).
(b) Anaerobic digestion takes place in a septic tank
(c) It is designed for a detention period of 5 to 10
Cowl
days
(d) It is usually rectangular in shape
OR Manhole
Vent Pipe
A septic tank is :- (RPSC ITI VP, 2016)
[RPSC VP ITI - 2012] Scum
(a) Aerobic method of on - site sewage treatment Inlet Outlet
(b) Physical method of water treatment Buffle wall
(c) Anaerobic method of on-site sewage treatment
Sludge
(d) Physio-chemical method of water treatment
OR
Septic tank is an (RSPCB JEE 09/01/2024) Sloping Floor
(a) Aerobic attached growth treatment system Septic Tank
(b) Aerobic suspended growth biological treatment
system Design Parameters of Septic Tank
(c) Anaerobic suspended growth treatment system  Flow of sewage 40 – 70 lpcd & rate of accumulation
(d) Anaerobic attached growth biological treatment = 30 1pc per year
system  Detention time 12 – 36 hrs & Cleaning period 6
OR month - 1 year
A septic tank is a  Length/width ratio = 2 to 3 & Depth 1.2 – 1.8 m
RHB Project Engineer, CIVIL Degree 10/09/2023  It is rectangular in shape.
HPSSC JE Code 459/2019  Volume of septic tank = (Sewage flow × Detention
BPSC AE 2019
PSSSB JE 22.11.2015 time) + (Sludge accumulation rate × cleaning
KPSC Code 41/2015 period).
SSC JE 2009  The effluents from the septic tank are discharged
(a) Sedimentation tank into soak pit.
(b) Digestion tank  Soak pit is effective only in highland (where water
(c) Combination of sedimentation and digestion table is well below ground level) & preferable at lo-
tank cations where soil is porous.
(d) Aeration tank 2. For high rate anaerobic sludge digesters, the typi-
cal value of Hydraulic Retention Time (HRT) is
Ans. (c)
(RPSC Lecturer Paper 2 2021)
(a) 5 days (b) 10 - 20 days
Method Contact Decomposition
(c) 20 - 30 days (d) 30 - 40 days
Trickling filter Attached growth Aerobic Ans. (b) Hydraulic Retention Time (HRT) is the average
Rotating biological time which wastewater spends in a treatment unit.
Attached growth Aerobic
contactor  For high-rate anaerobic sludge digesters, the general
Activated sludge value of Hydraulic Retention Time is generally 10 - 20
Suspended growth Aerobic
process days.This range is generally used to ensure efficient di-
Oxidation pond Suspended growth Aerobic gestion of organic matter while maintaining a manageable
Septic tank Suspended growth Anaerobic volume of sludge in the digestor.
I mhoff tank Suspended growth Anaerobic 3. Which one of the following sewage treatment units
has a parshall flume ? DLB AE Shift III, 2015
Septic Tank JPSC AE Paper-II 23.10.2021
JPSC AE Paper-II 10.04.2021
 Septic Tank functions as both a sedimentation & UPPCL JE CE 2015
digestion tank. It allows solids to settle at the bottom Maharashtra PSC HOD 5.10.2014
(sedimentation) while anaerobic bacteria break down the (a) Aeration tank (b) Trickling filter
organic matter (digestion). (c) Oxidation pond (d) Grit chamber
 It is an anaerobic method of on site treatment.
 SEWAGE TREATMENT 485

OR  Two channels are provided, one for normal flow

Parshall flume is a grit chamber can be used as and other for peak flow.
(RPSC Lecturer Paper 2 2021)  A parshall flume can be used both as a discharge
(a) A velocity control, flow measuement and energy measuring & velocity control device. It is preferable
dissipation device because head loss is less.
(b) A flow measurement and energy dissipation  Only one section (channel) is required if velocity
device control devices has been used (Proportional flow
(c) A velocity control and energy dissipation device weir, Sutro weir Parshall flume).
(d) A flow measurement and velocity control device Free board (0.3 m)
OR
What is the detention time of grit chamber when the Inlet
depth is 1-1.8 m. (RSPCB JEE 09/01/2024)
(a) 10 minutes (b) 30 minutes Designed
Silt removing
depth (1 to 1.2 m)
(c) 60-90 sec. (d) 40-60 sec pipe
OR
The detention period usually adopted in grit cham-
bers is: [RPSC A.En. - 2013]
(a) 30 seconds (b) 120 minutes
(c) 60 minutes (d) 60 seconds Design Criteria:
OR
 Removes particles of size  0.2 mm & G S  2.65
Horizontal flow and aerated are common terms as-
sociated with (RPSC Lecturer Paper 2 2021)  For 0.2 mm particle, settling velocity = 0.025 m/s
(a) Screens (b) Shredder  Surface over flow rate = 2160 m3/m2/day & free
(c) Comminutor (d) Grit chamber board is 0.3m
OR  Detention time is 30 – 60 sec & Depth is 1 – 1.5 m
Which of the following sewage treatment units has  Horizontal critical flow velocity V c =
a parshall flumes ? (RPSC ACF 2021) K c g(G s  1)d
(a) Trickling filter (b) Oxidation ditch
KC = 3 to 4.5
(c) Aerated Lagoon (d) Grit Chamber primary treatment secondary treatment
Ans. (d) disinfectant
aeration tank
raw sewage secondary
clarifier
screens
inlet outlet
fats, oil and grease primary secondary
effluent effluent
comminutor discharge to
surface water
air compressor activated (or tertiary
primary treatment
clarifier return sludge sludge
grit if needed)
particle

return sludge pump

compressed
air (optional) grit chamber
raw or primary sludge
grit disposal

sludge treatment and disposal

4. One litre of sewage, when allowed to settle down for
grit
30 minutes give sludge of 27 cm3. If the dry weight
of sludge is 3.0 gm, then its sludge volume index
will be- (RSMSSB JE (DEGREE) 2022)
(MPPSC AE 2017)
Grit removal basin, such as grit chambers or grit channels (a) 24 (b) 9
or Detritus tanks are the sedimentation basins placed in (c) 30 (d) 81
front of the waste water treatment plant to remove the Ans. (b) Sludge Volume Index (SVI)
inorganic particles like as sand, gravel grit, egg shells, etc.  It indicate physical state of sludge in the
Grit Chamber: Grit chambers are provided in the form of Biological Aeration system.
longer length & smaller X-sec area & should not allow
settlement of organic materials.  Vob 
 Stokes law can not be applied to grit chamber Sludge volume index (SVI) (in m  /gm) =  X 
 ob 
4 (  s   w )d Xob = Concentration of suspended solids in the
Vs2 = mixed liquor in gm/lt.
3 C D . w
 It is Located either before or after sewage pumps. It Vob = settled sludge volume in m  /  t.
separate non-decaying heavy inorganic material  27ml / lt 
from sewage. SVI     9 ml/gm
 3gm / lt 
 486 (Civil Ki Goli Publication 9255624029)

5. In the treatment of sewage, identify the aerobic bio- sedimentation tank is discrete.
logical unit from the following: 2. Type II - Flocculent settling: It refers to settling of
flocculent particles of low concentration (1000 ppm)
M. Oxidation ponds Ex.-Bioflocs in upper part of SST. In it, particle
N. Septic tanks velocity increase with time.
O. Imhoff tanks 3. Type III - Zone settling /Hindered settling: When
P. Anaerobic lagoons the concentration of flocculated particles is in
(HPCL BARMER, 07/08/2021) intermediate range, they are close enough together
(a) P (b) O so that their velocity fields overlap causing
(c) M (d) N Hindered settling. Particle concentration increase
Ans. (c) Oxidation Pond during settling. Ex.-found in SST following ASP.
O2 4. Type IV - Compression: In it, concentration of par-
ticles becomes so High that particles are in physi-
Algae Effluent cal contact with each other.Particle concentration
Raw Photosynthesis: CO2+ HO2= O2+ Algal Cells
Algae
Aerobic
Zone
increase during settling. Ex.-bottom of SST follow-
Wastewater
Settleable Organic matter CO + NO, PO3+ H O
ing TF.
Solids + Oxygen
2 2
Facultative 8. The correct series of unit processes in a water
Bacteria Zone
treatment plant is: (RIICO Ast. Site Engr. 2021)
Rising Gases
Biomass Anaerobic
Zone
(a) Coagulation, Filtration, Flocculation,
Sludge CO2+ CH 4+ NH3+ H2S (gases)
Sedimentation
Layer + New Cells (b) Coagulation, Flocculation, Sedimentation,
Solids removal mechanism in a faculative stabilization pond Filtration
6. The first stage of natural process of sludge diges- (c) Coagulation, Sedimentation, Flocculation,
tion is (RPSC ACF 2021) Filtration
(a) Acid fermentation (d) Flocculation, Coagulation, Filtration,
(b) Acid regression Sedimentation
(c) Alkaline fermentation Ans.(b)
(d) None of these
Ans. (a) Stages in Sludge Digestion Process
Cl2
(a) Hydrolysis Raw
Aeration
(b) Fermentation water
NH3
(c) Methane formation (Methanogenesis)
7. In a water treatement process by sedimentation the It becomes necessary when water comes from fast flowing
flocculent particles follows. streams. It removes large suspended solid

(RCDF Ast. Manager, Env. & ETP 18/08/2021) alum

(a) Type - I settling (b) Type - II settling Mixing, flocculation
and settling
(c) Type - III settling (d) Type - IV settling
Polymers
OR
A dilute suspension of particles that is flocculating It removes turbidity by coagulating the colloids & settling them out
during sedimentation process in water treatment is It may be remove colour caused by large organic molecules
an example of: (RIICO Ast. Site Engr. 2021)
Micro Strainer
(a) Type III settling (b) Type II settling
(c) Type IV settling (d) Type I settling
OR Filtration Cl2
Type of settling in plain sedimentation tank is :
(RSPCB JEE 2016) It polishes the water and removes remaining turbidity
(a) Flocculent (b) Discrete
(c) Zone (d) Hindered Adsorption
OR
Type III settling refers to (RSPCB JEE 2016) It becomes necessary if water contains dissolved organics. The
unit may consist of activated carbon added in powdered form
(a) Hindered settling in operation similar to mixing, flocculation & settling.
(b) Compression settling
(c) Flocculant settling
(d) Discrete settling Disinfection Cl2
Ans. (b) Basically four types of settling accurs depending
on the tendency of particles to intract and the It destroys the pathogens & enough quantity of chlorine is added
concentration of solid. to provide residual in the distribution system.
Types of Settling : Storage
1. Type I - Discrete settling: It occurs when particles
do not change its size, shape or mass during settling
Ex.-settling in Grit chamber.In it, particle velocity
Distribution system
do not change.Discrete settling, the phenomenon
can be analysed by the classic sedimentation laws
of Newton and Stokes.The type of settling in a plain
 SEWAGE TREATMENT 487
9. The two main gases liberated from an anaerobic Fly nuisance: Due to the presence of insects over
sludge digestion tank would include the filter media, the entire media is flooded with flies which
(RCDF Ast. Manager, Env. & ETP 18/08/2021) makes the operation of filter difficult.Theses flies moves
(RSPCB JEE 2016) along with waste water in the filter medium leading to
(a) NH3 and CH4 (b) CO2 and CH4 choking of filter, thereby affects the working of trickling
(c) CH4 and Ne (d) NH3 and CO2 filter.
OR Odour problem: It is generally found if spray nozzle
The two main gases obtained from anaerobic method of distribution method is adopted.
decomposition are (RPSC AE 2018) 11. The working conditions in Imhoff tanks are -
(a) Ammonia and CO2 (RVUNL AE 2021)
(b) CO2 and CH4 (RPSC AE 2018)
(c) CH4 and Hydrogen sulphide (a) Aerobic only
(d) Ammonia and CH4 (b) Anaerobic only
Ans. (b) Anaerobic Digestion (c) Aerobic in lower compartment and anaerobic in
 It is the degradation of organic material by microbial upper compartment
activity in the absence of air transforming it into biomass (d) Anaerobic in lower compartment and aerobic in
& biogas, a mixture of methane (CH4), carbon dioxide (CO2)
upper compartment
& some trace gases.
(e) None of these
 When sludge is digested anaerobically, the volume of
Ans. (d) Imhoff tanks are designed with two compartments:
1 Upper Compartment: It has a circular or rectangular shape
original sludge is reduced to about of the original value. with V-shaped walls and a slot at the bottom, allowing
3
 The table shows the amount of different gases liberated solids to settle into the digestion compartment, while
in this process - preventing foul gas from rising up and disturbing the
settling process. It is exposed to atmosphere, and thus
Methane (CH 4 ) 50% to 75% working condition is aerobic.
Lower Compartment: In it, sludge accumulates and is
Carbon Dioxide (CO2 ) 25 to 50% compacted & partially stabilized through anaerobic
Hydrogen (H) 5 to 10% digestion. Thus, working condition is anaerobic
Imhoff Tanks: Two storey digestion tank.
Nitrogen (N 2 ) 1 to 2%
Sludge Removal
Hydrogen sulfide (H 2S) Traces Gas Removal
Gas Chamber
 Anaerobic digestion is generally adopted for primary Raw sewage
Liquid Removal
sludge because primary sludge contains large amount of Sedimentation
readily available organics which would induce rapid Chamber
growth, of biomass if treated anaerobically. Slot
Neutral Zone
 Anaerobic decomposition process produce less biomass
& primary function of anaerobic digester is to convert as Digested sludge
Digestion
much of sludge as possible to end products like as liquid chamber
& gases while producing little biomass.
Imhoff Tank
 Waste-water contains wide varieties of organism.
 The organisms are broadly classified as • This tank is three phase system (solid, liquid, gas),
 Methane former: They are strictly (anaerobic) & convert collect gases from the top.
acids and alcohol along with hydrogen and carbon dioxide • It is an improvement over septic tank.The limitation
to methane. of septic tank is the disturbance of sludge digestion
 Acid formers: They consists of (facultative) & (anaerobic) due to flow of water.
bacteria and organisms which stabilized the organic acids • Imhoff tank is a two storey tank which divides the
through hydrolysis. flow from digesting sludge.
10. Which of the following sewage treatment methods • Primary sedimentation chamber consists of flow of
has inherent problems of odour, ponding and fly waste water only.
nuisance? (RPSC AE 2018) • Digestion chamber includes the collection of sludge
(a) UASB system & its decomposition reaction.
(b) Activated sludge process 12. Excess sludge in a oxidation ditch may be sent to
(c) Trickling filters (RIICO 2015
(d) Stabilization ponds (a) Aerobic digester (b) Anaerobic digester
Ans.(c) Operational troubles in Standard rate Trickling (c) Incinerators (d) Drying beds
Filter are: Ans. (d) Oxidation ditch is based on extended aeration pro-
Ponding Problem: Due to growth of algae & fungi cess. There is no need of separate sludge digester as the
in the voids of the filter media which leads to ponding of detention time is very large. So sludge can be directly
the waste water over the filter media. taken to sludge drying beds.
 488 (Civil Ki Goli Publication 9255624029)

13. One of the following units works in anaerobic con-  Generally SVI value should be (80–150) ml/gm.
ditions? (RIICO 2015)  SVI represents the degree of concentration of sludge in
(a) Activated sludge treatment the system
(b) Sedimentation tank  Its unit ml/gm
(c) Sludge digestion tank QR X 106
(d) Trickling filters  , Xu  , where X is MLSS in
Q0 X u  X SVI
Ans. (c)
tank (ppm)
Contact
Method Decomposition QR X
Mechanism  6
Q
Recirculation Ratio = 0 10
Trickling Attached X
Aerobic SVI
filter growth
Vob (ml /l ) Vob
Rotating Where, SVI = X (mg/l )  X ml /mg
ob ob
biological Attached growth Aerobic
contractor 270
SVI 
2700
Activated
Suspended 1000
sludge Aerobic
growth SVI  100
process 17. The growth of algae is useful in
Oxidation (RSMSSB PHED JE (DEGREE) 2015)
Suspended growth Aerobic (a) Sedimentation tank
pond
(b) Slow sand filter
Septic (c) Oxidation pond
Suspended growth Anaerobic
tank (d) Sludge digestion tank
Imhoff Suspended growth Anaerobic Ans. (c) Oxidation pond
• They are open flow through earthen basin such
14. Sewage treatment units for a city are designed for
ponds provide comparatively long detention period
(RIICO 2015)
during which the waste get stabilized by the action
(a) Average flow only
of natural forces.
(b) Minimum flow only
• In a totally aerobic pond stabilization is brought
(c) Maximum flow only
about by aerobic bacteria. The oxygen demand is met
(d) None of the above
by combine
Ans. (a) Average flow is considered in the design of sew-
action of algae and other micro-organism. This pro-
age treatment units owing to economy.
cess is called algal photosynthesis or algal symbio-
15. The maximum efficiency of the BOD removal is
sis.
obtained in (RSMSSB PHED JE (DIPLOMA) 2015)
• In this symbiosis, the algal while growing in the pres-
(a) Oxidation ditch (b) Oxidation pond
ence of sunlight produces oxygen by photosynthe-
(c) Aerated lagoon (d) Trickling filter
sis and this oxygen is utilized by the bacteria for
Ans. (a)
oxidising the waste organic matter.
BOD removal unit % (Removal) 18. For the same solid content, if the quantity the
1. Trickling filter 70  80% quantity of sludge with moisture content of 98% is
2. Oxidation ditch 98% 100m3 /day, then the quantity of sludge with moisture
3. Oxidation 90% content of 96% will be
4. Aerated lagoon 65  90% (RAJASTHAN DLB AE (SHIFT 1) 2015)
(a) 25 (b) 75
16. Consider the following condition (c) 50 (d) 100
A. Volume of mixed liquor suspended solids 2700 mg/
l Ans. (c) V1  100m3 / day
B. Settled volume 270 ml V1 (100  98)  V2 (100  96)
Based on this data, the sludge volume index would
be 100  2  4V2
(RSMSSB PHED JE (DEGREE) 2015)
(a) 2700 (b) 100
V2  50m3 / day
(c) 270 (d) 27
Ans. (b) Sludge Volume Index (SVI)
 It indicate physical state of sludge in the Biological
Aeration system.
 SEWAGE TREATMENT 489
19. The Hydraulic loading for a high rate trickling (a) Hydrolysis
filter varies between (b) Fermentation
(RAJASTHAN DLB AE (SHIFT II) 2015) (c) Methane formation (Methanogenesis)
(a) 110 to 330 ML per hectare per day 21. Air-binding in rapid sand filters is encontered when
(b) 50 to 60 ML per hectare per day (RPSC Lecturer Paper 2 2021)
(c) 500 to 600 ML per hectare per day (a) The water subjected to prolonged aeration
(d) 11 to 33 ML per hectare per day (b) There is excessive negative head
Ans (a) Trickling filters are a type of attached growth bio- (c) The filter bed comprises largely of coarse sand
logical treatment process. (d) The raw water contains dissolved gases
Ans. (b) Air Binding : The clogging of a filter due to re-
Standard High-rate sistance offered by the medium particle along with the im-
purities exceeds the static head of water. Due to the above
Parameters rate trickling trickling resistance, the negative head is developed at the bottom
filter filter layer of the filter medium.
Hydraulic loading 22. The absorbent most commonly used in water and
waste treatment is [RPSC Lecturer - 2014]
rate (M.L./hectare 10  40 100  400
(a) Sand of grain size from 0.1 to 2 mm
/day) (b) Activated carbon granules of size 0.1 to 2 mm
Organic Loading (c) Ordinary wood shavings of fine size
rate (Kg/BOD/ 0.11  0.37 0.37  1.85 (d) Coal-tar
3 Ans. (b) Activated carbon is a specially treated carbon,
m / day)
that possesses the property of absorbing & attracting, im-
Depth (m) 1.5  3.0 1.0  2.0 purities, like as gases, liquids & finely divided solids. Due
Re circulation to this excellent property of absorbing impurities, it is widely
0 1.0  4.0 used for removing tastes and odours from public supplies.
Ratio
23. Chlorine is sometimes used in sewage treatment
[RPSC Lecturer - 2014]
20. Hydrolysis, acid formation and methane formation (a) To avoid flocculation
are the stages of which of the following processes (b) To increase biological activity of bacteria
(RPSC Lecturer Paper 2 2021) (c) To avoid bulking of activated sludge
(a) Disinfection of sludge (d) To help in grease separation
(b) Digestion of sludge Ans. (c & d)Sludge Bulking
(c) Settling of sludge  Sludge with poor settling characteristics
(d) Removal of sludge  Sludge Bulking can be reduce by
Ans. (b) Sludge Digestion (a) Reduction of the sludge age to less than 6 days
 The sludge digestion serves both to reduce the (b) Chlorination of returned activated sludge
volume of thickened sludge still further & to render (c) Addition of Nutrient
the remaining solids inert and relatively pathogen 24. From amongst the following sewage treatment op-
free. tions, largest land requirements for a given discharge
Aerobic Digestion will be needed for [RPSC Lecturer - 2014]
 Adopted only for biological sludge (2º-sludge) that (a) Trickling filter (b) Anaerobic pond
does not contain primary sludge. (c) Oxidation ditch (d) Oxidation pond
 The sludge dewaters poorly. Ans. (d) Design Parameters for Oxidation Pond
 An extension of extended aeration process &
 Depth 1–1.5 m & (L/B) = 3 (but L   750 m)
process is energy consumption
 Detention time 20–30 days & sludge accumulation
Anaerobic Digestion
 The organisms are broadly classified as is 2 – 5 cm/year
 Methane former: They are strictly (anaerobic) &  Pathogenic bacteria removal = 99.9% & BOD
removed = 80 – 90%
convert acids and alcohol along with hydrogen and
 Minimum depth of water 0.3 m
carbon dioxide to methane.
 It is very cheap but required large land (0.5-1
 Acid formers: They consists of (facultative) &
hectare).
(anaerobic) bacteria and organisms which stabilized
the organic acids through hydrolysis. 25. The dominating micro-organisms in an activated
 When sludge is digested anaerobically, the volume sludge process reactor are [RPSC Lecturer - 2014]
(a) Aerobic heterotrophs
1 (b) Anaerobic heterotrophs
of original sludge is reduced to about of the
3 (c) Autotrophs
original value. (d) Phototrophs
Stages in Sludge Digestion Process Ans. (a) The dominating micro-organisms in an activated
sludge process reactor are aerobic heterotrophs.These or-
 490 (Civil Ki Goli Publication 9255624029)

ganisms uses oxygen to break down organic matter into the surface of which settled sewage is sprinkled.
wastewater, converting it into carbon dioxide, water & bio-  2 m deep circular bed, angular stone about 50 mm in
mass. size, supported on a tile floor
26. Which one of the following types of samples in rela-  Discharge under a head of 0.5-0.8 m
tively employed for the design of wastewater treat-  Design of TF is done for a average flow.
ment plant: [RPSC GWD A.En. - 2014]  It is very useful in removal of dissolve organic matter.
(a) Composite sample 29. The most common cause of acidity in water is
(b) Grab sample [RPSC ACF - 2011]
(c) Integrated sample (a) Carbon dioxide (b) Oxygen
(d) None of these (c) Hydrogen (d) Nitrogen
Ans. (a) Ans. (a) Carbon dioxide is the most common cause of acid-
(1) Grab sample:- A sample collected at a particular time ity in water.
and place can represent only the composition of the source 30. Which of the following unit works in anaerobic con-
at that time and place.Grab samples’ are simple scoops of ditions? [RPSC ACF - 2011]
the wastewater being sampled and are appropriate where (a) Sludge digestion tank
conditions are constant or well mixed & slow to change. It (b) Sedimentation tank
can be used for instance for Balance Tank sampling or mea- (c) Activated sludge treatment
suring sludge solids (MLSS) in the aeration basin. (d) Trickling filters
(2) Composite Samples: Composite samples are relatively Ans. (a) Sludge digestion tanks are used to break down
employed for the design of wastewater treatment plants as organic matter in sludge through anaerobic processes. It
they provide a more representative picture of the average produces biogas, which can be used for energy generation.
wastewater composition over a specified time period. 31. Pathogenic bacteria enter wastewaters, primarily
27. The hydraulic loading rate of a high rate trickling from [RPSC ACF - 2011]
filter, including recirculation, in m3/m2/day is : (a) Industrial wastes
(a) 1 - 4 (b) 10 - 40 (b) Domestic wastes
(c) 40 - 100 (d) 100 - 200 (c) Both industrial as well as domestic wastes
[RPSC A.En. - 2013] (d) Infiltration in sewers from the surrounding soil
Ans. (b) Ans. (c) Pathogenic bacteria can be present in both indus-
trial & domestic wastewater sources, depending on the ac-
Standard High rate
tivities & sources contributing to the wastewater.
Hydralic loading 10  40 (including 32. Process that removes inorganic nutrients such as
1 4
(in m3 / m 2 / day) recirculation) phosphate, from waste water are considered:
Organic loading 0.32  1.0 (excluding (a) Primary treatment
0.08  0.32
(in kg BOD5 / m3 / day) recirculation (b) Secondary treatment
Depth (m) 1.5  3.0 1 2 m (c) Tertiary treatment
Recirculation ratio
0 0.5  3.0
(d) All of the above
Q R / Q0 Ans. (a) Primary Treatments: It removes approx 60–
70% of suspended solid.
28. The removal of dissolved organic matter occurs in :-  The purpose of primary treatment is to remove
[RPSC VP ITI - 2012] suspended solid materials from the incoming waste water.
(a) Slow sand filters  Floating material (oil & grease) are removed by skimming
(b) Trickling filters tank.
(c) Rapid sand filters  Primary treatment of waste water is made with the
(d) Dual media filters application of gravitational pull mostly.
Ans. (b) Standard Rate Trickling Filter  Organic suspended solids are removed in sedimentation
tank & Inorganic solids (sand and grit) are removed in
grit chamber.
Influent Inlet 33. The stage in which of the biological processes is
(Influent) Peripheral Effluent
Pipe Collection Channel used to purify water in a waste water treatment plant
Rotating
Distribution Arm (a) Secondary sewage
Outlet chamber (b) Primary sewage treatment
Effluent
Laterals To SST (c) Waste water reduction
(d) Biochemical reduction
Ans. (a) Secondry treatment (Biological treatment) :- It is
Filter Media generally carried out aerobically. It leads to stable end prod-
uct. No foul gases are evolved in the process and rate of
reaction is faster (almost 3 times faster than anaerobic pro-
cess for domestic sewage).
 Filters are constructed as bed of stone/gravel onto • The goal of secondary sewage treatment is to achieve a
 SEWAGE TREATMENT 491
water quality, which is suitable for reuse or disposal.
34. The effective method used for reducing Biochemi-
cal Oxygen demand (BOD) Influent Inlet
Peripheral Effluent
(Influent)
(a) Primary (b) Secondary Pipe Collection Channel
Rotating
(c) Tertiary (d) All of the above Distribution Arm
Ans. (b) The goal of secondary sewage treatment is to Outlet chamber
achieve a water quality, which is suitable for reuse or dis- Effluent
Laterals To SST
posal.
35. Anaerobic digestion of solid waste mainly produce
(a) Carbon dioxide (b) Methane
Filter Media
(c) Hydrogen (d) Nitrogen dioxide
Ans. (b) Anaerobic Digestion
 The organisms are broadly classified as
 Methane former: They are strictly (anaerobic) & convert
 Filters are constructed as bed of stone/gravel onto the
acids and alcohol along with hydrogen and carbon dioxide
surface of which settled sewage is sprinkled.
to methane.
 2 m deep circular bed, angular stone about 50 mm in size,
 Acid formers: They consists of (facultative) &
supported on a tile floor
(anaerobic) bacteria and organisms which stabilized the
 Discharge under a head of 0.5-0.8 m
organic acids through hydrolysis.
 Design of TF is done for a average flow.
 When sludge is digested anaerobically, the volume of
38. Thickening of sludge occurs by type ____ settling
1 (RSPCB JEE 2020)
original sludge is reduced to about of the original value. (a) III (b) IV
3
Stages in Sludge Digestion Process (c) I (d) II
(a) Hydrolysis Ans. (b) Types of Settling:
(b) Fermentation 1. Discrete settling (Type I): It occurs when particles do
(c) Methane formation (Methanogenesis) not change its size, shape or mass during settling Ex.-
36. The activated sludge process consists of returning settling in Grit chamber.In it, particle velocity do not
a fraction of the (RSPCB JSO 2020) change.
(a) Effluent leaving secondary clarifier 2. Flocculent settling (Type II): It refers to settling of
(b) Sludge leaving the secondary clarifier flocculent particles of low concentration (1000 ppm) Ex.-
(c) Sludge leaving the primary clarifier Bioflocs in upper part of SST. In it, particle velocity increase
with time.
(d) Effluent leaving primary clarifier
3. Zone settling /Hindered settling (Type III): When the
Ans. (b) Activated Sludge process:
concentration of flocculated particles is in intermediate
range, they are close enough together so that their velocity
1° – ST ASP 2° – ST
fields overlap causing Hindered settling. Particle
QR Sludge form concentration increase during settling. Ex.-found in SST
2° – ST following ASP.
Wasted Alternate return 4. Compression (Type IV): In it, concentration of particles
sludge
becomes so High that particles are in physical contact with
The returned sludge from SST is active & rich in microbial each other.Particle concentration increase during settling.
masses. Hence, It is called activated sludge. Ex.-bottom of SST following TF. This process is common in
It is an aerobic suspended growth type biological process sludge thickening & dewatering operations.
that uses the active micro-organisms kept in suspension 39. Which of the following is not a secondary treatment
in the reactor to decompose & stabilize the soluble & par- unit (RSPCB JEE 09/01/2024)
ticulate (colloidal & suspended) organic matter present in (a) Grit chamber (b) ASP
waste water. (c) Trickling filter (d) UASB reactor
The settled sludge in secondary clarifier containing living Ans. (a) Grit chamber: It belong to the primary treatment
micro-organisms is returned to the available biomass & stage. Their primary function is to remove heavy inorganic
thus speed up the reaction. materials such as sand, gravel & cinders from the
37. Which of the following is not a suspended growth wastewater through sedimentation. These materials can
treatment system? (RSPCB JSO 2020) damage pumps & other equipment in the later stages of
(a) Activated Sludge Process treatment.
(b) Trickling Filter
(c) Sequencing Batch Reactor
(d) Membrane Bio-Reactor
Ans. (b) Trickling filters are not a suspended growth treat-
ment system.It is based on attached growth system.
Standard Rate Trickling Filter
 492 (Civil Ki Goli Publication 9255624029)

and other for peak flow.

 A parshall flume can be used both as a discharge
measuring & velocity control device. It is preferable
inlet outlet
fats, oil and grease because head loss is less.
 Only one section (channel) is required if velocity
control devices has been used (Proportional flow
weir, Sutro weir Parshall flume).
grit
particle
40. The detention period in primary sedimentation tank
in a sewage treatment plant is:
compressed
air (optional)
(RSPCB JEE 09/01/2024)
(a) 2-4 hours (b) 4 to 8 hours
(c) 8 to 12 hours (d) 12 to 18 hours
Ans. (a) The detention period in primary sedimentation
tank in a seepage treatment plant is 2-4 hrs.

grit

B
C

E A

A. Wastewater inlet
B. Floating scum scraper
C. Scum scraper
D. Sludge scraper
E. Sludge outlet

Data for Sedimentation Tanks

 Overflow rate : 12000 – 18000 litre/m2/day for plain
sedimentation, 24000 – 30000 litre/m 2/day for
sedimentation with coagulation.
 Design for Maximum daily flow = 1.8 × average daily
flow.
Treatment
Unit Function  Detention time is 3-4 hours for plain sedimentation tank
stage
& 2-2.5 hrs for sedimentation with coagulation.
Grit Removes heavy
P rimary Q × detention time = volume of tank
chamber inorganic materials
 Vf = 0.3 m/minute., Vf × detention time = length of tank
Removes dissolved and
 Width 10 to 12 m., Length - 4 times the width. (2-5
Secondary ASP organic matter through times)
aerobic biological processes  Depth of tank 3 m (It do not have any effect on efficiency
Removes dissolved and organic of tank)
Trickling
Secondary matter through aerobic biological 41. The efficiency of biochemical oxygen demand
filter
proceses using fixed-film media (BOD) removal in trickling filter is:
Removes dissolved and (RSPCB JEE 09/01/2024)
UASB
Secondary organic matter through anaerobic (a) 98% (b) 75%
reactor
biological processes (c) 56% (d) 65%
Ans. (b) Advantages of Trickling filters
Grit Chamber: Grit chambers are provided in the form of 1. Rate of filter loading is high as such requiring lesser
longer length & smaller X-sec area & should not allow land areas & smaller quantities of filter media for their
settlement of organic materials. installations.
 Stokes law can not be applied to grit chamber 2. Effluent obtained from the trickling filter is sufficiently
nitrified & stabilized, They can remove about 75% of BOD
4 (  s   w )d & about 80 % of suspended solids.
Vs2 = 3 C .
D w 3. Moisture content of sludge, obtained from trickling filter
 It is Located either before or after sewage pumps. It is as high as 99% or so.
separate non-decaying heavy inorganic material
from sewage.
 Two channels are provided, one for normal flow
 SEWAGE TREATMENT 493
tank.
 Primary treatment of waste water is made with the
Rotating Domed
Influent Enclosure application of gravitational pull mostly.
Distributer  Organic suspended solids are removed in sedimentation
tank & Inorganic solids (sand and grit) are removed in grit
chamber.
 Primary treatment removes 60 - 70% suspended solids
Media
present in the sewage and satisfied 30 - 40% BOD
associated with it.
Air Air
Special Point: Primary treatment will not remove the
Influent
colloidal & soluble (dissolved) organic content of waste

Effluent
Filter
Trickling Filter water.
Treated Water 43. Coagulation after SST is provided for which of the
following? (RSPCB JEE 09/01/2024)
Clarifier
(a) Dechlorination
Recycle
(b) Metal removal
Sludge
(c) Suspended solids removal
Pump (d) pH adjustment
Ans. (c) The primary purpose of coagulation after SST is
the removal of suspended solids. Coagulants added
Standard Rate Trickling Filter during this process neutralize the charges on colloidal and
suspended particles, promoting their aggregation into
Influent Inlet larger flocs. These larger flocs settle more readily in
(Influent) Peripheral Effluent
Pipe Collection Channel sedimentation tanks, contributing to the removal of
Rotating
Distribution Arm suspended solids.
Outlet chamber 44. What is the range of Mesophilic and Thermophilic
Effluent range of decomposition ? (RSPCB JEE 09/01/2024)
To SST
Laterals (a) 20-35 and 55-60
(b) 15-20 and 40-50
(c) 30-40 and 50-60
Filter Media (d) 40-50 and 60-70
OR
Generally for the satisfactory working of a sludge
 Filters are constructed as bed of stone/gravel onto digestion unit, the pH range of a digested sludge
the surface of which settled sewage is sprinkled. shall be maintained as
 2 m deep circular bed, angular stone about 50 mm in (RAJASTHAN DLB AE (SHIFT 1) 2015)
size, supported on a tile floor.Over this trickling filter, (a) 10.5 to 12.0 (b) 4.5 to 6.0
the rotary distributors spray the incoming sewage (c) 6.5 to 8.0 (d) 8.5 to 10.0
continuously over the filter bed under the action of Ans. (c) Factors Affecting Sludge Digestion and their
gravity. Control
 Discharge under a head of 0.5-0.8 m (a) Temperature
 Design of TF is done for a average flow.  Mesophilic bacteria have an optimal temperature for
BOD removal for various units is : growth between 30-40 oC & consequently mesophilic
digesters are usually operated at temperatures around
Treatment method BOD Removal Efficiency 35oC.
Oxidation Ditch 98%  Thermophilic bacteria have an optimal temperature range
Oxidation Pond 90% of 50-60oC.
Aerated Lagoons 65  90%
Mesophilic Thermophilic
range range
42. The BOD removal efficiency, during primary treat- digestion digestion
ment, under normal conditions are Period of by mesophilic by thermophilic
bacteria bacteria
(RSPCB JEE 09/01/2024) digestion
(days)
(a) 65% (b) 10%
(c) 55% (d) 30% 30
Ans. (d) Primary Treatments: It removes approx 60–
70% of suspended solid. 15
 The purpose of primary treatment is to remove suspended
solid materials from the incoming waste water. 20ºC 29ºC 40ºC 54ºC 60ºC
 Floating material (oil & grease) are removed by skimming Temperature
 494 (Civil Ki Goli Publication 9255624029)

(b) Mixing and Steering of Raw Sludge or circular tank in the primary sewage treatment plant that
(c) pH is designed to reduce the velocity of the flow of sewage
 pH decreases due to overdosing of raw sewage, over to eliminate the grit materials such as sand, ash and
withdrawal of digested sludge & sudden admission of clinkers, eggshells, bone chips, and many inert materials
industrial waste. inorganic in nature.
(d) Seeding with Digested Sludge: It helps in achieving • It is based on transition law.
quick balance condition. • The grit chamber should not allow the settlement of
(e) Nuisance Organisms: Remedy is to add iron to ppt. organic materials.
iron sulphide. •Two channels are provided one for normal flow and the
45. Formation of Ammonia in the process of sludge di- other for peak flow obviously used with normal flow.
gestion (RSPCB JEE 09/01/2024)
(a) Alkaline Formation
(b) Acid Formatting
(c) ACID Regression inlet outlet
(d) Other fats, oil and grease

Ans. (a) Sludge digestion:

Sludge digestion is a biological process in which organic
solids are decomposed into stable substances. Digestion
reduces the total mass of solids, destroys pathogens, and
makes it easier to dewater or dry the sludge. Digested grit
particle
sludge is inoffensive, having the appearance and
characteristics of rich potting soil. It includes aerobic compressed
air (optional)
suspended culture.
It is done in three stages:
1. Acid Fermentation: Important things in this stage is-
• Anaerobic and facultative bacteria start to act upon the
sludge
• Acid former bacteria's stabilize the matter by hydrolysis
• Soluble products are fermented to volatile acids and
grit
organic alcohol of low molecular weight
• Evolution of CH4 and H2S gases
• Low pH - Highly acidic in nature.
• High BOD
2. Acid Regression:
• Volatile organic acids and nitrogenous compounds remain
in the mix
• Bacteria's forms acid carbonates and ammonia primary treatment secondary treatment
compounds
aeration tank disinfectant
• The evolution of foul gases is reduced raw sewage secondary
• pH rises to more than 6.5 clarifier
• BOD remains high screens
• Separate layers of scums are formed. primary secondary
3. Alkaline Fermentation: effluent effluent
• Proteins and organic acids are broken by anaerobic comminutor discharge to
surface water
bacteria air compressor (or tertiary
primary activated
• The formation of ammonia, organic acids and gases return sludge sludge treatment
clarifier if needed)
continues
• Liquid separates out from solids and the digestive sludge
return sludge pump
is formed.
• Digestive sludge is formed at the bottom of the tank grit chamber
raw or primary sludge
• pH value increases and the mix becomes alkaline grit disposal
• BOD rapidly falls down.
46. Which is used to remove grit, sand and such other sludge treatment and disposal
inorganic matter from sewage?
(RSPCB JSO 09/01/2024)
(a) Screen chamber • Grit removal basin, such as grit chambers or grit channels
(b) Grit chamber or Detritus tanks are the sedimentation basins placed in
(c) Primary settling tank front of the waste water treatment plant to remove the
(d) Sludge digester inorganic particles such as sand, gravel grit, egg shells,
Ans. (b) Grit chamber: -A grit chamber is a long narrow
 SEWAGE TREATMENT 495
etc. A detention time of about 40 to 60 seconds is generally Phosphates are to be added in nitrates, sulphates, and
sufficient for a water depth of about 1 to 1.8 m phosphates form for effective sludge digestion.
Secondary settling tank: In the secondary settling tank,
the concentration of particles increases from top to bottom
leading to the thickening of sludge. mixed sludge effluent
dewatering
dissolved-air flotation tank from secondary to secondary
oSuch secondary clarifiers where zone settling occurs are treatment treatment
designed on the basis of solid loading and checked for
surface overflow rate. Both of which can be determined by filtered water
conducting a settling column analysis.
Activated sludge process: The activated sludge process is
a multi-chamber reactor unit that uses highly concentrated dried
microorganisms to degrade organics and remove nutrients air compressor sludge
air
from wastewater with an aerobic process, producing quality water grit tank
filters
effluent. The goal is to maintain aerobic conditions and to disposal biogas
keep the activated sludge suspended. boiler
thickened
Trickling filter: Biological Trickling filters are aerobic sludge
treatment device that removes organic material from steam
wastewater by using microorganisms attached to a media.
• This mechanism is employed in many different auger
technologies, including rotating biological contactors and
packed bed reactors. These processes are known as steam anaerobic
attached-growth processes. thermal flash digester
sludge
hydrolysis tank
• Suspended-growth techniques, on the other hand, are pulper
reactor
systems that keep microorganisms alive in a solution. pulped sludge hydrolyzed sludge heat exchanger
47. Sludge digestion is (RSPCB JSO 09/01/2024)
(a) Disposal of sludge
(b) Dilution of sludge 48. The anaerobic digestion process produces gas com-
(c) Stabilisation of sludge
posed of ____known as biogas
(d) Removal of waste products from sludge
(RSPCB JSO 09/01/2024)
Ans. (c) Sludge Digestion - The sludge digestion serves
(a) Methane (CH4) and carbon dioxide (CO2)
both to reduce the volume of thickened sludge still further
& to render the remaining solids inert and relatively (b) Oxygen & Nitrogen
pathogen free. (c) Oxygen & CO2
Sludge digestion process: (d) Nitrogen & methane
(i) The sludge withdrawn from the sedimentation basins Ans. (a) Bio-gas = Biogas is a mixture of gases, primarily
contains a lot of putrescible organic matter and if disposed consisting of methane and carbon dioxide, produced from
of without any treatment, the organic matter may raw materials such as agricultural waste, manure, municipal
decompose, producing foul gases and a lot of nuisance, waste, plant material, sewage, green waste and food waste.
pollution, and health hazards. It is a renewable energy source.
(ii) In order to avoid such pollutions, the sludge is first of Biogas is produced by anaerobic digestion with anaerobic
all, stabilised by decomposing the organic matter under organisms or methanogen inside an anaerobic digester,
controlled anaerobic conditions and then disposed of biodigester or bioreactor.
suitably after drying on drying beds, etc. The process of Biogas is primarily methane (CH4) and carbon dioxide (CO2)
stabilization is called the sludge digestion, and the tank and may have small amounts of hydrogen sulfide (H2S),
where the process is carried out is called the sludge moisture and siloxanes.
digestion tank. The gases methane, hydrogen, and carbon monoxide (CO)
Factor affecting anaerobic sludge digestion: can be combusted or oxidized with oxygen.
(i) Temperature: In the zone of thermophilic digestion, This energy release allows biogas to be used as a fuel; it
thermophilic organisms are very active, and the temperature can be used in fuel cells and for any heating purpose, such
ranges from 35° C to 60° C. The digestion period for such as cooking.
digestion is about 7 to 10 days only. It can also be used in a gas engine to convert the energy
In the zone of mesophilic digestion, mesophilic organisms in the gas into electricity and heat.
are very active, and the temperature ranges from 26° C to Biogas can be compressed after the removal of carbon
35° C. The digestion period for such digestion is about 30 dioxide, the same way natural gas is compressed to CNG
days. and used to power motor vehicles.
(ii) pH: The desired range of pH values for the efficient It qualifies for renewable energy subsidies in some parts
digestion of sludge is 6.5 to 7.5. of the world.
(iii) Carbon, Oxygen, Hydrogen, Nitrogen, Sulphur, and Biogas can be cleaned and upgraded to natural gas
Phosphorous are the important nutrients required for standards when it becomes bio-methane.
bacterial growth in sludge. Carbon, Oxygen, and Hydrogen
are plentiful in Indian sludge. Nitrogen, Sulphur, and
 496 (Civil Ki Goli Publication 9255624029)

Biogas is considered to be a renewable resource because Advantages of Trickling filters

its production-and-use cycle is continuous, and it 1. Rate of filter loading is high as such requiring lesser
generates no net carbon dioxide. land areas and smaller quantities of filter media for their
As the organic material grows, it is converted and used. installations.
It then regrows in a continually repeating cycle. 2. Effluent obtained from the trickling filter is sufficiently
From a carbon perspective, as much carbon dioxide is nitrified and stabilized, They can remove about 75% of
absorbed from the atmosphere in the growth of the primary BOD and about 80 % of suspended solids.
bio-resource as is released, when the material is ultimately 3. Moisture content of sludge, obtained from trickling filter
converted to energy. is as high as 99% or so .
49. The depth of tank for a high rate trickling filter 52. Septic tank works on the principle of ______.
may vary from (RPSC AE DLB 2022) (RVUNL AE 2021)
(a) 1.0 to 1.8 m (b) 2.0 to 2.7 m (a) Aerobic digestion
(c) 3.0 to 1.8 m (d) 4.0 to 5.3 m (b) Anaerobic digestion
Ans. (a) Media depths of 0.9-2.5 m have been used for high (c) Continuous flow
rate filters with an optimum range of 1.5-20.0 m for the first (d) Aeration system
stage & 1-2 m for the second-stage filters. (e) None of these
50. The specific gravity of sewage is
HPSSC JE CODE 502/2018
Ans. (b) Septic tanks work on the principle of anaerobic
DDA JE 23.04.2018 Shift - II digestion.In it, bacteria break down organic matter in the
M.P. VYAPAM Sub Engg. 4.9.2018 9.00 am absence of oxygen. The bacteria produce methane gas as
NPCC 2017 a byproduct.
MP VYAPAM Sub. Engg. 3.04.2016, 9.00 am
UPSSSC JE 2015 Paper-I
53. Which of the following statements about the tube
DLB AE Shift III, 2015 settler is INCORRECT ?
HPSSC JE 2015 (RCDF Ast. Manager, Env. & ETP 18/08/2021)
MH Nanded ZP 2014 (a) It is the combination of multiple channels at an
(a) Slightly greater than 1 angle of 60°
(b) Slightly less than 1 (b) Settling depth is less than conventional one.
(c) 0 (c) Settling time is less than conventional one.
(d) Equal to 1 (d) Effective settling area is decreased by providing
Ans. (a) Sewage normally contain very small amount of multiple tubular channels sloped at an angle of
solid (0.1%) as compare to water (99.9%). The specific grav- 60° and adjacent to each other.
ity is slightly greater than 1. Ans. (d)Tube settlers are designed to increase the surface
 Solids in sewage consist of both the organic and area available for settling by using inclined plates or tubes,
inorganic solids. The organic matter work out to be about which shortens the settling path & increases the efficiency
45% of total solid & the remaining about 55% is the of the sedimentation process.
inorganic matter.  Clarifiers/basins equipped with tube settlers can operate
51. The efficiency of biochemical oxygen demand at 2 - 4 times the normal rate of clarifiers/basins without
(BOD) removal in trickling filter is: tube settlers.
(RVUNL AE 2021)  It is possible to cut coagulant dosage by up to half
(a) 98% (b) 75% while maintaining a lower influent turbidity to the treatment
(c) 56% (d) 65% plant filters.
(e) 82%  Less filter backwashing equates to significant operating
Ans. (b) Trickling Filter: cost savings for both water & electricity.
 New installations using tube settlers can be designed
Inlet
smaller because of increased flow capability.
Influent
(Influent) Peripheral Effluent  Flow of existing water treatment plants can be increased
Pipe Collection Channel
Rotating through the addition of tube settlers.
Distribution Arm  Tube settlers increase allowable flow capacity by
Outlet chamber
Effluent
expanding settling capacity & increasing the solids
Laterals To SST removal rate in settling tanks.
Sludge Discharge

Tube Settlers
Filter Media
Fine sand backflow

Clear Water
A trickling filter is an aerobic attached growth system. This
filter consists of a 2 m to 3 m deep circular bed of packed
granular stone or gravel about 50 to 70 mm in size. Over
this trickling filter, the rotary distributors spray the Raw Water

incoming sewage continuously over the filter bed under

the action of gravity. Reaction Tank Clarifier Tank
 SEWAGE TREATMENT 497
 Tube settlers offer an inexpensive method of upgrading Ans. (a) Sludge drying beds primarily work on a bed of
existing water treatment plant clarifiers & sedimentation sand & gravel, allowing both drainage & evaporation to
basins to improve performance. reduce sludge volume.
 They can also reduce the tankage/footprint required in  Sand is one of the commonly used media in sludge
new installations or improve the performance of existing drying beds. It provides a porous surface which allows
settling basins by reducing the solids loading on water to drain efficiently while supporting the sludge
downstream filters. during drying.The sand bed allows for adequate aeration
 Made of PVC, tube settlers can be easily supported with & drainage, which are essential for the dewatering & drying
minimal structures which often incorporate the effluent of sludge.Sand beds are relatively low-cost & easy to
trough supports. maintain, making them a popular choice for sludge drying
 The settlers use multiple tubular channels sloped at an applications.
angle of 60° & adjacent to each other, which combine to  Sludge drying beds are used in wastewater treatment
form an increased effective settling area. plants for dewatering & drying sludge, which is the solid
54. Sludge thickening will reduce the volume of sludge residue left after the wastewater treatment process. These
by_______ conditioning beds typically consist of a specific type of media or support
(RCDF Ast. Manager, Env. & ETP 18/08/2021) system which facilitates the dewatering & drying process.
(a) Physical (b) Chemical 57. Read the given statements related to secondary
(c) Biological (d) Physico-chemical treatment of wastewater and identify whether they
Ans. (a) Sludge thickening primarily relies on physical are correct or incorrect. Statements : -
processes to increase the concentration of solids and (A) Secondary treatment of wastewater can be achieved
reduce the volume of sludge. by chemical unit processes such as chemical oxida-
Sludge Thickener tion, coagulation-flocculation and sedimentation and
 To reduce the volume to be handled in sludge chemical precipitation.
digestor, sludge thickner is provided. (B) Secondary treatment of wastewater cannot be
 These are types of thickening: achieved by employing biological processes (aero-
1. Gravity thickening bic) where bacteria is used as a catalyst for removal
2. Centrifugation of pollutant.
3. Air floating (RCDF Ast. Manager, Env. & ETP 18/08/2021)
 It is a procedure used to increase solids content by (a) Statement A is correct but B is incorrect
removing a portion of liquid fraction. (b) Statement B is correct but A is incorrect
 Relation between solid levels & water content: Water (c) Both statements are correct
content in the sludge is given by (d) Both statements are incorrect
Water Content (%) = 100 -Dry solids level (%) Ans. (d) Statement A : "Secondary treatment of wastewater
55. Fresh sludge has moisture content of 99%, and af- can be achieved by chemical unit processes like as
ter thickening, its moisture content is reduced to chemical oxidation, coagulation-flocculation &
96%.The reduction in the volume of sludge is: sedimentation, and chemical precipitation is incorrect
(RCDF Ast. Manager, Env. & ETP 18/08/2021) statement. Secondary treatment of wastewater primarily
(a) 197% (b) 5% relies on biological processes rather than chemical unit
(c) 75% (d) 3% processes.
Ans. (c) Statement B : "Secondary treatment of wastewater can not
Given that, P1 = 99%, P2 = 96% be achieved by employing biological processes (aerobic)
 The volume of the digested sludge at given moisture where bacteria is used as a catalyst for removal of pollutant
content is given by the formula as is incorrect statement. In fact, secondary treatment of
wastewater heavily relies on biological processes,
V2 (100  P1 ) V2  100  99  particularly aerobic processes where bacteria play a
  
V1 (100  P2 ) V1  100  96  important role in breaking down organic matter & removing
pollutants.
V2 1 58. The term 'Sludge Volume Index' (SVI), used for the

V1 4 = 0.25 determination of sludge settleability is defined as:
(RCDF Ast. Manager, Env. & ETP 18/08/2021)
 V2  (a) Volume occupied in ml by one gram of solids in
% decrease in volume = 1    100 the mixed liquor after settling for 30 min
 V1  (b) Volume occupied in ml by one gram of solids in
= (1 – 0.25) × 100 = 75% the mixed liquor after settling for 150 min
56. On what media does the sludge drying bed work? (c) Percentage decrease in the volume of sludge
(RCDF Ast. Manager, Env. & ETP 18/08/2021) after secondary treatment of waste water
(a) Sand (d) Mass of sludge occupied in 1 ml of mixed liquor
(b) Poypropylene membrane after settling for 150 min
(c) Geotextile cloth Ans. (a) Sludge Volume Index (SVI)
(d) Plastic  Sludge volume index is the volume occupied in mL by
 498 (Civil Ki Goli Publication 9255624029)

one gm of solids in the mixed liquor after settling for 30 60. Sewage treatment works are normally designed for
minutes. a design period of [RPSC ACF - 2011]
 It indicate physical state of sludge in the Biological (a) 40 - 50 years (b) 30 - 40 years
Aeration system. (c) 15 - 20 years (d) 5 - 10 years
 Generally SVI value should be (80–150) ml/gm. Ans. (c) Sewage treatment works are normally designed for
 SVI represents the degree of concentration of a design period of 15 to 20 years.
sludge in the system 61. Which method is employed in cold countried to main-
 Its unit ml/gm tain temperature of ASP in skimming tank
(RSPCB JEE 09/01/2024)
QR

X 106 (a) Passing hot air
, Xu  , where X is MLSS in
Q0 X u  X SVI (b) Boiling
tank (ppm) (c) Passing hot water
(d) Direct burning
QR X
 6 Ans. (c) In cold countries, to maintain the temperature of
Recirculation Ratio = Q 0 10  X the Activated Sludge Process in skimming tanks, the
SVI method of passing hot air is generally used.It helps to keep
the biological activity at optimal levels by preventing the
Vob (ml /l ) Vob temperature from dropping too low.
Where, SVI = X (mg/l )  X ml /mg
ob ob 62. The hydraulic retention time in extended aeration
When 1 litre sample of mixed liquor is allowed to settle activated sludge systems is [RPSC A.En. - 2013]
for half an hour & the settled sludge volume (Vob) in ml. (a) 4 - 5 hours (b) 4 - 6 hours
Concentration of suspended solids in the mixed liquor in (c) 8 - 10 hours (d) 12 - 24 hours
ppm be XOb. Ans. (d) The hydraulic retention time in extended aeration
59. Read the given statements related to biological treat- activated sludge systems is generally 12 to 24 hours.
ment of waste water and identify whether they are 63. The design period (in years) for sewage treatment
correct or incorrect. Statements : - units are (RSMSSB JE (DEGREE) 2022)
(A) The objective of the biological treatment of waste- (a) 40-50 (b) 5-10
water is NOT to remove nutrients such as nitrogen (c) 15-20 (d) 30-40
and phosphorous from the wastewater. Ans. (c) The design period treatment unit is taken as 15 - 20
(B) The objective of the biological treatment of waste- years.
water is to remove organic matter from the waste- Design life of different units in a sewerge system
water, which is present in soluble and colloidal form
(RCDF Ast. Manager, Env. & ETP 18/08/2021) Component Design Life
(a) Statement A is correct but B is incorrect
(b) Statement B is correct but A is incorrect Sewer system 20 years
(c) Both statements are correct Land acquisition 30 years
(d) Both statements are incorrect Conventional sewer 30 years
Ans. (b) Statement A: "The objective of the biological
Pumping station 30 years
treatment of wastewater is NOT to remove nutrients such
as nitrogen and phosphorous from the wastewater is Non-conventional sewer 15 years
incorrect statement. One of the primary objectives of Pumping mains 15 years
biological treatment of wastewater is indeed to remove Pumping Machinery 15 years
nutrients, including nitrogen & phosphorus. Excessive
levels of these nutrients in wastewater can lead to Sewage treatment 15 years
environmental issues like as eutrophication, where Effluent disposal 30 years
overgrowth of algae occurs, leading to oxygen depletion Effluent utilization 15 years
& harm to aquatic life. Biological treatment processes, like
as activated sludge, can effectively remove nutrients
through processes like nitrification & denitrification. 64. The detention period for oxidation pond is generally
Statement B : "The objective of the biological treatment kept as : (RSPCB JEE 2016)
of wastewater is to remove organic matter from the (a) 30 hours (b) 10 to 15 days
wastewater, which is present in soluble and colloidal form (c) 2 to 6 days (d) 24 hours
is correct statement. The primary objective of biological Ans. (b) Oxidation Pond
treatment of wastewater is to remove organic matter, which These ponds flow through the earthen channel. In these
is generally present in soluble & colloidal forms. Biological ponds, a longer detention period is provided during which
treatment processes, like as activated sludge, trickling the organic matter get decomposed by the action of micro-
filters, & sequencing batch reactors, utilize microorganisms organism. In it, a special type of relationship exists between
to break down organic pollutants into simpler, more stable algae & micro-organisms which is known as a symbiotic
substances like carbon dioxide & water. relationship.
 SEWAGE TREATMENT 499
66. Trickling filter is : (RSPCB JEE 2016)
O2 (a) Suspended growth process
(b) Stationary media attached film
Algae Effluent (c) Moving media fixed film
Raw Photosynthesis: CO2+ HO2= O2+ Algal Cells Aerobic
Wastewater Algae Zone (d) Stationary media suspended growth
Settleable Organic matter CO + NO, PO3+ H O
Solids 2 2
Ans. (c) A trickling filter is an attached growth process i.e.
+ Oxygen Facultative
Bacteria Zone process in which microorganisms responsible
Biomass
Rising Gases
Anaerobic for treatment are attached to inert packing mate-
Zone

Sludge
rial.
CO2+ CH4+ NH3+ H2S (gases)
Layer + New Cells  The packing material used in attached growth
Solids removal mechanism in a faculative stabilization pond processes includes rock, gravel, slag, sand, red-
wood, and a wide range of plastic & other syn-
 Minimum depth of water 0.3 m thetic materials.
67. The unit in which both, sedimentation and digestion
 It is very cheap but required large land (0.5-1
processes of sludge take place simultaneously is :
hectare).
(RSPCB JEE 2016)
 For sewage treatment using an oxidation pond, when (a) Detritus tank (b) Digestion tank
it gets overload, sodium nitrate is added to stimulate (c) Skimming tank (d) Imhoff tank
the algal growth. Ans. (d) The Imhoff tank is the unit which simultaneously
 The difference between oxidation ponds and oxida- performs both sedimentation & digestion of sludge.
tion lagoons or ditches is that the detention period of la- 68. UASB process is classified as :
goons is 1 month to 3 months. (RSPCB JEE 2016)
Design Criteria (a) Attached film process
(b) Suspended growth process
(c) Fixed film process
Area (each unit) 0.2  0.4 hectare (d) Stationary growth process
Depth 1  1.8 Ans. (b) The UASB (Upflow Anaerobic Sludge Blanket)
2  6 weeks (168- process is a suspended growth process where wastewater
Detention time flows upward through a sludge blanket containing anaero-
1008) hrs.
bic microorganisms.
B.O.D. removal 80  90% 69. The end products of aerobic and anaerobic biological
Pathogenic bacteria processes contain one common constituent. This
99.9%
remove common end product in aerobic and anaerobic pro-
Sludge accumulation 2  5 cm / year cesses is : (RSPCB JEE 2016)
(a) H2S (b) CO2
Length of tank (L > 750 m) 3 × width of tank (c) CH4 (d) H2O
Suspended solid removal 90% Ans. (b) Carbon dioxide (CO2) is a common end product in
Organic loading Hot countries-150-300 both aerobic & anaerobic biological processes.
(In kg/hectare/day) Cold countries-60-90 70. The maximum and minimum flows of sewage are of
great concern in the design of :
(RSPCB JEE 2016)
65. The size of clear opening in case of bar-rack (coarse- (a) SST (b) PST
screen) is usually (RSPCB JEE 2016) (c) ASP (d) Grit chamber
(a) 10 mm to 20 mm Ans. (a) The maximum & minimum flows of sewage are
(b) 20 mm to 50 mm important considerations in the design of a grit
(c) Less than 10 mm chamber because grit chambers are designed to
(d) 50 mm or more remove heavier inorganic materials (such as
Ans. (d) Screens: A bar screen is a mechanical filter use to sand, gravel, etc.) from sewage, & the flow rate
remove larger objects, like as rags & plastics, affects the sedimentation process. If the flow is
from wastewater. It is the part of primary filtra- too fast, the grit may not settle; if it's too slow,
tion flow and generally is the first, or prelimi- organic materials might also settle, which is un-
nary, level of filtration, being installed at influ- desirable.
ent to a wastewater treatment plant.
Spacing between bars :
(i) Coarse screen = 50 mm or more
(ii) Medium Screen = 6 mm - 40 mm
(iii) Fine screen 1.5 mm 3 mm
 500 (Civil Ki Goli Publication 9255624029)

71. The F/M ratio in aeration tank of an ASP treating

0.96 MLD of wastewater having a BOD of 250 mg/l
with MLSS concentration of 3000 mg/l and HRT of 6
hrs is : (RSPCB JEE 2016)
(a) 0.303 (b) 0.33
(c) 3.3 (d) 3.03
 Lt. 
Ans. (b) ASP (Q) = 0.96 MLD or 0.96 × 106  
 day 
BOD = 250 ppm
MLSS (n) = 3000 ppm
6
Hydraulic Retention Time = 6 hours or days
24
Volume = HRT × ASP (t × Q)
6
v  0.96  106  24 104 litres
24
Mass of BOD applied aeration tank
F/M 
Mass of MLSS
Q  S0
F/M 
Vn

0.96  106 (L / day)  250(mg / L)

F/M   0.33
24  104 (L)  300(Mg / L)