# GATE Questions & Answers of Water and Waste Water Civil Engineering

#### Water and Waste Water 76 Question(s)

A rapid sand filter comprising a number of filter beds is required to produce 99 MLD of potable water. Consider water loss during backwashing as 5%, rate of filtration as 6.0 m/h and length to width ratio of filter bed as 1.35. The width of each filter bed is to be kept equal to 5.2 m. One additional filter bed is to be provided to take care of break-down, repair and maintenance. The total number of filter beds required will be

A water sample analysis data is given below.

 Ion Concentration, mg/L Atomic Weight $\style{font-family:'Times New Roman'}{\mathrm{ca}^{2+}}$ 60 40 $\style{font-family:'Times New Roman'}{\mathrm{Mg}^{2+}}$ 30 24.31 $\style{font-family:'Times New Roman'}{{\mathrm{HCO}}_3-}$ 400 61

The carbonate hardness (expressed as mg/L of CaCO3, up to one decimal place) for the water sample is ______

The ultimate BOD (L0) of a wastewater sample is estimated as 87% of COD. The COD of this wastewater is 300 mg/L. Considering first order BOD reaction rate constant k (use natural log) = 0.23 per day and temperature coefficient $\style{font-family:'Times New Roman'}{\mathrm\theta}$ = 1.047, the BOD value (in mg/L, up to one decimal place) after three days of incubation at 27oC for this wastewater will be ______

A waste activated sludge (WAS) is to be blended with green waste (GW). The carbon (C) and nitrogen (N) contents, per kg of WAS and GW, on dry basis are given in the table.

 Parameter WAS GW Carbon (g) 54 360 Nitrogen (g) 10 6

The ratio of WAS to GW required (up to two decimal places) to achieve a blended C:N ratio of 20:1 on dry basis is ______

As per IS 10500:2012, for drinking water in the absence of alternate source of water, the permissible limits for chloride and sulphate, in mg/L, respectively are

A flocculation tank contains 1800 m3 of water, which is mixed using paddles at an average velocity gradient G of 100/s. The water temperature and the corresponding dynamic viscosity are $30^\circ C$ and $0.789\times10^{-3}$ Ns/m2 , respectively. The theoretical power required to achieve the stated value of G (in kW, up to two decimal places) is ______

At a small water treatment plant which has 4 filters, the rates of filtration and backwashing are 200 m3/d/m2 and 1000m3/d/m2 , respectively. Backwashing is done for 15 min per day. The maturation, which occurs initially as the filter is put back into service after cleaning, takes 30 min. It is proposed to recover the water being wasted during backwashing and maturation. The percentage increase in the filtered water produced (up to two decimal places) would be ______

A schematic flow diagram of a completely mixed biological reactor with provision for recycling of solids is shown in the figure.

$S_0,S$ = readily biodegradable soluble BOD, mg/L Q,

$Q,Q_r,Q_w$ = flow rates, m3 /d

$X_0,X,X_e,X_u$ = microorganism concentrations (mixed-liquor volatile suspended solids or MLVSS), mg/L

The mean cell residence time (in days, up to one decimal place) is ______

The wastewater from a city, containing a high concentration of biodegrable organics is being steadily discharged into a flowing river at a location S. If the rate of aeration of the river water is lower than the rate of degradation of the organics, then the dissolved oxygen of the river water

The spherical grit particles , having a radius of 0.01 mm and specific garvity of 3.0, need to be separated in a settling chamber. It is given that

$\bullet$ g = 9.81 m/s2

$\bullet$ the density of the liquid in the settling chamber - 1000 kg/m3

$\bullet$ the kinematic viscosity of the liquid in the settling chamber = 10-6m2/s

Assuming laminar condition , the settling velocity (in mm/s, up to one decimal place) is ___________

Two wastewater streams A and B having an identical ultimate BOD are getting mixed to form the stream C. The temprature of the stream A is 20ºC and the temprature of the stream C is 10ºC. It is given that

$\bullet$ the 5-day BOD of the stream A measured at 20ºC = 50mg/l

$\bullet$  BOD rate constant (base 10) at 20ºC =0.115 perday

$\bullet$ temperature coefficiant = 1.135

The 5-day BOD (in mg/l,up to one decimal place) of the stream C, calculated at 10ºC, is __________

The wastewater having an organic concentration of 54 mg/l is flowing at a stady rate of 0.8m3/day through a deteniton tank of dimensions 2 m × 4 m × 2 m. If the contents of the tank are well mixed and the decay constant is 0.1 per day, the outlet concentration (in mg/l, up to one decimal place) is ___________

The most important type of species involved in the degradation of organic mater in the case of activated sludge process is

For a given water sample, the ratio between  $\style{font-family:'Times New Roman'}{{\mathrm{BOD}}_{5-\mathrm{day}.20^\circ\mathrm c}}$ and the ultimate $BOD$ is 0.68. The value of the reaction rate constant $k$(on base $e$) (in day-1,up to two decimal places) is __________

A municipal corporation is required to treat 1000 m3/day of water. It is found that an overflow rate of 20m/day will produce a satisfactory removal of the discrete suspended particles at a depth of 3 m. The diameter (in meters, rounded to the nearest integer) of an circular settling tank designed for the removal of these particles would be ____________

The analysis of a water sample produces the following results:

 Ion milligram per milli-equivalent for the ion Concentration (mg/L) Ca2+ 20.0 60 Mg2+ 12.2 36.6 Na+ 23.0 92 K+ 39.1 78.2 Cl- 35.5 71 SO42- 48.0 72 HCO3- 61.0 122

The total hardness (in mg/L as CaCO3 ) of  the water sample is ___________

Crown corrosion in a reinforced concrete sewer is caused by:

Match the items in Group – I with those in Group – II and choose the right combination.

 Group - I Group - II P. Activated sludge process 1. Nitrifiers and denitrifiers Q. Rising of sludge 2. Autotrophic bacteria R. Conventional nitrification 3. Heterotrophic bacteria S. Biological nitrogen removal 4. Denitrifiers

Effluent from an industry 'A' has a pH of 4.2. The effluent from another industry 'B' has double the hydroxyl (OH-) ion concentration than the effluent from industry 'A'. pH of effluent from the industry 'B' will be __________

The 2-day and 4-day BOD values of a sewage sample are 100 mg/L and 155 mg/L, respectively. The value of BOD rate constant (expressed in per day) is _________

A water supply board is responsible for treating 1500 m3/day of water. A settling column analysis indicates that an overflow rate of 20 m/day will produce satisfactory removal for a depth of 3.1 m. It is decided to have two circular settling tanks in parallel. The required diameter (expressed in m) of the settling tanks is __________

The hardness of a ground water sample was found to be 420 mg/L as CaCO3. A softener containing ion exchange resins was installed to reduce the total hardness to 75 mg/L as CaCO3 before supplying to 4 households. Each household gets treated water at a rate of 540 L/day. If the efficiency of the softener is 100%, the bypass flow rate (expressed in L/day) is _________

A sample of water has been analyzed for common ions and results are presented in the form of a bar diagram as shown.;

The non-carbonate hardness (expressed in mg/L as CaCO3) of the sample is

For a wastewater sample, the three-day biochemical oxygen demand at incubation temperature of 20°C (BOD3day, 20°C) is estimated as 200 mg/L. Taking the value of the first order BOD reaction rate constant as 0.22 day-1, the five-day BOD (expressed in mg/L) of the wastewater at incubation temperature of 20°C (BOD5day, 20°c) would be _________

Total Kjeldahl Nitrogen (TKN) concentration (mg/L as N) in domestic sewage is the sum of the concentrations of :

Consider a primary sedimentation tank (PST) in a water treatment plant with Surface Overflow Rate (SOR) of 40 m3/m2/d. The diameter of the spherical particle which will have 90 percent theoretical removal efficiency in this tank is ________ $\mu$m. Assume that settling velocity of the particles in water is described by stokes’s Law.

Given: Density of water = 1000 kg/m3; Density of particle = 2650 kg/m3; g = 9.81 m/s2; Kinematic viscosity of water (v) = 1.10 X 10-6 m2/s.

A groundwater sample was found to contain 500 mg/L total dissolved solids (TDS). TDS (in %) present in the sample is ____

A water treatment plant of capacity, 1 m3/s has filter boxes of dimensions 6 m × 10 m. Loading rate to the filters is 120 m3/day/m2. When two of the filters are out of service for back washing, the loading rate (in m3/day/m2) is __________.

Ultimate BOD of a river water sample is 20 mg/L. BOD rate constant (natural log) is 0.15 day-1. The respective values of BOD (in %) exerted and remaining after 7 days are :

In a wastewater treatment plant, primary sedimentation tank (PST) designed at an overflow rate of 32.5 m3/day/m2 is 32.5 m long, 8.0 m wide and liquid depth of 2.25 m. If the length of the weir is 75 m, the weir loading rate (in m3/day/m) is _________.

A conventional flow duration curve is a plot between

Some of the nontoxic metals normally found in natural water are

The amount of CO2 generated (in kg) while completely oxidizing one kg of CH4 to the end products is ____________

16 MLD of water is flowing through a 2.5 km long pipe of diameter 45 cm. The chlorine at the rate of 32 kg/d is applied at the entry of this pipe so that disinfected water is obtained at the exit. There is a proposal to increase the flow through this pipe to 22 MLD from 16 MLD. Assume the dilution coefficient, n = 1. The minimum amount of chlorine (in kg per day) to be applied to achieve the same degree of disinfection for the enhanced flow is

The potable water is prepared from turbid surface water by adopting the following treatment sequence.

The dominating microorganisms in an activated sludge process reactor are

A suspension of sand like particles in water with particles of diameter 0.10 mm and below is flowing into a settling tank at 0.10 m3/s. Assume g = 9.81 m/s2, specific gravity of particles = 2.65, and kinematic viscosity of water =1.0105×10−2 cm2/s. The minimum surface area (in m2) required for this settling tank to remove particles of size 0.06 mm and above with 100% efficiency is _______________

A surface water treatment plant operates round the clock with a flow rate of 35 m3/min. The water temperature is 15°C and jar testing indicated an alum dosage of 25 mg/l with flocculation at a Gt value of 4×104 producing optimal results. The alum quantity required for 30 days (in kg) of operation of the plant is ____________

An effluent at a flow rate of 2670 m3/d from a sewage treatment plant is to be disinfected. The laboratory data of disinfection studies with a chlorine dosage of 15 mg/l yield the model Nt=Noe-0.145t where Nt = number of micro-organisms surviving at time t (in min.) and No = number of micro-organisms present initially (at t = 0). The volume of disinfection unit (in m3) required to achieve a 98% kill of micro-organisms is ______________

A waste water stream (flow = 2 m3/s, ultimate BOD = 90 mg/l) is joining a small river (flow = 12 m3/s, ultimate BOD = 5 mg/l). Both water streams get mixed up instantaneously. Cross-sectional area of the river is 50 m2. Assuming the de-oxygenation rate constant, k' = 0.25/day, the BOD (in mg/l) of the river water, 10 km downstream of the mixing point is

Some of the water quality parameters are measured by titrating a water sample with a titrant. Group-I gives a list of parameters and Group-IIgives the list of titrants.

 Group-I Group-II P. Alkalinity 1. N/35.5 AgNO3 Q. Hardness 2. N/40 Na2S2O3 R. Chloride 3. N/50 H2SO4 S. Dissolved oxygen 4. N/50 EDTA

The correct match of water quality parameters in Group-I with titrants in Group-II is:

A water treatment plant is designed to treat 1 m3/s of raw water. It has 14 sand filters. Surface area of each filter is 50 m2. What is the loading rate (in $\frac{{m}^{3}}{day.{m}^{2}}$)with two filters out of service for routine backwashing? __________

A settling tank in a water treatment plant is designed for a surface overflow rate of 30 $\frac{{m}^{3}}{day.{m}^{2}}$.Assumespecific gravity of sedimentparticles = 2.65, density of water($\rho$)= 1000 kg/m3, dynamic viscosity of water (μ)=0.001 N.s/m2,and Stokes’ law is valid.The approximate minimum size of particles that would be completely removed is:

A student began experiment for determination of 5-day, 20°C BOD on Monday. Since the 5thday fell on Saturday, the final DO readings were taken on next Monday. On calculation, BOD (i.e. 7 day, 20°C) was found to be 150 mg/L. What would be the 5-day, 20°C BOD (in mg/L)? Assume value of BOD rate constant (k) at standard temperature of 20°C as 0.23/day (base e). __________

The percent voids in mineral aggregate (VMA) and percent air voids (Vv) in a compacted cylindrical bituminous mix specimen are 15 and 4.5 respectively. The percent voids filled with bitumen (VFB) for this specimen is:

A sample of domestic sewage is digested with silver sulphate, sulphuric acid, potassium dichromate and mercuric sulphate in chemical oxygen demand (COD) test. The digested sample is then titrated with standard ferrous ammonium sulphate (FAS) to determine the un-reacted amount of

Assertion [a]: At a manhole, the crown of the outgoing sewer should not be higher than the crown of the incoming sewer.

Reason [r]: Transition from a larger diameter incoming sewer to a smaller diameter outgoing sewer at a manhole should not be made.

The CORRECT option evaluating the above statements is :

A water sample has a pH of 9.25. The concentration of hydroxyl ions in the water sample is

A town is required to treat 4.2 m3/min of raw water for daily domestic supply. Flocculating particles are to be produced by chemical coagulation. A column analysis indicated that an overflow rate of 0.2 mm/s will produce satisfactory particle removal in a settling basin at a depth of 3.5 m. The required surface area (in m2 ) for settling is

An activated sludge system (sketched below) is operating at equilibrium with the following information. Wastewater related data: flow rate = 500 m3/hour, influent BOD = 150 mg/L, effluent BOD = 10 mg/L. Aeration tank related data: hydraulic retention time = 8 hours, mean-cell-residence time = 240 hours, volume = 4000 m3, mixed liquor suspended solids = 2000 mg/L.

The food-to-biomass (F/M) ratio (in kg BOD per kg biomass per day) for the aeration tank is

An activated sludge system (sketched below) is operating at equilibrium with the following information. Wastewater related data: flow rate = 500 m3/hour, influent BOD = 150 mg/L, effluent BOD = 10 mg/L. Aeration tank related data: hydraulic retention time = 8 hours, mean-cell-residence time = 240 hours, volume = 4000 m3, mixed liquor suspended solids = 2000 mg/L.

The mass (in kg/day) of solids wasted from the system is

consider the following unit processes commonly used in water treatment; rapid mixing (RM), flocculation (F), primary sedimentation (PS), secondary sedimentation(SS),chlorination(C) and rapid sand filtration(RSF). The order of these unit processes(first to last) in a conventional water treatment plant is

Anaerobically treated effluent has MPN of total coliform as 106/100 mL. After chlorination ,the MPN value declines to 102/100mL. The percent removal(%R) and log removal (log R) of total coliform MPN is

Chlorine gas(8 mg/L as Cl2) was added to a drinking water sample. If the free chlorine residual and pH was measured to be 2 mg/L(as Cl2) and 7.5, respectively, what is the concentration of residual OCl- ions in the water? Assume that the chlorine gas added to the water is completely converted to HOCl and OCI- . Atomic weight of Cl:35.5

The sludge from the aeration tank of the activated sludge process(ASP) has solids(by weight) of 2%. This sludge is put in a sludge thickener, where sludge volume is reduced to half. Assume that the amount of solids in the supernatant from the thickener is negligible, the specific gravity of sludge solids is 2.2 and the density of water is 1000 kg/m3.

What is the density of the sludge removed from the aeration tank?

The sludge from the aeration tank of the activated sludge process(ASP) has solids(by weight) of 2%. This sludge is put in a sludge thickener, where sludge volume is reduced to half. Assume that the amount of solids in the supernatant from the thickener is negligible, the specific gravity of sludge solids is 2.2 and the density of water is 1000 kg/m3.

What is the solids content (by weight) of the thickened sludge?

If the BOD3 of a wastewater sample is 75 mg/L and reaction rate constant k (base e) is 0.345 per day, the amount of BOD remaining in the given sample after 10 days is

Ion concentrations obtained for a groundwater sample (having pH=8.1) are given below.

 Ion Ca2+ Mg2+ Na+ HCO3 SO4 2- Cl- Ion concentration (mg/L) 100 6 15 250 45 39 Atomic Weight Ca=40 Mg=24 Na=23 H=1, C=12, O=16 S=32, O=16 Cl=35.5 Total hardiness (mg/L as CaCO3) present in the above water sample is

Ion concentrations obtained for a groundwater sample (having pH=8.1) are given below.

 Ion Ca2+ Mg2+ Na+ HCO3 SO4 2- Cl- Ion concentration (mg/L) 100 6 15 250 45 39 Atomic Weight Ca=40 Mg=24 Na=23 H=1, C=12, O=16 S=32, O=16 Cl=35.5 Carbonate hardness (mg/L as CaCO3) present in the above water sample is

A horizontal flow primary clarifier treats wastewater in which 10%, 60% and 30% of particles have settling velocities of 0.1 mm/s, 0.2 mm/s, 0.2 mm/s, and 1.0 mm/s respectively. What would be the total percentage of particles removed if clarifier operates at a Surface Overflow Rate (SOR) of 43.2 m3/m2.d?

An aerobic reactor receives wastewater at a flow rate of 500 m3/d having a COD of 2000 mg/L. The effluent COD is 400 mg/L. Assuming that wastewater contains 80% biodegradable waste, the daily volume of methane produced by the reactor is

 Column I Column II P. Grit chamber 1. Zone settling Q. Secondary settling tank 2. Stoke’s law R. Activated sludge process 3. Aerobic S. Trickling filter 4. Contact stabilisation

Following chemical species were reported for water sample from a well:

Total hardness in mg/L as CaCO3 is

Following chemical species were reported for water sample from a well:

Alkalinity present in the water in mg/L as CaCO3 is

A wastewater sample contains 10-56 mmol /l of OH- ions at 25oC. The pH of this sample is

Group I lists estimation methods of some of the water and wastewater quality parameters. Group II lists the indicators used in the estimation methods. Match the estimation method (Group I) with the corresponding indicator (Group II).

 Group I Group II P. Azide modified Winkler method for dissolved oxygen 1. Eriochrome Black T Q. Dichromate method for chemical oxygen demand 2. Ferrion R. EDTA titrimetric method for hardness 3. Potassium chromate S. Mohr or Argentometric method for chlorides 4. Starch

Determine the correctness or otherwise of the following Assertion [a] and the Reason [r]
Assertion : The crown of the outgoing larger diameter sewer is always matched with the crown of incoming smaller diameter sewer.

Reason: It eliminates backing up of sewage in the incoming smaller diameter sewer.

The 5-day BOD of a wastewater sample is obtained as 190 mg/I (with k = 0.01h-1). The ultimate oxygen demand (mg/l) of the sample will be

A water treatment plant is required to process 28800 m3/d of raw water (density = 1000 kg/m3, kinematic viscosity = 10-6m2/s). The rapid mixing tank imparts a velocity gradient of 900 s-1 to blend 35mg/l of alum with the flow for a detention time of 2 minutes. The power input (W) required for rapid mixing is

Match Group I (Terminology) with Group II (Definition/Brief Description) for wastewater treatment systems

 Group I Group II P. Primary treatment 1. Contaminant removal by physical forces Q. Secondary treatment 2. Involving biological and / or chemical reaction R. Unit operation 3. Conversion of soluble organic matter to business S. Unit process 4. Removal of solid materials from incoming wastewater

The presence of hardness in excess of permissible limit causes

The alkalinity and hardness of a water sample are 250 mg/L and 350 mg/L as CaCO3, respectively. The water has

A completely mixed activated sludge process is used to treat a wastewater flow of 1 million litres per day (1 MLD) having a BOD5 of 200 mg/L. The biomass concentration in the aeration tank is 2000 mg/L and the concentration of the net biomass leaving the system is 50 mg/L. The aeration tank has a volume of 200 m3.

What is the hydraulic retention time of the wastewater in aeration tank?

A completely mixed activated sludge process is used to treat a wastewater flow of 1 million litres per day (1 MLD) having a BOD5 of 200 mg/L. The biomass concentration in the aeration tank is 2000 mg/L and the concentration of the net biomass leaving the system is 50 mg/L. The aeration tank has a volume of 200 m3.

What is the average time for which the biomass stays in the system?

A plain sedimentation tank with a length of 20m, width of 10 m, and a depth of 3 m is used in a water treatment plant to treat 4 million litres of water per day (4 MLD). The average temperature of water is 20°C. The dynamic viscosity of water is 1.002 × 10-3 N.s/m2 at 20°C. Density of water is 998.2 kg/M3. Average specific gravity of particles is 2.65.

What is the surface overflow rate in the sedimentation tank?