# GATE Questions & Answers of Sewage and sewerage treatment

## What is the Weightage of Sewage and sewerage treatment in GATE Exam?

Total 20 Questions have been asked from Sewage and sewerage treatment topic of Water and Waste Water subject in previous GATE papers. Average marks 1.80.

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 ______

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 ______

Crown corrosion in a reinforced concrete sewer is caused by:

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 _________

The dominating microorganisms in an activated sludge process reactor are

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

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 :

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

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?

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

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

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?