(a) Discuss the following operational difficulties encountered in the operation of Activated Sludge process :
(i) Rising Sludge or Floating Sludge
(ii) Sludge Bulking
What kind of impact these are having in the operation of Activated Sludge process ?

Question

(a) Discuss the following operational difficulties encountered in the operation of Activated Sludge process :
(i) Rising Sludge or Floating Sludge
(ii) Sludge Bulking
What kind of impact these are having in the operation of Activated Sludge process ? 20

(b) Explain the following :
(i) Specific capacity of a well
(ii) Specific yield of an aquifer
(iii) Perched water table
(iv) Intrinsic permeability
(v) Bulk pore velocity 15

(c) (i) Define Delta, Duty and Base Period, and derive the relationship among them. 7
(ii) Discuss the factors influencing the selection of site for a proposed dam. 8

UPSC Answer Check · Accepted Answer

The activated sludge process depends on stable floc formation, adequate oxygen transfer and controlled sludge age; when these fail, the plant loses settling and effluent quality. In Indian STPs, variable influent strength and limited clarifier space make these failures common.

Rising or floating sludge. In the final clarifier, sludge may rise or float as a loose, dark, odorous layer. The main cause is denitrification in the clarifier under anoxic/facultative conditions: nitrate-rich mixed liquor is reduced to nitrogen gas, and N2 bubbles adhere to flocs, lowering their density. It is common in nitrifying plants where return sludge carries high nitrate. It is favoured by high nitrate, low DO, low temperature, high sludge age and long clarifier detention. It is identified by poor settleability, rising sludge and abnormal or high sludge volume index. Impact is loss of clarifier capacity, short-circuiting, high TSS and BOD in effluent, reduced return sludge quality and operational instability. Remedies include increasing sludge wasting to reduce sludge age and nitrate, reducing MLSS, improving aeration/DO, reducing return sludge nitrate, and, where needed, adding denitrification control or clarifier modifications.

Sludge bulking. Bulking is failure of flocs to settle, usually due to excessive filamentous bacteria such as Sphaerotilus natans and Thiothrix competing with floc-formers. It is promoted by low DO, high or fluctuating F/M ratio, organic shock, low pH, nutrient deficiency and toxic loads. SVI above about 150 mL/g is an indicator. Impact is poor sludge blanket, carry-over of solids, high effluent TSS/BOD, reduced plant capacity, increased sludge handling and difficulty meeting discharge standards. Both reduce clarifier solids residence time and increase downstream load. Control includes maintaining DO, adjusting F/M and sludge age, chlorination or selective oxidation, adding selector basins, nutrient balancing and avoiding toxic shocks.

Hydrogeological terms. Specific capacity of a well is the discharge obtained per unit drawdown, Q/s, usually m³/day/m; it indicates well efficiency. Specific yield of an aquifer is the volume of water drained by gravity per unit volume of aquifer, Sy = ΔV/(Δh A), dimensionless. A perched water table is a local saturated zone above the regional water table, separated by an aquitard or low-permeability layer. Intrinsic permeability is a property of the porous medium, k, with units m², independent of the fluid. Bulk pore velocity, or actual interstitial velocity, is v = K i / n, where K is hydraulic conductivity, i hydraulic gradient and n porosity; it is the true velocity of water through pores. They guide well yield, aquifer storage and contaminant travel estimates.

Irrigation water requirement. Delta (Δ) is the depth of water required by a crop over its base period. Duty (D) is the area irrigated per unit discharge, expressed in hectares per cumec, i.e. 1 m³/s throughout the base period. Base period (B) is the total duration of irrigation. If a discharge Q m³/s irrigates area A ha for B days, total volume is Q×86400×B m³. Since 1 ha-cm = 100 m³, Δ cm = (Q×86400×B)/(100 A). As D = A/Q, Δ = 8.64 B/D. It shows higher duty or shorter base period lowers delta.

Dam site selection. A suitable site should have a narrow gorge with a wide upstream valley for storage, sound foundation rock with low seepage, adequate catchment and reliable inflow, low sediment load, proximity to demand centres, stable geology and low seismic risk, and acceptable environmental and social impacts. Geotechnical investigation, flood routing, cost-benefit and clearances also matter. In India, sites such as Tehri and Sardar Sarovar show that geological soundness, reservoir submergence, rehabilitation and environmental clearance are decisive. Thus, operational sludge problems, aquifer behaviour and irrigation/dam planning all require matching design assumptions with field conditions; remedies, monitoring and site-specific assessment are essential for reliable performance.

(a) Discuss the following operational difficulties encountered in the operation of Activated Sludge process : (i) Rising Sludge…

Model answer

What "Discuss" is asking you to do

Structure that answers it

Where marks are lost

How this answer will be evaluated

Approach

Key points expected

Evaluation rubric

Must cover

Loses marks

Earns more

Extra mark

Must cover

Loses marks

Earns more

Extra mark

Must cover

Loses marks

Earns more

Extra mark

Must cover

Loses marks

Earns more

Extra mark

Practice this exact question

More from Civil Engineering 2023 Paper II