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

(a) Rising sludge and sludge bulking in the activated sludge process

Both are settling failures in the secondary clarifier. They cause solids to escape with the effluent, which raises its suspended solids and BOD.

(i) Rising (floating) sludge: sludge that settled well first floats to the clarifier surface in clumps, carrying small gas bubbles. The usual cause is denitrification in the clarifier: where the mixed liquor is nitrified, nitrate is reduced in the anoxic sludge blanket to nitrogen gas, which lifts the flocs. It is favoured by high nitrate, long sludge age, warm water, low return rates and long sludge retention in the clarifier, and the sludge volume index (SVI) remains normal. Impact: loss of solids, scum, odour and poorer effluent. Remedies: return sludge faster, waste more sludge, reduce the sludge age, raise the dissolved oxygen or add an anoxic zone to denitrify earlier.

(ii) Sludge bulking: the sludge is light and settles badly, with a high SVI, above about 150 mL/g. It arises from filamentous organisms such as Sphaerotilus and Thiothrix, which dominate under low dissolved oxygen, a low food-to-microorganism (F/M) ratio, nutrient deficiency, low pH or septic, high-strength feed. Impact: a rising sludge blanket, washout of solids, loss of return sludge concentration, process upset and failure to meet discharge standards. Remedies: control DO, F/M and nutrients, use selector tanks, waste more sludge, and chlorinate the return sludge in emergencies.

(b) Groundwater terms

(i) Specific capacity of a well is the discharge per unit drawdown, Q/s (m³/h per m), a measure of well performance.

(ii) Specific yield of an aquifer is the volume of water that drains freely by gravity from a unit volume of saturated aquifer, as a fraction or percentage; it is smaller than porosity by the specific retention.

(iii) A perched water table is a local saturated zone held above the main water table by a lens of impermeable or low-permeability material.

(iv) Intrinsic permeability k is the property of the porous medium alone, independent of the fluid, with dimensions of length squared (m² or darcy). K = kρg/μ links it to hydraulic conductivity.

(v) Bulk pore velocity is the average velocity of water in the pores, the seepage velocity v_s = v/n = Ki/n, where v is the Darcy velocity and n is the porosity. It is always larger than the Darcy velocity.

(c)(i) Delta, duty and base period

Base period B is the time, in days, from the first watering of a crop to its last. Duty D is the area irrigated per cumec of continuous supply during the base period, in ha/cumec. Delta Δ is the total depth of water required by the crop over the base period.

Derivation: a discharge of 1 cumec for B days gives a volume of 86 400 B m³. This irrigates D hectares = 10⁴ D m², so the depth is Δ = 86 400 B/(10⁴ D) = 8.64 B/D metres (with B in days and D in ha/cumec). A larger duty therefore means a smaller delta for the same base period.

(c)(ii) Factors in selecting a dam site

A good site has a narrow gorge with a wide valley upstream, so that a short dam stores a large volume. The foundation must be sound rock or a competent stratum with low permeability and freedom from faults, and the reservoir basin must be watertight. There must be adequate, reliable inflow from the catchment with a low silt load. Suitable construction material and access should be near the site, and there should be a good site for the spillway. The submerged area should be small, with minimum displacement of people, land and forest. The site should also have low seismic risk, acceptable environmental impact and favourable cost per unit of storage.