Which is the correct order of layers from top to bottom?

Supernatant water, schmutzdecke on fine sand, graded gravel, under-drains

Supernatant water, graded gravel, fine sand, under-drains

Fine sand, supernatant water, gravel, schmutzdecke

Schmutzdecke, gravel, fine sand, under-drains

Which sand specification suits a slow sand filter?

Effective size 0.2 to 0.3 mm, uniformity coefficient 2 to 3, depth about 0.9 to 1.2 m

Effective size 0.45 to 0.7 mm, uniformity coefficient 1.3 to 1.7

Effective size 0.2 to 0.3 mm, uniformity coefficient 10, depth 0.3 m

Effective size 2 to 3 mm, depth 3 m

How is a slow sand filter cleaned when head loss reaches 0.6 to 1.2 m?

By scraping off the top 1 to 3 cm of sand, then running filtrate to waste for 1 to 2 days

By backwashing with upward flow of clean water

By chlorinating the bed overnight

2073 Chaitra · 8 marks

Slow sand filter construction and design

Short answer

A slow sand filter consists of a rectangular tank with a supernatant water layer 1.0–1.5 m deep over a fine sand bed (0.9–1.2 m, effective size 0.2–0.3 mm) and graded gravel under-drainage. Design rate is 0.1–0.4 m/h; area is determined from Q/rate with one extra unit; cleaning is by scraping the top 1–3 cm when head loss reaches 0.6–1.2 m.

Describe the construction of a slow sand filter with neat sketches and its design consideration.

A slow sand filter (SSF) is a gravity filter in which water passes slowly (0.1 to 0.4 m/h) through a fine sand bed. It works by straining, sedimentation, adsorption and especially biological action, and it needs no chemicals.

Construction

   Inlet          supernatant water 1.0-1.5 m
     |      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     v      | schmutzdecke (biological film)
  ______    |:::::::::::::::::::::::::::::|
 |      |   | fine sand 0.9-1.2 m         |
 |      |   |-----------------------------|
 |      |   | gravel 0.3-0.45 m (graded)  |
 |______|   |==under-drain pipes (laterals)|
 Free board 0.5 m       |             |
                   main drain -> clear water

Tank: rectangular RCC or masonry, with watertight floor, L : B about 2:1 to 3:1; area up to about 2000 m² per bed, with a minimum of two units.
Supernatant water: 1.0 to 1.5 m depth above the sand, plus 0.5 m free board.
Filter sand: depth 0.9 to 1.2 m, effective size 0.2 to 0.3 mm, uniformity coefficient 2 to 3 (not more than 5), free of clay and organic matter.
Base material: gravel 0.3 to 0.45 m in 3 or 4 layers from 3 mm (top) to 40 mm (bottom).
Under-drainage: central main drain with lateral perforated pipes or open-jointed tiles covered with gravel, bed slope 1 in 100.
Inlet and outlet chambers with valves, weirs, a filter-to-waste valve, and a rate (flow) controller.

Design considerations

Rate of filtration: 0.1 to 0.4 m/h (100 to 400 l/h/m²).
Area: $A=Q/\text{rate}$ , with one extra unit as standby for cleaning.
Sand cleaning: when the head loss reaches 0.6 to 1.2 m, remove the top 1 to 3 cm by scraping, and re-sand when the bed depth is below 0.4 to 0.5 m (cleaning every 1 to 3 months).
Influent turbidity not above 30 NTU (use a roughing filter or sedimentation for turbid water).
After scraping, the filtrate of the first 1 to 2 days is run to waste until the schmutzdecke is formed again.
Efficiency: removes 98 to 99 % of bacteria and the turbidity to below 1 NTU; chlorination is still required.

✓ Answer checked by Claude Opus 5.5 (high effort)

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