Water supply and sanitation management

Managing water supply and sanitation

1. Water supply: use a proper layout (sump, pump, overhead tank, distribution pipes) giving adequate quantity (135 L/person/day) and pressure; clean the tanks regularly; fix leaks; use water-saving fixtures and recycle grey water; supplement the supply by rainwater harvesting.
2. Sanitation: separate soil (toilet) and waste (kitchen, bath) pipes; traps with a water seal on every fixture; vent pipes to release gases; inspection chambers at bends and junctions; a minimum slope of about 1 in 40 to 1 in 60 for house drains; and proper disposal to a sewer, or to a septic tank where there is no sewer.
3. Operation and maintenance: regular inspection, desludging of the septic tank every 1-2 years, prevention of misuse (no solids, oils or chemicals), and no mixing of storm water.

Septic tank and soak pit in sanitation

The septic tank receives toilet sewage, settles the solids, digests the sludge anaerobically and discharges clarified effluent to the soak pit, where it infiltrates into the soil. Together they provide on-site treatment and disposal where there is no public sewer.

Septic tank

A septic tank is a watertight, underground, single-storey sedimentation and digestion tank that treats the sewage of a household or small building where no public sewer exists. Solids settle at the bottom as sludge and are digested by anaerobic bacteria; the partly clarified liquid (effluent) flows out for further treatment in a soak pit.

Parts: inlet pipe with a T-baffle (dip pipe), settling chamber, sludge zone, scum zone, outlet T-pipe, vent pipe, access manhole with cover. Often two chambers are provided, with a dividing wall (the first chamber is 2/3 of the length).

  inlet    manhole        vent   outlet
    |        |             |       |
 ===v========v=============v=======v==== GL
 | T          scum layer       |   T   |
 |___|                         |___|   |
 |   liquid (effluent zone)    |       |
 |   sludge digestion zone  [] |       |
 |_____________________________|_______|
     chamber 1 (2/3)         chamber 2

Working: sewage enters and its flow velocity drops; heavy solids settle and light fats float as scum; anaerobic bacteria reduce the sludge volume (by about 30-50%), producing gas that escapes through the vent. Clear effluent leaves through the outlet.

Design points (IS 2470 / Nepal practice): detention time about 24 hours; sewage 90-135 L/person/day; sludge allowance about 30 L/person/year with a desludging interval of 1-2 years; length : width = 2:1 to 4:1; width not less than 0.75 m; liquid depth 1.0-1.8 m; free board 0.3 m; walls of brick in cement mortar, plastered inside with waterproofing; floor and cover of RCC.

Soak pit

A soak pit (soakaway) is a circular pit that receives effluent from the septic tank and lets it percolate into the surrounding soil.

Construction: circular, diameter 1.0-2.5 m (typically 1.5 m) and depth 1-3 m; lined with dry brick masonry with honeycomb openings; filled with brick bats/stone aggregate (coarse at the bottom, finer above); covered with an RCC slab with a manhole; the inlet pipe enters near the top.

  from septic tank
        |      cover slab
   =====v======+====== GL
   |  honeycomb brick wall |
   |  ooo brick bats ooo   |
   |  ooo aggregate  ooo   |
   |_______________________|
        |   |  seepage -> soil

Conditions: the soil must be permeable, the water table well below the pit bottom, and the pit at a safe distance (about 15 m or more) from drinking-water sources and 3 m from building foundations. Where the soil is impermeable, use a leach/drain field or an absorption trench instead.