Sewerage system planning & design; sewer appurtenances

Terminology

Systems of sanitation

Sewerage systems

Choice: the separate system suits Indian conditions (short intense monsoon, long dry periods) — storm water can go to natural drains while sanitary sewers remain small; the combined system suits places with evenly distributed rainfall and flat areas where pumping is needed anyway.

Patterns of collection systems

• Perpendicular pattern — sewers perpendicular to a stream; shortest lengths; for storm water or combined sewers discharging to rivers (untreated — not suitable for sanitary sewage now).
• Interceptor pattern — an interceptor sewer along the river collects branch sewers and carries sewage to a treatment plant.
• Radial pattern — sewers radiate outward to the periphery (land disposal around the town).
• Fan pattern — all sewers converge to a single outlet/trunk sewer.
• Zone pattern — area divided into zones with separate interceptors (hilly or large towns).

Sewer materials

Sewer shapes

• Circular — most common: minimum perimeter for a given area, easy to make, strong, hydraulically efficient.
• Egg-shaped (ovoid) — narrow invert gives higher velocity at low flows — good for combined sewers with widely varying flows; costlier and less stable.
• Horseshoe, semi-elliptical, basket-handle, U-shaped, rectangular — large sewers, tunnels, limited headroom.

Sewer appurtenances

Manholes

Openings to allow inspection, cleaning and repair of sewers.

• Provided at every change of direction, gradient or diameter, at junctions, at the head of sewers, and at regular intervals on straight runs (spacing increases with sewer size — from about 30 m for small sewers to much larger intervals for big trunk sewers).
• Types by depth: shallow manholes (inspection chambers, up to about 0.9 m deep), normal manholes (about 1.5 m), deep manholes (greater depth, with steps/ladders and a larger working chamber).
• Parts: access shaft, working chamber, bottom benching with channels shaped to the sewer, cover and frame (cast iron or ductile iron/ FRP), steps.

Drop manhole — used when an incoming branch sewer is considerably higher than the outgoing sewer (drops of more than about 0.6 m); a vertical drop pipe outside (or inside) the manhole brings the sewage down so that it does not splash and damage the benching or endanger workers.

Other appurtenances

Quantity of sanitary sewage

Variation of flow

Sewage flow varies hourly (peaks a little after water-demand peaks, lag in the sewers), daily and seasonally.

Peak factors for sewers (CPHEEO, based on contributory population):

• Sewers are designed for the peak flow at the end of the design period and checked for self-cleansing velocity at the minimum (present) flow.
• Minimum flow is commonly taken as a fraction (about one-third to one-half) of the average flow.
• Design period for sewers is long (about 30 years), since relaying is costly; pumping machinery and treatment units shorter (about 15 years).

Quantity of storm water

Runoff coefficients (indicative)

Time of concentration

• Inlet time $t_i$ — time for overland flow to reach the first inlet (commonly 5–30 min depending on slope and surface).
• Time of flow $t_f$ — travel time in the drain = length ÷ velocity.

Rainfall intensity

Intensity–duration–frequency relations are used where available. Classical formulas (British Ministry of Health, $i$ in mm/h, $t$ in min):

The design frequency (return period) is chosen by the importance of the area — higher for commercial centres and critical areas than for residential streets.

Hydraulic design of sewers

Velocity limits

• Self-cleansing velocity — the minimum velocity that prevents deposition of solids. Commonly adopted minimum values: about 0.6 m/s for sanitary sewers at peak flow (with a check at present peak flow) and about 0.75–1.0 m/s for storm and combined sewers (grit is heavier).
• Camp's (Shields) formula:

($\beta$ ≈ 0.04 for starting motion, up to 0.8 for complete cleansing of sticky material; $f$ = Darcy friction factor; $S_s$, $d$ = specific gravity and size of particles.)

• Non-scouring (maximum) velocity — to prevent erosion of the sewer: about 2.5–3 m/s for concrete and stoneware (higher for cast iron); steep ground needs drop manholes to keep velocities down.

Minimum size and depth

• A minimum sewer diameter (commonly 150 mm for laterals, larger in big cities) is used to avoid choking.
• Sewers are laid deep enough to receive house connections from basements/ground floors and to be below water mains; minimum cover protects against traffic loads.