Wastewater Sewerage Engineering

WATER POLLUTION

• Definition: water becomes polluted when the addition of materials renders it unfit for its intended use.
• Example mentioned: Pang River.
• Ethical / practical implication: polluted water endangers public health, ecosystems and economic activities.

POINT vs. NON-POINT SOURCES OF POLLUTION

• Point sources
– Pollutants discharged directly to water through a pipe/sewer.
– Technology exists for monitoring / regulating.
– Example icons: factory, wastewater treatment plant.
• Non-point sources
– Pollutants conveyed indirectly across the landscape (run-off).
– Example: fertilizer carried from a field to a stream by rainfall.
– Harder to control.
• Schematic examples given:
– Non-point: city streets, rural homes, cropland, animal feedlot.
– Point: factory, WWTP.

SEWERAGE ENGINEERING – GENERAL

• Sewerage = the collection & conveyance of wastewater to a disposal/treatment point.
• Liquid wastes must be treated before discharge to protect health & aesthetics.

CATEGORIES OF SEWER SYSTEMS

• Separate Systems – independent pipes for sanitary sewage & stormwater.
– Sanitary sewer sub-types (by hydraulics):
• Gravity (most common; elevation-driven).
• Pressure / pumped (for areas where gravity unsuitable; small, tight, exclude I/I).
• Vacuum (when neither gravity nor pressure feasible).
– Storm sewer: gravity flow, handles street & roof run-off; sanitary wastewater excluded (except via cross connections).
• Combined Systems – one pipe carries both sewage & rainfall (only historic, now discouraged).

INDUSTRIAL WASTEWATER OPTIONS

• Direct discharge to sanitary sewer.
• On-site partial treatment then sanitary sewer (pre-treatment).
• Complete on-site treatment to meet receiving-water specs.

SOURCES & COMPONENTS OF COMMUNITY WASTEWATER

• Sanitary (domestic) wastewater.
• Industrial wastewater.
• Infiltration/Inflow (I/I) – groundwater or stormwater entering sewers.
• Stormwater runoff.

Infiltration (subsurface) paths

• Defective pipes, leaking joints, poor connections, cracked manhole walls, root intrusion.

Inflow (surface) paths

• Leaking covers, roof gutters, foundation drains, cross-connections, yard drains, cooling-water discharges, uncapped cleanouts, etc.

SANITARY SEWER OVERFLOWS (SSOs)

• Occur in separate systems when capacity exceeded or defects exist.

Environmental impacts

– Nutrients/toxicants → algal blooms, O₂ depletion, aquatic life loss.

Public-health impacts

– Skin/ear infections, gastroenteritis, paralytic shellfish poisoning, red-tide exposure via contact, ingestion or inhalation.

CAUSES OF SSOs (data from 6 communities)

• Pipe blockages 43 %
• Infiltration/Inflow 27 %
• Pipe breaks 12 %
• Insufficient system capacity (design) 7 %
• Power/vacuum failures (pressure systems) 11 %

SEWER CORROSION

• Acid attack (low-pH industrial wastes) below waterline.
• Hydrogen sulfide mechanism (biogenic): \text{SO}4^{2-}\to \text{H}2\text{S}\uparrow\xrightarrow[O2]{\text{aerobic}}\text{H}2\text{SO}_4 attacks crown.
• Consequences: leaks, I/I, reduced flow capacity, eventual collapse.

FLOW ESTIMATION – DOMESTIC & OTHERS

• If no data: assume 70\,\% of domestic water use returns as wastewater.

Typical Residential Per-Capita Wastewater

• Apartment 260\,\text{L·cap⁻¹·d⁻¹}; Luxury home 380\, etc.

Commercial, Institutional, Recreational tables

• (Full value ranges provided in transcript; include in study sheet for design).

Industrial

• Without internal reuse: 85{-}95\,\% of water use returns.
• With reuse: site-specific data needed.

SAMPLE PROBLEM (given)

• All housing types total wastewater: 0.8025\,\text{ML·d}^{-1}.
• Peak factor =2.5 → peak flow =2.00625\,\text{ML·d}^{-1} (transcript rounded to 2.00\,\text{MLD}).

HIERARCHY OF PIPES IN A SEPARATE SANITARY SYSTEM

• Building connecting pipe → Lateral/branch → Main → Trunk → Interceptor → Treatment plant.
• Principle: lay sewers near every occupied building; favour gravity.

CHOICE OF COLLECTION SYSTEM

• Prefer gravity. If slopes inadequate use pump station (wet well + force main) or vacuum/pressure hybrids.

DESIGN OF GRAVITY SEWERS – PROCEDURE

1. Preliminary investigation & mapping (streets, contours, utilities, groundwater, soil, existing structures).
2. Establish design data:
   – Peak wastewater flow Q.
   – Hydraulic equation (Manning recommended, n\ge0.013 for new).
   – Material roughness, min/max velocities, slopes, cover.
   – Alignments & appurtenances (manholes, etc.).
3. Calculate pipe diameters/grades.

FUNDAMENTAL HYDRAULIC EQUATIONS

• Continuity: Q1 = Q2 = AV (incompressible).
• Bernoulli: \tfrac{P}{\gamma}+\tfrac{V^{2}}{2g}+z = \text{const} - h_{\text{loss}}.
• Reynolds number: Re = \dfrac{DV\rho}{\mu}
– Laminar Re

Manning Formula

V=\dfrac{1}{n} R^{2/3} S^{1/2}
For circular pipe R=\dfrac{D}{4}, Q=0.312\,D^{8/3}S^{1/2}/n (metric).

Darcy-Weisbach

hf = f\,\dfrac{L}{D}\,\dfrac{V^{2}}{2g} or hf =0.051f\,\dfrac{L}{D}\,V^{2} (metric).

Hazen-Williams

V = 0.849 C R^{0.63} S^{0.54} ; or Q = 0.278 C D^{2.7} S^{0.54}.
Table provided: C ranges 50–160 (e.g., PVC C≈150, corrugated steel C≈60).

Series / Parallel Pipe Relations

• Series: Q1=Q2=…=Q, h{f,\text{total}}=\sum hf.
• Parallel: Q=Q1+Q2+…; equal headloss across each branch.

STORM RUNOFF ESTIMATION

• Rational Method peak: Q=CIA where C=run-off coefficient (table: rooftops 0.70{-}0.95, asphalt 0.80{-}0.95, etc.), I rainfall intensity, A area.
• Unit-hydrograph useful for various return periods.

PUMP POWER

• Horse-power (metric): \text{HP}=\dfrac{\gamma Q H}{\text{Eff}}.
• Head components: H = Hs + Hf + Hm + Hv (static + friction + minor + velocity).
• Overall efficiencies: Ew = Pw/Pp, Em = Pp/Pm.

SEWER PIPE MATERIALS – PROS & CONS

• Ductile Iron – strong, river crossings, root-proof, but acid/H₂S/ brackish corrosion.
• Reinforced concrete – large sizes, available, H₂S & sulfate attack risk.
• Pre-stressed concrete – long mains, leakage-tight, same corrosion caveats.
• PVC – 4–15 in, corrosion-proof, not for large mains.
• Vitrified clay – 4–36 in, acid/alkali resistant, brittle, min 8 in for sewers.

Minimum Sizes & Velocities

• Building laterals often 6 in; smallest public sewer > building size.
• Velocity targets:
– V{min}=0.6\,\text{m·s}^{-1} (2 ft/s) at half-to-full depth. – V{max}=2.5{-}3.0\,\text{m·s}^{-1} (8-10 ft/s).
• Slopes adjusted to maintain self-cleansing.

SEWER APPURTENANCES

• Manholes – at size/slope/ direction changes; spacing:
–

TRENCHING & EXCAVATION

• Provide ≥150\,\text{mm} clearance each side; in rock excavate 150 mm below pipe & backfill with sand.

SEWER LOADS & MARSTON EQUATION

• Total load types: hydraulic, earth, groundwater, superimposed.
• Vertical earth load on pipe: W = C w B^{2} (Marston).
– C depends on trench width/depth and soil interface friction K u' (nomograph).
– Typical Ku' range 0.10{-}0.16; max by soil type: cohesionless 0.192, saturated clay 0.110, etc.

INFILTRATION TREATMENT COST

\text{A.C.}= D L I \times365 \times \text{U.C.}
• D diameter, L length, I infiltration rate per dia-length per day, \text{U.C.} cost per m³ treatment.

SUMMARY CONNECTIONS & IMPLICATIONS

• Proper separation & design of sewer systems avert pollution, SSOs & public-health crises.
• Hydraulic equations & material choices ensure adequate capacity, longevity & economy.
• Managing I/I, corrosion & load protects infrastructure investment and environment.
• Ethical responsibility: engineers must balance cost, safety, sustainability and regulatory compliance in wastewater projects.