Design of Sewage Treatment Systems – Comprehensive Study Notes

Overview of Sewage Treatment Requirements in Hong Kong’s Un-Sewered Areas

• Hong Kong contains numerous districts where public foul sewers are absent, obliging private developers to provide on-site sewage treatment.
• Engineering challenges are intensified by:
– Dense urban fabric & very limited land parcels.
– Steep, varied topography making gravity flow difficult.
– Immediate proximity to ecologically sensitive coastal waters, rivers and catchments.
– Extremely high population densities that elevate design loadings and odour concerns.
• Three principal treatment/containment options sanctioned by the authorities:

  1. Sewage Treatment Plants (STPs) – secondary treatment or higher.

  2. Septic Tanks – primary treatment with soil infiltration.

  3. Cesspits – storage only, no treatment.

Regulatory Framework

• Environmental Protection Department (EPD)
– Administers Water Pollution Control Ordinance.
– Issues discharge licences; sets effluent quality limits.
• Buildings Department (BD)
– Approves design & construction under Building Regulations.
– Verifies dimensional, material and safety compliance.
• Drainage Services Department (DSD)
– Publishes technical manuals; inspects maintenance.
• All submissions must demonstrate compliance with effluent standards, structural integrity and operational safety across the three agencies.

Sewage Treatment Plants (STPs)

Application & Suitability

• Appropriate for private developments with 50≤PE≤2,00050 \le \text{PE} \le 2{,}000 (Population Equivalent).
• Preferable when high effluent quality or marine discharge is required.
• Unsuitable when \text{PE}<50, unreliable power, or limited O&M budgets exist.

Location Requirements

• Structures must be fully visible for inspection; flood-free siting is compulsory.
• Provide direct vehicular access for chemical delivery & sludge haulage.
• Maintain buffers from residences to reduce odour; locate near discharge point to cut pumping costs.
• Safeguard space for future expansion where growth is likely.

General Design Provisions

• Portable equipment – at least one portable submersible pump for recirculation during low inflows.
• Headroom – 3 m3\,\text{m} (can locally reduce to 2.5 m2.5\,\text{m} under beams) in enclosed/underground plants.
• Ventilation – minimum air volume ≥14 m3\ge 14\,\text{m}^3 with ≥10\ge 10 air changes·hr⁻¹; exhaust stack ≥1 m\ge1\,\text{m} above roof.

Access, Walkways & Safety

• Walkways: clear width ≥0.75 m\ge0.75\,\text{m}; gradient 1:251:25 cross-fall to prevent ponding.
• Provide staircases not cat-ladders where level changes occur; stainless-steel safety rails + toe-boards.
• Tanks: stainless ladders or step irons @ 300 mm300\,\text{mm} (horiz.) & 250 mm250\,\text{mm} (vert.) spacing. Mild steel prohibited.
• Cover slabs: stainless-steel/Al open-mesh designed for 5 kPa5\,\text{kPa} UDL; avoid Al in confined chlorination rooms.

Utilities & Control Systems

• Fresh-water taps + backflow preventers; hand-washing stations.
• Electrical boards above flood level; incorporate de-humidifier heaters & bilingual mimic diagrams.
• Labelling – every valve, penstock & pump tagged in English + Chinese for safe operation & emergency response.

Sedimentation Tank Design

• Two geometric options:

  1. Rectangular horizontal-flow – L:W≥2\text{L:W}\ge2 (avoid 1<\text{L:W}<2); mechanical scraper on sloped floor; twin tanks encouraged.

  2. Square/Circular upward-flow – hopper slope 60∘60^\circ to ease sludge draw-off; occupies smaller footprint.

Sedimentation Performance Requirements

• Adjustable V-notch weirs where flow varies; side-wall height ≥1 m\ge1\,\text{m} to prevent carry-over.
• Inlet structures must never be submerged in sludge.
• Provide recirculation chamber downstream of final clarifier when initial flows « design flows.

Hydraulic & Organic Design Loads (Residential)

• Average daily flow: 0.30–0.46 m3 PE−1 d−10.30\text{–}0.46\,\text{m}^3\,\text{PE}^{-1}\,\text{d}^{-1}.
• Organic/solids loadings: 55 g BOD PE−1 d−155\,\text{g BOD}\,\text{PE}^{-1}\,\text{d}^{-1} and 55 g SS PE−1 d−155\,\text{g SS}\,\text{PE}^{-1}\,\text{d}^{-1}.
• Peak-to-dry-weather factor:
– PE≤1000\text{PE}\le1000 → Q<em>peak=6 DWFQ<em>{peak}=6\,DWF – \text{PE}>1000 → Q</em>peak=4 DWFQ</em>{peak}=4\,DWF (never < value for 1 000 PE)
• Alternative: design for 3 DWF3\,DWF with an equalisation tank sized for surplus; duty pumps modulate inflow.

Flow Equalisation Concept

• Balances diurnal peaks, stabilising biological process loads; enables smaller downstream units.

Sludge Management

• Routine withdrawal – remove sludge daily via submersible pumps/air-lifts; scum removed separately.
• Provide inspection troughs for quality checks.
• Dewatering – machine must reach 30%30\% dry solids for landfill.
• Emergency storage: ≥14\ge14 days of sludge volume.
• Small STPs (<100 PE) – may omit dewatering but require wet-storage tank sized for 6060 days; tanker access mandatory.

Effluent Disinfection & Discharge

• Standard disinfectant: sodium hypochlorite; design contact tank for regulated bacterial kill.
• Chlorine residual at outfall must satisfy licence.
• Submission documents to include site plan, discharge point, and submarine outfall specs where relevant.

Vehicle & Service Access

• Roads to sustain fully-laden sludge tankers; radius & gradients to suit local truck fleet.
• Containment curbing near loading bays to prevent accidental spillage.

Advantages / Limitations Recap

• Advantages – highest pollutant removal (secondary +); compliant with sensitive receivers.
• Limitations – highest capital + O&M cost; skilled staff; uninterrupted power supply essential.

Septic Tanks

Application

• Ideal for small-scale (typically \text{PE}<50) rural or low-density developments where STPs are disproportionate.
• Requires permeable soils for soakaway disposal.

Mandatory Separation Distances

• ≥18 m\ge18\,\text{m} from any potable water source (spring, stream, well).
• Site to minimise odour along prevailing wind; adopt surface runoff diversion to prevent inundation.

Dimensional & Structural Criteria

• Depth: 1.2 m≤D≤1.8 m1.2\,\text{m}\le D \le1.8\,\text{m} from inlet invert to floor.
• Length-to-width ratio: 3:1≤L:W≤4:13:1 \le L:W \le 4:1.
• Walls: 215 mm215\,\text{mm} brick or 125 mm125\,\text{mm} concrete; floor ≥150 mm\ge150\,\text{mm} concrete.
• Smooth, impervious internal render; gas-tight covers with inspection openings.

Treatment Mechanism

• Stratification → scum | liquor | sludge layers.
• Anaerobic digestion over ≈2\approx2 months reduces organics & solids, producing biogas (methane) which is occasionally recoverable in large systems.

Sizing Formulae & Rules

• Volume provides 16–48 h16\text{–}48\,\text{h} hydraulic retention.
• Minimum tank capacity: 3.5 m33.5\,\text{m}^3 regardless of population.
• Sludge accumulation allowance: 0.8 L person−1d−10.8\,\text{L person}^{-1} \text{d}^{-1}.
• Two-chamber layout and baffles to discourage short-circuiting.

Sludge & Effluent Disposal

• Desludging interval: 1–3 years depending on use; always leave ≈1 cm1\,\text{cm} sludge to retain bacterial seed.
• Vacuum tanker distance ≤30 m\le30\,\text{m}; vertical lift limit ≈4–5 m\approx4\text{–}5\,\text{m}.
• Effluent infiltration options (subject to BD approval):
– Soakaway pits (single point).
– Drainage fields (pipe networks in gravel).
– Alternative polishing (mounds, wetlands) where soils unsuitable.

Cesspits

Application & Limitations

• Employed where no treatment option is feasible (rocky soil, temporary camps, extremely low use, emergency backup).
• Purely a watertight holding tank → requires frequent emptying ⇒ high operating cost.

Location & Construction

• Offset ≥20 m\ge20\,\text{m} from potable water sources; ≥15 m\ge15\,\text{m} from occupied buildings.
• Must be fully impervious; reinforced cover and adequate venting.

Sizing Principles

• Storage period: 1 month between pump-outs.
• Design flow: 135 L person−1 d−1135\,\text{L person}^{-1}\,\text{d}^{-1} → monthly volume per person =135×30=4,050 L=135\times30=4{,}050\,\text{L}.
• Example – remote office (10 staff): V=40.5 m3V=40.5\,\text{m}^3; weekend home (6 occupants, 8 days/month): V=6.5 m3V=6.5\,\text{m}^3.

Operations

• Monthly (or more frequent) vacuum tanker emptying; direct road access essential regardless of weather.
• Hybrid cesspit-to-absorption outlets occasionally used but must meet effluent rules & monitoring.

Comparative Analysis & Selection

Quantitative Comparison (Typical)

• Population range: STP 50–2,00050\text{–}2{,}000 | Septic 5–505\text{–}50 | Cesspit 1–201\text{–}20.
• Treatment levels: High vs Medium vs None.
• Life-cycle cost (20-yr): STP highest capital + O&M; Cesspit smallest capex but very high haulage cost over time.
• Relative pollutant removal (indicative):
– STP: BOD ≈ 90 %, TSS ≈ 90 %, Nitrogen removal possible, path. reduction >3 log with disinfection.
– Septic: BOD/TSS ≈ 60 % (primary only), negligible N.
– Cesspit: 0 % (storage).

Decision Matrix

• Choose STP when PE≥50\text{PE}\ge50, sensitive receiver, high standard, budget & expertise available.
• Choose Septic when \text{PE}<50, suitable soils, limited O&M skills, moderate standard acceptable.
• Choose Cesspit for temporary, very low use, poor soils, or when discharge impossible.

Hybrid / Emerging Solutions

• Septic + constructed wetland; package STP + equalisation; membrane bioreactors for ultra-compact footprints; electrochemical polishers for niche small flows; urine diversion to lower liquid load.

Common Design Pitfalls

• Undersizing future growth & peaks → overload.
• Poor access for maintenance vehicles → unreliable desludging.
• Inadequate ventilation in enclosed STPs → corrosion & safety hazards.
• Wrong materials (e.g.
mild steel) in corrosive sewage.
• Insufficient monitoring points → cannot prove compliance.

Key Takeaways for Exam Revision

• Population threshold is the primary selector: PE≥50\text{PE}\ge50 → STP, <50 → septic, with cesspit as last resort.
• Observe minimum separation distances (18 m septic vs water, 20 m cesspit vs water).
• Comply with all dimensional ratios and structural specs (STP headroom 3 m3\,\text{m}; Septic L:W=3–4:1L:W=3\text{–}4:1).
• Design for maintainability: daily sludge removal (STP) vs 1-3 yr (septic) vs monthly (cesspit).
• Consider life-cycle cost and operational skill availability, not just initial capital.
• Provide bilingual labelling, accurate schematic controls and sufficient sampling points to satisfy EPD, BD & DSD.