🏠➡️🏢 The Definitive Guide to Sewage Systems: Sizing, Standards, and Selection
Choosing the right sewage system is a critical engineering decision for any construction project. It ensures public health, legal compliance, and environmental protection. This step-by-step guide breaks down the specifications and standards required for a successful system, whether you're building a cozy home or a massive hospital
Phase 1: The Foundation — Public Sewer vs. On-Site System
The first choice is the most important: centralization or self-contained treatment.
Step 1: Check Availability
Public Sewer (Centralized): If available and mandatory by local utility boards, this is the simplest and lowest-maintenance choice. You pay connection fees and monthly utility bills.
On-Site System (Septic/STP): Required in rural or remote areas. This puts the responsibility for maintenance (pumping, monitoring) on the owner but avoids monthly utility bills.
Step 2: Site Suitability (For On-Site Systems Only)
You must hire a professional to perform a Site Assessment before any design work begins.
| Test | Purpose | Required Outcome for Standard System |
| Percolation Test (Perc Test) | Measures how quickly soil absorbs treated water (effluent). | Required to accurately size the Drainfield (Soakaway). |
| Soil Analysis/Boring | Identifies soil composition (clay/sand) and depth of suitable soil. | Sufficient depth of permeable soil must exist above bedrock and groundwater. |
| High Water Table Check | Determines the highest seasonal groundwater level. | The entire drainfield must be a minimum distance (e.g., 600mm to 1 meter) above the high water mark. |
Phase 2: Sizing — Calculating Daily Wastewater Flow (Q)
System sizing is based on the maximum expected flow, not current occupancy. This ensures the system won't fail when the property is fully used.
A. Small House/Residential Standards (Based on Bedrooms)
Residential systems are sized using a minimum Estimated Population (P- Population Equivalent) and a standard flow rate (often 150 Litres per person per day
| Property Size | Estimated Population (P - Population Equivalent) | Daily Flow Rate (Q) (at ≈150 L/person/day) |
| Up to 3 Bedrooms | 5 Persons | 750 L /day |
| 4 Bedrooms | 6 Persons | 900 L/day |
| 5 Bedrooms | 7 Persons | 1,050 L/day |
Specific Sizing Note: If a garbage disposal unit is installed, the tank capacity must be increased by at least 50% due to the higher solids load.
B. Large Building/Commercial Standards (Based on Usage)
| Building Type | Flow Rate Unit | Standard Flow Rate (Approximate) |
| Large Office Complex | Per Employee/Shift | 40 - 70 Litres per person per day |
| Hotel/Motel | Per Bed/Room | 150 - 250 Litres per bed per day |
| Large Hospital | Per Bed | 350 - 450 Litres per bed per day |
Phase 3: System Selection and Specifications
Step 3: Determine Tank/Plant Capacity
Septic Tank (Anaerobic): For residential use with good drainage. Must provide 2-3 days of retention.
Minimum Capacity Formula: Capacity ( L) = Daily Flow (Q) + 2,000 L
The tank must have a minimum of two compartments for effective solid separation.
Sewage Treatment Plant (STP) / Aerobic Treatment Unit (ATU): For commercial flows, poor soils, or sensitive environmental areas. This mechanical system requires power and maintenance but produces high-quality effluent. Sizing is based directly on the peak flow rate.
Step 4: Design the Effluent Disposal (Drainfield)
This area is sized based on the Perc Test results and the daily flow.
| Soil Condition | Effluent Disposal System | Rationale |
| Good Permeability | Conventional Drainfield/Leach Field | Least expensive and simplest to install; relies on the natural soil to provide final filtration and water dispersal. |
| Poor Permeability / High Water Table | Mound System | Uses a constructed bed of specified sand and gravel above the natural grade to provide the necessary treatment layer and separation distance from the water table or impermeable soil. |
Step 5: Define Pipe Specifications (Self-Cleansing Velocity)
Sewer pipes must be correctly sloped to prevent solids from settling (silting), which causes blockages. The goal is to achieve a Self-Cleansing Velocity of at least 0.6 meters per second
| Pipe Requirement | Standard Specification |
| Minimum Diameter | 100 mm (4 inches) for residential laterals; 150 mm (6 inches) for commercial main drains. |
| Minimum Slope (Gradient) | 100 mm pipe: 1 in 50 (Drop of 20 mm per meter) |
| Maximum Flow Depth | Pipe should run no more than 75% to 83% full to allow for ventilation and non-pressurized flow. |
Phase 4: Final Compliance and Installation
Step 6: Regulatory Approval (The Mandate)
Before starting construction, you must obtain a Site Permit. This requires submitting the soil analysis, engineering plans, and system specifications to your local Health Department or Environmental Agency. All designs must adhere to local setback requirements (minimum distances from wells, foundations, and property lines).
Step 7: Installation and Final Inspection
The system must be installed by a licensed contractor exactly according to the approved design. A final inspection is always required by the local authority to verify correct pipe placement, tank installation, and drainfield size before the system is commissioned.
Key Takeaway: While this guide outlines the standards, the final design is always site-specific. The only way to guarantee a compliant and functional system is to engage a Licensed Engineer or Site Designer who can certify the flow calculations and site tests.
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