Design Scheme for Sewage System in Building Construction

1.0 General Design Principles

1. Design Objectives: To safely, efficiently, and environmentally collect, treat, and discharge sewage generated within the building into the municipal sewer network or a designated receiving water body. Ensure the system is free from blockages, leaks, noise, and odors.

2. Design Basis:

   • Code for Design of Building Water Supply and Drainage(GB 50015)

   • Code for Design of Outdoor Wastewater Engineering(GB 50014)

   • Code for Acceptance of Construction Quality of Building Water Supply, Drainage, and Heating Works(GB 50242)

   • Relevant local regulations and requirements from municipal authorities.

   • Architectural design drawings (to understand layout, functions, floor height, pipe shaft locations, etc.).

2.0 Design Process

The design of a building's sewage system is a systematic process. The core design workflow can be summarized into several key stages, starting from basic data collection, progressing through hydraulic calculations, pipeline network design, and system selection, ultimately resulting in a complete construction plan. The specific process is illustrated in the flowchart below:

flowchart TD
    A[Phase 1<br>Collect and Analyze Basic Data] --> B[Phase 2<br>Determine Drainage System Type and Outfall]
    B --> C[Phase 3<br>Estimate Sewage Flow and Load]
    C --> D[Phase 4<br>Plan and Vertical Pipeline Design]
    D --> E[Phase 5<br>Special Structures and Equipment Selection]
    E --> F[Phase 6<br>Finalize Design Scheme]

The following sections provide detailed explanations for each phase:

Phase 1: Collect and Analyze Basic Data

• Building Information: Building purpose (residential, office, hotel, commercial complex), floor area, number of stories, story height, location and quantity of functional zones (kitchens, bathrooms, laundry rooms, etc.).

• Municipal Conditions: Connection point to the municipal sewer network, pipe diameter, buried depth, flow direction, and permitted flow rate and quality requirements for discharge.

• Site Conditions: Geotechnical investigation report to understand groundwater level, soil properties, etc.

Phase 2: Determine Drainage System Type and Outfall

• Drainage System Type: Typically, a separate system is used, meaning sanitary sewage (blackwater from toilets) and greywater (from sinks, showers, laundry) are separated. This facilitates potential greywater reuse later. Based on municipal requirements and project positioning, a combined system might also be determined.

• Discharge Outfall:

  1. Discharge to Municipal Sewer: The most common method. Ensure effluent quality meets municipal connection standards.

  2. On-site Wastewater Treatment Plant (WWTP): Designed when no municipal sewer is available or the distance is too far. Requires designing septic tanks or small-scale wastewater treatment facilities for discharge or reuse after meeting standards.

Phase 3: Estimate Sewage Flow and Drainage Load

• Domestic Sewage Flow Rate: Calculated based on building type, using the maximum daily water consumption per capita from the Code for Design of Building Water Supply and Drainage, multiplied by a sewage discharge coefficient (typically 0.85-0.95).

• Design Drainage Flow Rate (Peak Instantaneous Flow): Calculated using probability formulas (e.g., Hunter's curve) to determine the maximum instantaneous flow generated by simultaneous drainage of fixtures served by a pipe section. This is the core basis for determining pipe diameter.

Phase 4: Plan and Vertical Pipeline Design

• Layout Planning:

  • Determine the locations of drainage stacks and horizontal main pipes, preferably located in pipe shafts or ceiling voids for easy installation and maintenance.

  • Follow the principle of "shortest pipeline, fewest bends" to reduce blockage risk.

• Vertical Design (Core):

  • Drainage Stack: Collects sewage from branch pipes on each floor. Must be equipped with a vent stack (stack vent or circuit vent) open to the atmosphere to balance internal air pressure, prevent water seal loss in traps, and ensure smooth drainage.

  • Horizontal Drainage Pipes: Must maintain a certain slope (typically 1% to 2%, depending on pipe diameter) to achieve gravity flow and prevent sediment deposition.

Phase 5: Special Structures and Equipment Selection

• Septic Tank / Grease Interceptor: If required, its volume is calculated based on number of users, sewage detention time. Used for primary sedimentation and anaerobic digestion (septic tank) or grease removal (grease interceptor for kitchens).

• Sump Pit and Sewage Ejector Pump: For basements, equipment rooms, or other locations where sewage cannot be discharged by gravity. A sump pit collects the sewage, and submersible sewage pumps (typically with duty/standby configuration) lift it to the outdoor network.

• Cleanouts / Access Points: Installed at the base of stacks, pipe bends, and size transitions for easy cleaning and maintenance.

Phase 6: Finalize the Design Scheme

• Draw detailed construction drawings, including system schematics, plan views, and detailed diagrams.

• Prepare equipment and materials lists and design specifications.

3.0 Brief Design Scheme (Example: High-Rise Residential Building)

1. System Type: Separate system for sanitary sewage (blackwater) and greywater. Blackwater is treated in a septic tank before combining with greywater for discharge to the municipal sewer.

2. Pipeline Network System:

   • Stack System: Each drainage stack is equipped with a dedicated vent stack, connected at each floor by circuit vents, forming a "dual stack" system to ensure stable air pressure during drainage in high-rises.

   • Pipe Material Selection:

     • Indoor: Stacks and mains use quiet drainage plastic pipes (U-PVC) or cast iron pipes with flexible connections (for higher acoustic requirements). Branch pipes can use standard U-PVC pipes.

     • Outdoor: Use High-Density Polyethylene (HDPE) corrugated pipes or PVC-U reinforced pipes, which have high pressure resistance and corrosion resistance.

3. Key Structures and Equipment:

   • Septic Tank: Sized based on the number of households. Precast concrete or fiberglass septic tanks can be selected, buried under green spaces or roads outdoors.

   • Basement Drainage: Wastewater from basement car wash drains, pump room drainage, etc., is collected in sump pits. Submersible sewage pumps (duty/standby) are installed in the pits, controlled automatically by liquid level for discharge.

4. Key Points of Hydraulic Calculation:

   • Pipe Diameter: Determined based on the design peak flow and the specified pipe fullness (indoor drainage pipes are typically designed for partially full flow, fullness ≤ 0.5-0.6) by consulting hydraulic tables. For example, the main stack diameter is usually not less than De110, and the underground building drain not less than De160.

   • Slope: Slope for De110 horizontal pipe is 1.2%; for De160 horizontal pipe, it is 0.8%.

5. Environmental Protection and Noise Reduction Measures:

   • Seal pipe penetrations through floors with waterproof oakum and then with sealing rings to prevent noise transmission and leakage.

   • Use rubber pads at pipe supports for vibration damping.

   • Prioritize the use of silent-drainage pipe materials (e.g., spiral silent pipes) to reduce drainage noise.

4.0 Summary

The design of a building's sewage system is a comprehensive engineering task, whose core principles are "smooth drainage, balanced air pressure, and ease of maintenance." An excellent design scheme must be based on accurate fundamental data, compliant hydraulic calculations, and a rational spatial layout. Simultaneously, full consideration must be given to the convenience of later operation and maintenance to ensure the long-term stable operation of the building's drainage system.