What is a Collection System

A collection system is a network of interconnected underground pipes, access chambers, pumping stations, and ancillary structures designed to receive and transport wastewater and/or stormwater from residential, commercial, industrial, and public premises to a centralised treatment facility or point of discharge. Depending on the type of wastewater conveyed and the design of the network, a collection system may include separate or combined pathways for sanitary sewage, industrial effluent, and stormwater runoff.

Collection systems form the foundation of modern urban sanitation infrastructure. They play a vital role in safeguarding public health, protecting the environment, and supporting the efficient functioning of wastewater treatment plants. Whether serving a single property or a complex metropolitan area, the effectiveness of a collection system depends on sound design, reliable operation, and ongoing maintenance.

Purpose and Function of a Collection System

The primary purpose of a collection system is to transport liquid waste and stormwater safely and efficiently from its source to a treatment location or final disposal point. This function is essential for:

• Protecting public health by removing pathogenic organisms from inhabited areas. 
• Preventing environmental pollution by controlling the discharge of untreated wastewater. 
• Avoiding property and infrastructure damage caused by flooding or backflow. 
• Enabling the centralised treatment and safe disposal or reuse of wastewater. 

In addition to these functional roles, modern collection systems are increasingly designed to incorporate climate resilience, energy efficiency, and pollution control mechanisms to meet evolving regulatory and environmental standards.

Types of Collection Systems

There are several types of collection systems, categorised primarily by the kind of wastewater they convey and their configuration.

1. Combined Sewer System

A combined sewer system collects both sanitary sewage (from toilets, sinks, showers, etc.) and stormwater runoff (from rooftops, roads, and open areas) in a single set of pipes. These systems were commonly constructed in the 19th and early 20th centuries in older urban centres.

Advantages:

• Simplified infrastructure with fewer pipes. 
• Lower initial construction cost. 

Disadvantages:

• Risk of combined sewer overflows (CSOs) during heavy rain, leading to untreated discharges into watercourses. 
• Greater hydraulic variability at treatment plants. 
• Difficult to retrofit for separation without major excavation. 

2. Separate Sewer System

In a separate sewer system, foul water and stormwater are carried in two distinct pipe networks:

• Sanitary sewer: Conveys only domestic and industrial wastewater to treatment works. 
• Stormwater sewer: Carries surface runoff to local watercourses or sustainable drainage systems (SuDS). 

Advantages:

• Prevents hydraulic overload of treatment plants. 
• Reduces pollution risk in wet weather. 
• Easier to manage flows independently. 

Disadvantages:

• Higher installation cost and spatial requirements. 
• Misconnections can cause cross-contamination and environmental harm. 

3. Simplified or Decentralised Systems

In some contexts, particularly in rural or peri-urban areas, collection systems may be simplified or decentralised. These can include:

• Small-bore sewerage: Utilises smaller-diameter pipes to carry settled sewage (after on-site primary treatment). 
• Vacuum sewerage systems: Use negative pressure to convey sewage, suitable for flat or waterlogged terrain. 
• Pressurised sewer systems: Employ grinder pumps at each property to push wastewater through narrow pipes to a treatment point. 

These systems reduce excavation requirements and offer flexibility where conventional gravity systems are impractical.

Components of a Collection System

An effective collection system includes several critical components, each with a specific function:

1. Pipes and conduits: 
   • Typically made from vitrified clay, plastic (PVC/HDPE), ductile iron, or reinforced concrete. 
   • Sized according to flow rates, gradient, and service life requirements. 
2. Manholes and inspection chambers: 
   • Provide access for cleaning, inspection, and maintenance. 
   • Located at pipe junctions, changes in direction, or regular intervals. 
3. Gullies and inlets: 
   • Collect surface water from roads, pavements, and open areas. 
   • Often equipped with silt traps to capture debris. 
4. Pumping stations: 
   • Installed where gravity flow is insufficient due to terrain. 
   • Lift wastewater to higher elevations or across long distances. 
5. Overflow structures: 
   • In combined systems, these allow excess flow to bypass the treatment plant during storms. 
   • May include screens or storage to reduce pollutant discharge. 
6. Ventilation systems: 
   • Prevent gas build-up and odour problems. 
   • Allow air movement through gravity sewers. 
7. Backflow prevention devices: 
   • Protect properties from flooding due to surcharging or downstream blockage. 
8. Emergency overflow connections: 
   • Permit controlled discharge during extreme events to reduce upstream damage. 

All components must be appropriately designed and integrated to ensure reliable function and long-term system integrity.

Design Principles

Designing a collection system requires a multidisciplinary approach, integrating hydraulics, topography, population projections, environmental impact, and regulatory compliance.

Key design considerations include:

• Capacity: The system must accommodate peak flow rates during storms or high usage periods. 
• Gradient and velocity: Pipes must be laid at appropriate slopes to maintain self-cleansing velocity and prevent sedimentation. 
• Pipe sizing and material selection: Based on expected loadings, chemical resistance, lifespan, and installation constraints. 
• Access and maintenance: Regular access points must be provided to support jetting, CCTV surveys, and repairs. 
• Infiltration and inflow control (I&I): Measures must be taken to minimise groundwater infiltration or surface water inflow that can overload the system. 
• Resilience: The system should withstand blockages, root intrusion, chemical corrosion, and changes in flow patterns due to urbanisation or climate shifts. 

Regulatory standards such as Sewerage Sector Guidance (SSG) and British Standards (e.g. BS EN 752) provide frameworks for design and installation.

Operation and Maintenance

The long-term success of a collection system depends on robust operation and maintenance (O&M) practices. Neglect can lead to pipe collapse, surcharging, pollution incidents, and costly emergency repairs.

Routine O&M tasks include:

• CCTV inspection surveys to identify defects, root ingress, or blockages. 
• Jetting and flushing to clear sediment and debris. 
• Grease and fat management in food service areas. 
• Monitoring of manholes and pumping stations for performance issues. 
• Repair and rehabilitation using patching, relining, or excavation as needed. 
• Public education to reduce misuse (e.g. wet wipes, fats, chemicals). 

Asset management systems and GIS mapping tools assist in tracking system condition and prioritising maintenance efforts.

Challenges and Considerations

Collection systems face a variety of technical, environmental, and societal challenges, including:

• Aging infrastructure: Many networks are over 50 years old and require rehabilitation. 
• Climate change: Increased rainfall intensity and urban runoff demand higher capacity and flexibility. 
• Pollution control: Reducing CSOs and nutrient loading to water bodies is a regulatory priority. 
• Urban growth: Rapid development can outpace infrastructure capacity without strategic planning. 
• Misconnections: Incorrect plumbing, where foul water enters surface drains or vice versa, causes pollution and treatment issues. 

Addressing these issues requires a combination of engineering innovation, policy development, and community engagement.

Integration with Sustainable Drainage

Modern best practices encourage the integration of Sustainable Drainage Systems (SuDS) with traditional collection infrastructure to manage runoff at source and reduce pressure on sewers. This includes:

• Permeable paving to reduce runoff. 
• Rainwater harvesting systems. 
• Swales, detention basins, and green roofs to attenuate flows. 
• Pre-treatment systems (e.g. silt traps, oil interceptors) to protect sewer integrity. 

Where possible, collection systems are designed with dual-functionality, allowing for both drainage and environmental enhancement.

Conclusion

A collection system is a vital part of wastewater and stormwater management, providing the infrastructure needed to convey flows from source to treatment or discharge. Whether simple or complex, gravity-fed or pressurised, its success hinges on thoughtful design, durable materials, proactive maintenance, and adaptability to changing conditions.

For professionals in the plumbing, civil engineering, and water sectors, understanding collection systems is fundamental to building resilient, safe, and efficient urban environments. As demands on infrastructure continue to evolve with population growth and climate variability, the role of integrated, well-managed collection systems becomes ever more critical in ensuring a sustainable future for water and sanitation services.