What is a Combined inflow

Combined inflow refers to the mixture of domestic wastewater, industrial effluent and stormwater that enters a combined sewer system. In such systems, both sanitary sewage and surface runoff are conveyed through the same network of pipes to a wastewater treatment plant or, in some cases, to a combined sewer overflow discharge point.

This concept is central to understanding how urban drainage systems function in many older cities across the United Kingdom and Europe. Combined inflow represents both an engineering solution and an environmental challenge: it enables efficient drainage under normal conditions but can lead to overflows and pollution during heavy rainfall.

A detailed understanding of combined inflow is essential for designing, operating and upgrading urban sewerage networks in a sustainable way.

The concept of combined sewer systems

Combined sewer systems were first introduced in the 19th century, when urban sanitation infrastructure was developed to prevent disease and remove wastewater from populated areas. At the time, separating foul sewage and stormwater was considered impractical and too costly. Instead, engineers designed a single pipe system that carried all types of flow together to rivers or later to treatment plants.

Under dry weather conditions, the system carries domestic and industrial wastewater only. When rainfall occurs, surface water from roofs, roads and paved areas also enters the network through gullies, drains and manholes. This creates the combined inflow, which can significantly increase the total flow within a short period.

While this approach was effective in early urban drainage, it presents challenges in modern cities, particularly as rainfall intensity increases and environmental standards tighten.

Composition and characteristics of combined inflow

Combined inflow consists of several distinct components that vary depending on weather, land use and system design.

1. Sanitary wastewater:
   Produced from households and commercial premises, this component includes blackwater from toilets, greywater from sinks, showers and washing machines, and industrial effluent from manufacturing or processing activities. It contains organic matter, nutrients, suspended solids, and microorganisms.

2. Stormwater runoff:
   Generated by rainfall, this component varies widely in quantity and quality. It may carry sediment, hydrocarbons, heavy metals, litter, oils and other pollutants washed off from impervious surfaces such as roads and car parks.

3. Groundwater infiltration and surface inflow:
   In older systems, groundwater may enter through cracks or joints in sewers, while surface water can enter through faulty manhole covers or cross-connections. These unintentional contributions further increase flow volumes, especially during wet weather.

The combined inflow therefore represents a dynamic mixture of flows with different chemical and physical characteristics. During dry weather, pollutant concentrations are relatively high but flow is low. During rainfall, dilution occurs, reducing concentrations but increasing hydraulic loading.

Hydraulic behaviour of combined inflow

The hydraulics of combined inflow are complex due to rapid fluctuations in flow rate and volume during storm events. When rainfall begins, runoff quickly enters the system, causing flow rates to rise sharply. This can lead to surcharging of pipes, increased pressure on pumping stations, and temporary storage in detention basins or tunnels.

Most combined sewer systems are designed with overflow structures known as combined sewer overflows (CSOs). These allow excess flow to be diverted to nearby watercourses during heavy rain, preventing flooding of streets and properties. However, CSO discharges contain diluted sewage and can negatively impact water quality.

Understanding the relationship between rainfall intensity, catchment area and sewer capacity is essential for managing combined inflow and minimising overflow events. Hydraulic modelling tools are widely used to predict system performance and plan mitigation measures.

Environmental implications of combined inflow

While combined systems provide efficient drainage under normal conditions, they pose significant environmental challenges during wet weather. The main concerns include:

• Combined sewer overflows: When inflow exceeds system or treatment plant capacity, untreated or partially treated wastewater is discharged directly into rivers, lakes or coastal waters. These events can cause contamination, oxygen depletion and harm to aquatic life.

• Pollution load variability: The quality of combined inflow varies widely, making treatment more complex. High concentrations of organic matter and nutrients can lead to eutrophication, while pathogens pose public health risks.

• Sediment accumulation and resuspension: During dry periods, solids settle in pipes and channels. Sudden high flows during rainfall can resuspend these materials, causing pollutant spikes in outflows.

• Odour and gas production: Organic matter decomposition within slow-moving sections of combined systems can generate hydrogen sulphide and other odorous gases.

Effective management of combined inflow is therefore essential to minimise environmental impact and ensure compliance with water quality regulations.

Treatment and management of combined inflow

The treatment of combined inflow depends on system design and available capacity. In modern facilities, flow management and storage play key roles in preventing pollution and maintaining treatment efficiency.

1. Flow separation and screening:
   At the inlet to treatment works, coarse screens remove large debris such as rags, plastics and grit. Flow is then distributed between primary treatment tanks and storm tanks based on capacity.

2. Stormwater storage and attenuation:
   When inflow exceeds the plant’s design limit, excess water is temporarily stored in storm tanks or tunnels. After rainfall subsides, this retained water is gradually returned for treatment.

3. Hydraulic control structures:
   Regulators, weirs and gates are used to balance flow between different parts of the network, directing excess water to storage or overflow points as needed.

4. Real-time control systems:
   Modern networks increasingly employ sensors and automated controls to monitor rainfall, flow levels and pump operation. These systems enable dynamic management of combined inflow to reduce overflow frequency and optimise treatment.

5. Treatment of overflow discharges:
   In some areas, CSO discharges are treated using screens, sedimentation tanks or disinfection systems to remove solids and pathogens before release into the environment.

By combining these approaches, utilities can significantly improve performance while meeting regulatory standards.

Combined inflow and treatment plant operation

Combined inflow poses several operational challenges for wastewater treatment plants. The high variability in flow and pollutant load during storms can disrupt biological processes and reduce treatment efficiency.

During heavy rain, the dilution effect reduces the concentration of organic matter and nutrients, leading to underloading of biological reactors. However, the sudden influx of large volumes can also wash out biomass from activated sludge systems, causing instability.

To address these challenges, treatment plants are often designed with flexible capacity and advanced process control. Key measures include:

• Installation of balancing tanks to equalise flow and load before biological treatment.

• Adjustable aeration systems that respond to changes in organic load.

• Bypass arrangements to divert heavily diluted inflows to separate treatment or storage facilities.

Maintaining a stable operation under variable combined inflow conditions requires continuous monitoring and adaptive management.

The shift toward separate sewer systems

Modern urban drainage design increasingly favours separate sewer systems, where wastewater and stormwater are collected in independent networks. This approach reduces hydraulic load on treatment plants and prevents overflow discharges of untreated sewage.

However, converting existing combined systems to separate networks is costly and often impractical, particularly in densely built urban areas. As a result, many cities continue to operate combined systems while implementing control measures to reduce the impacts of combined inflow.

These measures include:

• Installing sustainable drainage systems (SuDS) such as green roofs, permeable pavements and rain gardens to reduce stormwater entering sewers.

• Disconnecting roof drains and surface runoff from the combined network.

• Expanding storage capacity through underground tunnels or retention basins.

• Implementing real-time control and predictive modelling for system optimisation.

By combining these strategies, cities can significantly reduce the frequency and severity of combined sewer overflows without full system replacement.

Climate change and future challenges

Climate change presents new challenges for managing combined inflow. More frequent and intense rainfall events increase the risk of sewer overflows, while urbanisation adds impervious surfaces that generate greater runoff.

Existing combined systems, many of which were designed decades ago, often lack the capacity to handle these increased loads. As a result, utilities must adapt by upgrading infrastructure, improving monitoring and adopting integrated urban water management approaches.

Predictive modelling, green infrastructure and smart control technologies are key tools in building climate resilience. These allow operators to anticipate extreme weather and optimise network performance in real time, reducing both flooding and pollution risks.

Monitoring and data management

Accurate monitoring of combined inflow is essential for effective management. Flow meters, rain gauges and level sensors installed throughout the sewer network provide real-time data on system behaviour.

Data analysis and hydraulic modelling help identify problem areas, such as bottlenecks or frequent overflow points. This information supports decision-making for maintenance, rehabilitation and investment planning.

In many regions, environmental regulators require utilities to record and report overflow events. Automated telemetry systems simplify compliance by logging data automatically and generating reports on system performance and discharge volumes.

Environmental regulations and standards

In the United Kingdom, the Environment Agency and water companies operate under strict regulatory frameworks to control combined sewer overflows and protect water quality. The Urban Waste Water Treatment Regulations and Water Framework Directive set limits on pollutant discharges and require monitoring of CSO events.

Utilities are investing in major infrastructure projects, such as storage tunnels and treatment upgrades, to meet these requirements. London’s Thames Tideway Tunnel, for example, is a large-scale solution designed to capture and treat combined inflow that would otherwise overflow into the River Thames.

Similar initiatives across Europe and North America aim to modernise ageing combined systems and align them with contemporary environmental standards.

Conclusion

Combined inflow represents the convergence of wastewater and stormwater within a single sewer system. While this approach has served cities for over a century, it now poses significant environmental and operational challenges due to increasing rainfall and stricter water quality regulations.

Understanding and managing combined inflow is essential for sustainable urban drainage. Through improved monitoring, storage, real-time control and green infrastructure, utilities can reduce overflow events and enhance treatment efficiency.

As cities evolve and climate pressures intensify, the effective management of combined inflow will remain a cornerstone of modern water and wastewater engineering, balancing the needs of urban resilience, public health and environmental protection.