What is a Gross solids

Gross solids refer to large, visible particles found in wastewater or stormwater that are either suspended in the flow or buoyant enough to float. These materials are typically not dissolved or finely dispersed, and they pose a significant pollution risk when discharged into receiving water bodies such as rivers, lakes, or coastal areas. Gross solids are a key concern in both urban drainage systems and combined sewer overflows (CSOs) due to their impact on water quality, aesthetics, and ecological health.

They are typically removed or retained during preliminary stages of wastewater treatment or within stormwater management infrastructure to prevent them from reaching natural environments.

Characteristics of Gross Solids

Gross solids include a wide range of anthropogenic and natural materials. They are often defined not only by their size but also by their persistence and potential for visual or biological pollution.

Common types of gross solids include:

  • Sanitary items (e.g. wet wipes, sanitary towels, cotton buds)
  • Plastic packaging and litter
  • Food waste particles
  • Fats, oils, and grease (when congealed and solidified)
  • Organic debris (e.g. leaves, twigs, or silt clumps)
  • Tissue and toilet paper fragments

These materials are typically greater than 6 mm in size, distinguishing them from finer suspended solids or dissolved pollutants.

Environmental Impacts

When gross solids enter receiving waters untreated or in large volumes, they can cause:

  • Visual pollution: Floating waste and debris spoil the appearance of rivers, canals, and coastal areas, affecting public perception and recreation.
  • Blockages and scum layers: Accumulated solids can form mats or scum layers on water surfaces, reducing oxygen exchange and disrupting aquatic life.
  • Ecosystem degradation: Plastics and fibrous materials can entangle or be ingested by wildlife, leading to injury or death.
  • Sedimentation issues: Larger solids may settle in slower-flowing areas, contributing to channel siltation and reduced capacity.
  • Public health risks: Items contaminated with faecal matter or chemicals may carry pathogens into water used for swimming or fishing.

For these reasons, controlling gross solids is an essential part of environmental protection strategies.

Gross Solids in Sewerage and Drainage Systems

Gross solids are particularly problematic in combined sewer systems, which carry both foul sewage and surface water. During heavy rainfall, these systems may become overloaded, resulting in combined sewer overflows (CSOs). CSOs discharge excess water directly into rivers or the sea to prevent flooding — but often carry untreated gross solids.

To address this, many water utilities and engineers incorporate gross solids removal technologies into system design. These may include:

  • Screens and bar racks: Installed at treatment works or overflow chambers to trap large debris.
  • Gross solids separators: Devices specifically designed to capture floating and suspended material in stormwater flows.
  • Sedimentation and detention tanks: Slow water to allow settling of heavier solids.
  • Hydrodynamic vortex separators: Use flow dynamics to capture and retain solids in stormwater systems.

In urban drainage systems, gross solids may also be intercepted through:

  • Litter traps in gullies or road drains
  • Vegetated swales or wetlands that filter debris
  • Trash racks at outfalls and culvert inlets

Monitoring and Regulation

Environmental agencies increasingly monitor gross solids as part of efforts to improve bathing water quality, aquatic biodiversity, and public amenity standards. In the UK:

  • Environment Agency permits for CSOs often require controls to limit the discharge of gross solids.
  • Water companies are under growing pressure to invest in storm overflow improvements, particularly in sensitive areas.
  • Water Framework Directive (WFD) objectives include reducing visible and physical pollutants, including litter and gross solids.

Best practice now encourages systems to be designed to retain gross solids during all but the most extreme rainfall events.

Mitigation and Public Education

In addition to infrastructure measures, public awareness campaigns play an essential role in reducing gross solids at the source. Examples include:

  • “Bin it, don’t flush it” campaigns to discourage disposal of wipes and hygiene products in toilets
  • Promoting proper disposal of litter and kitchen waste
  • Installing clear signage and public bins near water bodies
  • Encouraging the use of fat traps in commercial kitchens to prevent FOG solidification

Reducing gross solids at source helps reduce both environmental impacts and the operational burden on treatment infrastructure.

Conclusion

Gross solids are a visible and harmful class of pollutant in urban drainage and sewerage systems. Composed of large, suspended or floating waste materials, they contribute to water pollution, ecological degradation, and public health risks when not properly managed. By combining effective screening, separation technologies, sustainable drainage design, and public education, engineers and water authorities can minimise the presence of gross solids in receiving waters and ensure compliance with environmental standards. For modern water management strategies, addressing gross solids is not just about system performance — it is a fundamental part of safeguarding clean, healthy, and usable water environments.