What is a Suspended solids

Suspended solids are a critical concept in the field of wastewater treatment, drainage engineering, and environmental protection. These solids refer to small particles that are not dissolved in water but remain suspended or floating. They are commonly found in natural water bodies, domestic sewage, industrial effluents, and stormwater runoff.

Professionals involved in plumbing, drainage system maintenance, wastewater treatment, and environmental regulation need a thorough understanding of suspended solids, as they directly impact water quality, treatment efficiency, environmental compliance, and infrastructure longevity.

This article explores the nature of suspended solids, their sources, measurement, effects, and management strategies in both water and wastewater systems.

What Are Suspended Solids?

Suspended solids (often abbreviated as SS or TSS for Total Suspended Solids) consist of tiny particles of inorganic or organic matter that are dispersed throughout a body of water but do not dissolve in it. These particles are kept suspended due to the movement of water and their own low settling velocity.

They can range in size, typically from a few microns to several millimetres, and may include:

  • Soil and silt particles

  • Plant fibres

  • Algae

  • Bacteria and protozoa

  • Organic detritus

  • Industrial residues

  • Rust and metal flakes

  • Plastic micro-particles

Suspended solids can be visually noticeable as turbidity, giving the water a cloudy, murky appearance. In laboratory analysis, their concentration is usually expressed in milligrams per litre (mg/L).

Sources of Suspended Solids

Suspended solids can enter water and wastewater systems through a wide variety of sources. These differ based on whether the water is natural, treated, or part of an industrial or municipal system.

Natural Water Sources

  • Surface runoff from soil erosion, construction sites, and unpaved roads

  • Decaying vegetation releasing fibres and organic matter

  • Natural algae blooms, particularly in nutrient-rich waters

  • Sediment resuspension due to water flow or wave action

Domestic and Municipal Wastewater

  • Food particles, fats, and fibres from kitchens

  • Toilet paper, organic waste, and hygiene products from bathrooms

  • Hair, lint, and dirt from laundry and personal care activities

  • Street debris and grit washed into combined sewers during rainfall

Industrial Effluents

  • Sludge and slurry from chemical or manufacturing processes

  • Paper pulp, sawdust, or textile fibres from processing industries

  • Fine metal shavings from machining and metalworking operations

  • Suspended pigments or residues from dyeing, painting, or printing industries

Understanding the source of suspended solids is critical in designing appropriate removal and treatment systems.

Measurement and Analysis

Suspended solids are most commonly measured as Total Suspended Solids (TSS). The typical method involves:

  1. Collecting a representative water or wastewater sample.

  2. Passing the sample through a pre-weighed fine filter (usually 1.5 micron pore size).

  3. Drying the filter to remove moisture.

  4. Weighing the retained solids to determine the mass per volume.

The result is expressed in mg/L. For example, a TSS value of 150 mg/L means that one litre of the sample contains 150 milligrams of suspended material.

High TSS levels can indicate:

  • Pollution from organic or inorganic sources

  • Overloaded or malfunctioning treatment systems

  • Erosion and sedimentation issues in catchment areas

Regular TSS monitoring is a requirement in many water discharge permits and environmental regulations.

Impact of Suspended Solids

Suspended solids have significant effects on water quality, wastewater treatment, aquatic ecosystems, and infrastructure. These effects vary depending on the type and concentration of solids present.

Environmental and Ecological Impact

  • Reduced light penetration affects aquatic photosynthesis and ecosystem balance.

  • Increased turbidity impacts fish and invertebrate feeding and breeding behaviour.

  • Oxygen depletion can occur as organic suspended solids decompose, affecting aquatic life.

  • Sedimentation in rivers and lakes alters habitats and may contribute to flooding.

Impact on Wastewater Treatment

  • Clogging of equipment such as pumps, filters, and screens

  • Reduced efficiency of biological treatment processes if solids overload the system

  • Higher sludge production, requiring more complex and costly handling

  • Straining of membrane technologies in advanced filtration systems

Infrastructure and Plumbing Implications

  • Blockages and reduced flow capacity in pipework due to sediment build-up

  • Accelerated wear and tear on valves, impellers, and mechanical components

  • Corrosion risk where solids interact with chemical residues or produce hydrogen sulphide

Given these challenges, effective control of suspended solids is essential across all water management sectors.

Treatment and Removal of Suspended Solids

There are several strategies and technologies used to remove or reduce suspended solids from water and wastewater streams. These vary in complexity depending on the application, volume, and quality requirements.

Primary Treatment Methods

  1. Screening: Coarse solids are removed using bar screens or rotating drum screens at the inlet of treatment works.

  2. Sedimentation: Particles settle by gravity in tanks or clarifiers. Common in municipal sewage works.

  3. Grit chambers: Specifically designed to remove heavier inorganic particles such as sand or gravel.

Secondary and Advanced Methods

  1. Filtration: Sand filters, fabric filters, and membrane filtration can trap smaller suspended particles.

  2. Flocculation and Coagulation: Chemicals such as alum or ferric chloride are added to cause fine particles to clump together, enhancing removal by sedimentation or filtration.

  3. Dissolved Air Flotation (DAF): Air bubbles attach to particles and lift them to the surface for removal. Often used in industrial settings.

  4. Constructed wetlands and SuDS: These systems use natural processes to settle and filter suspended solids from surface water runoff.

In some applications, sludge management becomes a parallel challenge, as the removal of solids results in the accumulation of semi-solid waste that must be processed, dewatered, and safely disposed of or reused.

Regulatory Standards and Compliance

In the UK and across the EU, suspended solids are a key parameter in environmental permits and water quality standards. The Environment Agency, Scottish Environment Protection Agency (SEPA), and Natural Resources Wales all regulate discharges to ensure TSS levels remain within safe and permitted limits.

Typical discharge consent levels for suspended solids may range from 20 to 60 mg/L, depending on the receiving environment and type of effluent. Exceeding these levels may result in:

  • Fines or enforcement actions

  • Increased operating costs due to required improvements

  • Reputational damage for polluting companies or public authorities

Industrial operators and wastewater treatment plants must carry out regular monitoring, reporting, and, where required, invest in treatment upgrades to remain compliant.

Relationship with Other Water Quality Parameters

Suspended solids are closely linked to several other key water quality indicators:

  • Biochemical Oxygen Demand (BOD): Suspended organic solids increase BOD levels, placing stress on receiving waters.

  • Chemical Oxygen Demand (COD): Suspended particles may include chemically oxidisable matter.

  • Turbidity: A direct visual effect of suspended solids; often used as a proxy indicator.

  • Nutrient loading: Particulates can adsorb and transport nitrogen and phosphorus, contributing to eutrophication.

As such, suspended solids are not merely a physical concern but are integral to the chemical and biological health of water systems.

Preventative Measures and Good Practice

To reduce the generation and discharge of suspended solids, a range of preventative and operational measures can be implemented. These include:

  • Regular cleaning and maintenance of gullies, catch pits, and drainage networks

  • Erosion control measures on construction sites and exposed soil

  • Proper segregation of industrial processes to prevent cross-contamination

  • Public education to discourage disposal of inappropriate materials into drains

In the design phase, systems should be sized to accommodate expected solids loading and allow for adequate retention, settling, and treatment time.

Conclusion

Suspended solids are a fundamental concern across all aspects of water and wastewater management. Their presence affects water quality, system performance, ecological health, and regulatory compliance. Understanding the sources, impacts, measurement methods, and treatment options is essential for professionals working in plumbing, environmental engineering, drainage design, and water treatment.

As environmental standards tighten and urbanisation increases, the need for effective suspended solids control is greater than ever. Through a combination of good design, robust maintenance, and advanced treatment technologies, the challenges posed by suspended solids can be successfully addressed, leading to cleaner water, healthier ecosystems, and more resilient infrastructure.