What Are Solids in Wastewater? Complete Guide to Types, Measurement & Treatment
Solids in wastewater are one of the most important indicators of water quality and treatment performance. Whether you operate a municipal treatment plant, manage industrial discharge, or simply want to understand what happens to water after it goes down the drain, understanding solids is essential.
In simple terms, solids in wastewater refer to any matter that remains after water is evaporated, including organic and inorganic particles, dissolved minerals, and suspended materials. These solids come from homes, businesses, industries, and stormwater runoff, and they must be removed or reduced before treated water can be safely discharged into the environment.
This article explains the different types of solids, how they are measured, why they matter, and how modern treatment systems remove them.
What Are Solids in Wastewater?
Solids in wastewater are all materials present in water that are not liquid. They include everything from large debris like paper and plastic to microscopic particles and dissolved salts. When a water sample is filtered and dried at a specific temperature, the remaining residue is classified as "solids."
The presence of solids affects:
Water clarity and appearance
Oxygen levels in receiving water bodies
Treatment efficiency
Sludge production and disposal costs
Regulatory compliance
High levels of solids can harm aquatic life, clog pipes, reduce disinfection effectiveness, and increase the cost of treatment.
The Main Types of Solids in Wastewater
Solids are typically classified based on size, solubility, and volatility. The four primary categories are total solids, suspended solids, dissolved solids, and volatile solids.
Total Solids (TS)
Total solids is the sum of all solids in a water sample. It includes both suspended and dissolved materials. TS is determined by evaporating a sample and weighing the remaining residue.
Formula: TS = TSS + TDS
Total Suspended Solids (TSS)
Total suspended solids are particles that do not dissolve and can be removed by filtration. TSS is one of the most regulated parameters in wastewater treatment. It includes organic matter, silt, clay, and microorganisms.
Total Dissolved Solids (TDS)
Total dissolved solids are materials that pass through a filter and remain dissolved in water. These include salts, metals, minerals, and some organic compounds. TDS is often associated with salinity and conductivity.
Volatile and Fixed Solids
Volatile solids (VS) represent the organic portion of solids that burns off at high temperature (usually 550°C). Fixed solids are the inorganic residue that remains. The ratio of volatile to fixed solids helps operators understand the organic load and stabilization level of sludge.
Table 1: Types of Solids in Wastewater at a Glance
| Type of Solid | Definition | Typical Measurement | Common Sources |
| Total Solids (TS) | All solids, dissolved and suspended | Evaporation at 103–105°C | Domestic, industrial, stormwater |
| Total Suspended Solids (TSS) | Particles retained on a filter | Filtration + drying | Human waste, food, paper, grit |
| Total Dissolved Solids (TDS) | Materials passing through a filter | Filtrate evaporation | Salts, minerals, detergents |
| Volatile Solids (VS) | Organic fraction that burns off | Ignition at 550°C | Food waste, fecal matter, biomass |
| Fixed Solids (FS) | Inorganic residue after ignition | Remaining ash | Sand, silt, minerals |
| Settleable Solids | Solids that settle in 60 minutes | Imhoff cone | Grit, heavy particles |
Why Solids Matter in Wastewater Treatment
Solids are more than just an aesthetic issue. They directly influence how a treatment plant operates and how well it protects public health and the environment.
Key reasons solids matter:
Regulatory compliance – Most discharge permits set strict limits on TSS and sometimes TDS.
Treatment performance – High solids loads can overwhelm biological processes.
Disinfection – Suspended particles can shield pathogens from UV light or chlorine.
Sludge management – Solids become sludge, and sludge handling is one of the largest operational costs.
Environmental impact – Solids can smother aquatic habitats and reduce oxygen levels.
How Solids in Wastewater Are Measured
Accurate measurement is critical for monitoring and controlling treatment processes. The most common methods follow Standard Methods for the Examination of Water and Wastewater.
TSS Measurement
A measured volume of sample is filtered through a glass fiber filter. The filter is dried at 103–105°C and weighed. The increase in weight represents the suspended solids.
TDS Measurement
The filtrate from the TSS test is evaporated and dried. The remaining residue is the total dissolved solids.
Settleable Solids
A sample is placed in an Imhoff cone for 60 minutes. The volume of material that settles at the bottom is recorded in mL/L.
Volatile Solids
Dried solids are ignited at 550°C. The weight loss represents volatile (organic) solids, while the remaining ash is fixed solids.
Table 2: Typical Solids Concentrations in Wastewater
| Wastewater Type | TSS (mg/L) | TDS (mg/L) | BOD (mg/L) |
| Raw domestic wastewater | 200–350 | 250–500 | 200–300 |
| Primary effluent | 60–120 | 250–500 | 100–150 |
| Secondary effluent | 10–30 | 300–600 | 10–30 |
| Tertiary treated water | 1–10 | 300–600 | 1–10 |
| Industrial effluent | Highly variable | Highly variable | Highly variable |
Note: Values are typical ranges and vary by location, season, and source.
How Solids Are Removed in Wastewater Treatment
Wastewater treatment plants use a combination of physical, chemical, and biological processes to remove solids.
Preliminary Treatment
Screens and grit chambers remove large debris, rags, plastics, and heavy grit. This protects downstream equipment.
Primary Treatment
Sedimentation tanks allow settleable solids to sink to the bottom as primary sludge. Floating materials are skimmed from the surface.
Secondary Treatment
Biological processes such as activated sludge or trickling filters convert dissolved and colloidal organic matter into biomass, which is then settled out in clarifiers.
Tertiary Treatment
Filtration, membrane bioreactors, and disinfection further reduce suspended solids. Advanced options like reverse osmosis remove dissolved solids.
Sludge Treatment
Sludge from primary and secondary treatment is thickened, digested, dewatered, and disposed of or reused. Volatile solids reduction is a key goal in sludge stabilization.
Common Problems Caused by Excess Solids
Excess solids can create serious operational and environmental problems:
Clogged pipes and pumps – Grit and debris cause wear and blockages.
Poor disinfection – Particles protect bacteria and viruses.
Sludge bulking – Filamentous organisms can cause poor settling.
Higher disposal costs – More sludge means more hauling and treatment.
Permit violations – Exceeding TSS limits can result in fines.
Ecosystem damage – Solids reduce light penetration and suffocate aquatic life.
Best Practices for Managing Solids
Effective solids management combines monitoring, maintenance, and process optimization.
Recommended practices:
Monitor TSS, TDS, and VS regularly.
Maintain proper screening and grit removal.
Optimize coagulant and flocculant dosing.
Control sludge age and return rates in biological systems.
Use real-time turbidity monitoring as an early warning.
Train operators on sampling and analysis procedures.
Review discharge permits and update treatment goals.
Need Help With Wastewater Solids Management? Contact us today to discuss your wastewater challenges and find the right solution for your facility. Reach out now for a consultation and let our experts guide you toward cleaner, compliant water.
Frequently Asked Questions
What is the difference between TSS and TDS?
TSS (total suspended solids) are particles that can be filtered out of water, while TDS (total dissolved solids) are materials that pass through a filter and remain dissolved. TSS affects clarity and can be removed by settling or filtration, while TDS requires processes like reverse osmosis or ion exchange.
Why is TSS regulated in wastewater discharge?
TSS is regulated because suspended particles harm aquatic ecosystems, reduce oxygen levels, carry pathogens and pollutants, and increase treatment costs downstream. High TSS can also indicate poor treatment performance.
How are solids in wastewater reduced?
Solids are reduced through a combination of physical, chemical, and biological processes, including screening, sedimentation, biological treatment, filtration, and sludge digestion. Each stage targets different types and sizes of solids.