How to Reduce Solids in Wastewater Treatment

Solids are an unavoidable challenge in wastewater treatment. Even highly efficient treatment systems must manage, remove, or dispose of some residual material.

The three primary types of solids discussed in this article are:

• Trash and solid waste

• Organic solids

• Inorganic solids, or grit

Understanding the differences between these materials can help wastewater operators choose the most effective strategy for reducing solids and maintaining treatment efficiency.

Types of Solids in Wastewater

Trash and Solid Waste

Trash includes large, non-biodegradable materials that enter the wastewater collection system.

Common examples include:

• Rags

• Plastic bottles

• Baby wipes

• Personal hygiene products

• Other non-flushable materials

These materials can clog sewer lines, damage pumps, and increase maintenance requirements throughout the collection and treatment system.

Organic Solids

Organic solids are biodegradable particles found in sewage and wastewater.

These materials contribute significantly to the organic loading of a wastewater treatment plant. Unlike trash and grit, many organic solids can be broken down by microorganisms during biological wastewater treatment.

Improving biological solids degradation can help reduce sludge accumulation and improve overall process efficiency.

Inorganic Solids and Grit

Inorganic solids include materials that generally cannot be biologically degraded.

Examples include:

• Sand

• Silt

• Clay

• Rocks

• Gravel

• Eggshells

• Coffee grounds

• Other dense particulate materials

These materials can settle in pipes, basins, and treatment equipment, increasing wear and reducing available treatment capacity.

Why Solids Management Is Important

Reducing solids is essential for maintaining wastewater treatment performance and effluent quality.

High concentrations of suspended or organic solids in treated wastewater can make it difficult for facilities to meet federal, state, and local discharge requirements.

Wastewater utilities and Publicly Owned Treatment Works (POTWs) must meet the limits established in their National Pollutant Discharge Elimination System (NPDES) permits.

Failure to meet permit requirements may result in regulatory action, increased monitoring, and potential penalties.

How Solids Affect Wastewater Lagoons

In wastewater lagoons, accumulated solids can gradually reduce the amount of space available for treatment.

As sludge builds up on the lagoon bottom:

• Available treatment volume decreases

• Wastewater detention time may be shortened

• Biological treatment efficiency can decline

• Lagoon capacity is reduced

• Mechanical dredging may eventually be required

Effective lagoon solids management can help preserve treatment capacity and delay costly sludge removal.

How Trash and Grit Affect Mechanical Treatment Plants

Trash and inorganic solids can also damage mechanical wastewater treatment equipment.

These materials may:

• Clog pumps and pipes

• Damage screens and grinders

• Increase equipment wear

• Reduce treatment efficiency

• Increase maintenance requirements

• Increase operating costs

Removing these solids early in the treatment process can help protect downstream equipment.

Solids Can Contribute to Sewer Overflows

Trash and other large solids entering wastewater collection systems can create blockages that restrict flow.

These blockages may contribute to:

• Sanitary Sewer Overflows (SSOs)

• Combined Sewer Overflows (CSOs)

• Sewer backups

• Emergency maintenance

• Environmental contamination

Preventing solids from entering the collection system is therefore one of the most effective ways to reduce wastewater maintenance and overflow risks.

Five Ways to Reduce Solids in Wastewater Treatment

The following strategies can help wastewater utilities reduce solids in collection systems, lagoon systems, and mechanical treatment plants.

Some methods are designed for specific treatment configurations, while others can be used across many types of wastewater facilities.

1. Prevent Trash from Entering the Collection System

The simplest way to reduce trash in wastewater systems is to prevent it from entering the collection system.

While complete prevention is difficult, wastewater utilities can reduce solid waste through physical screening and public education.

Two common approaches include:

• Installing screens or grates to capture large debris

• Educating residents about what should and should not be flushed or poured down drains

These strategies can be used by communities with many different collection and treatment system designs.

Install Screens and Trash Racks

Screens and grates can help prevent large objects from entering or moving through wastewater collection systems.

Common screening equipment includes:

• Coarse screens

• Bar screens

• Trash racks

• Trash grates

These systems can be installed at sewer inlets, treatment plant headworks, or other strategic locations within the collection system.

Prevent Trash in Combined Sewer Systems

Combined sewer systems are particularly vulnerable to trash accumulation because they collect both wastewater and stormwater.

During rainfall events, litter from:

• Streets

• Parking lots

• Sidewalks

• Storm drains

can enter the sewer system.

Large floating debris can contribute to blockages and increase the risk of combined sewer overflow events.

Preventing trash from entering these systems can help reduce emergency maintenance, overflow risks, and long-term wastewater management costs.

Educate the Community About Trash in Sewer Systems

Community education is an important part of preventing trash and non-biodegradable solids from entering wastewater collection systems.

Domestic wastewater contains many different types of solid material. Some organic materials can be biologically degraded during wastewater treatment, while plastics, wipes, and other complex products do not break down easily.

Items that should never be flushed or rinsed down drains include:

• Baby wipes

• Sanitary wipes

• Dental floss

• Plastic bottle caps

• Paper towels

• Personal hygiene products

• Other non-flushable materials

Reducing all unnecessary solids entering the collection system helps Publicly Owned Treatment Works (POTWs) operate more efficiently.

Simple household practices can make a difference. Sink screens can capture food particles and other debris before they enter drains, while placing a trash can near the toilet can encourage proper disposal of wipes, floss, and hygiene products.

Wastewater utilities can also use community events, websites, social media, utility bills, and printed materials to educate residents about proper disposal.

Effective education can help residents understand that their everyday habits directly affect sewer blockages, treatment costs, and environmental protection.

Benefits of Community Education

Public education can:

• Reduce non-flushable materials entering sewer systems

• Lower the risk of blockages

• Improve wastewater treatment efficiency

• Reduce maintenance requirements

• Encourage environmentally responsible disposal habits

Education programs are also relatively affordable and can be distributed through websites, social media, newsletters, and public outreach.

Limitations of Community Education

Educational programs depend on residents changing their behavior.

Reducing trash in wastewater systems through public outreach may therefore require consistent messaging, long-term education, and continued participation from the community.

Benefits and Limitations of Collection System Screens

Mechanical screens and grates can effectively capture trash before it moves farther into the wastewater collection system.

These devices are relatively simple and can often be incorporated into municipal wastewater infrastructure.

However, screens require routine cleaning to maintain proper wastewater flow.

Collected materials must also be handled, transported, and disposed of appropriately, creating additional labor and disposal costs.

2. Establish and Maintain Effective Wastewater Headworks

Wastewater headworks systems remove trash, floatable materials, and inorganic solids before wastewater enters the main treatment process.

A typical headworks may include:

• Screens

• Bar racks

• Grinders

• Compactors

• Grit chambers

Effective pretreatment protects downstream wastewater equipment and reduces the amount of material entering biological treatment systems.

Headworks can be installed ahead of many different types and sizes of wastewater treatment facilities.

Screening Trash and Large Solids

Screens remove large solids such as:

• Plastics

• Paper products

• Rags

• Wipes

• Woody debris

• Other large objects

Several types of wastewater screens are available.

Manual Bar Screens

Manual bar screens typically use vertical bars spaced approximately 1 to 2 inches apart to capture large debris.

Operators manually rake the accumulated solids from the screen for disposal.

Automatic Bar Screens

Automatic bar screens perform a similar function but use mechanical equipment to continuously or periodically remove collected material.

This reduces manual labor and can improve reliability at facilities receiving larger solids loads.

Integrated Headworks Systems

Some modern headworks systems combine screening, grinding, washing, and compacting into a single piece of equipment.

Wastewater passes through the screen while captured solids are processed, compacted, and transferred into a container for disposal.

Removing Inorganic Solids with Grit Chambers

Grit removal is another important headworks function.

Grit includes dense inorganic materials such as:

• Sand

• Gravel

• Silt

• Eggshells

• Coffee grounds

• Other heavy particulate material

Removing grit before biological treatment can reduce wear on pumps and mechanical equipment while lowering downstream solids-handling requirements.

How Grit Chambers Work

Traditional grit chambers slow wastewater velocity enough for heavier inorganic particles to settle.

The settled material is then collected and removed.

Other grit removal systems use vortex technology to circulate wastewater and separate heavier solids from the liquid stream.

Depending on the equipment, grit removal systems may be cleaned manually or automatically.

Benefits of Wastewater Headworks

Headworks provide several advantages for wastewater treatment facilities.

Screening equipment offers options for both small and large POTWs, allowing facilities to remove trash before it reaches sensitive treatment equipment.

Grit removal systems can also:

• Reduce equipment wear

• Protect pumps and mechanical systems

• Lower downstream maintenance requirements

• Reduce inorganic solids accumulation

• Improve overall treatment reliability

Limitations of Wastewater Headworks

Installing new headworks infrastructure can require a substantial capital investment.

Construction, screening equipment, grit removal systems, electrical controls, and maintenance requirements can represent a significant expense for smaller utilities.

Headworks equipment also requires regular cleaning, inspection, and maintenance to operate effectively.

3. Improve Biological Solids Reduction

Most wastewater treatment facilities rely on naturally occurring microorganisms to break down organic material.

However, the existing microbial population may not always have the diversity or abundance needed to efficiently process high organic solids loads.

Bioaugmentation involves introducing selected microbial cultures into the wastewater treatment process to support biological treatment and improve organic solids degradation.

Microorganisms selected from diverse natural environments can provide additional biological capabilities for breaking down materials such as cellulose and other biodegradable organic solids.

Bioaugmentation can be used in both wastewater lagoons and mechanical treatment plants.

Bioaugmentation in Wastewater Lagoons

When microorganisms are added to wastewater lagoons specifically to reduce accumulated organic sludge, the process is often referred to as biological dredging or biodredging.

The added microbial populations help break down organic solids that have accumulated on the lagoon bottom.

Over time, biological treatment may reduce sludge depth and recover valuable treatment capacity.

Microbial products are often introduced at the lagoon influent, lift station, or another upstream location so the microorganisms have sufficient contact time before entering the treatment cells.

Bioaugmentation in Mechanical Wastewater Plants

Mechanical wastewater treatment facilities can also use bioaugmentation to support organic solids reduction and biomass management.

High organic loading may overwhelm the existing microbial community, reducing treatment efficiency.

Adding selected wastewater microorganisms can help support:

• Organic matter degradation

• BOD reduction

• Biomass stability

• Improved sludge settling

• Easier solids handling

Improving Activated Sludge Settling

Wastewater solids include both living and dead microbial biomass.

The type and balance of microorganisms within an activated sludge system can significantly affect how well those solids settle.

Excessive filamentous bacteria can contribute to poor settling, bulking, and elevated total suspended solids (TSS) in plant effluent.

High effluent TSS concentrations can make it more difficult for wastewater facilities to meet NPDES discharge requirements.

Bioaugmentation may help support a more balanced microbial population and improve sludge settling when biological conditions are properly managed.

Microbial cultures are often introduced near the headworks or another upstream treatment location to maximize contact time.

Benefits of Biological Solids Reduction

Lagoon Systems

Biological sludge reduction can be especially useful in lagoons because their long wastewater detention times provide microorganisms with more time to degrade organic material.

Potential benefits include:

• Reduced sludge accumulation

• Increased treatment capacity

• Delayed mechanical dredging

• Lower long-term solids removal costs

Mechanical Treatment Plants

Bioaugmentation in mechanical plants may help:

• Reduce organic loading

• Improve biomass stability

• Support solids settling

• Improve solids recycling

• Reduce treatment disruptions

Limitations of Biological Solids Reduction

Lagoon Systems

Biological solids reduction generally requires consistent treatment and sufficient time for microorganisms to break down accumulated organic material.

Repeated product applications also create an ongoing operating expense.

Mechanical Treatment Plants

Mechanical plants typically have shorter wastewater residence times than lagoon systems.

Because of this, microorganisms have less time to degrade solids before wastewater moves through the treatment process.

Treatment performance therefore depends heavily on factors such as flow, loading, temperature, microbial populations, and retention time.

4. Mechanical Dredging

Mechanical dredging is a common method for removing accumulated sludge from wastewater lagoon systems.

Lagoon treatment naturally allows solids to settle to the bottom of the ponds while treated wastewater remains closer to the surface.

Over time, the settled material can accumulate and reduce available lagoon capacity.

When biological solids reduction is not sufficient, accumulated sludge may need to be physically removed.

Mechanical dredging typically involves three main steps:

1. Removing accumulated sludge from the lagoon

2. Dewatering the sludge

3. Transporting and disposing of the remaining solids

Mechanical dredging can effectively restore lagoon capacity, but the process may require specialized equipment, significant labor, and substantial hauling and disposal costs.

How Mechanical Dredging Removes Lagoon Sludge

A floating dredge or similar mechanical equipment can be used to remove accumulated sludge from the bottom of a wastewater lagoon.

Removing this material helps restore lagoon treatment and holding capacity.

Greater available volume can also increase wastewater detention time, giving biological treatment processes more time to work effectively.

The frequency of mechanical dredging depends on several factors, including:

• Rate of sludge accumulation

• Lagoon depth and available capacity

• Organic and inorganic solids loading

• Treatment performance

• Facility design

• Regulatory and operational requirements

Some lagoon systems may operate for years before dredging is required, while others may need more frequent solids removal.

Dewatering and Disposing of Dredged Sludge

After sludge is removed from a wastewater lagoon, it typically contains a large amount of water.

The dredged material must therefore be dewatered before transportation and final disposal or beneficial use.

Dewatering equipment removes much of the liquid from the sludge, reducing its overall volume and weight.

The dewatered solids can then be loaded into trucks and transported to an approved disposal or processing facility.

Benefits of Mechanical Dredging

Mechanical dredging provides a relatively fast and direct method for restoring lagoon capacity.

Benefits may include:

• Rapid removal of accumulated sludge

• Increased wastewater holding capacity

• Improved detention time

• Restoration of treatment volume

• Removal of years of accumulated solids

Depending on lagoon size and site conditions, mechanical equipment can remove substantial quantities of sludge within a relatively short period.

Limitations of Mechanical Dredging

One of the primary disadvantages of mechanical dredging is cost.

The overall expense may include:

• Dredging equipment

• Labor

• Sludge dewatering

• Transportation

• Disposal fees

• Site preparation and restoration

Dredging, dewatering, hauling, and disposal costs can reach approximately $300 to $500 per dry ton of solids, depending on the project and disposal requirements.

For small municipalities, these expenses can represent a significant portion of the wastewater operating or capital budget.

5. Solids Digestion in Activated Sludge Treatment Plants

Mechanical activated sludge treatment plants are designed to separate and manage biological solids as part of the wastewater treatment process.

Typically, solids are:

1. Settled in a secondary clarifier

2. Returned to the biological process as return activated sludge (RAS), or removed as waste activated sludge (WAS)

3. Sent to a thickening or digestion process

4. Dewatered for disposal or beneficial use

Wasting excess biomass helps control solids concentrations within the activated sludge process and maintain stable biological treatment.

Sludge Thickening and Dewatering

Waste activated sludge may first be sent to a solids thickening process.

Thickening reduces the amount of water associated with the sludge, decreasing the volume that must be treated, transported, or disposed of.

Following thickening or digestion, the remaining solids may be dewatered.

Depending on treatment quality and applicable regulations, the resulting material may be:

• Transported to a landfill

• Further processed

• Used in approved land-application programs

• Beneficially reused as biosolids

Anaerobic Digestion and Biogas Production

Some larger wastewater treatment facilities use anaerobic digestion to stabilize organic solids.

Inside an anaerobic digester, microorganisms break down biodegradable organic material in the absence of oxygen.

This process produces biogas, which typically contains a high percentage of methane.

Depending on the facility, recovered biogas may be used to:

• Heat buildings or digesters

• Generate electricity

• Produce combined heat and power

• Fuel properly equipped vehicles

• Be upgraded to renewable natural gas

Anaerobic digestion can therefore reduce solids volume while also recovering energy from wastewater residuals.

Beneficial Use of Digested Biosolids

After digestion, the remaining stabilized solids may be further processed into biosolids.

When treatment quality and regulations allow, biosolids may be applied to agricultural land as a soil amendment and nutrient source.

Beneficial use can reduce disposal requirements while returning organic matter and nutrients to the soil.

Benefits of Solids Digestion

Activated sludge systems are effective at removing suspended and biodegradable organic material from wastewater.

Solids wasting and digestion help:

• Maintain stable biomass concentrations

• Remove excess biological solids

• Reduce sludge volume

• Stabilize organic material

• Improve solids handling

• Reduce the quantity of material requiring final disposal

When anaerobic digestion is used, facilities may also recover energy from the resulting biogas.

Limitations of Activated Sludge Solids Management

Activated sludge treatment depends heavily on good biomass settling.

Excessive growth of filamentous bacteria can contribute to poor settling, sludge bulking, and solids carryover.

When sludge does not settle properly, operators may have difficulty:

• Returning activated sludge to the biological process

• Wasting excess solids

• Maintaining target biomass concentrations

• Meeting effluent total suspended solids limits

Solids thickening, digestion, dewatering, hauling, and disposal can also create significant operating expenses.

Closing Thoughts: Choosing the Right Solids Management Strategy

There is no single method for managing solids in every wastewater treatment facility.

The most appropriate strategy depends on the type of solids, treatment process, facility design, available equipment, operating budget, and treatment objectives.

Several strategies can be used across many types of wastewater systems:

1. Prevent trash from entering the collection system through screening and community education.

2. Install and maintain effective headworks to remove trash and inorganic grit.

3. Use bioaugmentation to support biological degradation of organic solids.

4. Mechanically dredge lagoon systems when accumulated sludge must be physically removed.

5. Waste, thicken, and digest solids in activated sludge treatment plants.

The Role of Bioaugmentation in Wastewater Solids Reduction

Bioaugmentation can be used to support organic solids degradation in both lagoon systems and mechanical wastewater treatment plants.

In lagoon systems, biological treatment may help reduce accumulated organic sludge and delay the need for mechanical dredging.

In mechanical plants, bioaugmentation may help support:

• Organic solids degradation

• Biomass stability

• Sludge settling

• BOD reduction

• More manageable solids handling

When applied under appropriate treatment conditions, improved biological solids management may also help reduce sludge hauling and disposal requirements.

Effective wastewater solids management often requires a combination of prevention, screening, biological treatment, mechanical removal, and solids processing.

Understanding the strengths and limitations of each option allows wastewater operators to select the most appropriate approach for their facility.