Sludge & Biosolids Fundamentals

Learn sludge and biosolids fundamentals, including sludge sources, solids concentration, volatile and fixed solids, thickening, stabilization, dewatering, storage, handling, odors, pathogens, and process monitoring.

Wastewater treatment removes pollutants from water, but many of those pollutants do not disappear. They become concentrated in sludge that must be collected, treated, stored, transported, and ultimately managed. Solids handling is therefore a major part of wastewater treatment rather than a separate disposal problem.

Operators should understand where sludge comes from, how solids concentration affects volume and handling, why stabilization is needed, how thickening and dewatering differ, and how poor solids handling can affect the liquid treatment process.

What Is Sludge?

Sludge is the concentrated mixture of water and solids removed during wastewater treatment.

Sludge can contain:

organic material;
microorganisms;
inorganic solids;
nutrients;
water;
other materials removed from wastewater.

Where Sludge Comes From

Common sludge sources include:

primary clarification;
activated-sludge wasting;
secondary clarification;
chemical treatment processes.

Primary Sludge

Primary sludge is removed from primary clarifiers.

It commonly contains:

settleable organic solids;
inorganic solids;
grease;
other material removed before biological treatment.

Waste Activated Sludge

Waste activated sludge, or WAS, is excess biological solids removed from the activated-sludge process.

WAS contains large amounts of biological material and generally has a relatively low solids concentration before thickening.

Secondary Sludge

Secondary sludge is biological sludge produced during secondary treatment.

In an activated-sludge plant, WAS is the major secondary biological sludge stream.

Chemical Sludge

Chemical treatment can produce additional solids when chemicals react with wastewater constituents.

Examples include solids formed during:

chemical phosphorus removal;
coagulation;
precipitation.

Sludge Is Mostly Water

Many sludge streams contain far more water than dry solids.

This is why a relatively small increase in percent solids can greatly reduce sludge volume.

Percent Solids

Percent solids describes the fraction of a sludge sample that consists of solids rather than water.

A sludge containing 2 percent solids contains approximately:

2 percent solids;
98 percent water by weight.

Total Solids

Total solids represent the material remaining after water is removed from the sample by the laboratory method.

Volatile Solids

Volatile solids represent the portion of total solids lost during the volatile-solids test.

They are commonly used as an estimate of the organic fraction of sludge.

Fixed Solids

Fixed solids are the solids remaining after the volatile fraction is removed during the test.

They represent the more inorganic portion of the sample.

Volatile Solids Percentage

A useful relationship is:

Volatile Solids, % = Volatile Solids ÷ Total Solids × 100

Volatile-Solids Example

A sludge sample contains:

total solids = 10,000 mg/L;
volatile solids = 7,000 mg/L.

Volatile Solids, % = 7,000 ÷ 10,000 × 100

Volatile Solids = 70%

Approximately 70 percent of the measured solids are volatile.

Why Volatile Solids Matter

Volatile solids are important because they relate to:

organic content;
biological activity;
stabilization;
odor potential;
digestion performance.

Solids Mass

Operators often need to know the dry mass of solids in a sludge stream.

A common formula is:

Solids, lb/day = Flow, MGD × Concentration, mg/L × 8.34

Sludge-Solids Example

WAS flow is 0.05 MGD and solids concentration is 8,000 mg/L.

Solids = 0.05 × 8,000 × 8.34

Solids = 3,336 lb/day

The waste stream removes approximately 3,336 pounds of solids per day.

Volume Is Not the Same as Solids Mass

A high sludge flow does not necessarily mean a high dry-solids mass.

Mass depends on both:

sludge flow;
solids concentration.

Why Solids Concentration Matters

Higher solids concentration generally means less water must be:

pumped;
stored;
heated;
dewatered;
transported.

Thickening

Thickening increases sludge solids concentration by removing part of the water.

The main goal is volume reduction.

Thickening Does Not Necessarily Stabilize Sludge

Thickening removes water but does not automatically reduce:

pathogens;
odor potential;
volatile organic content.

Common Thickening Methods

Depending on the plant, thickening may use:

gravity thickening;
dissolved-air flotation;
gravity belt thickening;
centrifuges;
other mechanical equipment.

Stabilization

Stabilization reduces the tendency of sludge to:

putrefy;
produce strong odors;
support excessive biological activity.

Common Stabilization Processes

Stabilization can include:

anaerobic digestion;
aerobic digestion;
lime stabilization;
other approved processes.

Digestion

Digestion reduces biodegradable organic material in sludge.

Depending on the process, digestion can:

reduce volatile solids;
reduce odor potential;
improve handling characteristics;
reduce biological activity.

Anaerobic Digestion

Anaerobic digestion stabilizes sludge without free dissolved oxygen.

Microorganisms convert biodegradable material into products that can include:

methane;
carbon dioxide;
stabilized solids.

Aerobic Digestion

Aerobic digestion uses aerobic microorganisms to reduce biodegradable solids.

It requires:

oxygen;
mixing;
adequate detention time.

Dewatering

Dewatering removes additional water from sludge to produce a higher-solids material that is easier to handle and transport.

Common Dewatering Equipment

Methods can include:

belt filter presses;
centrifuges;
screw presses;
filter presses;
drying beds.

Thickening Versus Dewatering

Both processes remove water, but dewatering generally produces a much higher solids concentration than thickening.

Sludge Cake

Dewatered sludge is often called sludge cake.

Cake should contain enough solids to make:

storage;
loading;
transport;
final management

more practical.

Polymer Conditioning

Many thickening and dewatering processes use polymer to improve solids separation.

Polymer helps particles form larger flocs that release water more effectively.

Too Little Polymer

Insufficient polymer can cause:

poor solids capture;
wet cake;
cloudy filtrate or centrate;
lower throughput.

Too Much Polymer

Excessive polymer can:

waste chemical;
increase operating cost;
interfere with dewatering performance;
create difficult handling conditions.

Solids Capture

Solids capture describes how effectively a thickening or dewatering process retains solids rather than losing them in the liquid return stream.

A simplified calculation is:

Capture, % = Solids Retained ÷ Solids Fed × 100

Why Poor Capture Matters

Solids lost in:

centrate;
filtrate;
pressate;
other return flows

can return to the liquid treatment process.

Recycle Streams

Solids-handling processes can create liquid recycle streams containing:

suspended solids;
ammonia;
phosphorus;
organic material.

Recycle Loads Can Affect the Main Plant

Returning concentrated solids-handling liquids too quickly can increase:

organic load;
nutrient load;
oxygen demand.

Sidestream Loading

A sidestream may have a relatively small flow but a high pollutant concentration.

Operators should evaluate mass loading rather than flow alone.

Storage

Sludge storage provides operational flexibility between treatment and final handling steps.

Storage capacity can help accommodate:

equipment outages;
weekend schedules;
transport interruptions;
variable sludge production.

Storage Can Create Problems

Excessive sludge storage can lead to:

septic conditions;
odor;
gas formation;
poor dewatering.

Mixing During Storage

Some sludge storage tanks require mixing to prevent:

settling;
stratification;
uneven solids concentration.

Odor

Odors can increase when sludge becomes septic or undergoes uncontrolled decomposition.

Odor problems may indicate:

excessive storage time;
poor mixing;
inadequate stabilization;
equipment problems.

Gas Hazards

Sludge handling can produce hazardous gases.

Depending on conditions, concerns can include:

hydrogen sulfide;
methane;
carbon dioxide;
low oxygen atmospheres.

Confined Spaces

Sludge tanks, pits, and related structures can present confined-space hazards.

Operators should follow applicable safety procedures before entry.

Hydrogen Sulfide

Hydrogen sulfide can be associated with septic sludge and anaerobic conditions.

It is hazardous and should not be evaluated by odor alone because smell is not a reliable exposure indicator.

Methane

Methane is combustible and can be generated during anaerobic digestion.

Digester gas systems require appropriate:

ventilation;
monitoring;
ignition control;
maintenance.

Pathogens

Untreated sludge can contain disease-causing organisms.

Solids treatment and final management therefore require procedures that address:

pathogen reduction;
worker hygiene;
safe handling.

Biosolids

Biosolids are sewage sludge that has been treated to meet applicable requirements for a particular beneficial use or disposal pathway.

The terms sludge and biosolids should not automatically be treated as interchangeable in every context.

Beneficial Use

Properly treated biosolids may be managed through approved beneficial-use pathways.

Final management depends on:

treatment level;
quality;
regulatory requirements;
available outlets.

Final Management Options

Depending on treatment and applicable requirements, solids may be managed through:

land application;
composting;
landfill disposal;
thermal processes;
other approved methods.

Solids Handling Affects Liquid Treatment

Poor solids handling can affect the wastewater plant through:

excessive return loads;
insufficient WAS removal;
equipment downtime;
storage limitations.

WAS Removal and Activated Sludge

If a solids-handling process cannot accept enough WAS:

MLSS may rise;
SRT may rise;
clarifier loading may increase;
process control can deteriorate.

Solids Handling Is Part of Process Control

Operators should coordinate:

WAS production;
thickening capacity;
digestion capacity;
dewatering schedule;
storage capacity;
transport schedule.

Sludge Pumping

Sludge is more difficult to pump than water because it can be:

viscous;
nonuniform;
high in solids;
prone to settling.

Sludge-Pump Problems

Possible problems include:

plugging;
loss of prime where applicable;
wear;
low flow;
high discharge pressure.

Pipe and Valve Problems

Sludge piping can experience:

solids buildup;
plugging;
corrosion;
valve obstruction.

Flow Measurement

Accurate sludge-flow measurement is important for:

mass calculations;
WAS control;
chemical dosing;
process monitoring.

Representative Sampling

Sludge concentration can vary significantly if the material is poorly mixed.

Samples should represent the actual stream being evaluated.

Useful Solids-Handling Measurements

Depending on the process, operators may track:

sludge flow;
percent solids;
total solids;
volatile solids;
polymer dose;
cake solids;
solids capture;
storage level;
digester temperature.

Mass Loading Is Often More Useful Than Flow Alone

If sludge concentration doubles while flow remains unchanged, solids mass loading also approximately doubles.

Percent Solids and Volume

For approximately the same dry-solids mass, increasing percent solids reduces the volume that must be handled.

Conceptual Volume Example

Suppose two sludge streams contain approximately the same dry-solids mass.

One is:

2 percent solids.

The other is:

4 percent solids.

The 4-percent-solids stream requires approximately half as much wet sludge volume to carry the same dry-solids mass, assuming similar density.

Why Thickening Saves Capacity

Reducing sludge volume can reduce demand on:

digesters;
storage tanks;
dewatering equipment;
transport vehicles.

Example: Sludge Production Increases

Review:

influent organic loading;
WAS rate;
chemical addition;
primary sludge production;
solids concentration.

Example: Thickened Sludge Becomes Dilute

Possible causes include:

equipment problem;
hydraulic overload;
poor polymer dose;
changing feed sludge;
sampling error.

Example: Dewatered Cake Becomes Wetter

Review:

feed solids concentration;
polymer dose;
equipment settings;
sludge characteristics;
mechanical condition.

Example: Centrate or Filtrate Becomes Cloudy

This can indicate reduced solids capture.

Review:

polymer dose;
feed rate;
equipment settings;
solids characteristics.

Example: Solids Storage Level Keeps Rising

Review:

sludge production;
dewatering schedule;
equipment availability;
transport schedule;
storage capacity.

Example: Strong Odor Develops in Storage

Possible causes include:

long storage time;
septic conditions;
poor mixing;
inadequate stabilization.

Example: Activated-Sludge MLSS Rises During Dewatering Outage

If the plant cannot process or store normal WAS production, operators may be forced to reduce wasting.

This can increase:

MLSS;
SRT;
secondary clarifier solids loading.

Preventive Maintenance

Critical solids-handling equipment may include:

sludge pumps;
mixers;
thickeners;
digesters;
centrifuges;
belt presses;
polymer systems.

Plan for Equipment Outages

Because sludge production continues during wastewater treatment, facilities should understand how they will operate when a solids-handling unit is unavailable.

Operating Records

Useful records include:

sludge volume;
solids concentration;
dry-solids mass;
polymer use;
cake production;
storage levels;
transport quantities;
equipment downtime.

Common Sludge and Biosolids Mistakes

Thinking that wastewater solids disappear after liquid treatment.
Using sludge volume without considering solids concentration.
Confusing thickening with stabilization.
Confusing thickening with dewatering.
Ignoring recycle loads from solids handling.
Ignoring the effect of dewatering outages on WAS control.
Using polymer dose without evaluating solids capture and cake quality.
Allowing excessive sludge storage time.
Ignoring gas and confined-space hazards.
Using the terms sludge and biosolids as if they always mean exactly the same thing.

A Practical Solids-Handling Review

Identify each sludge source.
Review sludge flow and solids concentration.
Calculate or review dry-solids mass.
Review thickening performance.
Review stabilization performance.
Review dewatering performance.
Review storage capacity and levels.
Review recycle-stream quality.
Review transport and final-management schedule.
Document abnormal conditions and equipment outages.

A Practical Rising-Sludge-Volume Review

Verify sludge-flow measurements.
Review percent solids.
Review dry-solids production.
Check thickening performance.
Review WAS and primary sludge production.
Check for dilution water entering the process.
Review downstream capacity.
Correct the cause rather than responding only to volume.

A Practical Dewatering Review

Review feed flow.
Review feed solids concentration.
Review polymer dose.
Review equipment settings.
Measure cake solids.
Review centrate or filtrate quality.
Evaluate solids capture.
Compare performance with normal operating trends.

A Practical Solids-Mass Review

Measure sludge flow.
Measure solids concentration.
Calculate pounds of solids per day.
Compare current mass with historical production.
Review process changes that may have changed solids production.
Use mass data to evaluate thickening, digestion, dewatering, and transport capacity.

What to Remember for the Exam

Sludge contains the solids removed or produced during wastewater treatment.
Primary sludge comes from primary clarification, while WAS is excess biological sludge from activated-sludge treatment.
Sludge streams commonly contain much more water than dry solids.
Total solids include volatile and fixed solids.
Volatile solids are commonly used as an estimate of the organic portion of sludge.
Sludge solids mass in lb/day can be calculated as MGD × mg/L × 8.34.
Sludge volume and dry-solids mass are different measurements.
Thickening increases solids concentration primarily by removing water and reducing sludge volume.
Stabilization reduces putrescibility, odor potential, and biological activity.
Dewatering removes additional water and produces a higher-solids material such as sludge cake.
Thickening and dewatering are not the same as stabilization.
Polymer can improve solids separation during thickening and dewatering.
Poor solids capture sends additional solids back to the liquid treatment process through recycle streams.
Solids-handling recycle streams can carry high concentrations of ammonia, phosphorus, organic matter, and suspended solids.
Long sludge storage can create septic conditions, odors, and gas production.
Sludge handling can involve hydrogen sulfide, methane, low-oxygen atmospheres, and confined-space hazards.
Biosolids are treated sewage sludge intended to meet applicable requirements for a particular use or management pathway.
Solids-handling capacity directly affects activated-sludge wasting and process control.
Mass loading is usually more informative than sludge flow alone.
Good solids management coordinates sludge production, thickening, stabilization, dewatering, storage, transport, safety, and final management.