Preliminary Treatment: Screens, Comminutors & Grit Removal

Learn wastewater preliminary treatment, including bar screens, fine screens, comminutors, grinders, grit removal, screenings handling, head loss, flow effects, maintenance, and troubleshooting.

Preliminary treatment is the first major physical treatment stage at many wastewater plants. Its main purpose is to protect pumps, piping, valves, clarifiers, biological processes, and other equipment by removing or reducing materials that should not continue through the treatment train.

Preliminary treatment commonly includes screening, size reduction, and grit removal. These processes do not remove most dissolved organic pollution, but they can prevent equipment damage, reduce clogging, limit abrasive wear, and improve downstream operation.

Purpose of Preliminary Treatment

Wastewater entering a treatment plant can contain:

rags;
plastics;
wipes;
sticks;
trash;
sand;
gravel;
other debris.

If these materials are not removed or controlled, they can interfere with treatment equipment.

Preliminary Treatment Protects the Plant

Good preliminary treatment helps protect:

influent pumps;
valves;
piping;
clarifier mechanisms;
aeration equipment;
sludge-handling equipment.

Screening

Screening removes large suspended and floating materials from wastewater by passing flow through openings of a defined size.

Materials retained by the screen are called screenings.

Coarse Screens

Coarse screens remove larger debris and commonly serve as the first physical barrier in the plant.

They can capture:

rags;
large plastics;
wood;
large wipes;
other bulky debris.

Fine Screens

Fine screens use smaller openings and remove smaller material than coarse screens.

They may reduce downstream:

ragging;
solids accumulation;
equipment fouling;
debris loading.

Manual Screens

Some smaller facilities use manually cleaned screens.

Operators must remove accumulated screenings before excessive blockage develops.

Mechanically Cleaned Screens

Mechanically cleaned screens use equipment to remove retained material automatically or intermittently.

Components may include:

rakes;
chains;
bars;
drive motors;
scrapers;
conveyors.

Screen Head Loss

As debris accumulates on a screen, resistance to flow increases.

This can produce a larger difference in water level across the screen.

Head-Loss Monitoring

Operators may monitor screen condition using:

upstream water level;
downstream water level;
level difference;
automatic differential-level controls.

Increasing Head Loss

Increasing head loss across a screen can indicate:

screen blockage;
high debris loading;
failure of the cleaning mechanism;
unusually high flow.

Why Screen Blockage Matters

A blocked screen can cause:

upstream flooding;
bypass conditions;
reduced plant flow capacity;
increased hydraulic stress;
equipment overload.

Screen Cleaning

Screen cleaning should remove retained debris before hydraulic performance becomes unacceptable.

The appropriate frequency depends on:

influent debris loading;
flow;
screen opening size;
screen type;
automatic control settings.

Screenings Handling

Removed screenings require proper handling.

Possible steps can include:

washing;
compaction;
dewatering;
conveying;
container storage;
disposal.

Why Wash Screenings

Washing can remove some organic material and reduce:

odor;
putrescible matter;
weight;
handling problems.

Compaction

Compaction reduces the volume of screenings and can improve handling and transport.

Screenings Safety

Screenings can contain:

sharp objects;
biological contamination;
needles;
chemicals;
other hazardous debris.

Operators should use appropriate PPE and handling procedures.

Ragging

Ragging occurs when stringy or fibrous material wraps around equipment.

Ragging can affect:

pumps;
mixers;
shafts;
clarifier mechanisms;
sludge equipment.

Wipes and Fibrous Material

Wipes and similar materials can create serious operating problems because they may remain strong and fibrous in wastewater.

Good screening can reduce their impact downstream.

Comminution

Comminution reduces the size of large solids rather than physically removing all of them from the wastewater.

A comminutor cuts or shreds material so it is less likely to clog downstream equipment.

Comminutors

Comminutors may include:

cutting surfaces;
rotating elements;
stationary elements;
drive motors;
protective controls.

Comminution Is Not Screening

The distinction is important:

screening removes debris from the wastewater;
comminution reduces debris size and allows much of it to continue downstream.

Advantages of Comminution

Potential advantages include:

reduced clogging;
smaller solids;
less manual screenings handling in some systems.

Limitations of Comminution

Because solids remain in the wastewater, they can still:

increase downstream solids loading;
accumulate in treatment units;
contribute to maintenance problems.

Grinders

Some plants use grinders to reduce the size of rags and other solids.

Like comminutors, grinders reduce particle size rather than necessarily removing the material from the liquid stream.

Comminutor or Grinder Problems

Possible problems include:

jammed cutting surfaces;
worn cutters;
motor overload;
foreign objects;
electrical failure;
excessive debris loading.

Bypass Channels

Preliminary treatment systems may include bypass channels or alternate flow paths for maintenance or emergencies.

Operators should understand:

when bypass is allowed;
what downstream equipment is affected;
how debris loading changes;
what procedures apply.

Grit

Grit consists mainly of relatively heavy, abrasive inorganic particles.

Examples include:

sand;
gravel;
cinders;
other dense mineral material.

Why Grit Must Be Removed

Grit can cause:

pump wear;
pipe abrasion;
tank accumulation;
reduced basin volume;
clogging;
higher maintenance requirements.

Grit Is Different from Organic Solids

Effective grit removal attempts to remove dense inorganic material while leaving much of the lighter organic matter in the wastewater for downstream treatment.

Grit Chambers

A grit chamber creates hydraulic conditions that allow heavy particles to settle while lighter organic material remains in suspension.

Common Grit-Removal Systems

Grit removal can use:

horizontal-flow grit chambers;
aerated grit chambers;
vortex grit systems;
other specialized equipment.

Horizontal-Flow Grit Chambers

Horizontal-flow systems depend on controlling wastewater velocity so heavy grit settles while lighter organic matter remains suspended.

Velocity Control

If velocity is too high:

grit may not settle;
abrasive material continues downstream.

If velocity is too low:

excess organic material may settle with the grit;
grit can become more odorous and difficult to handle.

Aerated Grit Chambers

Aerated grit chambers use controlled air to create a rolling or spiral flow pattern.

The goal is to separate heavy grit from lighter organic material.

Aeration in Grit Removal

Air rate should be appropriate for the chamber and treatment objective.

Too much agitation can keep grit suspended.

Too little can allow excessive organic solids to settle.

Vortex Grit Removal

Vortex systems use hydraulic or mechanically induced circular flow to separate dense grit from wastewater.

Operators should understand the specific controls and equipment used at their facility.

Grit Collection

Settled grit may be removed using:

pumps;
air lifts;
screws;
scrapers;
other collection equipment.

Grit Washing

Grit washing removes some organic matter from collected grit.

Cleaner grit is generally:

less odorous;
less putrescible;
easier to handle.

Grit Classification

Some systems separate grit from organic material by settling, washing, or classification equipment before disposal.

Organic Content in Removed Grit

If removed grit contains excessive organic matter, investigate:

flow velocity;
air rate;
equipment settings;
grit-removal frequency.

Too Little Grit Removal

Signs can include:

grit accumulating in downstream basins;
pump wear;
abrasion;
reduced tank volume;
frequent cleaning requirements.

Too Much Organic Material in Grit

This may indicate that the grit process is capturing material that should remain in the liquid stream for biological treatment.

Hydraulic Loading

Preliminary treatment performance depends strongly on plant flow.

High flow can affect:

screen head loss;
channel velocity;
grit settling;
equipment capacity.

Wet-Weather Flow

Wet-weather conditions can increase:

plant flow;
grit loading;
debris loading;
hydraulic stress.

Storm Events

During storms, operators may observe:

more sand and grit;
higher screenings volume;
higher head loss;
more frequent equipment cycling.

Channel Velocity

A simplified velocity relationship is:

Velocity = Flow ÷ Cross-Sectional Area

Velocity Example

A channel carries 6 ft³/s through a flow area of 3 ft².

Velocity = 6 ft³/s ÷ 3 ft²

Velocity = 2 ft/s

Whether this velocity is appropriate depends on the specific channel and treatment purpose.

Why Velocity Matters

Changing wastewater depth or flow can change channel velocity.

This affects:

grit settling;
debris movement;
hydraulic performance.

Detention Time

Grit systems also depend on available hydraulic residence time.

A simplified relationship is:

Detention Time = Volume ÷ Flow

Detention-Time Example

A grit chamber holds 18,000 gallons and receives 1.5 MGD.

Detention Time = 18,000 ÷ 1,500,000 day

Detention Time = 0.012 day

Convert to minutes:

0.012 × 1,440 = 17.3 minutes

The theoretical detention time is approximately 17 minutes.

Actual Hydraulic Conditions

Actual flow patterns may differ from theoretical calculations because of:

short-circuiting;
uneven inlet conditions;
deposits;
equipment configuration.

Influent Pump Protection

Depending on plant design, screening can occur before or after influent pumping.

If pumps receive unscreened wastewater, they may require designs capable of handling larger solids.

Preliminary Treatment and Pumping

Poor preliminary treatment can increase:

pump clogging;
impeller fouling;
seal wear;
maintenance frequency.

Odor Control

Screenings and grit containing organic material can produce odors.

Good operation includes:

prompt removal;
washing where provided;
good housekeeping;
covered containers where appropriate.

Housekeeping

Preliminary treatment areas can become difficult work environments if debris is allowed to accumulate.

Operators should control:

spills;
screenings;
grit;
standing wastewater;
slip hazards.

Biological Exposure

Influent wastewater contains microorganisms and contaminated debris.

Operators should follow appropriate:

PPE practices;
handwashing;
decontamination procedures;
site safety rules.

Confined Spaces

Channels, wet wells, pits, and related structures can present confined-space hazards.

Operators should follow the facility's confined-space program and never enter a hazardous space without proper procedures.

Mechanical Hazards

Screening and grit equipment can contain:

chains;
rakes;
rotating shafts;
cutters;
screws;
conveyors.

Lockout/tagout procedures are critical during maintenance.

Routine Screen Inspection

Operators should inspect:

screen condition;
cleaning mechanism;
drive system;
water levels;
screenings conveyor;
odor and housekeeping.

Routine Grit-System Inspection

Operators should review:

flow distribution;
air rate where applicable;
grit pumps;
collection equipment;
removed grit condition;
downstream grit accumulation.

Example: Screen Head Loss Rises Rapidly

Possible causes include:

heavy debris loading;
failed rake mechanism;
high influent flow;
blocked screen openings.

Example: Screenings Volume Suddenly Increases

Review:

storm conditions;
collection-system changes;
industrial or commercial discharges;
flow increase.

Example: Screen Rake Stops

Check:

motor overload;
jammed debris;
chain or drive condition;
electrical controls;
emergency-stop condition.

Example: Grit Appears in Primary Clarifiers

This can indicate inadequate grit removal.

Review:

grit chamber velocity;
air rate;
equipment operation;
high-flow conditions.

Example: Removed Grit Is Very Odorous

This can indicate excessive organic material in the grit.

Review:

velocity;
aeration rate;
washing equipment;
removal frequency.

Example: Grit Pumps Wear Rapidly

Grit is abrasive, but unusually rapid wear can also indicate:

incorrect pump operation;
excessive grit concentration;
poor maintenance;
improper equipment selection or condition.

Example: Downstream Pumps Rag Frequently

Review:

screen opening size;
screen condition;
screen bypass;
cleaning frequency;
comminutor or grinder operation.

Example: Grit Chamber Collects Little Material During High Flow

Possible causes include:

velocity too high;
short detention time;
equipment failure;
poor flow distribution.

Example: Excess Organic Solids Settle in Grit Chamber

Possible causes include:

velocity too low;
insufficient aeration where applicable;
poor hydraulic control.

Example: Preliminary Treatment Bypass Is Used

Operators should anticipate greater downstream loading of:

rags;
debris;
grit.

Monitor pumps and treatment units closely and restore normal preliminary treatment as soon as operating procedures allow.

Process Records

Useful records can include:

influent flow;
screenings volume;
grit volume;
screen head loss;
equipment run time;
motor overloads;
maintenance activities;
wet-weather conditions.

Trend Screenings and Grit

Changes in screenings or grit production can reveal:

seasonal patterns;
storm impacts;
collection-system changes;
equipment deterioration.

Maintenance

Important maintenance items include:

lubrication;
chains;
bearings;
cutters;
motors;
pumps;
conveyors;
air systems.

Preventive Maintenance Matters

Preliminary treatment equipment operates in a severe environment.

Waiting for complete failure can expose downstream processes to large debris and grit loads.

Common Preliminary Treatment Mistakes

Thinking preliminary treatment removes most dissolved wastewater pollution.
Allowing screens to plug before cleaning them.
Confusing screening with comminution.
Ignoring head-loss trends across screens.
Operating grit chambers without considering flow and velocity.
Allowing too much organic material to settle with grit.
Ignoring grit accumulation in downstream basins.
Failing to maintain grinders, rakes, conveyors, and grit pumps.
Ignoring wet-weather changes in debris and grit loading.
Neglecting safety and housekeeping in preliminary treatment areas.

A Practical Screen Review

Review influent flow.
Inspect screen openings.
Review upstream and downstream levels.
Observe the cleaning mechanism.
Inspect screenings handling.
Check drive and electrical equipment.
Review recent head-loss trends.

A Practical Grit-System Review

Review influent flow.
Review chamber hydraulics.
Check air rate where applicable.
Inspect grit-collection equipment.
Observe removed grit.
Check for excessive organic material.
Check downstream tanks for grit accumulation.

A Practical High-Flow Review

Verify plant flow.
Increase attention to screen head loss.
Monitor screen-cleaning frequency.
Review grit-chamber performance.
Check downstream equipment for debris or grit.
Document abnormal loading.

A Practical Equipment-Failure Review

Identify the failed component.
Protect personnel and apply lockout/tagout where required.
Use alternate treatment equipment or bypass procedures if authorized.
Monitor downstream consequences.
Repair the equipment.
Restore normal operation.
Document the failure and response.

What to Remember for the Exam

Preliminary treatment protects downstream equipment and processes from debris and grit.
Screens remove large material from wastewater.
Screenings are the solids retained by a screen.
Fine screens remove smaller debris than coarse screens.
Screen blockage increases head loss and can cause upstream hydraulic problems.
Comminutors and grinders reduce solids size rather than necessarily removing the material.
Screening and comminution are different processes.
Grit consists mainly of dense, abrasive inorganic material such as sand and gravel.
Grit can wear pumps, abrade pipes, and accumulate in basins.
Effective grit removal separates heavy inorganic material while leaving much of the lighter organic matter in suspension.
Horizontal-flow grit removal depends on appropriate velocity.
Excessive velocity can carry grit downstream.
Velocity that is too low can allow excessive organic solids to settle.
Aerated grit chambers use controlled air to help separate grit from organic material.
Wet-weather flow can increase both hydraulic loading and grit loading.
Velocity equals flow divided by cross-sectional area.
Theoretical detention time equals volume divided by flow.
Removed screenings and grit require safe handling and proper disposal.
Lockout/tagout and biological-exposure precautions are important around preliminary treatment equipment.
Good preliminary treatment operation depends on hydraulic control, equipment inspection, debris handling, grit removal, preventive maintenance, and safety.