Primary Treatment & Primary Clarification

Primary treatment removes settleable and floatable solids from wastewater before biological treatment. In many plants, the main primary-treatment unit is the primary clarifier, also called a primary sedimentation tank.

Primary clarification reduces the solids and organic load that downstream biological processes must handle. Good primary treatment can improve overall plant stability, while poor clarification can increase biological loading, solids accumulation, and downstream treatment demand.

Purpose of Primary Treatment

Primary treatment is designed mainly to remove material that can be separated physically by settling or flotation.

Primary treatment can remove:

• settleable suspended solids;
• floating material;
• oil and grease associated with floatable matter;
• a portion of the influent organic load.

Primary Clarification

Primary clarification provides relatively calm hydraulic conditions that allow heavier solids to settle while lighter floatable material rises to the surface.

The primary clarifier therefore has two major solids-removal functions:

• collect settled sludge from the bottom;
• remove floating scum from the surface.

Primary Influent

Wastewater entering a primary clarifier has usually passed through preliminary treatment such as screening and grit removal.

Primary influent can still contain:

• fine suspended solids;
• settleable organic solids;
• grease;
• floatable material;
• dissolved organic matter.

Primary Effluent

Water leaving the primary clarifier is called primary effluent.

Primary effluent normally still contains:

• dissolved organic matter;
• fine suspended solids;
• nutrients;
• microorganisms.

Primary treatment is therefore not complete wastewater treatment. Biological and other downstream processes are normally required.

Primary Sludge

Solids collected at the bottom of the primary clarifier are commonly called primary sludge.

Primary sludge can contain:

• settleable organic solids;
• inorganic solids;
• water;
• other accumulated material.

Scum

Scum is floating material collected from the clarifier surface.

Scum can include:

• grease;
• oil;
• floating solids;
• other low-density material.

Clarifier Zones

A primary clarifier generally includes functional areas for:

• influent distribution;
• settling;
• sludge collection;
• scum removal;
• effluent withdrawal.

Influent Distribution

Wastewater entering a clarifier should be distributed as evenly as practical.

Poor inlet conditions can create:

• short-circuiting;
• turbulence;
• uneven solids loading;
• reduced settling performance.

Short-Circuiting

Short-circuiting occurs when some wastewater travels through the tank much faster than intended.

This reduces effective settling time and can increase suspended solids in the primary effluent.

Dead Zones

Areas with little effective flow can allow:

• solids accumulation;
• septic conditions;
• odor formation;
• loss of useful basin volume.

Settling

Settling depends on the difference between particle density and water density, along with particle size, shape, and hydraulic conditions.

Larger and denser particles generally settle more readily than very small or low-density particles.

Flocculent Settling

Some wastewater particles can combine while settling.

As particles join together, they may form larger aggregates and settle more rapidly.

Detention Time

A simplified theoretical detention-time relationship is:

Detention Time = Basin Volume ÷ Flow

Detention-Time Example

A primary clarifier has an operating volume of 300,000 gallons and receives 2.0 MGD.

Detention Time = 300,000 ÷ 2,000,000 day

Detention Time = 0.15 day

Convert to hours:

0.15 × 24 = 3.6 hours

The theoretical detention time is approximately 3.6 hours.

Theoretical Versus Actual Detention Time

Theoretical detention time assumes the full basin volume is used evenly.

Actual hydraulic behavior can differ because of:

• short-circuiting;
• dead zones;
• sludge accumulation;
• poor inlet distribution;
• uneven effluent withdrawal.

Surface Overflow Rate

Another important clarifier relationship is:

Surface Overflow Rate = Flow ÷ Surface Area

This expresses hydraulic loading on the clarifier surface.

Surface Overflow Example

A clarifier receives 1,800,000 gallons per day and has a surface area of 3,600 ft².

Surface Overflow Rate = 1,800,000 ÷ 3,600

Surface Overflow Rate = 500 gpd/ft²

The significance of a specific value depends on the clarifier design and operating requirements.

Higher Flow Increases Hydraulic Loading

If clarifier surface area remains constant while flow increases, surface overflow rate increases.

Higher hydraulic loading can reduce settling performance and increase solids carryover.

Weir Loading

Clarified wastewater commonly leaves over outlet weirs.

A simplified relationship is:

Weir Loading = Flow ÷ Effective Weir Length

Uneven or excessive withdrawal near portions of the weir can affect clarifier hydraulics.

Effluent Weirs

Operators should inspect weirs for:

• uneven flow;
• blockage;
• debris;
• poor leveling;
• algae or solids buildup.

Sludge Collection

Settled solids must be moved toward sludge hoppers or withdrawal points.

Clarifiers may use:

• scrapers;
• flights;
• chains;
• rotating collectors;
• other mechanical systems.

Sludge Withdrawal

Primary sludge should be removed often enough to prevent excessive accumulation.

If sludge remains in the clarifier too long, it can:

• become septic;
• produce gas;
• create odors;
• float;
• increase solids carryover.

Excessive Sludge Accumulation

Too much sludge can reduce effective clarifier volume and interfere with normal operation.

Operators should monitor sludge accumulation and adjust withdrawal according to plant conditions.

Excessive Sludge Withdrawal

Removing sludge more frequently than needed can:

• waste water;
• increase pumping;
• increase downstream solids-handling volume.

The goal is appropriate withdrawal, not maximum withdrawal.

Sludge Blanket

A sludge blanket is the accumulated layer of settled solids in the lower portion of the clarifier.

Monitoring blanket depth can help operators identify:

• excessive solids buildup;
• collector problems;
• sludge-pumping problems;
• changes in solids loading.

Scum Removal

Scum should be removed from the clarifier surface and directed to the appropriate solids or residuals process.

Poor scum removal can result in:

• surface accumulation;
• odor;
• scum carryover;
• equipment fouling.

Floating Sludge

Settled sludge can sometimes rise to the surface because gas forms within the sludge layer.

Possible causes include:

• excessive sludge detention;
• septic conditions;
• biological gas formation.

Odors

Strong odors around primary clarifiers can indicate:

• septic influent;
• long sludge storage;
• stagnant areas;
• poor scum handling.

Primary Treatment and Organic Loading

Primary clarification removes a portion of the organic solids before biological treatment.

If primary performance deteriorates, the biological process may receive a higher organic load.

Mass Loading

A common wastewater operator relationship is:

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

BOD Loading Example

A plant receives 3.0 MGD with primary influent BOD of 220 mg/L.

BOD Loading = 3.0 × 220 × 8.34

BOD Loading = 5,504.4 lb/day

Primary Effluent Loading

If primary effluent BOD is 160 mg/L at the same 3.0 MGD flow:

Primary Effluent BOD Load = 3.0 × 160 × 8.34

Primary Effluent BOD Load = 4,003.2 lb/day

The approximate BOD mass removed in primary treatment is:

5,504.4 - 4,003.2 = 1,501.2 lb/day

Removal Efficiency

A simplified concentration-based removal calculation is:

Removal Efficiency, % = (Influent - Effluent) ÷ Influent × 100

TSS Removal Example

Primary influent TSS is 240 mg/L and primary effluent TSS is 120 mg/L.

Removal Efficiency = (240 - 120) ÷ 240 × 100

Removal Efficiency = 50%

Use Concentration and Mass Together

Percent removal is useful, but operators should also consider flow and mass loading.

A high flow can create a large downstream solids load even when percent removal remains similar.

Primary Effluent TSS

Primary effluent suspended solids are an important indicator of clarifier performance.

A rising trend can indicate:

• high hydraulic loading;
• sludge accumulation;
• poor solids settling;
• mechanical problems;
• short-circuiting.

Primary Effluent BOD

Primary effluent BOD helps indicate how much organic load is being transferred to biological treatment.

An increase can result from:

• higher influent strength;
• poorer primary settling;
• higher flow;
• industrial or commercial discharges.

Wet-Weather Flow

Wet weather can increase influent flow through infiltration and inflow.

Higher flow can:

• reduce theoretical detention time;
• increase surface overflow rate;
• increase solids carryover;
• change influent concentration.

Hydraulic Surge

Sudden flow increases can disturb clarifier operation.

Possible effects include:

• solids washout;
• uneven weir flow;
• higher primary effluent TSS;
• higher downstream biological loading.

Flow Equalization

Some systems use flow equalization to reduce rapid variations in flow or loading.

More stable flow can support more stable downstream clarification and biological treatment.

Temperature

Temperature can affect:

• wastewater viscosity;
• settling behavior;
• biological activity in accumulated sludge;
• odor generation.

Septic Wastewater

Wastewater that has remained anaerobic for a long period can arrive at the plant in a septic condition.

Possible signs include:

• strong odors;
• dark color;
• gas formation;
• corrosion concerns.

Primary Clarifiers and Downstream Biology

Poor primary clarification increases the load on downstream biological processes.

This can increase:

• oxygen demand;
• solids production;
• aeration requirements;
• secondary clarifier loading.

Too Much Primary Removal Can Also Affect Downstream Processes

Some downstream biological nutrient-removal processes rely on available organic carbon.

Operators should understand the intended process design rather than assume maximum primary solids removal is always the only objective.

Mechanical Equipment

Primary clarifier equipment can include:

• drives;
• scrapers;
• chains;
• flights;
• skimmers;
• sludge pumps;
• scum pumps.

Mechanical Failure

A failed collector can allow sludge to accumulate even when influent conditions are normal.

Mechanical problems can therefore become treatment problems.

Routine Clarifier Inspection

Operators should routinely observe:

• surface condition;
• scum accumulation;
• weir flow;
• collector operation;
• sludge pumping;
• odors;
• effluent appearance.

Example: Primary Effluent TSS Rises Suddenly

Review:

• influent flow;
• influent solids;
• sludge blanket depth;
• collector operation;
• sludge withdrawal;
• weir condition.

Example: Sludge Blanket Rises

Possible causes include:

• insufficient sludge withdrawal;
• sludge-pump failure;
• higher solids loading;
• collector malfunction.

Example: Sludge Floats to the Surface

Possible causes include:

• gas formation;
• septic sludge;
• excessive sludge detention.

Review sludge removal and clarifier operating conditions.

Example: Scum Carries Over the Effluent Weir

Review:

• scum skimmer operation;
• scum beach or collection equipment;
• surface hydraulics;
• grease loading.

Example: One Clarifier Performs Worse Than Another

Compare:

• flow split;
• sludge withdrawal;
• collector operation;
• weir leveling;
• inlet distribution.

Example: Both Clarifiers Deteriorate During Rain

A common hydraulic cause is more likely.

Review:

• influent flow;
• infiltration and inflow;
• surface overflow rate;
• downstream biological response.

Example: Strong Odor Develops

Review:

• influent condition;
• sludge age in the clarifier;
• sludge withdrawal frequency;
• scum accumulation;
• stagnant zones.

Example: Biological Process Loading Increases

If influent strength is unchanged, investigate whether primary clarification is removing less material than normal.

Compare:

• primary influent BOD and TSS;
• primary effluent BOD and TSS;
• flow;
• clarifier condition.

Process Records

Useful primary-treatment records can include:

• influent flow;
• primary influent TSS;
• primary effluent TSS;
• primary influent BOD;
• primary effluent BOD;
• sludge withdrawal;
• sludge blanket depth;
• scum removal;
• equipment condition.

Trend Clarifier Performance

Useful trends include:

• flow versus effluent TSS;
• sludge blanket depth versus solids carryover;
• sludge withdrawal versus blanket depth;
• primary removal versus biological loading.

Primary Clarifier Safety

Hazards can include:

• open tanks;
• slippery surfaces;
• moving mechanical equipment;
• biological exposure;
• confined spaces;
• hazardous gases.

Lockout/Tagout

Collector drives, skimmers, pumps, and other moving equipment should be isolated according to facility lockout/tagout procedures before maintenance.

Confined-Space Hazards

Clarifier structures, sludge pits, and associated chambers may present confined-space hazards.

Operators should follow the facility's confined-space program.

Common Primary Clarification Mistakes

• Assuming primary treatment removes dissolved pollution completely.
• Ignoring flow when clarifier performance changes.
• Allowing excessive sludge accumulation.
• Withdrawing sludge without considering actual blanket conditions.
• Ignoring uneven effluent weir flow.
• Using theoretical detention time as if actual hydraulics were perfect.
• Ignoring scum accumulation.
• Trying to correct mechanical problems with process adjustments.
• Evaluating removal efficiency without considering mass loading.
• Failing to connect primary-treatment performance with downstream biological loading.

A Practical Primary Clarifier Review

1. Review influent flow.
2. Review influent BOD and TSS.
3. Observe inlet hydraulics.
4. Inspect clarifier surface conditions.
5. Check sludge blanket depth.
6. Verify sludge withdrawal.
7. Check scum removal.
8. Inspect effluent weirs.
9. Review primary effluent BOD and TSS.
10. Compare downstream biological loading.

A Practical High-TSS Review

1. Verify the laboratory result.
2. Review current and recent flow.
3. Check sludge blanket depth.
4. Check collector operation.
5. Review sludge withdrawal.
6. Inspect weir flow.
7. Check for hydraulic short-circuiting.
8. Review influent solids loading.

A Practical Sludge-Handling Review

1. Inspect sludge collection equipment.
2. Review blanket depth.
3. Verify sludge pump operation.
4. Review withdrawal frequency.
5. Inspect sludge piping.
6. Check downstream solids-handling capacity.

A Practical Wet-Weather Review

1. Verify influent flow.
2. Review surface overflow rate.
3. Monitor primary effluent TSS.
4. Watch for solids carryover.
5. Review sludge blanket depth.
6. Monitor downstream biological loading.

What to Remember for the Exam

• Primary treatment removes settleable and floatable solids before biological treatment.
• Primary clarifiers separate settled sludge from floating scum.
• Primary effluent still contains dissolved organic matter, fine solids, nutrients, and microorganisms.
• Primary sludge is collected from the bottom of the clarifier.
• Scum is removed from the clarifier surface.
• Poor inlet hydraulics can cause turbulence and short-circuiting.
• Theoretical detention time equals basin volume divided by flow.
• Surface overflow rate equals flow divided by clarifier surface area.
• Higher flow increases hydraulic loading when clarifier area remains constant.
• Excessive sludge accumulation can reduce effective basin volume and cause odors or solids carryover.
• Sludge that remains too long can become septic and produce gas.
• Primary clarification reduces the solids and organic load reaching biological treatment.
• Mass loading in lb/day can be calculated as MGD × mg/L × 8.34.
• Removal efficiency equals influent concentration minus effluent concentration, divided by influent concentration, times 100.
• Percent removal should be evaluated together with flow and mass loading.
• Wet-weather flow can reduce detention time and increase solids carryover.
• Mechanical collector failure can become a treatment problem.
• Effluent weirs should be kept clean and hydraulically balanced.
• Operators should trend primary influent, primary effluent, flow, sludge withdrawal, and blanket depth together.
• Good primary clarification depends on stable hydraulics, effective sludge and scum removal, reliable equipment, monitoring, and connection to downstream process loading.