Coating factories generate various types of water.

Water used in pretreatment.
Water used in degreasing and cleaning.
Water discharged from rinsing processes.
Wet booth circulation water.
Rinse water from electrodeposition coating.
Water used to wash floors and fixtures.
Mixed wastewater entering the wastewater treatment system.

These are often all viewed as the same “wastewater.”
In reality, however, their properties differ greatly.

Water with a high oil content.
Water containing paint components.
Water containing chemicals.
Water containing metals.
High-pH water.
Low-pH water.
Water with abundant sludge.
Water with abundant organic matter.

Wastewater treatment will not be stable if it is planned without checking these differences.

Before introducing wastewater treatment equipment, the first requirement is water quality analysis.

Addressing coating wastewater begins with knowing which processes generate what types of water, and in what quantities.

1. Coating Wastewater Properties Differ by Source

There is more than 1 type of wastewater in a coating factory.

Even within the same factory, water composition varies greatly between processes.

For example, pretreatment processes may involve degreasers, acids, alkalis, phosphates, and metals.
Wet booths involve paint mist, resins, pigments, solvents, booth treatment chemicals, and sludge.
Floor wash water may contain paint residue, oil, dust, and cleaning chemicals.

Looking only at combined wastewater therefore makes causes difficult to identify.

When considering wastewater treatment, start by examining sources separately.

• Pretreatment wastewater
• Rinse wastewater
• Wet booth water
• Electrodeposition rinse water
• Cleaning wastewater
• Floor wash water
• Whether domestic wastewater is mixed in
• Whether rainwater enters the wastewater

Understanding wastewater properties requires checking wastewater from individual processes, as well as wastewater from the factory as a whole.

2. Choosing a Treatment Method Without Water Quality Analysis Is Risky

When selecting wastewater treatment equipment, people sometimes begin with the treatment method.

Would coagulation and sedimentation be suitable?
Would flotation separation be suitable?
Is filtration needed?
Is biological treatment needed?
Which chemicals should be used?
How much treatment capacity is required?

These questions are, of course, important.

However, something else needs to be checked first.

It is the nature of the water you want to treat.

Without knowing the water’s properties, an appropriate treatment method cannot be selected.

For example, water with a high oil content and water with abundant paint sludge require different treatment.
Water with a pH far from neutral and water close to neutral also require different chemicals.
The approach to treatment changes between water rich in organic matter and water rich in inorganic matter as well.

Designing wastewater treatment without water quality analysis makes treatment prone to instability.

3. Water Quality Parameters to Check First

Considering wastewater does not need to be overly complicated.
Start by checking the basic parameters.

Typical parameters to check are as follows.

pH

A numerical value indicating whether water is acidic or alkaline.
A pH far from the appropriate range affects chemical reactions and compliance with wastewater standards.

SS

The amount of fine solids suspended in water.
Paint residue, sludge, and dust contribute to this parameter.

COD

One indicator of the amount of organic matter in water.
It is relevant when water contains large amounts of paint components, solvents, chemicals, or organic contamination.

BOD

An indicator of the amount of organic matter that microorganisms can break down.
It is important when considering biological treatment.

Oil

Oil may enter from degreasing processes, machinery oil, or wash water.
It makes coating wastewater treatment more difficult.

Metals

Iron, zinc, aluminum, nickel, and other metals may be involved, depending on the pretreatment and workpiece material.

Color, turbidity, and odors

Appearance and smell are important information, as well as numerical values.
They provide readily recognizable warning signs on the shop floor.

Even checking these basic parameters makes wastewater properties easier to understand.

4. pH Is a Basic Condition for Wastewater Treatment

pH is one of the most fundamental parameters in wastewater treatment.

An excessively high or low pH can prevent compliance with wastewater standards and also affect how treatment chemicals work.

For example, when coagulants are used to aggregate contaminants, unsuitable pH conditions prevent effective aggregation.
The pH conditions are also important when precipitating metals.

Factories with unstable pH are also prone to unstable wastewater treatment.

Check the following points in wastewater treatment.

• Which processes generate acidic water
• Which processes generate alkaline water
• Whether pH changes at different times
• Whether pH changes abruptly when chemicals are added
• Whether neutralization equipment is needed
• Whether pH measurement records are available

pH is fundamental to understanding wastewater conditions.

5. SS Relates to Sludge and Paint Residue Management

SS means solids suspended in water.
In coating factories, these include paint residue, pigments, sludge, dust, and precipitates from pretreatment.

Wastewater with high SS requires treatment such as sedimentation, filtration, or coagulation.

In wet booths, paint mist enters the water and becomes sludge.
Promptly separating and collecting this sludge may reduce SS.

Conversely, leaving sludge unattended can allow it to disperse into fine particles in the water, making treatment difficult.

Check the following points in SS management.

• Which processes discharge solids
• Whether sludge floats or settles
• Whether solids are finely dispersed
• Whether filtration can remove them
• Whether coagulants can aggregate them
• Whether sludge quantities are recorded

SS relates to booth management, cleaning, and paint waste, as well as wastewater treatment.

6. COD and BOD Are Indicators of Organic Contamination

Coating wastewater may contain organic matter.

Paint resins.
Solvents.
Cleaning agents.
Degreasers.
Booth treatment chemicals.
Oil.
Other organic contamination.

Large amounts of this organic matter can make wastewater treatment difficult.

COD and BOD are typical indicators used to assess organic contamination in water.

COD indicates the amount of substances that can be chemically oxidized.
BOD indicates the amount of organic matter that microorganisms can break down.

Although these are technical measurements, the following practical interpretation makes them easier to understand.

The more organic contamination water contains, the greater the treatment load.
The greater the treatment load, the more important treatment equipment, chemicals, and management become.
Identifying processes with high organic contamination allows action at the source.

Wastewater treatment requires checking which processes increase COD or BOD, as well as looking at the numerical values.

7. Oil Makes Wastewater Treatment Difficult

Oil can enter wastewater in coating factories.

Degreasing processes.
Machining oil on substrates.
Machinery oil.
Floor wash water.
Maintenance work.
Pretreatment processes.

Oil entering wastewater can make treatment difficult.
It may float on the surface or disperse into fine droplets and form an emulsion.

Emulsification is a condition in which oil disperses finely in water and becomes difficult to separate easily.

A high oil content causes the following problems.

• Coagulation treatment becomes unstable
• Sludge increases
• Compliance with wastewater standards is affected
• Odors develop
• More treatment chemicals are required
• Wastewater treatment equipment becomes dirty

Oil control requires measures to prevent oil carryover in preceding processes, as well as wastewater treatment.

Managing oil on substrates before the coating process also helps control wastewater.

8. Metals Relate to Pretreatment and Substrates

Wastewater from coating factories may contain metals.

Iron.
Zinc.
Aluminum.
Nickel.
Chromium.
Copper.

These relate to substrates, pretreatment chemicals, plating, degreasing, and surface treatment processes.

Metals may be subject to wastewater standards.
Treatment methods also need to be selected carefully.

For example, precipitating metals may require pH adjustment and chemical treatment.
However, the presence of other components can make treatment unstable.

Check the following points when assessing metals.

• Which substrates are handled
• What the pretreatment chemicals contain
• Whether plated products are handled
• Whether metal powder or grinding dust enters the wastewater
• Which components are subject to wastewater standards
• Whether analysis results are checked regularly

Metals can be difficult to detect by appearance.
That is why water quality analysis is necessary.

9. Wastewater Volume and Discharge Timing Also Matter

Wastewater volume matters as well as water quality.

Even at the same concentration, a larger wastewater volume creates a greater treatment load.
Large discharges over a short period may also exceed the treatment system’s ability to keep up.

Check the following items.

• Daily wastewater volume
• Maximum hourly wastewater volume
• Wastewater volume by process
• Times when wastewater is discharged
• Whether discharge is batch or continuous
• Whether large quantities are discharged only during cleaning
• Whether rainwater or domestic wastewater is mixed in

Wastewater treatment equipment designed only for average volume may become unstable.
Peak wastewater volume also needs to be checked.

In coating factories, wastewater volume may increase temporarily during color changes, washing, periodic cleaning, or booth water replacement.

Checking volume and timing is important in wastewater treatment.

10. Examine Each Source Before Mixing Wastewater

Wastewater may eventually collect in a single treatment system.

However, examining all wastewater as a mixture from the outset makes causes difficult to identify.

For example, suppose a wastewater limit is exceeded.
If all wastewater has already been mixed, it is difficult to identify which process caused it.

Checking individual sources is therefore important.

Pretreatment wastewater.
Booth water.
Wash water.
Floor wash water.
Electrodeposition rinse water.
Wastewater from cleaning.

Analyzing each water stream separately reveals which processes increase the treatment load.

In some cases, action at the source is less expensive and more stable.

For example, collect sludge promptly inside the booth.
Reduce oil before pretreatment.
Manage highly concentrated cleaning wastewater separately.
Separate and neutralize water streams with very different pH levels.

Wastewater control should consider reducing contaminants at the source before mixing and treating them.

11. Select Wastewater Treatment Chemicals to Match Water Quality

Wastewater treatment may use chemicals such as coagulants, neutralizing agents, defoamers, and treatment aids.

However, these chemicals need to be selected to match water quality.

Using chemicals unsuitable for the water quality causes the following problems.

• Contaminants do not aggregate effectively
• Excessive sludge is generated
• Treated water is turbid
• pH adjustment becomes unstable
• Chemical consumption increases
• Treatment costs rise
• Wastewater standards cannot be met

Testing with actual wastewater is important when selecting chemicals.

Coating wastewater quality changes particularly when paints or chemicals change.
A chemical selected once therefore does not necessarily remain optimal indefinitely.

When paint, pretreatment chemicals, or products change, their effects on wastewater treatment also need to be checked.

12. Common Wastewater Management Challenges in Vietnamese Factories

Vietnamese factories are prone to the following coating wastewater challenges.

Water quality before entering the treatment system is not understood.
Wastewater volumes by process are not recorded.
Booth water and pretreatment wastewater are considered together.
pH is measured irregularly.
Sludge quantities are not recorded.
Treatment chemical dosage is left to shop-floor judgment.
Action is taken only after wastewater standards are not met.
Wastewater effects are not checked when paints change.
Management of cleaning and wash wastewater is weak.
Information sharing between wastewater treatment personnel and coating operations is insufficient.

Wastewater treatment is not stable under these conditions.

Wastewater management is not solely the environmental staff’s job.
Coating operations also need to understand the types of water they discharge.

13. Basic Items to Check in Coating Wastewater

The following checks provide a practical approach to managing coating wastewater.

1. Sources

Identify which processes discharge wastewater.

2. Wastewater volume

Check daily, hourly, and peak volumes.

3. pH

Check whether the water is acidic or alkaline.

4. SS

Check solids, sludge, and paint residue in the water.

5. COD and BOD

Check the amount of organic contamination.

6. Oil

Check the effects of degreasing, machining oil, and floor wash water.

7. Metals

Check metals originating from substrates and pretreatment.

8. Color, odors, and turbidity

Check warning signs that are recognizable on the shop floor.

9. Chemical consumption

Check the amounts of coagulants, neutralizing agents, defoamers, and other chemicals used.

10. Sludge quantity

Check the amounts generated, collected, and disposed of.

14. The Value of Wastewater Analysis According to CoatureVietnam

CoatureVietnam does not regard wastewater treatment as merely an equipment issue at the end of the factory process.

Wastewater is the result of the coating process.

Which paint was used?
Which pretreatment was performed?
How much overspray occurred?
How was booth water managed?
How was washing performed?
How was sludge collected?
How were floors cleaned?

These factors ultimately appear in the wastewater.

Wastewater analysis therefore addresses environmental concerns and also makes the coating process visible.

CoatureVietnam examines wastewater from the following perspectives.

• Which processes increase the treatment load
• Whether contamination can be reduced at the source
• Whether sludge management can improve conditions
• Whether changes in paint or chemicals have an effect
• Whether the wastewater treatment system is excessively burdened
• Whether operators are performing overly demanding cleaning or chemical dosing work

This puts CoatureVietnam’s valued principle, “Coating the Future with Science and Care”, into practice.

Science means measuring water quality, separating sources, and making the relationship between processes and wastewater visible.
Care means creating a system that can be managed consistently so that wastewater problems and environmental risks do not leave the shop floor under constant pressure.

Summary

Coating wastewater measures should not begin with selecting treatment equipment.

The first requirement is water quality analysis.

Which processes discharge water?
What quantities are discharged?
What is the pH?
Is SS high?
Are COD or BOD high?
Is oil present?
Are metals present?
How much sludge is generated?
Are there odors or turbidity?

Only after checking these points can appropriate treatment methods be identified.

Wastewater does not suddenly appear at the end of a factory.
It results from the coating process, pretreatment, booths, washing, cleaning, and chemical management.

That is why coating wastewater should first be considered through water quality analysis.
Then improve conditions at the source, as well as in the treatment equipment.

CoatureVietnam regards wastewater as an important indicator for coating process improvement, as well as an environmental issue, and proposes improvements suited to Vietnamese factories.