Treatment of wastewater from printing and finishing in the paper industry

• 1. Wastewater Generation in the Paper Industry
• 2. Characteristics of Effluents Generated in Printing and Finishing Processes
• 3. Wastewater treatment line
• 4. Water Reuse in the Paper Industry
• 5. Conclusions

1. Wastewater Wastewater Generation in the Paper Industry

The effluents generated during the printing and finishing stages of the paper industry have a heterogeneous composition, characterized by the presence of inks, pigments, chemical additives, dissolved organic compounds, and suspended solids.

These discharges exhibit high variability in flow rate and pollutant load. The treatment of these effluents typically combines physicochemical, biological, and filtration processes to ensure compliance with environmental regulations and, in some cases, partial reuse within the same industrial facility.

Effluents are generated during the following stages of the production process:

Ink preparation. This includes operations such as formulation, transfer, and cleaning of inking systems that generate wastewater with high levels of dyes, resins, solvents, and surfactants. The rinsing of tanks, pipes, and equipment contributes significant amounts of organic load and color to the overall effluent.

Printing. This includes discharges from the cleaning of rollers and cylinders, from the purging of recirculation circuits, and from wash water from various production stages, such as flexography, rotogravure, and offset printing. These effluents are characterized by their strong color, high COD, and the presence of organic compounds.

Surface finishing. Coating processes such as stucco application or varnishing generate wastewater containing adhesives, polymers, and mineral fillers (primarily kaolin and calcium carbonate). Cleaning the machinery results in high levels of suspended solids.

Discharges resulting from equipment washing and maintenance tasks—which contain traces of chemicals, fine solids, oils, and greases—must also be taken into account; on the other hand,

Finally, effluents from floor cleaning, leaks in production areas, and system purges must also be taken into account.

2. Characteristics of Effluents Generated in Printing and Finishing Processes

Effluents from printing and finishing processes in the paper industry are characterized by a heterogeneous composition and high temporal variability, associated with changes in ink and coating formulations and cleaning operations.

In general, they contain a combination of organic and inorganic pollutants and suspended solids, which determine the design of the wastewater treatment system.

Here are some of the typical characteristics of these effluents:

• High color intensity.
• High turbidity levels due to the presence of suspended solids such as pigments, mineral fillers, or paper fibers. Colloidal content is typically high, which makes it difficult to separate the solids unless the coagulation–flocculation process is precisely controlled.
• The use of resins, adhesives, solvents, and additives found in inks and coatings results in high levels of organic compounds.
• The biodegradability of the effluent varies depending on the balance between degradable and refractory substances.
• Traces of metals from certain pigments, as well as surfactants and biocidal compounds used in cleaning products, can contribute to chronic toxicity that hinders biological degradation processes.

As a general rule, these effluents are usually suitable for biological treatment after appropriate physicochemical pretreatment.

The main control parameters fall within the following ranges:              

PARAMETER	UNIT	CONCENTRATION
pH	-	5.5 – 9.0
COD	mg/l	1,000 – 5,000
DBO5	mg/l	300 – 1,500
SST	mg/l	200 – 1,500
Conductivity	µS/cm	1,000 – 4,000
Oils and fats	mg/l	20–150
Temperature	ºC	20 - 40

These values vary depending on the type of paper produced and the printing technologies used. Therefore, it is essential to accurately characterize the wastewater in order to properly size the treatment plant.

3. Wastewater treatment line

The treatment of effluents generated during printing and finishing processes in the paper industry typically involves the following stages:

Pre-treatment

Pre-treatment, which is intended to protect subsequent stages and balance the effluent, includes:

• Grates and screens for retaining coarse solids.
• Homogenization tank with pH control

Physicochemical treatment

This is the key stage for removing some of the color and suspended solids. It consists of:

• Coagulation-flocculation using Fe and Al salts and polymers to separate the colloids present in the effluent.
• Clarification is typically achieved using dissolved air flotation (DAF) systems, which separate high-concentration sludge (between 3 and 5% dry matter) by flotation and have a suspended solids removal efficiency of over 90%.
• Sludge management through mechanical dewatering, using either a decanter, a drum dewatering machine, or a filter press to achieve a higher dryness (up to 30–35%).

Biological treatment

After physicochemical treatment, the effluent is sent to a biological treatment process to remove dissolved organic matter:

Membrane bioreactors (MBR) are the most widely used process, as they achieve a high degree of biodegradation through a micro- or ultrafiltration process using membranes. This significantly reduces the concentration of contaminants and colorants in the effluent.

The activated sludge is recirculated to the bioreactor, while the excess sludge is sent to the sludge tank along with the sludge separated during the physicochemical process for dewatering (mechanical drying), stabilization, and subsequent disposal at a landfill.

Tertiary treatment (optional)

When water reuse is required or strict discharge limits must be met, additional treatment steps are incorporated:

• Adsorption using activated carbon.
• Advanced oxidation processes.
• Disinfection.

Diagram of a wastewater treatment plant for paper printing and finishing

4. RReuseWater reuse in the paper industry

Reusing effluents generated during printing and finishing processes in the paper industry reduces water consumption, thereby improving the sustainability of the production process. Recovering treated water requires the use of advanced treatment methods capable of removing solids, colloids, residual color, dissolved salts, and refractory organic compounds.

The most commonly used processes for recovering treated effluent are:

Membrane filtration: Nanofiltration (NF) and reverse osmosis (RO) processes are used to reduce the concentration of dissolved salts, low-molecular-weight organic compounds, and residual color. This results in a high-quality permeate suitable for reuse in industrial processes or as service water.

Filtration using activated carbon beds: Activated carbon, whether in granular or powdered form, removes refractory organic compounds, micropollutants, and traces of color through adsorption, improving the physical and chemical quality of the reclaimed water.

Advanced oxidation: Technologies such as ozonation, UV/peroxide, or the Fenton process can degrade refractory organic compounds and reduce residual color. These processes are used when high-quality treated effluent is required.

Disinfection: It is common to use chemical oxidation or UV radiation systems to ensure that recycled water does not cause microbiological contamination in the process circuits or service systems for which it is intended.

The selection and combination of treatment processes depend on the end use of the reclaimed water, the system’s economic and energy balance, and the established quality requirements. In many cases, the partial reuse of treated water in internal operations—such as cleaning, cooling towers, or pulp preparation—allows for significant reductions in municipal water consumption and discharge volume, thereby impacting supply costs and discharge fees.

5. Conclusions

The complexity and variability of the wastewater generated during printing and finishing processes in the paper industry require the implementation of combined wastewater treatment solutions that ensure compliance with discharge regulations or the reuse of water

From the perspective of wastewater treatment, these streams are typically treated in a common system that includes:

• Homogenization and pH adjustment pretreatments.
• Physicochemical coagulation-flocculation processes.
• Solid-liquid separation by DAF flotation, for the removal of color and solids.
• Biological treatments for reducing dissolved organic load.
• In some cases, tertiary treatment aimed at water reuse or compliance with stricter discharge limits.

Effluent recovery systems used in the printing and finishing processes of the paper industry allow for the reuse of between 40 and 85 percent of the treated water, depending on the treatment method used.

Through a combination of filtration, membrane processes, and advanced treatment methods, water suitable for uses such as cleaning, cooling, or process preparation is obtained. Although they require a higher initial investment, these systems reduce fresh water consumption, discharge costs, and environmental impact, thereby improving the plant’s overall water efficiency.

Proper characterization of effluents, appropriate design of the treatment line, and efficient management of associated waste make it possible to significantly reduce environmental impact, optimize water consumption, and improve the overall efficiency of the facilities.

In this context, wastewater treatment is not only a regulatory requirement but has also established itself as a strategic tool for cleaner, more efficient paper production that is aligned with the principles of the circular economy.

Bibliography and consultations

Metcalf & Eddy. Wastewater Engineering: Treatment and Resource Recovery. McGraw-Hill.

Paper and Pulp Industry – Adiquimica

Effluent treatment in the printing and dyeing industry