Water is one of the most important concerns of various industries. Therefore, its supply, purification, and recovery are essential. The paper and cardboard industry has always been of interest due to its high water consumption in the production process on one hand and the pollution of the wastewater produced on the other hand.

Characteristics of wastewater from the paper industry:

Toxicity

High COD of produced wastewater

Limited quantity and quality of chemical pollutants (approximately 250 chemical substances including resins, heavy metals, cyanides, phenols, etc.)

It is worth mentioning that among the various stages of paper production, including pulping, washing the pulp, screening, bleaching, and paper making, the most significant sources of pollution occur during the pulping stage, which includes chloride and organic compounds.

The most suitable wastewater treatment system for the paper and cardboard industry is selected after a thorough review of environmental regulations and identification of various pollutants. In fact, all wastewater treatment methods are designed to reduce pollution load, but what matters is choosing the best and most suitable method.

In the biological treatment of wastewater from the paper and cardboard industry, the most important stage of treatment is the neutralization process. This is because high pH affects bacterial growth. After that, by performing steps such as primary sedimentation, suspended and solid materials in the wastewater are removed, and by passing the wastewater through various stages, its turbidity and color are reduced.

Generally, biological treatment, chemical oxidations, coagulation, ion exchange processes, and surface adsorption processes are among the most important stages utilized in the treatment of wastewater from the paper industry.

The most important parameters of wastewater in the treatment of wastewater from the paper industry:

Before examining various wastewater treatment methods for these industries, we will take a look at some of the most important parameters for evaluating treated wastewater. One of the main indicators is biological oxygen demand (BOD) and chemical oxygen demand (COD). The entry of treated wastewater with BOD and COD levels exceeding the standard into rivers, springs, and other aquatic environments consumes the dissolved oxygen present in the water, thus endangering the life of aquatic animals and plants. Another important factor is color.

Due to the use of various dyeing and decolorization processes in the paper industry, one of the common pollutants present in the wastewater of these industries is dyes and their solvents. These chemical and organic compounds can be highly toxic and lead to environmental degradation and endanger the health of humans and other living beings. Another important indicator that requires special attention is turbidity and total suspended solids (TSS). The presence of suspended materials in wastewater disrupts the passage of light through the water and the respiration of aquatic organisms. The methods for removing these pollutants from wastewater are usually simple and can be eliminated using appropriate processes (sedimentation, flotation, membrane methods, etc.).

Other parameters:

One of the biggest concerns in the treatment of wastewater from the paper industry is the presence of stable and resistant compounds. These organic compounds are not biodegradable and remain in the wastewater even after various aerobic or anaerobic biological processes. The removal of these materials will be possible by employing appropriate advanced treatment processes (such as advanced oxidation or electrocoagulation). The next parameter is sulfur-containing compounds. The main problem associated with these compounds is the potential reaction with sulfur-reducing bacteria present in the environment. Anaerobic conditions can lead to the production of toxic H2S gas and other sulfur-containing compounds with an unpleasant odor. Another important factor is salts and ionic compounds. The presence of inorganic salts and ionic compounds such as sodium chloride in industrial wastewater leads to salinity and increased conductivity of receiving waters.

Sources and characteristics of wastewater from the paper industry:

The first important point regarding wastewater from the paper industry is to note their diversity and variability compared to each other. Due to the high diversity of processes, materials, and equipment used, as well as the products produced, the characteristics of the effluent generated in this sector of the industry are very broad and varied. The following figure shows the main sources and common pollutants present in the wastewater of these industries.

One of the first stages of the paper production process is soaking wood and stripping its bark. The effluent produced in this step is comparatively much less polluted than in other stages. The primary application of the mechanical pulping stage is for producing papers where the separation of wood fibers is done mechanically (magazines and newspapers).

The main pollutants present in this stage are suspended and dissolved materials entering the water, as well as small amounts of BOD and COD. During the chemical pulping process, a very large volume of chemicals (sodium sulfate, resin soaps, fatty acids), suspended and dissolved solids enter the wastewater stream, and a significant portion of the produced wastewater is related to this stage. The recovery and bleaching stages also lead to the introduction of a large volume of chemicals such as solvents and detergents, suspended and dissolved materials (such as pieces of paper) into the wastewater stream.

The main goal of most treatment systems is to achieve the following three objectives:

Removal of suspended materials

Removal of dissolved pollutants

Addressing the toxicity issue of wastewater and produced effluent

This is usually done in two stages. Initially, some pollutants are removed using gravity separation, and then primary treatment is performed (using clarifiers and in some cases flotation). The second stage (secondary treatment) is usually biological treatment of the wastewater.

Primary treatment: Clarification using sedimentation

Gravity separation systems depend not only on the density difference between the pollutant and the liquid but also on the size of the solid particles (large enough). Usually, the second issue is resolved by creating larger masses and increasing the sedimentation rate. Sedimentation tanks can be very simple or equipped with skimmers and sludge collection equipment. The skimmers' job is to collect foam and other light materials present on the water surface.

Sludge collection equipment continuously directs the settled materials at the bottom of the tank to the sludge collection system. In some sedimentation systems, baffles are used to direct the flow and provide calm conditions for the flow to pass through the unit. As expected, the primary sludge resulting from paper production processes contains large amounts of cellulose materials and calcium carbonate and clay minerals.

Dissolved Air Flotation Units

In addition to sedimentation, flotation is another method for clarifying wastewater from the paper industry. The commonly used system is dissolved air flotation (DAF) units. As shown in the figure below, this equipment introduces a saturated air stream into the water resulting from the paper production process. When this stream enters the tank, very small air bubbles come out of solution and start rising to the surface. These bubbles bring with them the solid materials that adhere to them to the surface. Then these solid materials are removed from the system by skimmers.

Treatment of wastewater from the paper industry

DAF systems are one of the most economical methods for treating wastewater that contains very high amounts of solids (5000 – 300 milligrams per liter). With the correct and proper use of DAF systems and the application of suitable coagulants and flocculants, it is possible to remove 80 to 95 percent of suspended solids as well as other pollutants such as ink particles, lipophiles, colloidal particles, etc. from the wastewater of the paper industry.

Secondary treatment: Activated Sludge

The most common method for removing organic materials from wastewater (with a history of over 100 years) that includes aeration and returning part of the sludge to the beginning of the process (system input) is the activated sludge system. The figure below is a schematic of a clarification unit that may be used in this method.

When higher efficiency is required or the space needed for gravity clarifiers is not available, membrane bioreactors (MBR) or other bioreactors with moving or fixed beds can be used as replacements. Also, in cases where sand filters, microfiltration (MF), or ultrafiltration (UF) are used to separate sludge, MBR systems will not have problems related to bulking. In addition, the use of membrane methods in biological processes leads to reactor performance with a higher concentration of dried solids, resulting in less biological sludge production and improved degradation of organic materials. These factors lead to a reduction in hydraulic retention time.

Sludge Concentration

The secondary sludge produced during biological treatment processes in the treatment of wastewater from the paper industry mainly consists of biological cells and the products of their decomposition, making it very difficult to separate it from water. In such sludges, each biological cell acts like a balloon filled with water. One of the most common methods to compensate for the inability to adequately dewater these sludges is to mix secondary sludge with primary sludge, which is primarily composed of fibrous materials. This results in a mixture with greater dewatering capability.

Various methods are used to increase the solids content in secondary sludge. This makes transportation easier, allows for more efficient use, or facilitates disposal. These methods include collecting sludge from the bottom of the clarifier (using scrapers), using various dewatering equipment, and centrifuges, one of the most efficient of which is the screw press (MDSP).

Advanced Treatment

In many cases, to achieve the discharge standards set by the environmental agency, it is necessary to use advanced treatment methods. Specifically, it can be said that the color and resistant compounds present in the wastewater produced during the decolorization and bleaching stages in the paper industry pass safely through the previous two treatment stages and are not removed from the wastewater. Some of the most important advanced treatment methods used in this sector include coagulation, adsorption by activated carbon, advanced oxidation systems, membrane technologies, electrocoagulation, etc.

Wastewater treatment in the paper and cardboard industry

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