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

Characteristics of wastewater from the paper industry:

Toxicity

High COD in produced wastewater

Limited diversity and quality of chemical pollutants (approximately 250 chemical substances, including resin acids, heavy metals, cyanides, phenols, etc.)

It should be noted that among the different stages of paper production, such as pulping, pulp washing, screening, bleaching, and paper making, the main sources of pollution are generated 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 the pollution load, but what matters is selecting the best and most appropriate method.

In the biological treatment of wastewater from the paper and cardboard industry, the most important treatment stage is neutralization, as high pH affects the growth of bacteria. After that, initial sedimentation is applied to remove suspended and solid materials from the wastewater, and passing the wastewater through various stages reduces its turbidity and color.

Generally, biological treatment, chemical oxidation, coagulation, ion exchange processes, and adsorption processes are some of the most important stages utilized in the wastewater treatment of the paper industry.

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

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 indices is biological oxygen demand (BOD) and chemical oxygen demand (COD). The entry of treated wastewater with BOD and COD levels exceeding the standards into rivers, springs, and other aquatic environments depletes the dissolved oxygen in water, 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 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 organisms. Another important indicator that requires special attention is turbidity and total suspended solids (TSS). The presence of suspended materials in wastewater disrupts light penetration in water and the respiration of aquatic organisms. The methods for removing these pollutants from wastewater are usually simple, and they 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 persistent and resistant compounds. These organic compounds are not biodegradable and remain in the wastewater even after performing various biological aerobic or anaerobic processes. The removal of these materials can be achieved by employing suitable supplementary treatment processes (such as advanced oxidation or electrocoagulation). The next parameter is sulfur-containing compounds. The main issue with these compounds is the potential reaction with sulfur-reducing bacteria present in the environment. Anaerobic conditions can lead to the production of the toxic gas H2S and other sulfur-containing compounds with unpleasant odors. Another important factor is salts and ionic compounds. The presence of inorganic salts and ionic compounds such as sodium chloride in industrial wastewater causes salinity and increases the conductivity of receiving waters.

Sources and characteristics of wastewater from the paper industry:

The first important point regarding wastewater from the paper industry is the diversity and variability compared to each other. Due to the high diversity of processes, materials, equipment used, and the variety of products produced, the properties of wastewater generated in this sector are very broad and different. The diagram below shows the main sources and common pollutants found 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 generated in this step is considerably less polluted than that of other stages. The main application of the mechanical pulping stage is to produce papers where the separation of wood fibers is done mechanically (e.g., magazines and newspapers).

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

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

Removal of suspended solids

Removal of dissolved pollutants

Addressing toxicity issues in wastewater and produced effluent

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

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 size). Usually, the second condition is met by creating large masses and increasing sedimentation rates. Sedimentation basins can be very simple or equipped with skimmers and sludge collection equipment. Skimmers are responsible for collecting foam and other light materials on the surface of the water.

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 create calm conditions for the flow to pass through the unit. As expected, the primary sludge resulting from the paper production processes contains large amounts of fibrous 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 diagram below, this device introduces a saturated air stream into the water produced from the paper manufacturing process. As this stream enters the tank, very small air bubbles are released from the dissolved state and begin to rise to the surface. These bubbles bring with them the solid materials that adhere to them to the surface. Then, these solids are removed from the system by the skimmers.

Wastewater treatment in the paper industry

DAF systems are one of the most cost-effective methods for treating wastewater that contains large amounts of solids (5000 - 300 mg/L). With proper and effective use of DAF systems and by employing appropriate coagulants and flocculants, 80 to 95 percent of suspended solids and other pollutants, such as ink particles, lipophilic substances, colloidal particles, etc., can be removed from the wastewater of the paper industry.

Secondary treatment: Activated sludge

The most common method for removing organic materials from wastewater (which has been in use for 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 diagram below is a schematic of a clarifier that may be used in this method.

When a higher efficiency is needed, or when the space required for gravity clarifiers is not available, membrane bioreactors (MBR) or other bioreactors with fixed or moving beds can be used as substitutes. In cases where sand filter, microfiltration (MF), or ultrafiltration (UF) is used for sludge separation, the MBR systems will not have the bulking problems. Additionally, the use of membrane methods in biological processes leads to reactors operating with higher concentrations of dried solids, resulting in the production of less biological sludge and improved degradation of organic materials. These factors lead to reduced hydraulic retention times.

Sludge concentration

The secondary sludge produced during biological treatment processes in wastewater treatment from the paper industry mainly consists of biological cells and the products resulting from their decomposition. Therefore, separating it from water is very difficult. In such sludge, each biological cell acts like a balloon of water. One of the most common methods to compensate for the inability to properly dewater these sludges is to mix secondary sludge with primary sludge, which consists mainly of fibrous materials. This process helps achieve a mixture with higher dewatering capabilities.

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

Supplementary treatment

In many cases, to meet discharge standards set by the Environmental Protection Agency, the use of supplementary treatment methods is necessary. Specifically, it can be said that colored compounds and those resistant to biological degradation present in the wastewater produced in decolorization and bleaching processes in the paper industry safely pass through the previous two stages of treatment and are not removed from the effluent. Some of the most important supplementary treatment methods used in this sector include coagulation, adsorption by activated carbon, advanced oxidation processes (complementary), membrane technologies, electrocoagulation, etc.

Wastewater treatment in the paper and cardboard industry

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