Towards a Sustainable Approach in the Treatment and Reuse of Water Resources in the Textile Industry

The textile industry is among the most polluting and water-consuming sectors in the global industrial landscape.

The need to reduce environmental impact and optimize the use of water resources has led to the development of closed circuit purification systems.

These systems allow the treatment and reuse of waste water, significantly reducing the consumption of fresh water and the release of pollutants into the environment. This article aims to explore how such systems are built and how they work, with a specific focus on their use in the recycling of textile waste.

Principles of Water Purification

Water purification relies on physical, chemical and biological processes to remove contaminants. These processes are adapted and optimized according to the specific needs of the textile industry, where contaminants can vary widely in nature and concentration.

The Textile Industry and the Use of Water Resources

The textile industry requires large volumes of water for dyeing, washing and treating fabrics. This consumption leads to a high volume of wastewater, which if left untreated can cause serious damage to the environment.

Environmental Impacts of Textile Waste

Textile waste contains a variety of pollutants, including toxic chemicals used in fabric finishing and dyeing processes. Inadequate management of this waste can lead to contamination of water resources, soil and air.

Closed Circuit Purification Technologies

Closed-loop water purification technologies in the textile industry aim to maximize water recovery and reuse, minimizing the consumption of water resources and the production of wastewater.

The main technologies used include:

Reverse Osmosis (OI)

Reverse osmosis uses semipermeable membranes to remove ions, unwanted molecules, and larger particles from water. It works by applying pressure that exceeds natural osmotic pressure, thus allowing water to be separated from contaminants. The membranes used must be resistant to chemicals, high temperatures and biological attacks to ensure efficiency and durability.

Ultrafiltration (UF)

Ultrafiltration uses porous membranes to separate nanosized substances, such as suspended particles, colloids and some macromolecules, from water. Operating at relatively low pressures, UF is less energy intensive than OI and particularly effective in pathogen removal and pretreatment for reverse osmosis processes.

Vacuum evaporation

This process is based on reducing the pressure above a liquid mass to lower the boiling point of water, allowing evaporation at temperatures lower than normal ones. The evaporated water is then condensed and recovered as pure distillate, while the contaminants remain in the concentrated residue.

Membrane Bioreactors (MBR)

MBRs combine activated sludge biodegradation with membrane filtration. This technology is particularly effective in the treatment of wastewater containing high organic loads, as it allows the maintenance of a high concentration of active biomass, improving the efficiency of the biological treatment.

Adsorption

Adsorption uses adsorbent materials, such as activated carbon, to remove organic contaminants, dyes, and some heavy metals from water. The capacity and efficiency of adsorption depend on the nature of the adsorbent material, pore size, temperature and the presence of other chemicals in the water.

These technologies, individually or in combination, allow the creation of an efficient water recycling and reuse system, significantly reducing the dependence on fresh water and the environmental impact of the textile industry.

Construction of a plant

Preliminary Assessment: It is essential to carry out detailed analyzes of the characteristics of the wastewater generated and the applicable environmental regulations. This step includes collecting data on the quantities, chemical composition and temporal variability of the wastewater.

Design: The design phase requires the selection of the most appropriate technologies based on the results of the preliminary evaluation, the determination of the capacity of the plant and the definition of the footprint and arrangement of the components. This step also includes estimating investment and operating costs.

Construction: During construction, it is essential to ensure that all components are installed according to technical specifications and that the system complies with safety and environmental standards.

Start-up: The start-up of the plant involves testing of the treatment systems, process optimization and staff training. This phase is crucial to ensure that the plant operates as expected.

Maintenance and Monitoring: Preventive maintenance and continuous monitoring are necessary to maintain plant efficiency, prevent failures and comply with environmental regulations. Monitoring includes checking operational parameters, output water quality and efficiency of treatment processes.

Operation of the Plant

The operation of a closed-loop purification plant begins with the collection of wastewater from various production processes. This water is then subjected to preliminary treatments, such as sedimentation and filtration, to remove suspended solids and larger particles.

Subsequently, the water passes through more sophisticated treatment systems, such as UF, OI, and MBR, depending on the specific purification needs. During these stages, contaminants such as dyes, heavy metals, and organic substances are removed.

The treated water is then reused in production processes, reducing the need for new fresh water and minimizing the production of waste water. Constant monitoring of water quality ensures that reuse parameters are always respected, guaranteeing both operational safety and environmental sustainability.

The operation of the system is divided into several phases:

Preliminary Treatment: Removal of suspended solids, fats and oils through sedimentation, flotation and filtration. This step prevents damage and clogging of membranes and subsequent treatment systems.

Primary and Secondary Treatment: Use of biological (MBR) and physico-chemical (UF, OI) processes for the removal of organic, inorganic and microbiological contaminants. This phase significantly reduces the polluting load of the water.

Tertiary Treatment: Further purification through reverse osmosis, adsorption or other specific processes to remove any residual contaminants and adapt the quality of the water to reuse requirements.

Water Reuse and Waste Management: Treated water is reused in production processes, reducing fresh water consumption. Concentrated waste from treatment is safely managed to minimize environmental impact.

This integrated approach ensures not only environmental sustainability but also the economic efficiency of the purification plant, making it a key component in the sustainable management of water resources in the textile industry.

Types of Pollutants in Textile Waste Recycling Waters and Italian Discharge Regulations

Wastewater treatment in the textile industry is a complex issue due to the wide range of pollutants present, which vary depending on the materials treated and the processes used. This chapter explores the main types of pollutants deriving from the recycling of textile waste and discusses the Italian legislation relating to the chemical limits of pollutants authorized for discharge into sewers.

Dyes and Pigments

Physicochemical Processes: Activated carbon adsorption is widely used to remove dyes from wastewater, due to its high specific surface area and ability to adsorb a wide range of colored organic compounds.

Additionally, advanced oxidation processes, such as ozonation or hydrogen peroxide treatment in the presence of UV, can degrade dyes into less harmful compounds or into water and carbon dioxide.

Biological Processes: Some microorganisms are capable of degrading dyes. However, this approach may be limited by the variability in biodegradability of dyes and requires relatively long processing times.

Auxiliary Chemical Substances

Coagulation/Flocculation: These processes are used to remove suspended particles and some dissolved compounds, including detergents and bleaches, by aggregating them into larger flocs that can be easily separated from the water via sedimentation or flotation.

Biological Treatment: Membrane bioreactors (MBRs) and activated sludge systems can effectively degrade auxiliary organic chemicals, transforming them into biomass, carbon dioxide and water.

Heavy metals

Chemical Precipitation: Heavy metals can be removed from wastewater through chemical precipitation, converting them to insoluble forms (e.g., sulfides, hydroxides) that can be easily separated by sedimentation.

Ion Exchange: This technique uses ion exchange resins to selectively remove heavy metals from water, replacing them with harmless ions such as sodium or hydrogen.

Volatile Organic Compounds (VOCs)

Steam Stripping: VOCs can be removed from wastewater using steam stripping, a process in which water is heated and the VOCs are vaporized and then removed from the water.

Adsorption: Adsorption on materials such as activated carbon is also effective for removing VOCs from wastewater, due to carbon's ability to adsorb a wide range of volatile organic compounds.

To ensure the effective removal of these pollutants and compliance with regulatory limits, textile waste recycling water treatment plants often implement a combination of these processes in a multimodal approach.

This strategy allows you to optimize the treatment according to the specific composition of the waste water and the quality objectives of the treated water, while ensuring compliance with environmental regulations.

Italian Discharge Regulations

In Italy, the reference legislation for the treatment and discharge of industrial waste water, including that of the textile industry, is Legislative Decree 152/2006, known as the "Environmental Code".

This decree establishes maximum concentration limits for pollutants present in waste water that can be released into the sewer system or aquatic environments.

For textile wastes, some of the specific limits include:

COD (Chemical Oxygen Demand): Indicates the amount of oxygen required to oxidize organic chemicals present in water. The limit varies based on the type of discharge and the capacity of the receiving body.

Dyes: Although there is no specific limit for each dye, the discharge must not cause a significant color change in the receiving body of water.

Heavy Metals: Specific limits are established for each metal, for example, total chromium has a limit of 0.5 mg/l for discharge into sewers.

pH: Must be maintained within a range that does not compromise the operation of the sewer system or purification station, generally between 6.5 and 9.5.

Companies must also obtain a discharge permit from the relevant authorities, which may include specific requirements in addition to the general limits imposed by the Environmental Code.

It is critical that textile industries implement effective wastewater treatment systems to comply with these regulations and minimize the environmental impact of their operations.

This regulatory approach aims to balance production needs with the protection of the aquatic environment, ensuring that the textile industry can operate in a sustainable and responsible manner.