Zero Liquid Discharge (ZLD)

ZERO LIQUID DISCHARGE IN INDUSTRIAL PROCESSES

Water is an indispensable raw material in practically all industrial processes. Water treatment therefore plays an important role in industry. The aim is always to achieve economical and resource-saving treatment, optimized for the respective application.

In a ZLD treatment process, wastewater is treated in such a way that this treatment produces purified water and a sludge that is suitable for landfill. Ideally, the water is treated in such a way that it can be reused as process water.  In addition to environmental aspects, economic and occasionally also legal aspects are decisive for the choice of process. ZLD processes are characterized by the fact that the final process stage is usually carried out using an energy-intensive thermal process.

ADVANTAGES OF THE ZLD TREATMENT PROCESS APPLICATION

The aims are to reduce the volume of wastewater produced, reduce the loads in the residual wastewater and/or recover valuable materials. The ideal approach is a holistic one, which involves reducing and optimizing the additional water required for the process on the one hand and reducing and reusing the wastewater produced on the other.

THE PROCESSES USED IN THE ZLD TREATMENT PROCESS

Different processes are used here. Chemical-physical wastewater treatment is often carried out first to separate poorly soluble and insoluble substances. If necessary, this is followed by biological wastewater treatment to break down organic impurities. Depending on the type of organic substances, chemical-oxidative treatment can also be carried out. The result of these treatment stages is usually a wastewater that fulfills the discharge criteria for discharge into a wastewater network.

THE TREATMENT STAGES FOR THE REUSE OF WASTEWATER AS PROCESS WATER

Additional treatment stages are required to reuse the wastewater as process water. These usually include a membrane process to produce process water - or to concentrate the pollutants. However, membrane processes are limited by the maximum permissible operating pressures, i.e. the maximum possible concentration is restricted by the osmotic pressure of the solution. For this reason, a final thermal concentration step, consisting of evaporation/crystallisation, is usually carried out in ZLD processes.

This produces practically pure water (vapour condensate) for further use as process water on the one hand and solid substances in the form of salts or sludge on the other hand. These are usually largely dewatered using a mechanical dewatering process and, depending on their composition, can be used as secondary raw materials or disposed of.

The NET POSITIVE WATER STRATEGIE

The recovery of water through ZLD can be supplemented by using additional raw water sources, such as rainwater, as part of a net positive water strategy. The aim here is to cover the process water requirement from the company's own resources and, ideally, to release more clean water into the environment than is extracted from other sources as part of the processes.

PWT Wasser- und Abwassertechnik GmbH builds treatment plants for the production of process water from various raw water sources as well as for wastewater treatment and reuse.