In water-short areas, wastewater reclamation has emerged as a practical option. Water shortage has emerged as a major issue as the world’s population grows and natural water supplies get depleted. It has been forecasted that the global demand for freshwater will exceed the supply by 40% by 2030.
It is expected that water scarcity will increase from about one-third to nearly half of the global urban population in 2050. Recovery and recycling of wastewater have become a growing trend in the past decade due to rising water demand. Most of the cost-effective water purification has been made possible via membrane treatment. Reverse Osmosis membranes have been demonstrated to significantly reduce total dissolved solids, organic pollutants, viruses, bacteria, heavy metals, and other dissolved contaminants.
Wastewater reuse, not only reduces the quantity and environmental threat of discharged wastewater, but it also alleviates the impact on ecosystems generated by freshwater withdrawal. Wastewater is no longer regarded as pure waste that may harm the environment if recycled, but rather as an additional resource that can be used to achieve water sustainability.
2. What is Reverse Osmosis?
Reverse osmosis (RO) is a membrane-based separation method that uses difference in the permeability of the water’s constituents. The membranes are made of a synthetic substance that is semipermeable; some constituents pass through it very easily, while others pass through it less readily.
To remove a constituent from water, water is forced across the surface of a membrane, resulting in product separation, which is why reverse osmosis (RO) is the best of all membrane filtration methods.
Schematic diagram of the Reverse osmosis process
The RO technique is utilized to remove dissolved solids because traditional municipal treatment methods are unable to do so. In chemical and environmental engineering, RO is increasingly employed as a separation process to eliminate organics and organic contaminants from wastewater.
Application of Reverse osmosis
The use of reverse osmosis in wastewater treatment is limited by the high running costs caused by membrane contamination. In the case of industrial wastewater, RO has been employed in industries where it is possible to increase process efficiency by recovering valuable components that can be recycled in the manufacturing process.
An RO plant for industrial usage has the following goals:
- 50% desalination of seawater and brackish water
- 40% ultrapure water production for the electronic, pharmaceutical, and energy production industries and
- 10% decontamination systems for urban and industrial water.
Some common applications of RO system include the following:
(a) Desalination of the sea and brackish water.
(b) Generation of high-purity water for pharmaceuticals.
(c) Generation of ultrapure fresh water for microelectronics.
(d) Generation of processed water for beverages (beer, bottled water, fruit juices, etc.);
(e) Processing of dairy products.
(f) Waste treatment for the recovery of process materials such as metals for metal finishing industries and dyes used in the manufacture of textiles.
(g) Water reclamation of municipal and industrial wastewater.
How does Reverse Osmosis work?
In Reverse osmosis, cellophane-like membranes separate pure water from polluted water. When pressure is applied to the concentrated side of the membrane, purified water is forced into the dilute side, and the rejected impurities from the concentrated side being washed away in the rejected water.
Permeate (or product) water is desalinated water that has been demineralized or deionized. The reject (or concentrate) stream is the water stream that contains the concentrated pollutants that did not pass through the RO membrane.
Salts and other contaminants are not allowed to pass through the semi-permeable membrane as the feed water enters the RO membrane under pressure (enough pressure to overcome osmotic pressure), and they are discharged through the reject stream (also known as the concentrate or brine stream), which goes to the drain or, in some cases, can be fed back into the feed water supply to be recycled through the RO system to save water.
Permeate or product water is the water that passes through the RO membrane and typically has 95% to 99% of the dissolved salts removed from it.
4.1. Stages of RO systems
Every RO system includes different types of filtrations. There are many filtration stages in a RO system. In addition to the RO membrane, every reverse osmosis water system also includes a sediment filter and a carbon filter. Depending on whether the filters are used before or after the membrane, the filters are referred to as prefilters or post-filters.
Each type of system contains one or more of the following filters:
- Sediment filter: filters out particles such as dirt, dust, and rust
- Carbon filter: Reduces the amount of volatile organic compounds (VOCs), chlorine, and other pollutants in water that give it an unpleasant taste or odor.
- Semi-permeable membrane: up to 98% of the total dissolved solids.
4.2. Technical Requirements of a RO System
Several fundamental technical prerequisites for a RO system include:
- Feed water needs to be prefiltered and pH adjusted. After prefiltration, the feed water’s TDS and suspended particles should be kept under the specified ranges.
- The microbiological quality of feed and product water should be monitored. If microbiological quality levels are exceeded, the system should be cleaned.
- Before disinfection, every system component needs to be mechanically cleaned. To ensure that chemicals used in disinfection are eliminated from the system, the proper tests should be run.
- It is best to avoid using filters or ion exchangers downstream of RO units.
- The chemical and microbiological quality of water should be evaluated at predetermined intervals during a production cycle.
- The RO system should be constructed for continuous flow without traps, dead ends, and pipe sections that may gather stagnant water. Installation of in-line conductivity sensors at strategic locations is necessary for ongoing water quality monitoring. The equipment should be qualified, and the RO system should be validated periodically, as well as operated and maintained according to the manufacturer’s instructions so that it can consistently produce water with acceptable quality.
5. What contaminants will Reverse Osmosis remove from water?
Reverse osmosis may remove up to 99%+ of dissolved salts (ions), particles, colloids, organics, bacteria, and pyrogens from the feed water.
- However, the RO system cannot remove 100% of bacteria and viruses.
Contaminants are rejected by a RO membrane based on their size and charge. A properly operating RO system will generally reject any contamination with a molecular weight greater than 200. (For comparison a water molecule has a MW of 18). Similarly, the higher the contaminant’s ionic charge, the less probable it is to flow through the RO membrane.
6. ZLD combined with RO
RO is a technique for cleansing contaminated water using a membrane and a pressure unit. Furthermore, RO generates a large amount of liquid discharge, i.e. saline water. The Zero Liquid discharge system is used to limit discharge into streams and create a self-sustaining system with zero effluents.
Zero-liquid discharge (ZLD) is a water treatment technique that purifies and recycles all wastewater, resulting in zero discharge at the end of the treatment cycle. It is a cutting-edge wastewater treatment technique that combines ultrafiltration, reverse osmosis, evaporation/crystallization.
ZLD eliminates any liquid waste from exiting the plant or facility perimeter, with most of the water recovered for reuse. ZLD eliminates the danger of pollution associated with wastewater discharge and maximizes water usage efficiency, achieving a balance between freshwater resource exploitation and aquatic environment protection.
In order to increase energy and cost savings, reverse osmosis (Ro) has been added into ZLD systems. However, while RO is far more energy efficient than thermal evaporation.
Reverse osmosis for ZLD/MLD is constantly evolving, and the most efficient plants now have two concentration stages. The filtered wastewater is first forced through semi-permeable membranes at pressures of up to 80 bar, which reduces the water content by 40 to 50%. The liquid is forced through membranes at ultra-high pressures of up to 120 bar during the second step of the process, which reduces the water content by an additional 30 to 40%. It means that by the time the concentrate enters the brine concentrator following the two-stage reverse osmosis process, the water content has been decreased by up to 60%.
RO is the best and most efficient desalination technique available today. The goal is to use RO to recover as much water from evaporation as feasible. Evaporation and Crystallisation is the most basic type of ZLD system, aiming for 90% water recovery and 100% crystallisation.
RO is currently the best and most energy efficient desalination method available. The goal is therefore to recover as much water as possible before evaporation using RO. As RO recovery rises, the cost of ZLD decreases.
Since water supplies are increasingly scarce, reuse options are growing in popularity. In this perspective, zero-liquid discharge (ZLD) is a new strategy for reducing waste, recovering resources, treating toxic industrial waste streams, and helping minimize water quality consequences in receiving water streams.
Although ZLD systems can reduce water contamination and increase water supply, their industrial-scale applications are limited due to their high cost and high energy consumption. Membrane-based technologies are an appealing future solution for industrial wastewater reclamation in ZLD systems.
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