Reverse Osmosis: Application in Zero liquid discharge


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. 

  1. 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.

  1. 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 filterfilters out particles such as dirt, dust, and rust
      • Carbon filterReduces the amount of volatile organic compounds (VOCs), chlorine, and other pollutants in water that give it an unpleasant taste or odor.
      • Semi-permeable membraneup 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%.


  1. Conclusion

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.

8. Reference :

Introduction to Pipe sizes and pipe schedule

  1. Introduction

Pipe size calculation is necessary for planning and identifying the pipe to be utilized during project execution. The size of the pipe is determined primarily by

  1. the flow rate required,
  2. the pressure of the piping system, and
  3. the end connection equipment.

Identification of pipe size is vital for transmitting the exact idea from the piping design team to the piping execution team for using the correct pipe size for spool production. Pipe size also has a significant impact on cost. Oversizing a pipe result in additional costs, more complex pipe design, more footing, and, in some cases, process difficulties.

In the field of engineering, there are many and varied materials & types of pipes used, each   with their own unique credentials, and often with their own sizing. This results in some confusion in the industry around the outside diameter of piping, and the sizing of required pipe supports. That is why understanding to pipe sizing is very important.

Pipe size in referred in various terms like – Nominal Diameter (DN), Nominal Pipe Size (NPS), or Nominal Bore (NB). However, despite having certain similarities and variances in terms of their uses and places of origin, these are all notations for pipe sizes.

  1. Pipe sizing

Pipe size is specified with two non-dimensional numbers,

      • Nominal Pipe Size (NPS)
      • Schedule Number (SCH)

And the relationship between these numbers determines the inside diameter of a pipe.

  • Nominal pipe size and its importance


The Nominal Pipe Size (NPS) is a North American standard size for pipes that are suitable for high or low pressures and temperatures. It defines the diameter of pipe. Nominal pipe size only refers to the outside diameter (OD) of a pipe. When it is said that pipe size is 2 NPS, it indicates any pipes with an outer diameter of 2.375-inch (or 60.3 mm), regardless of wall thickness and hence internal diameter.

Nominal pipe sizing

Nominal Bore (NB) and Nominal Diameter (DN) are sometimes used alternately with Nominal Pipe Size (NPS). Nominal Bore (NB) is the European equivalent for NPS. The Nominal Diameter (DN) for NPS 5 and larger is equal to the NPS multiplied by 25.

Pipe size illustration

2.1.1 Nominal diameter (DN): The Nominal Diameter (DN), is frequently referred to as “mean or average outer diameter.” The metric unit system uses nominal diameter, sometimes known as Diameter Nominal (DN). The pipe’s Inner Diameter (ID) and Outer Diameter (OD) are not equal to the nominal diameter.

Although not exactly equal, the value of DN is close to the inner diameter of the pipe. The connecting dimensions of pipes and pipe fittings are denoted using this notation. Pipes come in a variety of DN sizes, and using standard tables and pipe schedule charts, the DN is used to determine the pipe’s final dimensions.

NPS vs DN measurement

2.1.2 Nominal Bore (NB): A European standard for indicating pipe size is Nominal Bore, or NB. In the case of pipes, the terms bore, and nominal refer to hollow structures, respectively. The nominal bore is a rough internal measurement across the diameter of the pipe. In other terms, a nominal bore refers to the pipe’s bore’s approximation in size.

 A pipe is exactly 10.75 inches long when measured in inches; hence, 10.75′′ x 25.4 = 273.05 mm. This is the reason why the outside diameter is not a straightforward number like 250NB.

Pipe Size Indication

  1. What is schedule Number?

The wall thickness of the pipe is described using steel pipe schedules. This crucial variable as it is directly related to the strength of the pipe and the suitability for specific applications.  Given the design pressure and permitted stress, a pipe schedule is a dimensionless number which is derived using the design formula for wall thickness.

Pipe wall thickness is determined by pipe schedule. The mechanical strength of the pipe increases as the wall thickness, allowing it to withstand higher design pressures.

Schedule number = P/S

  • P is the service pressure in (psi)
  • S is the allowable stress in (psi)

Two pipes of the same diameter with different schedules will have different wall thicknesses. So, when specifying a pipe for a high-pressure application, a larger number signifies a larger schedule (wall thickness).

Pipe schedule illustration

The schedule numbers 40 and 80 are the most popular ones. As the schedule number increases, the wall thickness of the pipe increases. Since there has been advancement of industrial age, pipe sizes and their standard has also been changed. There is wide range of wall thicknesses: SCH 5, 5S, 10, 10S, 20, 30, 40, 40S, 60, 80, 80S, 100, 120, 140, 160, Standard (STD), Extra Strong (XS) AND Double Extra Strong (XXS).

Steel pipe schedules are a way to specify the pipe’s wall thickness. This is an important metric since it is directly related to the pipe’s strength and applicability for various applications. A pipe schedule is a dimensionless number that is calculated using the design formula for wall thickness and the permissible stress.

4.  Conclusion

Pipe sizing is one of the first important actions a process engineer performs throughout the P&ID preparation process.  Pipe size is a key consideration in a well-designed process. It will have an impact on fluid velocity, pressure drop, flow regime, and so on. A poorly sized pipe can disrupt the entire process and, in extreme situations, lead to plant shutdown That is why one should always know about the differences between Nominal pipe sizes and pipe schedule numbers.

5.  Reference

Introduction to Air Quality standards for the classification of Compressed air quality

  1. Introduction

ISO (International Standards Organisation) is the world’s largest developer and publisher of international standards. An intermediary between the public and private sectors, ISO is a non-governmental organization. It has developed international standards to test the quality of compressed air. There are now three standards in use that are specifically related that directly relate to compressed air quality (purity) and testing:

  1. ISO8573 Series
  2. ISO12500 Series
  3. ISO7183

The most used standard is the ISO8573 Series and in particular ISO8573-1:2010. The ISO air quality standard measures three types of contaminants present in compressed air: water, oil content, and solid particles. It does not consider microorganisms and gases.

2.     What is compressed air quality & why it is needed?

When air is gathered to be compressed, more particles are also gathered, resulting in contaminated air. When air is compressed, the number of impurities present exponentially rises. Additionally, when compressing air, other pollutants could be included. To clean up the compressed air system’s pollutants, air treatment is required. The types of contaminants found in a compressed air system include:

  1. Solid particles / Dust
  2. Water (in liquid or vapor form)
  3. Oil content (in vapor or aerosol form)

Schematic presentation of contaminants present in compressed air

When air is adequately treated, it is considered clean and safe. However, the quality of compressed air is determined not just by how clean it is, but also by how dry it is. The number of particles of a specified size present in one cubic meter of air, the dew point, and the number of oil aerosols and vapor must all be counted to assess how clean and dry the compressed air is.

Compressed air is utilized in a variety of industries, including mining, manufacturing, textile manufacturing, and food processing. The air quality utilized in industrial applications has a direct impact on the work process, installed machines, and product quality. As a result, it is critical that the compressed air be clean and devoid of pollutants.

The smaller the chance of contamination, breakdowns, and product rejection, the cleaner the air. This is especially important in businesses like food and beverage and pharmaceuticals. There is a possibility that the air will come into direct touch with the product or will come into indirect contact with the packaging.

  1. Classification of Compressed air quality

Compressed air is the only utility created by the end user of all the key utilities used in the food manufacturing setting. This means that the end-user has a direct impact on the quality of this energy source.

High-quality compressed air is essential for producing food that is not only cost-effective to process but also safe to consume. Therefore, choosing the appropriate compressed air equipment for food processors is in the best interests of everyone. The basis for choosing air treatment products is made much easier by the ISO 8573 air quality standards and ISO 12500 compressed air filter standards.


      Standards used for air quality measurement

  1. What is ISO 8573 & how it defines different air purity classes?

ISO 8573 is the compressed air quality standard. ISO8573 is a series of international standards that address compressed air quality (or purity). Part 1 of the standard describes the compressed air quality requirements, and parts 2 through 9 detail the testing techniques for various pollutants.

  • ISO 8573-1: Contaminants & purity classes
  • ISO 8573-2: Oil aerosol test methods
  • ISO 8573-3: Humidity test methods
  • ISO 8573-4: Solid particle test methods
  • ISO 8573-5: Oil vapor test methods
  • ISO 8573-6: Gas test methods
  • ISO 8573-7: Viable microbiological content tests
  • ISO 8573-8: Solid particle test methods by mass concentration
  • ISO 8573-9 Liquid water test method


       Various air quality classes


This above standard also determines air quality, which is designated by the following nomenclature: Compressed Air Purity Classes A, B, C:
A= Solid particle class designation
B= Humidity and liquid water class designation
C= Oil class designation

Air quality designation

5.     Why is it important to consider the air quality?

Compressed air can contain unwanted substances, such as water in drop or vapor form, oil in drop or aerosol form, and dust. Depending on the compressed air’s application area, these substances can impair production results and even increase costs. Air treatment aims to produce the compressed air quality specified by the consumer. When the role of compressed air in a process is clearly defined, finding the system that will be the most profitable and efficient in that specific situation is simple. This will be determined by your finished product and the working environment of your application.

6.     What does Class 0 mean for air quality?

It is recommended that only compressed air classified as Class 0 be used in critical processes to eliminate the risk of air contamination. This level of classification does not mean zero contamination. Class 0 refers to the highest air quality possible with minimum contamination present in the air and must be lower in contamination than Class 1.

A combination of compressed air equipment can be installed to produce clean air. This may include various air filters and dryers. Identifying which contaminants need to be removed will help you to determine which equipment you need.

  1. Conclusion:

A specific compressed air class is assigned depending on the number of contaminants found. The air quality class is set according to ISO 8573-1. This standardized system defines parameters from the least to most contaminated sources of compressed air. These standards are very much useful when it comes to selecting air compressors/ compressed air systems for industrial purposes. As Compressed air is a vital energy source and is utilized in multiple operations in a food processing facility.

When properly treated, compressed air is regarded as a safe, clean utility, as compared to other energy sources. It is ultimately used to package, wrap, seal, palletize and label food products prior to storage or shipment. ISO 8573 is a European Standard that describes contaminants in compressed air and defines purity classes for them. This multi-part standard also defines approved measurement methods for testing contamination levels.

  1. Reference