Reverse Osmosis in Industrial Wastewater Treatment Units

Abstract

The MENA region faces a severe water crisis, prompting governments to take action by improving irrigation methods, treating and reusing sewage and agricultural wastewater, and issuing restrictions regulating industrial wastewater discharge. As a result, many large factories have established industrial wastewater treatment plants to recycle water, reduce reliance on external sources, comply with environmental regulations, and implement MLD or ZLD principles. This chapter will focus on industrial wastewater treatment using reverse osmosis (RO) membranes. It will cover the treatment of various contaminants such as nitrogen, phosphorus, COD, BOD, TOC, and heavy metals. It will discuss different treatment methods and technologies to produce reusable water while achieving MLD and ZLD principles.

1. Introduction

How is water recycled in your space? Water reuse and recycling have become inevitable, especially in areas where water is scarce. Water scarcity is a global problem as most of the water on the surface of the planet is salty water, whether it is of high salinity, as in the seas, oceans, and salty lakes, or water of medium salinity, as in most of the wells waters, and that water represents about 97% of the total water present. While fresh water is mostly confined to snow in the north and south poles, which represents about 2% of the amount of water present, and the remaining 1% is divided between 0.6% fresh water in wells, and 0.3% represents moisture water in the atmosphere, and the remaining only about 0.1% of all available water resources is fresh surface water in rivers and freshwater lakes. Besides that, the easy-to-use surface freshwater is limited to about 0.1% of all water resources, and that represents a natural physical water scarcity; however, most of that water is exposed to different kinds of anthropogenic pollution, making it needs further treatment before using and leading to continued pressure on that limited water resources.

The massive industrial development in the last century and the spread of huge industrial complexes and their need for large quantities of water of different quality led to another kind of pressure on the limited water resources, in addition to the negative impact of the industrial wastewater of those industrial complexes in case it was discharged without treatment or with partial treatment to different water bodies, whether fresh or not fresh or seawater, so the safe disposal of these liquid wastes or recycling water and reusing it in various industrial processes has also become an indivisible necessity.

“Save it before it is too late”, this slogan must have its meaning present in the minds of all those responsible for the industrial units and facilities that already exist or are under construction because one of the biggest reasons for the lack of proper and safe disposal of the industrial wastewater is the lack of interest or full knowledge of those responsible for the industry about the extent of the danger of these wastes on the environment and in the core of its water sources and its exposure to pollution. Furthermore, let us look from a narrow perspective and it will negatively affect the quality of the feed water for those factories themselves, which may lead to an increase in the cost of water treatment required for industry or affect the efficiency of the industrial process itself in an endless cycle of increasing pollution and additional treatments.

One of the best-applied methods to saving water is reusing or recycling it, where the most appropriate is to apply this at the industrial level, as the water required for industry varies in quality from one industry to another, as well as for various uses within the same industry, as (cooling water – manufacturing products – steam production – a carrier of raw materials or waste – or a solvent), and also as water reused at the industrial level that will not be affected by psychological and societal acceptance, as in the reuse of water for drinking purposes.

There are many water treatment technologies used in the treatment of industrial wastewater; probably the most prominent of them are; physical treatment like (screening, mixing, sedimentation, flotation, filtration, and gas transfer) and chemical treatment in which the removal or conversion of contaminants is carried by the addition of chemicals or by other chemical reactions like (precipitation, oxidation/reduction, neutralization, adsorption, and disinfection), also biological treatment in which the removal of contaminants is carried by biological activity to remove the biodegradable organic substances whether colloidal or dissolved and nutrients like nitrogen and phosphorus from the industrial wastewater using one or all of aerobic, anaerobic, and anoxic biological treatment methods.

Industrial wastewater treatment is a general concept, and reuse is a particular case where after applying the recommended treatment method to remove different contaminates, the question remains, is this water suitable for the type of application that will be reused through it? Possibly one of the substantial and essential treatment methods is the removal of salts through reverse osmosis (RO) membrane technology.

Despite the wide use of reverse osmosis (RO) membrane technology in the treatment of industrial wastewater for reuse, this requires several critical challenges, one of which is; due to the higher sensitivity of these membranes; they require complex primary treatment, which is considered not only every industry has its industrial wastewater case or every factory, but every stream inside the factory is evaluated as a certain case study and needs unique treatment methods that achieve the best-needed quality with the lowest costs. While the other is how to safely dispose of the resulting concentrated solution, whether by achieving the principle of minimum liquid discharge (MLD) using evaporation lakes, deep injection wells, or drainage on seawater after fulfilling the necessary environmental conditions, or thermal evaporation and crystallization achieving the principle of zero liquid discharge (ZLD), while the resulting desalinated water may not be suitable for use directly in some cases, and it needs certain additions or additional treatments before using it, depending on the type of application. On the other hand, the selection of the RO unit’s proper design recovery, membrane types, flux, and configuration is another one of the most important points for sustainable RO technology application in industrial wastewater treatment and reuse.

In this chapter, we will address some important points that must be taken into consideration when designing and implementing the various stages of the treatment and reuse of industrial wastewater.

2. Pretreatment of industrial wastewater

The primary treatment of industrial wastewater is the cornerstone and depends on to what extent its efficiency could achieve the maximum benefit from that wastewater, whether by direct reuse, partially desalting using RO technology, or reaching demineralized water.

The first step is an accurate knowledge of the nature and sources of industrial wastewater based on a good knowledge of the industry processes, places of drainage, and the nature of its being continuous or patched (intermittent) streams of industrial wastewater, so you need to know the amount and frequency of each stream as well as their specifications, the next table help to know the nature of each stream flow (Table 1).

Streams		Units	Stream 1	Stream 2	Stream 3	Stream 4
Name
Continuous Flow		m^3/h
Peak	Flow	m^3/h
	Duration	Hrs
Patch (Intermittent)		Volume/time

Table 1.

Demonstrates a detailed approach to documenting different sources and quantities of industrial wastewater streams.

Also, it is essential to know the sources of raw feed water for the industry in which industrial waste treatment is to be done, as it gives an initial idea of the nature of wastewater composition, or at least the general tendency. For example, if the wastewater results from raw water from wells, it is necessary to analyze wastewater for elements such as silica, iron, manganese, calcium, magnesium, barium, and strontium. But if the raw water is surface fresh water, the the big focus will be on its organic load, silica, and so on. Also, kind of water treatment techniques used in the utility section of the industry where it is possible to maximize its recovery or rearrange it to make it more suitable and reduce the waste resulting from it, or parts of it can be used to treat wastewater or mix portion of the pretreated industrial wastewater with feed water for some of its units.

In the following, we will review some of the significant parameters of industrial wastewater, which must be closely monitored, and some of their effects on the treatment stages will be shown.

2.1 Temperature

The continuous follow-up of the change in the temperature of the industrial wastewater is a heightened concern because the shift in it may be unlike surface water or the well water is not always linked to the change in the ambient temperature during the different seasons, as there may be sources of industrial wastewater associated with a big rise in temperatures such as steam condensate drain which could be ranged from 50° C to 90° C, or some exothermic reactions which make the industrial waste stream temperature reaches 80° C. So identifying the temperature ranges of each industrial wastewater stream are very important because, based on the type of subsequent treatment of that water, the extent to which it needs to be cooled or not will be determined, and whether this stream (which may be small in quantity) can be separated and cooled separately before mixing it with the rest of the waste streams, to prevent the high temperature of the mixed (equalized) industrial wastewater above the recommended temperature for the subsequent treatment technologies like for ultrafiltration (UF) organic membrane (maximum temperature is 40° C) or RO membranes (maximum temperature is 45° C). Or there is a type of treatment that will be applied to this stream individually. Or on the contrary, blending it with the rest of the industrial wastewater sources may maintain an average temperature in different seasons, leading to improving some types of treatments that are greatly affected by the extreme drop in temperatures, such as biological treatment, coagulation processes, and also reverse osmosis, which requires certain precautions during the design phase to maintain the desired efficiency of these units.

2.2 Organic content

There are several ways to express the organic load of industrial wastewater, including the chemical oxygen diamond (COD), the biological oxygen diamond (BOD), and also the total organic carbon (TOC).

2.2.1 COD measurement

COD represents the quantity of dissolved oxygen in the water that must be present to oxidize organic materials. As a pollution measuring tool, COD is used to measure the short-term influence that wastewater effluents will have on the receiving water bodies’ oxygen levels. So, COD is an essential measurement that helps detect the organic pollutant amount and follows the efficiency of different treatment techniques to ultimately limit pollution in water.

COD measuring may be essential when treated wastewater is discharged into the environment. High levels of industrial wastewater COD indicate concentrations of organics that could deplete dissolved oxygen in the water, leading to adverse environmental and regulatory significances. But when using the pretreated wastewater as feed water for RO system, TOC measuring is the best way to get the total organic loads that may be above the RO membranes manufacturers recommendations.

COD measurement has many interferences which may be present on the industrial wastewater and cannot be dependent on measurement for assets the organic loads before the RO system as interference from chloride, florid, and bromide, chromium, nitrite, sulfite ions where mercuric sulfate that eliminates chloride interference up to 2000 mg/L and samples with higher chloride concentrations are typically diluted or may be removed by precipitation with silver ion and filtration before digestion. Some aromatic compounds like Pyridine and related compounds resist oxidation, and volatile organic compounds will react in proportion to their contact with the oxidant; in contrast, straight-chain aliphatic compounds are oxidized more effectively in the presence of a silver sulfate catalyst. Also, some organic compounds are not oxidized completely with the COD method like urea compounds not properly appearing in the COD measurement, so using of COD only to measure the effectiveness of the pretreated wastewater is not accurate, and TOC measuring is a more accurate, faster, and more sharp method for the organic content [1].

The total organic carbon (TOC) measurement is essential to assessing the pretreatment process capabilities before the RO membrane system in industrial wastewater treatment units. TOC is a measure of the total amount of organic compounds present in water and is used as an indicator of the quality and suitability of water and can provide valuable information about the efficiency of the pretreatment process and the potential for fouling of membranes in the RO system.

TOC limits before the RO membrane system will depend on the specific design consideration of the RO system, like; RO system recovery and choosing the type of RO membrane in terms of its fouling resistance capabilities. In general, it is recommended to keep the TOC levels as low as possible to minimize the risk of fouling the RO membranes. For industrial water treatment applications, the American Membrane Technology Association (AMTA) recommends a maximum TOC concentration of 5 mg/L [2].

The presence of nutrients in the pretreated wastewater can potentially impact the permissible limits of TOC before the RO membrane system. Nutrients, such as nitrogen and phosphorus, can stimulate the growth of microorganisms, even if it is less than 5 mg/l, so it is preferred to decrease TOC concentration to less than 2 ppm in case of the presence of residual nutrition in the pretreated wastewater, or it may be required to dose nonoxidizing biocide like 2,2-dibromo-3-nitrilopropionamide (DBNPA) whether continuous or intermittent shock doses to prevent or reduce RO membrane biofouling rates.

Both TOC and COD are commonly used to measure the concentration of organic matter in water. TOC measures the total amount of organic compounds present in water, while COD measures the oxygen-depleting capacity of organic compounds in water.

In general, TOC is considered a more comprehensive measure of organic matter in water because it includes a wider range of organic compounds, including both bio/chemical degradable and nondegradable compounds. COD, on the other hand, only measures the oxygen-depleting capacity of biodegradable organic compounds.

TOC is generally considered to be more accurate than COD because it includes a wider range of organic compounds. Therefore, in terms of interferences, TOC is less subject to interference from various sources.

For this reason, TOC has generally been considered a more reliable indicator of the quality and suitability of water for various applications, including use in RO systems.

2.2.2 Biodegradability of an industrial wastewater

It is important to note that the biodegradability of industrial wastewater may vary depending on the specific contaminants present and the conditions in which the wastewater is treated. Therefore, it is important to conduct thorough testing in order to accurately determine the biodegradability of a particular industrial wastewater.

BOD5 is commonly used to measure the strength of wastewater and the effectiveness of the biological treatment processes.

There are several factors that can interfere with BOD measurements in industrial wastewater. These include:

Inorganic substances: Inorganic substances, such as sulfur, chlorine, and ammonia, can interfere with BOD measurements by reacting with the oxygen that is used in the test. This can lead to inaccurate results [1].

• pH: The pH of a sample can affect the availability of oxygen to microorganisms and can also interfere with the accuracy of BOD measurements. So, if the pH of the sample is not between 6 and 9, the pH of the sample can be adjusted to a neutral value (around pH 7) to minimize interference from changes in pH.

• Nutrient levels: The presence of nutrients, such as nitrogen and phosphorus, can affect the rate of biological activity and the accuracy of BOD measurements, where the addition of nutrients, such as nitrogen and phosphorus, can help to stimulate the growth of microorganisms and improve the accuracy of BOD measurements.

• Interference by other compounds: Other compounds, such as certain types of surfactants and detergents, may interfere with the BOD measurement by reacting with oxygen or by inhibiting the growth of microorganisms [3].

• Incomplete decomposition: If the sample is not allowed to decompose completely, the BOD measurement may be underestimated, so you may need to allow for complete decomposition to obtain accurate BOD measurements, like using BOD20.

BOD20 [4, 5] is a similar measure, but the test is conducted for 20 days rather than 5 days. This can provide a more accurate measure of biodegradability, as some substances may take longer than 5 days to break down fully. However, the BOD20 test is not as widely used as the BOD5 test, as it takes longer to conduct and requires more resources.

In general, the BOD5 test is considered sufficient for most purposes, but the BOD20 test may be used in cases where a more accurate measure of biodegradability is required or if the substance being tested is known to take longer than 5 days to break down.

There are several factors that can hinder the biological treatment of industrial wastewater. Some of these factors include:

High levels of toxins or other contaminants: If the wastewater contains high levels of toxins or other contaminants, it may be more difficult for microorganisms to break down the organic matter in the wastewater.

There are many toxins or chemical types that can hinder the biological treatment of industrial wastewater. Some examples of toxins that have been shown to have negative impacts on the biological treatment process include:

• Cellulose is a type of organic matter that is resistant to decomposition, and it can interfere with the microorganisms that are used in the BOD test to measure the amount of oxygen required to break down the organic matter in the wastewater. This can result in an underestimation of the actual BOD of the wastewater, leading to false low readings. Cellulose compounds can be degraded by biological treatment of industrial wastewater, but it may be more challenging than other organic matter types. Cellulose is a complex carbohydrate that is found in plant cell walls and is resistant to decomposition due to its complex structure and the fact that it is highly crystalline, which makes it difficult for microorganisms to access and break down. However, certain microorganisms, such as fungi and some bacteria, can break down cellulose or may need to reduce its concentrations using physical treatment methods like sedimentation, filtration, and centrifugation to remove cellulose from the source. Also, chemical treatment methods involve using chemicals to break down the cellulose, like the use of enzymes or chemicals like sodium hydroxide or hydrochloric acid [6, 7].

• Heavy metals: Heavy metals such as lead, mercury, and cadmium can be toxic to the microorganisms responsible for breaking down organic matter in wastewater. For example, a study published in the Journal of Environmental Management found that high concentrations of lead and mercury inhibited the degradation of organic matter in wastewater treatment systems [8].

• Pesticides: Pesticides such as organophosphates and carbamates can be toxic to the microorganisms responsible for breaking down organic matter in wastewater. For example, a study published in the journal Water Research found that pesticides in wastewater significantly reduced the rate of organic matter degradation [9].

• Polycyclic aromatic hydrocarbons (PAHs): PAHs are a group of chemicals formed during the incomplete burning of organic matter. They are toxic to the microorganisms responsible for breaking down organic matter in wastewater. For example, a study published in the journal Environmental Science and Technology found that high concentrations of PAHs inhibited the degradation of organic matter in wastewater treatment systems [10].

• Endocrine disrupting chemicals (EDCs): EDCs can interfere with the normal functioning of the endocrine system. They are toxic to the microorganisms responsible for breaking down organic matter in wastewater. For example, a study published in the journal Environmental Science and Technology found that EDCs in wastewater significantly reduced the rate of organic matter degradation [11].

It is important to note that these are just a few examples of toxins that can hinder the biological treatment of industrial wastewater, and there are many other types of toxins that can have similar impacts.

Lack of nutrients: Some industrial wastewater may be deficient in essential nutrients, such as nitrogen and phosphorus, which are required for the growth and activity of microorganisms.

Overall, the success of a biological treatment process for industrial wastewater will depend on various factors, including the specific contaminants present in the wastewater and the conditions in which the treatment is carried out.

2.3 Oil and grease

Oil and grease can be difficult to remove completely from industrial wastewater before the RO membranes system. It is necessary to effectively remove oil and grease from wastewater before it is treated using RO membranes to minimize their adverse impacts, where oil and grease can attach and accumulate on the surface of the RO membrane causing severe and may lead to irreversible organic fouling. RO Membrane manufacturers recommended that oil and grease concentrations should be less than 0.1 mg/l. Several techniques can be used to remove oil and grease from industrial wastewater, depending on the concentration of these contaminants. It may be necessary to use several successive methods to achieve the required quality and also according to the extent of the tendency and types of other pollutants associated with this industrial wastewater and the possibility of separating the sources of oils to treat them alone or not, especially if the concentration is high.

Gravity separation (Skimming): This involves using a floating device to physically remove oil and grease from the surface of the wastewater, where allowing the wastewater to sit in a settling tank, where the oil and grease float and can be separated by a skimmer [12].

Coagulation and flocculation: In this method, chemicals are added to the wastewater to cause the oil and grease to clump together, forming larger particles that can be more easily removed [13].

One of the important technologies that provide effective removal of many wastewaters contaminates is dissolved air flotation (DAF) is a wastewater treatment process that uses coagulation/flocculation combined with dissolved air to create tiny air bubbles that can attach to contaminants in the water, causing them to float to the surface and heavy suspended solids can sink down and then removed by a scrubber [14]. DAF is often used to remove oil and grease from industrial wastewater, as well as other suspended solids and some types of organic matter [15]. Here are a few reasons why DAF systems may be particularly important in the treatment of oil and grease in industrial wastewater:

• High removal efficiency: DAF systems are generally able to achieve high removal efficiencies for oil and grease, often in the range of 95–99%. This makes them an effective option for reducing the amount of oil and grease in wastewater to levels that meet discharge standards.

• Ability to treat a wide range of wastewater flows: DAF systems can be designed to treat a wide range of wastewater flows, from small to large volumes. This makes them a flexible option for treating oil and grease in industrial wastewater.

• Simple operation and maintenance: DAF systems are relatively simple to operate and maintain, which can reduce the overall cost of treatment.

However, DAF systems have some limitations that can affect their ability to effectively remove oil and grease from industrial wastewater. Some of these limitations include:

• High oil and grease levels: DAF systems are generally most effective at removing relatively low levels of oil and grease from wastewater. At higher concentrations, the oil and grease may not be fully removed, or the DAF system may need a pretreatment for high oil and grease concentration using API, or cyclones techniques.

• Emulsified oil and grease: As mentioned in a previous answer, emulsified oil and grease can be more difficult to remove from wastewater than nonemulsified oil and grease. DAF systems may be less effective at removing emulsified oil and grease.

• The pH of the wastewater: The effectiveness of DAF systems can be affected by the pH of the wastewater. At high pH values (above 9), the air bubbles may not dissolve as effectively, which can reduce the overall effectiveness of the DAF system. At low pH values (below 6), the air bubbles may dissolve too quickly, which can also reduce the effectiveness of the DAF system.

Biological treatment: Certain types of bacteria can break down oil and grease into simpler, water-soluble compounds. This process can be done in a specialized bioreactor or as part of a broader wastewater treatment process. High concentrations of oil and grease can also interfere with the biological processes used to treat the wastewater, which can reduce the overall performance of the treatment plant [16].

Absorption: This involves using a solid material, such as clay or a synthetic polymer, to absorb the oil and grease from the wastewater.

Carbon filters can be used to treat industrial wastewater to remove residual traces of oil and grease, and other contaminants such as refractory organic compounds. Carbon filters work by adsorbing contaminants onto the surface of the carbon, then after accumulating on carbon media it can then be removed by backwashing and rinsing or either regenerating or replacing the carbon media. Carbon filters are typically used as a final step in the treatment of industrial wastewater to remove any remaining contaminants that other treatment methods may not have effectively removed or to reach out to a high-quality effluent. In addition, they are often used in combination with other treatment technologies, such as physical separation, chemical treatment, and biological treatment, to provide a high level of contaminant removal [17].

One of the main advantages of using carbon filters for oil and grease removal is their ability to effectively remove a wide range of contaminants. Carbon filters can also be effective at removing contaminants that have a low solubility in water, which can make them difficult to remove using other methods. However, carbon filters have some limitations that should be considered when using them for oil and grease removal. For example, carbon filters may become saturated with contaminants over time, which can reduce their effectiveness and increase the frequency of filter changes.

Membrane filtration: This method involves passing the wastewater through a membrane with small pores that can remove oil and grease by size exclusion. But also, it is important to effectively remove oil and grease from wastewater before it is treated using membranes like ultrafiltration (UF) to minimize these negative impacts, and it is considered a final removal step of only residual concentrations [18].

It is important to carefully consider the type and volume of oil and grease in the wastewater and the desired treatment level when selecting a treatment method.

One of the important contaminants that hurt the removal efficiency of oil and grease from industrial wastewater is the presence of emulsified agents in industrial wastewater which can make the removal of oil and grease more challenging and reduce the efficiency of treatment processes. Emulsified agents can cause oil and water to mix and form an emulsion, a stable mixture of oil droplets suspended in water [19].

Some common emulsified agents that may be present in industrial wastewater include surfactants, soaps, and detergents. These agents can interfere with the ability of physical separation methods, such as skimming, floating, and sedimentation, to effectively remove oil and grease from the wastewater. They can also make it more difficult to use chemical methods, such as coagulation and flocculation, to remove oil and grease by forming stable emulsions that are resistant to flocculation.

Biological methods, such as biodegradation, can also be affected by the presence of emulsified agents. Some microorganisms may be able to break down the emulsified agents, but this can also consume some of the oxygen in the wastewater, which can be detrimental to the overall treatment process.

Membrane filtration may be less affected by the presence of emulsified agents, as it relies on size exclusion rather than chemical or biological processes to remove contaminants. However, the efficiency of membrane filtration can still be reduced if the emulsified agents coat the membrane or if they form stable emulsions that are too small to be effectively removed by the membrane.

There are some considerations that should be taken in the design and operations of the oil and grease removal process, and the following are the most common two of them;

• De-emulsification is the process of breaking down an emulsion, or a mixture of two immiscible liquids, such as oil and water, into their components. In wastewater treatment, de-emulsification is often used to separate oil and grease from the water so that the water can be treated and returned. The chemical methods for de-emulsifying oil and grease in wastewater include mechanical separation and chemical separation;

• Mechanical separation involves physically separating the oil and grease from the water. This can be done using devices such as centrifuges.

• Chemical separation involves adding chemicals to the wastewater to help break down the emulsion. Common chemicals used for this purpose include surfactants, molecules that can help destabilize the emulsion, and emulsion breakers, encouraging the oil and water to separate.

• Saponification can occur in industrial wastewater treatment as part of the process of removing oil and grease from the water. The saponification process involves reacting an alkali, such as sodium hydroxide, with a fat or oil to produce soap. This can lead to solubilizing the oil and grease, making it difficult to separate from the water. Several factors can affect the occurrence of saponification in wastewater treatment, including the pH of the wastewater and the type of oil and grease present, where some oils and greases are more easily saponified than others due to their chemical composition. For example, animal fats and vegetable oils are more easily saponified than mineral oils. In general, higher pH values and the presence of more easily saponifiable oils and greases will tend to promote the saponification process.

2.4 Heavy metals occurrence and treatment

Heavy metals are naturally occurring elements with a high atomic weight and density at least five times greater than that of water. They are commonly found in industrial wastewater, and their presence can negatively impact the environment and human health. Also, have a negative impact on the performance of RO membranes, which are commonly used to treat and reuse industrial wastewater. When present in high concentrations, heavy metals can foul RO membranes. Fouling of RO membranes by heavy metals can occur through a variety of mechanisms, including adsorption of the metal ions onto the membrane surface, precipitation of the metal ions within the membrane pores, and the formation of metal hydroxides or metal oxides or insoluble metal compounds on the membrane surface. In addition to fouling, heavy metals in the feed water can also lead to toxic disinfection byproducts, which can occur when the heavy metals react with disinfectants (such as chlorine) used to pretreat the wastewater, resulting in the formation of harmful compounds.

Heavy metals are commonly found in industrial wastewater due to their use in various industrial processes. Some examples of heavy metals that may be present in industrial wastewater include:

• Lead: Lead is often used to produce batteries, pigments, and metal alloys. It can be found in industrial wastewater due to its use in these processes and from the corrosion of lead pipes.

• Mercury: Mercury is used to producing chemicals, pesticides, and pharmaceuticals. It can also be found in industrial wastewater due to its use in the extraction of gold and silver from ore.

• Chromium: Chromium is used in the production of stainless steel, leather tanning, and wood preserving. It can be found in industrial wastewater due to its use in these processes.

• Cadmium: Cadmium is used in the production of batteries, pigments, and coatings. It can be found in industrial wastewater due to its use in these processes and from the corrosion of cadmium pipes.

• Arsenic: Arsenic is used in the production of pesticides and herbicides. It can also be found in industrial wastewater due to its presence as a natural contaminant in some ore deposits.

It is important to properly treat industrial wastewater containing heavy metals before it is processed using RO membranes (mostly should be less than 0.05 mg/l) to minimize the negative impact on the membrane’s performance [20]. This may involve using physical, chemical, or biological treatment methods to remove or neutralize the heavy metals in the wastewater.

There are several important considerations to take into account when removing heavy metals from industrial wastewater:

• Type and concentration of heavy metals: The specific type and concentration of heavy metals present in the wastewater will determine the most appropriate treatment method. Some treatment methods are more effective at removing certain heavy metals than others, so it is important to carefully analyze the wastewater to determine the most suitable approach.

• Environmental impact: Disposal of heavy metal-containing waste, it is important to properly dispose of any waste generated during the treatment process that contains heavy metals. This may involve safely storing the waste in a secure location or properly disposing of it through a licensed waste management facility. Also, treatment methods may have negative impacts on the environment, such as by producing harmful byproducts or consuming large amounts of energy.

• Cost and feasibility: The cost and feasibility of the treatment method should be considered when selecting the most appropriate approach. Some treatment methods may be more expensive or logistically challenging to implement, so it is important to carefully weigh the costs and benefits of each option.

• Effectiveness: The effectiveness of the treatment method in removing the heavy metals from the wastewater should be a top consideration. It is important to select a treatment method that can effectively reduce the concentration of heavy metals in wastewater to acceptable levels.

• Health and safety: The health and safety of workers involved in the treatment process should be a top priority. It is important to ensure appropriate safety measures are in place to protect workers from exposure to hazardous materials.

There are a variety of treatments that can be used to remove heavy metals from water and other materials. These include physical, chemical, and biological treatments.

One example of a physical treatment for heavy metal removal is sedimentation, in which the heavy metals are separated from the wastewater by settling. Heavy metals can precipitate from wastewater by raising the pH to a level above their respective precipitation pH. Precipitation pH is the pH at which a particular metal ion will begin to precipitate out of the solution as a solid. Different metal ions have different precipitation pH values, so the specific pH required to precipitate a particular metal will depend on the type of metal. For example, if the wastewater contains lead ions and the pH is raised to 9.5, the lead ions may begin to precipitate out of the solution as a solid. It is important to note that simply raising the pH of the wastewater may not be sufficient to completely remove all heavy metals. Other treatment methods, such as chemical precipitation or ion exchange, may be necessary to effectively remove the heavy metals from the wastewater [21, 22].

Chemical treatments for heavy metal removal include the use of chelating agents, which can bind to the heavy metals and allow them to be removed from the water.

Biological treatments for heavy metal removal include the use of bacteria that can absorb and remove heavy metals from the water.

Once the heavy metals have been removed from the water, they can often be recovered and reused. This can be done through metal recovery, which involves separating the heavy metals from the material they were removed from and purifying them for reuse. By using this treatment process, the factory can reduce its environmental impact and recover valuable resources for reuse [23].

Several methods can be used to remove heavy metals from industrial wastewater. Here are some examples of treatment methods that can be used to remove specific heavy metals from wastewater:

• Lead: Chemical precipitation, ion exchange, and electrocoagulation are all effective methods for removing lead from industrial wastewater.

• Mercury: Activated carbon adsorption and chemical precipitation effectively remove mercury from industrial wastewater.

• Chromium: Chemical precipitation, ion exchange, and electrocoagulation are all effective methods for removing chromium from industrial wastewater.

• Cadmium: Chemical precipitation, ion exchange, and electrocoagulation are all effective methods for removing cadmium from industrial wastewater.

• Arsenic: Chemical precipitation, ion exchange, and coagulation/flocculation are all effective methods for removing arsenic from industrial wastewater.

2.5 Hardness removal

Hardness in industrial wastewater is often caused by high concentrations of calcium, magnesium, carbonate, and sulfate ions, which can come from various industrial processes. These ions can cause various problems, including scale formation in pipes and equipment, and can interfere with the effectiveness of wastewater treatment and reuse in certain industrial processes. Softening industrial wastewater before it is treated with RO can help to increase the operated recovery of the RO unit with less dosage of antiscalant, so increasing the wastewater reused.

Hardness in industrial wastewater can come from a variety of sources, including:

• Industrial processes: Many industrial processes, such as power generation, oil and gas production, and metal finishing, can produce wastewater with high levels of hardness.

• Cooling water: Water used for cooling industrial equipment, such as cooling towers blowdown, can become hard due to the accumulation of calcium and magnesium ions depending on the cycle of concentration (COC).

• Boiler water: Boiler blowdown involves the removal of hard water from a boiler to maintain specific parameters within certain limits and prevent issues such as corrosion, scale, and carryover. It is also used to remove suspended solids that may be present in the system.

• Groundwater: In some cases, the source of raw feed water used in industrial processes may have a naturally high hardness level, like the groundwater.

Several methods can be used to remove hardness from industrial wastewater, including:

• Chemical treatment: This involves adding chemicals such as lime or soda ash to the wastewater, which can react with the calcium and magnesium ions to form a precipitate that can be separated from the water.

• Ion exchange: This involves passing the wastewater through a bed of resin beads that are charged with sodium ions. The calcium and magnesium ions in the water are attracted to the resin beads and exchange places with the sodium ions, leaving the water with a lower hardness level.

• Nanofiltration: This filtration process uses a membrane to remove ions and other contaminants from the water. It effectively removes hardness but requires a comprehensive pretreatment like RO membranes.

• Electrodialysis: This process uses an electric current to separate ions in the water based on their charge. It effectively removes hardness but is expensive and requires specialized equipment.

It is important to choose the most appropriate method for removing hardness from industrial wastewater based on the specific needs and constraints of the application.

Some important considerations that should be taken into account when deciding lime soda softening and caustic soda softening for the treatment of industrial wastewater:

Cooling tower blowdown water contains a dispersant and antiscalant that hindered or disturb the coagulation-precipitation process. Where dispersants are chemicals added to the cooling water to prevent the formation of scale and the precipitation of solids, they work by inhibiting the aggregation of particles and keeping them suspended in the water. This can make it difficult or impossible to remove contaminants through coagulation-precipitation. Also, antiscalants are chemicals added to the cooling water to prevent scale formation on surfaces. They work by inhibiting the precipitation of minerals such as calcium and magnesium. However, these chemicals can interfere with the coagulation-precipitation process by preventing the formation of the necessary flocs or aggregates of particles that are necessary for the process to be effective. So it is important to degrade this chemical and inhibit its functions by using a strong oxidizing agent like chlorine with a sufficient concentration and contact time to effectively inactivate those chemicals before the coagulation-precipitation process.

Ferric chloride and alum is a commonly used coagulants in the treatment of industrial wastewater. However, there is a negative impact that should be considered when using ferric chloride as a coagulant before the RO membrane system especially at softening clarifiers where pH is high enough to dissolve part of those minerals: membrane fouling: where ferric chloride can cause fouling of the RO membrane, which can reduce its efficiency and require more frequent cleaning.

It is important to consider these negative impacts carefully when deciding whether to use ferric chloride or alum as a coagulant before an RO system.

In the chemical softening process of wastewater treatment, the pH of the water is typically raised during the softening process and then lowered during the neutralization step. The alkalinity of the wastewater can play a role in the amount of chemicals (lime soda or caustic soda) required to raise the pH to the desired level for softening. Alkalinity is a measure of the water’s ability to neutralize acids and is typically expressed in terms of the concentration of bicarbonate, carbonate, and hydroxide ions in the water.

If the alkalinity of the wastewater is high, it may take more chemicals to raise the pH to the desired level for softening. This is because the water’s high alkalinity indicates the presence of a large amount of bicarbonate, carbonate, and hydroxide ions, which can neutralize the alkaline chemical added to raise the pH. As a result, more alkaline chemicals may be required to overcome the buffering effect of the alkalinity and achieve the desired pH.

On the other hand, if the alkalinity of the wastewater is low, it may take less chemicals to raise the pH to the desired level for softening. In this case, fewer bicarbonate, carbonate, and hydroxide ions are present to neutralize the alkaline chemical, so fewer chemicals are required to achieve the desired pH.

After the softening process, the pH of the wastewater is typically lowered during the neutralization step. To neutralize the excess lime or caustic soda, an acid such as sulfuric acid or hydrochloric acid is added to the water. The acid reacts to effectively neutralize the pH of the water. It is important to carefully control the amount of acid added to the water, as adding too much acid can result in a pH that is too low, which can have negative effects on the environment or downstream processes.

The buffering effect in wastewater is a result of the presence of ions that can neutralize acids or bases, preventing significant changes in pH. High concentrations of ions such as ammonia/nitrate or bicarbonate can contribute to the buffering effect in wastewater.

Ammonia (NH₃) and nitrate (NO3-) ions can act as weak bases in water, neutralizing acids and helping to maintain a relatively stable pH. Bicarbonate (HCO3-) ions can also act as a buffer in water, neutralizing both acids and bases and helping to maintain a relatively stable pH.

The buffering capacity of wastewater can have an impact on the effectiveness of pH adjustment or neutralization processes. If the wastewater has a high buffering capacity, it may take more acid or alkaline chemicals to achieve the desired pH change. Conversely, if the wastewater has a low buffering capacity, it may take less acid or alkaline chemical to achieve the desired pH change.

It is important to carefully monitor the pH and buffering capacity of wastewater during treatment processes and adjust the chemical dosage as needed to effectively adjust the pH or neutralize the water.

The addition of lime or caustic soda to wastewater during the softening process will typically result in an increase in the total dissolved solids (TDS) of the water. This is because the lime or caustic soda reacts with the hardness-causing ions in the water to form solid precipitates, which contribute to the TDS of the water. The neutralization step, in which acid is added to the water to neutralize the excess lime or caustic soda, will not typically result in a significant change in the TDS of the water.

It is important to note that the TDS of the water can also be affected by other factors, such as the presence of other dissolved solids in the water, the volume of water treated, and the efficiency of any downstream treatment processes. In general, it is desirable to keep the TDS of wastewater as low as possible, as high TDS can have negative effects on the environment and on any downstream processes.

2.6 Total dissolved solids

It is important to accurately determine the final TDS and other component ions of pretreated wastewater before it is treated with a reverse osmosis (RO) system. This information is critical for properly designing the RO unit and optimizing its performance.

During the pretreatment process, various chemicals may be added to the wastewater to remove contaminants or adjust the water’s properties. For example, chemical precipitation or softening may be used to remove hardness-causing ions, and pH neutralization may be used to adjust the pH of the water. However, these processes can increase the TDS of the wastewater, as the chemicals added can contribute to the dissolved solids content of the water.

Accurately measuring the TDS and other component ions of the pretreated wastewater is important because it allows the RO system to be properly sized and configured to meet the specific treatment needs of the water. It also helps to ensure that the RO system is operating at optimal efficiency and can effectively remove contaminants from the water.

It is generally recommended to measure the TDS and other component ions of the wastewater at various points throughout the treatment process, to gain a comprehensive understanding of the water’s quality and to make any necessary adjustments to the treatment process.

Also, it is important to consider the potential for changes in the characteristics of the wastewater during the design and operation of a reverse osmosis (RO) system. For example, in situations where different streams of wastewater are combined and neutralized, there is a risk that changes in the flow or concentration of one of the streams could affect the final TDS and other ion concentrations of the RO feed water.

To address this risk, it is important to carefully design the RO system to be able to handle the worst-case scenario. This may involve selecting RO membranes that are resistant to fouling and able to handle a wide range of water qualities, selecting a high-pressure pump that is capable of operating effectively under varying conditions, and specifying piping materials that are compatible with the wastewater being treated.

By designing the RO system to be able to handle the worst-case scenario, it is possible to ensure that the system is sustainable and durable and able to effectively treat the wastewater even if there are changes in the flow or concentration of the different streams. It is also important to regularly monitor the water quality and adjust the treatment process as needed to maintain the efficiency and effectiveness of the RO system.

3. Conclusions

Industrial wastewater treatment and reuse are gaining high importance in many parts of the world. This chapter gives an overview of the considerations involved in the treatment and reuse of industrial wastewater, with a focus on the pretreatment of wastewater before the reverse osmosis (RO) treatment. It highlights the various factors that have contributed to the growing importance of industrial wastewater treatment and reuse, including environmental regulations, limited water resources, cost savings, and sustainability. The conclusion also describes the various pretreatment technologies and techniques that can be used to prepare industrial wastewater for RO treatment, such as chemical precipitation, softening, pH adjustment, oil and grease removal, biological processes, and filtration. It emphasizes the importance of properly designing and operating the pretreatment system, as well as accurately measuring the TDS and other component ions of the wastewater, to ensure the efficiency and effectiveness of the RO system. Finally, the conclusion notes that there is no one-size-fits-all treatment scheme for industrial wastewater and that the specific pretreatment steps required will depend on the characteristics of the wastewater and the specific requirements of the RO system being used.