Water Reuse and Wastewater Recycling

Water that has already been used and sullied can still be recycled and used, even for drinking. This process is commonly known as water recycling, water reuse, or water reclamation. Water reuse or wastewater recycling can be done on a variety of sources. The resulting recycled water could be used for various purposes such as agriculture and irrigation, potable water supplies, groundwater replenishment, industrial processes, and environmental restoration. Treatment methods utilized in water reuse would vary depending on the source and the intended purpose of the resulting recycled water. Water reuse and wastewater recycling can provide alternatives to existing water supplies and be used to enhance water security, sustainability, and resilience.

Types of Water Reuse

Municipal wastewater, industrial process and cooling water, stormwater, agricultural runoff and return flows, and produced water from natural resource extraction activities are all potential sources of water for reuse. These sources of water would undergo treatment processes that are designed to improve their quality so that they would meet certain specifications. These specifications are dependent on what the resulting treated water would be used for. For instance, whatever source water it may be, water for crop irrigation needs to meet the following criteria; is of sufficient quantity, does not harm the plants or soils, does not make the subsequent crop unsafe for consumption, and does not harm the farmworkers or whoever may handle the irrigation water. If the intent of the water reuse would result in greater human exposure to the treated water, like recycling water for drinking, the source water needs to be treated thoroughly to remove any chemicals and microbes to prevent diseases or cause any negative health effects. In essence, recycling water for drinking is more intensive because the treated water affects human health directly.

Uses for Recycled Water

Besides reusing water for drinking, there are other things that recycled water can be used for:

• Irrigation for agriculture

• Irrigation for landscaping such as parks, rights-of-ways, and golf courses

• Municipal water supply

• Process water for power plants, refineries, mills, and factories

• Indoor uses such as toilet flushing

• Dust control or surface cleaning of roads, construction sites, and other trafficked areas

• Concrete mixing and other construction processes

• Supplying artificial lakes and inland or coastal aquifers

• Environmental restoration

Alternate Water Sources, End Uses, and Implementation Scale

There are several alternate water sources. Facilities and projects that aim to reuse these water sources can range from portable reuse projects to water reclamation facilities that could conduct large-scale recycling.

Alternate Water Sources

Urban areas which contain buildings, both commercial and residential, are capable of generating several types of alternate water sources. These are some of the most common types of alternate water sources:

• Roof runoff is an alternate water source that collects in roof surfaces that are not frequently accessed by the public. The water that collects in roof surfaces comes from rain or snowmelt events.

• Stormwater is large amounts of water that flows over and or collects on impervious surfaces such as streets, parking lots, and rooftops during storms.

• Domestic wastewater or blackwater are used water or wastewater that comes from toilets and/or kitchen sinks and dishwashers.

• Graywater is wastewater collected from non-blackwater sources such as bathroom sinks, showers, bathtubs, clothes washers, and laundry sinks.

If additional alternate water sources are unveiled, authorities can with proper regulations recycle them for whatever purposes that they may serve. However, emphasis on proper regulations should be upheld. This means that the authorities should implement directives that prevent the exposure of humans to chemical, microbial, and whatever hazards recycled water may possess.

Non-potable End Uses

Alternate water sources are not just for drinking, water sources can be treated so that they can be used for a variety of non-potable uses. The most common indoor use of non-potable reuse water is in toilets and urinal flushing. This could significantly reduce the amount of water used in them by representing approximately 25 percent of the total water demand in a residential building and up to 75 percent of the total water demand in a commercial building. Other non-potable end uses of recycled water include irrigation, cooling or heating applications, process water, and clothes washers. Promoting the use of non-potable water wherever appropriate could reduce the cost associated with water treatment and conserve water. Estimates have shown that additional applications of non-potable water would increase their demand by up to 50 percent for residential buildings and up to 95 percent for commercial buildings.

Water Treatment Standards for Alternate Water Sources

The use of recycled water has its associated health risk. To address this, standards for treating alternate water sources were put into place. These standards seek to have the alternate water sources meet or exceed what is known as Log Reduction Targets. Log Reduction Targets are defined as “a measure of how thoroughly a decontamination process reduces the concentration of a contaminant.” The Log Reduction Targets were established by taking into account the tolerable levels of risk and the concentrations of pathogens in different source waters. The National Blue Ribbon Commission recommends the use of Log Reduction Targets as a stringent risk goal for protecting public health. Their goal is to bring down the annual risk of infection to less than 10-4 through the implementation of the Log Reduction Targets alone.

Older wastewater treatment parameters that focused on reducing biological oxygen demand or chemical oxygen demand, total dissolved solids, nitrogen, and phosphorus should not be phased out. They still hold value especially in determining the appropriate treatment steps for meeting the Log Reduction Targets and for performance monitoring.

Contaminants present in Potable Reuse Water Sources

Potable reuse or recycling water for drinking is a process that must meet the criteria set by several standards such as those set by the Clean Water Act and Safe Drinking Water Act. To effectively meet these standards, the constituents or the pollutants that may be present in the source water must be considered.

Pathogenic microorganisms

There are several microorganisms that are present in nature and in water. Most of them do not cause any harm. However, a large number of pathogenic microorganisms including bacteria, protozoa, helminths, and viruses are usually present in wastewater or in the source water that is to undergo recycling. Exposure to water that possesses these pathogenic microorganisms could lead to the introduction of the said microorganisms into the human body which could lead to negative health effects. The most common route of infection when it comes to domestic water is the fecal-oral route. This route of infection involves the contamination of the water with fecal material which acts as the primary source of pathogens. The pathogens are transmitted from the fecal matter and into the water. The pathogens remain viable while they are in the water and exposure to this contaminated water leads to an infection caused by the pathogen. Exposure to the contaminated water could come in various ways such as ingestion of the contaminated water or inhalation of aerosolized water containing suspended pathogenic microorganisms. There are several factors that affect how long pathogens can remain in the water. These factors include the distance of travel, rate of transport, temperature, exposure to sunlight, water chemistry, and predation by other organisms. Exposure to pathogens usually leads to the acute presentation of their diseases. In other words, exposure to pathogens associated with potable reuse scenarios usually leads to the appearance of symptoms in a matter of hours or days.

Chemicals

Chemicals can get into water from several sources including atmospheric contact, geology, natural products, pesticides, runoff, and discharges from industrial facilities. The chemicals present in water would vary depending on the activities that occur near or at the wastewater source. Pharmaceutical-associated chemicals could wind up in domestic water when their byproducts are excreted as part of human waste. Aside from that, pharmaceuticals are sometimes flushed down the toilet. Wastewater could also become contaminated with pesticides and other agricultural chemicals which are carried towards them from stormwater runoff. Chemicals that are often found in wastewater include metals, salts, nutrients, chemicals present in liquid detergents, and oils.

Inorganic chemicals

Water sources can get contaminated with inorganic chemicals when minerals from the soil and rocks are dissolved in the water. The presence of minerals in the water can make them hard which could necessitate the usage of salt-based water softeners. These would introduce more inorganic chemicals into the water. Additionally, commercial and industrial discharges and chemicals used in water treatment would also introduce inorganic chemicals to the water sources.

Organic chemicals

Natural organic matter, soluble microbial products, fecal matter, kitchen wastes, liquid detergents, oils, grease, consumer products, and low concentrations of an extensive range of organic chemicals from industrial and domestic sources are organic chemicals that can contaminate water. These chemicals can originate from pharmaceutical and personal care products, pesticides, preservatives, surfactants, flame retardants, disinfection byproducts and more.

Trace chemical constituents

Trace chemical constituents can either be inorganic or organic chemicals that can have a negative effect on the human body even at small doses. They are often described further based on their specific health effects. For instance, trace chemicals that can disrupt our endocrine system are called endocrine-disrupting chemicals. Pharmaceuticals are a subgroup of trace chemicals that have been detected in surface waters since the 1970s. Per- and polyfluoroakyl substances are a group of trace chemical constituents that are well studied. In the 1990s steroid hormones have been detected in surface waters.

Treatment Technologies

Advanced wastewater treatment facilities employ individual unit processes that are assembled in a range of combinations to achieve water quality appropriate for potable reuse. They are responsible for further purifying treated effluent from conventional wastewater treatment facilities. To fully take advantage of these individual units, they must be thoroughly understood.

Technologies for the removal of Suspended Solids

Conventional wastewater treatment facilities rarely remove all suspended solids. Bacteria, viruses, protozoan cysts, and oocysts are suspended solids that could remain in the effluent of conventional wastewater treatment facilities. Exposure to this contaminated water, without the reduction of the contaminants to acceptable levels, can lead to negative health effects. Even if the suspended solids are not associated with any negative health effects, they can render the subsequent treatment processes ineffective. This is especially true for reverse osmosis and treatment units.

Current technologies, microfiltration and ultrafiltration membranes are capable of removing large pathogens such as protozoan cysts. To effectively remove smaller pathogenic microorganisms such as viruses and bacteria, membranes with smaller pore sizes such as nanofiltration or reverse osmosis membranes are required. To improve the effectiveness of membrane filtration, it is better to have them in stages where each succeeding stage has a smaller pore size than the previous one.

Media Filtration

Water can be filtered by forcing them to go through a media that has porous beds either through gravity or pressure differentials. This process is what is known as particle filtration. Particle filtration units serve two purposes when they are utilized in a treatment train. First, they remove solids including microbial agents and pathogens associated with particulates, colloids, or organics. Second, they remove particles so that the effluent would be easier to disinfect and the efficacy of subsequent treatment processes is improved. Using water filtration membranes can reduce organics and other forms of particulates which could reduce the demand for chemical oxidants in subsequent disinfection processes. High turbidity can reduce the efficacy of UV disinfection by shielding the target pathogens from UV light. By reducing water turbidity, filtration membranes can increase the effectiveness of UV light disinfection. Physical absorption and size exclusion are the two general mechanisms for particle removal through medial filtration. Physical absorption is a mechanism whereby smaller particles and pathogens are adsorbed to the surface of large particles that are present in the filter media. Since the smaller particles and pathogens are adsorbed by the filter media, they are subsequently removed from the water. Size exclusion is a mechanism whereby suspended solids that are larger than the pores in the filter media are physically excluded at the media surface.

Microfiltration and Ultrafiltration

Microfiltration and ultrafiltration, collectively known as microporous membrane filtration, were used for large-scale municipal water treatment at first. Later on, they were adapted for use in wastewater treatment facilities. Membrane filtration technologies can be used as pretreatment to Reverse Osmosis to prevent fouling or clogging of the Reverse Osmosis membrane.

The microporous membrane is the most important part of the membrane filtration system. It is a thin porous polymer film or a ceramic structure that acts as a selective barrier that prevents unwanted materials from passing through. Some membrane filters are polymeric and are usually made from one of the following materials:

• Polypropylene.

• Polyvinylidene fluoride.

• Polysulfone and polyethersulfone.

Polymeric membranes have a chance of breaking which can allow the contaminants that are supposed to be eliminated to pass through. Ceramic membranes are more durable than polymeric membranes. However, they are significantly more expensive than polymeric membranes.

Size exclusion is a membrane filters’ mechanism for removing contaminants. In working order, they are capable of completely removing all contaminants greater than their nominal pore size. However, since they work this way, anything smaller than their pore size gets through and that includes dissolved solids. Although membrane filter’s mechanisms are similar to media filtration, factors such as flow rate, feed water quality, or filtration run-length affect their ability to remove contaminants. Anything that is dissolved in water can be removed by membrane filtration by increasing its size. For instance, dissolved organic material can be coagulated or adsorbed by powdered activated carbon. Afterwhich, they can be removed by membrane filtration. Most facilities that utilize membrane filtration do so without using coagulants or powdered activated carbon which means that dissolved organic constituents can pass through.

Microfiltration membranes have a nominal pore size between 0.1 and 0.2 micrometres. This membrane blocks suspended solids, protozoa, bacteria and larger viruses from going through into the filtrate. Ultrafiltration membranes have a nominal pore size between 0.02 and 0.08 micrometres which are notably smaller than the pores in microfiltration membranes. Since that is the case, they are more effective in removing viruses. Microfiltration and ultrafiltration membranes are not able to remove dissolved organic matter and do not have any measurable impact on total dissolved solids.

The operation of microfiltration and ultrafiltration systems leads to the reduction of their efficacy and fouling. To address this issue, these systems contain a backwash system and a chemical clean-in-place system. This cleaning system maintains the filtration systems’ efficiency and reverses membrane fouling. The clean-in-place systems of these filtration systems generally clean the filtration membrane once a month. However, some facilities clean their filtration systems with chemicals more than once a month. The frequency of clean-in-place periods is determined by the organic content, microbial presence, and coagulant doses of the source water. If these are high, the filtration system needs to have an increased frequency of clean-in-place periods. To clean the filtration membrane, clean-in-place systems utilize chemicals such as acids and sodium hypochlorite. Other systems utilize proprietary detergents.

Water is usually pretreated before they undergo membrane filtration. This is done to prevent fouling and membrane damage. This does not enhance the filtration process but it maintains the membrane’s efficiency and improves its longevity. Filtration membranes are not designed to block large particulates because they could damage the membrane or plug them up. Automatic strainers with large pores between 100 and 500 would remove these large particles to protect the filtration membrane. Some systems do chlorination of the feed water to prevent biofouling but this is not necessary for filtration systems with low filtration rates.

To remove contaminants from water through filtration a bit of force needs to be applied to get the water to pass through the membrane. Submerged and pressurized are the two configurations for pushing the water through the filtration membrane. In submerged systems, the membrane is suspended in a basin and the feed water is forced through the membrane by adding a vacuum pressure on the membrane’s filtrate side. Pressurized systems use pumps to apply a transmembrane pressure to the feed but the filtrate is at roughly atmospheric pressure.

Technologies for Reducing the Concentration of Dissolved Chemicals

Some pollutants would be completely dissolved in water forming a homogeneous solution. Filtration membranes are not capable of purifying homogeneous solutions. To remove pollutants that have completely dissolved in water, unit processes such as Reverse Osmosis, Electrodialysis, Electrodialysis Reversal, Nanofiltration, Granular Activated Carbon, Ion Exchange, and Biologically Active Filtration are needed.

Reverse Osmosis

Reverse osmosis is similar to microfiltration and ultrafiltration systems in that feed water is forced through a porous material. In the case of reverse osmosis, the semipermeable membrane is the porous material and the water is forced through this membrane using the pressure gradient to separate the pollutants from the water. In the reverse osmosis system, pressure needs to be applied to the feed water so that the osmotic pressure difference between the feedwater and the permeate can be overcome. This is necessary to drive the water through the semipermeable membrane leaving the pollutants in the feed water. Reverse osmosis systems are usually utilized for the removal of salts and minerals from seawater and brackish groundwater. Potable reuse systems could also include reverse osmosis units to effectively remove dissolved chemical substances, total organic carbon, trace organic compounds, total dissolved solids, and pathogens from water.

Most reverse osmosis membrane designs include a thin film composite, a thick support structure, and a thin membrane skin. There are five major components of a Reverse Osmosis system:

• First, you have the high-pressure pump which is responsible for applying pressure to the feed water. This pressure is needed to overcome the osmotic force, the flow of water from the weak solution to the strong solution, so that the water is forced through the reverse osmosis membrane, leaving behind the impurities.

• Second, is the membrane modules, usually around six to eight of them arranged in series within cylindrical pressure vessels. The serial arrangement allows for multiple stages of treatment. The concentrate that comes out of the membrane of the first membrane module is treated again by the second membrane module, and so on. This allows for the maximum recovery of the product water.

• Third, is the structure of the membrane modules themselves. Each membrane module possesses multiple membrane envelopes in a spiral wound configuration. They use a feed or concentrate channel on the outside of the envelope and a permeate channel on the inside. Water passing through the semipermeable membrane to the inside of the envelope becomes the treated permeate and is collected in a central permeate tube.

• Fourth, the permeate piping which collects treated water from the permeate tubes inside the membrane modules and conveys the permeate from the reverse osmosis system.

• Fifth, the concentrate piping, which conveys the concentrated waste stream to the final disposal.

Reverse osmosis membranes work better with sufficient pretreatment. That is because organic colloids, biological growth, and inorganic scale can all impede the production of water which could entail elevated feed pressures, increased cleaning frequency, and higher operating costs. To improve the efficacy of reverse osmosis further, the pretreatment methods utilized on the feed water need to be specifically chosen depending on the water’s characteristics. Thus, before a Reverse Osmosis system is put into place, a thorough analysis of the characteristics of the water is required.

Microfiltration and ultrafiltration units were traditionally used as a pretreatment process for reverse osmosis systems. They prevent fouling which results in lower operating costs, reduced chemical cleaning, and less frequent membrane replacement. The use of pretreatment procedures is essential because when fouling does occur, it decreases the membrane system efficiency, requiring more energy to treat the water. The membranes also need to be periodically chemically clean to remove foulants. Without removing foulants, the membrane may get damaged permanently which may require membrane replacement.

Common types of membrane fouling that could be improved with pretreatment:

• Scaling - water with high amounts of calcium and magnesium or other salts that do not dissolve in water easily can cause scaling in the membrane. The feedwater can be pretreated with softeners to prevent scaling.

• Colloidal fouling - this type of fouling occurs when organic material, silica, and clarifier treatment chemicals are present in the water in high amounts. Pretreating the feed water with processes that are capable of removing the aforementioned constituents prevents colloidal fouling.

• Biological fouling - If a large number of microorganisms are present in the water they can stick to the membranes and form biofilms which can block the membrane surface. Chloramine is usually added to the feedwater to prevent biofouling or biological growth on the membranes.

Nanofiltration (NF)

Nanofiltration units are very similar to microfiltration and ultrafiltration units. They are usually made of the same materials and the same processes but their membrane has pores that are only nanometers in size. They are less selective than reverse osmosis membranes because they allow the passage of more monovalent ions. Compared to reverse osmosis units, nanofiltration requires less feed pressure which translates to reduced cost. Despite their drawbacks, nanofiltration is still capable of producing effluent water quality that is similar to reverse osmosis in instances where total dissolved solids reduction is not required.

When compared to Reverse Osmosis, Nanofiltration has two major disadvantages. First, the Nanofiltration’s membranes are less efficient in removing Total Dissolved Solids compared to the membranes of Reverse Osmosis units. Second, nanofiltration membranes are less efficient in removing nitrate.

Electrodialysis or Electrodialysis Reversal (ED or EDR)

Electrodialysis units possess ion-selective membranes that are utilized in an electrically driven process to transport mineral salts and other similar constituents from one solution to another, forming a solution with high amounts of solutes known as a concentrate, and the filtered water or the dilute solution. An electrodialysis system contains both cation, positively charged, and anion, negatively charged membranes. They are stacked and arranged in an alternate pattern between spacers with the anode on one end and the cathode on the other. To move the ions through the membranes, a direct current is applied. Due to the fact that the membrane carries a certain charge, either positive or negative, ions present in the water that have a charge opposite that of the membrane are rejected and exit the system in the form of a concentrate. Electrodialysis reversal as the name would suggest is similar to electrodialysis but utilizes periodic reversal of the direct current’s polarity. This is done as a self-cleaning mechanism. For maximum efficiency, feed water for electrodialysis reversal units needs to be pretreated so that their Total Dissolved Solids concentration would fall between 1,000 to 5,000 micrograms per litre. However, electrodialysis reversal units are still capable of treating water with Total Dissolved Solids reaching 10,000 to 12,000 micrograms per litre. Although similar, there are some differences between Electrodialysis and Electrodialysis Reversal units, and Reverse Osmosis and Nanofiltration membranes. Electrodialysis and electrodialysis reversal units are incapable of reducing suspended solids, pathogens, or non-charged contaminants of emerging concern, but they can reduce Total Dissolved Solids. One advantage of electrodialysis reversal over reverse osmosis and nanofiltration is that they are more effective in eliminating bromide. If the goal is to reuse water and make them potable, electrodialysis and electrodialysis reversal units can only be included in the treatment train if treatment units capable of removing total suspended solids, pathogens, and contaminants of emerging concern are placed ahead of them.

Ion Exchange

Ion exchange units include a solid phase ion exchange material that is used to replace an ion in the aqueous phase, which is water in the case of water reuse, for an ion that is in the solid phase. Resins are the most common solid phase used in ion exchange. These resins could contain different kinds of ions depending on the target ion that is to be removed. For instance, if the goal is to remove magnesium and calcium ions from the water, resins with sodium ions can be utilized so that the magnesium and calcium ions would be drawn into the resins and the sodium ions would be released in exchange. This is the reason why ion exchange processes are usually used for water softening. They are capable of removing magnesium and calcium ions which are the two foremost causes of water hardness. The solid phases used in ion exchange units have a fixed charge and are dependent on the functional groups attached to the material itself. If the functional groups has a positive charge, the resin is said to be cationic. On the other hand, if the functional groups have a negative charge, they are anionic. Ion exchange units are capable of removing constituents such as boron, barium, radium, arsenic, perchlorate, chromate, Na+, Cl-, SO42-, NH4+, and NO3-. Technically, ion exchange units are not essential for potable reuse applications. However, they are an important pretreatment process for nitrate removal.

Activated Carbon

A single particle of activated carbon has a surface that is highly porous that a gram of this substance has a surface area that is greater than 1000 square meters. These pores are the reason why activated carbon is highly effective in adsorbing molecules on their surface. Because of their ability to adsorb molecules in their surface, activated carbon can work really well for water treatment. When used in water treatment, activated carbon can remove organic chemicals and other chemical compounds that are hydrophobic and have a large molecular weight. However, they are not efficient in eliminating smaller-chain hydrophilic aliphatic hydrocarbons.

Biologically Active Filtration

Biologically active filtration is a practice in water treatment that involves the promotion of biological activity within the filter for the purpose of enhancing the removal of organic and inorganic constituents. Any filter can be a biofilter if they would allow a biologically active layer to establish and colonize the filter media surface. Some filter media that could work as biologically active filters include slow sand filters, rapid rate filters with or without preoxidation, Granular Activated Carbon filters with or without preoxidation, riverbank filters, aquifer filters, and anoxic biological treatment filters. What typically happens in a biologically active filter is that indigenous microorganisms would populate the biofilter, and contaminants are biodegraded through direct substrate utilization or cometabolism. Biofilters are capable of a lot of things including removal of turbidity, natural organic matter, disinfection by-product precursors, taste and odour compounds, iron, manganese, ammonia, algal toxins, and trace chemical constituents. Factors that affect the efficacy of biofilters include pH, temperature, and hydraulic loading rates.

Technologies for the Disinfection and Removal of Trace Organic Compounds

Some disinfection processes are capable of reducing pathogenic microorganism populations and eliminating chemical contaminants at the same time. Most of these disinfection processes involve oxidation. Most disinfection technologies utilized in potable reuse treatment trains are aimed at treating water so that they would be able to meet the water quality criteria for recreational water. Whenever trace organic contaminants become an issue, oxidation and advanced oxidation methods can be implemented. Technologies for the disinfection and removal of trace organic compounds include the following; Ultraviolet light, chlorination, peracetic acid disinfection, pasteurization, chlorine dioxide, ozone, and advanced oxidation processes.

UV

UV light can disinfect water by causing the microorganisms’ nucleic acids to form dimers which would effectively kill the microorganism and or prevent them from replicating. With that said, UV disinfection can be considered a biophysical disinfection method. A microorganism’s genetic material is composed of organic molecules chemically bonded together. At high enough UV doses, the UV light can break these chemical bonds which kills the microorganism.

The three most common types of UV lamps currently being used in water treatment include low-pressure low output, low-pressure high output, and medium pressure. However, among the three, the low-pressure high output lamp is more common. It has a monochromatic output at a wavelength of 254 nanometers. Between medium-pressure UV lamps and low-pressure UV lamps, the low-pressure ones consume less energy. Most wastewater discharges to surface water are treated with low-pressure high output systems because they are the type of UV system that is permitted in low-dose applications. The main disadvantage of low-pressure high-output UV systems is that they require a lot of space. Medium pressure UV lamp systems require less space but they are more energy-intensive and have a lower germicidal efficiency.

There are several factors that affect UV intensity, these include the extent of sleeve fouling, power input, the age of the lamps, and UV transmittance. Increased Total Suspended Solids may increase water turbidity which, in turn, reduce UV transmittance and UV intensity. It is important that water is pretreated to remove the suspended solids because this improves UV transmittance and intensity. UV intensity is important because it determines the UV dose. Turbidity lowers UV intensity and shields pathogenic microorganisms from UV light which may lead to inadequate disinfection. When used in direct potable reuse treatment trains, high UV doses can remove both pathogens and carcinogenic by-products such as N-nitrosodimethylamine.

Chlorine or Chloramines

Among the different forms of disinfection in water and wastewater treatment, chlorine disinfection is the most common. Chlorine that is used for water treatment comes in various forms. It could be applied as chlorine gas, liquid sodium hypochlorite, or solid calcium hypochlorite. Chlorine gas is the most cost-effective but it poses significant safety challenges regarding storage and handling.

Peracetic Acid

Using peracetic acid as a wastewater disinfectant is a relatively new method of treatment. However, they have been used in the disinfection of food, beverage, medical, and pharmaceuticals for a long time. Peracetic acid is not delivered as is, it is introduced as a mixture of equal parts acetic acid, hydrogen peroxide, peracetic acids, and water. There are several factors that can affect the performance of peracetic acid as a disinfectant; these factors include water quality and operating conditions. For instance, water pH below seven improves peracetic acid’s disinfection efficacy. The great thing about using peracetic acid as a disinfectant is that only very low doses and short contact times are required to inactivate bacteria. The widespread usage of peracetic acid in medical and agricultural industries revealed that this particular acid is effective against viruses and protozoa. Peracetic acid disinfection works well when paired with UV disinfection. Moreover, studies have shown that this chemical disinfectant is not known to produce harmful disinfection by-products.

Pasteurization

Pasteurization has just recently gained attention in the wastewater disinfection field. In sewage sludge processing, pasteurization produces dewatered and heated sewage sludge known as Class A Biosolids. There are several factors that affect the efficacy of pasteurization; these factors include temperature and exposure time, characteristics of the organisms of interest, and characteristics of the medium. Compared to disinfection methods such as UV disinfection, pasteurization processes have less operational costs because any waste heat can preheat the water that is still to be disinfected.

Chlorine Dioxide

This chemical is an excellent germicide because it has a high oxidation potential. Chlorine dioxide is commonly used in drinking water treatment. However, they’ve never been used in publicly owned treatment works for wastewater disinfection or reuse in the United States. Compared to chlorine or chloramines, chlorine dioxide is more effective in killing and inactivating bacteria and viruses. Major factors that determine the required chlorine dioxide dose for meeting disinfection objectives include pH and the amount of microorganisms in the water. One major drawback of chlorine dioxide is that they readily decompose which is why they are usually prepared before use. Another major disadvantage of chlorine dioxide is that they would generate disinfection byproducts such as chlorate and chlorite. Methodologies for getting rid of these disinfection byproducts can be costly.

Ozone

Ozone is another chemical that is commonly used in drinking water treatment. Like chlorine dioxide, it is a powerful oxidant that is capable of breaking down organic compounds that may affect taste and odor and trace chemical constituents. Since they are capable of disinfection and oxidation of organic carbon, they are gradually being adopted for water treatment.

Ozone is an unstable gas composed of three oxygen atoms. Due to the fact that they are unstable, they can decompose readily into free radicals. When used in water treatment, ozone interacts with water and forms hydroperoxyl (HO2) and hydroxide (HO-) which are responsible for a significant portion of the oxidation in the disinfection process. 25 degrees Celsius is the optimal temperature at which ozone acts as a chemical disinfectant. At this temperature they are more potent than chlorine and monochloramine. Due to ozone’s instability, they quickly decompose to elemental oxygen very rapidly after generation which is why they are generated just before use.

Advanced Oxidation Processes

Advanced oxidation processes are capable of destroying trace chemical constituents. This process produces hydroxyl radical (HO-), a very powerful oxidant. Advanced oxidation processes breakdown products at different doses. Current data suggests that each constituent can be broken down effectively if a specific dose is observed but information relating to this is still lacking. Still, organic compounds would be oxidized to carbon dioxide at extreme doses. Advanced oxidation processes are usually used at the end of treatment trains in potable reuse application because they are capable of destroying organic compounds. Due to this, advanced oxidation processes effectively polishes the water. Advanced oxidation processes aren’t always necessary, they are only needed when organic compounds that cause taste and odor issues are present.

UV and Hydrogen Peroxide

Both UV and hydrogen peroxide treatment methods are capable of breaking down trace chemical constituents. Using both UV and hydrogen peroxide leads to UV photolysis, an oxidation process catalyzed by UV light, and the generation of hydroxyl radicals through the UV light and hydrogen peroxide reaction. The combination of UV and hydrogen peroxide is effective in removing trace chemical constituents even at low concentrations.

Ozone and Hydrogen Peroxide

The combination of ozone and hydrogen peroxide in advanced oxidation processes eliminates taste and odor in drinking water. Compared to UV and hydrogen peroxide, the ozone and hydrogen peroxide combination has lower power costs. Moreover, the combination of ozone and hydrogen peroxide is more effective in eliminating some select trace chemical constituents. The absence of UV light means that light-sensitive species such as N-nitrosodimethylamine (NDMA) are removed at lower rates compared to UV-based advanced oxidation processes. If the water that is to be treated contains low levels of UV sensitive constituents or if UV light treatment is a limiting factor, the ozone and hydrogen peroxide combination may be a viable process alternative.

UV and Chlorine

The combination of UV and chlorine works similarly with UV and hydrogen peroxide in that the UV light breaks down small trace chemical constituents in reclaimed water through photolysis and the formation of hydroxyl radicals. To improve the efficiency of the UV and Chlorine treatment combination, a large amount of free chlorine residual needs to be present, chlorine demand should be low, and the pH should be between 6 to 6.5. When used in direct potable reuse, the UV and chlorine combination provides three major advantages. First, besides toxoplasma and cryptosporidium oocysts, chlorine can kill all other microbiological pathogens. This means that if this treatment combination is used at the end of a treatment train, or somewhere near the end, it provides a disinfection redundancy that could kill any microorganism that may survive the disinfection processes before it. This property is not present in the UV and hydrogen peroxide combination. Second, peroxide quenching prior to the drinking water treatment plant, or prior to introducing the purified water into a drinking water system is no longer required for this system. Finally, monitoring the integrity of this system does not cost as much because you only need to measure free chlorine residual.

Technologies for Aesthetic Improvement

How the consumers would perceive the final treated wastewater or reused water is something that needs to be considered. The water properties that most consumers would be more concerned about are the taste, odour, and colour, collectively known as the water’s aesthetics, because these are the properties that are apparent to them. Consumers are very likely to think that treated wastewater or reused water is not safe for use because of aesthetic concerns even though they are completely safe. If the goal of wastewater treatment or water reuse is to produce potable water, the final water should be safe and free of questionable taste, odour, or colour. Otherwise, the consumers wouldn’t be satisfied with the treated water.

Taste and Odor Control

There are a lot of factors that affect the taste and odor of water. For instance, algal blooms can impart unfavourable odour and taste to water that originates from surface waters sources. For water sources that originate from groundwater, minerals such as iron and manganese can have a negative impact on their taste and odour. Moreover, even the treatment methods that are meant to make the water safer can cause taste and odour problems. The water’s salinity also causes taste issues. Putting too much chlorine for the purpose of disinfecting water could impart a bleach taste or odour in it. Phenols could also enter the source water and if they are not removed through treatment would impart a medicine-like taste to the water. Reverse osmosis and nanofiltration systems are capable of removing most of the water’s minerals. If these minerals are not replaced, the water would take on a metallic taste.

Geosmin and 2-methylisoboreneol are naturally occurring compounds that are responsible for imparting an earthy, musty smell and taste in water. They are not associated with any negative health effects but they can cause public discontent.

Treatment processes that utilize powerful oxidants are usually effective in removing taste and odor. However, using powerful oxidants could lead to the formation of harmful byproducts. Fortunately, water aesthetics can be improved by using methods such as activated carbon. When combined with ozone, they can effectively resolve a lot of taste, odour, and colour issues. The combination of UV and chlorine is also an effective method for taste and odour removal.

Colour

Water should be colourless and clear. Water that has hue contains dissolved organic material or inorganic constituents that impart colour. Even small amounts of dissolved organic matter or inorganic constituents that are imperceptible to the naked eye can stain sinks and plumbing fixtures over time. It is important to determine the cause of the water’s colour issue before treating them. Incorrect treatment methods may exacerbate this issue. For instance, ozone is effective in removing colour caused by organic compounds dissolved in water. However, if the cause of the issue is manganese, the ozone can oxidize this chemical to permanganate which imparts a purple coloration to the water.

Technologies for Stabilization

There are water treatment processes, such as reverse osmosis and nanofiltration, that are capable of removing minerals from the water. However, water that has been stripped of its minerals is extremely corrosive which could cause corrosion in metal piping or concrete tanks. The treated water could be stabilized by using methods such as decarbonation, the addition of lime, caustic soda, and or calcium chloride.

Decarbonation

Decarbonation is the removal of carbon from the water. This can be achieved through aeration. When the carbon dioxide is removed, the water’s pH would increase. Essentially, decarbonation can lower a water’s pH and is a good stabilization method. Packed tower aerators can be attached to reverse osmosis units so that stabilization of the permeate can be achieved through decarbonization without needing to add any chemicals. Decarbonation can be a low-cost means of increasing the pH when sufficient carbonate alkalinity is present. Moreover, this method provides advantages if other dissolved gases or volatile chemicals such as trihalomethanes, hydrogen sulfide, methane, or radon, are present in the water. Despite this, decarbonation alone does not affect the total alkalinity of the water.

Sodium Hydroxide

Permeate, the treated water that comes out of a Reverse Osmosis unit is subjected to pH adjustments and sodium hydroxide or caustic soda are the usual chemicals used in making these adjustments. Unlike decarbonation, the use of sodium hydroxide will increase the total alkalinity of the water while increasing its pH as well. The permeate is often low in hardness as well as alkalinity. Sodium hydroxide alone may not be enough to produce a stable product water.

Lime Stabilization

The beauty of lime or calcium oxide is that it can be used for product water stabilization, increasing the alkalinity of water, increasing water hardness, and increasing water pH. There are two forms of lime that can be purchased, quicklime (CaO) and hydrated lime (Ca(OH)2). The former requires the use of a slaker while the latter can be added directly. The main drawback of lime is that it is challenging to work with because it clumps in the dry feed equipment, can cause dust accumulation, and increases turbidity in the water.

Calcium Chloride

Calcium chloride is a stabilizer that can increase the hardness of the water but does not affect pH or alkalinity. Since that is the case, it needs to be used with other chemicals that could increase the water’s pH such as sodium hydroxide. Calcium chloride for water treatment can be purchased in liquid form, and this does not cause turbidity. Although the combination of calcium chloride and sodium hydroxide is more expensive than lime, they are used more often because they are more manageable.

Blending

Blending is the stabilization method of adding water of appropriate quality, also known as blend water, to the water that has been treated by reverse osmosis or nanofiltration methods. This way, the treated water’s hardness and alkalinity levels can be restored. This is a cost-effective restabilization method but requires the availability of blend water. Blending could be done at different stages within direct potable reuse schemes. It could be done before entry into an engineered storage buffer, after storage in the buffer, or before introduction into the potable water system. This stabilization method is not always applicable, its usage will depend on on-site-specific constraints. Blending conventional water with direct potable reuse water leads to the remineralization of the treated water. Thus, when it is used in direct potable reuse schemes, remineralization may no longer be needed.

Residuals Management

The operation of water and wastewater treatment plants leads to the generation of residuals that needs to be managed. Management of residuals can include treatment, reuse, and or disposal. Advanced wastewater treatment facilities are capable of producing residuals such as screenings, backwash solids and liquid streams, and reverse osmosis concentrate. Backwashing and screening produce solids that are usually macerated and returned to the wastewater treatment plant where they are mixed with other process solids, removed or disposed of, and or incinerated. With the exception of reverse osmosis concentrate, reject streams and backwash water are often returned to the wastewater treatment plant or advanced wastewater treatment facility’s inlet for retreatment. Concentrate management needs to be considered in advanced wastewater treatment facilities that utilize reverse osmosis methods. There are several ways of doing concentrate management. In coastal areas where it is permitted, the concentrate is sent to ocean outfalls. In cases where ocean outfalls are not practical or permitted, the concentrate can be treated using various brine concentration and crystallization methods or other salt recovery techniques. Managing the concentrate this way leads to the generation of solid waste which can be sold if the quality of the final product is sufficient for the market. Other methods of concentrate disposal include deep-well injection, surface water discharge, land application, or discharge to the wastewater collection system.

Sources

https://www.epa.gov/waterreuse/basic-information-about-water-reuse

https://www.epa.gov/waterreuse/epa-water-reuse-resources

https://www.epa.gov/sites/default/files/2018-01/documents/potablereusecompendium_3.pdf