S. Kumar, Fluor New Delhi, New Delhi, India; B. EKMEKCI, Fluor Amsterdam, Amsterdam, the Netherlands; and A. RANWE and S. G. S., Fluor New Delhi, New Delhi, India
Although approximately 71% of the Earth’s surface is covered with water, the issue of global water scarcity is a popular topic of conversation. Of the total amount of water available on the planet, < 1% is available as freshwater. The remaining water on earth is either inaccessible or has excessive salinity. Additionally, the growing global population, rapid industrialization and a negligible rate of wastewater recycling further exacerbate the situation.
To procure freshwater, seawater is viewed as a primary source of water and desalination is perceived as the most viable and feasible solution to address this problem. The desalination process reduces the salt content of saline water and produces freshwater. However, in this process, the recovery is generally limited to 30%–50% due to the high pressure requirements to achieve high recovery.
This means that for every 100 m3/hr of feed saline water, 50 m3/hr–70 m3/hr of reject (brine) is generated. Techniques to treat this brine further have been required to maximize recovery. Similarly, for brackish water treatment and industrial water treatment, brine is generated to the extent of 10%–30%, depending upon the process employed to generate fresh or recycled water. Among the available technologies of wastewater treatment to recover reusable water, zero-liquid discharge (ZLD) has become the most frequently used term in oil and gas refineries and petrochemical plants. It has been effectively used in the treatment of brine to maximize the recovery of freshwater or recycled water and to recover salt and other chemicals that are used in other industries.
ZLD, as the name itself suggests, is a technology where no liquid effluent is discharged into surface water—rather, it is treated and reused in operations, eliminating the liquid effluent and minimizing discharge/disposal volumes. The main objective of ZLD is to produce clean water and economically reduce wastewater that is suitable for reuse. There are several reasons to adopt the ZLD concept, but the following are the most important key drivers:
Conventional disposal methods. The reject water (brine) composition and concentration in a typical treatment plant depend primarily on the recovery of the reverse osmosis system that is part of the pre-concentration stage in a ZLD system. A higher recovery of freshwater provides a higher concentration of rejected substances and a smaller volume of brine.
If ZLD techniques are not employed to further reduce the reject water volume and maximize freshwater recovery, several conventional disposal methods are used to discharge this brine. These include surface water discharge, wastewater treatment plant discharge, deep well injection, solar evaporation and land application.
Surface water discharge releases brine into surface water bodies (e.g., oceans, seas, rivers, lakes). Consequently, this degrades the water quality of the receiving water body and impacts its aquatic life. In some countries, brine disposal or discharge are not feasible options due to statutory restrictions. Discharge to the ocean can have a lesser effect on the environment compared to other surface water bodies discharge options; however, this option can be quite expensive and requires extensive infrastructure to transport brine to the ocean (up to 2 km–3 km) before discharging.
Discharge to a wastewater treatment plant requires a detailed assessment of the design and capacity of the wastewater treatment plant being considered to receive the brine. Evaporation ponds are considered a comparatively eco-friendly solution; however, they are only applicable in hot and dry locations that provide a high evaporation rate. Since this method uses solar energy to evaporate the water out of the brine, operational costs are minimal. Deep well injection—the injection of brine back into the ground—depends mainly on local regulations. If allowed, this requires a detailed techno-commercial analysis before implementation.
Lastly, the land application is also a viable option for reject disposal. In this method, brine is spray irrigated on salt-tolerant plants and grasses. This disposal method works well with low volumes of brackish water brine and desalinated water brine. Its application is also restricted by various parameters such as climatic conditions, groundwater conditions, seasonal demand, the availability of suitable land, etc.1
ZLD as a solution. While these disposal methods have been used widely, each has its pros and cons (as discussed above)—ultimately, the main concern is the environmental impact of their long-term use and on human health. This leads to the need to adopt a different approach: treatment rather than disposal. This approach effectively reduces the quantity of wastewater by producing reusable water and salt as a byproduct that can be quite useful in some industries.
Numerous factors impact the reject treatment process selection and its implementation. The first is the implementation of a high recovery system apart from other factors, such as effluent composition, the characteristics and the cost of ZLD treatment. The higher the recovery of the upstream treatment system, the smaller the ZLD system will be. Wastewater must undergo several types of pre-treatment operations before utilizing a ZLD system. Subsequently, ZLD can be achieved by membrane-based and thermal-based technologies.
FIG. 1 provides a generic overview of various treatment steps of a typical ZLD system. The process mainly consists of primary and secondary treatments, the pre-concentration of brine for ZLD to maximize recovery, and finally ZLD, which is further divided into the concentration and crystallization stages.
The primary treatment stage may include an API separator (API), a corrugated plate interceptor (CPI) and dissolved gas flotation (DGF) to remove oil, grease and other impurities. It may also include filters and clarifiers to remove suspended solids.
The secondary treatment stage is for biological treatment of water in which the main aim is to reduce biochemical/biological oxygen demand (BOD)/chemical oxygen demand (COD) levels. This can be done in both anaerobic and aerobic conditions. After treatment from the primary and secondary treatment stages, the pre-concentration stage is employed. Its main aim is to treat water to a higher purity while concentrating the stream to a high salinity so that the size of the concentration stage is reduced as much as possible. This may comprise various treatment steps such as clarifiers, multimedia/sand filtration, and different types of membrane filtration like micro/ultra filtration and reverse osmosis (RO). This is used to remove solid impurities, reduce total dissolved solids (TDS) and remove organic impurities, chemicals, colloids, viruses, bacteria and heavy metals like nitrate, sodium, arsenic, mercury, lead, etc. Brine from this stage is sent to the ZLD system to further recover water for reuse and further concentrate brine to minimize the disposal.
Classification of ZLD. After pre-concentration through the RO system as described in the preceding section, the concentration stage is employed. As it indicates, the concentration stage further concentrates the brine to minimize its volume. Membrane separation or thermal separation technologies are employed in this stage. Finally, the crystallization stage is adopted to achieve complete ZLD.
The following section and TABLE 1 include, in detail, various thermal-based and membrane-based technologies adopted for the concentration and crystallization stages.
Membrane separation technologies. Membrane separation-based technologies are gaining attention due to their effective separation capabilities in wastewater treatment for water reuse. Membrane separation involves the passage of the brine stream through a semi-permeable membrane so that the salt contents are separated into reject and permeate based on their size and charge. Various membrane-based technologies, and shown in TABLE 1, include ED/EDR, EDM, MD, MCr and FO.
ED/EDR and EDM are electrical-driven membrane separation technologies, whereas MD and MCr are thermal-driven membrane separation technologies. ED/EDR is a membrane separation process where ions are transported through a semi-permeable membrane from one stream to another stream under the influence of applied voltage potential difference. When the polarity of the electrodes is reversed at certain time intervals, the movement of ions gets reversed and charged particles that have been precipitated on the membranes are removed—this is called EDR. ED/EDR theoretically has no concentration limit of brine to be treated, however, it does not remove organic contaminants and micro-organisms.2 Also, in ED/EDR, when the concentration rate increases, water recovery decreases as multivalent ions [such as sulfates (SO42- ), bicarbonates (HCO3- ) and phosphates (PO43- )] are intended to scale with magnesium (Mg2+) or calcium ions (Ca2+) due to the presence of sparingly soluble salts on the concentrate side of these membranes.3 EDM offers a solution for this. It is a variation of EDR in which a metathesis reaction occurs. The advantage of EDM is that it offers lower fouling potential due to the absence of applied pressure and, concurrently, offers high water recovery.
On the other hand, MD is based on a thermally-driven separation process where separation is done by phased change that uses a hydrophobic membrane. Due to its hydrophobic nature, the membrane acts as a barrier for the liquid phase while allowing vapors to pass through the pores. The difference in vapor pressure that is caused by the temperature difference between the two sides of membrane is the driving force for the separation process.4 Another thermal-driven membrane separation technology, MCr, is basically an extension of MD where a solution becomes supersaturated to simultaneously achieve solution separation and component solidification.5 The advantage of this technology is that it has well-controlled nucleation, growth kinetics, fast crystallization rates and reduced induction time.6 However, the treatment cost of freshwater in MCr is slightly higher than that in MD.
Lastly, FO, also known as direct osmosis (DO), relies on the osmotic pressure differential (Δπ ) across the membrane. In this process, an osmotic pressure gradient is produced across a semi-permeable membrane by use of a remarkably high concentration ‘draw’ solution, which results in the transportation of water molecules from the less concentrated feed brine solution to the highly concentrated draw solution.7
Thermal separation technologies. Thermal separation-based technologies are used in combination with membrane-based technologies to achieve complete ZLD. Thermal separation technologies are based on evaporation (mechanically, thermally or naturally) and crystallization. In a new phase, either vapor or a solid is created by the separation of one component from others present in the liquid phase using thermal energy, mainly steam.
Thermal separation-based technologies can be broadly categorized as evaporators and brine crystallizer, multi-effect evaporation (MEE), multi-stage flash distillation (MSF) and eutectic freeze crystallization (EFC). Mostly, evaporators consist of four main components: an evaporator heating section (through which heat is supplied from an external source, such as steam); a section in which liquid-vapor separation takes place (flash chamber, separator, etc.); a pump to circulate the brine; and a structural body to house these elements and to separate the process and heating fluids.8
Evaporators are further classified into six sub-categories: falling film evaporator, rising film evaporator, forced circulation evaporator, agitated thin film evaporator, MVR and TVR (TABLE 1). This categorization of evaporators is typically based on their configuration in the process scheme. For Instance, in a falling film evaporator, the feed brine is fed with the help of a circulation pump to the tube side of a shell-and-tube type heat exchanger that is arranged in vertical orientation in an evaporator vessel to create a falling film of brine on the inside surface of the evaporator tubes. Heating supplied on the shell side by heating media (usually steam) results in the formation of vapor on the tube side that get separated in a separator vessel and condensed to form clean water.
Conversely, a rising film evaporator works on thermo-siphon without the use of a circulation pump. In a rising film evaporator, feed brine rises through the evaporator tubes under the effect of thermo-siphon as it gets heated and evaporated in the evaporator tubes due to the heat supplied by the steam on the shell side in the evaporator section. A forced circulation evaporator uses pumps and, as its name indicates, forces the feed brine upwards through the evaporator tubes where it gets heated near to its boiling temperature in evaporator tubes. Then, it is flashed in a flash vessel located on the discharge side of the evaporator vessel. This sudden flashing converts it into vapor, which is then separated and condensed in a separator vessel.
An agitated thin film evaporator is mainly comprised of a jacketed shell and a rotor that rotates inside the shell. As the feed brine enters from the top of the evaporator, the rotor starts spreading it in the form of thin film on the heated wall of the shell where the heat transfer takes place and forms vapor to separate out clean water. A jacketed shell is supplied with steam as heating media.
MVR and TVR use the thermal energy of produced vapor by recompressing it mechanically and thermally, respectively, thereby eliminating or minimizing the requirement of external thermal energy. In an MVR evaporator, the produced steam (vapors generated in the evaporator vessel tube side) is removed and compressed by a mechanical compressor with the aim to raise its temperature, which is then supplied into the evaporator (shell side) as heating media to evaporate the feed brine. The evaporation process starts with the external source of energy (steam) supply to the evaporator. Once the cycle has started, an external heating source is no longer required as the steam gets mechanically compressed by a compressor that provides enough heat to maintain the evaporation of the liquid in evaporator.
In a TVR evaporator arrangement, vapor is compressed thermally instead of mechanically. In this type of evaporator, the produced steam (vapors generated in the evaporator vessel tube side) is removed and compressed thermally by a steam jet ejector by supplying motive steam as an external source with the aim to raise its temperature, which is then supplied into the evaporator (shell side) as a heating media to evaporate the feed brine. The principles of MVR and TVR can be applied to different types of evaporators.
A BCr is the most commonly used technology to concentrate brine feed into solid crystals and clean water. A typical brine concentrator can recover 95%–99% of wastewater for reuse. Similar to evaporators, a BCr also consists of a shell-and-tube type heat exchanger that is arranged in a vertical orientation in an evaporator vessel provided with heat input from an available steam source or steam ejector (TVR) or compressor (MVR) to recycle the vapor to reduce energy consumption. A forced circulation crystallizer is the most commonly used in which the brine is initially fed into the bottom sump where it mixes with the recirculating brine, which is then pumped into the tube side of the shell-and-tube heat exchanger section of an evaporator vessel by a recirculation pump. The tubes in the heat exchanger are submerged and the brine is under pressure, so brine does not evaporate. After gaining heat from the evaporator vessel, recirculating brine is inserted into the crystallizer vessel at an angle so that it generates a vortex. A small portion of the brine evaporates, resulting in the formation of crystals. Generated vapors are fed to the vapor compressor/steam ejector, which compresses/raises its temperature before being supplied to the shell side of the heat exchanger in the evaporator section, where it heats up the circulating brine while condensing. The remaining large amount of brine is recirculated to the evaporator tube-side section through a circulation pump, while a small stream of the recirculating loop is transferred to a centrifuge to remove the remaining water from the crystals. As a result, dry solid is produced and freshwater is collected.9
In an MEE, several vessels are used in a sequential manner to achieve continuous evaporation where each next vessel is set at a lower pressure than the previous one. As a result, the boiling temperature of the feed brine gets lowered as pressure decreases. The first vessel only needs a heating source, then the vapor formed in the vessel acts as a heating source for the next vessel. As the next vessel is at a lower pressure than the previous one, it requires less energy to evaporate. In this manner, the MEE uses heat in an efficient manner. Generally, evaporation is stopped before the precipitation of solutes gets started in the operation of an evaporator.
In an MSF system, feed brine is supplied to a series of brine preheaters where it is preheated utilizing condensing vapors from the multiple flash units. It is then fed to a feed brine heater where it is heated to a maximum temperature of up to 110°C–120°C with an external heat source, such as steam. Sometimes, this temperature is reduced to decrease the formation of scale. This hot feed brine is fed to multiple flash units that are arranged in a series maintained at successively lower vapor pressures and temperatures. While passing through these flash units, a portion of the heated feed brine solution is evaporated and further condensed in the feed brine preheaters. Thus, the condensed water vapor is the freshwater, whereas the concentrated brine is the liquid that exits from the final flash unit in the series.10
Salt and pure water can also be separated from brine via EFC. The process of EFC involves cooling the water. As it cools, a portion of it solidifies into ice and can be extracted from the solution. Because of its increased density, the other part gets denser and settles down. Salts can be extracted from the solution by crystallizing where the salt content is highest. In comparison to traditional evaporation-based separation procedures, EFC works at potentially low temperatures with comparatively less energy consumption.
To be continued…Part 2 will appear in the September issue and will discuss the evaluation of ZLD technologies, and the preliminary treatment, disposal or further treatment for a preliminary treated system, among other topics. HP
NOMENCLATURE
API
API separator
BOD
Biochemical/biological oxygen demand
BWRO
Brackish water reverse osmosis
COD
Chemical oxygen demand
CPI
Corrugated plate interceptor
DGF
Dissolved gas flotation
EDM
Electrodialysis metathesis
ED
Electrodialysis
EDR
Electrodialysis reversal
EFC
Eutectic freeze crystallization
FO
Forward osmosis
FWRO
Freshwater reverse osmosis
MCr
Membrane crystallization
MD
Membrane distillation
MEE
Multi-effect evaporator
MED
Multi-effect distillation
MSF
Multi-stage flash Distillation
MVR
Mechanical Vapor recompression
OP
Operating pressure
RO
Reverse osmosis
SWRO
Seawater reverse osmosis
TDS
Total dissolved solids
TVR
Thermal vapor recompression
UF
Ultra filtration
VOCs
Volatile organic compounds
WWTP
Wastewater treatment package
ZLD
Zero liquid discharge
DISCLAIMER
The views and conclusions presented in this article are solely those of the authors and cannot be ascribed to Fluor Corp. and/or any of its subsidiaries.
LITERATURE CITED
Sandeep Kumar is a Senior Mechanical Design Engineer at Fluor India with more than 20 yr of industry experience. In addition to being the Fluor Global Subject Matter Expert (SME) for water and wastewater treatment, he is also India office lead of Fluor Global Water Technology Group.
Kumar has experience working on pre-front-end engineering design (FEED), FEED, detailed engineering and construction support for refineries, chemical, petrochemical, oil and gas, and infrastructure projects. His core strength is in the detailed design and engineering of mechanical utility and process packages, including water and wastewater treatment packages, ZLD plants, sludge incineration plants, condensate polishing and DM packages, PSA nitrogen/hydrogen packages, HVAC and process refrigeration, etc. He has worked on large-scale, schedule-driven industrial wastewater treatment plants in recent years, including a ZLD plant in the Middle East region.
Burcu Ekmekci is a Fluor Fellow in Water and Wastewater Treatment and leads Fluor’s Global Water Technology Group, which comprises more than 10 SMEs and more than 50 members distributed over 10 offices globally.
Ekmekci has more than 20 yr of experience in the design and operation of industrial water and wastewater treatment systems. Over the years, she has led several projects ranging from feasibility studies to commissioning and troubleshooting of treatment plants for various industries. Since 2020, she has served as Vice Chairwoman of the American Institute of Chemical Engineers (AiCHE) for the Netherlands/Belgium section.
Anshul Ranwe is a process engineer with 2 yr of industry experience and has worked for Fluor since the beginning of his industrial career. From the start, he has been an integral part of Fluor’s energy transition team, working on biomass gasification, green ammonia, and pink hydrogen projects (which involved ultra-pure water generation). Ranwe now works on liquefied natural gas (LNG) projects. He has a keen interest in water and wastewater treatment and is a member of the Fluor Global Water Technology Group.
Sonia G S is a process engineer working for Fluor India. She has worked on various renewables energy projects that have involved pre-treatment, RO and wastewater treatment. She has a keen interest in water and wastewater treatment and is a member of the Fluor Global Water Technology Group.