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.
Part 1 of this article (August 2024) discussed conventional disposal methods, membrane and thermal separation technologies, among other topics, and Part 2 will evaluate zero-liquid discharge (ZLD) technologies and the preliminary treatment, disposal or further treatment for a preliminary treated system.
EVALUATION OF VARIOUS TECHNOLOGIES
In the present technological era, ZLD is becoming a must have technology for most industrial wastewater treatment plants to maximize water recovery for reuse, yet it is still cost-intensive. It is crucial to identify the best applicable ZLD technology—among the technical description of available technologies provided in Part 1—and an approach to have a ZLD selection diagram—as included in Part 2—may prove helpful in selecting a best fit for the end user. This provides a comparison of available solutions to evaluate various technologies based on their abilities and limitations to treat industrial wastewater brine. The technology selection criteria (FIG. 2) can be broken down into the following two steps:
Step 1: Preliminary treatment. The first step in a typical industrial treatment plant is wastewater collection, segregation and primary treatment. The primary treatment stage is mainly used to remove suspended solids, oil, grease, volatile organic compounds (VOCs) and other such impurities. It may include an API separator (API), a corrugated plate interceptor (CPI), dissolved gas flotation (DGF)/DAF in combination or as needed, depending on the feed effluent and to achieve < 10-mg/l oil concentration at the primary treatment outlet. The waste stream generated from primary treatment is oily sludge that is dealt with separately in a sludge handling system. Another waste stream from this stage may be VOCs that are routed to an incinerator or thermal oxidizer.
Depending on the organic contents in the treated effluent from primary treatment, it is processed further in a secondary treatment stage that mainly aims to reduce biochemical/biological oxygen demand (BOD)/chemical oxygen demand (COD) levels and suspended solids (FIG. 3). Conventional methods include the activated sludge process and clarification followed by pressure/sand filtration to remove suspended solids. The generally accepted limit of organic contents after biological treatment are BOD5 (20 mg/l–30 mg/l), COD (150 mg/l–200 mg/l) and total suspended solids of 50 mg/l–100 mg/l. Post pressure/sand filtration-treated water is good for reuse as utility water, etc.
Step 2: Disposal or further treatment for preliminary treated streams. After fulfilling the demand for utility water, the remaining treated water can be discharged into an external water body, municipal drain, etc., depending on compliance with environmental regulations. As an alternative, the further recovering/treating of the effluent is an option. To further recover reusable water from the treated water stream and to minimize the discharge, a detailed evaluation is required to decide the best-fit ZLD technology. The ZLD treatment process is broadly divided into three sub-categories: the pre-concentration stage, the concentration stage and the crystallization stage.
In the pre-concentration stage (FIG. 4), treated water is further treated with membrane-based technologies, such as ultra filtration (UF) and reverse osmosis (RO). These technologies treat water to a higher purity while concentrating the stream to a high salinity so that the concentration stage size is reduced to the best extent possible. If the salinity of the treated water of the preliminary treated stream is < 3,000 mg/l, it is treated with a low-pressure RO system operating at a bar pressure of < 15. Water with salinity between 3,000 mg/l and 10,000 mg/l is treated with a medium-pressure RO system operating in the range of 15 bar–25 bar, and a salinity beyond 10,000 mg/l is treated with a high-pressure RO system operating at a bar pressure above 25.
The RO system is usually preceded by an ultra-filtration system. Permeated or clean water from the RO system is collected for further reuse. Recovery of the RO system depends upon the salinity of the feed water to the RO system and may vary from 30%–85%. Reject from the pre-concentration stage has a salinity in the range of 20,000 mg/l–75,000 mg/l, or higher. Depending on the environmental assessment, there are several options as outlined in Part 1 of the article to dispose of this concentrate.
The next stage needed to accomplish ZLD is the concentration stage (FIGS. 5 and 6), which aims to further concentrate the brine to the extent of 200,000 mg/l–250,000 mg/l total dissolved solids (TDS) and recover clean reusable water by employing membrane-based technologies or thermal-based technologies.
If thermal energy is unavailable, membrane-based separation technologies are employed. If an economic draw solution is available, which has a higher osmotic pressure than the feed brine solution being treated, forward osmosis (FO) can be selected as it is a very cost-effective solution.
Sodium chloride, seawater, glucose, potassium nitrate and ammonia/carbon dioxide (CO2) solutions are used as draw solutions in this separation technology. Longer membrane life, a decreased use of chemicals and potentially low energy consumption to recover clean water are some of the advantages of this technology.16 However, high-viscosity operations are tricky, as they can clog the membrane and cause pressure buildup within the system. These operations also have a much lower flux rate than RO. Membrane distillation (MD) can be used if a draw solution is unavailable; however, this requires low thermal energy. Also, there is no such limit on feed concentration, as it is theoretically a rejection of non-volatile components. This technology has relatively high module costs and low flux compared to other pressure driven membranes.11
MD also presents issues, such as the unavailability of enhanced membrane fabrications like FO, as well as other issues like membrane pore wetting and poor thermal efficiency. Additional challenges include flux reductions due to concentration polarization.12 The electrical-driven membrane separation technologies [electrodialysis (ED)/electrodialysis (EDR)/electrodialysis metathesis (EDM)] are less efficient than FO/MD, but they are suitable for treatment of high silica-content brine, as silica is neutrally charged—EDM is the most promising of these three technologies.13 However, this technology (EDM) is not yet mature and requires the addition of an NaCl current to achieve equilibrium in the mass balance.
If thermal energy is available, multi-stage flash distillation- (MSF-), multi-effect evaporator- (MEE-) or evaporator-based technologies are employed. MEE and MSF are conventional desalination technologies. Because MEE uses the generated vapor at each stage as its heat source, it significantly reduces energy consumption and therefore has a significantly lower operating cost. In MEE, a higher concentration of brine can be achieved due to the use of multiple effects. However, it has high capital and maintenance costs as well as problems like fouling, the accumulation of solids, and impurities on the heat transfer surface. In a similar way, MSF also produces particularly satisfactory results in terms of distillate generated—it has a rejection of up to 99.9% and a low operating cost. MSF, however, also has high capital and maintenance costs and requires exotic metallurgy.14
Evaporation technology. This technology has evolved as a very promising solution among thermal separation technologies. Evaporators can be broadly classified as a recirculating type and once-through type. Falling film evaporators, rising film evaporators and forced circulation evaporators are recirculating types, whereas an agitated thin film evaporator is a once-through type evaporator. In recirculating-type evaporators, as the brine circulates through the evaporator heating section, only a part of it is vaporized in each pass; un-vaporized brine is returned to the brine sump and recirculated to reach its maximum concentration. Therefore, the residence time of brine in these types of evaporators is more than that in a once-through type evaporator. They can operate over a wide range of brine concentrations, however, due to the large residence time, they are unsuitable for heat-sensitive liquids.8
Conversely, an agitated thin film evaporator is a once-through type evaporator and offers the shortest resistance time of liquid brine among all evaporator types. Due to its short residence time, it is suitable for heat-sensitive liquids. Also, because it agitates the liquid with the help of rotors—which maximizes heat transfer—it is the most suitable for aggressive, corrosive and even highly viscous brines.8
Next in the evaporator series are mechanical vapor recompression (MVR) and thermal vapor recompression (TVR), which are proven energy-saving evaporator technologies that can minimize evaporation energy use by 95% or more compared to other evaporation technologies like MEE. The concepts of MVR and TVR can be applied to any type of evaporator specified above, and they are the most energy efficient evaporation technologies to date. While MVR completely eliminates the thermal energy requirement (except for startup), it does require electrical energy for mechanical compression of the vapors generated. On the other hand, TVR requires motive stream as minimal thermal energy.
A brine concentration of 200,000 mg/l–250,000 mg/l can be achieved in the concentration stage with the use of the thermal-based and membrane-based separation technologies described above. The concentration is then fed to the final stage of ZLD (i.e., the crystallization stage).
Crystallization stage. This stage aims to further concentrate the brine to crystallized form (producing solids, crystals) with a target TDS as high as 350,000 mg/l. Available options for brine crystallization are a brine crystallizer (BCr), membrane crystallization (MCr) and eutectic freeze crystallization (EFC). BCr is the commercial ZLD technology for treatment of high-TDS brine and boasts excellent performance (95%–99% recovery) with production of high-quality fresh water (10 mg/l–20 mg/l); however, its capital cost is quite high due to the use of exotic metallurgy.13
MCr is also a promising crystallization technology that works on low-grade thermal energy and may utilize non-conventional energy sources such as solar, geothermal and even waste heat, if available. However, membrane issues such as low flux, membrane pore wetting and poor thermal efficiency limits this technology usage.14 Moreover, this technology is still evolving.
The third option is EFC; however, due to its extremely excessive cost and because it is also still evolving, this method is only used when it is infeasible to adopt other crystallization methods.
The evaluation details above are useful to identify the best applicable ZLD technology. However, the cost of the system is also an important driving parameter for the end user when deciding the best fit for their project. TABLE 2 indicates the specific energy consumption of both membrane-based and thermal-based separation ZLD technologies and may be used to evaluate the economics of the system in addition to the technical evaluation above.
Takeaways. Recovering reusable water from industrial wastewater treatment plants is a necessity for the water processing industry. Industrial effluent plants discharge brine into open water bodies—in addition to other disposal methods, this is unsustainable and has a larger environmental impact. Apart from environmental concerns, in some countries, brine disposal is not a feasible option due to statutory requirements restricting the discharge of brine. Selecting ZLD for the treatment of brine minimizes environmental pollution and yields the highest recovery of reusable water or production of fresh water. As discussed here, a single type of technology (i.e., membrane-based or thermal-based) is insufficient to achieve a complete ZLD. To achieve a complete ZLD, a holistic approach with a hybrid/integrated system that combines membrane-based and thermal-based technologies should be used. The section titled “Evaluation of various technologies” may be used to evaluate various available options to choose the best fit considering the effluent feed quality as well as the techno-commercial evaluation of the capital and operating costs of the system. 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.