An Overview on Zero Liquid Li quid Discharge Technology and Promoting ZLD in Bangladesh
CHE 6405 Water Pollution and Control
Submitted to DR. MD SHAHINOOR ISLAM Assistant Professor Department of Chemical Engineering, BUET
Submitted by TANVIR AHMED Student No. 1015022027 SABRINA KHAN Student No. 1015022029 Department of Chemical Engineering, BUET
ABSTRACT Zero liquid discharge (ZLD) refers to a treatment process in which the plant discharges no liquid effluent into surface waters, in effect completely eliminating the environmental pollution associated with treatment. Apart from this benefit, a ZLD process also makes effective use of wastewater treatment, recycling, and reuse, thereby contributing to water conservation through reduced intake of fresh water. This paper includes effluent treatment methods, the prospect of ZLD in Bangladesh, the benefits & challenges and equipment overviews. Zero discharge solutions can be accomplished by concentrating the effluent using various techniques, including membrane-based and multiple effect evaporation-based systems, and recovery and recycling of water.
i
TABLE OF CONTENTS ABSTRACT................................................................................................................ i TABLE OF CONTENTS ...........................................................................................ii LIST OF FIGURES AND TABLES......................................................................... iv 1
INTRODUCTION ............................................................................................................. 1 1.1
2
Zero Liquid Discharge ............................................................................................... 1
Effluent Treatment Methods .............................................................................................. 3 2.1
Physical Unit Operations [4] ...................................................................................... 3
2.1.1
Screening................................................................................................................ 3
2.1.2
Flow Equalization .................................................................................................. 3
2.1.3
Sedimentation and Filtration .................................................................................. 4
2.1.4
Aeration.................................................................................................................. 4
2.2
Chemical Unit Processes [4] ...................................................................................... 4
2.2.1 pH Control ............................................................................................................. 4 2.2.2 2.3
3
Chemical Coagulation and Flocculation ................................................................ 5 Biological Unit Processes [4]..................................................................................... 5
2.3.1
Activated Sludge .................................................................................................... 6
2.3.2
Fixed Film ..............................................................................................................6
2.4
Physio-chemical Treatment ....................................................................................... 7
2.5
Physio-chemical and Biological Treatment ............................................................... 7
Introduction of ZLD in bangladesh.................................................................................... 8 3.1 3.1.1
Reasons to Introduction of ZLD in Bangladesh ........................................................ 8 Ground Water Level .............................................................................................. 8
3.1.2 National Standards for Wastewater ....................................................................... 9 3.2
Benefits and Challenges Facing the ZLD ................................................................ 10
3.2.1
Benefits of ZLD ................................................................................................... 10
3.2.2
Challenges of ZLD ............................................................................................... 11
ii
4
5
6
Overview on Zero Liquid discharge ................................................................................ 12 4.1
Process Selection ..................................................................................................... 13
4.2
Some ZLD Process Examples:................................................................................. 16
4.2.1
Ionics’ EnChem. A ZLD-solution for the semiconductor industry ..................... 16
4.2.2
Tenergys’ plating waste water recovery system .................................................. 17
Achieving ZLD: Equipment Overview ............................................................................ 18 5.1
Reverse Osmosis (RO) ............................................................................................. 18
5.2
Electro-dialysis reversal (EDR) ............................................................................... 19
5.3
Evaporator ................................................................................................................ 19
5.4
Crystallizer ............................................................................................................... 21
5.5
Spray Dryer .............................................................................................................. 21
CONCLUSION................................................................................................................ 22 REFERENCES ........................................................................................................ 23
iii
LIST OF FIGURES AND TABLES FIGURES:
Figure 2-1: Typical Flow Diagram of a Biological Treatment Plant ......................................... 6 Figure 4-1: General ZLD Technology Process ........................................................................ 12 Figure 4-2: Zero liquid discharge system flow diagram .......................................................... 14 Figure 4-3: Example of how a present ZLD-system is integrated in a power plant ................ 15 Figure 4-4: Ionics’ ZLD-system for the semiconductor industry ............................................ 16 Figure 4-5: Tenergys’ ZLD-system for the plating industry ................................................... 17 Figure 5-1: Industrial Reverse Osmosis Process ..................................................................... 18 Figure 5-2: Industrial Electro-dialysis reversal system ........................................................... 19 Figure 5-3: A falling film evaporator, also called a brine concentrator [16] ........................... 20 Figure 5-4: The heat flows in a crystallizer ............................................................................. 21
TABLES:
Table 2-1: Wastewater Treatment Levels and Processes [3] ..................................................... 3 Table 2-2: Biological Treatment Processes ............................................................................... 5
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CHAPTER 1 1
INTRODUCTION
In the past several decades, industrial production has increased in Bangladesh. Water consumption for industrial use has consequently risen and will continue to rise. Ecological issues are an integral and important part of the environmental issues challenging Bangladesh. Poor air quality, water pollution, and garbage all affect the quality of food and the environment necessary for ecosystems to thrive. Nowadays, Environmental concerns are steadily increasing, and regulatory authorities are constantly tightening the environmental standards, insisting that industries adopt advanced wastewater treatment technologies, including ZLD solutions. Today few industries are concerned with developing ZLD solutions that address some of the following challenges: 1. Innovative and customized solution offerings 2. Highly corrosive effluent and the selection of metallurgy 3. Recovery of pure process condensate for reuse and recycling 4. Operating temperature and pressures and scaling and fouling tendency 5. Continuous operation of the system with minimization of cleaning-in-place (CIP) effluents 6. Selection of appropriate type of evaporator effects in multi effect evaporation system 7. Optimization of CAPEX and OPEX for ZLD solutions. 1.1
Zero Liquid Discharge
Any process or combination of processes, by virtue of which there is no liquid effluent, or discharge from a process plant It implies that wastewater is treated and effectively recycled and reused such that there is no effluent discharge. ZLD is usually accomplished by concentrating the effluent using various techniques, including membrane-based and multi effect evaporation based systems. ZLD involves:
1
• The elimination of the liquid waste effluent stream from the plant • The recycling of recovered water and solids • The establishment of no liquid pollutant nor ms. If implemented correctly, ZLD has the potential not just to alleviate concerns of effluent discharge but also to lead to water conservation, which is critical for regions experiencing water scarcity. Typically, ZLD systems are used where there are environmental limitations such as water insufficiency and where strong environmental regulations are obligatory; e ither because water resources are scarce or an industry’s activity is highly polluting. In principle, the end goal of ZLD is to eliminate (to the extent feasible) the discharge or disposal of liquid waste from a facility, wherein no wastewater gets discharged to surface water bodies. This is ultimately accomplished by recycling, reusing or reducing the volume of waste stream, primarily including water-based streams. The non-water components of a wastewater come from either the manufacturing process itself or from a cleaning process and can include any number of materials, chemicals, oils or solids. Typical waste streams that produce large volumes of wastewater include cooling tower blowdown, gas scrubber blowdown, ion-exchange regeneration effluent and rinses, plant wash down and rain water runoff, and process wastes. These come from a wide variety of industries, including but certainly not limited to:
Metal Working
Metal Finishing
Manufacturing/Production
Transportation
Water Treatment
Material Recycling
Pharmaceutical
Food and Beverage
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CHAPTER 2 2
EFFLUENT TREATMENT METHODS
Effluent can be treated in a number of different ways depending on the level of treatment required. These levels are known as preliminary, primary, secondary and tertiary (or advanced) [1]. Conventional wastewater treatment consists of a combination of physical, chemical, and biological processes (Table 2-1) and operations to remove solids, organic matter and, sometimes, nutrients from wastewater [2]. Many of these processes will be used together in a single treatment plant. Tabl e 2-1: Wastewater Tr eatment L evels and Pr ocesses [3]
Treatment Level
Description
Process
Preliminary
Removal of large solids such as rags, sticks, grit and grease that may damage equipment or result in operational problems Removal of floating and settle able materials such as suspended solids and organic matter Removal of biodegradable organic matter and suspended solids Removal of residual suspended solids / dissolved solids
Physical
Primary Secondary Tertiary/Advanced
2.1
Physical and chemical Biological and chemical Physical, chemical and biological
Physical Unit Operations [4]
Common physical unit operations include among other processes screening, flow equalization, sedimentation, clarification and aeration. 2.1.1
Screening
A screen with openings of uniform size is used to remove large solids such as plastics, cloth, polythene etc. which may damage process equipment, reduce the effectiveness of the ETP or contaminate waterways. 2.1.2
Flow Equalization
There are several different steps in the industrial processes and therefore wastewater quality and quantity varies over time. ETPs are usually designed to treat wastewater that has a more or less constant flow and a quality that only fluctuates within a narrow range. The equalization tank overcomes this by collecting and storing the waste, allowing it to mix and become a regular quality before it is pumped to the treatment units at a constant rate. To
3
determine the required volume of an equalization tank the hourly variation of flow needs to be determined. 2.1.3
Sedimentation and Filtration
The flocs formed in flocculation are large enough to be removed by gravitational settling, also known as sedimentation. This is achieved in a tank referred to as the sedimentation tank, settling tank or clarifier. Sedimentation is also used to remove grit and suspended solids, to produce clarified effluent, and to thicken the sludge produced in biological treatment. Flocculation and sedimentation should remove most of the suspended solids and a portion of the BOD [5]. 2.1.4
Aeration
Aeration is required in biological treatment processes to provide oxygen to the microorganisms that breakdown the organic waste. Two main methods are used for this, either mechanical agitation of the water so that air from the atmosphere enters the water, or by introducing air into the tank through diffusers.
2.2
Chemical Unit Processes [4]
Chemical unit processes are always used with physical operations and may also be used with biological treatment processes, although it is possible to have a purely physio-chemical plant with no biological treatment. Chemical processes use the addition of chemicals to the wastewater to bring about changes in its quality. They include pH control, coagulation, chemical precipitation and oxidation. 2.2.1
pH Control
Waste from textile industries is rarely pH neutral. Certain processes such as reactive dyeing require large quantities of alkali but pretreatments and some washes can be acidic. It is therefore necessary to adjust the pH in the treatment process to make the wastewater pH neutral. This is particularly important if biological treatment is being used, as the microorganisms used in biological treatment require a pH in the range of 6-8 and will be killed by highly acidic or alkali wastewater. Various chemicals are used for pH control. For acidic wastes (low pH) sodium hydroxide, sodium carbonate, calcium carbonate or calcium hydroxide, may be added among other things. For alkali wastes (high pH) sulphuric acid or hydrochloric acid may be added. Acids can cause corrosion of equipment and care must be taken in choosing which acid to use. Hydrocholoric acid is probably better from an
4
environmental view point but can corrode stainless steel therefore plastic or appropriately coated pumps and pipes must be used. 2.2.2
Chemical Coagulation and Flocculation
Coagulation is a complex process but generally refers to collecting into a larger mass the minute solid particles dispersed in a liquid. Chemical coagulants such as aluminium sulphate (alum) or ferric sulphate may be added to wastewater to improve the attraction of fine particles so that they come together and form larger particles called flocs. A chemical flocculent, usually a polyelectrolyte, enhances the flocculation process by bringing together particles to form larger flocs, which settle out more quickly Flocculation is aided by gentle mixing which causes the particles to collid e.
2.3
Biological Unit Processes [4]
Biological treatment is an important and integral part of any wastewater treatment plant that treats wastewater from either municipality or industry having soluble organic impurities or a mix of the two types of wastewater sources. The obvious economic advantage, both in terms of capital investment and operating costs, of biological treatment over other treatment processes like chemical oxidation; thermal oxidation etc. has cemented its place in any integrated wastewater treatment plant. Biological treatment using aerobic activated sludge process has been in practice for well over a century. Increasing pressure to meet more stringent discharge standards or not being allowed to discharge treated effluent has led to implementation of a variety of advanced biological treatment processes in recent years. The objective of biological treatment of industrial wastewater is to remove, or reduce the concentration of, organic and inorganic compounds. Biological treatment process can take many forms (Table 2) but all are based around microorganisms, mainly bacteria. Tabl e 2-2: B iological T r eatment Processes
Treatment Processes Definition Suspended-growth processes e.g. activated The micro-organisms are maintained in sludge suspension in the liquid Attached-growth processes or fixed-film The micro-organisms are attached to some processes inert medium such as rock or inert plastics
Combined processes
A combination of suspended-growth and fixed-film
5
These microorganisms use components of the effluent as their “food” and in doing so break them down to fewer complexes and less hazardous compounds. In the process the microorganisms increase in number.
F igur e 2-1: Typical F low Di agram of a Bi ological Tr eatment Plant * A sludge recycle line is essential for activated sludge systems but is not needed for fixed film systems. ** The aeration unit can be either activated sludge or a fixed film reactor.
There are two main types of processes, these involve suspended microbial growth (e.g. activated sludge) and attached microbial growth (e.g. fixed film). With both approaches large populations of microorganisms are brought into contact with effluent in the presence of an excess of oxygen. In both systems the microbial population has to be retained in a tank referred to as the reactor. With suspended growth systems microbes grow in small aggregates or “flocs” (this is known as activated sludge). 2.3.1
Activated Sludge
Activated sludge (AS) leaves the reactor with the treated effluent but is settled out in a clarifier and returned to the aeration unit to recycle the bacteria. If the amount of AS is excessive some may be disposed of rather than being recycled. 2.3.2
Fixed Film
In fixed film systems the microbial population grows as a thin layer (a “bio -film”) on the surface of an inert support medium. The classical fixed film system is known as a percolating 6
or biological filter and uses small stones as a medium to support microbial growth. In the more modern system microbes grow on plastic supports. In the traditional percolating filters effluent is sprayed over the medium and trickles through a packed bed with oxygen entering from the air. In more recent reactor designs, the medium (usually plastic) is submerged in effluent and air is blown into the base of the reactor. Submerged fixed film reactors using plastic media require much less land. Fixed film systems require a final clarifier to remove particles of biofilm that become detached from the medium. Biological treatment plants must be carefully managed as they use live microorganisms to digest the pollutants. For example some of the compounds in the wastewater may be toxic to the bacteria used, and pre-treatment with physical operations or chemical processes may be necessary. It is also important to monitor and control pH as adverse pH may result in death of the microorganisms. The ETP must be properly aerated and must be operated 24 hours a day, 365 days a year to ensure that the bacteria are provided with sufficient “food” (i.e. wastewater) and oxygen to keep them alive. Like humans, microorga nisms need a “balanced diet” with sources of carbon, nitrogen, phosphorus and sulphur.
2.4
Physio-chemical Treatment
The basic units needed for a stand-alone physio-chemical treatment plant are screening, an equalization unit, a pH control unit, chemical storage tanks, a mixing unit, a flocculation unit, a settling unit and a sludge dewatering unit.
2.5
Physio-chemical and Biological Treatment
In this type of treatment a combination of physical operations, and physio-chemical and biological processes are used. The basic units needed for a physio-chemical and biological treatment plant are screening, an equalization unit, a pH control unit, chemical storage tanks, mixing units, flocculation units, a primary settling unit, an aeration unit, and a secondary settling unit. The physio-chemical unit always comes before the biological unit.
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CHAPTER 3 3
INTRODUCTION OF ZLD IN BANGLADESH
The textile sector is the backbone of Bangladesh’s economy. However, the industry is faced with many challenges due to high resource (energy, water and chemical) footprint and its consequent environmental impact. Water usage by the textile industry in Bangladesh is estimated to be 1,500 million cubic meters, which is principally made of groundwater. Around 70% of this water consumption takes place in the wet processing of textiles, which involves washing, dyeing, and finishing of textiles. Besides high water footprint, the textile industry also faces the challenge of dealing with the problem of effluent discharge and the use of coagulants and chemicals for its treatment. Realizing the scale of these issues and the urgent need for addressing them, Bangladesh Government has issued the Zero Liquid Discharge (ZLD) Regulation for the textile sector. This poses a huge challenge for the industry, particularly for the small and medium sized units. The key to its successful implementation would be a cautious and practical approach and view this as an opportunity to deal with the pressing environmental challenges in a sustainable manner [6].
3.1
Reasons to Introduction of ZLD in Bangladesh
Reasons behind, why Zero Liquid Discharge (ZLD) technology should introduced in Bangladesh as soon as possible are listed below: 3.1.1
Ground Water Level
Like many natural resources, groundwater is being exploited at an increasing rate all over the world. Groundwater is generally preferred as a source of potable water in the developing world because of its ready availability and natural protection from contamination. It is commonly used for irrigation and to supply industrial and domestic needs. A lack of proper understanding of the groundwater system, in terms of resource utilization, is one of the major limitations to the effective management of groundwater resources [7]. Dhaka is dependent primarily on groundwater for the urban water supply; about 84% of the present municipal water supply comes from groundwater and 16% is from surface water [8]. The dependence on groundwater for domestic, industrial, and commercial water supply in the city area was more than 95% prior to the commissioning of a large surface water treatment plant (Sayedabad Surface Water Treatment Plant) in 2002. 8
The capital’s groundwater levels are falling rapidly due to excessive extractions to meet the needs of its growing population, said Dhaka Water Supply and Sewerage Authority. The capital’s underground water level has dropped to 60 meters below ground level, according to DWASA. In 2011, the capital’s ground water level was 51 meters below the ground level, according to a study of Bangladesh Agricultural Development Corporation. The groundwater levels in the capital are falling by two to three meters each year, according DWASA [9]. 3.1.2
National Standards for Wastewater
Effluent from textile dyeing industries must meet the national effluent discharge quality standards set by the Government of Bangladesh, including the “Quality Standards for Classified Industries” (Table 3-1), and may also need to meet additional standards set by international textile buyers. Consequently any ETP must be designed and operated in such a way that it treats the wastewater to these standards [10]. Tabl e 3-1: D ischarge Qual i ty Standard f or Cl assif ied I ndu str ies - Composi te Textil e Plan t and L arge Processi ng U ni ts
Parameter
Limit, Bangladesh Standard (mg/L)
pH
6.5-9
Total Suspended Solid (TSS)
2100
Total Dissolved Solid (TDS)
100
Biochemical Oxygen Demand for 5 days (BOD5) Chemical Oxygen Demand (COD)
50 200
SO42-
400
S2-
2
Total Chromium
2
Phenolic Compounds
5
The regulations state that these quality standards must be ensured from the moment of going into trial production for industrial units. They also state that the Department of Environment can undertake spot checks at any time and the pollution levels must not exceed these quality standards. Furthermore, the quality standards may be enforced in a more stringent manner if considered necessary in view of the environmental conditions of a particul ar situation [11].
9
The waste discharge quality standards differ according to the point of disposal. So, the standards are different for inland surface water (ponds, tanks, water bodies, water holes, canals, river, springs or estuaries); public sewers (any sewer connected with fully combined processing plant including primary and secondary treatment); and irrigated land defined as an appropriately irrigated plantation area of specified crops based on quantity and quality of wastewater [11]. Bangladesh Government is planning to lower the current limiting standards such as BOD level to 30 mg/L from 50 mg/L, COD level to 150 mg/L from 200 mg/L. But with current technology this is not possible to attain such result, so improvement of technology and current process must be introduced.
3.2
Benefits and Challenges Facing the ZLD
While successful implementation of ZLD in the textile sector in Bangladesh would have significant environmental, economic and social benefits, it is also thwart with many challenges. It is important that before the ZLD mandate is rolled out in Bangladesh, the regulatory authorities and the industry are fully aware of the pros and cons of such a program in terms of the possible benefits and the challenges or roadblocks that can come in the way of its successful implementation. 3.2.1
i.
Benefits of ZLD
Implementation of ZLD had encouraged the industry to closely monitor water usage, avoiding of wastages and to promote recycling. For example, the textile dyeing industry moved from conventional Winches which used more water (1:16 Liquor of Fabric weight to Water volume) to less water consuming Soft Flow Machines with 1:8 Liquor ratio and many are increasingly moving towards even lower water consuming “Air flow Machines” with 1:3.5 Liquor ratios to increase their production while generating lower volumes of effluent.
ii.
The high recovery of water (>90-95%) and the recovery of salt has mitigated the higher cost of operation of a ZLD system.
iii.
The implementation of ZLD paved the way for a more sustainable growth of the industry while meeting most stringent environmental norms.
iv.
Reduction in water demand from the Industry by implementation of ZLD enabled in freeing up water for Agriculture and Domestic demands.
10
v.
The environmental problems created earlier has been arrested and the degraded land and water bodies are slowly recovering back
vi.
Sustainable growth of the industry also implies growth in economy and sustainable livelihood for many people who are dependent on the textile dyeing cluster either directly or indirectly.
3.2.2
i.
Challenges of ZLD
ZLD results in generation of hazardous solid wastes (particularly waste mixed salt) causing disposal challenges, which is being stored in storage yards within the CETPs.
ii.
For the chemical sludge, the best way to dispose it off is its gainful utilization for cement co-processing but it needs tie up with a willi ng/recipient cement company.
iii.
The high cost of operation of a ZLD is also a major challenge. The recovery of water and salt (Sodium sulphate and brine) offsets this costs significantly, but it would apply only to water scarce areas where the cost of water is high.
iv.
High Carbon foot print of a ZLD facility is another major concern. The typical power consumption ranges from 8 to 10 kW/m 3. The thermal evaporators alone consume about 20-40 kW/m 3 in addition to several tons of firewood for the boilers.
v.
Non uniform application of ZLD standards across the country for similar industries has serious impact on the competitiveness of the local industry.
vi.
Implementation of ZLD requires a host of advanced wastewater treatment technologies. Implementation of ZLD in Tamilnadu has highlighted several Technology shortcomings such as in Thermal evaporation & brine concentration, Salt separation and Crystallization, Colour removal etc.
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CHAPTER 4 4
OVERVIEW ON ZERO LIQUID DISCHARGE
Zero Liquid Discharge (ZLD) represents the ultimate cutting-edge treatment system for the total elimination of wastewater effluent into neighboring waterways. The ZLD System removes dissolved solids from the wastewater and returns distilled water to the process (source). Reverse osmosis (membrane filtration) may be used to concentrate a portion of the waste stream and return the clean permeate to the process. In this case, a much smaller volume (the reject) will require evaporation, thus enhancing performance and reducing power consumption. In many cases, falling film evaporation is used to further concentrate the brine prior to crystallization [12].
F i gur e 4-1: General Z L D T echnol ogy Process
For over 30 years vapor compression evaporation has been the most useful technology to achieve zero liquid discharge. Evaporation recovers about 95 % of a wastewater as distillate for reuse. Waste brine can then be reduced to solids in a crystallizer/dewatering device. However, evaporation alone can be an expensive option when flow rates are considerable. One way to solve this problem is to integrate membrane processes with evaporation. These 12
technologies are nowadays often combined to provide complete ZLD-systems. The most common membrane processes used so far are reverse osmosis (RO) and electro-dialysis reversal (EDR). By combining these technologies with evaporation and crystallization ZLDsystems have become less expensive. They are however combined differently depending on the circumstances, see chapter general guidelines. Together with these components, a variety of other well-known water treatment technologies are used in ZLD-systems for pre-treatment and polishing treatment [13]. These treatments are:
pH adjustment
de-gasifier
mixed/separate bed
oil/water separator
neutralization
oxidation (UV, ozone, sodium hypochlorite)
dissolved air flotation (DAF)
carbon adsorption
anaerobic or aerobic digestion
The variation of ZLD-systems are, as previously mentioned, endless. Below, I have tried to display a couple of ZLD-systems applied in different industries. The power plant system, see figure 1, is the most general system, while the other two, see figure 2 and 3, are commercial ZLD-systems from two water treatment companies.
4.1
Process Selection
The individual processing steps (and their abbreviations used in the illustration) of the five conventional ZLD processing schemes for treating wastewater include:
Reverse osmosis (RO)
Lime softening (LS)
Thermal brine concentrator (BC)
Thermal crystallizer (CRYST)
Spray dryer (SD) (used only for low-volume flows)
Evaporation pond (EP)
Landfill (LF)
13
Product water is produced by the RO, BC, and CRYST process steps. Processing steps for the wastewater treatment, beginning with concentrate (conc.), are as follows [14]:
Scheme 1A:
conc. →
BC →
EP
Scheme 1B:
conc. →
BC → CRYST → EP
Scheme 2A:
conc. → LS → RO → BC →
Scheme 2B:
conc. → LS → RO → BC → CRYST → EP
Scheme 3:
conc. → LS → RO →
EP
EP
Solids produced from the lime softening and crystallization steps go to a landfill. If the water flow rate is small, not many components are necessary. The following general guidelines are accepted today [13]:
Below 10 gpm of feed – crystallizers and/or spray dyers can be combined.
10 – 50 gpm of feed – use a crystallizer alone.
50 – 100 gpm of unsaturated feed – use an RO/EDR/crystallizer combination.
50 – 100 gpm of saturated feed – use an evaporator/crystallizer combination.
100 – 500 gpm of feed – either an RO/crystallizer or an evaporator/crystallizer combination may be the most economical.
500 – 1000 gpm of feed – all three should be used
F igu r e 4-2: Zero li qui d di schar ge system fl ow diagr am
14
F igur e 4-3: Ex ample of how a present Z L D-system is i ntegrated in a power pl ant
15
4.2 4.2.1
Some ZLD Process Examples: Ionics’ EnChem. A ZLD-solution for the semiconductor industry
Ionic claims that their water treatment system, the EnChem, removes more than 99% of the contaminating materials. The EnChem technology is also capable of reducing water usage during the semiconductor manufacturing process by up to 85% through reclaim. Ionics’ EnChem technology is a low pressure water treatment solution. It is specifically designed to reduce water consumption and operating costs [15].
F igur e 4-4: Ionics’ ZLD-system for the semi conductor i ndu str y
The water to be treated has to go through four steps; through the reaction tank, on to the filtration, on to the backflush and finally to de-watering. Each step is described below: Step 1. Reaction step. Influent waste water is pH adjusted, then mixed with organic and
inorganic coagulant additives. The polymers react with the contaminants to form spheres. The reaction is complete in a few minutes. Step 2. Filtration. The filtration is accomplished through low pressure membranes. The clean
water then exits the top of the filter while solids are retained on the filter membrane. Operating pressures remain below 15 psi (1 bar). Step 3. Backflush. The membranes are pulsed to remove solids and then solids are pumped to
a settling tank. Step 4. Solids formation/De-watering. Solids are pumped to a holding tank for further
settling. Conventional filter presses can be used to further separate and de-water solids. Overflow filter press water is returned to the reaction tank.
16
4.2.2
Tenergys’ plating waste water recovery system
This ZLD-system is used for the Ni and Cr plating industry. Filtration is combined with separated bed and an evaporator. The system can handle flow rates of 50 gpm.
F igur e 4-5: Tenergys’ ZLD-system f or the platin g in dustr y
In the pre-treatment step ultrafiltration (UF) is combined with carbon filter. UF is used for removal of volatile organics, virus and bacteria and suspended solids. The carbon filter contain granular activated carbon media that adsorb impurities within molecule-sized pores. Oxidants such as chlorine are also removed during their interaction with the carbon surface. After pre-treatment the water passes through the polishing step, which consists of separate bed demineralizers, where the salts in the water are separated into positively charged cations and negatively charged anions. The process begins when the water is passed through cation exchange resin. The cation resin is in the h ydrogen form (H+) and exchanges all the positively charged ions for hydrogen, thus converting all the impurities in the water into acids. The water from the cation exchange is then passed through anion exchange resin. Separate bed means that there are two tanks, one containing cation resin, and the other containing anion resin. The anion resin is in the hydroxyl form (OH -) and exchanges all the negatively charged ions into the hydroxyl form, completing the conversion of all impurities into water (H + + OH→ H2O), thus providing pure demineralized water . The concentrates finally feed an evaporator providing clean water for plating, thus completing the closed ZLD-system.
17
CHAPTER 5 5 5.1
ACHIEVING ZLD: EQUIPMENT OVERVIEW Reverse Osmosis (RO)
Reverse osmosis is a process where water is pressurized so that it passes through a semi permeable membrane, leaving dissolved inorganic salts and silica behind. As a rough guide to performance, RO can produce a concentrate containing 30000 ppm total dissolved solids (TDS). Two problems with RO are that organics will seriously foul RO systems and that RO requires a feed stream that is free of suspended solids. Because of this it is advisable to remove organics from wastewater before it enters the RO, so extensive front-end filtration equipment is required. Some membranes are pH and temperature sensitive, so pH control and feed equalization may be necessary. RO is also quite energy-intensive. The advantage of RO over evaporation is that the life cycle costs of RO are about half those of evaporators.
F i gur e 5-1: I ndu str ial Rever se Osmosis Pr ocess
18
5.2
Electro-dialysis reversal (EDR)
Electro-dialysis reversal (EDR) is a membrane process in which electrolytes migrate across charge-selective membranes in response to an electrical field. In EDR, the polarity of the electrodes is reversed several times an hour and the fresh water and the concentrated wastewater are exchanged within the membrane stack to remove fouling and scaling. EDR differs from RO in that the ions are removed and the water is left behind, whereas in RO, the water is removed and the ions are left behind. Because of this, silica and dissolved organics are not removed with an EDR process, which is an important aspect to remember when the clean stream is reused. Like RO, EDR requires solids and organics removal from the feed for reliable operation.
F igu r e 5-2: I ndu str ial El ectro-di alysis rever sal system
5.3
Evaporator
Evaporators come in all sizes and shapes, for example; falling film-, rising film-, forced circulation and scraped surface/thin film- and combination evaporator. Evaporators produce a distillate stream that is very clean, typically containing less than 10 ppm of TDS one of the main reasons why evaporators are used in a ZLD-system. The most prevalent type is the
19
falling film evaporator, also called brine concentrator, see figure 4. This evaporator can treat RO or EDR concentrates to a total solids (TS) concentration of 300 000 ppm. At this value the boiling point rise of the brine results in either an excessively large heat-transfer area (large capital cost) or an excessively large temperature difference (large operating cost). Values higher than this makes the combination of a crystallizer and an evaporator more economical than an evaporator alone.
F igur e 5-3: A f all in g fi lm evapor ator, also call ed a bri ne concentr ator [ 16]
20
5.4
Crystallizer
F igur e 5-4: T he heat fl ows in a crystalli zer
The crystallizer reduces highly saturated wastewater to dry solids for disposal. High purity water is recovered from the crystallizer for recycling. A crystallizer may also recover specific salts from a mixed salt waste stream. The crystallizer is a forced circulation evaporator which uses a mechanical vapor compressor or plant steam as the energy source.
5.5
Spray Dryer
When a crystallizer is not appropriate, the spray dryer is another method for dewatering the concentrated slurry left over from the brine concentrator. The spray dryer transforms the slurry into a fine powder of mixed salts for disposal. The spray dryer atomizes the wastewater slurry inside a hot chamber, instantly vaporizing the water droplets and leaving only dry salts behind.
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CHAPTER 6 6
CONCLUSION
ELGs (Effluent Limiting Guidelines) do change and that requires the manufacturer to act or to face the consequences of not meeting the guidelines. More often, ELGs become stricter over time, and the manufacturing facility ends up needing to purchase additional water treatment equipment and chemicals or to replace their wastewater treatment system altogether. Adding or replacing water treatment equipment often means adding personnel to operate it and capital money to purchase it. The additional water treatment costs also contribute to higher operating costs overall. A zero liquid discharge system eliminates the need to react to changing ELGs because there is no longer any wastewater that has to meet the limits. These systems can be used as a stand-alone treatment following wastewater treatment, or they can be used in conjunction with an industrial water reuse system. Combining water reuse and ZLD means the added benefit of cost savings on water purchases. In fact, with the right pretreatment process, the solid waste from the ZLD system can be disposed in a landfill as non-hazardous waste.
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