An Overview on Zero Liquid 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

i

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.

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TABLE OF CONTENTS

ABSTRACT ................................................................................................................ i

TABLE OF CONTENTS ........................................................................................... ii

LIST OF FIGURES AND TABLES......................................................................... iv

1 INTRODUCTION ............................................................................................................. 1

1.1 Zero Liquid Discharge ............................................................................................... 1

2 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 Chemical Coagulation and Flocculation ................................................................ 5

2.3 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

3 Introduction of ZLD in bangladesh.................................................................................... 8

3.1 Reasons to Introduction of ZLD in Bangladesh ........................................................ 8

3.1.1 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

iii

4 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

5 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

6 CONCLUSION ................................................................................................................ 22

REFERENCES ........................................................................................................ 23

iv

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:

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• The elimination of the liquid waste effluent stream from the plant

• The recycling of recovered water and solids

• The establishment of no liquid pollutant norms.

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; either 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.

Table 2-1: Wastewater Treatment Levels and Processes [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

Physical

Primary Removal of floating and settle able materials

such as suspended solids and organic matter

Physical and

chemical

Secondary Removal of biodegradable organic matter

and suspended solids

Biological and

chemical

Tertiary/Advanced Removal of residual suspended solids /

dissolved solids

Physical, chemical

and biological

2.1 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

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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 micro-

organisms 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

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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 collide.

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.

Table 2-2: Biological Treatment Processes

Treatment Processes Definition

Suspended-growth processes e.g. activated

sludge

The micro-organisms are maintained in

suspension in the liquid

Attached-growth processes or fixed-film

processes

The micro-organisms are attached to some

inert medium such as rock or inert plastics

Combined processes A combination of suspended-growth

and fixed-film

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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.

Figure 2-1: Typical Flow Diagram of a Biological Treatment 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

7

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, microorganisms 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.

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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].

Table 3-1: Discharge Quality Standard for Classified Industries - Composite Textile Plant

and Large Processing Units

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)

50

Chemical Oxygen Demand (COD) 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 particular situation [11].

10

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 Benefits of ZLD

i. 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.

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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 Challenges of ZLD

i. 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 willing/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/m3. The thermal evaporators alone consume

about 20-40 kW/m3 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].

Figure 4-1: General ZLD Technology 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

13

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 ZLD-

systems 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)

14

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 → EP

Scheme 2B: conc. → LS → RO → BC → CRYST → EP

Scheme 3: conc. → LS → RO → 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

Figure 4-2: Zero liquid discharge system flow diagram

15

Figure 4-3: Example of how a present ZLD-system is integrated in a power plant

16

4.2 Some ZLD Process Examples:

4.2.1 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].

Figure 4-4: Ionics’ ZLD-system for the semiconductor industry

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.

17

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.

Figure 4-5: Tenergys’ ZLD-system for the plating industry

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 hydrogen 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.

18

CHAPTER 5

5 ACHIEVING ZLD: EQUIPMENT OVERVIEW

5.1 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.

Figure 5-1: Industrial Reverse Osmosis Process

19

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.

Figure 5-2: Industrial Electro-dialysis reversal 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

20

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.

Figure 5-3: A falling film evaporator, also called a brine concentrator [16]

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5.4 Crystallizer

Figure 5-4: The heat flows in a crystallizer

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.

23

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