ZLD Systems Installed for ENEL Power 

Plants in Italy M.N. RAO, Aquatech International Corporation, Canonsburg, PA, 

USA, and SERGIO DONADONO, ENEL, Milano, Italy 

IWC-08-33 

KEYWORDS: Flue gas Desulfurization (FGD), Zero Liquid Discharge (ZLD), Brine concentrator (BC), Forced 

Circulation Crystallizer (FCC), Wastewater Treatment, Falling film Evaporator, Clarifier, Softening, Scrubber, 

Blow-down. 

ABSTRACT: Aquatech International Corporation was awarded the contract to supply and install ZLD systems to 

treat FGD waste water from five coal-fired power projects for ENEL (Ente Nazionale Energia Elettrica) in Italy. 

Each plant receives a wide variety of Coal that throws challenges in the treatment of this wastewater. Some of 

the challenges include high concentrations of suspended solids, dissolved ionic impurities e.g. calcium, 

magnesium, sulfate, heavy metals and chloride ions as well as large fluctuations in the FGD wastewater blowdown quantity and quality. Company has supplied the systems with softening the FGD waste stream as a 

pretreatment step before it is routed into the ZLD systems. This paper discusses design considerations and 

concept applied and compares the initial operational data from the plants commissioned. 

Introduction: Physical and chemical treatment to 

remove heavy metals and suspended solids from 

FGD waste water from coal fired power plants has 

been successfully implemented for many years. 

General Treatment scheme includes pH 

adjustment, Sulfide precipitation, Ferric coprecipitation, flocculation, clarification and finally the 

filtration of Clarifier effluent. In some of the plants 

COD/BOD Oxidation with Hydrogen Peroxide is 

also employed. More recently new technologies, in 

particular biological treatment for better removal of 

trace metals and other contaminants such as BOD 

have been considered, but treated water still 

contains high dissolved solids that make recycle 

and reuse impossible. With increased emphasis on 

waste minimization Zero Liquid Discharge approach 

is gaining momentum across the globe. 

ENEL is the largest Power Company in 

Italy. Following the European legislation and 

corporate objective of reducing environmental 

impact ENEL has been in the forefront in installing 

the SCR-DeNOx (for removing/minimizing Nitrogen 

Oxide gases), high efficiency ESP (Electrostatic 

Precipitator) or bag filters for removing fly-ash 

(particulate matter) and wet limestone gypsum 

forced oxidation DeSOx (De-sulfurizer for removing 

Sulfur Oxide gases). The early DeSOx (Brindisi #1-

4, Sulcis #3,Fusina #3-4) are equipped with 2X50% 

lines consisting Scrubbers fed with Sea Water for 

final dedusting and gas saturation and an absorber, 

while more recently (La Spezia #3, Fusina #1-2) a 

configuration with 1X100% line without any preScrubber has been installed. The blowdown waste 

water from Pre-scrubber (if any) and absorber along 

with other plant wastewaters was being sent to 

wastewater treatment plant where metal removal 

was effected by addition of lime and Sodium Sulfide 

and further by addition of Ferric chloride and 

settling suspended solids were removed and after 

final oxidation with Hydrogen peroxide the treated 

wastewater was being discharged into the nearby 

sea. 

In 2003 ENEL started the conversion of 

Torrevaldaliga North Power station from Oil firing 

(no more economical) to Coal firing. For this Project 

ENEL considered to apply all the best available 

technologies for ensuring environmental protection 

and finally decided to go with Zero Liquid Discharge 

approach. While doing this, ENEL also decided to 

upgrade all other DeSOx wastewater treatment 

plants with ZLD systems. Careful review was 

undertaken in final selection of the scheme for 

installing the ZLD systems at all the five coal-fired 

power plants namely Brindisi, Fusina, 

Torrevaldaliga and Sulcis. All the plants operate on 

imported coals received from various countries. 

Three (Brindisi, Fusina and La Spezia) ZLD 

systems out of five plants have been successfully 

commissioned and fourth one (Sulcis) is under 

commissioning at the time of writing this paper and 

fifth one (Torrevaldaliga) is planned to be 

commissioned during the last quarter of 2008. By 

the end of the year all the ZLD systems are 

scheduled to be in operation and no DeSOx 

wastewater will be discharged into the Sea from 

any of the ENEL plants in Italy. 

 
The Process Scheme: As most of us know, there 

are two types of ZLD configurations that could be 

used for treating FGD waste water. 

A. Direct feed to ZLD system after removing bulk 

quantity of suspended solids. This involves 

Brine Concentrator (BC) followed by Spray 

dryer for dewatering the pre-concentrated brine. 

This approach typically requires natural gas 

source for spray dryer operation, 

B. Softened feed to ZLD system. This treatment 

involves lime and Soda Ash softening before 

the feed is routed to the BC followed by Forced 

Circulation Crystallizer (FCC) and belt filter 

press for final dewatering. 

In all ENEL projects, Aquatech supplied ZLD 

systems with softening as pretreatment step. The 

main reason for going with softening is due to (in 

Brindisi, Fusina and Sulcis in particular) a part of 

softened water was required to be recirculated to 

the Pre-scrubber. To maintain uniformity of the 

process the same scheme was applied to all the 

five plants. Lime and sulfide addition and 

precipitation is carried out in the first clarifier and 

soda ash softening is done in the second clarifier. 

This allows recycle of clean Calcium Carbonate 

sludge from the soda ash softening clarifier back to 

the FGD scrubber. In three of the five projects 

(Brindisi, Fusina and Sulcis) the metals removal 

and suspended solids removal clarifier with lime 

and Sodium sulfide addition has been existing prior 

to installation of the ZLD systems .In these three 

projects soda ash softening system was installed by 

Aquatech along with the ZLD system. In La Spezia 

and Torrevaldaliga Projects Aquatech supplied 

complete pretreatment system along with ZLD 

plant. 

All the ZLD plants are designed to be operated 

in “seeded Mode” where all the scaling compounds 

are preferentially attached to seed crystals of 

Calcium sulfate. The feed to ZLD system is 

saturated in Sulfate and to provide the required 

natural seed level certain amount of Calcium is fed 

into the BC. For this purpose a bypass is provided 

across the softening Clarifier that maintains the 

required calcium hardness in the overall feed to the 

BC. 

 The overall flow schematic is as given below: 

Figure 1: Overall Flow Schematic 
DESIGN BASIS OF ZLD PROJECTS AT ENEL

1. Brindisi: This is a 2640 MW (4x660 MW) Coalfired power facility located on the Adriatic Sea 

coast. Prior to installing the ZLD system the 

FGD Scrubber blow-down was being treated for 

removal of heavy metals and suspended solids 

before being discharged into the sea. Pretreatment treats Influent flow of 140 m3

/hr in 

2x50% clarifiers for reducing the permanent 

hardness. About 70 m3

/hr of softened water is 

re-circulated back to FGD Scrubber units. 

Balance 70 m3

/hr of softened water is routed to 

the ZLD system and treated through 2x50% BC 

(each of 35 m3/hr capacity) and 1x100% FCC 

(10 m3

/hr) and 2x50% Belt Filter presses (about 

2.0 Ton/hr total solids handling capacity)

Table 1.0 given below indicates four (4) 

different cases and range of analyses 

considered at Brindisi for plant design 

depending on the coal sources used. 

2. Fusina: This is a 970 MW (2X320+2x165) 

power facility near the city of Venice. Here too 

the FGD scrubber blow-down waste water was 

being treated for heavy metals and suspended 

solids removal prior to installing the ZLD 

system. The pre-treatment plant is designed for 

70 m3/hr capacity, to remove permanent 

hardness in 2x50% clarifier units (each of 35 

m3/hr ). 35 m3/hr of the softened wastewater is 

recirculated back to the Scrubber units. The 

evaporation/crystallization section is designed 

to treat the remaining 35 m3/hr softened 

wastewater in 2x50% BC units (each of 17.5 

m3/hr capacity), 1x100% FCC (about 5 m3/hr) 

and 2x50% belt filter presses. 

Table 2.0 given below indicates five (5) 

different cases and range of analyses 

considered at Fusina for plant design 

depending on the coal sources used. 

3. La Spezia: This is a 1300 MW power plant with 

1x600 MW coal-fired power unit and 2x350 MW 

gas-fired combined cycle power units. Aquatech 

supplied here the complete physical chemical 

treatment and the ZLD system. The pretreatment plant consisting of metals removal 

and magnesium reduction is designed for a net 

capacity of 30 m3

/hr of wastewater coming from 

the FGD Scrubber. The softening clarifier and 

ZLD Section is designed for treating 15 m3/hr of 

softened water. Pre-treatment and ZLD 

Sections are configured in 1x100% capacity. 

Table 3.0 given below indicates five (5) different 

cases and range of analyses considered at La 

Spezia for plant design depending on the coal 

sources used.

4. Sulcis: This is a 585 MW (1x345 MW fluidized 

bed bolier+1x240 MW conventional boiler) 

Coal-fired power plant located in Sardinia 

Island. FGD scrubber blow-down is softened in 

the pre-treatment section that is designed for a 

capacity of 45 m3/hr and ZLD Section is 

designed to treat 12 m3/hr of softened water. 

Remaining softened water is recycled back to 

the FGD scrubber. 

Table 4.0 given below indicates four (4) 

different cases and range of analyses 

considered at Sulcis for plant design depending 

on the coal sources used. 

5. Torrevaldaliga: This is a 1980 MW (3x660 

MW) newly converted coal fired power plant 

being built about 60 miles from the city of 

Rome. FGD blow down water is treated for 

heavy metals and suspended solids removal 

and softening in the pre-treatment section that 

is designed for a flow rate of 50 m3

/hr (1x100%) 

configuration and the ZLD section is designed 

to treat 35 m3

/hr (2x50%) configuration. Table 

5.0 given below indicates five (5) different 

cases and range of analyses considered at 

Torrevaldaliga for plant design depending on 

the coal sources used.

Actual Feed Data for Brindisi ZLD Plant: The 

following Table (Table 6.0) gives the actual FGD 

wastewater quality at the inlet of the pretreatment to 

ZLD section i.e. soda ash softening clarifier for 

Brindisi. Figure 2.0 below the table illustrates the 

same data graphically 

The Table 7.0 gives the softened water quality at 

the inlet of the ZLD section i.e. soda ash softening 

clarifier for Brindisi. Figure 3.0 below the table 

illustrates the same data graphically. 
Table 1.0: Brindisi Analysis

 Case A Case B Case C Case D 

Ca++, mg/l 2400 1400 1700 4200 

Mg++, mg/l 200 to 250 

Na+ & K+, mg/l balance balance balance balance 

SO4

--, mg/l 3000 8000 11900 1700 

Cl-, mg/l 18700 18200 15200 22800 

NO3- max, mg/l 300 

F- max, mg/l 25 

HCO3 -(as CaCO3), mg/l 80 

CO2 (as CaCO3), mg/l 5 

SiO2 max, mg/l 10 

Table 2.0: Fusina Analysis

 Case A Case B Case C Case D Case E 

Ca++, mg/l 3400 1550 900 6800 8400 

Mg++, mg/l 1900 2500 4500 660 1700 

Na+ & K+. mg/l balance balance balance balance balance 

SO4--, mg/l 2200 4600 10900 1100 1200 

Cl-, mg/l 16600 13400 13400 20000 25000 

Table 3.0: LaSpezia Analysis

 Case A Case B Case C Case D Case E 

Ca++, mg/l 3400 1550 900 6800 8400 

Mg++, mg/l 1900 2500 4500 660 1700 

Na+ & K+, mg/l balance balance balance balance balance 

SO4

--, mg/l 2200 4600 10900 1100 1200 

Cl-, mg/l 16600 13400 13400 20000 25000 

NO3

-

 max, mg/l 300 

F- max, mg/l 350 

HCO3

-

(as CaCO3 ) max, mg/l 600 

Table 4.0: Sulcis Analysis

 Case A Case B Case C Case D 

Ca++, mg/l 1000 820 810 1200 

Mg++, mg/l 340 550 1200 100 

Na+ & K+, mg/l balance balance balance balance 

SO4

--, mg/l 16000 20500 24500 8000 

Cl-, mg/l 10700 8200 5800 15400 

Table 5.0: Torrevaldaliga Analysis

 Case A Case B Case C Case D Case E 

Ca++, mg/l 3500 1150 650 8750 12000 

Mg++, mg/l 2400 3300 6400 450 2350 

Na+ & K+, mg/l Balance balance balance balance balance 

SO4

--, mg/l 2200 6600 17700 900 1000 

Cl-, mg/l 14000 9200 9200 18600 30000 

NO3

-

 max, mg/l 300 

F- max, mg/l 1000 

HCO3

-

(as CaCO3) max, mg/l 600 
Table 6.0: Actual Feed Data for Brindisi Power Plant 

3/5 3/18 3/31 4/10 4/30 5/12 5/29 6/10 6/30 7/21 

pH 8.9 9.4 9.3 9.2 8.9 8.8 8.8 8.7 9.1 8.50 

Ca, mg/l 946 1142 778 661 621 1010 681 902 962 701 

Mg, mg/l 51 203 63 70 97 110 121 134 49 75 

Na, mg/l 3105 2010 1258 1409 4234 1356 1916 1652 2002 4022 

Cl, mg/l 3600 3800 1480 1220 2600 2500 2000 2360 2900 3900 

SO4, mg/l 4050 2538 2683 3091 7160 2250 3318 2912 2694 5019 

NH4, mg/l 1.7 2 3 1.6 3.7 1.6 0.8 1.9 4 3.1 

F, mg/l 12 21 21 21 19 19.0 34.2 12.0 20.0 25.0 

NO3, mg/l 28 19 16 27 21 22.0 18 22.8 32.6 54.5 

COND, μS/cm 16670 13250 8460 8124 18130 10140 10700 9730 13000 18550 

Figure 2.0: FGD Waste Water Analysis for Brindisi Power Plant 

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

PPM ION 

DATE

FGD WASTE WATER ANALYSIS 2008

pH

Ca

Mg

Na

Cl 

SO4

NH4

F

Table 7.0: Softened Water Quality for Brindisi Power Plant 

3/5 3/18 3/31 4/10 4/30 5/12 5/29 6/10 6/30 7/21 

pH 9 9 9.1 9.2 8.9 8.8 8.7 8.5 8.7 9 

Ca, mg/l 561 800 761 501 573 450 477 481 421 450 

Mg, mg/l 97 140 114 56 114 60 46 117 49 40 

Na, mg/l 3671 2155 1223 1390 3123 1873 1829 1690 2306 3617 

Cl , mg/l 2500 3500 1600 1060 2440 2200 1730 1900 2500 3500 

SO4, mg/l 5980 2200 2591 2848 4995 2200 2740 2510 2590 4000 

NH4, mg/l 1.2 3 3 1.0 2 1.5 3 3.0 0.9 1.3 

F, mg/l 16 12 26 12.0 21 18 22 23.0 14.0 22 

NO3, mg/l 20 22 21 30.5 23 20 26 21.4 30.5 25 

Cond., μS/cm 16980 11209 8400 8380 17800 12000 10760 10410 12330 12600 
0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

PPM ION

DATE

ANALYSIS FOR BC FEED 2008 

pH

Ca

Mg

Na

Cl 

SO4

NH4

F

NO3

Figure 3.0: BC Feed Analysis for Brindisi Power Plant 
Chemistry Control: As can be seen from the 

above FGD blow-down wastewater data that there 

is significant variability in the feed characteristics to 

the pretreatment and ZLD section compared to the 

original design and added to that the flow variability 

can be expected as any other wastewater treatment 

plant. Chloride level in the actual feed water is 

varying the range 1360 - 3500 mg/l, while the 

design cases had chloride in the range 15200 - 

23800 mg/l. 

The fact that these parameters vary put 

higher responsibility on the ZLD plant operator of 

correct operation. The operator needs a simple way 

of determining where the ZLD plant should operate 

and how much softening is required to avoid scaling 

in the pre-heater or the downstream heat 

exchanger surface and minimizing or eliminating 

the calcium and magnesium chlorides into the ZLD 

system. Higher levels of these salts will lead to 

higher boiling point elevation as these do not 

crystallize at concentrations similar to sodium 

chloride. 

Some of the major operational issues of 

importance are: 

1. Gypsum scaling in the pre-heater and 

deaerator 

2. Calcium and Magnesium chloride build up 

in the evaporator 

3. Glauberite scaling in the evaporator 

4. Insufficient amount of Calcium Sulfate 

(CaSO4) seed slurry in the evaporator 

5. Evaporator/crystallizer pH 

6. Volatility of Ammonia and Boron 

7. Foaming 

8. Moisture content of dewatered cake 

Company has developed programs based on 

the laboratory level research and actual field 

experience and prepared charts that can be used 

for predicting and correcting some of the operating 

parameters. However, they are quite involved. 

Keeping in view the ease of operator needs, site 

specific spreadsheets have been developed to 

arrive at the amount of softening and bypass 

required, the amount of suspended solids need 

recycled via Hydrocyclone, and the concentration 

factor needs to be maintained in the Brine 

concentrator. 

Recently there was an opportunity to open one 

of the Brine concentrators at Fusina after it was in 

seeded mode operation for about two months. The 

tubes were found to be as clean as that of new unit. 

This demonstrates the operation of the systems is 

reliable provided proper attention is paid to the 

chemistry issues and necessary chemistry 

adjustments are made in a timely manner. 

 

Figure 4.0: BC Tubes after 2 months operation 

 (Magnified view) 

Figure 5.0: BC Tubes after 2 months operation

Distillate Quality: The recovered distillate from BC 

and FCC units has conductivity in the range 20 - 

100 μS/cm and pH in the range 7.5 - 9.0. Higher 

conductivity is primarily due to the presence of 

ammonia at higher range of pH. The distillate is 

being recycled as partial make-up to the prescrubber, scrubber and cooling tower. The distillate 

quality is under further testing and review to 

determine its suitability as demineralization plant 

make-up. 

Cake Leachability Analysis: The salt cake 

produced from belt filter has been tested for heavy 

metals and their leachability using Toxicity 

Characteristics Leachate Procedure (TCLP) per 

local regulatory requirement and it is established 

that the regulated metals are below the specified 

discharge limits. As a result, the salt cake can be 

disposed off in a regular landfill. 
SOME OF THE PROBLEMS ENCOUNTERED 

Cake Quality: During the initial stabilization period 

it was found that Cake quality was not up to the 

expectation of 85% dry solids. On review of the 

analysis at various stages it was determined that 

lack of Sodium Chloride. Partially this was also due 

to operating the Evaporation/crystallization system 

at full load for testing purposes, though not 

necessary. This of course was helping recycle of 

low saline water to FGD Pre-scrubber& scrubber, 

however low Sodium Chloride to ZLD system was 

leading to higher percentage of Calcium Sulfate to 

total solids and in turn resulting in formation of 

sloppy cake. Calcium sulfate forms fine crystals and 

generally dewatering is difficult from its cake. After 

feed cycles timing in belt filter press was adjusted 

and air drying duration was increased this problem 

was mitigated. 

Figure 6.0: Wet Cake 

Figure 7.0: Dry Cake

Post precipitation: 

Per the design scheme the softened water was to 

be blended with FGD wastewater going into the 

ZLD system and the blended feed was to be 

acidified before routing to the Pre-heater and 

deaerator. To maintain stable chemistry the 

blending was effected in a holding tank. This 

allowed ensuring required levels of Calcium for 

maintaining the natural seed level before being fed 

into the ZLD system. This however was resulting in 

suspended solids build up in the holding tank. Also 

the piping was found getting coated with 

precipitates due to presence of excess Soda Ash in 

the softened water. Even after attempts of 

optimizing the dosage of Soda Ash in the softening 

clarifier, this phenomenon of post precipitation 

could not be completely eliminated. Site 

modification is being planned to add Acid dosing 

and pH measurement at the discharge of Softened 

water pump with a pH control loop to maintain the 

pH around 8.3. This modification is expected to 

reduce post precipitation effect. 

Future of ZLD for FGD Wastewater Treatment: 

Coal is expected to remain a major energy (electric 

power) source in foreseeable future. With the 

opportunity for expedited project permitting process, 

ever-increasing regulations from the governments 

and local bodies, the ZLD technology solution is 

becoming acceptable to the Power industry. Up 

until recently the general perception has been that 

ZLD systems are more expensive involving high 

capital and operating investment. However, the 

actual cost of ZLD approach needs to be properly 

evaluated against the tangible and intangible costs 

associated with treatment and disposal of FGD 

wastewater with current technology and the impact 

of waste disposal on the environment. 

Conclusion: The successful implementation of 

ZLD systems at ENEL is indicative of future trend in 

the industry that coal fired power plants are willing 

to pursue generating cleaner power. The 

technology exists that is viable and reliable and it is 

up to the industry for emulating the success of ZLD 

systems at ENEL power stations and ensure 

cleaner environment.