# 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 Disc harge (ZLD), Brine concentrator (BC), Forced Circulation Crystallizer (FCC), Wastewater Treatment, Fa lling 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 suspen ded 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 syst ems 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 pHadjustment, 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 larges t 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) aconfiguration 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 plant s 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 discha rged 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 SchematicDESIGN BASIS OF ZLD PROJECTS AT ENEL

1. Brindisi: This is a 2640 MW (4x660 MW) Coalfired power facility loca ted 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 solidsbefore being discharged into the sea. Pretreatment treats Influent flow of 140 m 3 /hr in 2x50% clarifiers for reducing the permanent

hardness. About 70 m 3 /hr of softened water is

re-circulated back to FGD Scrubber units.

Balance 70 m 3 /hr of softened water is routed to the ZLD system and treated through 2x50% BC (each of 35 m 3 /hr capacity) and 1x100% FCC (10 m 3 /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/crystallizati on 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 chemicaltreatment and the ZLD system. The pretreatment plant consisting of metals removal and magnesium reduction is designed for a net capacity of 30 m 3 /hr of wastewater coming from the FGD Scrubber. The softening clarifier and ZLD Section is designed for treating 15 m 3 /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 m 3 /hr (1x100%) configuration and the ZLD section is designed

to treat 35 m 3 /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 in let 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

SO 4 -- , mg/l 3000 8000 11900 1700

Cl - , mg/l 18700 18200 15200 22800

NO3 - max, mg/l 300

F - max, mg/l 25

HCO 3 -(as CaCO 3 ), mg/l 80

CO 2 (as CaCO 3 ), mg/l 5

SiO 2 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

SO 4 -- , mg/l 2200 4600 10900 1100 1200

Cl - , mg/l 16600 13400 13400 20000 25000

NO 3 - max, mg/l 300

F - max, mg/l 350

HCO 3 - (as CaCO 3 ) 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

SO 4 -- , 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

SO 4 -- , mg/l 2200 6600 17700 900 1000

Cl - , mg/l 14000 9200 9200 18600 30000

NO 3 - max, mg/l 300

F - max, mg/l 1000

HCO 3 - (as CaCO 3 ) max, mg/l 600Table 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

SO 4 , mg/l 4050 2538 2683 3091 7160 2250 3318 2912 2694 5019

NH 4 , 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

NO 3 , 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 20000PPM 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

SO 4 , mg/l 5980 2200 2591 2848 4995 2200 2740 2510 2590 4000

NH 4 , 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

NO 3 , 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 126000 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000PPM 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 PlantChemistry 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 responsibilit y 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 concentratio ns 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 (CaSO 4 ) 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 isbeing 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 Evaporati on/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 emul ating the success of ZLD systems at ENEL power stations and ensure

cleaner environment.