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Zero‐Liquid Discharge System at Duke Energy Mayo Plant

Dr. Matthias Loewenberg, GEA Process Engineering Inc., Columbia MD 

Danny Johnson PE, Duke Energy, Raleigh NC 

John Edelen PE, Duke Energy, Raleigh NC 

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Keywords: Mayo Plant, Zero Liquid Discharge, Falling Film Evaporator, Brine 

Concentrator, Mechanical Vapor Recompression, Forced Circulation Evaporator, Brine 

Cooler, Boron Scrubber, Water Reuse, Fly Ash Mixing, Redundant System, NPDES 

Permit. 

ABSTRACT 

Duke Energy is currently installing a Zero-Liquid Discharge treatment system, provided 

by GEA Processing Engineering Inc., for Flue Gas Desulfurization Wastewater at their 

Mayo Plant, located near Roxboro NC. The paper outlines the various concepts which 

were considered as well the selected solution that is currently in the engineering phase 

of the project 

 
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Introduction

Duke Energy is currently installing a 

Zero-Liquid Discharge treatment system 

for Flue Gas Desulfurization (FGD) 

Wastewater (Figure 2) at their Mayo 

Plant located near Roxboro North 

Carolina. The Mayo Plant started 

operating in 1983, is located about 60 

miles north of Raleigh NC, and has a 

generation capacity of 742MW. 

Figure 1: Mayo Plant is located 

approximately 60 miles north of Raleigh NC

The system, designed and supplied by 

GEA Process Engineering Inc., supports 

Duke Energy’s commitment to provide 

safe, reliable and environmental friendly 

energy generation. The Zero-Liquid 

Discharge addition to their existing FGD 

scrubber supports 

compliance with tight 

National Pollutant 

Discharge Elimination 

System (NPDES) 

wastewater discharge 

regulations. 

The project is completing 

the engineering phase 

and beginning the 

construction contracting 

effort. It is scheduled 

to be operational by the 

end of 2013. 

This paper outlines the methodology 

behind the selection of the Partial ZeroLiquid Discharge approach to support 

environmental compliance. 

Background		

With the addition of a wet FGD system 

at the Mayo Plant, new discharge limits 

were imposed in the site’s renewal of 

the NPDES permit in 2008. To meet 

discharge limits for selenium and 

mercury, a gypsum settling pond and 

bioreactor were installed to treat FGD 

wastewater. The Mayo Plant treated 

wastewater discharges to the low 

turnover Mayo Lake. Water quality 

monitoring since the FGD went into 

service showed an increasing 

concentration of several constituents in 

the Mayo Lake. Since the Plant’s outfall 

consists of different wastewater 

Figure 2: Simplified Plant overview for FGD Waste Water source 

(Liquid Waste Products)
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sources, a metal loading study was 

performed to identify the source and to 

quantify the impact of different 

constituents on the lake. The study 

identified the key contaminants of 

concern as aluminum, boron, copper, 

manganese, selenium, silver, and 

thallium. Chloride concentration in the 

lake was also shown to be trending 

upwards. Although the concentration 

was below the permit limits, there were 

operational concerns with the elevated 

levels of chlorides. 

Several treatment options were 

considered for the facility to support 

compliance with the permit limits. 

Figure 3: Traditional Zero-Liquid Discharge 

Based on a high level evaluation, the 

following three options were further 

examined in more detail: 

 Pipe wastewater to a nearby river 

 Electrocoagulation 

 Zero-Liquid Discharge, traditional 

and partial 

Traditional Zero-Liquid Discharge (see 

Figure 3) evaporates and concentrates 

either raw or pretreated FGD 

wastewater past its crystallization point 

which results in a recovered water 

stream and salt. The Partial Zero-Liquid 

Discharge (see Figure 4) concentrates 

the wastewater and reduces its volume 

significantly such that the remaining 

liquid can be solidified with binders like 

cement or fly ash. 

Figure 4: Partial Zero-Liquid Discharge 
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The evaluation considered the impact of 

changes in fuel sources, life cycle costs, 

as well as the anticipated future 

regulatory environment. 

The study concluded that a Zero-Liquid 

Discharge approach is preferred for 

Mayo Plant’s needs based on an 

evaluation criteria that included multipollutant removal ability, demonstrated 

technology, time constraints, and life 

cycle cost. 
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ZLD Detailed Study Overview

To further detail the scope of the ZeroLiquid Discharge solution, a subsequent 

study was performed considering a 

typical and estimated maximum water 

analysis for the FGD blowdown. 

Three ZLD alternatives were 

considered, two traditional and one 

partial ZLD solution. The traditional ZLD 

options included one with softening 

pretreatment followed by an evaporator 

with a high temperature crystallization, 

and one with an evaporator using low 

temperature crystallization process. 

Table 1: Evaluated ZLD Alternatives 

 Alternative 

 1 2 3 

Softening ● 

Evaporation ● ● ●

Low Temperature 

Crystallization ●

High Temperature 

Crystallization ● ● 

Fly Ash Mixing ● 

The chloride content of the ZLD 

feedwater is dependent on the chlorides 

in the coal and the FGD scrubber 

blowdown concentration. A total of 9 

flow versus inlet chloride scenarios were 

considered for each alternative. These 

scenarios allowed for a range of 

equipment configurations to provide the 

plant high operational reliability and 

flexibility. 

A caustic vapor scrubber for the 

reduction of boron and droplets from the 

vapor stream was included for all 

alternatives to ensure a clean distillate 

stream for use within the plant. 

Selected ZLD Solution for

Mayo Plant

Based on the data provided by the 

detailed study and a comparison of 

alternatives, Duke Energy selected the 

Partial Zero-Liquid Discharge solution 

as most appropriate. 

The Partial Zero-Liquid Discharge 

system consists of falling film evaporator 

technology (Primary Evaporator [PE]) 

for total of 370 GPM feed with a forced 

circulation evaporator (Secondary 

Evaporator [SE]) in order to reduce the 

FGD blowdown volume significantly. A 

following Brine Cooler [BC] decreases 

the brine temperature prior to storage. 

The resulting concentrated brine will be 

mixed with Mayo Plant fly ash and 

disposed in a new on-site landfill with 

leachate control. The distillate water will 

be used in the Plant systems, reducing 

the make-up water demand. 

As outlined in Figure 5, three parallel 

trains of Primary Evaporators with feed 

of 150 GPM each (3x40%) FGD blowdown as well as two 100% Secondary 

Evaporators are utilized. A brine cooler 

system reduces the brine temperature 

for easier handling as well as to lower 

cost materials of the brine system. 
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Figure 5: Mayo ZLD-System Process Flow 

Diagram 

A maximum discharge of 18 GPM of 

brine is set as the objective, which is 

balanced based on the estimated 

amount of fly-ash for mixing. 

The design provides redundancy in the 

form of parallel operating trains or 

system in back-up configuration to 

minimize the risk of down time of the 

ZLD system. 

ZLD‐System Components		

The three major parts of the ZLD 

System (Primary Evaporator, Secondary 

Evaporator and Brine Cooler) are 

discussed in more detail below. 

Primary Evaporator

The Primary Evaporator is used for the 

majority of the concentration 

(Concentration Factor ~8-12). The PE is 

a falling film type evaporator with a 

mechanical vapor compression system, 

outlined in Figure 6. Due to the nature of 

FGD blowdown the system is operated 

as a seed system. 

To improve the evaporation system, 

various GEA in-house evaporation pilot 

tests with actual Mayo FGD blowdown 

were performed. These were to quantify 

boron content in the vapor stream and 

to optimize the vapor scrubber to 

remove the boron from the water vapor 

stream. 

The FGD blowdown is currently 

discharged from the scrubber to a 

settling pond. From here the blowdown 

is fed to a bioreactor. The bioreactor will 

be decommissioned once the ZLD 

system becomes operational. The 

Gypsum Settling Pond will remain and 

acts as a large holding tank (~30 days 

holding capacity), which provides the 

ZLD System with a steady feed and 

storage capacity.
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The FGD wastewater is introduced into 

one of two 100% plate heat exchangers 

per PE to preheat the wastewater 

against the hot distillate from the falling 

film evaporator. Calcium sulfate, one of 

the main scaling components, is 

inversely soluble with temperature which 

means that the solubility decreases with 

increase in temperature. In addition, the 

blow-down itself is already saturated 

with calcium sulfate from the scrubber 

operation. Because both aspects have 

an undesirable impact on the scaling 

within the plate heat exchanger, seed 

crystal is added prior to the heat 

exchanger to help mitigate scaling. 

Also, a redundant heat exchanger is 

installed having one unit operational and 

the second either in stand-by or 

available for cleaning. 

The preheated wastewater is 

additionally heated by the vent stream 

from the falling film evaporator shell side 

to further optimize the energy 

consumption of the system. This 

preheating takes place in a two-stage 

flash system called a deaerator. While in 

the upper chamber steam is partially 

condensed into the wastewater, the 

lower chamber flashes off a fraction of 

water again. This flashing action 

removes inert gases and therefore 

deaerates the wastewater. Releasing 

inert gases has two main purposes; on 

one hand it reduces the oxygen level in 

the water to reduce the corrosion 

potential in combination with the high 

concentration of dissolved chlorides and 

on the other hand it removes inert 

components before they otherwise 

would be recompressed in the 

mechanical vapor recompressor (MVR) 

and reduces its efficiency. 

The wastewater subsequently enters the 

sump of the falling film evaporator. Here 

the recirculation pump transfers the 

liquid to the top tube sheet where the 

wastewater enters the heat transfer 

tubes, creating a thin film of cascading 

water. Inside the tubes the FGD 

Figure 6: One of Three Primary Evaporator, Falling Film Type Evaporator with Mechanical Vapor

Compression
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blowdown is concentrated by 

evaporating water from the solution. 

Both the water vapor and concentrated 

wastewater travel down the tubes. At 

the bottom, the vapor mist is separated 

from the liquid using centrifugal forces in 

the vapor-liquid separator which is 

wrapped around the lower sump area. 

Concentrated water droplets are then 

washed down and combined with the 

wastewater stream in the sump. 

Concentrating brine solutions with high 

concentrations of dissolved chlorides, in 

most cases result in exotic materials of 

constructions. While the heat transfer 

tubes are made of Titanium-Gr.16 the 

other product wetted materials are made 

of AL6XN and Hastelloy. 

Water vapor leaves the falling film 

evaporator and is introduced into the 

vapor scrubber. Boron, as a component 

of concern with a discharge permit limit, 

could potentially carry over. Therefore in 

order to reduce the boron concentration 

in the distillate (condensed process 

water vapor) the water vapor is 

scrubbed. 

The scrubbing occurs by an intensive 

vapor-liquid exchange with a caustic 

solution. Boron is “captured” in the 

liquid and therefore removed from the 

vapor stream. This vapor stream is 

compressed in a mechanical vapor 

recompression system. By increasing 

the pressure of the water vapor, the 

water vapor can be condensed at a 

higher saturated temperature and 

consequently used as the heating 

medium within the evaporation process. 

The resulting distillate is used for 

preheating the incoming FGD 

blowdown. 

During the concentration of the FGD 

blowdown calcium sulfate will 

precipitate. In order to reduce the 

possibility of precipitation on the heat 

transfer surfaces the Primary 

Evaporation system is operated in a 

seed configuration. Precipitation or 

scaling at the heat transfer surface area 

would reduce the evaporators’ efficiency 

and therefore limit the evaporation 

capacity. 

In a seed evaporation system, the 

predominate crystal (seed) in the 

system is used and artificially introduced 

into the feed stream. The seed crystals 

act as nucleation sites for the 

subsequent calcium sulfate precipitate 

and crystal growth as the FGD 

blowdown is concentrated. The seeding 

process is self-sustaining and after an 

initial seeding further seed addition is 

not required. 

In order to recover the seed crystals 

from the concentrated solution, a 

hydrocyclone is utilized where the 

overflow is sent to the Secondary 

Evaporator Feed Tank and the 

underflow, seed rich fraction, is 

collected in a Seed Slurry Tank and 

later combined with the un-concentrated 

FGD blowdown. 
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Secondary Evaporator

The concentrated FGD blowdown from 

the three Primary Evaporator trains will 

be further concentrated in one of the two 

Secondary Evaporators, since the 

primary evaporation step is limited to 

concentrate the wastewater close to the 

saturation point of the main salts in the 

solution, sodium chloride and calcium 

chloride. 

Figure 7: One of Two Secondary Evaporator, 

Forced Circulation Type Evaporator 

For the remaining concentration a 

successive evaporation step is required 

to reduce the wastewater volume to 

match the available fly ash for mixing. 

Fluctuations in the initial wastewater can 

result in crystallization at various 

concentration factors. To successfully 

manage variations, a forced circulation 

evaporator system was used, which is 

typically used for crystallization of salts 

in the food and chemical industry. 

The concentrated wastewater is 

introduced into the recirculation loop 

were the temperature is increased in a 

shell and tube heat exchanger that is 

heated with plant steam. Boiling in the 

heat exchanger is suppressed by its 

elevated pressure. By releasing this 

pressure to atmospheric conditions in 

the flash vessel the liquid starts to 

flash/boil. Since boiling is not occurring 

on the heat transfer areas, in contrast to 

the falling film evaporator, the system is 

able to tolerate precipitation and 

crystallization much easier. The 

generated water vapor is subsequently 

condensed in a shell and tube 

condenser and combined with the 

Primary Evaporator distillate for use 

within the Mayo Plant. 

Brine Cooler

Prior to storage of the concentrated 

wastewater (Brine) in the Brine Tanks 

the solution is cooled in a Brine Cooler. 

The solution is cooled to approximately 

140°F, which allows the use of lessexpensive FRP. 

Figure 8: One of Two Brine Cooler, 

Recirculated Type Cooler 
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Fly Ash Mixing

The brine is continuously pumped to the 

two fly ash silos in one of two brine 

piping recirculation loops (highways). 

Each brine loop can feed each of two 

new fly ash mixers. The fly ash and 

brine mixture is loaded into dump trucks 

and hauled to a new on-site lined ash 

storage landfill with a leachate collection 

and use system. 

Summary

The installation of the Zero-Liquid 

Discharge system at Duke Energy’s 

Mayo Plant is the culmination of site 

specific situation and constraints, and 

development of alternative FGD 

wastewater treatment methods. The 

capability to remove multiple 

constituents from the FGD blowdown 

and recover high quality distillate makes 

this the solution of choice for the unique 

conditions at Mayo Plant. Flexibility and 

redundancy are key requirements to the 

design and of upmost importance to 

Duke Energy. 

The ZLD project started at the beginning 

of 2011 and is scheduled to be 

operational by the end of 2013. 

References: 

Environmental Protection Agency (2007), Permit NC0038377, December 14 

Melches C., Loewenberg M., Hofmann G. (2011), “Concepts in Zero-Liquid 

Discharge”, International Water Conference, Paper IWC 11-49 

Shaw W. (2009), “New Low-Temperature ZLD Process”, International Water 

Conference, Paper IWC 09-52 

Mandigo G. (2009), “ZLD Achieved for Treatment of FGD Waste Water at Iatan 

Station”, International Water Conference, Paper IWC 09-51