The Influence of NOM in Raw Water on the Power Industry – A Review of

the Eskom Experience

Speaker / Author: G Gericke

Eskom RT&D

Private Bag 40175, Cleveland, Johannesburg, 2022

Email:gerhard.gericke@eskom.co.za

Phone: 011 629 5721

Abstract

The focus of this paper is on the influence of natural organic matter (NOM) in the Eskom

power generation plants of South Africa and the strides taken by the Eskom power utility

to deal with the challenges presented by the occurrence of NOM. A review of recent

literature concludes the impact of NOM on power plant chemistry and the detrimental

impact of NOM in the power industry was highlighted. This paper concludes the research

programme that was established by Eskom to identify and address the organics presents

in the raw water and their impact on power plant operation. The treatability and removal

of the NOM in a water treatment train was investigated by determining the character and

composition of the NOM samples taken at various stages of the water treatment process.

Identification of NOM in Eskom Identification of NOM in Eskom raw water supplies was

performed by making use of LC-OCD and SUVA-254 to determine the treatability of the

water from the various catchments supplying the Eskom power plants as well as the

impact of peroxide catalyzed UV -254 nm oxidation.

1. Introduction

Water treatment forms an integral part of thermal electric power generation (coal fired as

well as nuclear power stations) [1,2]. There are generally two main water cycles

associated with thermal power generation, namely the steam/water cycle that uses demin

water for the production of steam and the cooling water circuit, which is used for

condensing the steam back to water. The two cycles are isolated from each other and

exchange heat via a condenser.

Power station make-up water treatment plants have traditionally been designed to produce

water of a very high quality (free from inorganic salts) by synthetic ion exchange resins

or reverse osmosis. Raw water quality, mainly total dissolved solids (TDS), oxygen

absorbed (OA) and silica (SiO2) is the key factor in the design of the plant. Any

deterioration of raw water quality during the life cycle of the plant, which is currently a

South African and worldwide phenomenon, will inevitably have a negative impact on

treatment plant performance and plant integrity. To meet the specifications of the water

quality required for proper plant operation, an extra burden, in terms of associated

treatment costs, will therefore have to be placed on existing treatment processes to

compensate for change in feed water quality.

Apart from increasing salt concentrations in raw waters, there are indications that natural

organic matter (NOM) present in the water is also increasing.

2. Demin water specifications and treatment processes employed by Eskom

Modern high-pressure power generating plants require very high purity feed water to

ensure plant integrity and to reduce the failure of plant components. Strict guidelines are

enforced by the power industry with regard to make-up water

quality (Table 1) [1].

Table 1. Demin water specifications for coal fired power stations measured at the mixed

bed outlet Parameter Target Limit

Turbidity (FTU) <0.1 0.2

Specific conductivity (µS/cm @ 25 °C) <0.08 0.1

Sodium (µg/l as Na⁺) <1 2

Silica (µg/l as SiO₂) ALARA 10

Chloride (µg/l as CI-) <1 2

Sulphate (µg/l as SO₄²-) <1 2

TOC (µg/l as C) 100 250

These specifications were developed over the years by institutions such as EPRI and the

VGB and were traditionally limited to the inorganic species only, but recently the

specifications were broadened to also include total organic carbon (TOC) values. These

values were however chosen as guidelines only with the intention that they could be

increased or decreased as more knowledge is gained with regard to the impact of organic

contamination in plant systems [1,3-6].

3. The nature and origin of natural organic matter

Natural organic matter (NOM) is present in all natural water sources and comprises

organic compounds with various chemical properties. The presence of NOM is as a result

of the decay of animal and vegetal matter [7-9] and it is normally quantified and

expressed as TOC. TOC is defined as the sum of the dissolved organic carbon (DOC),

non-purgeable organic carbon (NPOC), particulate organic carbon (POC) and volatile

organic carbon (VOC) fractions [10, 11].

NOM can be divided into a hydrophilic and hydrophobic fraction. The hydrophilic

fraction comprised of microbial by-products and colloidal matter, whilst the hydrophobic

fraction consists mainly of humic substances (HS) which are highly aromatic, high

molecular polyfunctional weak organic acids and sparingly soluble in water [1,6,7]. It is

also generally accepted that the composition of NOM in aquatic systems is not uniform

due to the varying quality and origin of different water sources. Humic substances are

responsible for the brown or yellow colour in surface waters containing high levels of

NOM (10 – 20 ppm C) [7-11].

By definition, DOC is classified as that fraction of NOM that can pass through a 0.45 µm

filter. DOC contributes to between 5 and 10 % of all anionic species in natural waters.

This ionic character is due to the dissociation of carboxylic acid groups. Calcium and

sodium ions are generally the counter ions associated with these negative groups. When

phenolic groups are present, metal ions can chelate or bond to these structures [7,10].

Humic substances can further be divided into three main groups of organic compounds

namely [12]:

• Humic acid: that fraction of humic substances that is not soluble in water under acidic

conditions (pH < 2.0),

• Fulvic acid: that fraction that is soluble in water under all pH conditions and finally

• Humin: that fraction that is not soluble in water at any pH level.

4. The impact of NOM within the power industry

Three classical examples indicating the impact of NOM on power plant chemistry are

summarised in the following section.

4.1 Impact of NOM on the water demineralisation process

Although water demineralisation processes can cope very well with the removal of ionic

inorganic compounds from raw water, the removal of organic species poses a greater

problem. It has been reported that anion exchange resin is ideal for the removal of NOM

from clarified and filtered water [8,9]. Fouling of the anion exchange resin (reversible as

well as irreversible fouling) normally occurs during this process, where fouling can be

considered as the incomplete removal of the adsorbed species during regeneration. The

impact of this on the entire demineralisation process as used within the power industry or

the use in ultra-pure water production, is well documented [10-12].

During the regeneration process (caustic for anion exchange resin) the organic

compounds are not completely removed due to the high affinity of the resin for the

organic molecules. Gradual fouling of the resin takes place over a period of time and the

resin starts to show the following symptoms [10-13]:

• The rinse water requirements increase.

• The carboxyl groups of the organic compounds interact with the Na+ ion of the caustic

to form sodium carboxylate (Figure 2) which eventually hydrolyses back to the free

acid releasing sodium ions into the product water.

• Gradual increase in product water conductivity and decrease in pH due to organic acid

leakage.

• Gradual silica leakage.

• A gradual decline in exchange capacity over a period of time.

• Shortened operating times.

To overcome this problem, caustic brine washes need to be carried out from time to time

and the frequency of these clean-up procedures will increase with increasing fouling

levels.

The phenomenon of sodium and TOC leakage from fouled anion exchange resin can

clearly be seen from results that were obtained from a demineralisation unit at Matimba

Power Station which had a design specification of 9 Ml in Fig. 2 and 3.

Figure 2: Sodium leakage levels from various stages in the demineralisation process

(Measured continuously with a SWAN -AMI™ on-line sodium analyser)

Figure 3: TOC leakage levels from various stages in the demin process (Measured

continuously with a Sievers - 900™ Total Organic Carbon Analyser)

4.2 Impact of tihalomethane on power plant steam/water cycle

It is generally accepted that organic matter will undergo thermal decomposition upon

entering the steam/water circuit forming carbon dioxide and lower molecular weight

organic acids [2, 13, 14] Superheated steam often contains low concentrations of volatile

acids which can range from sub-ppb to ppb levels [2 – 6].

These low concentrations of organic acids and carbon dioxide will impact on the cation

conductivity (or acid conductivity) of the steam, condensate and feed water. Cation

conductivity is the conductivity of an aqueous solution measured at the outlet of a strong

cation exchange resin column. The cations are exchanged for hydrogen ions resulting in

the formation of an acidic solution which has a relatively much higher conductivity than

the saline solution. This is due to the high specific ion conductance of the hydrogen ion

(SHC). Due to the low ionic strength of ultra-pure water, trace concentrations of

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6

So

diu

m c

on

cen

tra

tio

n (

mg

/l)

Volume of water produced (Ml)

Cation out

Anion out

Mixed bed out

contaminants are very difficult to detect by conventional conductivity measurement,

however the measurement is much more sensitive if the equivalent acid is measured.

Cation conductivity is a critical measurement in steam/water circuits and a target of

0.200 µScm-1 has been set to ensure material protection and plant performance [1].

In many cases inorganic compounds associated with the organic molecules will also be

released in the form of inorganic acids [14]. Organics in the raw water for instance, will

react with chlorine during the treatment process forming halogenated organic compounds

that are not removed by the ion exchange process. When decomposed, these organics will

not only form the lower molecular weight organic acids and carbon dioxide, but also

hydrochloric acid. This will give rise to a lowering of the pH and increase in conductivity

in the regions of first condensate formation on the low pressure stage of the

turbine [15 -17]. It is these regions that are more susceptible to acidic attack. Eskom’s

own experience at both Koeberg Nuclear Power Station and Kendal Power Station bear

testament to this [14 – 17, 18].

Traditionally, raw water was pre-chlorinated to reduce algal growth within the clarifiers

and sand filters. This practice was however terminated due to the formation of

trihalomethane (THM) which cannot be removed via the ion exchangers. These

compounds would then thermally break down within the steam water circuit resulting in

an increase in chloride ions. This situation is quite evident at Koeberg Power Station as

the station receives potable water that requires treatment for station make-up. By the time

the water reaches the station it already contains THM. This water is then further

chlorinated resulting in even higher THM concentrations.

Chloride in contact with metal surfaces has a detrimental effect on the structural integrity

for power plants. Inconel, a metal alloy consisting of nickel, chromium and iron is used

for the manufacture of steam generator tubes of Pressurised Water Reactors worldwide

because of its physical properties. Steam generator tubing is by far the most vital

component of this type of nuclear power plant and Inconel is susceptible to chloride

induced pitting corrosion. Chloride also causes stress corrosion cracking in austenitic

stainless steels that constitute the balance of the plant systems. For these reasons it is

imperative that plant chemists must continue to strive to keep the chloride concentration

as low as achievable [14].

Table 2 shows the chloride concentrations obtained in the make-up water (ultra-pure

water) after UV/TiO2 oxidation [14] (chloride levels were measured by ion

chromatography).

Table 2. Chloride levels detected in demineralised water after UV/TiO2 oxidation.

4.3 Impact of NOM on the calcium precipitation potential in cooling water systems

The role of the cooling water system at large steam generating plants is to cool down the

exhaust steam (used to turn the turbines) for recycling. The cooling water flows through

condenser tubes and the steam passes over these cold tubes which enables condensation.

Due to the high TDS concentration, scaling of the condenser tubes can occur and result in

decreased thermal efficiency [19].

Various studies have shown that the presence of NOM can influence the precipitation

potential in cooling water systems. Metal complexation with NOM differs depending on

the type of metal present, size and type of the organic molecule, pH and temperature of

the water [20]. This complexation of metals with organics affects the scaling potential of

the water [21]. Visual MINTEQ is a modelling programme that takes this metal

complexation into consideration when calculating the saturation index of the cooling

water and was used during the Eskom study [19].

Due to evaporation and concentration of chemical species in the water, NOM values in

excess of 70 mg/l C (as TOC) are commonly measured in Eskom power plants.

Traditionally Eskom used the EDTA complexation method to determine Ca2+ and Mg2+

concentrations in the cooling water system and based on the results would adjust the

chemical dosing to prevent scaling if required [18]. To protect the plant material from

acid attack, Eskom opted for a slight scaling regime. In recent years however, inductively

coupled plasma emission spectroscopy (ICP-OES) is being used to determine the Ca and

Mg concentrations. By adjusting the chemical dosing and control, plant operators found

that they cannot maintain the thin scale layer in the pipe work anymore resulting in bare

metal being exposed to the harsh cooling water conditions.

Upon investigation it was found that the ICP method reported higher Ca/Mg values than

what was measured through titration. The reason for this was that Ca/Mg complexation

rendered these ions unavailable for the complexation with EDTA. Due to the high plasma

temperature of the ICP the organic-metal complex was broken, resulting in the total

Ca/Mg concentration being measured.

The impact on Langelier Saturation Index (LSI) due to the different analytical techniques

can clearly be seen in Table 3.

Table 3. Effect of LSI (using ICP Ca2+ as well as titrated Ca2+ concentrations) at different

temperatures.

Through additional studies conducted, it was concluded that metals do bind with NOM in

the water. The results from this study clearly indicated that the organics in the water play

an important role when it comes to determining whether the water has a potential to scale

(Scaling Index, SI). The raw water quality in particular greatly affects the cooling water

in terms of the scaling potential.

5. The Eskom research approach

The real questions that need to be answered for effective water treatment are the

treatability of the water and the impact of the NOM on the treatment process. Specific

ultra violet analysis at 254 nm (SUVA-254) can be used to determine the treatability of

the water from the various catchments supplying Eskom power plants [20]. Based on

these results, targets can be set for organic removal at a power station based on the raw

water supply to that specific station.

In order to fully understand the impact of NOM on power plant operations, proper

identification of the species has become of importance. To facilitate this, liquid

chromatography organic carbon detection (LC-OCD) supported with fluorescence

emission excitation measurements (FEEM) can be employed [23-25].

Eskom embarked on a research programme which included the following:

Evaluation of selected water treatment chemicals (coagulants and flocculants) on

various raw water streams using standard laboratory jar tests to determine the

efficiency of organic removal.

The most suitable treatment chemicals were subjected to plant trials to determine

their efficacy in terms of organic and turbidity removal.

Optimisation of the operating conditions for each water plant [12].

Treatability of various water streams in terms of NOM removal through clarification

and flocculation.

Quantification of NOM and removal efficiency of treatment processes in Eskom raw

and treated waters by liquid chromatography organic carbon detector (LC-OCD).

5.1 Plant optimisation, chemical selection and treatability

The first step in ultra-pure water production is the clarification process and filtration in

order to remove suspended solids to a turbidity of less than 0.1 NTU. These processes

have been optimised to obtain maximum removal trough the practice of enhanced

coagulation and flocculation using poly-aluminium chloride and a minimum of organic

coagulant, not exceeding 0.8 mg/l [20]. Further to this, operating personnel has to ensure

that sand filters are kept clean and free from organic matter and biological growth through

proper sanitation. Proper chemical selection and mixing as well as optimised clarifier

operation (sludge control and up flow rate selection) is also of importance. Figure 4

shows typical results for optimum organic removal through process optimisation.

The real questions for water treatment are however the treatability of the water and the

impact of the NOM on the treatment process. SUVA-254 was used to determine

treatability of the water from the various catchments supplying Eskom power plants over

a number of dry and wet seasons and the results are depicted in Table 4.

Table 4. SUVA-254 nm values of Eskom raw water

Figure 4. Organic removal as measured at UV-254 nm across a clarifier before and after

process optimisation

5.2 Species removal across a treatment process as measured by LC-OCD

Although TOC/DOC measurements are widely being used for NOM quantification, the

technique suffers from lack species identification. Species identification is of special

interest where the performance of a treatment needs to be investigated and to answer

Before optimisation

After optimisation0

20

40

60

80

100

Clarifier

out Sand filter

out Cation outAnion out

Mixed bed

out

questions such what is being removed or what is being added. LC-OCD can distinguish

between humics, polysaccharides, so-called building blocks of humic structures, acids and

low molecular acids. The analysis clearly shows (Figures 5 and 6) impact of the

clarification process and hydrogen peroxide catalysed ultra violet (254 nm) on organic

removal and oxidation respectively.

Figure 5. The impact of clarification on TOC removal on water from the Vaal River

system (Measured with a Doc Labor LC-OCD organic analyser)

Table 5 shows the % species removal, measured by LC-OCD, of clarified water (30 mg/l

PAC and 0.8 mg/l poly-amine) from the Crocodile West River which will be the feed water

to Medupi Power Station. The TOC concentration, prior to treatment, was 6.96 mg/l and

3, 87 mg/l after coagulation and sedimentation.

Table 5. Organic species removal through coagulation and flocculation of Crocodile West

water (Measured with a Doc Labor LC-OCD organic analyser)

Species Before treatment (mg/l) After treatment (mg/l) % Removal

Building blocks 1.13 0.75 34

LMW neutrals 0.40 0.16 61

LMW acids 0.047 0.011 77

Biopolymers 1.76 0.043 97

Humic substances 3.62 2.86 21

Total 6.96 3.82 45 overall

Figure 6. The influence of hydrogen peroxide UV-254 nm oxidation on NOM species in

water from the Crocodile West River (Measured with a Doc Labor LC-OCD organic

analyser)

6 . Conclusion

The differing levels and composition of NOM in South African water sources within the

different geographical regions suggest that a water treatment strategy by a treatment plant

should address the issues relating to specific NOM occurring in the source water and its

treatability. The use of LC-OCD for NOM identification on the treated water at the Eskom

power station indicated the efficiency of the various steps in the pre-treatment process to

reduce NOM compounds. This identification and quantification of the various NOM

species is far superior to a standard TOC analysis. By monitoring individual organic

matter species (specifically using LC-OCD), problem areas within a process can be

highlighted and action can be taken towards plant optimisation to improve removal of the

identified fractions.

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