Khanna A.S. (Ed.) High-Performance Organic Coatings: Selection, application and evaluation

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High-performance organic coatings for

wastewater treatment plants

D DHAGE, EWAC Alloys Ltd., India

18.1 Introduction

particularly to aggressive environments. High-performance coatings

perform under harsh exposure conditions such as extremely polluted indus-

trial environments, marine environments, splash zones and spillage areas.

High-performance coatings show resistance over a wide range of pH, strong

resistance to acids and solvents, and also properties of high electrical resis-

tance, strong adhesion to the substrate and resistance to marine organisms.

Typical applications where high-performance coatings are extensively used

are storage tanks, pipelines, scrubbers, ducts, marine constructions, harbour

facilities, etc.

The high-performance anticorrosion coatings that are designed today are

able to achieve longer service life with consistent performance over 15

years. These coatings are used to protect metals, concrete structures, tanks,

pipes and processing equipment from deterioration, caused by exposure to

corrosive environments. This chapter deals with the application of high-

wastewater treatment plants.

In recent years, a lot of attention has been given to reducing the effect

of waste material on the natural environment and to recover resources from

it. Clean water is essential for everyday activities, personal health and

industrial usage. These activities result in production of unwanted or unde-

sired materials generally called waste. Wastewater travels from collection

systems to wastewater treatment plants where it is treated and discharged

to rivers, lakes and streams.

In India, nearly 27.022 million litres of sewage waste is generated per day

across the country. The current wastewater treatment plants are capable of

treating only 18.6% of the total amount. In Mumbai, 50% of the sewage

goes untreated into the Arabian Sea. With such marginal sewage treatment,

many Indian rivers are heading towards environmental disaster [1].

High-performance coatings can be defined as coatings that are applied at

high film build, in order to provide exceptional resistance and durability,

performance glassflake reinforced coating systems to combat corrosion in

High-performance organic coatings 389

Wastewater treatment plants control processes and equipment to remove

or destroy harmful substances, chemical compounds, and microorganisms

in the water. The waste exists in different forms, viz., solid, liquid and gas.

With increase of industrial activity and the various complex processes

involved, the complexity of the waste has increased manifold in waste treat-

ment plants.

A total investment of Rs 7655 crore is estimated for treating the entire

sewage waste throughout India. Often, the present treatment plants are

running beyond their capacities, in turn demanding higher life for waste

treatment plants in the future.

18.2 Categories of waste treatment plants

Waste treatment plants can be categorized as below:

• Industrial waste treatment plants

• Municipal waste treatment plants.

18.2.1 Industrial wastewater treatment plants

Industrial wastewater treatment plants treat water that has been generated

in various industrial processes by contamination due to industrial activities,

before releasing it into the environment or reusing it. The various industries

that generate wastes are as follows:

• Iron and steel industry. The iron and steel industry waste involves con-

as benzene, naphthalene, phenols, cresols and range of organic hydro-

carbons in waste streams. Wastewater also includes pickling acids and

soluble oils.

• Mining industry. In the mining industry, the waste is in the form of

surfactants and oils, especially hydraulic oils.

• Food and beverage industry. Waste from the food and beverage industry

has a high biological oxygen demand (BOD) and much suspended

solids. The waste is often very complex due to its wide range of pH. The

materials with acids and alkalis.

• Chemicals industry. Chemical industries like pesticides, pharmaceuti-

cals, dyes, petrochemicals, detergents, etc., involve various complex

processes. Therefore the resultant waste is also more complex. The

waste consists of material in dissolved or particulate form, cleaning

agents and solvents.

taminated products like ammonia, cyanide gasification products such

quarries and slurries. The contaminants are fine particulate haematite,

• Common effluent treatment plants

waste also includes insecticides, antibiotics, flavouring and colouring

390 High-performance organic coatings

• Nuclear industry. Nuclear waste is generated from fuel that contains

small amounts of uranium and thorium. A high level of waste is gener-

ated from nuclear reactors containing transuranic elements.

Treatment of industrial wastewater

Industrial wastewater treatment plants require different procedures for

wastewater treatment plant is shown in Fig. 18.1.

With exponential increase in small-scale industrial units, a need for afford-

able and cost-effective waste treatment measures for treating the waste is

foreseen. Setting up individual waste treatment plants may not be a feasible

imperative.

18.2.3 Municipal waste treatment plants

These are generally sewage treatment plants or domestic wastewater treat-

ment plants containing physical, chemical, organic, inorganic and mainly

biological contaminants. Municipal wastewater treatment plants face a

more aggressive corrosive environment than do industrial waste treatment

plants. In industrial wastewater treament plants, the corrosion is often

Collecting chamber

Cooling

chamber

Water

conditioning

tank

Settling tank

Reactor

Treated

effluents

Chemical dosing

Gas filter

18.1 Typical industrial wastewater treatment plant.

treating their waste and are specific to their industry. A typical industrial

18.2.2 Common effluent treatment plants

solution for small-scale units. Under such circumstances common effluent

treatment plants facilitate the proper management of effluent as per the

standard norms. Common effluent treatment plants cater for more than one

kind of industry and so segregation and pretreatment of the effluent is

High-performance organic coatings 391

below the waterline and predictable. In municipal waste treatment plants,

the corrosion is above the waterline and highly unpredictable. Corrosion is

mostly induced by biological organisms. Also there are various chemicals

in the system that cause severe corrosion.

A typical municipal wastewater treatment plant is shown in Fig. 18.2. In

a typical municipal wastewater treatment plant, waste is treated as

follows:

1. Preliminary chamber. Raw sewage passes through bar screens and grit

removal.

3. Sludge holding tanks. Biosolids are removed in the primary and sec-

ondary processes and blended prior to further treatment.

4. Aerating plants. These generate pure oxygen for biological growth.

5. Secondary aeration reactor. This mixes oxygen and returns sludge to

activate organisms for decomposition of pollutants.

6. Chemical dosing. Bleaching agent is mixed with water and injected

7. Dechlorination. The remaining bleach is removed by adding sodium

bisulphite.

9. Sludge thickening and treatment. This process uses polymer and dis-

solved air to reduce water content in the wasted secondary biosolids.

Biosolids are removed in the primary and secondary processes and

blended prior to further treatment.

10.

wasted secondary biosolids.

11.

Landfill

River

Sewage waste

Sludge treatment

18.2 Typical sewage treatment plant (see text for description of each

numbered part).

classifiers.

Bioroughing chambers. Ultra high-rate trickling filters assist in BOD

into the final effluent.

Sludge solidification. This treats sludge to reduce water content in the

Dewatering systems. Sludge is conditioned by vacuum filters followed

by landfilling.

Effluent pump station. Treated effluents are finally pumped out.

392 High-performance organic coatings

18.3 Materials of construction and their deterioration

Concrete is the state-of-the-art construction material for wastewater

tures are made of concrete. Even though the concrete used is as per

hydrogen sulphide and sulphuric acid generated by microbial activity and

various other chemicals present in the treatment plants. Due to the omni-

present nature of microbes and the available nutrients in aqueous environ-

ments, there exists microbially induced corrosion and its by-products.

These agents ensure loss in concrete and the metal surface of treatment

plants.

Hydrogen sulphide generation in municipal wastewater treatment plants

has always been associated with severe deterioration of concrete. The bio-

engender:

• Anaerobic microbially induced corrosion

• Aerobic microbially induced corrosion.

Sulphate-reducing bacteria (SRB), a type of anaerobic bacteria, have a high

growth rate of reproduction. Large colonies are formed over shorter

periods, which becomes potential sites for the onset of corrosion.

In aerobic conditions, the SRB that are present in submerged structures

convert the natural sulphates to H

S. This process lowers pH, encouraging

growth of Thiobacillus bacteria. The gaseous H

S, methane and CO

become

condensed on wet surfaces in the presence of sulphate-oxidizing bacteria

(SOB). These SOBs oxidize H

S to H

and other mild acids as their

waste products. This mechanism makes the surrounding environment highly

acidic, resulting in rapid corrosion [2].

Deterioration of concrete in highly aggressive media leads to the corro-

sion of reinforced bars inside the concrete structure. Coupled with micro-

bial activity and carbon dioxide gases from the environment, there is

breakdown of passivity by neutralization of concrete. Associated with this,

if ingress of chlorides crosses the threshold value, pitting corrosion of rebars

starts, resulting in degradation of the steel reinforced structures embedded

in the concrete [3]. Rebar corrosion inside the concrete structure over a

period of time is shown in Fig. 18.3.

18.3.1 Corrosion prevention

Concrete corrosion costs crores of rupees every year in repairs. Corrosion

reduces the life expectancy of concrete structures and sometimes results in

structures like dry sludge beds, clarifiers, vessels, trenches and other struc-

the design, specification and standard, it deteriorates mainly due to the

logical species are classified in two categories based on the corrosion they

treatment plants. The primary effluent chamber, collecting chambers,

High-performance organic coatings 393

catastrophic failures. There are various ways to combat corrosion, which

are as follows:

• Application of protective coating

• Application of cathodic/anodic protection

• Use of inhibitors and chemical additives.

Protective coatings are not the only means by which corrosion can be pre-

vented, but they are the most important, often the most economic and

sometimes the only appropriate method for protecting the structure against

corrosion. The performance of a coating depends on the bond between the

substrate and the concrete where the concrete should have high surface

strength. Concrete having pH between 6 and 9, tensile strength greater than

1.5 N/mm

, cured and aged at a temperature of 24°C for a minimum of 30

days is recommended for coating [4].

Coating performs the role of a barrier, thereby preventing the external

environment from reaching the substrate and thus inhibiting corrosion.

Since over the service life the environment penetrates and reacts with the

coating, the porosity of the coating is a major factor governing the corrosion

of the steel inside. The effect of the environment is evident from changes

in the properties of the coating such as resistivity, alkalinity, carbonation

and diffusion of chlorides, sulphates, etc. The properties of the steel, such

ence the corrosion rates.

provide protection. Coatings provide protection by becoming a physical

barrier, or shield and isolate the steel substrate from its immediate environ-

ment. A barrier coating must prevent aggressive liquids and gases from

Microbes

Crack

Corroded steel

Deteriorated concrete

Rigid concrete

Reinforced bars

Corrosive environment

–

, CO

S/H

Time

18.3 Corrosion of reinforced bars embedded in concrete structure.

Protective coatings do not require inhibitive or sacrificial pigments to

as composition, mechanical working and deformation pattern, also influ-

394 High-performance organic coatings

passing through it and reaching out to the steel substrate. One important

property of a barrier coating is called permeability. The permeability of a

barrier coating layer depends on its moisture vapour transmission rate. A

lower rate of permeability yields better barrier properties to the coating.

Along with this, the basic resin chemistry of coatings also plays an impor-

tant role. The higher degree of resin crosslinking results in lower permeabil-

ity and better adhesive bond of the coating to the surface [5].

18.4 Protection by coatings

The purpose of the coating is to act as a barrier between the environment

and the substrate metal, as described above, and is a combination of:

coating

• A barrier providing resistance to chemical attack

•

electrons.

The reactants necessary for corrosion to proceed in normal environments

are water, oxygen and sulphur dioxide (SO

). Aggressive ions (Cl

−

ions) can

products.

ment is well established. A couple of decades ago, the concentration of H

in the municipal sewerage system was marginal. Presently, the average H

concentration reported is about 2–7 ppm. In most of the system headspaces,

sulphuric acid concentration has increased from 1% to 7%. Increase of

industrial activity, with more complex processes involved therein, and also

growing concern with environmental protection activity with the enforce-

ment of stringent laws such as the environmental protection acts of 1986,

prohibiting the release of pollutants, heavy metals and creating more head-

spaces for odour control measures in the environment, have increased the

S concentration many times. The resultant effect is a severe increase in

the corrosion rate in the treatment plants.

Protective coatings such as coal-tar epoxies have been largely used to

protect waste treatment plants against corrosion over the years. But in

recent years there has been tremendous change in the exposure conditions

of wastewater treatment plants with higher sulphuric acid and H

S concen-

tration. These changes in exposure should be taken into account when

selecting a coating system.

Conventional coating systems such as coal-tar epoxy and amine-cured

epoxy with proper surface preparation were providing protection for 10–12

years. But in changed exposure conditions these conventional coating

• A physical barrier – a non-porous, flawless, defect-free, insoluble

An electrical resistor: it offers high resistance to the flow of ions and

also influence the rate of corrosion by formation of soluble corrosion

Concrete corrosion in effluent treatment plants due to the H S environ-

High-performance organic coatings 395

systems are showing premature failure within six months to two years.

to acid and the high permeation level to gaseous environments [6].

18.4.1 Criteria for selecting coatings for wastewater

treatment plants

Due to changes in exposure conditions, the following criteria should be

considered for protecting the concrete and structures in closed and exposed

conditions:

• Resistance to higher sulphuric acid concentration

• Resistance to hydrogen sulphide

• Low permeation to water vapour transmission

• High coating build-up

• High adhesive/bond strength.

Other physical properties of the coating, e.g. abrasion resistance and coef-

selecting the coating.

Over the years, the most universally adopted practice for corrosion preven-

tion is to use anticorrosion coatings, since they provide a practical and

economical solution. If all corrosive ions were prevented from reaching the

substrate, corrosion would never occur. However, in practice an imperme-

able barrier may not be possible to achieve. It is impossible to form a totally

permeation through the coating allows corrosion to take place.

Conventional coatings consist of an inhibitive prime coat followed by a

barrier coat system. The primer is intended to protect the corroding mate-

never be permanent because the inhibitive pigments are consumed in pre-

venting corrosion. Topcoats in the system have the purpose of protecting

the primer from chemical attack, providing a barrier from the environment,

and reducing permeation of corrosive ions to a level that can be coped with

by the primer. When the topcoat system is effective in reducing permeation,

ence accelerates transport of corrosion ions, thereby increasing the perme-

ation rate.

There are two ways of looking at a problem of utilizing coatings for cor-

rosion protection:

ficient of linear thermal expansion, should be taken into consideration for

impermeable barrier to corrosive ions with organic materials, and sufficient

rial by either passivation or sacrificial means, but the effectiveness can

18.5 Glassflake- filled coating systems

the primer will consume rapidly and corrosion will result. Electrical influ-

These failures can be attributed to the insufﬁ cient resistance of the coating

396 High-performance organic coatings

• either reduce permeation of corrosive species to a level where corrosion

• use an adequate inhibitive system with good topcoat barrier.

are permeable to water vapour because of their molecular structure. No

brought to a lower level. The permeability depends upon various factors,

ness applied.

lel and overlap over each other, thereby forming a barrier in the coating

system for gaseous molecules and increasing the path of corrosive media

to reach the substrate. This effect increases the path for corrosive ions –

known as the ‘tortuous path’ effect. In conventional coating systems, the

particles or barrier material are granular or spherical in nature and cannot

coating matrix is represented in Fig. 18.4.

The principle lies in providing a longer tortuous path through the resin

binder in order that corrosive ions travelling through the permeable binder

will have to travel a much longer distance. The tortuous path property gives

orientation. The distance travelled by corrosion ions will depend upon:

•

achieved in any given thickness of coating

Conventional coatings

Steel surface Steel surface

Spherical/angular particles

Glass flakes

Glassflake coatings

Corrosive

media

is stopped or becomes insignificant, or

polymer is impermeable in itself, but with fillers the permeability can be

including the resin chemistry and the fillers that are incorporated in the

theoretically 20 times longer path length than unfilled binder, but in prac-

tice 9–11 times longer path length can be achieved due to difficulty in

A glassflake coating system falls into the first category. Organic coatings

polymer matrix. Glassflake coating forms the best barrier on the surface

with approximately 100 layers of flake for every millimetre of coating thick-

Glassflakes in a coating matrix, due to their high aspect ratio, align paral-

overlap, thereby offering limited resistance [7]. Glassflake alignment in the

obtaining symmetry of flakes despite the use of leafing agents and physical

Thickness of flakes, which determines the number of layers that can be

18.4 Glassflake alignment in coating matrix.