waste water effluent technology

7/29/2019 Waste Water Effluent Technology

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Wastewater effluents

Technology

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Thermal Generation Study Committee20.04 THERCHIM

20.05 THERRES

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April 1997

Ref : 02005Ren9775


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Thermal Generation Study Committee

Groups of Experts: Thermal Power Plant Chemistry Mangement and Thermal

Power Plant Residue Management

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Wastewater Effluents

Technology

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Paper prepared by:

Roger ROOFTHOOFT (LABORELEC, Belgium); William S. KYTE (POWERGEN,United Kingdom); Eric SMITSHUYSEN (VESTKRAFT, Denmark); Antonio TOSI

(ENEL, Italy); Gianmaria GUARDIANI (ENEL, Italy); Stefano CONCARI (ENEL,

Italy)


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C O N T E N T

1. INTRODUCTION AND OBJECTIVES OF THE REPORT 1

1.1. GENERAL INTRODUCTION 1

1.2. OBJECTIVES OF WATER EFFLUENT TREATMENT TECHNOLOGY REPORT 1

2. WATERS USED IN POWER PLANTS 2

2.1. CONTINUOUS OR REGULAR USED WATER 2

2.1.1.COOLING WATER 2

2.1.2.BOILER MAKEUP WATER 2

2.1.3.SLAGFLUSH,COOLING WATER,ASH TRANSPORT WATER 3

2.1.4.DESULPHURISATION PLANT MAKE UP WATER 3

2.1.5.FLUSH AND DRAIN WATER 3

2.1.6.LABORATORY WATER 4

2.1.7.SANITARY WATER 4

2.2. PERIODICALLY OR SELDOM USED WATER 4

2.2.1.WASHING OF BOILERS,AIRPREHEATERS AND ASH PRECIPITATORS 4

2.2.2.BOILERACID CLEANING AND CONDENSERACID CLEANING 4

3. OVERVIEW OF WASTEWATER TREATMENT TECHNOLOGY 5

3.1. INTRODUCTION 5

3.1.1.BASIC PRINCIPLES 5

3.1.2.THE BASIC TECHNOLOGICAL PRINCIPLES 6

3.2. TECHNOLOGIES 8

3.2.1.OILY WASTEWATER 8

3.2.2.SANITARY WATER 19

3.2.3.SUSPENDED SOLIDS 22

3.2.4.INORGANIC POLLUTANTS 27

3.2.5.SPECIFIC INORGANIC POLLUTANTS 36


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3.2.6.FINAL TREATMENT 40

4. SURVEY OF THE SITUATION IN UNIPEDE COUNTRIES 43

5. CONCLUSIONS AND RECOMMENDATIONS 56

5.1. CONCLUSIONS 56

5.2. RECOMMENDATIONS 57


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EXECUTIVE SUMMARY

In the past, the subject of waste water from power plants has received little attention and was not considered

as a high priority topic. Increasing focus on water quality in recent years however resulted in more severe

restrictions on discharges.

Therres and Therchim, two Unipede Expert Groups, recognised the need for a report in this area and agreed

that a joint study had to be undertaken. Four major topics were identified. This report focuses on Water

effluent treatment technology. It gives an overview of available technologies and effluents of thermal

power plants. This overview is the result of an inquiry performed in Unipede member countries. The

different flows are identified in nature and flow rates or annual volumes (chapter 4).

Continuous waste streams are mainly cooling water, slag flush and ash transport water, FGD-effluents,

drains, laboratory water and sanitary water (chapter 2).

Discontinuous or periodic waste effluents are generated by acid cleanings and washing waters from boilers,

air preheaters and ash precipitators.

Specific technologies are available for oily waste water, sanitary water, suspended solids removal, heavy

metals and specific inorganic pollutants (mainly nitrogen compounds). These are described in chapter 3.

Possibilities and limits of the processes are given as well as small comparison tables taking economic

evaluations into account.

The report shows that :

many technologies, simple and advanced, are available for solving almost every problem of pollution

however good knowledge of the different steams and an appropriate design are necessary as well as a

good process control and appropriate supervision and maintenance

approaches vary from country to country in function of national or local regulations and depending on

age and design of the power plants

economic considerations can be of prime importance

almost all countries use technologies to reduce heavy metals and oils in water

water re-use and integrated water management get more attention.

It is recommended that integrated water and waste management should receive a high priority. The power

industry must keep a continuing awareness of developments in water treatment technology.

Exchange of information in this area should continue within the Unipede countries.


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1. INTRODUCTION AND OBJECTIVES OF THE REPORT

1.1. GENERAL INTRODUCTION

The subject of waste water from power stations has received little attention in the past and has

not in general been regarded as a high priority issue. However increasing focus in recent years on water

quality has resulted in ever tightening restrictions on the discharge of FGD and other emission control

technologies has greatly increased the number of chemical species that have to be considered.

These developments have meant that the complexity of waste water treatment has increased as well as the

risk of exceeding consent limits. Recognising this the two UNIPEDE Groups THERRES and THERCHIM,

independently decided that there was a need for a UNIPEDE report in this area. At the initiative of the two

chairmen the Thermal Generation Study Committee (THER) agreed that a joint study should be undertaken

in order to take advantage of the combined resources and expertise.

Accordingly a series of joint studies has been initiated under the generic title "Waste Water and Water

Residues Management. Four reports will be produced:

Waste Water and Water Residue Management

Regulations (THERRES)

Water effluent treatment technology (THERCHIM/THERRES)

Utilisation and disposal (THERRES/THERCHIM)

Monitoring and analysis (THERCHIM)

1.2. OBJECTIVES OF WATER EFFLUENT TREATMENT TECHNOLOGY

REPORT

The objectives of this report on Water effluents treatment technology are:

a) to give a general overview of the waters used and the technologies available for waste water treatment in

thermal power plants;

b) to survey the situation in thermal power plants in UNIPEDE member Countries:

c) to compare the treatment technologies used and to comment on their applicability with respect to

specific situations;

d) to make appropriate recommendations.


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2. WATERS USED IN POWER PLANTS

Beside the fuels, water is the most used chemical substance in the energy producing industry.

This section gives an overview of the water consumption on a power station and the waste water caused by

this consumption.

The quality of the water used varies from station to station depending on the resources accessible that often

depend on the geographical location. The water qualities/sources can be classified as:

clean water (e.g. municipal water, well water, etc.),

low salinity water (e.g. river water, lake water, etc.),

brackish water,

seawater.

At the power station these water qualities are processed/cleaned to meet the quality demands.

2.1. CONTINUOUS OR REGULAR USED WATER

2.1.1. COOLING WATER

Cooling water causes the steam from the turbines to condense in a large heat exchanger

(condenser). The demand for cooling water depends on the operation mode of the power station (back

pressure or condensing mode). When operating at condensing mode, a large amount of cooling water at

lowest possible temperature is required. Depending on the geographical location, cooling water is supplied

from a closed circuit or from an open circuit. A cooling tower is necessary in a closed circuit.

When flood or sea water is used, the circuit has to be constructed in a way to avoid corrosion. When a

closed circuit with a cooling tower is used, make up water has to be processed to avoid scaling and corrosion

in the system.

The effluent stream can contain small amounts of corrosion products from the condenser, bio and corrosion

inhibitors like biocides, chlorine, iron sulphate, etc.

The temperature increase of the cooling water may also cause problems, especially from systems without

cooling towers.

2.1.2. BOILER MAKEUP WATER

The water/steam system at the power plant contains high purity water. The water is

evaporated in the boiler to high pressure and high temperature steam. The steam is expanded through theturbine and condensed in the condenser. The condensate is returned through the feed water tank to the

boiler.

To avoid corrosion and depositions in the water/steam system there are very high demands for the purity of

boiler make up water. The conductivity of make up water is typically below 0.2 S/cm. The acid

conductivity of the steam is normally held well below 0.2 S/cm.

Waste water from the boiler (blow down) is normally very clean only containing small amounts of for

example ammonia and/or sodium hydroxide/phospates. This type of waste water seldom causes treatment

problems and the water is often reused for other purposes.


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To achieve this high purity water the available raw water has to be processed. The traditional way of

producing high purity water is to demineralise water in an ion exchange plant, after a pretreatment if

necessary. Other technologies like reverse osmosis, electrodialysis and distillation have been developed and

together with pretreatments like oxidation, flocculation, filtration, etc. give the possibility to use a very wide

range of water qualities in the production of boiler make up water.

The waste water from an ion exchange plant contains the concentrated impurities from the raw water as

well as salts from the regeneration chemicals (hydrochloric acid or sulphuric acid and sodium hydroxide).

Usually the ion exchange process is the most efficient water process ( 10 % wastewater) and reverse

osmosis is the less efficient process (25 - 30 % wastewater). The other processes are intermediate. The ion

exchange process is the most chemical demanding process.

To maintain the very high purity in the water/steam system it is possible to clean the condensate; this is

done by ion exchange technology (condensate polishing) and/or filtration.

Waste water from the condensate polishing plant contains ammonia and salts from regeneration chemicals.

Waste water from filters is flush water containing solids (corrosion products and precoats products, if any).

Some steam can be extracted from the main thermal cycle to preheat heavy fuel oil. The condensate could

be polluted by fuel oil itself.

2.1.3. SLAG FLUSH,COOLING WATER,ASH TRANSPORT WATER

The cooling and transportation of slag from the boiler to the storage can be performed by

means of water.

All types of water can be used. Restrictions can occur to avoid corrosion.

Slag cooling and transport water will contain suspended matter, dissolved salts, trace metals and may also

have an elevated pH.

When wet transportation of fly ash is used, demands and problems are similar.

2.1.4. DESULPHURISATION PLANT MAKE UP WATER

The quantities of water used in flue gas cleaning plants depend on the technology used, for

example dry, semidry, wet absorption and the absorption chemicals used.

Most water qualities can be used. At plants with a wet process often the make up water quality (salinity) sets

the demands on the system bleed off.

Only wet processes produce waste water. This waste water will typically contain chloride, sulphate, nitrate,

suspended matter and heavy metal salts.

2.1.5. FLUSH AND DRAIN WATER

Water is also used for all round cleaning purposes at the power station. Large areas and a

great part of machinery are flushed with water.

The quality of flushing water normally depends on its use so no damage is done to the flushed item.

Water that has been used for flushing/cleaning outdoors as well as indoors typically contains cleaning

chemicals, waste oil, coal and ash as well as desulphurisation and regeneration chemicals.


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2.1.6. LABORATORY WATER

In the laboratory there is normally a demand of two qualities of water: service water and high

purity water.

Waste water from laboratories can be characterised as either sanitary waste water or chemical waste, (often

a combination depending on the organisation of the laboratory, the discipline concerning the use of waste

receptacles and so on). The amount of laboratory waste water is extremely limited compared with other

waste water streams.

2.1.7. SANITARY WATER

Water for "human" use is usually well water or municipal water.

Sanitary wastewater from for example kitchens, bathing facilities, etc., will contain organic matter,

detergents and bacteria.

2.2. PERIODICALLY OR SELDOM USED WATER

2.2.1. WASHING OF BOILERS,AIRPREHEATERS AND ASH PRECIPITATORS

Power stations have different traditions in terms of washing the boiler and the flue gas path.

The boilers are typically flushed prior to the maintenance work on them. Washing is carried out to improve

the working environment and also to facilitate inspection of the boiler.

Most types of water can be used but low salinity water is preferred.

Used wash water contains solid combustion residues as well as dissolved salts and trace metals produced by

the combustion process. Depending on the type of fuel used the flush water may contain considerable

amounts of sulphuric acid or alkalising agents used to neutralise acid during operation.

2.2.2. BOILERACID CLEANING AND CONDENSERACID CLEANING

The purpose of the acid cleaning of the boiler is to dissolve and remove oxide film on the

inner walls of the boiler tubes. This is performed immediately before commissioning a new boiler or if the

amount of deposits on the inner tube surface gives rise to problems.

Water quality requirements are normally the same as for boiler make up water.

The contents of the waste water are dependent on the cleaning chemicals used. The typical contents are

hydrofluoric acid, hydrochloric acid, organic acids, ammonia, organic inhibitors, suspended matter and salt.

Condenser acid cleaning is by analogy with boiler acid cleaning but the water quality demands are less strict

and the acid used depends on the materials used for the construction of the condenser.


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3. OVERVIEW OF WASTEWATER TREATMENT TECHNOLOGY

3.1. INTRODUCTION

Superficially the treatment of wastewater from thermal power plants would appear to be a

very simple operation. Historically this has been the case and in many old power plants little or no

treatment was required. The presence of large quantities of cooling water meant that very often simple

treatments together with dilution were sufficient.

However, a number of developments have meant that the treatment of wastewater can now be quite

complicated, even sometimes approaching that on a chemical plant. The main developments have been:

1. increasingly stringent environmental constraints

2. more chemical processes, for example FGD

3. integrated plant water managementThe use of dilution to achieve concentration constraints is no longer an option.

These developments mean that increasing amounts of water have to be treated with ever increasing

complexity.

3.1.1. BASIC PRINCIPLES

The wastewater treatment technologies that are used on power plants are not significantly

different from those that are used in other industries. Complications can arise because many of the streams

that need to be treated are intermittent or of high volume. A further complication is often that waste

streams have been added during the lifetime of the plant so that water treatment may not be integrated but

built up on a piecemeal basis. This often means that technologies of different generations are utilised on the

same plant.

Another distinction that can make a significant difference is whether the power station is completely self-

contained or whether it is connected to an external sewage system, either domestic or industrial. In many

cases, power stations have been built in rural areas and have therefore had to be completely self-sufficient

and have their own domestic sewage system. In some cases the power plant has the benefit of being

connected to either or both domestic and industrial sewage systems. In these cases the treatment, on plant,

is very much simplified, though treatment may still be carried out in order to minimise sewage charges.

Water management, both on the process and wastewater side, is an exiting new concept that is being takenup on many power plants. This management is usually restricted to the power plants site but can in some

circumstances be extended to outside sources and sinks. Power stations are well used to integrating their

sources of fuel and uses of by-products with outside industries but the integration of water resources is a

much more novel concept. This can have very significant advantages for the environment. The concept is

however not completely new as, in many cases, advantage has always been taken of mutual properties of

waste streams. Thus on probably every power plant the neutralisation of acid waste streams with alkaline

waste streams (as for example with regeneration waste streams of ion exchange) has been practised for

many years. It is now realised that there are many more opportunities for minimising the use of process


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water and the discharge of wastewater, by careful examination of the properties of the waters required and

of the wastewater produced. The introduction of new technologies, such as FGD, has greatly accelerated

this tendency.

The minimisation of the use of water has become of increasing importance as it has become more difficult

to use clean sources. Hence it is necessary for a rigid prioritisation of the use of water with the cleanest

waters being preserved for those processes which absolutely need them, and thus in many cases there can be

a cascading of waters down through the processes so that the waste from one process can become the input

to another process. This not only minimises the use of clean waters but also minimises the amount of

wastewater produced. The price that has to be paid is an increasing complexity of water management, of

treatment processes and of the interdependence of processes on each other. It also leads to a fundamental

debate of whether there should be distributed wastewater treatment or whether wastewater is best treated in

a single dedicated plant. At the present time this debate has not been resolved and both systems are in

common use. There is still much room for development and increasing attention is being paid to what once

was a very neglected area.

3.1.2. THE BASIC TECHNOLOGICAL PRINCIPLES

Wastewater treatment systems are based upon physical, chemical, physico-chemical and

biological processes. These types of processes are used both independently and in series depending upon

the wastewater that needs to be treated.

3.1.2.1. PHYSICAL TREATMENT SYSTEMS

Physical treatment systems are used on every power plant. These include screening,

sedimentation, filtration (including centrifuges), decanting and other such simple physical separation

processes. They rely on the physical principle of separation through a membrane in such as screening and

filtration, or by using differential gravity processes such as sedimentation or decanting.

Evaporation/crystallisation is the only technology that can be used when zero liquid discharge is required.

This can also be accomplished by injection into the flue gas upstream of the ESP

3.1.2.2. CHEMICAL PROCESSES

Simple chemical treatment processes have been used on power stations for many years. The

simplest chemical process is that of neutralisation or pH control to produce a wastewater within acceptable

limits. In some cases, oxidation to remove Chemical Oxygen Demand has been used. There is now an

increasing focus on the presence of trace metals, particularly from ash effluents or the effluent from FGD,

and these are being treated by chemical means. The usual method is by precipitation using lime and/or

sulphides to produce the insoluble hydroxides and sulphides of toxic metals. These are then removed by

sedimentation or filtration.

3.1.2.3. PHYSICO-CHEMICAL PROCESSES

Physico-chemical processes that are used for wastewater treatment include ion-exchange,

absorption and membrane separation methods. All these technologies tend to be relatively expensive and


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are therefore usually used for polishing streams or when a particular compound or element must be

removed. Membrane processes are occasionally used when wastewater is purified for recycling

3.1.2.4. BIOLOGICAL PROCESSES

Many power plants have conventional sewage treatment processes for domestic wastes.

However some power plants, particularly those fitted with advanced NOx controls, may have biological

processes to remove nitrogen compounds such as ammonia or nitrates.

In a few cases bacteria are being used to metabolise organic matters such as oil and resins.


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3.2. TECHNOLOGIES

3.2.1. OILY WASTEWATER

Water polluted by oil is mainly treated by gravity separation. This technology takes advantage

of the natural insolubility of most hydrocarbons present in the oils and of the consequential natural

separation of organic and aqueous layers based on the different densities.

A complete separation of the layers can not however be always ensured only by the different densities of

immiscible oil and water, thus often a complementary treatment (for example filtration) can be very helpful

to obtain a higher efficiency in oil removal.

In some particular cases it is possible that organic and aqueous phases create an emulsion or that the

densities of the different phases are very similar. This can lead to the application of specific treatments that

enhance the separation of organic compounds from the water phase before gravity separation is applied (for

example flotation, coalescing filtration).The concentration of organic compounds in wastewater can also be reduced by the application of non-

specific processes such as the combination of coagulation, flocculation and sedimentation that are usually

applied for the removal of suspended solids and inorganic pollutants.

The treatment applied for the removal of oil from wastewater offers also partial removal of suspended

solids. Thus some considerations reported here are relevant to this subject that is presented in the following

section with more details.

3.2.1.1. GRAVITY OIL SEPARATION

The process relies on the different densities of immiscible oil and water for successful

operation. The organic substances present in the oil can be lighter or heavier than water and thus in general

two immiscible organic phases separate from water. One (mainly composed by light oils) floats at the top

due to its density lower than water, and the other (mainly composed by tar and pitch and including often

also suspended solids) settles to the bottom.

The wastewater stream is generally fed to a basin designed to provide a quiescent zone of sufficient

retention time to allow the various phases to separate.

The best efficiency in separation is obtained when the density differences among the various phases are

large.

Gravity separation is generally used as an initial treatment step for oil polluted wastewater if no emulsions

are present.

Typically oil/water gravity separators are of two different types:

- those designed in accordance with guidelines established by the American Petroleum Institute (API),

- those made with corrugated plate interceptors (CPI).

3.2.1.1.1. API separators

This kind of separator is one of the most widely used in wastewater treatments.


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As can be seen from the scheme presented in figure 1, a typical API oil/water separator resembles a

rectangular primary clarifier in construction, though specific sizing and design details differ in order to

better suit the purpose of oil removal rather than solids removal.

Figure 1 - API Separator schematic

Wastewaterfeed

Decanted water

Baffle

Sludge

hopper

Sludge

pump

to thickening

and disposal

Flight scraper

Chain drive

pipe oil skimmer

Adjustable slotted

Decant tank

Treated

wastewater

Skimmingpump

oil pump

Recovered

to tank

The wastewater firstly passes through an inlet section (pre-separator flume, trash rack, oil skimmer and

forebay) and after enters into the tank, where a velocity diffusion device (vertical slot baffle or reaction jet

inlet) is positioned immediately downstream of the influent entrance. The function of this diffusion device is

to reduce flow turbulence and to distribute the flow equally over the cross sectional area of the channel.

Removal of settled sludge and floating oil is usually accomplished with a chain drive mechanism. Attached

between a pair of endless chains are crosspieces or "flights", extending the full width of the tank and spaced

at about 3 m intervals. Settled sludge is dragged to a sludge hopper (there can be more than one) at one end

of the basin where it is pumped out for further processing. Floating materials are usually skimmed to the

effluent end of the basin by the "flights" returning at the liquid surface. The floating material may then be

manually scraped up an inclined apron or removed hydraulically or mechanically.

Rotating discs and oleodynamic "ropes" are often used devices to mechanically remove oil from the liquid

surface and collect it separately from water for recovery.

Typical process sizing and design criteria for API separators are given in table I.


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Table I - API design criteria and process sizing

Horizontal velocity Maximum = 0.9 m/min

(or 15 times the rate of rise of an oil globule,

whichever is smaller)

Depth 0.9 - 2.4 m

Width 1.8 - 6 m

Length max. 90 m

Depth to width ratio 0.3 - 0.5

Velocity in the pre-separator section 3 - 6 m/min

Retention time in pre separator section 1 - 2 min

3.2.1.1.2. Corrugated Plate Interceptors

CPI are typically vendor supplied and are based on proprietary designs. A CPI consists

however in general of a tank containing a number of parallel corrugated plates mounted from 2 to 4 cm

apart and inclined at a certain angle. Wastewater flows upwards between the plates, whit the oil droplets

floating upward and collecting on the underside of adjacent plates where they coalesce. The coalesced oil

droplets move up the plates to form a floating layer that is skimmed from the surface of the process tank.

Skimming of oil can be helped mechanically or hydraulically in the same way as for the API process.

The solids and the oil fraction with densities higher than water settle from the wastewater stream and

collect on the top of the adjacent plates; they then move down the plates and drop off into the bottom of the

CPI vessel. Laminar flow in the zone between the plates enhances the separation efficiency of oil from

water.

A typical CPI separator is presented in figure 2.

Typical process sizing and design criteria for CPI separators are proprietary information.

3.2.1.1.3. API and CPI comparison

The two types of gravity separators described above can produce about the same effluent

quality.

CPI are in general smaller and easier to cover (for control of atmospheric emissions). However the smaller

size is at the same time a disadvantage since it may not provide sufficient volume to accommodate slugs of

oil, and may not provide sufficient retention time for breaking emulsions.


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Figure 2 - CPI Separator schematic

Sludgepump

to thickening

and disposal

Wastewaterfeed

Decant tankSkimmingpump

Recoveredoil

pump

to tank

Treated wastewater

In general API separators may be preferred because of their simplicity and lack of internals subjected to

fouling and plugging. The best application for a CPI separator in fact is for the wastewater streams that do

not contain significant solids or heavy oil that would foul the unit.

In both types of separator chemicals, such as iron and aluminium salts, and polymers, are sometimes added

to the wastewater to improve the efficiency of gravity separation.

These chemicals help in the destabilisation of colloidal particles and the enhancement of the agglomeration

of fine particles to promote settling at a faster rate.

Particle growth is often improved by gentle mechanical mixing.

Higher removal efficiency achieved by chemical addition can also increase the hydraulic throughput rate.

Both types of gravity separators have an oil removal efficiency that can vary from 66 to 99 % and at the

same time give a suspended solid removal efficiency in the range 10 to 50 %. The actual removal efficiency

depends on many factors, including the relative densities of the oil and aqueous phases, the configuration of

the separator, the retention time, and the size of the oil and tar droplets.

In addition to those mentioned above, other parameters such as temperature, pH or the type of solids present

in the wastewater can also affect the separation efficiency.

In both types of separator the by-products obtained from wastewater purification are the skimmings and the

settled solids and heavy oil. The skimmings may have enough oil content to justify recovery and reuse of the

oil. The settled solids and heavy oil are typically dewatered prior to final disposal. The possibility to reuse

this solid residue for energy recovery should however be evaluated case by case in relation to its calorific

value and to the legal requirements.

Final treatment either for the reutilisation of wastewater or to meet very severe limitations on discharge are

often used on the effluent from both types of gravity separator.


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3.2.1.2. FLOTATION

Flotation is used to remove oil from industrial wastewater, especially in the case where the

more simple treatment by gravity separation could not ensure the required oil removal efficiency. The main

application of flotation is related to oil polluted wastewater streams where difficult to break emulsions are

present.

In the flotation process, small gas bubbles rising through the wastewater adhere to solid particles or oil

globules, decreasing the density (prior to flotation, oil has density very similar to that of water) of the

particle-bubble combination to less than that of water. As a result of this density decrease, the particles rise

to the water surface where they can be skimmed off.

The bubbles may be added to the wastewater by entraining a gas in the liquid by mechanical or diffused

aeration, or they can be induced by a sudden pressure reduction of a supersaturated portion of the

wastewater stream, causing the excess gas to come out of solution as bubbles.

The gas used in closed flotation units is generally nitrogen, especially in the petroleum industries, in order

to reduce the possibility of fires. Air flotation units represent however the majority of flotation units

currently in service.

The two types of flotation most commonly used in wastewater treatment are:

- dissolved air flotation (DAF)

- induced air flotation (IAF).

3.2.1.2.1. Dissolved air flotation

In this type of flotation units, three different variations are currently being used:

full flow pressurisation consists of saturating all of the wastewater stream with air under pressure and

releasing the pressure at the inlet to the flotation chamber,

split-flow pressurisation consists of isolating a portion of the treated stream and saturating it with air

under pressure before combining it again with the remainder portion in the flotation chamber,

recycle pressurisation consists of saturating a portion of the clarified effluent stream with air under

pressure and combining this stream with the raw influent in the flotation chamber.

The air is generally added under a pressure of 200 - 500 KPa. The pressurised flow is held in a retention

tank for at least 1 - 3 minutes, to allow air dissolution.

On reducing the pressure of the saturated wastewater stream, dissolved air in excess of saturation at

atmospheric pressure is released in extremely small bubbles. The air solubility in wastewater is lower than

the air solubility in clean water and in full scale systems the ratio between the two solubilties is normally

between 0.5 and 0.8. Because of this, in general the preferred DAF system is recycle pressurisation, a

scheme of this kind of plant is presented in figure 3. The main advantages of this type of DAF system are

the minimisation of emulsion formation by pressurising treated effluent rather than oil containing influent,

reducing at the same time the cost of the pressurising system.

Usually flotation tanks are available in circular (diameter ranging from 3 to 30 m) and rectangular (width as

great as 6 m and lengths as great as 24 m) units with a wastewater depth varying from about 0.9 to 3 m.


19/81

Figure 3 - Dissolved air flotation with recycle pressurisation schematic

chemicalfeed

system

wastewaterfeed

flotationtank

settledsolids

skimmedfloat

float/bottomspump

to thickeningand disposal

treatedwastewater

recyclepump

air

pressure

retentiontank

Typical process sizing and design criteria for DAF systems are given in table II.

Table II - DAF systems design criteria and process sizing

Recycle pressure 200 -500 KPa

Air solubility correction factor 0.5 - 0.8

Air to solids ratio (w/w) 0.01 - 0.1

Hydraulic retention time 20 - 60 min.

Surface loading 0.02 - 0.14 m/min

Recycle rate 5 - 100 %

Pressure tank retention time 1 - 3 min

3.2.1.2.2. Induced air flotation

Induced air flotation systems can be of two types:

- mechanically induced IAF,

- hydraulically induced IAF.

In both cases the air dissolution at elevated pressure is not required, although they operate on the same

principle as pressurised air DAF as far as bubble oil interaction.

Mechanically induced IAF

In this case the air is self-induced by a rotor-disperser mechanism. The rotor is submerged in the liquid and

forces the liquid through the disperser openings, thereby creating a subatmospheric pressure. The air is

pulled downward into the liquid promoting the air-liquid contact. The liquid normally moves through a


20/81

series of four cells, each one with a retention time of about one minute. The float skimmings overflow weirs

on each side of the unit.

Hydraulically induced IAF

In this case, to create the induction of air, liquid is recirculated from the outlet chamber, by a centrifugal

pump, through a delivery tube to each cell. The delivery tube extends down into the cell through a standpipe

or draft tube. Liquid flowing through the delivery tube and induction nozzle creates a Venturi effect,

evacuating liquid from the standpipe and inducing the air into the water.

Typical process sizing and design criteria for IAF systems are given in table III.

Table III - IAF systems design criteria and process sizing

Air to liquid ratio (v/v) 6

Hydraulic retention time 4 min

Surface hydraulic loading rate 0.4 - 0.6 m/min

Recycle rate 25 - 50 % for hydraulic IAF

3.2.1.2.3. Comparison of IAF and DAF systems

IAF typically generates greater volumes of float than DAF. Typical hydraulic loading rates for

IAF are greater than for DAF while IAF retention times are significantly lower than those for DAF.

IAF is more sensitive than DAF to proper chemical addition (higher dosages are however required with

IAF) and to upsets from large slugs.

Both DAF and IAF are often used in conjunction with flocculation.

The chemicals (for example alum, demulsifiers, polyelectrolytes and acids) are usually added prior to

flotation and can significantly improve the removal efficiency. Laboratory specific tests with wastewater are

usually performed to determine if chemical addition is required. Pilot or bench-scale tests are conducted to

define the chemicals required and the optimal dosages.

Removal efficiencies of about 60 - 80 % for oil and of about 40 - 65 % for suspended solids are normally

achieved without chemical addition in both types of flotation devices. With adequate chemical addition the

removal efficiencies can be improved to 85 - 89 % for oil and grease and to 80 - 93 % for suspended solids.

Process sizing and design criteria (reported separately above for DAF and IAF systems) are normally based

on the oil and total suspended solid concentration in the wastewater to be treated.

One of the limitations of the DAF process is the reduced effectiveness for treating wastewater at elevated

temperatures due to the decreased solubility of air in water (especially important for temperatures > 65 C).

In flotation systems the by-products obtained from wastewater treatment are the froth layer skimmed off the

top of the unit and the settled sludge. Typically these two by-products are dewatered together prior to final

disposal.


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3.2.1.3. FILTRATION

Filtration is one of the most widely used processes employed for the removal of suspended

solid and can be helpful also for the removal of moderate concentrations of oil and grease from wastewater.

Many systems of filtration can be used depending on the filtering media and on the configuration of the

filter.

Filtration in most wastewater treatments is inserted as the final or polishing step, to reduce suspended solid

and/or oil and grease to relatively low levels. If the suspended solid concentration in the influent stream is

greater than 100 mg/l, or if immiscible oils are present in concentrations greater than 25 mg/l, pretreatment

facilities (for example sedimentation, flotation, etc.) should be inserted prior to filtration.

Filtration can be carried out through membranes, cartridges, precoat filters, granular media, etc.

For the removal of suspended solid and oil from wastewater and for the pretreatment of fresh water

withdrawn from natural sources, the most applied filters are those that use granular media.

For the removal of suspended solid typical industrial processes are:

single media filtration (usually sand whose particle size can vary depending on the quality of the water

to be treated),

dual media filtration (typically sand and anthracite)

multi media (for example garnet, sand and anthracite).

For the removal of oil, especially as the final step after gravity separation or other oil removal treatment,

granular oleophilic media such as activated carbon are applied. Sometimes activated carbon can be also

combined with conventional granular materials in a single vessel. More widespread is the application of

granular oleophilic media in a treatment configuration after a separate granular material filter (typically

sand) to remove suspended solids.

Granular media filters can be of the gravity or pressure type and the flow of liquid stream through the bed

can be downflow or upflow. Conventional filtration units are operated in either filtration or backwash

modes. During backwashing the filter is taken out of service and due to this, filter systems often consist of

two or more units to allow off-line cleaning.

Oleophilic media, that are more commonly applied in pressure filters, usually swell when oil is absorbed,

and in some cases backwashing is not recommended. In the case the media is spent, the unit is taken out of

service and reloaded with new or regenerated media (typically thermal regeneration).

The efficiency of oil and solid removal can vary greatly depending on the type of granular media filter used,

the nature and fluctuations in wastewater solid content and on the specific operation of the filter.

Granular media filtration units may require doses of fungicides or microbiocides from time to time to

remove biological slime layers that can accumulate on the media. If the oleophilic media is activated

carbon, the presence of oxidative compounds (for example residual chorine from biofouling treatment) in

the water to be treated can cause partial consumption of the media and, after regeneration, it is more

frequently necessary to add new material.


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Some polymers such as flocculant aids are sometimes used immediately prior to the filter units to increase

the strength and size of particles entering the filter. Polymer dosage is usually in the range 0.01 to 0.1 mg/l.

By-products coming from filtration systems are backwash and/or regeneration effluents, and spent filter

media.

A typical downflow gravity filter generally consists of a granular media layer of 0.3 to 0.6 m. Hydraulic

loading is typically 0.08 to 0.24 m/min. Pressure filters that are similar to gravity filters in most respects

can have higher hydraulic loading (0.20 to 0.40 m/min) than gravity filters. Backwashing or regenerating

cycles can be controlled on a time schedule or more commonly by pressure drop across the filter.

Oil removal efficiencies from 50 to 85 % can be obtained in granular media filtration following oil/water

gravity separation. Oil and grease concentrations below 1 mg/l have been registered in granular media

filtration after secondary clarification.

3.2.1.4. COALESCING FILTERSThis process is a variation on multimedia filtration and is particularly used for the removal of

free and emulsified oils ensuring a good removal of suspended solids. Coalescing filters work by promoting

collisions of small dispersed oil droplets so that they can coalesce to form larger oil droplets that can be

more easily separated from the water phase.

The surface of the coalescing media can be wetted by oil and enhances the oil droplets to collide and to form

a film. The larger oil particles are then shed from the surface with the assistance of gravity and/or viscous

forces caused by fluid flow.

Three different types of coalescing filters are available:

- loose filter media separators (the most widely used),

- fixed media coalescers (involving foam materials or cylindrical cartridges)

- parallel plate separators (corresponding to CPI as described in the gravity separation chapter).

Coalescing filters can be of the upflow or downflow type but the most interesting are upflow coalescing

filters. These filters collect droplets of oil that break free from the coalescing surfaces during normal filter

operation. Because of the upward flow configuration, gravity and viscous forces work in combination to

release droplets of coalesced oil.

As with granular media filtration systems, coalescing filters also find application mainly as the final

polishing step at the end of oil and solid removal processes. They are particularly efficient in the treatmentof low to intermediate concentrations of finely divided oil droplets with low specific gravity (0.85 kg/l).

Removal efficiency tends to decrease slightly with increasing specific gravity of the oil, and in these cases

coalescing filters might be preceded by flotation and gravity separation.

The coalesced droplets migrate upward through the media and are released into a gravity separation section

at the top of the vessel where the oil collects in a layer and is periodically removed. The outlet for treated

water in the separation section is located in the lower part of this section.

Solid and heavy oil, if any, are trapped in the media as in conventional filtration. The build-up of solid in

the media contributes to increased pressure drop and requires the filter to be taken out of service for


23/81

backwashing. In order to obtain good removal of solids included in the filter media, backwashing should be

done in different steps: upward flow with strong agitation, simultaneous water backwashing and high rate

gas flow. Backwashing normally occurs at scheduled time intervals (for example once a day) or when the

pressure drop reaches a preset limit (typically 135 to 170 kPa).

Polymers are often added upstream of the filter to assist in coalescing fine oil droplets by reducing the

surface charges on oil droplets that result in repulsive forces preventing collisions and coalescence.

Surfactant chemicals, added to backwash water or steam, are sometimes used to help remove the oil film

from sand grains.

The oil removal efficiency of coalescing filters is mainly affected by the stability of oil in water emulsions in

the feedwater. In fairly difficult heavy oil applications, with feed wastewater oil concentrations from 30 to

100 mg/l, treated wastewater with 3 to 10 mg/l is typical. Solid concentration in the feed wastewater must

be kept below 100 mg/l to keep the backwashing frequency within a reasonable range.

Typical process sizing and design criteria for coalescing filters are given in table IV.

Table IV - Coalescing filters design criteria and process sizing

Surface loading rate 0.16 to 0.24 m/min

Backwash flow 1 m/min

Backwash duration 10 to 20 min.

Gas scour rate 0.9 to 1.8 m/min

Gas scour duration 5 to 20 min

Media depth 1.5 to 3.5 m

Vessel diameter 1.2 to 4.2 m

The by-products produced by this process are dirty backwash water, oily sludge, coalesced oil and spent

media. Dirty backwash can be settled and further retreated in coalescing filters. Oily sludge and exhausted

media should be dewatered and disposed of or incinerated. Coalesced oil can be recovered and reutilised in

the same manner as the surface oil from gravity separation process.

3.2.1.5. EVALUATION OF THE TECHNOLOGIES FOR THE TREATMENT OF OILY WASTE WATER

Table V presents a comparative evaluation of the technologies that can be applied to treat oily

waste water.

This evaluation is a very general one and must be read keeping in mind that the techologies discussed are

not completely interchangeable.


24/81

Table V - Oily water treatment technologies

Technology Performance Economic factors Technical factors Comments

installation

costs

operation

costs

complexity environmental

impact

API L L L L M not very efficient for heavy oil removal

CPI L M L L M not very efficient for heavy oil removal

Flotation H H H H H

Filtration H M H M L effective only asfinal treatment

Coalescing filters H H H H M

General graduation: Higher, Medium, Lower and Equal

Equal is used if there is less than 10-15 % between highest and lowest values

Two levels Higher and Lower are used if there is more than 15 % an less than 50 % between highest and lowest values

Three levels Higher, Medium and Lower can be used if there is more than 50 % between highest and lowest values

ITEMS

Performance:

The possibility of the technology to achieve the goal. H = best, L = worst. the meaning of L is only in comparison with other technologies

in the table. A technology with the mark L can still have high performance.

Economic factors:

Installation costs and operating costs. H = high costs, L = low costs.

Technical factors, complexity:

H = high complexity, L = low complexity

Technical factors, environmental impact:

An overall impact on the environment. In consideration is taken performance, energy consumption, toxicity of residues, handling

problems of residues, etc. H = worst, L = best

Technologies are only compared inside the single table and there can not be made comparison

between different tables. Besides the economical factors the comparisons are widely based on author

evaluation.


25/81

3.2.2. SANITARY WATER

Sanitary water is often sent to municipal sewage duct and treated outside of the power station.

In the power station where sanitary wastewater treatment plants are installed, they are only of the aerobic

biological oxidation (activated sludge) kind. The other type of plants that can be utilised to treat sanitary

wastewater are only briefly described in this section.

3.2.2.1. ACTIVATED SLUDGE (BIOLOGICAL AEROBIC OXIDATION WITH NITRIFICATION)

Activated sludge is an aerobic biological wastewater treatment process that oxidises organic

matter and, in some cases, ammonia contained in a wastewater by using microorganisms living in the

presence of oxygen.

Biological oxidation processes remove the organic compounds and, in some cases, ammonia from the

wastewater through the production of new bacterial cells. Because dissolved oxygen is necessary for aerobic

bacterial metabolism, it must be available continuously to ensure proper functioning of the process. Theproduction of the bacteria also requires a source of carbon, phosphorous, nitrogen, and other nutrients. The

degree and rate of organics and ammonia removal are dependent on the biodegradability of the particular

waste, on the temperature, on the presence of inhibitory compounds, and on the process design sizing, as

well as on adequate operation of the plant.

The activated sludge process consists of two primary components: an aeration basin and a clarifier. A

schematic of the activated sludge process is shown in figure 4. In the aeration basin the influent wastewater

contacts biological organisms in the presence of air or oxygen. The suspension of organisms is known as

mixed-liquor suspended solids. The main parameter that affects the sizing of this basin is the solid

retention time, which is the average retention time of solids in the activated sludge system.

Figure 4 - Activated sludge process

The mixed-liquor flows from the aeration basin to the clarifier, where the suspended solids are separated

from the waste stream. Most of these settled biological solids are returned to the aeration basin (return

activated sludge).

Aerationblowers

Return activated sludgeWaste activatedsludge

To solidshandling

Treatedeffluent

Aeration tank Clarifier

InfluentWastewater


26/81

The biological oxidation of organics and ammonia usually results in a net production of biological solids.

These excess solids must be removed from the system (waste activated sludge).

The activated sludge process can also be designed to achieve ammonia conversion to nitrate (commonly

termed nitrification) either in the same system with organics removal, or in a separate system, following

organics removal. An activated sludge system designed for nitrification generally requires a larger aeration

volume, longer residence time of solids, and greater aeration capacity than in a system designed for organics

removal alone.

Several aeration basin configurations are commonly used for the activated sludge process. The

conventional system uses long rectangular aeration basins with aeration being provided uniformly along

the basin length.

A significant recent improvement in aeration basin designs is the use of biological selectors. The selector

is a short retention time oxic, anoxic or anaerobic basin upstream of the aeration tank. The purpose of the

selector is to promote conditions for development of good settling and flocculated bacteria, and to

discourage the growth of poor settling filamentous bacteria. Anoxic selectors also provide denitrification

and the associated return of some alkalinity destroyed by nitrification.

Aeration basins are typically constructed of concrete or steel in either square, rectangular, or circular

configuration. The vessels usually are constructed at depths of 3 m to 6 m, although depths as high as 18 m

have been used. Air is introduced to the wastewater using diffused aeration equipment located on the reactor

bottom or by floating surface aerators. The aeration system serves the dual purpose of wastewater

oxygenation and reactor mixing to maintain the biological solids in suspension.

Variations of the activated sludge process include the use of pure oxygen instead of air and the addition of

powdered activated carbon to enhance removal of absorbable pollutants.

Organic removal as measured by BOD5 should exceed 90 to 95 % with activated sludge. Because of the

presence of bioresistant organics in the wastewater the COD removal can be sometimes lower

The expected performance values for activated sludge are summarised in table VI.

Table VI - Expected performance values for activated sludge process

Parameter Expected effluent concentration

or percent removal

BOD5 94 to 98 %

COD 60 to 75 %

TSS 30 mg/l

As already said, the most important process performance variable of the activated sludge process is the solid

retention time or mean cell residence time. To ensure adequate treatment, activated sludge systems are

generally designed for a solid retention time of 4 to 30 days. With warm (> 20 C) wastewater a solid

retention time above 8 to 15 days, nitrification can be expected to occur simultaneously with organics

removal. Conservative sizing for an activated sludge system with nitrification would be a design for a 20

day solid retention time.


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3.2.2.2. BIOLOGICAL NITRIFICATION AND DENITRIFICATION

Biological nitrification is the conversion of ammonia to nitrate which occurs under aerobic

conditions. The conversion of ammonia to nitrogen is a two step process involving two different classes of

bacteria. Biological denitrification is the conversion of the produced nitrate to gaseous nitrogen. The

conversion of nitrate to nitrogen occurs under anoxic conditions by means of microorganisms

Biological nitrification and denitrification plants, at present, have not found application in power plants to

treat sanitary water. Considering however that this process can be applied to remove ammonia, nitrite and

nitrate in the treatment of other wastewater streams, biological nitrification and denitrification process is

described in detail in the following section 3.2.5 relevant to the treatment of wastewater containing specific

inorganic pollutants.

3.2.2.3. ANAEROBIC PROCESSES

This kind of alternative process to reduce/remove organics from wastewater, at present hasnot found application in power stations in Europe and thus only a short presentation of the basic principle is

included in this report.

Anaerobic biological treatment is an alternative to more commonly applied aerobic wastewater treatment

processes used to remove organic constituents from a waste stream. Anaerobic degradation is a multi step

biological process involving two basic groups of bacteria. One group (acid formers) consists of bacteria

that hydrolyse and ferment complex organic compounds into simple organic acids. The other group

(methane formers) converts the organic acids produced by the acid formers into methane gas and carbon

dioxide gas (the combination of which is termed biogas). Both groups of organism live in the absence of

oxygen and thus are classified as anaerobic. The two groups of bacteria can co-exist and their metabolic

reactions can take place simultaneously. From a microbiological standpoint, all anaerobic process

configurations are similar. However, different reactor configurations have been developed for the different

process variations.


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3.2.3. SUSPENDED SOLIDS

Often the presence of suspended solids in wastewater is tied to the presence of other pollutants

that can be dissolved in the water phase or to organic immiscible compounds. Because of this the suspended

solid removal is often performed by one or more treatment steps included in a more or less complex

treatment that removes most pollutants from wastewater.

Coagulation and flocculation followed by sedimentation represent however the typical treatment that leads

to the removal of the suspended solids from wastewater.

While precipitation is used to remove dissolved wastewater pollutants, coagulation/flocculation followed by

sedimentation represents the specific treatment for the removal of colloidal and finely divided solids. In

general the most common wastewater treatment plants of various industries use coagulation and

precipitation reactions at the same time and in the same reaction vessels, since precipitation of dissolved

constituents aids in the coagulation of colloidal solids.

Suspended solids can also be removed from wastewater by filtration processes whose principles of operation

have already been described. In the following presentation only additional aspects relevant to filtration of

suspended solid are reported.

3.2.3.1. COAGULATION/FLOCCULATION/SEDIMENTATION

3.2.3.1.1. Coagulation

Colloidal solids in an aqueous phase typically carry an electrical charge and since most

particles have the same charge, they tend to repel each other, thereby impeding sedimentation. In the

coagulation process the mechanism of removal is twofold:

destabilisation of the repellent electrical charges on the colloidal particles

formation of a "blanket" of chemical precipitate that enmeshes suspended particles as it settles and thus

sweeps them from suspension.

The neutralisation of particle charges and the precipitate formation are promoted by coagulant chemicals

that are added to the water to be treated. Aluminium (sulphate or alum) and iron (ferric chloride) salts,

polymers and lime are commonly used coagulants, that are added in a small rapid-mix vessel in order to be

rapidly dispersed and to let the initial coagulation reaction take place essentially instantaneously. Bench

scale tests are normally performed for coagulant selection and dosage optimisation.

3.2.3.1.2. FlocculationFlocculation consists of the agglomeration of individual destabilised particles to form larger

and heavier solid masses (flocs) which can be more readily settled. The flocculation is the result of chemical

addition and/or special mixing techniques. Flocculated solids are commonly removed by sedimentation

and/or filtration.

Polyelectrolytes (cationic, anionic or non-ionic organic polymers) are added to the water stream as a

flocculation aid. The addition of polyelectrolytes is often made directly following the rapid-mix coagulation

or in later stages, and no vigorous mixing is required. As with coagulants the polyelectrolyte selection and

dosage optimisation must be determined by scale bench testing. The floc formed during flocculation can


29/81

help to trap additional colloidal and larger suspended solids; attention should be paid to not overdosing

polyelectrolyte because restabilisation of colloidal particles can be caused. There are many types of

flocculators, including hydraulic, air mechanical and turbine mixing units. Multiple basins of reduced size

are preferable to one large basin of equal total size. A progressively reduced mixing action can be helpful to

promote floc formation and to prevent shear force action.

The retention time required depends largely on the wastewater characteristics and generally is inversely

proportional to the concentration of destabilised colloidal solids; in this case combined

coagulation/flocculation/sedimentation units may be more appropriate. Combined units offer cost and space

savings, and because of their increased solids contact they promote the formation of floc, thereby affording

a reduction in overall hydraulic retention time.

3.2.3.1.3. Sedimentation

Sedimentation is used to separate flocculated solids from the wastewater. The sizing and the

performance of sedimentation units are thus strongly influenced by the degree of particulate and flocculant

settling and by the degree of solid compaction achieved, and thus also strongly dependent on any previous

coagulation and flocculation treatment steps.

The tests on bench or pilot scale, performed for evaluating best chemicals for coagulation and flocculation

steps, should thus also consider the impact on the sedimentation efficiency.

Sedimentation is due to gravity separation to the bottom of chemically flocculated sludge. This can happen

in three different types of clarifiers: horizontal flow basins, upflow basins, and solid contact basins.

Horizontal flow basins are usually employed for the process where separated tanks are provided for

coagulation, flocculation and sedimentation. The other two types of clarifier, combine the coagulation,

flocculation and sedimentation steps in one integral unit. In all three types of clarifiers, a blanket of settled

sludge is maintained as aid to thickening sludge and to promote floc formation. Sludge may be withdrawn

continuously or intermittently and sent to thickening and dewatering units.

Performance obtained by the combination of coagulation, flocculation and sedimentation in wastewater

treatment ensure an effluent suspended solid concentration below 100 mg/l also at high suspended solid

concentration of the influent stream. Generally removal of total suspended solids varying from 55 to 70 %,

of colloidal solids from 25 to 75 %, of total solids from 25 to greater than 75 %, of turbidity from 50 to 90

% and of oil and grease from 75 to 85 % can be expected from a properly designed coagulation, flocculation

sedimentation facility treating concentrated wastewater.

Conventional design criteria for separated steps of coagulation, flocculation and sedimentation are given in

table VII.

Conventional design criteria for a combined coagulation/flocculation/sedimentation unit are given in table

VIII.


30/81

Table VII - Conventional design criteria for separated coagulation, flocculation and sedimentation

Coagulation

Rapid mixing retention time 1 second - 2 minutes

Flocculation

Retention time

Power input

20 - 30 min

3.95 W/m3

Sedimentation

Overflow rate

Retention time

Clarifier depth

Weir overflow rate

0.015 - 0.03 m/min

1-4 h

2 - 4 m

0.07 -0.43 m3/min/m

Table VIII - Conventional design criteria for combined coagulation/flocculation/sedimentation

Hydraulic retention time

Mixing zone

Flocculation zone

Sedimentation zone

2 - 5 min

10 - 25 min

2 - 3h

Sedimentation zone

Rise rate

Average liquid depth

0.02 - 0.04 m/min

3 - 6 m

3.2.3.2. FILTRATION

Filtration has already been considered for the removal of oil from wastewater (see chapter

3.2.1).

If the concentration of suspended solid is greater than 100 mg/l, pretreatment facilities (for example

coagulation, flocculation, sedimentation) should be considered prior to filtration. In many wastewater

treatment plants, filtration is applied as final treatment for the removal of suspended solids not eliminated

in previous treatments.

The granular media usually applied for suspended solids filtration are sand, anthracite and garnet that can

be in single media, dual media or multi media filter, as described in the previous chapter.


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In case of utilisation of dual media or multi media filters, often a train configuration, with a sand filter as

protection of the more expensive media, is designed.

In multi media filters the media are usually graded with granule size increasing and granule density

decreasing from the bottom to the top. After backwashes, density differences between the filter media allow

the layers to settle out in the proper order.

The advantage in using dual or multi media filters versus single media is that in this case the entire depth of

the bed is used to remove solids, rather than just the media surface. In downflow multimedia filters larger

solids are removed by the top coarser layer saving the capacity of the sand and garnet layers for increasingly

finer solids. Dual media and multi media filters can increase filter run times (time between backwashes) by

200 to 300 % compared to single media sand filters.

As far as chemical addition, typical sizing and design criteria, and removal efficiency are concerned the

same consideration already reported for oil polluted wastewater has to be considered.

3.2.3.3. EVALUATION OF THE TECHNOLOGIES FOR THE REMOVAL OF SUSPENDED SOLIDS

Table IX briefly presents a comparative evaluation of the treatment processes that can be

applied to remove suspended solids from wastewater.

This evaluation is a very general one and must be examined bearing in mind that the techologies are not

completely interchangeable; in particular filtration can be effectively utilised only for low suspended solid

waters and/or as a final treatment.


32/81

TABLE IX - Wastewater with suspended solids treatment technologies

Technology Performance Economic factors Technical factors Comments

installation

costs

operation

costs

complexity environmental

impact

Coagulation+Flocculation+

Sedimentation

L H H E E

Filtration H L L E E It needs a low TSSinfluent.

General graduation: Higher, Medium, Lower and Equal

Equal is used if there is less than 10-15 % between highest and lowest values

Two levels Higher and Lower are used if there is more than 15 % an less than 50 % between highest and lowest values

Three levels Higher, Medium and Lower can be used if there is more than 50 % between highest and lowest values

ITEMS

Performance:

The possibility of the technology to achieve the goal. H = best, L = worst. the meaning of L is only in comparison with other technologies

in the table. A technology with the mark L can still have high performance.

Economic factors:

Installation costs and operating costs. H = high costs, L = low costs.

Technical factors, complexity:

H = high complexity, L = low complexity

Technical factors, environmental impact:

An overall impact on the environment. In consideration is taken performance, energy consumption, toxicity of residues, handling

problems of residues, etc. H = worst, L = best

Technologies are only compared inside the single table and there can not be made comparison

between different tables. Besides the economical factors the comparisons are widely based on author

evaluation.


33/81

3.2.4. INORGANIC POLLUTANTS

The inorganic pollutants dealt with in this paragraph are ionic substances dissolved in water.

This includes in general metal (salts), acids, bases and in particular ammonia, nitrate and nitrite.

In general the removal of inorganic pollutants is based on precipitation. Dissolved inorganic pollutants are

treated by changing pH and/or adding chemical substances whereby the dissolved pollutants precipitate and

become removable from the waste water by sedimentation. This kind of treatment is primarily aimed at the

removal of heavy metals but other components that coprecipitate with metal hydroxides can be removed.

The actions (number of steps in the chemical treatment) necessary to remove pollutants are dependant on

the amount (flow) of waste water, the load (concentrations) of the pollutants and the final destination.

3.2.4.1. PH INCREASE

By increasing pH a broad range of heavy metals precipitates as metal hydroxides. The

concentrations of dissolved heavy metals that can be achieved in the waste water are given by the solubilityproducts KSP [table X] for the reaction

Me aOH Me OHa a+

+ ( )

and K Me OHSP Mea

OH

a a

+ [ ] [ ]

Table X - Solubility products of hydroxides

Component K SP

Ca(OH)2 7,610-5

Mg(OH)2 1,910-10

Cd(OH)2 4,210-13

Fe(OH)2 1,110-14

Zn(OH)2 1,810-14

Ni(OH)2 2,710-14

Hg(OH)2 10-25

Pb(OH)2 1,410-20

Fe(OH)3 2,810-36

With a pH range of 8.5 to 9, low concentrations of dissolved heavy metals can be achieved, even if the i

differ from 1 due to high salinity. However it will be the capability to remove the insoluble particles of metal

hydroxides (normally by sedimentation) from the waste water that give the achievable total concentration of

remaining heavy metals.

The increase in pH is achieved by adding chemicals such as limestone, quicklime, slaked lime or sodium

hydroxide. The choice of chemical depends on the amount and the composition of the waste water.


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3.2.4.1.1. Limestone

Limestone (CaCO3) is only used for pre-treatment when the waste water has a very low pH

(< 1) or for neutralisation of a single batch of waste water.

Limestone is added to the wasterwater either as a powder or as a slurry.

The advantage is a very low price. The disadvantages are: the handling of the powder, the need of large

quantities, the release of CO2.

3.2.4.1.2. Quicklime/Lime

Quicklime (CaO) can be used as an alternative to limestone or lime.

The advantages are: smaller quantities, no release of CO2, faster reaction. The disadvantages are: the

handling of the alkaline and hygroscopic powder, very large heat production.

3.2.4.1.3. Slaked lime

Slaked lime (Ca(OH)2) is commonly used in the waste water treatment at power stations.

Slaked lime is added to the waste water either as a powder or as a slurry (milk of lime). When used in pre-

treatment or in the first step (in two steps pH increment) slaked lime can be added as powder, but normally

slaked lime is added as a slurry.

The advantage is a low price. The disadvantage is the handling of the powder. Advantages and

disadvantages in comparison with sodium hydroxide are summarised in table XI.

3.2.4.1.4. Sodium hydroxide

Sodium hydroxide (NaOH) is also commonly used but not as frequent as slaked lime.

Sodium hydroxide is always added to the waste water as a solution.

Advantages and disadvantages in comparison with slaked lime are shown in table XI.

Table XI - Advantages and disadvantages in the use of sodium hydroxide and slaked lime

Sodium Hydroxide, NaOH Slaked lime, hydrated lime, calcium

hydroxide, Ca(OH)2

Delivery and storage

By road tanker or in pallet tanks.

When used at high concentrations (46%),

care should be taken to prevent temperatures

below 15 C because of freezing.

By road tanker or in sacks.

Silos for storage have to be insulated to

prevent condensation and subsequent

formation of lumps.

Storage volume need

1 kg 100% NaOH is equivalent to 25 moles

of divalent heavy metal (e.g. 5,0 kg of Hg)

Normally delivered in concentrations of 32 to

46 %.

1 kg Ca(OH)2 is equivalent to 27 moles of

divalent heavy metal (e.g. 5,4 kg of Hg)

Normally delivered as a powder with a bulk

density of 600 kg/m3.

A 20 % suspension (slurry) has to be

prepared before use.


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Preparation

None A 5-20 % suspension (slurry) has to be

prepared before use. When used in two stageneutralisation, powder can be used in the first

step.

Dosage

Dosage is done by pump with variable length

of stroke and/or variable speed

Slurry has to be pumped continuously

through pipes and be recycled to prevent

precipitation. Dosage from pipes by using

valves

Reaction/Regulation

Added sodium hydroxide solution will be

mixed quickly in the reaction tank. This

gives a fast regulation of pH with a minimum

risk of unacceptable pH variation

Because the slaked lime is dosed as a slurry

or a powder, it has to be dissolved and mixed

before reaction. This gives a slow regulation

of pH with the risk of unacceptable pH

variation. To minimise this pH variation

neutralisation can be done in two steps (or a

large reaction tank can be used)

General problems

Risk of scaling

Health and safety

Cauterising fluid

Closed pipe and tank systems. Possible to

clean by flushing before maintenance. Risks

are well known at power stations due to

experience from water treatment.

Cauterising powder

Closed pipe and silo systems. Cleaning before

maintenance is necessary. Contact with

powder is difficult to avoid

Price

A NaOH mole equivalent costs 4 times the

price of a Ca(OH)2 mole equivalent.

Price based on road tanker delivery

(Denmark, July 1994)

3.2.4.1.5. pH reduction

High pH is seldom a problem but, if necessary, hydrochloric or sulphuric acid at a convenient

concentration can be used.


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3.2.4.2. TREATMENT WITH SULPHIDE

Environmental demands can seldom be met by pH increment only. By adding sulphide or

complexing agents, the solubility of the heavy metal can be further lowered. The concentrations of dissolved

heavy metals that can be achieved in the waste water are given by the solubility products KSP [table XII] for

the reaction

xMe yS Me Sa x y+

+ 2

and K Me SSP Mex a x

S

y y

+ [ ] [ ]2

Table XII - Solubility products of sulphides

Component K SP

FeS 9,610-16

ZnS 1,210-23

NiS 1,410-24

PbS 3,410-28

CdS 3,610-29

HgS 10-52

Fe2S3 1,410-86

The treatment with sulphide is achieved by adding sodium sulphide, trimercapto-s-triazin (TMT-15) or

similar. The choice of chemical is depending on the treatment plant construction, environment demands

and the composition of the waste water.

Advantages and disadvantages in the use of sulphide and trimercapto-s-triazin are compared in table XIII.

3.2.4.3. ADDITIONAL CHEMICALS

To optimise the precipitation of the heavy metals additional chemicals can be added to the

treated (pH and sulphide) waste water. The chemicals to be added are coagulants, flocculants and

polyelectrolytes.

3.2.4.3.1. Coagulants

To ensure the formation of flocks a coagulant can be added. The coagulant is normally a non

toxic metal that precipitates with increasing pH: iron or aluminium as iron sulphate/chloride or aluminium

sulphate.

When coagulants typically iron sulphate/chloride is added to the waste water in large quantities, it will

precipitate in large flocks and secure that smaller particles are adsorbed to the surface of the hydroxide floc.

Addition of iron sulphate/chloride is necessary to neutralise sulphide or similar added in a previous step of

the process.


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Table XIII - Advantages and disadvantages in the use of sulphide and TMT15

Sodium sulphide, Na2S TMT15, trimercapto-s-triazin, C3N3S3Na3

Delivery and storage

In 33 kg bags on pallets. In 25 L cans on pallets. Delivered in 15 %

concentration.

Preparation

Flake or powder has to be dissolved in water. None

Dosage





Reaction

Sodium sulphide reacts very effectively with

all heavy metals that precipitate with

sulphide.

Iron sulphate/chloride has to be added

afterwards in sufficient amounts to neutralise

the remaining sulphide

TMT15 reacts primarily with divalent heavy

metals that precipitate with sulphide.

Remaining TMT15 has to be neutralised.

Health and safety

Poisonous and cauterising fluid/powder

Bags with flakes or powder have to be

handled. Closed pipe and tank systems.Possible to clean by flushing before

maintenance. It produces poisonous gas

(H2S) when in contact with acid.

Non toxic

Price

The use of TMT15 is 40 to 50 times more

expensive than the use of sodium sulphide

3.2.4.3.2. Flocculants

Flocculants are organic polymers based on acrylamid, neutral or weak basic, that help to

increase the size of flocs and optimises sedimentation.

3.2.4.3.3. Polyelectrolytes

Polyelectrolytes are organic polymers, neutral or weak acid, that optimise the water release

from the sludge when de-watered in the filter press.

3.2.4.4. TYPICAL FLOW SHEETS

Some typical waste water treatment plants flow sheets are described in this section. The flow

sheets show only principles, for example mixing can be achieved in several ways, here shown as mechanical

steering but recirculation with pumps, steering by compressed air, etc. will serve the same purpose. The


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choice of the steering principle depends on the amounts of waste water, batch/continuous treatment, plant

construction, economics, etc.

The particle removal function is shown as a clarifier in all figures but a lamella precipitator, a clarification

basin, etc. will have the same purpose.

The pH increment is shown (figures 6 10) as addition of slaked lime, milk of lime or sodium hydroxide

but any of the in paragraph 3.2.4.1 mentioned chemicals can be used.

Figure 5 and 6 show the most simple waste water treatments.

The removal of solids is the only treatment taking place in figure 5. This simple treatment is only used for

the batch treatment of waste water and only when the demands are low or when treated waste water can be

reused. This kind of treatment is commonly used for waste water coming from boiler rinse, air preheater

rinse, ash precipitator rinse, flush water from mechanical filters, flush water, floor drains, etc.

When there is a need for pH adjustment, or if there are some demands for removal of heavy metals this can

be done with the treatment shown in figure 6. The treatment is used for the waste water streams mentioned

in figure 5 and for example: slag flush water, slag cooling water, ash transport water, waste water from acid

cleaning, regeneration waste water, etc.

Figure 5 and figure 6

Lime

Sludge

EffuentInfluent

Sludge

EffuentInfluent

High pH waste water coming from slag flushing or regeneration can be dealt with or whitout pH adjustment

over a wide range. The amounts of slag flushing water are normally small in comparison with other waste

water streams. Slag flush water (with a high pH) can be used to increase pH in other waste water streams or

just be diluted into those streams. Regeneration waste water with high pH can be avoided by regenerating

anions and cations simultaneously and with balanced amounts of acid and base.

Both figures 7 and 8 show flow sheets of waste water cleaning plants with full treatment for removal of

heavy metals. The two flow sheets differ in the pH increasing (neutralisation) step. The two step

neutralisation will be necessary in a system with a high flow rate and slaked lime neutralisation to ensure

correct pH. When sodium hydroxide is used for neutralisation two steps can be avoided. If pH in the waste

water is very low, two step neutralisation can be necessary. When the waste water contains a large amount

of solids, the first step in a two step neutralisation can be used to remove coarse sludge from the waste water

and thereby reduce the load on the clarifier. Recycling some percentage of the sludge will optimise the


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building of flocs and reduce chemical consumption. This kind of system is normally used for the treatment

of regeneration waste water or waste water from small flue gas cleaning plants.

Figure 7

Sludge

Effluent

Na2S FeCl3 Poly

Lime

Influent

Water

Figure 8

Sludge

Effluent

Lime

Influent

Water

Na2SFeCl3 Poly

Figure 9 and figure 10 show flow sheets of waste water cleaning plant commonly used with wet gypsum

desulphurisation plants. The first removal of solids and supersaturated calcium sulphate is necessary to

avoid scaling in the remaining system. The major difference is the sludge management system (gypsum and

fly ash removal). By not increasing pH in the first step in figure 9 the amount of heavy metals in the sludge


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from this step is minimised and the amount of heavy metal sludge from the following treatment is reduced.

When the first step is running completely without chemicals (no use of flocculants) the sludge from this step

can be recycled to the absorber slurry in the desulphurisation plant.

The system in figure 10 combines the removal of solids and supersaturated calcium sulphate and the

necessity of two step neutralisation when using slaked lime.

Figure 9

Sludge

Effuent

Na2S FeCl3 PolyNaOH

Influent

Poly

Figure 10

Lime

Influent

Na2S FeCl

3PolyFeCl

3Poly

Sludge

Effuent

NaOH

3.2.4.5. EVALUATION OF TREATMENT FOR REMOVAL OF INORGANIC POLLUTANTS

Table XIV briefly presents the comparative evaluation of different technologies that can

waste water effluent technology

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