Type of measurement

Averaging period

Definition

Continuous

Daily average

Average over a period of 1 day based on valid hourly or half-hourly averages

Periodic

Average over the sampling period

Average of three consecutive measurements of at least 30 minutes each(1) (2)

Equation 1:

[Bild bitte in Originalquelle ansehen]

Equation 2:

[Bild bitte in Originalquelle ansehen]

Averaging period

Definition

Average of values obtained during one month

Flow-weighted average value from 24-hour flow-proportional composite samples obtained during 1 month under normal operating conditions(3)

Average of values obtained during one year

Flow-weighted average value from 24-hour flow-proportional composite samples obtained during 1 year under normal operating conditions(3)

Equation 3:

[Bild bitte in Originalquelle ansehen]

Term used

Definition

BAT-AEPL

Environmental performance level associated with BAT, as described in Commission Implementing Decision 2012/119/EU(4). BAT-AEPLs include emission levels associated with the best available techniques (BAT-AELs) as defined in Article 3(13) of Directive 2010/75/EU

BTX

Collective term for benzene, toluene and ortho-/meta-/para-xylene or mixtures thereof

CO

Carbon monoxide

Combustion unit

Any technical apparatus in which fuels are oxidised in order to use the heat thus generated. Combustion units include boilers, engines, turbines and process furnaces/heaters, but do not include waste gas treatment units (e.g. a thermal/catalytic oxidiser used for the abatement of organic compounds)

Continuous measurement

Measurement using an ‘automated measuring system’ permanently installed on site

Continuous process

A process in which the raw materials are fed continuously into the reactor with the reaction products then fed into connected downstream separation and/or recovery units

Copper

The sum of copper and its compounds, in dissolved or particulate form, expressed as Cu

DNT

Dinitrotoluene

EB

Ethylbenzene

EDC

Ethylene dichloride

EG

Ethylene glycols

EO

Ethylene oxide

Ethanolamines

Collective term for monoethanolamine, diethanolamine and triethanolamine, or mixtures thereof

Ethylene glycols

Collective term for monoethylene glycol, diethylene glycol and triethylene glycol, or mixtures thereof

Existing plant

A plant that is not a new plant

Existing unit

A unit that is not a new unit

Flue-gas

The exhaust gas exiting a combustion unit

I-TEQ

International toxic equivalent – derived by using the international toxic equivalence factors, as defined in Annex VI, part 2 to Directive 2010/75/EU

Lower olefins

Collective term for ethylene, propylene, butylene and butadiene, or mixtures thereof

Major plant upgrade

A major change in the design or technology of a plant with major adjustments or replacements of the process and/or abatement units and associated equipment

MDA

Methylene diphenyl diamine

MDI

Methylene diphenyl diisocyanate

MDI plant

Plant for the production of MDI from MDA via phosgenation

New plant

A plant first permitted on the site of the installation following the publication of these BAT conclusions or a complete replacement of a plant following the publication of these BAT conclusions

New unit

A unit first permitted following the publication of these BAT conclusions or a complete replacement of a unit following the publication of these BAT conclusions

NOX precursors

Nitrogen-containing compounds (e.g. ammonia, nitrous gases and nitrogen-containing organic compounds) in the input to a thermal treatment that lead to NOX emissions. Elementary nitrogen is not included

PCDD/F

Polychlorinated dibenzo-dioxins and -furans

Periodic measurement

Measurement at specified time intervals using manual or automated methods

Process furnace/heater

Process furnaces or heaters are:

It should be noted that, as a consequence of the application of good energy recovery practices, some of the process furnaces/heaters may have an associated steam/electricity generation system. This is considered to be an integral design feature of the process furnace/heater that cannot be considered in isolation.

Process off-gas

The gas leaving a process which is further treated for recovery and/or abatement

NOX

The sum of nitrogen monoxide (NO) and nitrogen dioxide (NO2), expressed as NO2

Residues

Substances or objects generated by the activities covered by the scope of this document, as waste or by-products

RTO

Regenerative thermal oxidiser

SCR

Selective catalytic reduction

SMPO

Styrene monomer and propylene oxide

SNCR

Selective non-catalytic reduction

SRU

Sulphur recovery unit

TDA

Toluene diamine

TDI

Toluene diisocyanate

TDI plant

Plant for the production of TDI from TDA via phosgenation

TOC

Total organic carbon, expressed as C; includes all organic compounds (in water)

Total suspended solids (TSS)

Mass concentration of all suspended solids, measured via filtration through glass fibre filters and gravimetry

TVOC

Total volatile organic carbon; total volatile organic compounds which are measured by a flame ionisation detector (FID) and expressed as total carbon

Unit

A segment/subpart of a plant in which a specific process or operation is carried out (e.g. reactor, scrubber, distillation column). Units can be new units or existing units

Valid hourly or half-hourly average

An hourly (or half-hourly) average is considered valid when there is no maintenance or malfunction of the automated measuring system

VCM

Vinyl chloride monomer

VOCs

Volatile organic compounds as defined in Article 3(45) of Directive 2010/75/EU

Substance/Parameter

Standard(s)(5)

Total rated thermal input (MWth)(6)

Minimum monitoring frequency(7)

Monitoring associated with

CO

Generic EN standards

≥ 50

Continuous

Table 2.1,

Table 10.1

EN 15058

10 to < 50

Once every 3 months(8)

Dust(9)

Generic EN standards and EN 13284-2

≥ 50

Continuous

BAT 5

EN 13284-1

10 to < 50

Once every 3 months(8)

NH3 (10)

Generic EN standards

≥ 50

Continuous

BAT 7,

Table 2.1

No EN standard available

10 to < 50

Once every 3 months(8)

NOX

Generic EN standards

≥ 50

Continuous

BAT 4,

Table 2.1,

Table 10.1

EN 14792

10 to < 50

Once every 3 months(8)

SO2 (11)

Generic EN standards

≥ 50

Continuous

BAT 6

EN 14791

10 to < 50

Once every 3 months(8)

Substance/Parameter

Processes/Sources

Standard(s)

Minimum monitoring frequency

Monitoring associated with

Benzene

Waste gas from the cumene oxidation unit in phenol production(12)

No EN standard available

Once every month(13)

BAT 57

All other processes/sources(14)

BAT 10

Cl2

TDI/MDI(12)

No EN standard available

Once every month(13)

BAT 66

EDC/VCM

BAT 76

CO

Thermal oxidiser

EN 15058

Once every month(13)

BAT 13

Lower olefins (decoking)

No EN standard available(15)

Once every year or once during decoking, if decoking is less frequent

BAT 20

EDC/VCM (decoking)

BAT 78

Dust

Lower olefins (decoking)

No EN standard available(16)

Once every year or once during decoking, if decoking is less frequent

BAT 20

EDC/VCM (decoking)

BAT 78

All other processes/sources(14)

EN 13284-1

Once every month(13)

BAT 11

EDC

EDC/VCM

No EN standard available

Once every month(13)

BAT 76

Ethylene oxide

Ethylene oxide and ethylene glycols

No EN standard available

Once every month(13)

BAT 52

Formaldehyde

Formaldehyde

No EN standard available

Once every month(13)

BAT 45

Gaseous chlorides, expressed as HCl

TDI/MDI(12)

EN 1911

Once every month(13)

BAT 66

EDC/VCM

BAT 76

All other processes/sources(14)

BAT 12

NH3

Use of SCR or SNCR

No EN standard available

Once every month(13)

BAT 7

NOX

Thermal oxidiser

EN 14792

Once every month(13)

BAT 13

PCDD/F

TDI/MDI(17)

EN 1948-1, -2, and -3

Once every 6 months(13)

BAT 67

PCDD/F

EDC/VCM

BAT 77

SO2

All processes/sources(14)

EN 14791

Once every month(13)

BAT 12

Tetrachloromethane

TDI/MDI(12)

No EN standard available

Once every month(13)

BAT 66

TVOC

TDI/MDI

EN 12619

Once every month(13)

BAT 66

EO (desorption of CO2 from scrubbing medium)

Once every 6 months(13)

BAT 51

Formaldehyde

Once every month(13)

BAT 45

Waste gas from the cumene oxidation unit in phenol production

EN 12619

Once every month(13)

BAT 57

Waste gas from other sources in phenol production when not combined with other waste gas streams

Once every year

Waste gas from the oxidation unit in hydrogen peroxide production

Once every month(13)

BAT 86

EDC/VCM

Once every month(13)

BAT 76

All other processes/sources(14)

Once every month(13)

BAT 10

VCM

EDC/VCM

No EN standard available

Once every month(13)

BAT 76

Technique

Description

Applicability

a.

Choice of fuel

See Section 12.3. This includes switching from liquid to gaseous fuels, taking into account the overall hydrocarbon balance

The switch from liquid to gaseous fuels may be restricted by the design of the burners in the case of existing plants

b.

Staged combustion

Staged combustion burners achieve lower NOX emissions by staging the injection of either air or fuel in the near burner region. The division of fuel or air reduces the oxygen concentration in the primary burner combustion zone, thereby lowering the peak flame temperature and reducing thermal NOX formation

Applicability may be restricted by space availability when upgrading small process furnaces, thus limiting the retrofit of fuel/air staging without reducing capacity

For existing EDC crackers, the applicability may be restricted by the design of the process furnace

c.

Flue-gas recirculation (external)

Recirculation of part of the flue-gas to the combustion chamber to replace part of the fresh combustion air, with the effect of reducing the oxygen content and therefore cooling the temperature of the flame

For existing process furnaces/heaters, the applicability may be restricted by their design.

Not applicable to existing EDC crackers

d.

Flue-gas recirculation (internal)

Recirculation of part of the flue-gas within the combustion chamber to replace part of the fresh combustion air, with the effect of reducing the oxygen content and therefore reducing the temperature of the flame

For existing process furnaces/heaters, the applicability may be restricted by their design

e.

Low-NOX burner (LNB) or ultra-low-NOX burner (ULNB)

See Section 12.3

For existing process furnaces/heaters, the applicability may be restricted by their design

f.

Use of inert diluents

‘Inert’ diluents, e.g. steam, water, nitrogen, are used (either by being premixed with the fuel prior to its combustion or directly injected into the combustion chamber) to reduce the temperature of the flame. Steam injection may increase CO emissions

Generally applicable

g.

Selective catalytic reduction (SCR)

See Section 12.1

Applicability to existing process furnaces/heaters may be restricted by space availability

h.

Selective non-catalytic reduction (SNCR)

See Section 12.1

Applicability to existing process furnaces/heaters may be restricted by the temperature window (900–1 050 °C) and the residence time needed for the reaction.

Not applicable to EDC crackers

Technique

Description

Applicability

a.

Choice of fuel

See Section 12.3. This includes switching from liquid to gaseous fuels, taking into account the overall hydrocarbon balance

The switch from liquid to gaseous fuels may be restricted by the design of the burners in the case of existing plants

b.

Atomisation of liquid fuels

Use of high pressure to reduce the droplet size of liquid fuel. Current optimal burner design generally includes steam atomisation

Generally applicable

c.

Fabric, ceramic or metal filter

See Section 12.1

Not applicable when only combusting gaseous fuels

Technique

Description

Applicability

a.

Choice of fuel

See Section 12.3. This includes switching from liquid to gaseous fuels, taking into account the overall hydrocarbon balance

The switch from liquid to gaseous fuels may be restricted by the design of the burners in the case of existing plants

b.

Caustic scrubbing

See Section 12.1

Applicability may be restricted by space availability

Technique

Description

Applicability

a.

Recovery and use of excess or generated hydrogen

Recovery and use of excess hydrogen or hydrogen generated from chemical reactions (e.g. for hydrogenation reactions). Recovery techniques such as pressure swing adsorption or membrane separation may be used to increase the hydrogen content

Applicability may be restricted where the energy demand for recovery is excessive due to the low hydrogen content or when there is no demand for hydrogen

b.

Recovery and use of organic solvents and unreacted organic raw materials

Recovery techniques such as compression, condensation, cryogenic condensation, membrane separation and adsorption may be used. The choice of technique may be influenced by safety considerations, e.g. presence of other substances or contaminants

Applicability may be restricted where the energy demand for recovery is excessive due to the low organic content

c.

Use of spent air

The large volume of spent air from oxidation reactions is treated and used as low-purity nitrogen

Only applicable where there are available uses for low-purity nitrogen which do not compromise process safety

d.

Recovery of HCl by wet scrubbing for subsequent use

Gaseous HCl is absorbed in water using a wet scrubber, which may be followed by purification (e.g. using adsorption) and/or concentration (e.g. using distillation) (see Section 12.1 for the technique descriptions). The recovered HCl is then used (e.g. as acid or to produce chlorine)

Applicability may be restricted in the case of low HCl loads

e.

Recovery of H2S by regenerative amine scrubbing for subsequent use

Regenerative amine scrubbing is used for recovering H2S from process off-gas streams and from the acidic off-gases of sour water stripping units. H2S is then typically converted to elemental sulphur in a sulphur recovery unit in a refinery (Claus process).

Only applicable if a refinery is located nearby

f.

Techniques to reduce solids and/or liquids entrainment

See Section 12.1

Generally applicable

Technique

Description

Applicability

a.

Condensation

See Section 12.1. The technique is generally used in combination with further abatement techniques

Generally applicable

b.

Adsorption

See Section 12.1

Generally applicable

c.

Wet scrubbing

See Section 12.1

Only applicable to VOCs that can be absorbed in aqueous solutions

d.

Catalytic oxidiser

See Section 12.1

Applicability may be restricted by the presence of catalyst poisons

e.

Thermal oxidiser

See Section 12.1. Instead of a thermal oxidiser, an incinerator for the combined treatment of liquid waste and waste gas may be used

Generally applicable

Technique

Description

Applicability

a.

Cyclone

See Section 12.1. The technique is used in combination with further abatement techniques

Generally applicable

b.

Electrostatic precipitator

See Section 12.1

For existing units, the applicability may be restricted by space availability or safety considerations

c.

Fabric filter

See Section 12.1

Generally applicable

d.

Two-stage dust filter

See Section 12.1

e.

Ceramic/metal filter

See Section 12.1

f.

Wet dust scrubbing

See Section 12.1

Technique

Description

Main pollutant targeted

Applicability

a.

Removal of high levels of NOX precursors from the process off-gas streams

Remove (if possible, for reuse) high levels of NOX precursors prior to thermal treatment, e.g. by scrubbing, condensation or adsorption

NOX

Generally applicable

b.

Choice of support fuel

See Section 12.3

NOX, SO2

Generally applicable

c.

Low-NOX burner (LNB)

See Section 12.1

NOX

Applicability to existing units may be restricted by design and/or operational constraints

d.

Regenerative thermal oxidiser (RTO)

See Section 12.1

NOX

Applicability to existing units may be restricted by design and/or operational constraints

e.

Combustion optimisation

Design and operational techniques used to maximise the removal of organic compounds, while minimising emissions to air of CO and NOX (e.g. by controlling combustion parameters such as temperature and residence time)

CO, NOX

Generally applicable

f.

Selective catalytic reduction (SCR)

See Section 12.1

NOX

Applicability to existing units may be restricted by space availability

g.

Selective non-catalytic reduction (SNCR)

See Section 12.1

NOX

Applicability to existing units may be restricted by the residence time needed for the reaction

Technique

Description

a.

Catalyst selection

Select the catalyst to achieve the optimal balance between the following factors:

b.

Catalyst protection

Techniques used upstream of the catalyst to protect it from poisons (e.g. raw material pretreatment)

c.

Process optimisation

Control of reactor conditions (e.g. temperature, pressure) to achieve the optimal balance between conversion efficiency and catalyst lifetime

d.

Monitoring of catalyst performance

Monitoring of the conversion efficiency to detect the onset of catalyst decay using suitable parameters (e.g. the heat of reaction and the CO2 formation in the case of partial oxidation reactions)

Technique

Description

Applicability

a.

Addition of inhibitors to distillation systems

Selection (and optimisation of dosage) of polymerisation inhibitors that prevent or reduce the generation of residues (e.g. gums or tars). The optimisation of dosage may need to take into account that it can lead to higher nitrogen and/or sulphur content in the residues which could interfere with their use as a fuel

Generally applicable

b.

Minimisation of high-boiling residue formation in distillation systems

Techniques that reduce temperatures and residence times (e.g. packing instead of trays to reduce the pressure drop and thus the temperature; vacuum instead of atmospheric pressure to reduce the temperature)

Only applicable to new distillation units or major plant upgrades

c.

Material recovery (e.g. by distillation, cracking)

Materials (i.e. raw materials, products, and by-products) are recovered from residues by isolation (e.g. distillation) or conversion (e.g. thermal/catalytic cracking, gasification, hydrogenation)

Only applicable where there are available uses for these recovered materials

d.

Catalyst and adsorbent regeneration

Regeneration of catalysts and adsorbents, e.g. using thermal or chemical treatment

Applicability may be restricted where regeneration results in significant cross-media effects.

e.

Use of residues as a fuel

Some organic residues, e.g. tar, can be used as fuels in a combustion unit

Applicability may be restricted by the presence of certain substances in the residues, making them unsuitable to use in a combustion unit and requiring disposal

Technique

Description

Applicability

a.

Identification of critical equipment

Equipment critical to the protection of the environment (‘critical equipment’) is identified on the basis of a risk assessment (e.g. using a Failure Mode and Effects Analysis)

Generally applicable

b.

Asset reliability programme for critical equipment

A structured programme to maximise equipment availability and performance which includes standard operating procedures, preventive maintenance (e.g. against corrosion), monitoring, recording of incidents, and continuous improvements

Generally applicable

c.

Back-up systems for critical equipment

Build and maintain back-up systems, e.g. vent gas systems, abatement units

Not applicable if appropriate equipment availability can be demonstrated using technique b.

Parameter

BAT-AELs(18) (19) (20)

(daily average or average over the sampling period)

(mg/Nm3, at 3 vol-% O2)

New furnace

Existing furnace

NOX

60–100

70–200

NH3

< 5–15(21)

Technique

Description

Applicability

a.

Tube materials that retard coke formation

Nickel present at the surface of the tubes catalyses coke formation. Employing materials that have lower nickel levels, or coating the interior tube surface with an inert material, can therefore retard the rate of coke build-up

Only applicable to new units or major plant upgrades

b.

Doping of the raw material feed with sulphur compounds

As nickel sulphides do not catalyse coke formation, doping the feed with sulphur compounds when they are not already present at the desired level can also help retard the build-up of coke, as this will promote the passivation of the tube surface

Generally applicable

c.

Optimisation of thermal decoking

Optimisation of operating conditions, i.e. airflow, temperature and steam content across the decoking cycle, to maximise coke removal

Generally applicable

d.

Wet dust scrubbing

See Section 12.1

Generally applicable

e.

Dry cyclone

See Section 12.1

Generally applicable

f.

Combustion of decoking waste gas in process furnace/heater

The decoking waste gas stream is passed through the process furnace/heater during decoking where the coke particles (and CO) are further combusted

Applicability for existing plants may be restricted by the design of the pipework systems or fire-duty restrictions

Technique

Description

Applicability

a.

Use of low-sulphur raw materials in the cracker feed

Use of raw materials that have a low sulphur content or have been desulphurised

Applicability may be restricted by a need for sulphur doping to reduce coke build-up

b.

Maximisation of the use of amine scrubbing for the removal of acid gases

The scrubbing of the cracked gases with a regenerative (amine) solvent to remove acid gases, mainly H2S, to reduce the load on the downstream caustic scrubber

Not applicable if the lower olefin cracker is located far away from an SRU. Applicability for existing plants may be restricted by the capacity of the SRU

c.

Oxidation

Oxidation of sulphides present in the spent scrubbing liquor to sulphates, e.g. using air at elevated pressure and temperature (i.e. wet air oxidation) or an oxidising agent such as hydrogen peroxide

Generally applicable

Technique

Description

Applicability

a.

Water-free vacuum generation

Use mechanical pumping systems in a closed circuit procedure, discharging only a small amount of water as blowdown, or use dry-running pumps. In some cases, waste-water-free vacuum generation can be achieved by use of the product as a barrier liquid in a mechanical vacuum pump, or by use of a gas stream from the production process

Generally applicable

b.

Source segregation of aqueous effluents

Aqueous effluents from aromatics plants are segregated from waste water from other sources in order to facilitate the recovery of raw materials or products

For existing plants, the applicability may be restricted by site-specific drainage systems

c.

Liquid phase separation with recovery of hydrocarbons

Separation of organic and aqueous phases with appropriate design and operation (e.g. sufficient residence time, phase boundary detection and control) to prevent any entrainment of undissolved organic material

Generally applicable

d

Stripping with recovery of hydrocarbons

See Section 12.2. Stripping can be used on individual or combined streams

Applicability may be restricted when the concentration of hydrocarbons is low

e.

Reuse of water

With further treatment of some waste water streams, water from stripping can be used as process water or as boiler feed water, replacing other sources of water

Generally applicable

Technique

Description

Applicability

a.

Distillation optimisation

For each distillation column, the number of trays, reflux ratio, feed location and, for extractive distillations, the solvents to feed ratio are optimised

Applicability to existing units may be restricted by design, space availability and/or operational constraints

b.

Recovery of heat from column overhead gaseous stream

Reuse condensation heat from the toluene and the xylene distillation column to supply heat elsewhere in the installation

c.

Single extractive distillation column

In a conventional extractive distillation system, the separation would require a sequence of two separation steps (i.e. main distillation column with side column or stripper). In a single extractive distillation column, the separation of the solvent is carried out in a smaller distillation column that is incorporated into the column shell of the first column

Only applicable to new plants or major plant upgrades.

Applicability may be restricted for smaller capacity units as operability may be constrained by combining a number of operations into one piece of equipment

d.

Distillation column with a dividing wall

In a conventional distillation system, the separation of a three-component mixture into its pure fractions requires a direct sequence of at least two distillation columns (or main columns with side columns). With a dividing wall column, separation can be carried out in just one piece of apparatus

e.

Thermally coupled distillation

If distillation is carried out in two columns, energy flows in both columns can be coupled. The steam from the top of the first column is fed to a heat exchanger at the base of the second column

Only applicable to new plants or major plant upgrades.

Applicability depends on the set-up of the distillation columns and process conditions, e.g. working pressure

Technique

Description

Applicability

a.

Selective hydrogenation of reformate or pygas

Reduce the olefin content of reformate or pygas by hydrogenation. With fully hydrogenated raw materials, clay treaters have longer operating cycles

Only applicable to plants using raw materials with a high olefin content

b.

Clay material selection

Use a clay that lasts as long as possible for its given conditions (i.e. having surface/structural properties that increase the operating cycle length), or use a synthetic material that has the same function as the clay but that can be regenerated

Generally applicable

Technique

Description

Applicability

a.

Techniques to reduce liquids entrainment

See Section 12.1

Generally applicable

b.

Condensation

See Section 12.1

Generally applicable

c.

Adsorption

See Section 12.1

Generally applicable

d.

Scrubbing

See Section 12.1. Scrubbing is carried out with a suitable solvent (e.g. the cool, recirculated ethylbenzene) to absorb ethylbenzene, which is recycled to the reactor

For existing plants, the use of the recirculated ethylbenzene stream may be restricted by the plant design

Technique

Description

Applicability

a.

Optimised liquid phase separation

Separation of organic and aqueous phases with appropriate design and operation (e.g. sufficient residence time, phase boundary detection and control) to prevent any entrainment of undissolved organic material

Generally applicable

b.

Steam stripping

See Section 12.2

Generally applicable

c.

Adsorption

See Section 12.2

Generally applicable

d.

Reuse of water

Condensates from the reaction can be used as process water or as boiler feed after steam stripping (see technique b.) and adsorption (see technique c.)

Generally applicable

Technique

Description

Applicability

a.

Condensation

See Section 12.1

Generally applicable

b.

Scrubbing

See Section 12.1. The absorbent consists of commercial organic solvents (or tar from ethylbenzene plants) (see BAT 42b). VOCs are recovered by stripping of the scrubber liquor

Technique

Description

Applicability

a.

Material recovery (e.g. by distillation, cracking)

See BAT 17c

Only applicable where there are available uses for these recovered materials

b.

Use of tar as an absorbent for scrubbing

See section 12.1. Use the tar as an absorbent in the scrubbers used in styrene monomer production by ethylbenzene dehydrogenation, instead of commercial organic solvents (see BAT 38b). The extent to which tar can be used depends on the scrubber capacity

Generally applicable

c.

Use of tar as a fuel

See BAT 17e

Generally applicable

Technique

Description

Applicability

a.

Addition of inhibitors to distillation systems

See BAT 17a

Generally applicable

b.

Minimisation of high-boiling residue formation in distillation systems

See BAT 17b

Only applicable to new distillation units or major plant upgrades

c.

Use of residues as a fuel

See BAT 17e

Generally applicable

Technique

Description

Applicability

a.

Send the waste gas stream to a combustion unit

See BAT 9

Only applicable to the silver process

b.

Catalytic oxidiser with energy recovery

See Section 12.1. Energy is recovered as steam

Only applicable to the metal oxide process. The ability to recover energy may be restricted in small stand-alone plants

c.

Thermal oxidiser with energy recovery

See Section 12.1. Energy is recovered as steam

Only applicable to the silver process

Parameter

BAT-AEL

(daily average or average over the sampling period)

(mg/Nm3, no correction for oxygen content)

TVOC

< 5–30(22)

Formaldehyde

2–5

Technique

Description

Applicability

a.

Reuse of water

Aqueous streams (e.g. from cleaning, spills and condensates) are recirculated into the process mainly to adjust the formaldehyde product concentration. The extent to which water can be reused depends on the desired formaldehyde concentration

Generally applicable

b.

Chemical pretreatment

Conversion of formaldehyde into other substances which are less toxic, e.g. by addition of sodium sulphite or by oxidation

Only applicable to effluents which, due to their formaldehyde content, could have a negative effect on the downstream biological waste water treatment

Technique

Description

Applicability

a.

Minimisation of paraformaldehyde generation

The formation of paraformaldehyde is minimised by improved heating, insulation and flow circulation

Generally applicable

b.

Material recovery

Paraformaldehyde is recovered by dissolution in hot water where it undergoes hydrolysis and depolymerisation to give a formaldehyde solution, or is reused directly in other processes

Not applicable when the recovered paraformaldehyde cannot be used due to its contamination

c.

Use of residues as a fuel

Paraformaldehyde is recovered and used as a fuel

Only applicable when technique b. cannot be applied

Technique

Description

Applicability

a.

Use of pressure swing adsorption or membrane separation to recover ethylene from the inerts purge

With the pressure swing adsorption technique, the target gas (in this case ethylene) molecules are adsorbed on a solid (e.g. molecular sieve) at high pressure, and subsequently desorbed in more concentrated form at lower pressure for reuse or recycling.

For membrane separation, see Section 12.1

Applicability may be restricted when the energy demand is excessive due to a low ethylene mass flow

b.

Send the inerts purge stream to a combustion unit

See BAT 9

Generally applicable

Technique

Description

Applicability

a.

Staged CO2 desorption

The technique consists of conducting the depressurisation necessary to liberate the carbon dioxide from the absorption medium in two steps rather than one. This allows an initial hydrocarbon-rich stream to be isolated for potential recirculation, leaving a relatively clean carbon dioxide stream for further treatment.

Only applicable to new plants or major plant upgrades

b.

Catalytic oxidiser

See Section 12.1

Generally applicable

c.

Thermal oxidiser

See Section 12.1

Generally applicable

Parameter

BAT-AEL

TVOC

1–10 g/t of EO produced(23) (24) (25)

Technique

Description

Applicability

a.

Indirect cooling

Use indirect cooling systems (with heat exchangers) instead of open cooling systems

Only applicable to new plants or major plant upgrades

b.

Complete EO removal by stripping

Maintain appropriate operating conditions and use online monitoring of the EO stripper operation to ensure that all EO is stripped out; and provide adequate protection systems to avoid EO emissions during other than normal operating conditions

Only applicable when technique a. cannot be applied

Technique

Description

Applicability

a.

Use of the purge from the EO plant in the EG plant

The purge streams from the EO plant are sent to the EG process and not discharged as waste water. The extent to which the purge can be reused in the EG process depends on EG product quality considerations.

Generally applicable

b.

Distillation

Distillation is a technique used to separate compounds with different boiling points by partial evaporation and recondensation.

The technique is used in EO and EG plants to concentrate aqueous streams to recover glycols or enable their disposal (e.g. by incineration, instead of their discharge as waste water) and to enable the partial reuse/recycling of water.

Only applicable to new plants or major plant upgrades

Technique

Description

Applicability

a.

Hydrolysis reaction optimisation

Optimisation of the water to EO ratio to both achieve lower co-production of heavier glycols and avoid excessive energy demand for the dewatering of glycols. The optimum ratio depends on the target output of di- and triethylene glycols

Generally applicable

b.

Isolation of by-products at EO plants for use

For EO plants, the concentrated organic fraction obtained after the dewatering of the liquid effluent from EO recovery is distilled to give valuable short-chain glycols and a heavier residue

Only applicable to new plants or major plant upgrades

c.

Isolation of by-products at EG plants for use

For EG plants, the longer chain glycols fraction can either be used as such or further fractionated to yield valuable glycols

Generally applicable

Technique

Description

Applicability

a.

Techniques to reduce liquids entrainment

See Section 12.1

Generally applicable

b.

Condensation

See Section 12.1

Generally applicable

c.

Adsorption (regenerative)

See Section 12.1

Generally applicable

Technique

Description

Applicability

a.

Send the waste gas stream to a combustion unit

See BAT 9

Only applicable where there are available uses for the waste gas as gaseous fuel

b.

Adsorption

See Section 12.1

Generally applicable

c.

Thermal oxidiser

See Section 12.1

Generally applicable

d.

Regenerative thermal oxidiser (RTO)

See Section 12.1

Generally applicable

Parameter

Source

BAT-AEL

(daily average or average over the sampling period)

(mg/Nm3, no correction for oxygen content)

Conditions

Benzene

Cumene oxidation unit

< 1

The BAT-AEL applies if the emission exceeds 1 g/h

TVOC

5–30

—

Parameter

BAT-AEPL

(average value from at least three spot samples taken at intervals of at least half an hour)

Associated monitoring

Total organic peroxides, expressed as cumene hydroperoxide

< 100 mg/l

No EN standard available. The minimum monitoring frequency is once every day and may be reduced to four times per year if adequate performance of the hydrolysis is demonstrated by controlling the process parameters (e.g. pH, temperature and residence time)

Technique

Description

Applicability

a.

Material recovery

(e.g. by distillation, cracking)

See BAT 17c. Use distillation to recover cumene, α-methylstyrene phenol, etc.

Generally applicable

b.

Use of tar as a fuel

See BAT 17e.

Generally applicable

Technique

Description

Applicability

a.

Water-free vacuum generation

Use of dry-running pumps, e.g. positive displacement pumps

Applicability to existing plants may be restricted by design and/or operational constraints

b.

Use of water ring vacuum pumps with recirculation of the ring water

The water used as the sealant liquid of the pump is recirculated to the pump casing via a closed loop with only small purges, so that waste water generation is minimised

Only applicable when technique a. cannot be applied.

Not applicable for triethanolamine distillation

c.

Reuse of aqueous streams from vacuum systems in the process

Return aqueous streams from water ring pumps or steam ejectors to the process for recovery of organic material and reuse of the water. The extent to which water can be reused in the process is restricted by the water demand of the process

Only applicable when technique a. cannot be applied

d.

Condensation of organic compounds (amines) upstream of vacuum systems

See Section 12.1

Generally applicable

Technique

Description

Applicability

a.

Use of excess ammonia

Maintaining a high level of ammonia in the reaction mixture is an effective way of ensuring that all the ethylene oxide is converted into products

Generally applicable

b.

Optimisation of the water content in the reaction

Water is used to accelerate the main reactions without changing the product distribution and without significant side reactions with ethylene oxide to glycols

Only applicable for the aqueous process

c.

Optimise the process operating conditions

Determine and maintain the optimum operating conditions (e.g. temperature, pressure, residence time) to maximise the conversion of ethylene oxide to the desired mix of mono-, di-, triethanolamines

Generally applicable

Technique

Description

Applicability

a.

Condensation

See Section 12.1

Generally applicable

b.

Wet scrubbing

See Section 12.1. In many cases, scrubbing efficiency is enhanced by the chemical reaction of the absorbed pollutant (partial oxidation of NOX with recovery of nitric acid, removal of acids with caustic solution, removal of amines with acidic solutions, reaction of aniline with formaldehyde in caustic solution)

c.

Thermal reduction

See Section 12.1

Applicability to existing units may be restricted by space availability

d.

Catalytic reduction

See Section 12.1

Technique

Description

Applicability

a.

Absorption of HCl by wet scrubbing

See BAT 8d.

Generally applicable

b.

Absorption of phosgene by scrubbing

See Section 12.1. The excess phosgene is absorbed using an organic solvent and returned to the process

Generally applicable

c.

HCl/phosgene condensation

See Section 12.1

Generally applicable

Parameter

BAT-AEL

(mg/Nm3, no correction for oxygen content)

TVOC

1–5(26) (27)

Tetrachloromethane

≤ 0,5 g/t MDI produced(28)

≤ 0,7 g/t TDI produced(28)

Cl2

< 1(27) (29)

HCl

2–10(27)

PCDD/F

0,025–0,08 ng I-TEQ/Nm3 (27)

Technique

Description

Applicability

a.

Rapid quenching

Rapid cooling of exhaust gases to prevent the de novo synthesis of PCDD/F

Generally applicable

b.

Activated carbon injection

Removal of PCDD/F by adsorption onto activated carbon that is injected into the exhaust gas, followed by dust abatement

Substance/Parameter

Plant

Sampling point

Standard(s)

Minimum monitoring frequency

Monitoring associated with

TOC

DNT plant

Outlet of the pretreatment unit

EN 1484

Once every week(30)

BAT 70

MDI and/or TDI plant

Outlet of the plant

Once every month

BAT 72

Aniline

MDA plant

Outlet of the final waste water treatment

No EN standard available

Once every month

BAT 14

Chlorinated solvents

MDI and/or TDI plant

Various EN standards available (e.g. EN ISO 15680)

BAT 14

Technique

Description

Applicability

a.

Use of highly concentrated nitric acid

Use highly concentrated HNO3 (e.g. about 99 %) to increase the process efficiency and to reduce the waste water volume and the load of pollutants

Applicability to existing units may be restricted by design and/or operational constraints

b.

Optimised regeneration and recovery of spent acid

Perform the regeneration of the spent acid from the nitration reaction in such a way that water and the organic content are also recovered for reuse, by using an appropriate combination of evaporation/distillation, stripping and condensation

Applicability to existing units may be restricted by design and/or operational constraints

c.

Reuse of process water to wash DNT

Reuse process water from the spent acid recovery unit and the nitration unit to wash DNT

Applicability to existing units may be restricted by design and/or operational constraints

d.

Reuse of water from the first washing step in the process

Nitric and sulphuric acid are extracted from the organic phase using water. The acidified water is returned to the process, for direct reuse or further processing to recover materials

Generally applicable

e.

Multiple use and recirculation of water

Reuse water from washing, rinsing and equipment cleaning e.g. in the counter-current multistep washing of the organic phase

Generally applicable

Technique

Description

Applicability

a.

Extraction

See Section 12.2

Generally applicable

b.

Chemical oxidation

See Section 12.2

Parameter

BAT-AEPL

(average of values obtained during 1 month)

TOC

< 1 kg/t DNT produced

Specific waste water volume

< 1 m3/t DNT produced

Technique

Description

Applicability

a.

Evaporation

See Section 12.2

Generally applicable

b.

Stripping

See Section 12.2

c.

Extraction

See Section 12.2

d.

Reuse of water

Reuse of water (e.g. from condensates or from scrubbing) in the process or in other processes (e.g. in a DNT plant). The extent to which water can be reused at existing plants may be restricted by technical constraints

Generally applicable

Parameter

BAT-AEPL

(average of values obtained during 1 month)

Specific waste water volume

< 1 m3/t TDA produced

Parameter

BAT-AEPL

(average of values obtained during 1 year)

TOC

< 0,5 kg/t product (TDI or MDI)(31)

Technique

Description

Applicability

a.

Evaporation

See Section 12.2. Used to facilitate extraction (see technique b)

Generally applicable

b.

Extraction

See Section 12.2. Used to recover/remove MDA

Generally applicable

c.

Steam stripping

See Section 12.2. Used to recover/remove aniline and methanol

For methanol, the applicability depends on the assessment of alternative options as part of the waste water management and treatment strategy

d.

Distillation

See Section 12.2. Used to recover/remove aniline and methanol

Technique

Description

Applicability

a.

Minimisation of high-boiling residue formation in distillation systems

See BAT 17b.

Only applicable to new distillation units or major plant upgrades

b.

Increased recovery of TDI by evaporation or further distillation

Residues from distillation are additionally processed to recover the maximum amount of TDI contained therein, e.g. using a thin film evaporator or other short-path distillation units followed by a dryer.

Only applicable to new distillation units or major plant upgrades

c.

Recovery of TDA by chemical reaction

Tars are processed to recover TDA by chemical reaction (e.g. hydrolysis).

Only applicable to new plants or major plant upgrades

Parameter

BAT-AELs(32) (33) (34)

(daily average or average over the sampling period)

(mg/Nm3, at 3 vol-% O2)

NOx

50–100

Technique

Description

Applicability

a.

Control of feed quality

Control the quality of the feed to minimise the formation of residues (e.g. propane and acetylene content of ethylene; bromine content of chlorine; acetylene content of hydrogen chloride)

Generally applicable

b.

Use of oxygen instead of air for oxychlorination

Only applicable to new oxychlorination plants or major oxychlorination plant upgrades

c.

Condensation using chilled water or refrigerants

Use condensation (see Section 12.1) with chilled water or refrigerants such as ammonia or propylene to recover organic compounds from individual vent gas streams before sending them to final treatment

Generally applicable

Parameter

BAT-AEL

(daily average or average over the sampling period)

(mg/Nm3, at 11 vol-% O2)

TVOC

0,5–5

Sum of EDC and VCM

< 1

Cl2

< 1–4

HCl

2–10

PCDD/F

0,025–0,08 ng I-TEQ/Nm3

Technique

Description

Applicability

a.

Rapid quenching

Rapid cooling of exhaust gases to prevent the de novo synthesis of PCDD/F

Generally applicable

b.

Activated carbon injection

Removal of PCDD/F by adsorption onto activated carbon that is injected into the exhaust gas, followed by dust abatement

Technique

Description

Applicability

a.

Optimisation of thermal decoking

Optimisation of operating conditions, i.e. airflow, temperature and steam content across the decoking cycle, to maximise coke removal

Generally applicable

b.

Optimisation of mechanical decoking

Optimise mechanical decoking (e.g. sand jetting) to maximise coke removal as dust

Generally applicable

c.

Wet dust scrubbing

See Section 12.1

Only applicable to thermal decoking

d.

Cyclone

See Section 12.1

Generally applicable

e.

Fabric filter

See Section 12.1

Generally applicable

Substance/Parameter

Plant

Sampling point

Standard(s)

Minimum monitoring frequency

Monitoring associated with

EDC

All plants

Outlet of the waste water stripper

EN ISO 10301

Once every day

BAT 80

VCM

Copper

Oxy-chlorination plant using the fluidised-bed design

Outlet of the pretreatment for solids removal

Various EN standards available, e.g. EN ISO 11885, EN ISO 15586, EN ISO 17294-2

Once every day(35)

BAT 81

PCDD/F

No EN standard available

Once every 3 months

Total suspended solids (TSS)

EN 872

Once every day(35)

Copper

Oxy-chlorination plant using the fluidised-bed design

Outlet of the final waste water treatment

Various EN standards available, e.g. EN ISO 11885, EN ISO 15586, EN ISO 17294-2

Once every month

BAT 14 and BAT 81

EDC

All plants

EN ISO 10301

Once every month

BAT 14 and BAT 80

PCDD/F

No EN standard available

Once every 3 months

BAT 14 and BAT 81

Parameter

BAT-AEPL

(average of values obtained during 1 month)(36)

EDC

0,1–0,4 mg/l

VCM

< 0,05 mg/l

Technique

Description

Applicability

a.

Fixed-bed design for oxychlorination

Oxychlorination reaction design: in the fixed-bed reactor, catalyst particulates entrained in the overhead gaseous stream are reduced

Not applicable to existing plants using the fluidised-bed design

b.

Cyclone or dry catalyst filtration system

A cyclone or a dry catalyst filtration system reduces catalyst losses from the reactor and therefore also their transfer to waste water

Only applicable to plants using the fluidised-bed design

c.

Chemical precipitation

See Section 12.2. Chemical precipitation is used to remove dissolved copper

Only applicable to plants using the fluidised-bed design

d.

Coagulation and flocculation

See Section 12.2

Only applicable to plants using the fluidised-bed design

e.

Membrane filtration (micro- or ultrafiltration)

See Section 12.2

Only applicable to plants using the fluidised-bed design

Parameter

BAT-AEPL

(average of values obtained during 1 year)

Copper

0,4–0,6 mg/l

PCDD/F

< 0,8 ng I-TEQ/l

Total suspended solids (TSS)

10–30 mg/l

Parameter

BAT-AEL

(average of values obtained during 1 year)

Copper

0,04–0,2 g/t EDC produced by oxychlorination(37)

EDC

0,01–0,05 g/t EDC purified(38) (39)

PCDD/F

0,1– 0,3 μg I-TEQ/t EDC produced by oxychlorination

Technique

Description

Applicability

a.

Use of promoters in cracking

See BAT 83

Generally applicable

b.

Rapid quenching of the gaseous stream from EDC cracking

The gaseous stream from EDC cracking is quenched by direct contact with cold EDC in a tower to reduce coke formation. In some cases, the stream is cooled by heat exchange with cold liquid EDC feed prior to quenching

Generally applicable

c.

Pre-evaporation of EDC feed

Coke formation is reduced by evaporating EDC upstream of the reactor to remove high-boiling coke precursors

Only applicable to new plants or major plant upgrades

d.

Flat flame burners

A type of burner in the furnace that reduces hot spots on the walls of the cracker tubes

Only applicable to new furnaces or major plant upgrades

Technique

Description

Applicability

a.

Hydrogenation of acetylene

HCl is generated in the EDC cracking reaction and recovered by distillation.

Hydrogenation of the acetylene present in this HCl stream is carried out to reduce the generation of unwanted compounds during oxychlorination. Acetylene values below 50 ppmv at the outlet of the hydrogenation unit are advisable

Only applicable to new plants or major plant upgrades

b.

Recovery and reuse of HCl from incineration of liquid waste

HCl is recovered from incinerator off-gas by wet scrubbing with water or diluted HCl (see Section 12.1) and reused (e.g. in the oxychlorination plant)

Generally applicable

c.

Isolation of chlorinated compounds for use

Isolation and, if needed, purification of by-products for use (e.g. monochloroethane and/or 1,1,2-trichloroethane, the latter for the production of 1,1-dichloroethylene)

Only applicable to new distillation units or major plant upgrades.

Applicability may be restricted by a lack of available uses for these compounds

Technique

Description

Applicability

a.

Optimisation of the oxidation process

Process optimisation includes elevated oxidation pressure and reduced oxidation temperature in order to reduce the solvent vapour concentration in the process off-gas

Only applicable to new oxidation units or major plant upgrades

b.

Techniques to reduce solids and/or liquids entrainment

See Section 12.1

Generally applicable

c.

Condensation

See Section 12.1

Generally applicable

d.

Adsorption (regenerative)

See Section 12.1

Not applicable to process off-gas from oxidation with pure oxygen

Parameter

BAT-AEL(40)

(daily average or average over the sampling period)(41)

(no correction for oxygen content)

TVOC

5–25 mg/Nm3 (42)

Technique

Description

Applicability

a.

Optimised liquid phase separation

Separation of organic and aqueous phases with appropriate design and operation (e.g. sufficient residence time, phase boundary detection and control) to prevent any entrainment of undissolved organic material

Generally applicable

b.

Reuse of water

Reuse of water, e.g. from cleaning or liquid phase separation. The extent to which water can be reused in the process depends on product quality considerations

Generally applicable

Technique

Description

a.

Adsorption

See Section 12.2. Adsorption is carried out prior to sending waste water streams to the final biological treatment

b.

Waste water incineration

See Section 12.2

Technique

Description

Adsorption

A technique for removing compounds from a process off-gas or waste gas stream by retention on a solid surface (typically activated carbon). Adsorption may be regenerative or non-regenerative (see below).

Adsorption (non-regenerative)

In non-regenerative adsorption, the spent adsorbent is not regenerated but disposed of.

Adsorption (regenerative)

Adsorption where the adsorbate is subsequently desorbed, e.g. with steam (often on site) for reuse or disposal and the adsorbent is reused. For continuous operation, typically more than two adsorbers are operated in parallel, one of them in desorption mode.

Catalytic oxidiser

Abatement equipment which oxidises combustible compounds in a process off-gas or waste gas stream with air or oxygen in a catalyst bed. The catalyst enables oxidation at lower temperatures and in smaller equipment compared to a thermal oxidiser.

Catalytic reduction

NOx is reduced in the presence of a catalyst and a reducing gas. In contrast to SCR, no ammonia and/or urea are added.

Caustic scrubbing

The removal of acidic pollutants from a gas stream by scrubbing using an alkaline solution.

Ceramic/metal filter

Ceramic filter material. In circumstances where acidic compounds such as HCl, NOX, SOX and dioxins are to be removed, the filter material is fitted with catalysts and the injection of reagents may be necessary.

In metal filters, surface filtration is carried out by sintered porous metal filter elements.

Condensation

A technique for removing the vapours of organic and inorganic compounds from a process off-gas or waste gas stream by reducing its temperature below its dew point so that the vapours liquefy. Depending on the operating temperature range required, there are different methods of condensation, e.g. cooling water, chilled water (temperature typically around 5 °C) or refrigerants such as ammonia or propene.

Cyclone (dry or wet)

Equipment for removal of dust from a process off-gas or waste gas stream based on imparting centrifugal forces, usually within a conical chamber.

Electrostatic precipitator (dry or wet)

A particulate control device that uses electrical forces to move particles entrained within a process off-gas or waste gas stream onto collector plates. The entrained particles are given an electrical charge when they pass through a corona where gaseous ions flow. Electrodes in the centre of the flow lane are maintained at a high voltage and generate the electrical field that forces the particles to the collector walls.

Fabric filter

Porous woven or felted fabric through which gases flow to remove particles by use of a sieve or other mechanisms. Fabric filters can be in the form of sheets, cartridges or bags with a number of the individual fabric filter units housed together in a group.

Membrane separation

Waste gas is compressed and passed through a membrane which relies on the selective permeability of organic vapours. The enriched permeate can be recovered by methods such as condensation or adsorption, or can be abated, e.g. by catalytic oxidation. The process is most appropriate for higher vapour concentrations. Additional treatment is, in most cases, needed to achieve concentration levels low enough to discharge.

Mist filter

Commonly mesh pad filters (e.g. mist eliminators, demisters) which usually consist of woven or knitted metallic or synthetic monofilament material in either a random or specific configuration. A mist filter is operated as deep-bed filtration, which takes place over the entire depth of the filter. Solid dust particles remain in the filter until it is saturated and requires cleaning by flushing. When the mist filter is used to collect droplets and/or aerosols, they clean the filter as they drain out as a liquid. It works by mechanical impingement and is velocity-dependent. Baffle angle separators are also commonly used as mist filters.

Regenerative thermal oxidiser (RTO)

Specific type of thermal oxidiser (see below) where the incoming waste gas stream is heated by a ceramic-packed bed by passing through it before entering the combustion chamber. The purified hot gases exit this chamber by passing through one (or more) ceramic-packed bed(s) (cooled by an incoming waste gas stream in an earlier combustion cycle). This reheated packed bed then begins a new combustion cycle by preheating a new incoming waste gas stream. The typical combustion temperature is 800–1 000 °C.

Scrubbing

Scrubbing or absorption is the removal of pollutants from a gas stream by contact with a liquid solvent, often water (see ‘Wet scrubbing’). It may involve a chemical reaction (see ‘Caustic scrubbing’). In some cases, the compounds may be recovered from the solvent.

Selective catalytic reduction (SCR)

The reduction of NOX to nitrogen in a catalytic bed by reaction with ammonia (usually supplied as an aqueous solution) at an optimum operating temperature of around 300–450 °C. One or more layers of catalyst may be applied.

Selective non-catalytic reduction (SNCR)

The reduction of NOX to nitrogen by reaction with ammonia or urea at a high temperature. The operating temperature window must be maintained between 900 °C and 1 050 °C.

Techniques to reduce solids and/or liquids entrainment

Techniques that reduce the carry-over of droplets or particles in gaseous streams (e.g. from chemical processes, condensers, distillation columns) by mechanical devices such as settling chambers, mist filters, cyclones and knock-out drums.

Thermal oxidiser

Abatement equipment which oxidises the combustible compounds in a process off-gas or waste gas stream by heating it with air or oxygen to above its auto-ignition point in a combustion chamber and maintaining it at a high temperature long enough to complete its combustion to carbon dioxide and water.

Thermal reduction

NOX is reduced at elevated temperatures in the presence of a reducing gas in an additional combustion chamber, where an oxidation process takes place but under low oxygen conditions/deficit of oxygen. In contrast to SNCR, no ammonia and/or urea are added.

Two-stage dust filter

A device for filtering on a metal gauze. A filter cake builds up in the first filtration stage and the actual filtration takes place in the second stage. Depending on the pressure drop across the filter, the system switches between the two stages. A mechanism to remove the filtered dust is integrated into the system.

Wet scrubbing

See ‘Scrubbing’ above. Scrubbing where the solvent used is water or an aqueous solution, e.g. caustic scrubbing for abating HCl. See also ‘Wet dust scrubbing’.

Wet dust scrubbing

See ‘Wet scrubbing’ above. Wet dust scrubbing entails separating the dust by intensively mixing the incoming gas with water, mostly combined with the removal of the coarse particles by the use of centrifugal force. In order to achieve this, the gas is released inside tangentially. The removed solid dust is collected at the bottom of the dust scrubber.

Technique

Description

Adsorption

Separation method in which compounds (i.e. pollutants) in a fluid (i.e. waste water) are retained on a solid surface (typically activated carbon).

Chemical oxidation

Organic compounds are oxidised with ozone or hydrogen peroxide, optionally supported by catalysts or UV radiation, to convert them into less harmful and more easily biodegradable compounds

Coagulation and flocculation

Coagulation and flocculation are used to separate suspended solids from waste water and are often carried out in successive steps. Coagulation is carried out by adding coagulants with charges opposite to those of the suspended solids. Flocculation is carried out by adding polymers, so that collisions of microfloc particles cause them to bond to produce larger flocs.

Distillation

Distillation is a technique to separate compounds with different boiling points by partial evaporation and recondensation.

Waste water distillation is the removal of low-boiling contaminants from waste water by transferring them into the vapour phase. Distillation is carried out in columns, equipped with plates or packing material, and a downstream condenser.

Extraction

Dissolved pollutants are transferred from the waste water phase to an organic solvent, e.g. in counter-current columns or mixer-settler systems. After phase separation, the solvent is purified, e.g. by distillation, and returned to the extraction. The extract containing the pollutants is disposed of or returned to the process. Losses of solvent to the waste water are controlled downstream by appropriate further treatment (e.g. stripping).

Evaporation

The use of distillation (see above) to concentrate aqueous solutions of high-boiling substances for further use, processing or disposal (e.g. waste water incineration) by transferring water to the vapour phase. Typically carried out in multistage units with increasing vacuum, to reduce the energy demand. The water vapours are condensed, to be reused or discharged as waste water.

Filtration

The separation of solids from a waste water carrier by passing it through a porous medium. It includes different types of techniques, e.g. sand filtration, microfiltration and ultrafiltration.

Flotation

A process in which solid or liquid particles are separated from the waste water phase by attaching to fine gas bubbles, usually air. The buoyant particles accumulate at the water surface and are collected with skimmers.

Hydrolysis

A chemical reaction in which organic or inorganic compounds react with water, typically in order to convert non-biodegradable to biodegradable or toxic to non-toxic compounds. To enable or enhance the reaction, hydrolysis is carried out at an elevated temperature and possibly pressure (thermolysis) or with the addition of strong alkalis or acids or using a catalyst.

Precipitation

The conversion of dissolved pollutants (e.g. metal ions) into insoluble compounds by reaction with added precipitants. The solid precipitates formed are subsequently separated by sedimentation, flotation or filtration.

Sedimentation

Separation of suspended particles and suspended material by gravitational settling.

Stripping

Volatile compounds are removed from the aqueous phase by a gaseous phase (e.g. steam, nitrogen or air) that is passed through the liquid, and are subsequently recovered (e.g. by condensation) for further use or disposal. The removal efficiency may be enhanced by increasing the temperature or reducing the pressure.

Waste water incineration

The oxidation of organic and inorganic pollutants with air and simultaneous evaporation of water at normal pressure and temperatures between 730 °C and 1 200 °C. Waste water incineration is typically self-sustaining at COD levels of more than 50 g/l. In the case of low organic loads, a support/auxiliary fuel is needed.

Technique

Description

Choice of (support) fuel

The use of fuel (including support/auxiliary fuel) with a low content of potential pollution-generating compounds (e.g. lower sulphur, ash, nitrogen, mercury, fluorine or chlorine content in the fuel).

Low-NOX burner (LNB) and ultra-low-NOX burner (ULNB)

The technique is based on the principles of reducing peak flame temperatures, delaying but completing the combustion and increasing the heat transfer (increased emissivity of the flame). It may be associated with a modified design of the furnace combustion chamber. The design of ultra-low-NOX burners (ULNB) includes (air/)fuel staging and exhaust/flue-gas recirculation.