Division 7 Emissions to water

United Kingdom

BAT 11: In order to reduce emissions of pollutants to water, BAT is to use an appropriate combination of the techniques given below.

	Technique	Description
Covered by BAT 1, 4, 12, 13, 14 and 15.
Within the scope of the BAT reference document on Common Waste Water and Waste Gas Treatment/Management Systems in the Chemical Sector (CWW BREF).
a	Process-integrated techniques^f1	Techniques that prevent or reduce the generation of pollutants
b	Waste water treatment at source	Techniques to abate or recover pollutants prior to their discharge to the waste water collection system
c	Waste water pre-treatment^f2	Techniques to abate pollutants before the final waste water treatment
d	Final waste water treatment	Final waste water treatment by mechanical, physico-chemical and/or biological techniques before discharge to a receiving water body

BAT 12: In order to reduce emissions of chloride to water from the chlor-alkali plant, BAT is to use a combination of the techniques given in BAT 4.

BAT 13: In order to reduce emissions of free chlorine to water from the chlor-alkali plant, BAT is to treat waste water streams containing free chlorine as close as possible to the source, to prevent stripping of chlorine and/or the formation of halogenated organic compounds, by using one or a combination of the techniques given below.

	Technique	Description
a	Chemical reduction	The free chlorine is destroyed by reaction with reducing agents, such as sulphite and hydrogen peroxide, in stirred tanks.
b	Catalytic decomposition	The free chlorine is decomposed to chloride and oxygen in catalytic fixed-bed reactors. The catalyst can be a nickel oxide promoted with iron on an alumina support.
c	Thermal decomposition	The free chlorine is converted to chloride and chlorate by thermal decomposition at approximately 70 °C. The resulting effluent requires further treatment to reduce emissions of chlorate and bromate (BAT 14).
d	Acidic decomposition	The free chlorine is decomposed by acidification, with a subsequent release and recovery of chlorine. Acidic decomposition can be carried out in a separate reactor or by recycling of the waste water to the brine system. The degree of recycling of waste water to the brine circuit is restricted by the water balance of the plant.
e	Waste water recycling	Waste water streams from the chlor-alkali plant that contain free chlorine are recycled to other production units.

The BAT-associated emission level for free chlorine, expressed as Cl₂, is 0,05 – 0,2 mg/l in spot samples taken at least once every month at the point where the emission leaves the installation. The associated monitoring is in BAT 7.

BAT 14: In order to reduce emissions of chlorate to water from the chlor-alkali plant, BAT is to use one or a combination of the techniques given below.

	Technique	Description	Applicability
a	High-performance membranes	Membranes showing high current efficiencies, that reduce chlorate formation while ensuring mechanical and chemical stability under the given operating conditions.	Applicable to membrane cell plants when renewing membranes at the end of their lifetime.
b	High-performance coatings	Coatings with low electrode overpotentials leading to reduced chlorate formation and increased oxygen formation at the anode.	Applicable when renewing coatings at the end of their lifetime. The applicability may be restricted by the quality requirements of the produced chlorine (oxygen concentration).
c	High-purity brine	The brine is sufficiently purified to minimise contamination of electrodes and diaphragms/membranes, which could otherwise increase the formation of chlorate.	Generally applicable.
d	Brine acidification	The brine is acidified prior to electrolysis, in order to reduce the formation of chlorate. The degree of acidification is limited by the resistivity of the equipment used (e.g. membranes and anodes).	Generally applicable.
e	Acidic reduction	Chlorate is reduced with hydrochloric acid at pH values of 0 and at temperatures higher than 85 °C.	Not applicable to once-through brine plants.
f	Catalytic reduction	In a pressurised trickle-bed reactor, chlorate is reduced to chloride by using hydrogen and a rhodium catalyst in a three-phase reaction.	Not applicable to once-through brine plants.
g	Use of waste water streams containing chlorate in other production units	The waste water streams from the chlor-alkali plant are recycled to other production units, most typically to the brine system of a sodium chlorate production unit.	Restricted to sites that can make use of waste water streams of this quality in other production units.

BAT 15: In order to reduce emissions of halogenated organic compounds to water from the chlor-alkali plant, BAT is to use a combination of the techniques given below.

	Technique	Description
a	Selection and control of salt and ancillary materials	Salt and ancillary materials are selected and controlled to reduce the level of organic contaminants in the brine.
b	Water purification	Techniques such as membrane filtration, ion exchange, UV irradiation and adsorption on activated carbon can be used to purify process water, thereby reducing the level of organic contaminants in the brine.
c	Selection and control of equipment	Equipment, such as cells, tubes, valves and pumps, is carefully selected to reduce the potential leaching of organic contaminants into the brine.

Annotations

Footnotes

f1: Covered by BAT 1, 4, 12, 13, 14 and 15.
f2: Within the scope of the BAT reference document on Common Waste Water and Waste Gas Treatment/Management Systems in the Chemical Sector (CWW BREF).