Division BAT 21 In order to reduce emissions to water and/or to facilitate water reuse and recycling from aqueous processes (e.g. degreasing, cleaning, surface treatment, wet scrubbing), BAT is to use a combination of the techniques given below.

United Kingdom

Techniques		Description	Typical pollutants targeted
Preliminary, primary and general treatment
(a)	Equalisation	Balancing of flows and pollutant loads by using tanks or other management techniques.	All pollutants.
(b)	Neutralisation	The adjustment of the pH of waste water to a neutral value (approximately 7).	Acids, alkalis.
(c)	Physical separation, for example, by using screens, sieves, grit separators, primary settlement tanks and magnetic separation		Gross solids, suspended solids, metal particles.
Physico-chemical treatment
(d)	Adsorption	The removal of soluble substances (solutes) from the waste water by transferring them to the surface of solid, highly porous particles (typically activated carbon).	Adsorbable dissolved non-biodegradable or inhibitory pollutants, e.g. AOX.
(e)	Vacuum distillation	The removal of pollutants by thermal waste water treatment under reduced pressure.	Dissolved non-biodegradable or inhibitory pollutants that can be distilled, e.g. some solvents.
(f)	Precipitation	The conversion of dissolved pollutants into insoluble compounds by adding precipitants. The solid precipitates formed are subsequently separated by sedimentation, flotation or filtration.	Precipitable dissolved non-biodegradable or inhibitory pollutants, e.g. metals.
(g)	Chemical reduction	Chemical reduction is the conversion of pollutants by chemical reducing agents into similar but less harmful or hazardous compounds.	Reducible dissolved non-biodegradable or inhibitory pollutants, e.g. hexavalent chromium (Cr(VI)).
(h)	Ion exchange	The retention of ionic pollutants from waste water and their replacement by more acceptable ions using an ion exchange resin. The pollutants are temporarily retained and afterwards released into a regeneration or backwashing liquid.	Ionic dissolved non-biodegradable or inhibitory pollutants, e.g. metals.
(i)	Stripping	The removal of purgeable pollutants from the aqueous phase by a gaseous phase (e.g. steam, nitrogen or air) that is passed through the liquid. The removal efficiency may be enhanced by increasing the temperature or reducing the pressure.	Purgeable pollutants, e.g. some adsorbable organically bound halogens (AOX).
Biological treatment
(j)	Biological treatment	Use of microorganisms for waste water treatment (e.g. anaerobic treatment, aerobic treatment).	Biodegradable organic compounds.
Final solids removal
(k)	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 a gentle mixing stage so that collisions of microfloc particles cause them to bond to produce larger flocs. It may be assisted by adding polymers.	Suspended solids and particulate-bound metals.
(l)	Sedimentation	The separation of suspended particles by gravitational settling.
(m)	Filtration	The separation of solids from waste water by passing them through a porous medium, e.g. sand filtration, nano-, micro- and ultrafiltration
(n)	Flotation	The separation of solid or liquid particles from waste water by attaching them to fine gas bubbles, usually air. The buoyant particles accumulate at the water surface and are collected with skimmers.

Table 5

BAT-associated emission levels (BAT-AELs) for direct discharges to a receiving water body

Substance/Parameter	Sector	BAT-AEL^f1
The averaging period is given in the general considerations.
The BAT-AEL for COD may be replaced by a BAT-AEL for TOC. The correlation between COD and TOC is determined on a case-by-case basis. The BAT-AEL for TOC is the preferred option because TOC monitoring does not rely on the use of very toxic compounds.
The BAT-AEL only applies if fluorine compounds are used in the processes.
The upper end of the BAT-AEL range may be 1 mg/l in the case of zinc-containing substrates or of substrates pretreated using zinc.
The BAT-AEL only applies if chromium compounds are used in the processes.
The BAT-AEL only applies if chromium(VI) compounds are used in the processes.
Total suspended solids (TSS)	Coating of vehicles Coil coating Coating and printing of metal packaging (only for DWI cans)	5–30 mg/l
Chemical oxygen demand (COD)^f2		30–150 mg/l
Adsorbable organically bound halogens (AOX)		0,1–0,4 mg/l
Fluoride (F^-)^f3		2–25 mg/l
Nickel (expressed as Ni)	Coating of vehicles Coil coating	0,05–0,4 mg/l
Zinc (expressed as Zn)	Coating of vehicles Coil coating	0,05–0,6 mg/l^f4
Total chromium (expressed as Cr)^f5	Coating of aircraft Coil coating	0,01–0,15 mg/l
Hexavalent chromium (expressed as Cr(VI))^f6	Coating of aircraft Coil coating	0,01–0,05 mg/l

The associated monitoring is given in BAT 12.

Table 6

BAT-associated emission levels (BAT-AELs) for indirect discharges to a receiving water body

Substance/Parameter	Sector	BAT-AEL^f7 ^f8
The BAT-AELs may not apply if the downstream waste water treatment plant is designed and equipped appropriately to abate the pollutants concerned, provided this does not lead to a higher level of pollution in the environment.
The averaging period is given in the general considerations.
The BAT-AEL only applies if fluorine compounds are used in the processes.
The upper end of the BAT-AEL range may be 1 mg/l in the case of zinc-containing substrates or of substrates pretreated using zinc.
The BAT-AEL only applies if chromium compounds are used in the processes.
The BAT-AEL only applies if chromium(VI) compounds are used in the processes.
Adsorbable organically bound halogens (AOX)	Coating of vehicles Coil coating Coating and printing of metal packaging (only for DWI cans)	0,1–0,4 mg/l
Fluoride (F^-)^f9		2–25 mg/l
Nickel (expressed as Ni)	Coating of vehicles Coil coating	0,05–0,4 mg/l
Zinc (expressed as Zn)	Coating of vehicles Coil coating	0,05–0,6 mg/l^f10
Total chromium (expressed as Cr)^f11	Coating of aircraft Coil coating	0,01–0,15 mg/l
Hexavalent chromium (expressed as Cr(VI))^f12	Coating of aircraft Coil coating	0,01–0,05 mg/l

The associated monitoring is given in BAT 12.

Annotations

Footnotes

f1: The averaging period is given in the general considerations.
f2: The BAT-AEL for COD may be replaced by a BAT-AEL for TOC. The correlation between COD and TOC is determined on a case-by-case basis. The BAT-AEL for TOC is the preferred option because TOC monitoring does not rely on the use of very toxic compounds.
f3: The BAT-AEL only applies if fluorine compounds are used in the processes.
f4: The upper end of the BAT-AEL range may be 1 mg/l in the case of zinc-containing substrates or of substrates pretreated using zinc.
f5: The BAT-AEL only applies if chromium compounds are used in the processes.
f6: The BAT-AEL only applies if chromium(VI) compounds are used in the processes.
f7: The BAT-AELs may not apply if the downstream waste water treatment plant is designed and equipped appropriately to abate the pollutants concerned, provided this does not lead to a higher level of pollution in the environment.
f8: The averaging period is given in the general considerations.
f9: The BAT-AEL only applies if fluorine compounds are used in the processes.
f10: The upper end of the BAT-AEL range may be 1 mg/l in the case of zinc-containing substrates or of substrates pretreated using zinc.
f11: The BAT-AEL only applies if chromium compounds are used in the processes.
f12: The BAT-AEL only applies if chromium(VI) compounds are used in the processes.