Treating the surfaces of plastics
2 APPLIED PROCESSES AND TECHNIQUES
2.13 Abatement of potential releases to the environment
2.13.1 Waste water
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PT/EIPPCB/STM_BREF_FINAL September 2005 135
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136 September 2005 PT/EIPPCB/STM_BREF_FINAL Waste water is contaminated by used reagents and the breakdown products from the processes.
The main ingredients of concern are metal ions (cations), which are conservative, see Section 1.4.4.1 and toxic anions such as cyanide or chromate. Some of the waste water treatments themselves may produce contaminants that require further treatment, so any or all of the following categories of constituents may be present and are discussed in this section [21, Agences de l'Eau de France, et al., 2002] and see Section 1.4.4:
• organic materials (Section 2.13.1.2)
o immiscible – non-halogenated oils, greases, solvents
o immiscible – halogenated oils, degreasing solvents, paint solvents
o soluble – wetting agents, brighteners, organic ions and ligands, e.g. acetate, EDTA (Section 2.13.1.8), organic materials expressed as COD
o AOX – potentially formed in effluent treatment
• particulates suspended solids – metal hydroxides, carbonates, powders and dusts, film residues, metallic particles, etc. (Section 2.13.1.4)
• acids and alkalis (Section 2.13.1.3)
• metals – soluble anions from process activities (Sections 2.13.1.5, 2.13.1.6 and 2.13.1.7)
• nitrogenous materials – NH4 +, NO3-, NO2- (Section 2.13.1.9) from greasing, scouring, coating, phosphate coating, heat treatment, chemical nickel plating, etc.
• cyanides – CN-, SCN-; from degreasing, coating, etc.(Section 2.13.1.10)
• fluorides –from scouring, passivation, polishing, coating, etc.(Section 2.13.1.12)
• phosphated compounds – from degreasing, phosphate coating, brightening, chemical nickel plating, etc.(Section 2.13.1.13)
• sulphides (Section 2.13.1.11)
• other salts – Cl-, SO42-, K+, Na+, Ca+.(Section 2.13.1.14).
All contaminants need either or both:
• chemical treatment to destroy or change them to less harmful chemical species, or more readily managed or removable chemical species
• separation from the water to predetermined levels. The removal of the contaminants from water can be by filtering and/or settlement techniques, followed by flocculation at the correct pH and settlement.
In some cases, the whole waste water treatment plant can operate with mixed waste streams.
With some substances, it is preferable to segregate the waste water streams for individual treatment prior to separate discharge, or subsequent mixed treatment.
Treatment may be by batches or continuously for the whole or portions of the flow [3, CETS, 2002]. Batch treatment may be easier to control and supervise, but requires more capital plant capacity to contain the flow to be treated and may require more direct supervisory time.
Continuous treatment requires more sophisticated control systems and their consequent maintenance.
Figure 2.42 describes the layout of a typical waste water treatment plant.
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PT/EIPPCB/STM_BREF_FINAL September 2005 137 Figure 2.42: Flow diagram for a typical waste water treatment plant
[21, Agences de l'Eau de France, et al., 2002]
Separation is normally by removing the contaminants from the water, but can also be by removing the water from the contaminants by:
• evaporation, with or without condensation of the water vapour, with a residual sludge
• by reverse osmosis, providing a purified, but not pure, water and with a waste water, which contains the concentrated impurities.
The residual concentrates may in turn be reduced to a solid by the addition of other materials or the evaporation of the remaining water [3, CETS, 2002].
2.13.1.1 Treatment techniques
Various treatment techniques are mentioned in the following sections. Some of them are also used for process solution maintenance, treatment of incoming water and for treating waste waters prior to recycling, see Section 2.7.
2.13.1.2 Organic materials Immiscible organics
These are split into two groups:
• non-halogenated - oils, greases, solvents
• halogenated - oils, degreasing solvents, paint solvents.
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138 September 2005 PT/EIPPCB/STM_BREF_FINAL These may first be reduced to their solubility limit by physical separation, such as flotation
(such as for oils, electropainting solids) or for volatile substances, by liquid/liquid phase separation. For volatile organics, when levels below the solubility limit are required, two options exist [3, CETS, 2002]:
• air stripping, with removal from the air, e.g. by activated carbon, to circa 1 mg/l and final polishing by passing the waste water through activated carbon
• oxidation to carbon dioxide (and halogen acid in the case of halogenated organics) using UV irradiation and hydrogen peroxide addition.
Soluble organic materials
Wetting agents, brighteners, organic ions and ligands, e.g. acetate, EDTA, etc.
Sequestering agents increase the difficulty in removing metals by flocculation and settlement by complexing them and, if in surplus, may solubilise metals in the outside environment [22, Fraunhofer, 2002]. The concentration of miscible organics may be reduced by oxidation (such as by hypochlorite) by UV irradiation and hydrogen peroxide addition (typically 30 minutes), or their deleterious effects reduced by the addition of a benign metal salt, e.g. calcium chloride/hydroxide [3, CETS, 2002].
Reduction of COD
Dissolved organics in waste water raise the chemical oxygen demand (COD). The types of compounds present have highly variable breakdown rates, both chemically and biologically.
Upstream prevention is usually the easiest option in reducing COD load.
Where COD requires treatment, biological treatment by arranging discharge to the municipal waste water treatment plant is usually the easiest treatment option. Note, however, some compounds in effluents can be resistant to biological oxidation, and it may be necessary to test the biodegradability of the effluent [121, France, 2003]. In some circumstances, chemical treatments may be necessary and they include [21, Agences de l'Eau de France, et al., 2002]:
• physico-chemical treatment
• chemical emulsion breaking
• adsorption on activated carbon or other similar materials
• membrane techniques
• concentration by evaporation
• oxidation techniques using hypochlorite, peroxide, etc.
AOX
Organic chlorine compounds can potentially be formed in effluent treatment when hypochlorite or chorine are used as the oxidising agents.
2.13.1.3 Acids and alkalis
Acid and alkali discharges usually require pH adjustment to a range dependent on the receiving water or sewer before discharging. Continuous flow streams of opposite pH may be partially neutralised by mixing together. Batch discharges such as end-of-service life process solutions may be stored and mixed with solutions of opposite pH.
The chemistries of effluent pretreatments, such as reduction of hexavalent chromium or oxidation of cyanide, require a pH close to that of the originating process solution and are usually carried out prior to neutralisation.
Usually, pH control is on a continuous flow basis with automatic controls, although some discharges are treated on a batch basis.
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PT/EIPPCB/STM_BREF_FINAL September 2005 139 2.13.1.4 Particulate material
Metal hydroxides, carbonates, powders and dusts, film residues, metallic particles, etc. may be removed by settling or filtration, see Section 2.13.2.1.
2.13.1.5 Metals – soluble anions
The concentration of metals re-use or for recycling, either directly or after further treatment, may be viable, depending on the chemistry of the solution and the technique used. Techniques are described in Section 2.7. The capture of precious metals, e.g. platinum, gold, silver, rhodium and ruthenium may be achieved from waste water by electrochemical recovery or by ion exchange (see Section 2.7.6 and 2.7.8 with the subsequent sale of the loaded resin or the concentrated regeneration liquor stream to specialist recyclers. The reclamation of other cations from waste water streams may be carried out individually or as a composite of several metals.
Figure 2.43 shows an example of a treatment plant using ion exchange.
Where multiple processes are operated and where metal recycling is carried out, it may be preferable to concentrate or precipitate the metals arising from different metal plating lines in segregated streams. This may improve the economics and/or the practicalities of recovering the metals.
Figure 2.43: Example of waste water treatment plant using ion exchange (Productmetal S.A. and Agence de l’eau Seine-Normandie)
2.13.1.6 Reduction of oxidation state of metal ions
In some cases, it is necessary to reduce the oxidation state of the metal as the higher oxidation state(s) may not be readily flocculated and precipitated by pH change [3, CETS, 2002]. For instance, the reduction of Cr(VI) by sodium dithionite:
4 Na2Cr2O7 + 3 Na2S2O3 + 13 H2SO4 → 4 Cr2(SO4)3 + 7 Na2SO4 + 13 H2O
2.13.1.7 Precipitation of metallic floc
The multivalent metal ions are most conveniently removed by precipitation as the hydroxide and concentrations of individual metals in the post-settlement effluent well below 1 mg/l are achievable in theory. As the transition metals are amphoteric, there is a minimum solubility requiring careful pH selection and control (Figure 2.44):
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140 September 2005 PT/EIPPCB/STM_BREF_FINAL Figure 2.44: Variation of solubility of dissolved metal with pH
Simultaneous removals of several metal ions to very low levels may require selection of different pH values and settlement sequentially. The treatment of segregated streams may be preferable to sequential pH adjustments with intervening solids removal stages. The pH may require further adjustment prior to discharge.
The precipitated metals may be separated by settling. There are various types of separator or settlement tank such as [21, Agences de l'Eau de France, et al., 2002, 87, EIPPCB, ]:
• longitudinal
• upward flow radial
• laminar.
The use of an anionic settling aid (a coagulant with high ionic charge density, such as ferric ions, aluminium chlorohydrate) or a polyelectrolyte may be beneficial in coagulating a stable floc and optimising settlement. They also assist with any subsequent sludge dewatering, see Section 2.13.2.1.
For low discharge levels (for example, below about 3 mg/l for zinc), the effluent will require polishing (tertiary treatment) by filtration using sand, mixed media, cartridge or pressure filters.
For small discharges, direct filtration of the suspension instead of settlement may be more cost effective.
The settled floc or filtrate will contain about 5 % solids and is normally further concentrated by dewatering (see Section 2.13.2.1).
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PT/EIPPCB/STM_BREF_FINAL September 2005 141 For residual metal ion concentrations below that achievable by hydroxide addition alone, insoluble sulphide based salts may be used in conjunction with post hydroxide addition. Dithio-carbamate (DDC) is one of a range of suitable materials which are capable of reducing the concentration of soluble transition metals to below 0.1 mg/l.
2.13.1.8 Complexing (sequestering, chelating) agents
Sequestering agents, particularly EDTA are being used in increasing quantities again in the printed circuit board industry to achieve the high specifications being demanded in modern printed circuit technology [22, Fraunhofer, 2002].
The success of precipitation processes is dependent on the reaction between the soluble metal ion and hydroxide. Complexed metals present problems as hydroxides are difficult to form and the presence of complexing agents can be a cause of failure in waste water treatment plants (i.e. breach of operating limits for metals in the effluent). Waste water streams containing the cyanide (which is a complexing agent) may be easily treated (see Section 2.13.1.10). Other sequestering agents present in a number of cleaners and proprietary electrolytes are more difficult to overcome. Where complexing agents are a problem, metal precipitation may be possible with the use of calcium hydroxide in place of sodium hydroxide or by the addition of calcium or magnesium chlorides which preferentially complex with the agent. Extensive prior digestion (>30 minutes) with strong oxidising agents has been found to be beneficial in reducing the effect of sequestering agents but will oxidise chromium and manganese which then require subsequent reduction prior to precipitation where these metals are present. Alternative organic removal treatments, e.g. activated carbon and non-ionic resins, prior to precipitation may be environmentally sound. Microbiological oxidation of organics remains a theoretical possibility.
2.13.1.9 Nitrogenous materials
Compounds containing nitrogen such as NH4+, NO3-, NO2- come from degreasing, scouring, coating, phosphate coating, heat treatment, chemical nickel plating, etc. [21, Agences de l'Eau de France, et al., 2002].
Ammonia
If recovery by steam stripping is not economic, then ammonia may be oxidised to nitrogen and water with sodium hypochlorite. Any excess hypochlorite can be reduced using sodium sulphite. Ammonia can also be oxidised biologically, usually in a municipal waste water treatment plant.
Oxidation of nitrites
Nitrites can be oxidised with sodium hypochlorite at pH 6 with control of rH, usually automatically. It can also be oxidised with hydrogen peroxide to nitrate [113, Austria, 2003].
Nitrites can be reduced to N2 with sodium bisulphite at pH 2; in this case, control by rH is not possible [121, France, 2003]. In an acid solution, nitrite can easily be reduced to nitrogen gas by using sulphamic acid [113, Austria, 2003].
Environmental considerations
AOX may be formed when using hypochlorite solution [113, Austria, 2003].
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142 September 2005 PT/EIPPCB/STM_BREF_FINAL 2.13.1.10 Cyanides
Cyanide (CN-, SCN-) from degreasing and coating may be oxidised. The cheapest and most widely used option is hypochlorite or chlorine gas used at high pH, which first oxidises the cyanide to sodium cyanate, and secondly to ammonium and carbonate after pH neutralisation (about pH 8.5). Any excess hypochlorite can be reduced using sodium sulphite. Sodium hypochlorite solution can be used, or chlorine gas can be used to generate hypochlorite ions in situ at larger facilities. Other oxidising agents such as hydrogen peroxide can be used, with pH varying according to oxidising agent [113, Austria, 2003] [159, TWG, 2004].
Environmental considerations
AOX may be formed when using hypochlorite solution or chlorine gas.
When using hypochlorite solution or chlorine gas, the pH for the first stage must be kept high to ensure a rapid rate of reaction and to prevent the formation and release of the volatile lachromate, cyanogen chloride.
2.13.1.11 Sulphide
Sulphide is normally controlled by the excess of the multivalent cations present in most waste water streams, with no further treatment necessary. Where it occurs in excess, it may be precipitated out as elemental sulphur on oxidation with hydrogen peroxide or iron III salts.
2.13.1.12 Fluorides
Fluoride occurs from scouring, passivation, polishing, coating etc, and is readily precipitated out as calcium fluoride at a pH above 7. The lowest solubility of calcium fluoride is 15 mg/l at pH 11.2.
2.13.1.13 Phosphated compounds
Phosphate compounds are used in degreasing processes, phosphate coating, heat treatment, brightening, chemical nickel plating, etc. Although they are not usually a problem, if necessary they may require control because of local environmental conditions: the release of several kilos of phosphorus per day can have an impact on a receiving river and its eutrophication [121, France, 2003], phosphate is most conveniently precipitated out as calcium hydroxide phosphate.
The solubility is less than 5 mg/l at a pH greater than 10.
2.13.1.14 Other salts
Other ions such as Cl-, SO42-, K+, Na+, and Ca+ are not normally a problem, but local environmental conditions may require their removal.
Sulphate can be readily precipitated as calcium sulphate; the solubility product is 2 g/l depending on concentration of other ions.
Concentration of other ions may be desirable, and ion exchange, reverse osmosis or evaporation may be used either prior to one of the other treatments (above) or to produce a concentrate for disposal as a waste.
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PT/EIPPCB/STM_BREF_FINAL September 2005 143 2.13.1.15 Final cleaning of effluent (polishing)
Whatever waste water treatment technologies are used, the treated water will contain small amounts of the treated components and a significantly higher concentration of more benign materials arising from the treatment reagents used. The effluent may be treated further and examples of this are [3, CETS, 2002]:
• fine (sand) filter (circa 5 µm) to remove residual particulate material
• activated carbon bed to remove organic material
• chelating, crown or thiol cation exchange resin bed to selectively remove multivalent ions.