«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
4.3.4 Selection of biodegradable/bioeliminable complexing agents in pretreatment and dyeing processes Description Description Complexing agents are applied to mask hardening alkaline-earth cations and transition-metal ions in aqueous solutions in order to eliminate their damaging effect, especially in pretreatment processes (e.g. catalytic destruction of hydrogen peroxide), but also during dyeing operations.
Typical sequestering agents are polyphosphates (e.g. tripolyphosphate), phosphonates (e.g. 1hydroxyethane 1,1-diphosphonic acid) and amino carboxylic acids (e.g. EDTA, DTPA, and NTA) (see figure below).
Figure 4.6: Chemical structure of some N- or P- containing complexing agents [179, UBA, 2001] The main concerns associated with the use of these substances arise from, their N- and Pcontent, their often-low biodegradability/bioeliminability and their ability to form stable complexes with metals, which may lead to remobilisation of heavy metals (see also Section 8.
Softening of fresh water, to remove the iron and the hardening alkaline-earth cations from the process water, and the techniques described in Section 4.5.6 are available options for minimising/ avoiding the use of complexing agents in various applications (e.g. in hydrogen peroxide bleaching, rinsing after reactive dyeing of cotton).
When complexing agents are used, polycarboxylates or substituted polycarboxylic acids (e.g.
polyacrylates and polyacrylate-maleic acid copolymerisates), hydroxy carboxylic acids (e.g.
gluconates, citrates) and some sugar-acrylic acid copolymers are convenient alternatives to the conventional sequestering agents. None of these products contains N or P in their molecular structure. In addition, the hydroxy carboxylic acids and sugar-acrylic acid copolymers are readily biodegradable.
Figure 4.7: Chemical structure of some N- and P-free complexing agents [179, UBA, 2001] The best complexing agent (in a technical, economical and ecological sense) is one that also achieves a good balance of ecological properties and effectiveness and has no detrimental effect in dyeing (demetalisation of dyes).
Effectiveness is measured as the capacity to complex alkaline-earth cations, the dispersing capacity and the capacity of stabilising hydrogen peroxide.
On the ecological side, the following factors are to be considered:
· biodegradability · bioeliminability · remobilisation of heavy metals · nitrogen content (eutrophication potential) · phosphorus content (eutrophication potential).
A qualitative assessment of the ecological properties of most common classes of complexing agents is given in Table 4.8, while Table 4.9 gives an analysis of the aspects related to their effectiveness.
Table 4.8: Qualitative assessment of commercially available complexing agents Main achieved environmental benefits The substitution of conventional complexing agents with the product mentioned above has the
following positive effects:
· reduced eutrophication in the receiving water · improved biodegradability of the final effluent · reduced risk of remobilisation of the heavy metals from sediments.
Operational data Complexing agents are applied in many different fields in textile chemistry. Recipes and application techniques are therefore process-specific. However, the use of the optimised products mentioned above does not imply major differences with respect to conventional complexing agents.
Bioelimination/biodegradation rates for some commercial products that do not contain P and N
in their molecular structure are:
· sugar-acrylic acid copolymer: readily biodegradable, (OECD 301 F, mineralisation: 100 %;
COD: 194 mg/g; BOD5 40 mg/g) (“CHT, 2000”) · sugar acrylic acid copolymer: readily biodegradable (OECD 301C; COD: 149 mg/g) (“Stockhausen, 2000”) · hydroxy carboxylic acid: bioeliminable (OECD 302 B, elimination: 92 %; COD: 144 mg/g;
BOD5 51 mg/g) (“CHT, 2000”) · carboxylates: bioeliminable (OECD 302B, elimination90 %; COD: 280 mg/g; BOD5 125 mg/g) (“Petry, 1998”) · modified polysaccharide: readily biodegradable (OECD 301E, biodegradability: 80 %;
COD: 342 mg/g; BOD5 134 mg/g) (“Clariant, 2000”).
NTA is biodegradable when treated in waste water treatment plants under nitrifying conditions (OECD 302B, elimination 98 % COD: 370 mg/g; BOD 30: 270 mg/g – “BASF, 2000”). Recent studies have shown that NTA plays a minor role, if any, in the remobilisation of heavy metals in aquatic sediments [280, Germany, 2002]. Phosphonates are not biodegradable, but they are bioeliminable and they do not contribute to the remobilisation of heavy metals (see also Section 8.5).
Taking as a reference the application of conventional complexing agents, there are no crossmedia effects of concern. With polyacrylate-based complexing agents, the residual monomer content in the polymer should be taken into account (note that acrylates are also widely used in large volume in other sectors as detergent builders, thus overloading the waste water treatment plants more significantly than textile effluents do).
The complexing agents described in this section can be used in continuous and discontinuous processes. The effectiveness of the various products has, however, to be considered when replacing conventional complexing agents by more environmentally-friendly ones (see table below).
Economics Costs for N- or P-free compounds, especially for sugar-acrylic copolymers, are comparable to other N- and P-free products, although higher quantities may be necessary in some cases, [179, UBA, 2001].
Driving force for implementation The enforcement of regulations at national and European level, together with the PARCOM recommendations and the eco-labelling schemes, are the main driving forces.
Reference plants N- and P-free complexing agents are applied in many plants world-wide. Consumption of polycarboxylates is significantly higher than for sugar-acrylic copolymers and hydrocarboxylic acids [179, UBA, 2001].
Reference literature [61, L. Bettens, 1999], [169, European Commission, 2001], [179, UBA, 2001] with reference
“CHT, 1999” Chemische Fabrik Tübingen, D-Tübingen Material Safety Data Sheet (1999) “CHT, 2000” Chemische Fabrik Tübingen, D-Tübingen Product information (1999) “Clariant, 2001” Clariant, D-Lörrach Material Safety Data Sheet (2001) “Stockhausen, 2000” Stockhausen, D-Krefeld Material Safety Data Sheet (2000) “Petry, 1998” Dr. Petry, D-Reutlingen Material Safety Data Sheet (1998) 4.3.5 Selection of antifoaming agents with improved environmental performance Description Excessive foaming causes uneven dyeing of yarn or fabric. There is a trend towards higher consumption of defoamers because of the growing preference for high speed and high temperature processing, reduction in water usage and continuous equipment/processes. Antifoaming agents are commonly applied in pretreatment, dyeing (especially when dyeing in jet machines) and finishing operations, but also in printing pastes. Low foaming characteristics are particularly important in jet dyeing, where agitation is severe.
Products that are insoluble in water and have a low surface tension are suitable for providing antifoaming effect. They displace foam-producing surfactants from the air/water boundary
layer. Nevertheless, antifoaming agents contribute to the organic load of the final effluent. Their
consumption should therefore be reduced in the first place. Possible measures in this respect are:
· using bath-less air-jets, where the liquor is not agitated by fabric rotation · re-using treated baths (see Section 4.6.22).
However, these techniques are not always applicable and cannot completely avoid the use of defoamers. Therefore the selection of auxiliaries with improved ecological performance is important. Antifoaming agents are often based on mineral oils (hydrocarbons). The presence of PAHs contaminants must also be taken into account when poorly refined oils are present in the formulation.
Environmentally improved products are free of mineral oils and are characterised by high bioelimination rates.
Typical active ingredients of alternative products are silicones, phosphoric acid esters (esp.
tributylphosphates), high molecular alcohols, fluorine derivatives, and mixtures of these components.
Main achieved environmental benefits Thanks to the use of mineral oil-free defoamers the hydrocarbon load in the effluent, which is often limited in national/regional regulations, is minimized. Furthermore, these alternative defoaming agents have lower specific COD and higher bioelimination rate than hydrocarbons.
For example, a product based on triglycerides of fatty acid and fatty alcohol ethoxylates (COD:
1245 mg/l; BOD5: 840 mg/l) has a degree of bioeliminability higher than 90 % (determined in the modified Zahn-Wellens-Test, according to OECD 302 B Test method or EN 29888, respectively) [179, UBA, 2001].
For air emissions, due to the substitution of mineral oil-based compounds, it is possible to reduce VOC emissions during high-temperature processes (caused by the carry-over of antifoaming agents on the fabric after wet operations).
Operational data The mineral oil-free defoamers can be used in a way similar to conventional products. Because silicone products are highly effective, the required amount can be considerably reduced.
Account must be taken that:
· silicones are eliminated only by abiotic processes in waste water. Furthermore, above certain concentrations, silicone oils may hinder the transfer/diffusion of oxygen into the activated sludge · tributylphosphates are odour-intensive and strongly irritant · high molecular-weight alcohols are odour-intensive and cannot be used in hot liquors.
Applicability There are no particular limitations to be mentioned concerning the application of the mineral oil-free formulations. However, the effectiveness of the various alternative products has to be borne in mind [179, UBA, 2001].
If antifoaming agents based on silicones are used there is risk of silicone spots on the textile and silicone precipitates in the machinery [179, UBA, 2001].
Restrictions in the use of silicones in some sectors have to be considered. For example, in the automotive industry restrictions have been put in place, which forbid the use of silicones in automobiles and textiles for this industry.
Economics Cost of mineral oil-free products is comparable to conventional ones [179, UBA, 2001].
Driving force for implementation Minimisation of hydrocarbons in the effluent is the main reason for substituting mineral oilcontaining antifoaming agents.
Reference plants Many plants in Europe. There are various suppliers for antifoaming agents free of mineral oils.
Reference literature [179, UBA, 2001] with reference to:
“Dobbelstein, 1995” Optimierung von Textilhilfsmitteln aus ökologischer Sicht. Möglichkeiten und Grenzen Nordic Dyeing and Finishing Conference 20.05.1995, F-Hämeenlinna “Petry, 1999” Dr. Petry GmbH, D-Reutlingen Material Safety Data Sheet
4.4 Wool scouring 4.4.1 Use of integrated dirt removal/grease recovery loops Description As already described in Section 220.127.116.11 (see Figure 2.4), a wool scouring plant operating in
countercurrent mode normally produces three liquid waste streams:
· a dirt-rich flow, from the bottoms of the scouring bowls · a less concentrated dirty flow, from the bottoms of the rinse bowls · a grease-rich flow, from the top of the first scour bowl, or from the side tank of the first scour bowl, which receives the liquor removed from the wool as it exits the bowl through the squeeze press.
All of these flows can be partially decontaminated and recycled to the scour, by means of grease recovery and dirt removal loops.
There is no consensus on the best way to operate the loop(s). Some mills prefer to treat the dirtrich flow and the grease-rich flow separately, whilst others combine the two streams and carry out sequential treatment, first for dirt removal, then for grease recovery.
For grease recovery, plate-type centrifuges are employed. They are usually protected from the abrasive effects of dirt by hydrocyclones in cases where separate rather than sequential grease recovery and dirt removal is practised. The centrifuge produces a top phase, known as “cream”, which is grease containing a small amount of water. This “cream” is usually passed to a secondary centrifuge, which produces an upper, a lower and a middle phase. The upper phase 272 Textiles Industry
Chapter 4consists of anhydrous grease, which can be sold as a by-product. The bottom phase is high in dirt and may be passed to the input side of the dirt recovery loop, or to the effluent treatment plant. The middle phase is impoverished in both grease and dirt and may be completely or partially recycled to the scour, by addition to the first scouring bowl. A portion of the middle phase may flow to effluent treatment.
Dirt removal may employ gravity settling tanks, hydrocyclones or decanter centrifuges – or combinations of these methods.
In mills with more than one scouring line, the lines normally share dirt removal/grease recovery facilities.
For fine and extra-fine wool, when carried out using machinery that has a separate continuous sludge flow output, the wool grease recovery loop also allows the elimination of the very fine dirt fraction without the need for a separate loop for dirt removal.
Main achieved environmental benefits
The implementation of dirt removal/grease recovery loops allows:
· a reduction in water consumption ranging from a minimum of 25 % to a maximum of more than 50 %, taking as reference point the consumption of water of a conventional plant operating in countercurrent (between 5 and 10 l/kg of greasy wool) · a reduction in energy consumption equivalent to the amount of thermal energy carried by the recycled liquor (the liquor temperature is generally about 60 ºC) · the production of a valuable by-product: wool grease · a reduction in detergent and builder consumption proportional to the water savings achieved · the conversion of suspended dirt into spadeable sludge · a reduction of the load (oxygen-demanding substances and suspended solids) sent to the effluent treatment plant, which means a reduction in the consumption of energy and chemicals for the treatment of the waste water. This reduction is proportional to the dirt removal and grease recovery rate achieved.
Medium-to-large scouring mills (say 15000 – 25000 tonnes greasy wool per year) employing dirt removal/grease recovery loops should be able to achieve net specific water consumption figures of 2 - 4 l/kg of greasy wool for most types of wool. Both coarse and fine wool scourers in the survey are already achieving these figures. However, there is insufficient data to define whether these performances are also applicable to extra-fine wool scourers.
The amount of grease recovered as sellable by-product in the surveyed companies ranges between 10 and 35g/kg of greasy wool. The best performance for a fine wool scourer is almost 35 g/kg greasy wool and for a coarse wool scourer about 13 g/kg. These recovery rates represent about 25 % of the grease estimated to be present in the wool scoured.
There is probably a maximum amount of grease that can be recovered centrifugally, which is governed by the ratio between hydrophobic and less-hydrophobic grease (top grease and oxidised grease) present on the scoured wool [187, INTERLAINE, 1999].
Cross-media effects The dirt and part of the grease which is not recovered as by-product may be transferred as pollutant, from water to land.