«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
No consumption data are reported for the second case study.
Applicability The technique is applicable to all textile finishing industries, provided that proper waste water segregation is practised and selection of membrane-compatible single waste water streams is made. Recipes have to be checked in terms of membrane compatibility and have to be changed if necessary (see description above).
Structural changes for additional pipelines are needed in existing mills for waste water segregation. Additional tanks (space demand) for interim storage have to be installed.
For salt re-use (plant B), since the salt will be in the water from the very start, the so-called “allin” dyeing method has to be applied. This is opposite to the normal way where the dyestuff is evenly distributed before salt is added (see Section 2.7.3 “Reactive dyes”).
The investment for the 10 m3/h membrane equipment (Plant A) is about 1 million euros. Taking into account capital cost and operating cost (labour, energy, chemicals for membrane cleaning, maintenance and concentrate disposal) the specific costs are 4.5 euros/m3 recycled water (capital cost: 1.3 euros/m3, operating cost: 3.2 euros /m3) [179, UBA, 2001].
In Plant B, payback time is reported as 5 years for reclamation and re-use of dye bath by treatment with activated carbon and 8 months for membrane treatment and recycling of rinsing water from dyeing [192, Danish EPA, 2001].
Driving force for implementation High costs for fresh water and waste water discharge are the main driving forces.
Reference plants Membrane techniques for treatment of segregated waste water streams are applied in many
plants in Europe. In particular the two examples described here are:
Plant A: Fa. van Clewe GmbH & Co.KG, D-46499 Dingden with a design flow of 10 m3/h Plant B: Martensens A/S, DK-7330 Brande Reference literature [179, UBA, 2001], [192, Danish EPA, 2001].
4.10.5 Treatment and recovery of waste water containing pigment paste Description This technique refers to membrane treatment of waste water containing pigment printing pastes with full re-use of the resulting permeate.
In the example described here, waste water comes from the printing paste preparation kitchen (mainly resulting from cleaning operations of stirrers, drums etc.). The pigment pastes contain organic dye pigments, organic thickeners (usually polyacrylates), organic binders (copolymerisates), fixation agents (organic resins), catalysts and softening agents.
Treatment (see flow sheet in the figure) consists of the following steps:
· coagulation to de-activate the organic dyes, binders and fixation agents (polyaluminium chloride sulphate is added) · precipitation of the resulting coagulates with bentonite at pH 6 · microfiltration of the precipitate. The applied membranes consist of polypropylene and have a cut off of 0.2 µm. Suspended solids in the concentrate are removed in a tube settler by dosage of a flocculant.
The sludge is sent to physico-chemical treatment offsite. It is planned to send it to incineration in the near future. The permeate is totally free of suspended solids and can be re-used for cleaning operations.
Figure 4.45 indicates that, beside the waste water from the pigment paste preparation station, the effluent from the scrubbers (treamtent of off-gases from three stenters) is also sent to the membranes.
The core of the plant are two micro-filtration modules consisting of 400 spiral membrane tubes each.
Figure 4.45: Layout of a plant for treatment and recycling of waste water from pigment printing paste preparation kitchen (water from scrubbers is treated in the same plant) [179, UBA, 2001]
Main achieved environmental benefits More than 90 % of the water is recycled. Non-biodegradable compounds, such as organic thickeners, binders and fixation agents, are completely removed and can then be mineralised by incineration (incineration is not yet done in the example referred to in this section, but it is planned for the near future). It should be noted, however, that due to the presence of chlorides, there is a potential for the production of hazardous substances (dioxins and furans) when the sludge is incinerated [281, Belgium, 2002]. Catalytic and high temperature incinerators are now available to prevent these emissions.
Operational data COD of input water to the treatment plant varies between 4000 and 10000 mg/l. COD in the permeate is about 600 mg/l, which means a removal efficiency of about 90 %.
Coagulation has to be carried out and controlled very carefully because of organic binders and fixation agents. If these compounds became completely inactivated, they would lead to scaling of the membrane and would block it within a short time.
The pressure difference of microfiltration is about 1 bar.
Cross-media effects Energy is required for waste water treatment and recycling [179, UBA, 2001]. No data were made available regarding consumption levels.
Applicability The technique is applicable to existing and new installations preparing pigment pastes for coating or printing operations.
Economics The described plant with a flow of 2.5 m3/h (comprising the two waste water streams mentioned) needed investment of 180000 euros. Operating cost, including external disposal of the concentrate (which is the major part) is about 4 euro/m3.
Driving force for implementation The company considered in this example is discharging the waste water to a municipal waste water treatment plant with strong limitations imposed on flow and COD.
Reference plants A plant for 1.25 m3/h has been operating since 2001 at van Clewe GmbH, D-46495 Dingden, where combined treatment with scrubbing water from purification of stenter off-gas takes place (additional 1.25 m3/h).
Reference literature [179, UBA, 2001] 4.10.6 Anaerobic removal of residual dyestuff from padding liquors and printing paste residues Printing pastes and padding liquors for continuous and semi-continuous dyeing contain high concentrations of dyestuffs (see Section 18.104.22.168.4). Residual padding liquors and printing pastes
can be treated in anaerobic digesters, preferably in co-fermentation with primary and excess sludge from biological treatment. In practice, the residues are fed into anaerobic digesters at municipal waste water treatment plants.
When azo dyestuffs are treated under anaerobic conditions, the azo groups (characteristic of this type of dyestuffs) are irreversibly destroyed, causing the dyestuffs to lose their colour.
However, the remaining aromatic systems still absorb light, so some slight yellowish colour often remains.
The water-soluble cleavage products (the ones with sulphonic groups) are present in the water phase and reach the activated sludge treatment both as overflow from the anaerobic digester and as filtrate from sludge dewatering. The more-substituted naphthalene derivatives are hardly biodegradable and may still be present in the final effluent. For this reason, the supernatant needs to be subsequently treated in an activated sludge system.
Main achieved environmental benefits Anaerobic treatments reach colour removal efficiencies of more than 90 % with azo dyes (determined as reduction of spectral absorption coefficients at the wavelengths 436, 525 and 620 nm) [179, UBA, 2001].
Also with printing pastes containing natural thickeners, such as alginates or galactomannans, there is a conversion to biogas thanks to degradation of these biopolymers.
Although the quantity of the mentioned concentrates represents a small percentage of the total discharged waste water (only a few tonnes per week even for large mills), there are cases in which the total residual colour in the effluent of treatment plants could be reduced by about 50 %.
In order to derive the most benefit from anaerobic treatment, this technique should be applied in combination with process-integrated techniques aimed at minimising printing paste residues.
Moreover, it is important to separate at source the residual padding liquors from other streams in order to keep them concentrated.
The dosage of reactive printing paste should not exceed 10 g/kg sludge because of possible inhibition effects on the anaerobic process. Laboratory tests may assist with determination of inhibition effects.
Padding liquors and printing pastes with heavy metal-containing dyestuffs should be separated unless the sludge resulting from the anaerobic treatment is incinerated or disposed of in appropriate landfill (see Landfill Directive 99/31/EC).
Cross-media effects The reductive cleavage of the azo bonds leads to aromatic amines. As for the potential for release of carcinogenic aromatic amines, investigations carried out so far have not confirmed this fear [179, UBA, 2001] (with reference to “Kolb, 1988”). Moreover, the supernatant from the anaerobic treatment is normally treated with activated sludge.
Applicability The technique can be applied to both new and existing installations.
Anaerobic treatment is particularly suitable for azo dyestuffs, which represent 50 % of the colourants currently available on the market.
However, other chromophoric systems cannot be treated substantially. Vat dyes for example, are reduced to the colourless form, but this process is reversible.
Pigment printing pastes cannot be treated in anaerobic digesters because all components are non-biodegradable and scaling problems occur because of polymer binders.
In conclusion, even if anaerobic treatments reach efficencies of 90 % with azo dyes, for companies using a broader range of dyes this technique has an average overall efficiency.
Economics The known cost for anaerobic treatment in municipal anaerobic digesters varies between 30 and 60 euros/t of padding liquor or printing paste [179, UBA, 2001].
Driving force for implementation Pressure related to non-compliance with existing standards for colour in the discharge from treatment plants.
Reference plants In Germany, residual printing pastes are treated in the anaerobic digesters of the municipal waste water plants of Ravensburg, D-Ravensburg and Bändlegrund, D-Weil. Residual padding liquors for dyeing are treated in the anaerobic digester of the municipal waste water treatment plant of Heidenheim, D-Heidenheim Reference literature [179, UBA, 2001] 4.10.7 Treatment of selected and segregated, non-biodegradable waste water stream by chemical oxidation Description Highly concentrated waste water streams result from various processes in the textile finishing chain. Depending on the efficiency of the washing machines (water consumption) and load of sizing agents on the fabric, COD-concentrations of desizing baths up to 20000 mg/l can be observed. Depending on the class of dyestuffs, exhausted dye baths have COD-concentrations between 1000 and 15000 mg/l are possible. Residual padding liquors from dyeing and finishing and residual printing pastes show even higher COD-concentrations.
Desizing baths with non-biodegradable sizing agents and exhausted dye baths can be treated by oxidation in a special reactor at 100 – 130 °C and about 3 bars pressure (max. 5 bars). The main oxidising agent is molecular oxygen. Hydrogen peroxide only initiates the oxidation reaction and keeps it running (delivering 1/5 of the reactive oxygen). Iron(II)-salt is added as catalyst in acid medium. With COD of the feed of more than 2500 mg/l, the reaction is exothermic. The process is called “Thermal Fenton Process”. The figure below shows the reactor and reaction conditions. More information about Advanced Oxidation Processes and the Fenton reaction is reported in Section 14.
Figure 4.46: Scheme of the reactor for treatment by catalytic oxidation with O2/H2O2 of some segregated highly concentrated streams; on the left, illustration of the reactor at Schoeller AG, CHSevelen in operation since 1996 [179, UBA, 2001] Main achieved environmental performance COD removal efficiencies of 70 – 85 % are achieved, depending on retention time, applied temperature and pressure and chemical properties of the compounds in the effluent to be treated.
Residual COD is largely biodegradable, because of modification of the compounds during the oxidation process. Given that the effluent is in most cases submitted to subsequent biological treatment (normally in the municipal waste water treatment plant), high COD removal efficiencies (95 % or higher) are achieved. This removal is real mineralisation, that is complete break-down of organic compounds. De-colouration is more than 90 % and treated exhaust dye baths are practically colourless.
Operational data Waste water streams from different processes (different compounds and concentrations) are treated in sequence to minimise running costs. The treatment is performed continuously and is fully automated. It needs low manpower for operation.
Although recycling of the iron catalyst is possible, it is not always necessary; for example, where subsequent treatment in a waste water treatment plant uses iron for phosphate removal or at least for sludge dewatering.
Typical dosage of chemicals for the oxidation process is (e.g. for COD = 8500 mg/l):
- 13 l H2O2-solution (35 %)/m3 waste water (1.53 l H2O2-solution/m3 and 1000 mg/l)
- 35 ml H2SO4 (30 %)/m3 waste water
- 120 g Fe2+/ m3 waste water.
Cross-media effects The operation of the oxidation reactor requires electricity, but the amount is not significant.
Applicability The oxidation technique is applicable to both new and existing installations.
Segregation of the selected streams (preferably automatically) is required, along with the necessary pipe-work and equalisation tanks. The space requirement for an oxidation reactor and chemicals feed tanks is not significant and does not represent a limitation.
Investment cost for a reactor with a flow of 4 - 5 m3/h (including reactor, dosing system for hydrogen peroxide and catalyst, heat exchanger, catalyst preparation unit, automated control and pipe-work) is about 230000 euros. Operation cost, including above-mentioned dosage of chemicals, maintenance, labour and electricity, is about 3 euros/m3. It should be emphasized that this number is for the treatment of the selected high-loaded waste water streams and not for the whole of the mixed waste water.
Driving force for implementation Difficulty in complying with standards set by municipal waste water treatment plant in terms of COD-load, biodegradability and toxicity.
Reference plants One plant that has been in operation at Schoeller Textil AG, CH-9475 Sevelen since 1996; the flow is 4 - 5 m3/h. A second and third plant are under construction for Tintoria di Stabio SA, CH-6855 Stabio and Givaudan Vernier SA, CH-1214 Vernier [179, UBA, 2001].
Reference literature [51, OSPAR, 1994], [179, UBA, 2001]
Description Treatment of textile waste water by precipitation/flocculation in order to reduce organic load and especially colour has been performed for more than 100 years. However, today there are techniques that minimise the quantity of sludge produced and reduce negative effects associated with its disposal. Instead of landfill disposal, the sludge can be incinerated using state-of-the-art technology.
In modern plants the precipitate is separated from the aqueous phase not just by sedimentation but also by dissolved air flotation. Flocculation agents are specifically selected in order to maximise COD and colour removal, and to minimise sludge formation. In most cases, best performances are obtained with a combination of aluminum sulphate, cationic organic flocculant and very low amounts of an anionic polyelectrolyte.