«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Table 4.43: Measures emission values: off-gas from the thermal regeneration plant [179, UBA, 2001] The composition of fresh and regenerated lignite coke is shown in the next table.
The carbon content of regenerated coke is slightly higher, but ash content is about 30 % lower. Thus, recycling is possible without limitations. Also the size distribution of regenerated coke particles is very similar to that of the fresh material.
Ash removed from off-gas after heat recovery has to be disposed of as hazardous waste. The specific quantity is 5 g/m3 treated effluent.
Although the equipment was made of stainless steel, there were corrosion problems at the Schiesser plant; but they have been solved by replacing the supply pipes with plastic and coating the reactors with polyurethane.
Since summer 1998 the reverse osmosis plant has been out of operation because of high operating cost and the fact that the company does not need a 60 % recycling rate because the actual flow is far below the designed one. Thus, a recycling rate of 25 % is sufficient at the moment.
Data on the plant at Palla Creativ Textiltechnik GmbH are not available.
Economics Investment costs for such a plant are very high. For Schiesser it was 10.1 million euros, of which 2.0 million were for building construction, 7.4 million for technical equipment and
0.7 million for planning, scientific investigations etc. The plant has been highly subsidised by the federal and state government (about 80 %). The following table shows the cost per year and flow-specific cost, ignoring the subsidies, i.e taking full account of the capital cost.
The total flow-specific cost has to be compared with the alternative cost of disharging the waste water to the municipal waste water treatment plant.
In addition, since the company is only allowed to use 1000 m3/d ground water, 1700 m3/d would have been purchased from public water supply to cover the design consumption (2700 m3/d).
Thus, the company would have had to pay 2.90 euros/m3. Thus, 1.63 euros/m3 could be saved, which means nearly one million euros per year.
Data on the plant at Palla Creativ Textiltechnik GmbH are not available.
Driving force for implementation Ground water supply is limited. This was an important reason to go for a waste water recycling technique. The very high investment costs could be accepted because of the generous subsidies granted.
Reference literature [179, UBA, 2001] 4.10.3 Combined biological physical and chemical treatment of mixed waste water effluent Description Treatment in activated sludge systems under low F/M ratio conditions (Section 4.10.1) enables the degradation of both readily and hardly biodegradable substances. However, this technique is not sufficient for degrading or eliminating non-biodegradable compounds. Effluents containing non-biodegradable compounds should undergo additional treatments to remove or destroy these substances.
Such treatments should preferably be carried out before the final biological treatment (see Section 4.10.7), but in practice this is done only in a few mills.
In most cases additional sequential treatments are carried out after the activated sludge such as flocculation/precipitation, coagulation/adsorption/precipitation, ozonation. However, ozonation, when applied at the end of the treatment process, mainly has the effect of degrading the chemicals into intermediate degradation by-products, whereas the other treatments listed ultimately just transfer to sludge the substances that escape bioelimination.
Another approach for improving the performance of the activated sludge treatment is represented by the Powdered Activated Carbon treatment. This consists in combining the different technologies (biological, physical, chemical), thus allowing simultaneous biodegradation, adsorption and coagulation. The process was introduced in the early seventies and industrialised with the commercial name of PACT and PACT® systems.
In the PACT system, powdered activated carbon and bacteria are maintained in an aerobic/anoxic treatment process for symbiotic activity [292, US Filter-Zimpro, 2002].
In the PACT® system, the excess sludge (a mixture of spent powdered carbon and biomass) from the aerobic aerator is regenerated by means of a hydrothermal treatment (wet oxidation).
This is a liquid phase reaction in water using dissolved oxygen (or air) to oxidise soluble and suspended oxidisable contaminants. When air is used as the source of oxygen the process is referred to as “Wet Air Oxidation”. The oxidation reaction is carried out at moderate temperatures of 150 – 315 °C and at pressures from 10 to 207 bar. The process destroys the large molecules in waste water, converting them predominantly to carbon dioxide, water and
short chain organic acids, which are highly biodegradable and more suitable for biological treatment. This regeneration process provides continual re-use of the activated carbon and ensures high levels of waste treatment.
A first application of the PACT® system in the European textiles industry appeared in 1975 (Desso in Belgium). The process was later improved by integrating it with simulateous coagulation treatment (the system is called PACT+, a cryptic term to indicate a first improvement).
A second improvement (the so-called PACT++) could only be acheived by changing and extending the conventional activated sludge process with a nitrification/denitrification step followed by a filtration of the effluent to retain suspended solids.
Another technique is the PACT3+ system. This concept is a combination of different available techniques, with the aim of improving performance, flexibility and economy of scale of the PACT® system.
In the PACT3+ system, activated carbon is added to the aerobic aerator together with iron, which is used as a coagulant to precipitate phosphate and increase the binding of dyes into the sludge. The reactivation of the spent sludges containing powdered carbon and iron, is carried out at low temperature (below 130°C) if hydrogen peroxide is used (a process referred to as “catalytic active carbon wet peroxidation”). Concentrated or adsorbed substances are destroyed by advanced oxidation using hydrogen peroxide, creating the conditions for the Fenton reaction (H2O2, Fe2+ at pH 3). The principle of this process is described in Section 4.10.7. Both the reactivated carbon and the iron are recycled back to the aerobic system.
In this enhanced process it is not necessary to add oxygen (pure oxygen or air) because this is already available in the biomass.
Main achieved environmental benefits The described pretreatment techniques increase the performance of the activated sludge treatment.
The main advantages over sequential tertiary treatments applied after the biological system are:
· the production of excess sludge is reduced · substances that are potentially hazardous (non-biodegradable, accumulatable, toxic) are preferentially removed and destroyed · the activated sludge system is better protected against shock loading, and as the adsorbed material is degraded, the risk of displacement of dyes and other adsorbed substances is much lower than it appears to be with post-adsorption (e.g. granular activated carbon) · the excess solids produced are dense and retain the remaining substances, which can therefore be sent to easier dewatering (anaerobic) and incineration · mineralisation of the organic pollutants is improved · energy use in aeration is lower.
Operational data With PACT® and PACT3+ treatments a good filtration is very important in order to efficiently separate the sludge from the treated effluent.
Applicability The technique is applicable to existing and new installations where a biological treatment is available and where solids are fully retained in the clarification system. A microfilter is to be added when there is a risk that solids could escape with the effluent.
The addition of the adsorbent (activated carbon) and coagulant can be done at any point where the utilisation is most effective (cost, performance) and do not necessarily need to be added directly in the aerobic aerator; this is because the countercurrent-flow from coagulation, adsorption and filtration has the effect of returning to the biological system the adsorbent and coagulant containing the substances removed from the water.
Reactivation using hydrogen peroxide allows the re-use of carbon and iron. The technique is most interesting for pretreating hot concentrates (somewhat comparable with the technique described in Section 4.10.7, but simpler because there is no need to inject oxygen gas) and for reactivating sludges from biological, physical and coagulation processes.
The following additional equipment is needed:
· dosing systems for powdered carbon and iron-sulphate · dosing system for peroxide · microfiltration · reactor for reactivation of concentrated streams.
Special types of activated carbon are known to give the best performance. Cost depends on the dosage (less than 100 g/m3 of mixed effluent is needed, when reactivation of the activated carbon is carried out).
Hydrogen peroxide is consumed in stoechiometric amounts to transform the concentrated substances into bioeliminable substances (under conditions of optimal pH and temperature).
Iron is added as iron sulphate. It is introduced as a coagulant but it also serves as a catalyst, nutrient and precipitant for sulphides and phosphates.
Driving forces for implementation PARCOM Recommendations 94/5, after critical review of the PACT process, suppports the implementation of the PACT3+ concept as one of the most promising upcoming technologies (based on advanced oxidation processes, technologies for the nineties).
Reference plants Different PACT processes to treat dyestuff containing effluents are in place world-wide.
The PACT® system has been in operation since 1980 for textile effluents in the city of Vernon.
The PACT+ system has been implemented by Desso, who has also evaluated the PACT++ system (no info about future directions).
PACT3+ is a concept that combines existing applied techniques.
Reference literature [314, L. Bettens, 2002], [292, US Filter-Zimpro, 2002].
4.10.4 Recycling of textile waste water by treatment of selected streams with membrane techniques Description Membrane techniques are applied in various ways for the treatment of segregated streams to allow water reclamation and re-use closely integrated with the process. Two case studies are presented where membrane techniques are applied to effluents from dyeing operations. These case studies, however, are examples only. Membrane techniques can also be applied to other types of effluents, such as, for example, the desizing effluents (see Section 4.5.1), including those resulting from the enzymatic desizing of fabrics treated with starch- and modified starch sizes [192, Danish EPA, 2001].
Plant A) [179, UBA, 2001]
The company treats woven fabric, mainly consisting of cotton. The process sequence includes pretreatment, dyeing (cold pad batch), pigment printing and finishing (application of softeners or fluorocarbon resins). Rinsing operations account for most of the waste water produced.
The next figure shows the flow sheet of the treatments applied to the segregated streams.
Membrane techniques applied include ultrafiltration, nanofiltration and reverse osmosis.
Figure 4.44: Treatment of selected segregated waste water streams using a series of membrane techniques (ultrafiltration, nanofiltration and reverse osmosis) – the cut-offs are expressed in Dalton (D) [179, UBA, 2001] Not all waste water is recycled.
Waste water from pretreatment (scouring and bleaching) and finishing (residual padding liquors) is not treated in the membrane plant but is discharged, after neutralisation, to the municipal waste water treatment plant.
In order to assess the potential for re-use, the single process streams have been carefully analysed and segregated according to their suitability for treatment with membranes. For
instance, streams containing pigment pastes cannot be treated with membranes because the binders would lead to irreversible scaling of the equipment. Moreover, some process modifications have been necessary. For example, use of water glass in cold pad batch dyeing had to stop because silicates would also block the membranes.
The first membrane step is an ultrafiltration tubular ceramic module, which is needed in order to remove all residual particles and polymers.
About 90 % of the feed can be recycled for most processes. However, the re-use of the treated water has to be carefully assessed. For example, fresh water and not recycled water is used for the preparation of bleaching, dyeing and finishing liquors.
Plant B) [192, Danish EPA, 2001]
The second example is also of a company finishing cotton fabric. The measures include:
· reclamation and re-use of dye bath and first rinsing water after reactive dyeing by treating the highly coloured and salted water with activated carbon; the carbon retains the dyestuff and other organic chemicals and delivers clear, hot water with sodium chloride and sodium hydroxide for re-use · reclamation and re-use of rinsing water after dyeing by membrane filtration (with nanofiltration or reverse osmosis).
Main achieved environmental benefits
A reduction in water consumption and waste water discharge of about 60 % is reported in plant A [179, UBA, 2001]. The COD load in the effluent discharged to the municipal treatment plant is reduced by about 50 %. Similar reductions in water consumption and discharge of chemicals (especially salt) are also claimed in the second example plant [192, Danish EPA, 2001].
Plant A went into operation at the end of 1995. Many problems had to be tackled, especially the removal of fibres and particles (e.g. dust from singeing) and the identification of chemicals that were causing scaling of the membranes. The ultrafiltration step had to be changed from spiral modules to ceramic tubular modules, which are much less sensitive to scaling.
The reference data for permeate fluxes are:
· ultrafiltration (UF): 85 - 130 l/m2 x h · nanofiltration (NF): 12 - 17 l/m2 x h · reverse osmosis (RO): 11 - 17 l/m2 x h The plant treats about 900 m3/week waste water (which is about 70 % of the overall waste water flow) and recovers about 800 m3/week water which can be used for all washing/rinsing operations.
The plant is operated batch-wise. The concentrate is physico-chemically treated in an external plant. For further optimisation, plans are in hand to treat the concentrate by evaporation (in order to achieve 15 % water content) and then send it to incineration.
The experience with Plant B reported by Denmark was a semi-full-scale test. The dimensioning parameters from the activated carbon test-plant were a retention time of 2 hours and a capacity of 4 kg carbon/kg dyestuff. The carbon type used was F400 from Chemviron Carbon. A fullscale plant can consist of two columns connected in series and with reversible flow, so that there is capacity for recharging when they reach dye breakthrough point.
Test dyeings showed that re-using warm, saline and decoloured dye baths was possible with no adverse effects on fabric shade or fastness.
The dimensioning parameters of the membrane treatment from the test-plant based on spiral wound elements were an average production of 25 l/ m2h at 25 °C and 7 - 10 bar. The selected elements in use were 50 mil Duratherm elements from OSMONICS DESAL.
Cross-media effects Energy consumption seems to be significant. For plant A, the energy consumption of the membrane plant is reported to be about 20 kWh/m3 treated waste water. Moreover, since membrane treatment is a separation technique, correct disposal of the concentrate is a crucial point.