«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Although at high concentration (500 mg/l) sulphates become aggressive against concrete, the use of sulphate is preferred to chloride. Sulphates are easier to remove from water than chlorides. Moreover, the use of sulphate avoids the introduction of chlorides in the waste water and in the sludge to be incinerated [281, Belgium, 2002].
Iron sulphate is equally effective for the removal of COD and can also be considered as coagulant (e.g. it is particularly effective for removing acrylates and other substances from
pigment printing waste water). The introduction of iron has advantages (it is responsible for the activation of redox processes, it can be recycled, etc.), but it forms coloured complexes that remain in solution giving it a yellowish brownish tint [281, Belgium, 2002].
Main achieved environmental benefits Typically COD removal is only about 40 – 50 %. When the effluent has a high content of waterinsoluble compounds (e.g. in waste water from pigment printing sections), COD-removal is higher. De-colouration is more than 90 %.
The sludge is fully mineralised in an incineration plant.
Operational data Before flocculation/precipitation, the textile waste water is equalised. However equalisation time can be shorter (about 12-h) than with biological treatment. Fibres are removed by a sieve.
The dosage of flocculants (e.g. for a mixed effluent with COD of ca. 1000 mg/l) is about:
· aluminium sulphate: 400 - 600 mg/l · cationic organic flocculant: 50 - 200 mg/l · anionic polyelectrolyte: 1 - 2 mg/l.
The quantity of sludge produced is about 0.7 - 1 kg of dry matter/m3 treated waste water.
Usually, the sludge is dewatered in a chamber filter press to reach a dry matter content of about 35 - 40 % (3 kg of sludge are therefore produced for 0.5 kg of COD removed).
Cross-media effects A considerable amount of organic compounds is shifted from the aqueous phase to the sludge.
However, the sludge is incinerated and thus mineralised.
Energy is consumed for dewatering, transport and incineration.
Applicability The technique is applicable to both new and existing installations.
4.10.9 Air emission abatement techniques Description
The following off-gas abatement techniques can be used in textile finishing:
· oxidation techniques (thermal incineration, catalytic incineration) · condensation techniques (e.g. heat exchangers) · absorption techniques (e.g. wet scrubbers) · particulates separation techniques (e.g. electrostatic precipitators, cyclones, fabric filters) · adsorption techniques (e.g. activated carbon adsorption).
The description and the performance of these techniques are treated in detail in another BREF ([196, EIPPCB, 2001]). Depending on the type of air stream and pollutants to be treated, they can either be used as single treatments or be applied in combination. Typical applied systems
Textiles Industry 431 Chapter 4 · wet scrubbers · combination of wet scrubber and electrostatic precipitation · combination of heat exchanger, aqueous scrubber and electrostatic precipitation · heat exchangers (primarily used for energy saving, but partial condensation of certain pollutants is also achieved) · adsorption on activated carbon.
Main achieved environmental benefits Reduction of volatile organic carbon (VOC), particulates and special toxic substances in the offgas as well as minimisation of odour nuisances.
Operational data To achieve high operational reliability, adjustment of the appropriate operating conditions and proper maintenance (in some cases a weekly inspection and cleaning of the devices) of the equipment is crucial [179, UBA, 2001].
Cross-media effects The high energy demand and corresponding high amounts of CO2 resulting from thermal and catalytic incineration need to be addressed (greenhouse effect). However, this disadvantage can be considered to be outweighed by the benefit in terms of organic compounds removed [179, UBA, 2001].
In wet scrubbers, the pollutants are shifted from off-gas to the waste water. Efficient waste water treatment (e.g. oil/water separators, biological waste water treatment) is required.
Applicability Off-gas cleaning can be installed in both new and existing installations. However if existing machinery has to be rebuilt, applicability can be limited by economic, technical and logistical factors.
In each case, for the installation of an off-gas cleaning system, a tailor-made solution using the above-mentioned techniques has to be developed. In general, however, the following considerations about the applicability of the different abatement techniques have to be borne in mind.
The disadvantage of thermal incineration is the high energy consumption for heating the off-gas to at least 750 °C. After incineration, the temperature of the cleaned off-gas is around 200 °C to 450 °C. The textile industry does not have a demand for thermal energy in this sort of amount so most of it would be wasted.
Another problem arises from the gas-air-mixture typical of exhaust air from textile finishing. In the textile industry, most of the emissions to be treated are characterised by high off-gas flows, but relatively low load.
Moreover, the characteristics of the off-gases are often subject to fluctuation, leading to inefficient thermal incineration.
In catalytic incineration, phosphorus compounds, halogens, silicones and heavy metals can poison the catalyst. These compounds are quite common in the textile industry, so special care has to be taken when using catalytic oxidation in this sector.
Catalytic oxidation with full heat recovery is applied in some mills for treating off-gases arising from singeing operations (see Section 188.8.131.52). The hot gas at the outlet of the catalytic afterburner, is drawn through air-to-water heat exchangers: the hot water generated by the cooling process is used in the pretreatment process. The gas (with its remaining thermal content) is further used in the drying step taking place after the pretreatment process [281, Belgium, 2002].
Condensation techniques Pollutants with a high volatility and, in most cases, odour-intensive substances are removed.
Absorption techniques The efficiency of wet scrubbers in textile finishing depends strongly on process-specific parameters. Normally the efficiency is in the range of 40 to 60 %. Applicability for waterinsoluble pollutants is limited.
Electrostatic precipitation Electrostatic precipitators can precipitate dusts and aerosols with a size of 0.01 to 20 µm.
Maximum efficiency will be reached at around 0.1 µm – 1.5 µm. Manufacturers therefore recommend installing a mechanical filter before the electrostatic filter, which precipitates most of the particles with size 20 µm.
The efficiency of electrostatic precipitators for particle-sized solid and liquid pollutants is in the range of 90 % to 95 %. Gaseous pollutants and odorous substances cannot be precipitated. For best overall efficiency, it is therefore important that almost all condensable substances, emitted as aerosols, are removed before reaching the electrostatic precipitator. This can be achieved by heat exchangers or scrubbers.
Electrostatic precipitation in combination with heat exchangers or scrubbers is successfully applied in the treatment of fumes emitted from the stenters where the fabric is submitted to thermofixation.
The combination of electrostatic precipitation with heat exchangers (dry electrofiltration) is particularly advantageous when this operation is carried out as a first treatment step before washing. The oils and preparation agents present on the grey fabric evaporate and give rise to a
dense smoke also associated with odour emissions. This off-gas can be treated in four steps:
1) mechanical filtration
2) cooling and condensation (the suspended condensable compounds are separated in the form of oily droplets and thermal energy is recovered)
3) ionisation/ electrofiltration
4) collection of the condensates and separation of the oily phase from the aqueous phase in a static decanter.
One of the advantages of this dry electrofiltration system is that the oily condensates (mineral oils, silicone oils, etc.) are collected separately and thus recovered instead of being transferred to the water effluent (e.g. via a scrubber). Energy recovery is another advantage of this technique. Recovered energy (35 – 40 % of the supplied amount) can be used to preheat the fresh air supplied to the stenter or to heat up process water.
Installation and running costs have to be considered. In particular, costs of equipment maintenance and energy should be considered. Detailed information about costs is reported in another BREF ([196, EIPPCB, 2001]). Among the above-mentioned techniques, oxidation techniques have by far the highest investment and operating costs.
Textiles Industry 433 Chapter 4 Specific information about dry electrofiltration (combination of heat exchangers and electrostatic precipitation) has been submitted for the present document. A capital investment of EUR 70000 is reported for a 10000 m3/h unit with a pay-back time of less than 3 years [44, Comm., 2000].
Driving force for implementation Need to comply with standards set by environmental legislation for air pollution and improving environmental performance in terms of odour nuisances.
Reference plants Many plants. Systems based on heat exchangers, aqueous scrubbers and electrostatic precipitators dominate [179, UBA, 2001].
Reference literature [179, UBA, 2001], [281, Belgium, 2002], [44, Comm., 2000].
4.10.10 Waste water treatment in wool scouring installations Description The INTERLAINE report describes a number of available options for the management of water emissions arising from wool scouring installations. Clearly not all options can be considered BAT. Nevertheless it is useful to discuss the environmental performances and economic
implications involved in each of the following scenarios:
A. treatment in external municipal sewage treatment plant. It consists of screening the effluent to remove gross solids (3mm), perhaps cooling the effluent, and/or adjusting its pH to the sewerage undertaker’s requirements and disposing of the effluent to sewer B. treatment in an integrated dirt removal/grease recovery loop followed by discharge to municipal sewer. It is assumed that the mill installs a dirt removal/grease recovery loop, which recovers 25 % of the grease and removes 50 % of the dirt and a further 10 % of the grease from its effluent as a sludge C. treatment by coagulation/flocculation followed by discharge to municipal sewer. This technique supposes that the small scourer, rather than installing a dirt removal/grease recovery loop, opts to install on-site end-of-pipe effluent treatment, using coagulation/flocculation. Treated effluent is discharged to sewer D. treatment in an integrated dirt removal/grease recovery loop followed by coagulation/flocculation before discharge to municipal sewer (B+C) E. treatment by evaporation. The technique consists of evaporating the effluent, recycling the condensate if practicable and disposing of the residual concentrate or sludge. Not all mills analysed in the survey using evaporators recycle the condensates. It may be significant that the two mills analysed in the survey which do recycle the condensate, both employ a biological treatment as well as evaporation within the effluent recycling loop. One mill uses anaerobic lagooning before evaporation and the other uses a rapid bio-reactor after evaporation. It is possible that the biological treatments destroy the compounds responsible for odours F. treatment in an integrated dirt removal/grease recovery loop combined with evaporation (B+E) G. biological treatment (no data was made available for this technique).
Achieved emission levels The environmental performance of the proposed techniques have been estimated based on the
· for coarse greasy wool: 315 g/kg COD with 50 g/kg of grease and 150 g/kg of dirt · for fine greasy wool: 556 g/kg COD with 130 g/kg of grease and 150 g/kg of dirt · untreated effluent contains 95 % of the COD and dirt from the fibre · the sewage treatment plant removes 80 % of the incoming COD · the dirt/grease recovery loop recovers 25 % of the grease and removes 50 % of the dirt. It is assumed that a further 10 % of the grease is removed from the effluent as a sludge, before discharge to the sewer. For scouring lines fitted with a dirt/grease recovery loop a net water consumption of 6 l/kg greasy wool has been assumed, but levels of 2 - 4 l/kg are possible · the coagulation/flocculation treatment removes 89 % of the grease and 86 % of the suspended solids from the effluent · in the absence of a dirt/grease recovery loop, effluent volume is assumed at 13 l/kg greasy wool · evaporation does not entirely remove pollutants. Here it is assumed that the evaporator removes 99.3 % of the grease and 99.9 % of suspended solids. In trials at one mill, removal of sheep ectoparasiticides was: OCs, 96.5 %; OPs, 71.5 %; SPs 100 %. Water from the evaporator can be recycled. The residual COD (200 - 900 mg/l) and suspended solids (20 - 40 mg/l) in the condensate are no detriment to adding the recovered water to the rinse bowls of the scour (even the final rinse bowl operates at much higher contaminant levels than this). On the other hand, recycling requires an extra treatment process in order to avoid ammonia and odorous compounds being carried back to the scour. Water saving achievable by recycling of the condensate is not considered in the tables below.
The results of the calculations are reported in Table 4.46 and Table 4.47 for coarse and fine wool scourers, respectively.
Table 4.47: Waste water treatment techniques: Environmental performance – fine wool No precise information has been submitted for effluent treatment by biological processes.
It is known that there are scourers in Europe using biological processes as their main methods of effluent treatment. Biological treatment of scouring effluent is particularly popular amongst Italian scourers. One medium-sized Italian mill is known to employ anaerobic biological treatment, flocculation and prolonged aerobic biological treatment in succession for effluent treatment (total biological treatment time is approximately 7 days). This mill claims to produce an effluent containing only 650 mg/l COD, which is discharged to sewer. Another Italian mill uses a 3-day anaerobic process, followed by coagulation/flocculation (FeCl3) to produce an effluent containing 1000 – 1200 mg/l COD, which is again discharged to sewer [187, INTERLAINE, 1999].
Several remotely-situated Australian mills use anaerobic/aerobic lagooning for effluent treatment, but it is doubtful if any European mill has the space for such a process, not to mention the lack of neighbours which its odour generation potential would affect [187, INTERLAINE, 1999].
The concentrate or sludge from evaporation contains suint as well as dirt and grease. Sludges from coagulation/flocculation contain only dirt and grease because suint is highly water-soluble and is not flocculated. The presence of suint (largely potassium salts) in the sludge from evaporation appears to alter its physical properties. Flocculated sludge is spadeable and, depending on its water content, varies in consistency from something resembling moist earth to semi-liquid mud. Evaporator condensate, however, may be liquid at relatively high temperature and solid at ambient temperature. It appears that the suint salts act as a flux at the temperatures prevailing in the evaporator. This makes evaporator sludge more difficult to handle and to dispose of.