«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
· the amount of COD contained in the raw wool (556 kg COD/tonne fine raw wool and 315 kg COD/tonne coarse raw wool, see also Section 184.108.40.206) · the amount of grease removed/recovered from the effluent (assuming that the grease is the main contributor to the COD).
The available data concerning the COD levels after waste water treatment from the surveyed mills are summarised in Table 3.4. The mills have been subdivided into direct dischargers (companies that discharge directly to surface water) and indirect dischargers (companies that discharge to sewer after an on-site pretreatment. One mill recycles the effluent completely by evaporative treatment and therefore does not have any waste water discharge at all.
Some of the figures in Table 3.4 are estimated or calculated from other data supplied. To distinguish values supplied directly by the mills from estimated or calculated data, the former are printed in bold type.
In calculating the COD load entering the environment as a result of the activities of those wool scourers who discharge pretreated effluent to sewer, it has been assumed that the rate of mass removal of COD in the sewage treatment works is 80 %. This is believed to be an appropriate removal rate, although there is no hard evidence to support the assumption.
The processes used by the mills which responded to the questionnaire include all process types (coagulation/flocculation, evaporation, membrane filtration and aerobic/anaerobic biological treatment).
Unfortunately, not all effluent treatment sub-types are represented. For example, none of the responding mills uses dissolved air flotation (DAF) as a means of separation after addition of coagulants/flocculants to the effluent stream (all use either decanter centrifuges or hydrocyclones).
Only one mill uses membrane filtration (in this case ultrafiltration (UF) on rinse effluent only) – other types of membrane filtration are not represented. There is no mill which uses anaerobic digestion only to treat scouring effluent although the existence of such a mill in Italy is known.
There are also mills in Italy using conventional aerobic biological treatment (plants similar to those used for the treatment of municipal sewage) and combinations of anaerobic and aerobic biological treatment [187, INTERLAINE, 1999].
Four of the mills discharge effluent directly to surface waters. Two of these (Mills C and N) treat to high standards before discharge. Surprisingly, the other two discharge untreated effluent.
Discharges from the mills vary widely, from zero to 73 kg COD/tonne of greasy wool processed, reflecting differences in the on-site treatments applied. However, all mills discharging more than 3.4 kg COD/tonne discharge to sewer and pay the sewerage operator for further treatment. This reduces the range of COD entering the environment to 0 – 15 kg/tonne.
The best performance for a mill which does not completely recycle treated effluent (from evaporative treatment) is 0.2 kg COD/tonne for Mill N, but the estimated COD emissions to the environment from Mill L, which discharges via sewer, are similar at 0.3 kg/tonne.
With regard to sludges arising from effluent treatment, many scourers did not state whether the weights given were wet or dry weight. These instances are noted in Table 3.4. Sludge production (dry basis) ranged from about 100 to 300 kg/tonne greasy wool except for two cases.
Mill J treats effluent by anaerobic lagooning followed by evaporation and yet states that sludge production is only 55 kg/tonne. This figure possibly refers to the sludge or concentrate arising from evaporation and does not include the sludge from the anaerobic lagoon. In any case, it is unlikely to represent total sludge production at Mill J. Mill N treats scour effluent by evaporation and incineration. This produces 20 kg/tonne of ash, but no sludge. Sludges arise at this mill from gravity settling, plus decanter centrifuging, in the grease recovery/dirt removal loop and from aerobic biological treatment of rinse effluent. The figures given, equivalent to 75 kg/tonne, are believed correct [187, INTERLAINE, 1999].
The sludge is sent to landfill without pretreatment, or it finds other uses such as brickmaking or soil conditioner in agricultural land after composting. In one case it is incinerated.
Ectoparasiticides Residues of veterinary medicines in wool scour effluent have the potential to cause harm in the environment. The most commonly found ectoparasiticides and the environmental issues related to their release in the effluent have already been described in Sections 220.127.116.11 and 18.104.22.168.
The questionnaire sent to scourers involved in the survey asked them to give quantitative information on the source countries of the wools they scour. Thanks to this information in combination with the ENco Wool and Hair Pesticide database (see Section 22.214.171.124) it was possible to estimate the average biocide content of the incoming raw material. The results of this calculation are reported in Table 3.5, which shows concentrations of individual ectoparasiticides in the range of 2 – 15 mg/kg of raw wool.
Table 3.5: Average organochlorine, organophosphate and synthetic pyrethroid biocide content of the wools processed by 12 scourers The emission loads of pesticides discharged in the effluent from the surveyed companies are not available.
However, they could be estimated based on the water-grease partition factors of these compounds.
Biocides are in fact removed by the dirt removal/grease recovery loops, which are integrated with the scour, as well as by the end-of-pipe effluent treatment plant. For example, a mill which removes 25 % of the total grease on the incoming wool in its grease recovery loop, perhaps a further 5 % in its dirt removal loop, and 80 % of the remaining 70 % (i.e. 56 % of the total) in its effluent treatment plant, has an overall grease recovery rate of 86 %. Removal of lipophilic biocides would be expected to follow a similar pattern to that of grease removal. Rinse water recycling loops, if used, may also remove some biocides.
Many studies of the fate of ectoparasiticides in the wool scouring process have been carried out and these issues have already been dealt with in Section 126.96.36.199. Possible assumptions are listed
as follows [103, G. Savage, 1998]:
· 96 % of the pesticides are removed from wool (4 % is retained on the fibre after scouring) · of this 96 %, a percentage (which is usually 30 %, but in some examples it has been shown to be lower) is retained on-site in recovered grease · the remaining fraction (which does not associate with wool, grease and dirt) is discharged in the effluent and submitted to waste water treatment.
Exceptions to this behaviour are represented by:
· water soluble pesticides (e.g. cyromazine and dicyclanil); in this case it is assumed that 4 % of the initial amount remains on the fibre, but no further pesticide is removed by wool grease recovery or on-site treatment; therefore 96 % of the initial amount is found in the waste water · triflumuron: recent studies ([103, G. Savage, 1998]) have shown that triflumuron associates partly with grease and partly with dirt and that consequently a higher proportion of this pesticide residue is likely to be retained on-site. In particular, it can be assumed that 90 % of residues is retained on-site (including the amount retained on wool fibre and in recovered wool grease).
148 Textiles Industry
Chapter 3Concerning the effect of the waste water treatment, Table 3.6 summarises the monitoring results for woolscour effluent treatment plants, carried out by ENco in 1997/984. The results in the table compare the effluent before and after treatment and were obtained by analysis of 24 h composite samples, taken on 10 separate days. The table also shows the reduced efficiency of evaporative systems in the removal of OPs due to their steam volatility (see also Section 188.8.131.52).
Additional data come from a separate ENco study5, where the mass loads of the three most commonly used sheep treatment chemicals – diazinon (OP), propetamphos (OP) and cypermethrin (SP) – discharged to sewer from seven scouring mills, were calculated and compared with the loads present on the incoming greasy wool. The latter values were obtained by using the average residue concentrations taken from the ENco database for the mix of wool sources scoured at each mill. The results are shown in Table 3.7.
By comparing the results in Table 3.6 and Table 3.7, it can be seen that the overall removal rates of sheep treatment chemicals from scouring effluent are significantly higher than the removal rates in the effluent treatment plants. As indicated earlier, the balance is presumably removed in the dirt removal/grease recovery loops.
The above discusses the removal of sheep treatment chemicals in physical and physico-chemical effluent treatment plants. It is possible that prolonged biological treatments will destroy at least some of the chemicals. One of the scourers in the European survey described here treats rinse effluent by prolonged (4 – 5 day) aeration and this is known to remove all SPs and all OPs except dichlofenthion6,7. OCs are only partly removed. Biological treatments of short duration are not expected to break down the chemicals but may remove them by absorption into the lipid components of the biomass.
Figure 3.6 attempts to define for 1 tonne of raw wool the consumption and emission ranges for the scouring process and the waste water treatment.
The ranges are defined based on the results of the survey integrated with some results obtained from previous surveys of scouring mills carried out by ENco in 1996 and 1998. It has to be noted that some of the given ranges are not generally applicable. For example, the range of values for flocculants used in on-site treatment is valid only for those companies with a coagulation/ flocculation effluent treatment plant.
Dichlofenthion is an OP which was formerly registered for sheep treatment in New Zealand. It is particularly resistant to biodegradation and its registration has been withdrawn.
G Timmer, Bremer Wollkämmerei, private communication, 1998.
Figure 3.6: Diagram showing the ranges of inputs to and outputs from the scouring processes and effluent treatment plants (on- and off-site) at the mills surveyed [187, INTERLAINE, 1999]
3.2.2 Cleaning and washing with solvent The inputs and outputs of the process are reported in Table 3.8. Inputs and outputs are based upon a Wooltech plant production of 500 kg of clean wool fibre per hour. Typically 852.6 kg/h of greasy wool feed comprises 500 kg/h wool fibre, 128 kg/h grease, 102 kg/h dirt, 42.6 kg/h suint and 80 kg/h moisture. Wide variations in contaminants (pesticides, dirt and grease) are possible. These figures, whilst typical, are therefore nominal only.
The following comments (see also Figure 2.5) apply to the reported data [201, Wooltech, 2001]:
· clean wool: it should be noted that this wool is pesticide free as any pesticides in the wool partition strongly to the solvent and are removed with the grease (analytical data have been submitted to support this statement). This has an additional advantage at the textile finishing mill as it makes it easier for the finisher to comply with the emission limit values for pesticides · solvent make-up: a nominal consumption of 10 kg/h is shown, however much lower consumptions are generally achieved; the consumption is dominated by maintenance issues · grease: this leaves as hot, liquid stream. Although it contains some dirt and suint, processes have been shown to separate this if higher quality grease is required (either acid cracking or no-acid cracking may be used). Also, final grease can be used for combustion to fuel the process · fleece and moisture: water emissions come from moisture introduced with the wool, water introduced to the process (steam used in vacuum ejectors) and moisture recovered from air drawn into the equipment. This water is treated in two steps (see also Section 184.108.40.206). In the first step, most of the solvent is recovered by heating the water and air stripping it in the Solvent Air Stripping Unit. This recovers 99.98 % of the solvent present. The solvent recovered is recycled through the plant. Second, the minute traces of residual solvent (ppm level) in the water are then destroyed with a free radical process based on Fenton’s reaction in the Residual Solvent Destruction Unit. Recovered solvent from, for example, maintenance activities is treated in the same manner.
· dirt: rinsing the solids and re-centrifuging prior to drying can eliminate the grease content.
Solids are suitable for landfill or use as soil. Seeds in the dirt have been found to be rendered sterile by their contact with the solvent · exhaust air: air is extracted from the plant to keep the processing equipment under a slight negative pressure. This air is treated through an adsorption system to recover solvent vapours. Destruction of the remaining solvent will involve a scrubbing treatment followed by oxidative destruction as for the process water.
3.3 Textile finishing industry In the following sections, emissions and consumption levels are illustrated for a group of different sites belonging to the categories identified in Chapter 2 (see Section 2.14).
Information comes from various sources ([179, UBA, 2001], [198, TOWEFO, 2001], [200, Sweden, 2001], [199, Italy, 2001], [193, CRAB, 2001], [31, Italy, 2000]) and is the result of surveys carried out in a number of textile finishing industries in Europe in a five year timeframe (1995 to 2001).
With respect to the consumption of chemicals, where not otherwise specified, calculations have been carried out on the so-called “telquel”-basis. This means that the quantities of ready-for-use products have been considered, including water in the case of liquid formulations. This must be kept in mind when comparing the consumption levels of different companies. For instance, companies using mainly liquid dyestuff formulations (often the case for big mills) show specific dyestuff consumption higher than companies using powder or granulates.
3.3.1 Mills finishing yarn and/or floc 220.127.116.11 Mills finishing floc: mainly CV, PES, PAC and/or CO For this category the only information available relates to emissions to water. The values are compiled in Table 3.9. Because of the low liquor ratios and small number of process baths, the specific waste water flow is low. The values in Table 3.9 are confirmed by “FhG-ISI, 1997” reporting specific flows for three further companies between 14 and 18 l/kg.
Table 3.9: Concentration values and textile substrate specific emission factors for waste water from two mills mainly finishing floc material consisting of CV, PES, PAC or CO 3.
3.1.2 Mills finishing tops/floc and yarn: mainly WO Table 3.10 shows emissions and consumption levels in the apparel sector for three typical wool commission dyehouses and one spinning and dyeing mill (TFI 4). Companies belonging to this category tend to differentiate their production and to process more than one make-up, which means that comparisons are not straightforward. All four mills assessed in the table treat mainly tops, but TFI 1, TFI 3 and TFI 4 dye also a variable percentage of yarn (both in package and hank form), whereas TFI 2 treats only tops and loose fibre.
Figures for specific water consumption partly reflect the predominant processed make-up. TFI 4 shows very high water consumption levels. Various factors may reasonably explain this value.