«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
The sludge has been reported to have been transported as a liquid in a heated tanker for disposal to a landfill accepting heated liquids ([187, INTERLAINE, 1999]). However, the implementation of the Landfill Directive (Council Directive 1999/31/EC) required the cessation of the landfilling of liquids (and other problem wastes) by July 2001. Alternatives need to be sought and include: incineration, pretreatment to an acceptable solid form prior to landfilling (e.g. by composting, see Section 4.10.11), or composting and use on land (see Section 4.10.11), or use in other processes such as brick-making (see Section 4.10.12).
Cross-media effects Options in which the effluent is treated on-site or off-site in municipal sewer, without dirt removal/grease recovery loop simply produce a shift of the organic load from the aqueous phase to the sludge.
Both mechanical and thermal energy are used in the process of evaporation. However, the mechanical energy is not high and most of the thermal energy is recovered in a well-designed evaporator [187, INTERLAINE, 1999].
Applicability Apart from option A (treatment in external municipal sewer) which could find applicability only in cases where the discharge of the effluent would not cause environmental harm to the sewer (small scourer discharging effluent in a large sewage treatment plant), the other options are all applicable to any scouring mill.
The economics aspects involved for each of the proposed options have been discussed in detail (INTERLAINE, 1999 #187]). Table 4.49 and Table 4.50 summarise the information in the reference for a small scouring plant (3500 tonnes/year) and a medium-sized plant (15000 tonnes/year), scouring coarse wool. Similar results can be found for fine wool scourers, except for the factors related to the higher grease content of fine wool: this produces negative on-site running costs (savings) for all options which include grease recovery. Economies of scale are not expected to increase very much for mills with higher throughput than 15000 tonnes/year.
The capital costs for installation of dirt/grease loops used in the tables below are 412500 euros for the small mill and 825000 euros for the medium mills. The loops in the larger mills carry a much greater flow and these costs are believed to be realistic. The capital cost of flocculation plants at the larger mills is believed to be about 275000 euros and it is not believed that a smaller capacity plant would be much less expensive.
The capital costs for evaporators are somewhat more speculative. The following costs are assumed: capacity 21000 m3/year, cost 1.2 million euros; capacity 45500 m3/year, cost
1.8 million euros; capacity 60000 m3/year, cost 2.4 million euros; capacity 90000 m3/year, cost
3.0 million euros; capacity 120000 m3/year, cost 3.6 million euros.
On-site running costs are net of grease sales, hence the negative costs in some cases where grease recovery loops are installed. Calculations were based on the following unit costs.
Table 4.48: Unit costs Sludge disposal costs are those prevailing at present in UK for landfill (41 euros/t for wet sludge from flocculation and dirt/grease recovery plant and 95 euros/t for concentrate from the evaporator).
They include landfill tax (15 euros per tonne) and transport.
Sludge disposal costs do not take account of any changes consequent to the introduction of the Landfill Directive.
Effluent disposal costs are also values from UK (calculated according to the standard Modgen strength formula) and are believed to reflect full economic cost [187, INTERLAINE, 1999].
Table 4.50: Costs of effluent treatment options for a scouring mill processing 15000 t/yr of coarse wool For small mills processing coarse wool, from an economic point of view, the installation of a dirt/grease loop and a flocculation plant appears to be the best option.
Rapid payback is achieved (versus discharge to sewer) and this option has the lowest total cost over 10 years as well as the lowest NPV of 10-year cashflow.
The environmental performance of an evaporation plant is far superior to that of the flocculation plant. However, the initial cost of the evaporation plant is much higher and payback (versus discharge to sewer) is not achieved for 4 – 5 years.
The value of installing a dirt removal/grease recovery plant is clearly illustrated in the costings, even for this small mill, processing wool with a low grease content. When used in combination
with an evaporator, the loop enables a reduction in capital outlay because a smaller evaporator can be used.
For medium-sized mills, evaporation is slightly cheaper than flocculation over 10 years and has superior environmental performance. Again, the use of a dirt removal/grease recovery loop enables a smaller evaporator to be installed and reduces initial capital outlay.
The use of a dirt removal/ grease recovery loop allows also a reduction in running costs thanks to the proceeds from the sales of the grease. This effect is more significant for fine wool scouring mills because of the higher percentage of good quality grease. The loop also has a significant effect in reducing sludge disposal costs when used in combination with the evaporator. This is because the physical properties of evaporator concentrate make its transport and disposal difficult and expensive, so it is sensible to remove as much material from the effluent as possible before it is evaporated.
Reference literature [187, INTERLAINE, 1999].
4.10.11 Disposal of wool scouring sludge to agricultural land Description In most member states, conditions set in existing regulations make it necessary to pretreat wool scour sludges before disposal to agricultural land. The pretreatment of choice appears to be composting, although anaerobic digestion (with recovery of the evolved methane) might conceivably be considered as an alternative.
Material for composting9 should ideally have a C:N ratio of 25 – 30:1, which means that wool scour sludges need an addition of carbon-rich material. Green waste, sawdust, woodchip and straw have reportedly been used successfully. Some “structural material” (size up to 50mm) is needed in the co-composted material in order to allow the ready ingress of air. Optimum moisture content of the material for composting is 50 – 60 %.
Aeration of the material is used to control the rate of composting and therefore the temperature.
In the earlier thermophilic phase a temperature of 45 – 60 °C is optimum, whilst in the later mesophilic phase, lower temperatures (20 – 45 °C) are preferred.
There are many methods of composting in commercial production. They include methods
described as follows:
· open air, turned windrows · open air, with forced aeration · covered building, forced aeration · simple tunnel, non-turning · complex tunnel, with turning · enclosed hall, operating under negative pressure · reactor systems.
Enclosed, or in-vessel composting systems have advantages over open air systems that make invessel composting particularly suited for industrial wastes. First, control of the process is improved, allowing the use of higher temperatures and higher rates of composting. Second, The State of Composting in the UK and A Guide to In-Vessel Composting plus a Directory of Systems, The Composting Association, Ryton Organic Gardens, Coventry CV8 3LG, UK.
control of odours, dust and leachate is superior, allowing operation closer to industrial waste sources. The disadvantage of enclosed systems is their greater initial cost.
After composting, the compost must be allowed to mature for a number of weeks during which time, further (bio-)chemical changes occur, making the compost more suitable for purpose.
Main achieved environmental benefits It is believed that composting is a sustainable means of disposal of wool scour sludge, whilst at the same time producing a useful soil conditioner. The carbon which is decomposed by the composting process is converted aerobically into carbon dioxide, rather than methane (as in landfill). Carbon dioxide is a much less potent greenhouse gas than methane [187, INTERLAINE, 1999].
For wool scour sludges, the purpose of composting is to destroy, as far as possible, the components of the sludges which would be undesirable if spread on agricultural land. In the main, these are wool grease and ectoparasiticide residues. Composts produced from mixtures containing wool grease should therefore be periodically monitored for grease and ectoparasiticide content.
In composting trials carried out in UK10 similar results were achieved by composting for 6 – 7 weeks in a 10-tonne open windrow and for 14 days in an enclosed tunnel composter. The grease, organochlorine and synthetic pyrethroid ectoparasiticide content of the compost were reduced by 60 % and organophosphate ectoparasiticides were reduced by 80 %. Further reductions were expected during the maturation phase, but were not monitored. Interestingly, the wood chip and sawdust used as co-compostable material in this trial was found to be contaminated with lindane (OC) [187, INTERLAINE, 1999].
Composting can cause air pollution by odours and dust and water pollution via leachate. In a well-conducted composting operation, however, these problems should not occur. The use of compost as a soil conditioner may also cause water pollution if ectoparasiticide residues are present. However, this is thought to be highly unlikely if the composts are used responsibly because of the very poor mobility of sheep ectoparasiticides in soil [187, INTERLAINE, 1999].
Applicability This measure is applicable on-site by scourers who have the required space. Off-site, it is widely available, though costs of transport (because of distance from merchant composters) might be a problem for some scourers.
Availability of this option, both on-site and merchant may increase after the implementation of the Landfill Directive (1999/31/EC).
Economics Composting is not an inexpensive technique. Capital costs are reported in the next figure for recent start-ups, while Table 4.51 gives information for in-vessel systems.
M Madden, ENco, personal communication, 1998.
Table 4.51: Composting and maturation times, capital and treatment costs, for three types of invessel composting plant The market for compost is uncertain.
The Composting Association (UK) reports that no composting plant in the country can recover its costs through sales of compost. In fact, most plants receive no payment at all for their product. The cost of composting therefore has to be met through gate charges, or in the case of local authorities composting municipal waste, through local taxes.
Driving force for this technique
The implementation of the Landfill Directive will stop landfill of liquids. The future landfill of sludge is not clear. Composting provides a relatively low-tech, low cost means of treating wool scour sludge to give a material that can be used or more readily disposed of to landfill.
Reference plants Mills C, F, G and M in the survey reported in Section 3.2.1 use this method of sludge disposal.
Reference literature [187, INTERLAINE, 1999] 4.10.12 Use of wool scour sludge in brick-making Description Clay for brick-making should contain a certain amount of organic material. The oxidation of this material during the brick-firing process improves the quality of the resulting bricks. Some
clays are deficient in organic content and an addition is desirable. Wool scour sludges are excellent in this application.
Main achieved environmental benefits The sludge is used in the manufacture of a useful product and the organic material required for admixture with the clay might otherwise be derived from fossil sources.
Operational data It might be necessary to monitor trial firings using wool scour sludges to check that air emissions from the brickworks are not adversely affected.
Cross-media effects Air emissions might be increased, although this is believed unlikely. If sludges have to be transported long distances, the question of pollution arising from transport has to be considered.
Applicability The problems encountered in implementing this measure are logistical rather than technical. The difficulty is to match a scourer’s sludge production with a brick-maker’s requirements and location.
Economics This is greatly dependent on the deal struck between the scourer and the brick-maker and the cost of alternative sources of suitable organic material available to the latter. It would seem likely that this technique should be cheaper than landfilling, composting or incineration.
Driving force for implementing this technique For the scourer, cost would be the driving force. For the brick-maker, the availability of a reasonably consistent source of suitable material (probably available cost-free).
Reference plants Mills L and N in the survey reported in Section 3.2.1 use this method for disposal of some of their sludge production. It is also known that the technique is utilised by some Italian scourers.
Reference literature [187, INTERLAINE, 1999]
5 BEST AVAILABLE TECHNIQUESIn understanding this chapter and its contents, the attention of the reader is drawn back to the preface of this document and in particular the fifth section of the preface: “How to understand and use this document”. The techniques and associated emission and/or consumption levels, or ranges of levels, presented in this chapter have been assessed through an iterative process
involving the following steps:
· identification of the key environmental issues for the sector · examination of the techniques most relevant to address those key issues · identification of the best environmental performance levels, on the basis of the available data in the European Union and world-wide · examination of the conditions under which these performance levels were achieved; such as costs, cross-media effects, main driving forces involved in implementation of the techniques · selection of the best available techniques (BAT) and the associated emission and/or consumption levels for this sector in a general sense all according to Article 2(11) and Annex IV of the Directive.
Expert judgement by the European IPPC Bureau and the relevant Technical Working Group (TWG) has played a key role in each of these steps and in the way in which the information is presented here.
On the basis of this assessment, techniques, and as far as possible emission and consumption levels associated with the use of BAT, are presented in this chapter that are considered to be appropriate to the sector as a whole and in many cases reflect current performance of some installations within the sector. Where emission or consumption levels “associated with best available techniques” are presented, this is to be understood as meaning that those levels represent the environmental performance that could be anticipated as a result of the application, in this sector, of the techniques described, bearing in mind the balance of costs and advantages inherent within the definition of BAT. However, they are neither emission nor consumption limit values and should not be understood as such. In some cases it may be technically possible to achieve better emission or consumption levels but due to the costs involved or cross-media considerations, they are not considered to be appropriate as BAT for the sector as a whole.
However, such levels may be considered to be justified in more specific cases where there are special driving forces.
The emission and consumption levels associated with the use of BAT have to be seen together with any specified reference conditions (e.g. averaging periods).