«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Today, many activated sludge systems meet these system conditions (see following examples), which also enables almost complete nitrification. In these conditions, both readily and hardly biodegradable compounds can be degraded. On the contrary, effluents containing nonbiodegradable compounds should be treated/pretreated at the source (see Section 4.10.7), but this is done only in a few mills. In most cases, in addition to activated sludge further treatment
steps are carried out, such as flocculation/ precipitation, coagulation/adsorption/precipitation, adsorption to activated carbon and ozonation.
Other techniques combine the biological degradation process with physical adsorption, coagulation and advanced oxidation processes. These techniques are described in Section 4.10.3.
The treatment plant receives municipal waste water and effluent from four large textile finishing mills. The textile waste water is equalised and then mixed with primarily treated municipal waste water. The textile waste water accounts for about 45 % of the hydraulic load and about 60 % of the COD load. After primary treatment and equalisation, there is a biological treatment, including nitrification/denitrification and flocculation with FeCl3 as final step (FeCl3 has the disadvantage of introducing additional chloride ions in the system, which are a source of corrosion problems). The system can be seen from Figure 4.34 while Figure 4.35 shows the daily measured average COD concentration of the final effluent. The values vary within a significant range, reflecting fluctuations during the week, rainy days (because storm water enters the same sewer) and holiday time (very low values are recorded at the end of August which is the holiday time for companies).
Figure 4.34: Plant 1 – combined treatment of textile effluent and municipal waste water [179, UBA, 2001]
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0 15 1.0 29 1.0 12 1.0 26 2.0 11 2.0 25 3.0 08 3.0 22 4.0 06 4.0 20 5.0 03 5.0 17 6.0 01 6.0 15 7.0 29 7.0 12 7.0 26 8.0 09 8.0 23 9.0 07 9.0 21 0.0 04 0.0 18 1.0 02 1.0 16 2.0 30 2.0 2.
Figure 4.35: Daily average COD concentration measured on the final effluent from Plant 1 in the year 2000 [179, UBA, 2001]
In this big plant, waste water from two cities and some villages is treated together with textile waste water from four large textile finishing industries which accounts for about 40 % of the hydraulic load and about 65 % of the COD load. Municipal waste water and textile waste water are already mixed in the public sewer. Figure 4.36 shows the layout of the plant. The tanks for primary treatment are also used for equalisation of the incoming waste water. After the activated sludge treatment, no additional treatment is applied for further reduction of organic compounds and colour. The daily variation of the COD-load at the outlet of the plant can be seen from Figure 4.37. There are high peaks which result from high flows of storm water. In such conditions, retention time is reduced and removal efficiency is therefore also reduced. As in example Plant 1, during holiday time in the industry (August), residual COD load is significantly lower.
7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 07.99 21.99 04.99 18.99 02.99 16.99 30.99 13.99 27.99 10.99 24.99 08.99 22.99 05.99 19.99 03.99 17.99 31.99 15.99 29.99 12.99 26.99 12.99 26.99 09.99 23.99 2.
.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.126.96.36.199.1.1.0 Figure 4.37: Daily average COD load measured on the final effluent from Plant 2 in 1999 [179, UBA, 2001]
This example also features combined treatment of municipal and textile waste water. The layout of the plant is shown in the next figure. The neutralised and equalised waste water of a big textile company is discharged to the treatment plant by a separate sewer. The water is specially pretreated in a high-loaded activated sludge system with F/M 1.1 kg BOD5/kg MLSS · d. Under these conditions PVA, which is present at high concentration in the textile waste water, is not degraded at all. More than 90 % PVA removal is achieved in the subsequent (second) activated sludge stage having F/M 0.05 kg BOD5/kg MLSS · d.
Ozonation of textile waste water reduces colour significantly, but COD is reduced only very slightly ( 10 %) because of low ozone dosage (about 50 g/m3). However, it is postulated that biodegradability is increased. Activated carbon treatment is added only where the standards are breached, which has not happened in the past three years. Flocculation /filtration as a polishing step reduces COD to 10 – 20 % and removes some colour as well.
This large plant is for the treatment of waste water from about 150 textile units. Textile waste water accounts for about 55 % of the hydraulic load, municipal waste water for about 23 % and infiltration and storm water for the remainder. The layout of the plant is shown in the following figure.
After biological treatment including nitrification/denitrification, the mixed waste water is precipitated/flocculated for further COD reduction. The effluent is subsequently treated with ozone in order to remove colour and recalcitrant surfactants. The F/M ratio is higher than
0.15 kg BOD5/kg MLSS · d, which means that complete nitrification cannot be achieved and hardly degradable compounds may not be removed to as high an extent as with a lower F/M ratio.
Figure 4.39: Plant 4 – combined treatment of textile effluent and municipal waste water [179, UBA, 2001]
In plant 5, the waste water from one textile finishing industry is treated. The company mainly finishes cotton fabric, including pretreatment (desizing, scouring, bleaching), dyeing (cold pad batch and exhaust dyeing), printing (mainly with pigment printing pastes) and finishing. About 5 % of the treated waste water is recycled for washing and cleaning operations (floor washing, cleaning of printing equipment such as pumps, pipes, squeegees and screens). Retention time in the activated sludge system is very high. Decolourisation is achieved through reductive cleavage of azo groups of dyestuffs by an iron(II)-salt. Figure 4.40 shows the layout of the plant.
Figure 4.40: Plant 5 –treatment of waste water from one textile finishing mill with 5 % water recycling [179, UBA, 2001]
In the plant, shown in Figure 4.41, waste water from about 30 textile finishing units is treated together with municipal waste water. Textile waste water accounts for about 30 % of the hydraulic load and COD for about 40 %. The textile finishing industries discharge their waste water to the public sewer after neutralisation on site. Various companies have pretreatment Textiles Industry 411 Chapter 4 plants, especially pigment printing units, which treat the waste water from cleaning the printing equipment by flocculation/precipitation. The layout of the plant is typical with bar screen, aerated grit and grease chamber, primary clarifier, denitrification and nitrification stage. It is exceptional as regards the presence of an additional treatment with activated carbon powder in order to minimise COD and colour in the final effluent. The dosage of activated carbon powder is about 30 g/m3 and dosage of alum sulphate and polyelectrolyte is about 3 g/m3 for complete removal of residual suspended carbon particles. Backwash water containing activated carbon is fed to the activated sludge system (this has a significant stabilising effect). Residual COD is very low (below 20 mg/l; the annual average is 11 mg/l). The final effluent is colourless.
Figure 4.41: Plant 6 –treatment of waste water from one textile finishing mill [179, UBA, 2001] Achieved emission levels The table below shows the influent and effluent values and F/M-ratios of the six described treatment plants.
In some cases of combined treatment, textile waste water and municipal waste water are discharged to the plant via different sewer lines. For these plants, the influent values are presented separately. Where textile and municipal waste water are already mixed in the sewer, the influent values are given under the heading "influent (textile ww)".
F/M-ratios below 0.15 kg BOD5/kg MLSS·d enable almost complete nitrification (residual ammonia concentrations lower than 0.5 mg/l).
Plant 4 has a F/M ratio of 0.2, resulting in higher ammonia concentrations and lower BOD5 removal efficiency.
Table 4.41: Characteristics of waste water parameters (input and output) for six treatment plants [179, UBA, 2001] Applicability Activated sludge systems with low F/M ratios are applicable to both new and existing plants for all kinds of textile waste water.
They can also be applied to municipal waste water treatment plants with low and high percentages of textile waste water as well as to purely industrial plants in which the waste water of one or more finishing mills is treated.
The low F/M ratio conditions in an activated sludge treatment can be achieved not only by
increasing the hydraulic retention time. Other methods are applicable, such as for example:
· removing the food from the activated sludge (like for example in the technique described in Section 4.10.3) · reducing the load by pretreatment of selected concentrated streams (see Section 4.10.7) · increasing the biomass in the activated sludge system (e.g. bio-membrane reactor, bioflotation).
Cross-media effects Treatment in activated sludge under low F/M conditions as such is not sufficient for removing the fraction of COD represented by non-biodegradable hazardous chemicals. Other or additional treatment are necessary to transfer or better tranform these substances.
When low F/M ratio conditions are achieved by increasing the retention time, this means bigger aeration tanks, resulting in higher investment costs. Broadly speaking, the size of activated sludge systems is inversely proportional with F/M. Precise data on investment costs are not available. Additional cost for additional aeration is about 0.30 euros/m3.
Reference literature [179, UBA, 2001] 4.10.2 Treatment of mixed waste water with about 60 % water recycling Description The example presented here ([179, UBA, 2001]) illustrates the on-site treatment of mixed textile waste water with partial recycling of the treated effluent.
The flow sheet is shown in Figure 4.42.
Before treatment, the hot streams ( 40°C) are submitted to heat recovery. The following steps
are then carried out on the mixed effluent:
· equalisation (about 20 h equalisation) and neutralisation · activated sludge treatment in a special system consisting of loop reactors (dry matter content in the reactors: about 35 g/l) and clarifiers (which are not shown in the figure). Here the biodegradable compounds are completely removed ( 5 mg/l). Biodegradation efficiency is improved and stabilised by lignite coke powder which acts as temporary adsorbent both for organic compounds and oxygen (buffer function); in addition, micro-organisms growing on lignite powder can be enriched in the system · adsorption stage: lignite coke powder (with a specific surface of 300 m2/g) is added with a dosage of about 0.8 - 1 kg/m3 in order to remove dyestuffs and other hardly or nonbiodegradable compounds (the content of dry matter in the reactors is about 40 g/l). After sedimentation, the lignite coke powder is recycled to the adsorbers as well as to the activated sludge loop reactors · flocculation/precipitation and removal of the sludge by flotation: this step is necessary to ensure the complete removal of lignite powder (otherwise incomplete due to the small size of the particles). Alum sulphate and an anionic polyelectrolyte are added as flocculants (about 180 g/m3). In addition, to avoid breaching local limits on colour, especially red, an organic cationic flocculant (forming water-insoluble ion pairs with the sulpho-groups of the dyestuffs) is dispensed · filtration in a fixed bed gravel filter to remove suspended solids and some organic compounds.
Then about one third of the flow is discharged to the river and the other two-thirds are first treated in an activated carbon filter in order to remove residual traces of organic compounds and are then desalinated in a reverse osmosis plant.
In the reverse osmosis plant (consisting of 10 modules containing 4 spiral modules each) the permeate is mixed with fresh water and is used for all finishing processes, whereas the salty concentrate is re-used for the preparation of the brine solution needed for reactive dyeing.
Figure 4.42: On-site treatment of mixed textile waste water with partial recycling of the treated effluent [179, UBA, 2001] The treated waste water is stored in a tank and conditioned with ozone (about 2 g/m3) in order to prevent any biological activity.
The effluent is colourless and the inorganic and organic load is very low.
Along with excess sludge from the activated sludge system, the sludge from flotation is dewatered in a thickener and decanter and is then thermally regenerated in a rotary kiln (Figure 4.43). The temperature of the off-gas from the kiln is about 450 °C. The flue-gas is submitted to post-combustion (about 850 °C) and then the heat from the final off-gas is recovered by heat exchange (final temperature of the emitted air is about 120 °C).
Figure 4.43: Thermal treatment of excess sludge from the activated sludge treatment and sludge from flotation [179, UBA, 2001] Main achieved environmental benefits The described treatment enables significant reduction of waste water flow, achieving about 60 % recycling of the treated water.
In addition, about 50 % of neutral salt is recovered and reused for exhaust dyeing. The non-recycled water is discharged with very low residual content of organic compounds.
Applicability The described technique is applicable to all types of textile waste water. It has been tested at pilot plant scale (1 m3/h) for waste water from textile mills finishing yarn, woven fabric and knitted fabric with or without a printing section [179, UBA, 2001].
Cross-media effects The treatment requires considerable amounts of energy (mainly for the reverse osmosis plant).
A) Plant for the treatment and recycling of waste water at Schiesser, D-09243 Niederfrohna (in operation since 1995) for a design waste water flow of 2500 m3/d. This company treats cotton knitted fabric and dyes almost exclusively with reactive dyestuffs. The present waste water flow is about 1300 m3/d. There are two lines for activated sludge treatment and adsorption; at any one time, one line is in operation and the second is on stand-by in case of increasing flow.
B) A second plant has been in operation since 1999 at Palla Creativ Textiltechnik GmbH, DSt. Egidien which is designed for a flow of 3000 m3/d and a 60 % recycling rate. This company mainly finishes woollen woven fabric.
Operational data The performance of the Schiesser plant is illustrated in the following tables.
The very low values for COD, BOD5, TOC, detergents, colour and heavy metals in the treated effluent (see table below) indicate that there are no limitations for recycling. However, as seen
earlier, additional treatment (ion exchange and reverse osmosis) is necessary to remove salt and hardness ions (mainly calcium extracted from cotton).
Table 4.42: Typical characteristics of the different water streams (mean values) at the treatment plant Schiesser, D-Niederfrohna [179, UBA, 2001] As regards the regeneration of lignite coke in the sludge, the following emission values are achieved.