The main environmental impacts of the textile chain derive from wet processesing, mainly implemented by the textile finishing industry. On average, 90% of the water input in textile finishing operations needs to be treated end-of-pipe and 1 kg of chemicals and auxiliaries is processed per kg of textile products. Because of the wide variety of process steps, textile wastewater typically contains a complex mixture of organic and inorganic chemicals. Desizing, in some cases combined with scouring, is one of the industry largest sources of wastewater pollutants. Of major concern are recalcitrant or hazardous organics, such as dyes or some surfactants, metals and salts. COD concentrations in effluents range from 1 000 to 12 000 mg/l, BOD loads from 200 to 2 500 mg/l. A combination of physical-chemical followed by biological treatment and physiochemical treatment for textile effluent is offered by Dew, followed by UF, RO and Evaporators to make the unit Zero Liquid Discharge (ZLD).
A complete treatment of a mixed textile effluent consists of the following steps
Meant to remove (part of) the recalcitrant COD and/or suspended solids.
Conventional activated sludge systems have the capability to remove large fractions of COD. Due to the low degradability of most pollutants however, loading rates are often low. Dye removal has been observed, but is most often achieved by sorption processes, rather than by biodegradation. For effluents with a COD over 5 000 mg/l, anaerobic treatment becomes more and more important. A combined anaerobic and aerobic treatment can also be effective for removal of azodyes.
Removal of dyes and remaining recalcitrant COD is possible by activated carbon treatment, sorption processes, membrane filtration, oxidation processes, etc. Concentrated streams with non-biodegradable substances should preferably be treated at source. If this is not the case, biological treatment can be optimized by the addition of activated carbon or can be improved by coupling with advanced oxidation processes. Alternatively, decentralized treatments can be used for the recovery of non-biodegradable chemicals. E.g. membrane treatments may allow the recovery and recycle of some sizes.
Major ongoing developments aim at a minimization and rationalization of water consumption and at the development of cost-effective treatment units for advanced wastewater treatment for increasing the degree of water loop closure. Because of lower water quality demands, water is often reused in washing and desizing processes.
Anaerobic wastewater treatment is the biological treatment of wastewater without the use of air or elemental oxygen. The organic pollutants are converted by anaerobic microorganisms to a gas containing methane and carbon dioxide, known as "biogas". Anaerobic wastewater treatment is becoming more and more interesting for the treatment of industrial wastewaters.
WASTE WATER TREATMENT BY MEMBRANE FILTERATION
Membrane separations are evolving as a solution to the many problems a mill may be experiencing. Membranes can provide a solution in such areas as: color removal, BOD reduction, salt reduction and reuse, PVA recovery, and latex recovery. Membrane technology is unique in that it can provide a return on investment as a solution to pollution abatement. Membrane solutions are generally in keeping with the Clinton/Gore philosophy of pollution prevention is better than pollution treatment. Capital investment is competitive with conventional end of pipe treatment because membranes have become more of a commodity, and because a point source strategy can be employed. In many cases, valuable products can be reclaimed and reused, adding to an overall cost reduction.
Micro filteration is a low pressure (10-100 psig) process for separating larger size solutes from aqueous solutions by means of a semi-permeable membrane. This process is carried out by having a process solution flow along a membrane surface under pressure. Retained solutes (such as particulate matter) leave with the flowing process stream and do not accumulate on the membrane surface.
UF operates based upon a "sieving" mechanism that rejects constituents based upon size. The membrane pores are generally too large to effect desalting, so all ionic salts readily pass through the membrane. Ultrafiltration is basically used to remove large organics, colloidal silica and microbiological constituents.Ultrafiltration is also used to separate polymers from salts and low molecular weight materials, with pores of 0.01 to 0.1 micron. Turbidity is sharply reduced by 99%. Polymers are retained for reduced TOC, BOD and COD. A wide range of molecular weight cutoffs are available from 1000 to 500,000 daltons.
Nanofiltration is a form of filtration that uses membranes to preferentially separate different fluids or ions. Nanofiltration is not as fine a filtration process as reverse osmosis, but it also does not require the same energy to perform the separation. Nanofiltration also uses a membrane that is partially permeable to perform the separation, but the membrane's pores are typically much larger than the membrane pores that are used in reverse osmosis.
Simply stated, RO is a membrane process, which removes both organic molecules and salt ions from a solution (typically water). The membrane sieves organic molecules and repels salt ions while passing pure water through the micropores in its surface.
The permeate stream contains the water which passes through the membrane and is purified. The concentrate stream contains the water, salt ions, and organic molecules that do not pass through the membrane; the concentrate is typically plumbed to drain.
Evaporation is widely used to concentrate water from textile units.
Dew Falling Film Evaporator contains long tubes which are surrounded by steam jackets. The solution enters and gains velocity as it flows downward. This gain in velocity is attributed to the vapor being evolved against the heating medium, which flows downward as well. This evaporator is usually applied to highly viscous solutions so it is frequently used in the water , textile, pharmaceuticals ,chemical, food, and fermentation industry.
Dew single-stage evaporators, these evaporators can be made of up to seven evaporator stages or effects. The energy consumption for single-effect evaporators is very high and makes up most of the cost for an evaporation system. Adding one evaporator to the original decreases the energy consumption to 50% of the original amount. Adding another effect reduces it to 33% and so on. A heat saving % equation can be used to estimate how much one will save by adding a certain amount of effects.
Dew flash evaporators process of evaporation occurs when a saturated liquid stream undergoes a reduction in pressure by passing through a throttling valve or other throttling device. This process is one of the simplest unit operations. If the saturated liquid is a single-component liquid, a part of the liquid immediately "flashes" into vapor.