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Textile ETP and WTP: Treating Process Water and Effluent

What WTP and ETP mean

A water treatment plant (WTP) and an effluent treatment plant (ETP) are not the same thing, although the two names are often mixed up, including in the original title of this post. A WTP treats incoming water — from a river, a borehole or a municipal supply — so that it is fit for the process. An ETP treats the used effluent from wet processing before it is discharged to a sewer or a receiving water, or polished for reuse.

Both plants matter in a dyehouse or finishing mill. The process itself, described in textile manufacturing and in textile finishing, is a wet sequence of scouring, bleaching, dyeing, printing and finishing. It needs a reliable supply of water, and it produces a liquor that is typically alkaline, coloured and high in organic load. That liquor cannot simply be let go. The ETP is the last process unit on the site, and it has to be designed and run to the discharge or reuse standard that applies to the mill. Those standards are set by the competent authority and vary with the country and with the receiving water; they are not a single worldwide number.

Schematic showing where a water treatment plant (WTP) and an effluent treatment plant (ETP) sit in a textile mill, and a typical ETP route: screening, equalisation, pH correction, coagulation and flocculation, primary clarification, biological treatment, secondary clarification, optional tertiary treatment, sludge handling, and discharge or reuse.

The figure is a schematic drawn for this article. The upper row shows where a WTP and an ETP sit in the mill. The lower rows are a typical biological textile ETP: screening, equalisation, pH correction, coagulation and flocculation, primary and secondary clarification, optional tertiary treatment, and sludge handling. It is not a plant design. It replaces a set of unlabelled photographs and generic textbook diagrams that were on the original post.

What textile effluent contains

The composition of the effluent follows the processes that produced it. Desizing and scouring contribute starch or other sizes, oils, waxes and detergents, and therefore a high biochemical oxygen demand (BOD) and chemical oxygen demand (COD). Bleaching may leave residual oxidant. Dyeing and printing contribute colour and auxiliaries. Reactive dyeing of cotton also contributes a large salt load and hydrolysed dye that is no longer able to fix. Vat dyes, disperse dyes and acid dyes each leave a different residue. Garment washing of denim, described in the stone washing and enzyme washing articles, can add pumice grit, unfixed indigo and residual enzyme.

Taken together, textile process wastewater is typically alkaline and carries a high organic load, together with suspended solids, residual oils and, often, poorly biodegradable surfactants. Colour is a particular difficulty: many dyes are only partly removed by a conventional biological plant, so the ETP may need a dedicated colour-removal step. The first rule is still prevention: substitute more treatable auxiliaries where they exist, recover and reuse baths where it is practical, and keep high-strength or unusual streams (for example residual peroxide, or a concentrated colour dump) from shocking the main plant.

The water treatment plant

Incoming water is treated so that it will not spoil dyeing or finishing. The exact train depends on the source. A typical WTP may include screening, coagulation and clarification or filtration to remove turbidity, softening or other hardness control, and, where needed, adsorption or membrane treatment and disinfection. The aim is a stable supply: the right pH, low suspended solids, controlled hardness, and the absence of chlorine, iron or manganese at levels that would interfere with dyes. A WTP does not make the outgoing effluent legal, and an ETP is not a substitute for treating the incoming water.

A typical textile ETP

Most mill ETPs are a sequence of physical, chemical and biological stages, followed by sludge handling.

Screening comes first. Lint, fibres, threads and other coarse solids would otherwise clog pumps and occupy sludge volume, so they are taken out on a screen or a similar device.

Equalisation is then essential. A dyehouse does not discharge a steady liquor: flows, pH, temperature and colour change with the shift, the machine and the recipe. An equalisation tank, mixed and often with a long holding time — in textile mills commonly in the region of 12–24 hours — evens out those swings so that the rest of the plant can be dosed and aerated on a reasonably stable feed. Without it, a sudden alkaline or coloured dump can collapse a biological stage or produce a non-compliant discharge, even if the daily average load looks modest.

pH correction follows, or is combined with equalisation. Coagulants, flocculants and biological treatment all have a working pH range. The liquor is brought into that range before the chemical stage.

Coagulation and flocculation are the usual chemical steps for colour and fine solids. A coagulant is added, with rapid mixing, to destabilise the charge on particles and on some dissolved colour; a flocculant is then used, with gentler mixing, so that the particles grow into a settleable floc. The floc is removed in a primary clarifier. This stage can take out a large part of the visible colour and of the suspended solids, and it produces a chemical sludge that has to be handled separately from the later biological sludge.

Biological treatment, tertiary steps and sludge

The settled liquor then goes to biological treatment, most often an aerobic, activated-sludge or extended-aeration system. Micro-organisms consume biodegradable organic matter, reducing BOD and part of the COD. The mixed liquor is settled in a secondary clarifier. Settled biomass is returned to the aeration tank to keep the population up, and surplus sludge is wasted to the sludge stream. Aeration has to be matched to the load: too little, and treatment fails; too much, and energy is wasted and the floc may not settle. Toxic shocks, residual oxidant, extreme pH and sudden colour dumps are the usual reasons a biological stage goes off.

Biological treatment does not remove everything. Residual colour, salts and some poorly degradable organics may remain. When the discharge standard, or a reuse scheme, requires it, a tertiary step is added: sand or multimedia filtration, activated carbon, a further chemical colour-removal dose, or a membrane such as ultrafiltration or reverse osmosis. Membrane plants can produce a water clean enough to send back to the process, but they also produce a concentrated reject that still has to be managed. Reuse is a process design, not simply a longer ETP.

Sludge is a product of the plant, not an afterthought. Primary (chemical) sludge and surplus biological sludge are thickened and dewatered, often on a filter press or a similar device, and then sent to a licensed outlet. An ETP that meets its liquid discharge standard but leaves sludge in open pits has not finished the job.

Running the plant and checking the result

An ETP has to be operated, not only built. Daily checks typically include flow, pH, dissolved oxygen in the aeration tank, sludge volume and the appearance of the clarifiers, together with regular laboratory measurements of BOD or COD, suspended solids and colour, and any other parameters required by the permit (for example residual chlorine, oils, or metals if they are used on the site). Sudden changes in shade in the colour kitchen should be treated as a process event that the ETP needs to know about, not as a surprise at the outfall.

The original version of this post listed a wide range of generic hazardous-waste treatments, including thermal and disposal routes that are not the normal design of a textile ETP. A mill plant is almost always a wet physical–chemical and biological sequence of the kind described above, sized for the site and for the legal standard it has to meet.

FAQ

What is the difference between a WTP and an ETP?

A WTP (water treatment plant) treats incoming water so that it is fit for process use. An ETP (effluent treatment plant) treats the used effluent before discharge or reuse. A mill may have both. They are not interchangeable names for the same unit.

What does WTP stand for?

Water treatment plant. In a textile mill it is the plant that prepares incoming water. It should not be confused with an ETP, which treats outgoing effluent. The same letters are sometimes used more loosely, which is why the two are worth keeping distinct.

Why is equalisation so important?

Because dyehouse effluent is not a steady stream. Equalisation evens out flow, pH, temperature and pollutant load so that coagulation and biological treatment can be controlled. Without it, a short, strong discharge can take the plant out of its working range.

Why is colour often still visible after biological treatment?

Many textile dyes are only partly biodegradable. A well-run biological stage will reduce the organic load, but residual colour may still need coagulation, adsorption or a membrane step, especially if the water is to be reused or if the discharge standard is tight on shade.

Can treated effluent be sent back to the dyehouse?

Sometimes, after a tertiary or polishing stage that brings the water within the process specification. Hardness, colour, residual chemicals and dissolved salts all have to be acceptable to the dyeing or finishing recipe. The concentrated reject from a membrane plant still needs an outlet.

What happens to the sludge?

It is thickened, dewatered and sent to a licensed disposal or recovery route. Primary chemical sludge and biological sludge may be handled together or separately, but they should not be left untreated on the site.

References

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