Summary
An effluent treatment plant (ETP) cleans industrial wastewater through four stages: preliminary (screening and grit removal), primary (settling and chemical treatment), secondary (biological treatment) and tertiary (polishing, disinfection and nutrient removal). Textile mill effluent carries dyes, auxiliaries, salts and organic matter that raise pH, colour, BOD and COD. Legal duties depend on location: permits under the EU Industrial Emissions Directive (revised by Directive (EU) 2024/1785), Germany’s Abwasserverordnung (Annex 38 for textiles), UK environmental permits, the US Clean Water Act effluent guidelines for textile mills and Canada’s Fisheries Act. Brands also require ZDHC wastewater compliance from suppliers.
An Effluent Treatment Plant (ETP) treats industrial wastewater by removing harmful compounds and reducing pollutant loads before discharge or reuse. Treatment combines physical, chemical and biological processes, and discharge is regulated by national law and by buyer standards such as the IFC Environmental, Health and Safety Guidelines and the ZDHC wastewater guidelines.
In textile mills, effluent usually contains organic and inorganic matter, dissolved and suspended solids, dyes, chemical auxiliaries and metals. They affect colour, alkalinity, pH, hardness, BOD and COD, which are the parameters in discharge standards.
Where does wastewater in textile mills come from?
Mainly from finishing, printing, bleaching, dyeing and washing operations.
Why treat wastewater?
- To protect water bodies and ecosystems
- To comply with discharge law and permit conditions
- To meet customer requirements, since major brands and buying houses audit ETP compliance
- To support ESG commitments and supply chain due diligence
- To protect public health and local water resources
- To enable water conservation and reuse
What are the categories of ETP processes?
By nature of treatment
- Physical: separation by physical forces (screening, sedimentation, filtration)
- Chemical: treatment by chemical reactions (coagulation, neutralisation, disinfection)
- Biological: degradation by microorganisms (activated sludge, trickling filters)
By degree of treatment
| Preliminary treatment | Primary treatment |
| Screening, scraper, grit chamber, skimming tank, aeration | Sedimentation, clarification, flocculation, equalisation tank, neutralisation tank |
| Secondary treatment | Tertiary treatment |
| Activated sludge process, trickling filter, aerated lagoons, oxidation pond | Sand filters, activated carbon filters, disinfection, ion exchange, nutrient removal |
What happens in preliminary treatment?
It removes large solids, grease and grit that could damage downstream equipment, using physical separation controlled by flow rate.
Screening
Incoming effluent passes through bar, coarse and fine screens that remove fabric pieces, plastic fragments and other large solids.
Scraper
Scrapers remove and concentrate settled solids, which reduces the sludge volume to dewater and dispose of.
Grit chamber
Long, narrow tanks slow the flow so that grit, sand and heavy particles settle according to size, specific gravity and flow velocity.
Skimming tank
Floating oils and greases rise to the surface and are removed, while clarified liquid leaves through the outlet below.
Aeration tank
Air injection helps remove residual oil, grease and volatile compounds from the stream.
What happens in primary treatment?
Sedimentation separates settleable solids, and coagulants can remove colloidal matter. Unit processes include clarification, flocculation, equalisation and neutralisation tanks.
What happens in secondary (biological) treatment?
Aerobic microorganisms break down dissolved and suspended organic matter. Key processes are the activated sludge process, trickling filters, aerated lagoons and oxidation ponds. Well-designed biological systems can remove a large share of BOD, and lagoon systems need long retention times, often several days.
What happens in tertiary treatment?
It is the final polishing stage. It removes dissolved inorganic compounds (phosphorus, nitrogen), residual organic matter, colour, odour and pathogens. Processes include sand filtration, activated carbon filtration, disinfection (UV, chlorine, ozone), ion exchange and nutrient removal. Treated water can be reused for irrigation, construction or industrial processes if local rules allow, which supports circular economy and resource efficiency goals.
Which discharge rules apply in the EU, Germany, the UK, the US and Canada?
- EU: larger textile and other industrial plants need permits under the Industrial Emissions Directive 2010/75/EU, which Directive (EU) 2024/1785 revised in 2024, and the textile BAT conclusions set emission levels. The recast Urban Wastewater Treatment Directive (EU) 2024/3019 applies to municipal plants that receive industrial discharges.
- Germany: the Abwasserverordnung (AbwV), Annex 38 on textile production and finishing, sets requirements for wastewater discharge.
- UK: the Environmental Permitting (England and Wales) Regulations 2016 and devolved equivalents govern discharge consents.
- US: the Clean Water Act effluent guidelines for the textile mills category (40 CFR Part 410) and state permits under the NPDES programme.
- Canada: the Fisheries Act prohibits the deposit of deleterious substances, and provincial permits set discharge limits.
For a formal approach, align your wastewater management with ISO 14001 and the ZDHC guidelines, especially if you supply apparel brands.
Frequently Asked Questions
What are the main stages of an ETP?
Preliminary, primary, secondary and tertiary treatment, which remove solids, settleable matter, dissolved organics and finally residual contaminants.
Why is an ETP legally required?
Because discharging untreated industrial wastewater harms water bodies and health, and permits and national law set discharge limits. Buyers also audit ETP performance.
Related reading: Best Available Techniques for the Leather Industry | Chemical Safety | What is Compliance? | ETP Daily Operations Calculations
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