A steady equalization tank feed and a compact biofilm reactor can strip 80–90% of organics from highly colored textile wastewater—while an anaerobic pre‑treatment slashes aeration needs for very high‑strength streams.
Textile mills rarely send a gentle trickle to their wastewater plants. Batch dye baths and rinses hit in surges, with wild swings in flow, pH, and pollutant load. That variability makes an equalization (EQ) basin non‑negotiable to “dampen” peaks and deliver a steady feed to biology. Design guidance calls for up to 24 hours of detention—or a tank sized to one day’s flow—before the biological stage (nepis.epa.gov), with one example citing 920 m³/d for a mid‑size plant (www.mdpi.com).
In practice, 6–12 hours is common, but heavy fluctuations (intermittent dumps) may demand longer storage. The EQ tank should be well‑mixed and aerated to prevent stratification or anaerobiosis (nepis.epa.gov). Aeration/mixing also helps strip H₂S or ammonia surges and enables neutralization—dye baths are often strongly alkali or acid—so pH control should be built in; operators typically meter acid/alkali via a dosing pump at this stage.
How extreme can raw textile effluent be? In one Indonesian batik case, the influent was strongly alkaline (pH≈8.7) with BOD about 715 mg/L and COD about 3,769 mg/L; the stream was first equalized and neutralized before biological treatment (repository.ub.ac.id). In short, an EQ basin sized for daily flow and roughly 6–24 hours hydraulic retention time (HRT, the average time water remains in a tank) with pH control smooths shocks and improves downstream performance (nepis.epa.gov) (nepis.epa.gov).
Aerobic stage: CAS versus MBBR
After equalization, the workhorse is an aerobic reactor that oxidizes organic load—measured by chemical oxygen demand (COD) and biochemical oxygen demand (BOD)—and removes soluble organics. Conventional activated sludge (CAS, a suspended‑growth, aerated basin) is the incumbent, while moving‑bed biofilm reactors (MBBR, plastic carriers that grow attached biofilm) are increasingly favored for high‑strength loads or limited footprints. Both routinely hit ~80–90% organic removal on typical textile effluent, with MBBR often running higher loadings in a smaller volume. See activated sludge and moving‑bed biofilm reactors for process overviews.
In a pilot with influent COD around 2,000 mg/L, a CAS plant with 2‑day HRT achieved 83% COD removal (effluent roughly 350 mg/L) (www.mdpi.com). An MBBR at 30% carrier fill and 1‑day HRT removed 82% COD (down to about 179 mg/L) at double the organic loading rate (OLR, kgCOD per m³ per day) of 2.0 vs 1.0 kgCOD/m³·d (www.mdpi.com). In other tests, MBBR removed 86% COD and about 50% color from real dye effluent (www.mdpi.com). A reasonable single‑stage aerobic target is final COD of ~100–200 mg/L (see compliance below).
Capex, opex and sludge production
MBBR’s edge is compactness and cost. In the cited comparison, it halved the HRT relative to CAS for similar COD removal (www.mdpi.com) and landed at about 30% of CAS capex—a ~68% savings driven by smaller tanks and no clarifier (www.mdpi.com). Operating costs dropped too: the MBBR consumed roughly half the electricity of the CAS plant (blower demand scales with aeration time) and used far less decolorant (www.mdpi.com), with the analysis showing ~0.55 € /m³ for CAS (mostly decolorant) versus ~0.26 € /m³ in electricity for MBBR (www.mdpi.com). MBBR also generates less waste sludge (attached biofilm) and rides out load swings more stably.
CAS remains conceptually simple and may win where land is abundant, but it needs longer basins, produces more sludge, and typically relies on a downstream clarifier. Color removal is generally weak: in the cited study, CAS removed ~55% of residual color versus ~61% for MBBR under similar conditions (www.mdpi.com), and both often need tertiary polishing (adsorbents or oxidation) to meet limits. To pass a 200 Pt‑Co color standard, the study dosed an external decolorizer; CAS needed 200 mg/L while MBBR needed 100 mg/L (www.mdpi.com).
Performance envelope and solids control
For influent COD around 1,500–2,000 mg/L, a single‑stage aerobic reactor typically drives final COD to ~100–200 mg/L, with BOD knocked down by ~80–90%. Total suspended solids (TSS) are largely removed by settling; one study found ~66% TSS removal for CAS versus 78% for MBBR, while a membrane bioreactor (MBR—a biological step coupled with ultrafiltration) would remove ~99% (www.mdpi.com). See membrane bioreactors for reuse‑grade solids control. High‑strength dyes are only partially biodegraded—typically 50–80% color removal aerobically (www.mdpi.com)—so color polishing is often required.
Anaerobic pre‑treatment for very high strength
When raw COD runs in the multiple grams per liter, an anaerobic first stage can transform the economics. Upflow anaerobic sludge blanket (UASB) and expanded granular sludge bed (EGSB) reactors—both anaerobic digestion systems—excel at high organic loads. Studies report 60–90% COD removal and substantial color loss under anaerobic conditions; Wijetunga et al. (2008) achieved >90% COD removal and ~92% color removal on “real textile wastewater” in a UASB, purely anaerobically (www.researchgate.net). A bench‑scale hybrid UASB with 48‑hour HRT cut COD about 90% (BOD650→50 mg/L) on a strong textile effluent (www.researchgate.net). Typical textile UASB operation spans HRT=1–3 days and OLR up to 5–10 kgCOD/m³·d, removing on the order of 60–80% COD while producing biogas, which yields energy and reduces sludge disposal costs.
This route is particularly attractive in tropical climates (warm Indonesia), where UASB is mature for wastes like palm oil mill effluent and brewery waste. By stripping 60–80% COD anaerobically, the downstream aeration basin can shrink dramatically. For example, digesting a 3,769 mg/L COD stream to ~300–500 mg/L before aeration—a plausible outcome—cuts required air and reactor size by ~70–80%. Limitations remain: refractory dye fragments (aromatic amines) may still need aerobic oxidation or additional polishing, and start‑up can be slow. As a rule of thumb, anaerobic pre‑treatment is best when influent COD exceeds ~1–2 g/L; in the batik example (~3.8 g/L), an anaerobic step is justified. See anaerobic and aerobic digestion systems for configuration options.
Compliance targets and design choices
Indonesian standards (KLHK Permen 5/2014, amended by P.16/2019) benchmark textile effluent at BOD₅ 35–60 mg/L, COD 115–150 mg/L, TSS 30–50 mg/L, and color 200 Pt‑Co (values vary by plant size) (it.scribd.com). A well‑operated aerobic stage can often reach the lower end of BOD/COD; in the cited study, a single‑stage MBBR left COD ≈179 mg/L (www.mdpi.com), just above the 115 mg/L COD limit for large plants, so meeting the strictest tiers may require longer HRT or a polishing filter/membrane stage—an application where ultrafiltration membranes are commonly deployed.
Putting it together, a robust flexible line for highly colored textile effluent starts with a buffered EQ tank (6–24 h, mixing/neutralization) and a sufficiently sized aerobic reactor. For most cases, a fixed‑bed biofilm choice like MBBR at ~1–2 days HRT—on the order of 50–100 m³ of reactor volume per 100 m³/day of flow, adjusted by load—delivers about 80% COD removal at high influent concentrations (www.mdpi.com) (www.mdpi.com). Design the layout modularly so an anaerobic front end (e.g., UASB at 24–48 h) can be inserted if raw loads reach multi‑gram‑per‑liter; this cut can remove 60–90% COD and generate biogas (www.researchgate.net). All sizing decisions—tank volumes, carrier fill ratio, aeration energy—should be set by pilot data or engineering curves for the specific effluent character. The resulting train balances proven removals (80–90% organics, 50–90% color) with cost/energy (MBBR’s lower capex/opex and optional methane credit) and steers post‑treatment concentrations toward regulatory targets (www.mdpi.com) (it.scribd.com).
