Liquid Coagulant Tops Powdered and Sludge Options for Textile Wastewater Treatment

Textile factories around the world produce some of the most chemically stubborn wastewater of any industry. Each kilogram of finished fabric can demand between 30 and 150 liters of water depending on the fiber and the dyeing method, and the resulting effluent carries a punishing cocktail of persistent dyes, organic matter and dissolved metals. A new study from researchers in Iran has now put several popular treatment strategies head to head on real, untreated textile effluent and, in doing so, has delivered a result that challenges common assumptions about which coagulant works best. The team, led by Leila Tabandeh, Keivan Arastou and Afshin Ebrahimi of Isfahan University of Medical Sciences, tested powdered and liquid forms of polyaluminum chloride alongside an unusual candidate: recycled sludge from a drinking water treatment plant. Their verdict, published in Cleaner Engineering and Technology, was scored not by laboratory removal percentages alone but by a water accounting metric that captures the full environmental burden of a discharge.

The wastewater at the heart of the study came straight from the dyeing process outlet of a textile factory in Isfahan, in central Iran, and it was as dirty as the researchers expected. Chemical oxygen demand, a measure of organic pollution, stood at 8100 milligrams per liter, roughly 135 times the permissible limit for discharge to surface water in Iran. Color intensity registered 23,480 platinum-cobalt units, turbidity 620 nephelometric turbidity units, and total suspended solids 3890 milligrams per liter. Elemental analysis added another layer of concern. The effluent carried 5500 micrograms per liter of aluminum, 5300 of iron, an extraordinary 4200 of the toxic metal thallium, along with antimony, manganese, zinc, copper and lead at levels far above natural background. In Iran only about 20 percent of industrial effluents receive proper treatment before discharge, so the stakes for finding an affordable, effective recipe are high.

Polyaluminum chloride, or PACl, has become a favored coagulant in water treatment because it works across a wide pH range, forms dense and fast-settling flocs and produces less sludge than traditional alum. It is sold in both powdered and liquid form, and the two differ in ways that matter to plant operators. The powdered grade used here contained 28.05 percent aluminum oxide, more than double the 10.48 percent of the liquid grade, but liquid PACl arrives ready to dose, skipping on-site dissolution. The researchers optimized each coagulant independently through standard jar tests on 1-liter samples, defining the optimum as the minimum dose that pushed residual turbidity to 20 NTU or below with the smallest settled sludge volume. Powdered PACl settled on 0.7 grams per liter while liquid PACl required 3.0 milliliters per liter. Every scenario also received sodium hypochlorite at 75 milliliters per liter, chosen because color removal plateaued beyond that dose, and calcium oxide at 1.0 gram per liter, enough to push pH above 10.5 so that dissolved metals could precipitate as hydroxides.

Five treatment configurations were tested in triplicate. The first used powdered PACl, the second liquid PACl, the third a blend of the two, and the fourth and fifth replaced commercial coagulant entirely, or nearly so, with clarifier sludge collected from a local drinking water plant that itself uses PACl. The idea behind the sludge experiments was elegantly circular: the sedimentation basin sludge contains residual polyaluminum chloride and amorphous aluminum hydroxides, so it might act as a free, waste-derived coagulant that simultaneously reduces chemical purchases and diverts waste from landfills. The sludge was mixed into wastewater at a ratio of one part sludge to five parts effluent, the ratio that preliminary trials showed produced maximum floc formation. For the fifth scenario, a modest half-milliliter dose of liquid PACl was added on top of the sludge to test whether virgin and recycled coagulants could work synergistically.

The results split the metals into two camps. Zinc and chromium were the consistent success stories, removed at better than 96 percent in every configuration, with residual zinc concentrations falling below 0.2 micrograms per liter in most scenarios. Thallium, iron, manganese and silicon also dropped by more than 98 percent in the liquid and mixed PACl scenarios. Barium and strontium, by contrast, proved stubbornly recalcitrant, with strontium removal never exceeding 33 percent anywhere in the study, marking these two elements as priority targets for future work. Most striking was the fate of aluminum itself. Clarifier sludge alone removed 99.8 percent of aluminum, the best figure of any scenario, and near-complete removal of lithium and barium besides. But in the fifth scenario, where a small PACl dose was layered onto the sludge, aluminum removal collapsed to zero, and the treated water actually carried more aluminum than the mixed inlet, 4800 micrograms per liter against 4567 going in.

That counterintuitive collapse points to a phenomenon known as overdosing or charge reversal. Coagulants work by neutralizing the negative charges that keep colloidal particles suspended, and beyond the optimum dose the excess positive charge can flip particle surfaces back to a stable, restabilized state, re-suspending material that had already clumped. The sludge, already laden with residual aluminum hydroxides, plus the added PACl apparently tipped the system past that threshold. The authors caution that they did not measure zeta potential, so the mechanism remains a hypothesis, but the practical lessons are unambiguous: more coagulant is not better, combining waste-derived and virgin coagulants without re-optimization can backfire, and fixed-dose recipes are inadequate for wastewater whose composition shifts from batch to batch.

On the conventional pollutants, the combined PACl scenario proved the most robust all-rounder, cutting chemical oxygen demand by 95 percent to a residual 380 milligrams per liter and turbidity by 95 percent, while removing nearly 97 percent of suspended solids. Clarifier sludge alone, despite its dazzling color removal of 99.6 percent, managed only 19.8 percent turbidity removal and 79.9 percent for chemical oxygen demand, confirming that recycled sludge cannot substitute for commercial coagulant on high-strength textile effluent. Electrical conductivity rose in every scenario because calcium oxide and PACl both add dissolved ions, and final pH ranged from 10.74 to 12.51, values that would require neutralization before any discharge or reuse.

The study’s most distinctive move was its scoring system. Rather than ranking treatments by individual removal percentages, the team calculated the grey water footprint, an indicator that translates each pollutant load into the volume of freshwater needed to dilute it to regulatory limits. The critical pollutant for the raw effluent was chemical oxygen demand, driving a footprint of 121,500 cubic meters per month at the facility’s assumed discharge of 750 cubic meters per month, meaning the factory’s pollution load would demand dilution water more than 160 times its own flow. Liquid PACl delivered the best outcome: a 92.96 percent reduction in the footprint, statistically the top performance by analysis of variance, with the combined PACl scenario statistically comparable at 90.85 percent and powdered PACl at 88.27 percent. The sludge-only scenario ranked last at 83.33 percent, despite its standout aluminum and color numbers, because its weak performance on organic matter and turbidity dominated the overall environmental burden.

The grey water footprint also exposed a hidden bottleneck that conventional single-parameter assessment would have missed. In the combined PACl scenario, chemical oxygen demand was cut so effectively that antimony, with only 49 percent removal, quietly became the limiting pollutant, its footprint climbing above the organic load’s. Optimizing one pollutant, in other words, can mask the factor that actually constrains environmental performance. The researchers argue that the metric offers an objective common currency for comparing heterogeneous treatment trains, and note that their chemically enhanced primary treatment approached the footprint reductions typically associated with secondary biological processes such as activated sludge.

For plant operators, the study lands on a concrete protocol: liquid PACl at 3.0 milliliters per liter, sodium hypochlorite at 3750 milligrams per liter of active chlorine, and calcium oxide at 1.0 gram per liter, a combination that cut the grey water footprint by 93 percent while stripping more than 99 percent of thallium and zinc. Clarifier sludge, meanwhile, earns a narrower but genuine role as a free, selective coagulant aid for specific targets such as aluminum and color, and as a landfill-diversion strategy, so long as its dose is optimized independently and its reactive aluminum content is properly characterized. The authors flag open questions, including chlorinated byproducts from hypochlorite oxidation, the economics of full-scale deployment, and the mechanistic differences between PACl formulations, but the headline conclusion stands: when environmental impact is tallied honestly across every pollutant, the liquid form of a familiar coagulant is the strongest tool yet tested for taming textile wastewater.

Subject of Research: Comparative evaluation of PACl coagulant variants and recycled clarifier sludge for treating real textile wastewater using the grey water footprint metric

Article Title: Comparative assessment of PACl variants and clarifier sludge for textile wastewater treatment: A grey water footprint approach

Article References: Tabandeh, L., Arastou, K., & Ebrahimi, A. (2026). Comparative assessment of PACl variants and clarifier sludge for textile wastewater treatment: A grey water footprint approach. Cleaner Engineering and Technology, 34, Article 101319. https://doi.org/10.1016/j.clet.2026.101319

Image Credits: AI Generated

DOI: 10.1016/j.clet.2026.101319

Keywords: textile wastewater, polyaluminum chloride, grey water footprint, coagulation-flocculation, clarifier sludge, heavy metal removal, chemical oxygen demand, sodium hypochlorite, calcium oxide, circular economy, water treatment, industrial effluent