Effect of dose methods of a synthetic organic polymer and PFC on floc properties in dyeing wastewater coagulation process

• Papermaking sludge was recycled to synthetize a flocculant.
• Polyferric chloride (PFC) was prepared using spent pickling liquor.
• The products from industrial wastes were used in treating dyeing wastewaters.
• Different dose methods of coagulant/flocculant were studied.
• Floc properties were comparatively investigated.

Papermaking sludge and spent pickling liquor were recycled to synthetize an organic polymer flocculant (LA) and polyferric chloride (PFC), respectively. They were used in synthetic dyeing wastewater treatment – disperse yellow (DY) and reactive blue (RB). Effect of PFC, dual-coagulant (PFC + LA) and second dose of LA after breakage period (PFC − LA) on color removal, floc regrowth and floc structure were comparatively investigated. The result showed that PFC could achieve excellent color removal, around 90% for the two dyes at optimal dosages. LA addition after PFC gave a little higher color removal efficiency but much larger flocs. Specifically, ratio values of PFC–flocs for DY and RB were approximately 1.43 and 0.59 respectively, compared with 1.75 and 0.72 for flocs formed by PFC + LA. Floc recoverability was also much higher at the usage of LA when there was a breakage period, and they were in the order of PFC − LA > PFC + LA > PFC. Additionally, comparing with those of PFC, roughly opener flocs were reformed by PFC − LA and PFC + LA after breakage because of LA bridging. For instance, TWV values (%) were 10.24, 13.24 and 16.16 respectively for PFC, PFC + LA and PFC − LA in RB treatment. LA favored charge neutralization, adsorption and bridging in the coagulation process.