The effectiveness of coagulation for water reclamation from a wastewater treatment plant that has a long hydraulic and sludge retention times: A case study

• Effluent of underutilized WWTPs has much higher quality than expected.
• PAC performed better at removing SS, AlCl3 performed better at removing DOM.
• High molecular recalcitrant BAP residues in the EfOM came from bio-activities.
• The protein dominated BAP were still the main component after coagulation.

Coagulation is a feasible process to reclaim municipal wastewater, however, the role of coagulation in removing effluent organic matter (EfOM) from underutilized wastewater treatment plants (WWTPs) has not been fully explored. We identified the characteristics of the EfOM from a typical underutilized WWTP (i.e., the ratio of actual capacity to design capacity is 50%–70%), and investigated the performance of coagulation on suspended solids (SS) and dissolved organic matter (DOM) removal. The effluent could even satisfy the highest national standard of China (Class 1 A) for WWTP effluent, as evaluated by the traditional parameters such as SS and chemical oxygen demand (COD). However, the DOM in the EfOM we studied contained considerable biomass-associated products (BAPs), which were dominated by proteins with a molecular weight of approximately 150 kDa. In addition, protein also dominated the DOM after coagulation. Fulvic acid and humic-like acid organics were poorly removed by either AlCl3 or polyaluminum chloride (PAC) coagulation, even with a dosage as high as 24 mg Al L−1. Biodegradability was very poor, as the ratio of biological oxygen demand (BOD5) to COD was less than 0.17. After coagulation the typical BAPs, protein and polysaccharide, remained as high as 1.6 mg L−1 and 1.2 mg L−1 respectively. In this study we found coagulation was ineffective for removal of recalcitrant BAPs.

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