Polyhexamethylene biguanidine used as a new type sewage sludge conditioning agent: Effect on sludge dewaterability and mechanism.

Dewatering is the basic procedure of sludge treatment and disposal, and environmentally friendly and efficient sludge conditioning methods are urgently needed. Polyhexamethylene biguanide (PHMB), a broad-spectrum germicide used in daily life and medicine, was proposed as a sludge conditioning reagent in this paper, and its effect on waste activated sludge (WAS) dewaterability was studied for the first time. Results showed that PHMB can improve sludge dewatering performance, and capillary suction time (CST) and water content (Wc) of dewatered sludge cake was reduced by 78.11% and 13.37% with 100 mg PHMB/g dry sludge (DS). Further investigation revealed that the sludge properties changed pronouncedly after PHMB conditioning, the bound water content decreased from 1.58 g/g DS to 1.29 g/g DS, the particle size (D50) increased from 34.3 μm to 39.2 μm, the zeta potential increased from -20.96 mV to -3.36 mV, and the flowability increased whilst the viscosity decreased. When the dose of PHMB was lower than 50 mg/g DS, it mainly reacted with extracellular polymeric substance (EPS), resulting in a decrease in its content, which was also manifested by the decrease of molecular weights. However, when the dose reached 100 mg/g DS, PHMB would disrupt the cytomembranes of microorganisms and release cellular contents, reflected by a corresponding growth of EPS contents and the intensity of Fourier transform infrared (FTIR) spectrum. And the scanning electron microscope (SEM) images showed that PHMB conditioning made cracks and holes on sludge microstructures. The key mechanism of PHMB improving sludge dewaterability was inferred as "organic molecules disrupting" and "sludge particles flocculating". These findings demonstrate that PHMB is promising to be a novel, effective, and environmentally friendly sludge conditioning reagent.