Effect of pH on polyacrylamide degradation and methane production

Polyacrylamide (PAM), widely used as a flocculant in wastewater treatment process. The existance of PAM in sludge discharged a remarkable amount of its toxic monomer acrylamide (AM) in the atmosphere and impose serious threats for human beings and the environment. The aim of this work was to investigate the PAM degradation and improvement of methane production by mesophilic AD under optimum pH value. Sludge used in this study was collected from the secondary sedimentation tank of (G) Municipal Wastewater Treatment Plant (China). The sludge was firstly concentrated by settling for 24 h at 4 0C. The laboratory analysis was done by 1L glass bottles that were used as fermentation reactors in triplicate at different pH (e.g., from 5.0 to 11.0) for 15-days batch tests. the reactors were fed with 800 mL of PAM-added sludge.The reactors were purged with nitrogen gas for 3 min to displace oxygen, and inserted with rubber plugs and gas bags were operated at 35 ± 1oC in rotary shakers at 165RPM, Main Characteristics of the experimental sludge were pH 6.54, TS(g/L) 23.35 ±0.27, VS(g/L) 17.84 ±0.19, TSS(g/L) 21.72±0.20, VSS(g/L) 15.88±0.14. Polyacrylamide (PAM) degradation during mesophilic AD at pH 11 was observed to be rapid. 74.3% PAM degradation occurred at pH 11 during 15 days of mesophilic AD. Fig.1. showed the results for methane production. It can be seen from fig.1, that methane yield of 135.6 -141.7 ml/g VSS was achieved at pH 11 whereas it was only 80.3 ml/gVSS in control. Methane degradation of 98.7 and 118.4 ml/g VSS at pH 5 and 7 was achieved .
The methane yield was increased at pH 11 while at pH 5 and 7 it was declined. Though results for methane production at pH 7 were higher than at pH 5. The pH 11 stood most suitable for higher methane yield and PAM degradation. PAM degradation at pH 11 during mesophilic AD was 74.3% with a higher methane yield of 135.6-14.7ml/gVSS.