This may be due to the interactions between free hydroxyls groups and carboxylic groups and contents of the sewage waste.
Anionic polymers are known to make larger flocs by bridging mechanism but in this case, extent of change observed in the patterns (a) and (c), suggest that apart from secondary bonding between flocculant and solid waste, there may also be involvement of primary bonding like chelation between crystalline matter of the waste and the polymer. Though the XRD patterns do not give any specific evidence for mechanism of flocculation but they may be used as supportive evidence.