3.2. Water absorbing polymer

Imai et al. studied the effects of adding water absorbing polymer (WAP) particles into the inoculated sludge. WAP is a resin, mainly composed of acrylic compounds and shows a complex network structure with a high specific surface for microbial attachment. Moreover, it shows a low density (wet density of 1.0 g/ml), which means that the contact between the particles and biomass is improved, when comparing to sand and other materials. Although not influencing the average granule size, the addition of WAP clearly enhanced the granulation in the lab-scale and pilot scale UASB reactors using glucose or VFA as substrates, serving as a bio-carrier to allow more biomass to attach on them. After the granules were formed, the WAP was slowly decomposed by the anaerobic bacteria, which caused the granules to split into several small fragments that grew up again forming more mature granules. Eventually, all particles were digested and the granules formed did not contain visible WAP particles anymore. Based on the experiments performed, the authors recommended a dosage of WAP of approximately

750 mg/l of reactor volume for the enhancement of granulation.