Comparing the growth and structure of flocs from electrocoagulation and chemical coagulation

This study compares the growth and structure of flocs from chemical coagulation (CC) and electrocoagulation (EC). Flocs were more compact and larger when using CC rather than EC (on average, scattering exponents were 2.60 versus 2.31, while floc sizes were 254 versus 144 μm), and in low rather than high salt (2.51 versus 2.40, 222 versus 181 μm). They were also more compact at final pH 8.3 rather than pH 6.0 (2.53 versus 2.38). Transmission electron microscopy (TEM) revealed that CC and EC flocs were structurally distinct, suggesting that the former is better at aggregating. In low salt and at pH 8.3, flocs were more stable and likely required more collisions to form, producing denser structures. Compact flocs tend to be more resistant to breakage, allowing them to grow to larger sizes. The time required for a floc size distribution to stabilize depended strongly upon the interaction between method of dosing and final pH. The CC-pH 6.0 and EC-pH 8.3 cases stabilized fastest (averaging 8.0 and 7.8 min), as they were always in the appropriate pH range for iron precipitation. The CC-pH 8.3 cases were initially adjusted close to pH 9 to counter coagulant acidity, while the EC-pH 6.0 cases possibly suffered from localized acidity near the anode, making precipitation less successful.