434 - HCO3-, pKa 6.3 - NH4+/NH3, pKa 9.3 H2S/HS-/S- pKa 7.1 13.3 H3PO4/H2PO4-/HPO42- /PO43-pKa 2.1 7.2 12.3 - 7.5-8.0 - Mixing Intensity) Scum Degradable fraction Inert fraction Easily degradable fraction Slowly degradable fraction 8-30 23