Application of a Flat Surface pH Electrode to Demonstrate the Effect of Lime Addition Point Relocation on Mixing Quality and pH Decay of Lime Stabilized Biosolids

Uniform lime incorporation into sewage sludge is critical for biosolids lime stabilization processes. There is no Class B biosolids regulation for lime incorporation. The slurry method is currently used to evaluate the pH of limed biosolids, but this method homogenizes the biosolids and potentially masks poor lime mixing. In this study, a flat surface pH electrode was used in a full-scale experiment to measure the pH of lime stabilized biosolids without creating slurries. The standard deviation of fifteen pH measurements at different locations in a biosolid sample was used to assess mixing quality and the effectiveness of relocating the point at which lime was added to dewatered sludge. Well mixed limed biosolids had consistently high pHs (near 12) with low standard deviations (2.0 pH units). Poorly mixed biosolids exhibited rapid and marked pH decay, while well mixed biosolids resisted pH decay.