Ammonia stripping methodologies for pre and post digested thermally hydrolysed waste activated sludge: Preliminary investigations

Cardiff Sewage Treatment Works’ digesters treat a large proportion of waste activated sludge (WAS) and currently operate at a high ammonium concentrations. As part of a study to increase the methane production at the site, the effect of ammonia removal is being investigated. Ammonia stripping was evaluated for the thermally hydrolysed WAS (THWAS) as well as for the digestate. The levels of ammonium monitored for off-line samples over three months period for the thermally hydrolysed WAS varied between 931 – 1836 mg/l and 3598 – 4612 mg/l within the digesters’ matrix (the higher levels were obtained only when sludges were stored for over one week prior to stripping). Ammonia stripping methodologies consisted of varying stripping duration, air flow rate, temperature and pH levels. Concentrations of 3534-4057 mg/L and 473-600 mg/L of ammonium were efficiently removed from the digestate and THWAS, respectively. The removal efficiency of ammonia from the digestate was 65% after 6 hrs with air stripping at 0.6 l/min, 70°C without pH adjustment, with a higher efficiency when pH was 10. The removal of ammonia from THWAS was between 50-73% depending on the stripping conditions. THWAS required at least an initial pH adjustment to 10-10.5 to allow stripping conditions. The characteristics of the sludge changed between collections, which made results difficult to compare.