Capacity Upgrade of Municipal Sludge Digesters - The Hamilton Experience

The Hamilton City Council (HCC) initiated in 2012 the upgrade of its Pukete treatment plant digesters for treatment of increased biosolids loads (> 60 % load increase expected in next decade). Capacity upgrade was achieved in existing digesters and without plant operation interruption. Improved mixing in the methane digesters and recuperative thickening increased the digester liquor solids content from 1.5 % TS to 3 % TS, uncoupled hydraulic and solids residence time and doubled solids residence time and treatment capacity. High digester process stability was observed indicated by low volatile fatty acids (VFA; typically less than 30 mg VFA/L), high pH (pH > 7), high alkalinity (> 2500 mg/L) and high solids content in the mixed liquor (2.9 % – 3.2 % TS). The constructed volume of the upgraded digesters was fully available for biosolids treatment (Lithium tracer test). The digester process robustness was significantly improved. At completion, the digesters could treat 200 % of the current sludge flow and 180 % of the current biosolids load at a fraction of the CAPEX costs that would have been incurred for construction of equivalent new sludge digesters. HCC was able to defer capital costs for construction of new digester tanks until 2020.