Techno-economic evaluation of ozone-oxidation for sludge reduction at the full-scale. Comparison between the application to the return activated sludge (RAS) and the sludge digestion unit

Ozone oxidation was applied to a fraction (30 %) of the return activated sludge (RAS) of a full-scale Wastewater Treatment Plant for reducing the high sludge production (20 t/week). The amount of sludge sent to final disposal was decreased by 50 % and 75 % at 0.55 kgO3/h and 0.80 kgO3/h dosage, respectively. When the same ozonation system was applied to the aerobic digestion (0.06, 0.50, 0.67 kgO3/h), the reduction was 27–77 %. A thorough economic analysis was carried out considering the overall sludge quantity finally extracted from the plant in the presence and absence of ozonation, the corresponding costs of final disposal and the ozone oxidation operation costs. The net cost saving for sludge disposal was calculated to be 66.6 €/d and 109.9 €/d due to ozone in the return activated sludge at the two dosages, respectively. When ozonation was applied to the recycled sludge of the digestion unit, the net cost saving was in the range 235–655 €/d, depending on the reduction percentage achieved. The ozone oxidation showed to become economically convenient for a sludge disposal cost lower than that sustained by the Company. The results obtained prove feasibility of ozonation for sludge reduction in both locations of the WWTP: therefore, with respect to other disintegration techniques, ozonation possesses a high technical and economic advantage along with a high versatility. Furthermore, it is also shown the feasibility of lowering the applied dosages with respect to the past experiences, since effectiveness of the system is still guaranteed.