Stabilisation of dewatered domestic sewage sludge by lime addition as raw material for the cement industry: Understanding process and reactor performance

• The sludge stabilisation with lime process was studied at a pre-industrial scale.
• The high alkalinity provided by lime promotes the reduction of the organic matter.
• Mass and energy balances were used to improve the performance of the process.
• Reaction enthalpy associated with the organic matter mineralisation was determined.
• Biological process was discarded by taking into account the monitoring data.

This paper describes the performance and feasibility of the domestic dewatered sewage sludge (DDSS) stabilised with lime as raw material for the cement industry. The stabilisation process uses a screw-based plug reactor to provide mixing conditions suitable to reduce the water content of the sludge to level of 10%a w/w by taking advantage of the exothermic reaction of the hydration of lime to form portlandite, which favours the evaporation of water. The high alkalinity provided by the added lime promotes the reduction of the organic matter content through a transformation of the complex organic structures of the biomass present in the DDSS to simple organic structures and partly to inorganic carbon in a form of the (CaCO3(s)) via a series of exothermic (neutralisation and saponification) and endothermic chemical processes (hydrolysis).The sludge stabilisation with lime process was studied at a pre-industrial scale using a pilot plant with a 6 t DDSS/h treatment capacity. The dewatered sludge and the stabilised product streams were properly characterised to quantify the mass and energy balances. Both balances were used to improve the performance of the process and to quantify the reaction enthalpy (−7914.7 ± 172 kJ/kg DDSS) associated with the mineralisation of the organic matter, using a numerical algorithm. The influence of any biological process was discarded by taking into account the microbiological monitoring data. The low residence time in the reactor (less than 40 min) and the high alkalinity of the reaction mixture provided by the produced Ca(OH)2(s) did not favour substantial biological activity which also requires hydroxyl ions and water, however, CaO(s) will generate hydroxyl ions only after being hydrated.