Anaerobic treatability of liquid residue from wet oxidation of sewage sludge.

Wet Oxidation (WO) of sewage sludge is a chemical oxidation of sludge at high temperatures and pressures by means of an oxygen-containing gas. The liquid stream originated by WO is easily biodegradable, and therefore, the recirculation to the biological Waste Water Treatment Plant (WWTP) may be a feasible solution. However, the WO effluent has a residual organic and nitrogen content so that its treatment may be required when the receiving WWTP has no surplus treatment capacity left. The aim of this research was the assessment of the anaerobic treatability of the WO liquid residue, in order to reduce the organic load to be recirculated to the WWTP, simultaneously promoting energy recovery. For this purpose, the liquid residue obtained during full scale WO tests on two different types of sludge was submitted to anaerobic digestion in a continuous flow pilot reactor (V = 5 L). Furthermore, batch tests were carried out in order to evaluate possible inhibition factors. Experimental results showed that, after the start-up/acclimation period (~130 days), Chemical Oxygen Demand (COD) removal efficiency was stably around 60% for about 120 days, despite the change in operating conditions. In the last phase of the experimental activity, COD removal reached 70% under the following treatment conditions: Hydraulic Retention Time (HRT) = 20 days, Volumetric Organic Loading Rate (VOLR) = 0.868 kg COD/m(3)/day, Organic Loading Rate per Volatile Suspended Solids (OLRvss) = 0.078 kg COD/kg VSS/day, temperature (T) = 36.5 °C, pH = 8. Energy balance calculation demonstrated anaerobic treatment sustainability.

Wet oxidation of sewage sludge: full-scale experience and process modeling.

Nowadays, sewage sludge management represents one of the most important issues in wastewater treatment. Within the European project "ROUTES," wet oxidation (WO) was proposed for sludge minimization. Four different types of sludge were treated in an industrial WO plant: (1) municipal primary sludge (chemical oxygen demand COD: 73.0 g/L; volatile suspended solid VSS: 44.1 g/L); (2) secondary sludge from an industrial wastewater treatment plant (WWTP) without primary sedimentation (COD: 71.8 g/L; VSS: 34.2 g/L); (3) secondary sludge from a mixed municipal and industrial WWTP without primary sedimentation (COD: 61.9 g/L; VSS: 38.7 g/L); and (4) mixed primary (70%) and secondary (30%) municipal sludge (COD: 81.2 g/L; VSS: 40.6 g/L). The effect of process parameters (temperature, reaction time, oxygen dosage) on WO performance was investigated. Depending on operating conditions, VSS and COD removal efficiency varied in the range 80-97% and 43-71%, respectively. A correlation between process efficiency and the initial VSS/TSS (total suspended solids) ratio was highlighted. Furthermore, a mathematical model of WO process for simulating VSS and COD profiles was developed.