Synergy between anaerobic digestion and a post-treatment based on Thermophilic Aerobic Membrane Reactor (TAMR)

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
byWO 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/m3/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.