Foam formation in the excess sludge treatment facilities of biological wastewater treatment plants (WWTPs) may represent a critical issue as it could lead to several operative problems and reduce the overall plant performance. This trouble also affects a novel technology recently proposed for sludge minimization, the thermophilic membrane reactor (TMR), operating with alternate aeration/non-aeration cycles. This technology, which has proven to be extremely resilient and suitable for treating industrial wastewater of different nature, demonstrated a high potential also as a solution for integrating existing WWTPs, aiming at the "zero sludge production." In this work, an experimental study was conducted with a TMR pilot plant (fed daily with thickened sewage sludge) by adjusting the duration of aeration/non-aeration alternate cycles. Extracellular polymeric substance (EPS) concentration (and its soluble and bound fractions) has been monitored along with foaming power indices. The results highlight that foaming can be correlated to the presence of soluble protein fraction of EPS. Moreover, EPS production seems to be reduced by increasing the duration of the non-aeration cycles: optimal operating conditions resulted 2 h of aeration followed by 6 h of non-aeration. These conditions allow to obtain an EPS concentration of 500 mg L with respect to 2300 mg L measured at the beginning of experimental work.

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This research was carried out on a full-scale pure oxygen thermophilic plant, operated and monitored throughout a period of 11 years. The plant treats 60,000 t y⁻¹ (year 2013) of high-strength industrial wastewaters deriving mainly from pharmaceuticals and detergents production and landfill leachate. Three different plant configurations were consecutively adopted: (1) biological reactor + final clarifier and sludge recirculation (2002-2005); (2) biological reactor + ultrafiltration: membrane biological reactor (MBR) (2006); and (3) MBR + nanofiltration (since 2007). Progressive plant upgrading yielded a performance improvement chemical oxygen demand (COD) removal efficiency was enhanced by 17% and 12% after the first and second plant modification, respectively. Moreover, COD abatement efficiency exhibited a greater stability, notwithstanding high variability of the influent load. In addition, the following relevant outcomes appeared from the plant monitoring (present configuration): up to 96% removal of nitrate and nitrite, due to denitrification; low-specific biomass production (0.092 kgVSS kgCODremoved⁻¹), and biological treatability of residual COD under mesophilic conditions (BOD5/COD ratio = 0.25-0.50), thus showing the complementarity of the two biological processes.

Wet Oxidation (WO) of sewage sludge is a chemical\ud oxidation of sludge at high temperatures and pressures by\ud means of an oxygen-containing gas. The liquid stream originated\ud byWO is easily biodegradable, and therefore, the recirculation\ud to the biological Waste Water Treatment Plant\ud (WWTP) may be a feasible solution. However, the WO effluent\ud has a residual organic and nitrogen content so that its\ud treatment may be required when the receiving WWTP has\ud no surplus treatment capacity left. The aim of this research\ud was the assessment of the anaerobic treatability of the WO\ud liquid residue, in order to reduce the organic load to be\ud recirculated to the WWTP, simultaneously promoting energy\ud recovery. For this purpose, the liquid residue obtained during\ud full scale WO tests on two different types of sludge was\ud submitted to anaerobic digestion in a continuous flow pilot\ud reactor (V=5 L). Furthermore, batch tests were carried out in\ud order to evaluate possible inhibition factors. Experimental\ud results showed that, after the start-up/acclimation period\ud (~130 days), Chemical Oxygen Demand (COD) removal\ud efficiency was stably around 60% for about 120 days, despite\ud the change in operating conditions. In the last phase of the\ud experimental activity, COD removal reached 70 % under the\ud following treatment conditions: Hydraulic Retention Time\ud (HRT)=20 days, Volumetric Organic Loading Rate\ud (VOLR)=0.868 kg COD/m3/day, Organic Loading Rate per\ud Volatile Suspended Solids (OLRvss)=0.078 kg COD/kg VSS/\ud day, temperature (T)=36.5 °C, pH=8. Energy balance calculation\ud demonstrated anaerobic treatment sustainability

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