At present, the activated sludge process is the cheapest way to remove colloidal and soluble organic pollutants from sewage, but it produces a considerable amount of waste sludge, with a low dry solid (DS) content, rich in biodegradable organic substances. Therefore, it needs further processes to reduce its volume, by decreasing its water content, and to lower its polluting potential, due to its high content of biodegradable organic matter. Industrially, mechanical dewatering (centrifuge, filter press and belt press) increases the DS of sewage sludge up to 20-25% to decrease transport and disposal costs. Electro-osmosis could be a suitable technique that can further reduce the water content of the dewatered sludge by the application of an electric field. Preliminary tests carried out by applying an electric field, from 10 to 20 V/cm, in a lab-scale device, confirmed the possibility to increase the final dry solid content (DSf) by 10% to 15% higher than the initial content, with a relatively low energy consumption if compared to thermal treatments. Here, we investigated the characteristics and properties of sludge that affect pressure-driven electro-dewatering. Sludge samples were taken from four different WWTPs around the city of Milan (Italy). First of all, we characterized the sludge samples by measuring capillary suction time (CST), time to filter (TTF) and the zeta potential of the filtered liquid fraction. Then, we measured the final solids percentage and energy consumption in a lab-scale device, under the application of an applied voltage of 15 V, at 3-bar pressure and tried to find a relation between the characteristics of sludge and DSf.
Conditioned and thickened sludge samples reached ΔDS between 8-17%. Results on mechanically dewatered sludge samples have shown an increase of DS content up to values around 14%, with a total primary energy consumption lower than the primary energy needed for thermal drying. This highlights the efficiency of electro-dewatering as a post dewatering treatment. Electro-dewatered sludge may self-sustain combustion at 850°C without any preliminary thermal drying. CST, TTF and zeta potential are not suitable predictors of the efficiency of the electro-dewatering process. A lab-scale test is necessary to assess the DSf that electro-dewatering can achieve....
Session: L14 - Filtration Enhancement by Electric and Magnetic Fields
Day: 13 October 2016
Time: 09:00 - 10:15 h