Rheological parameters are very important in sewage sludge management, not only as designing parameters in transporting, storing, landfilling and spreading operations, but also as controlling ones in many treatments, such as stabilisation and dewatering. To study how different treatments affect sludge rheological behaviour, research has been undertaken at CNR-Istituto di Ricerca Sulle Acque (CNR-IRSA), and the preliminary results are discussed in this paper. Sludge samples taken from three municipal wastewater treatment plants at different steps of treatment have been tested by a rotational viscometer and data interpreted through both Bingham plastic and Ostwald pseudoplastic models. Coefficients of those models have been then correlated to solids concentration. Results confirmed that solids concentration is the main parameter affecting sludge rheology and evidenced that a single parameter is not sufficient to represent the rheological behaviour, thus indicating that other parameters should be introduced and characteristics considered for a better understanding of the phenomenon.

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Laboratory investigations have been carried out to study the influence of process failure modes on organics emission during the incineration of hazardous sludge. The thermal, temporal and fuel-oxidant mixing failure modes were tested. They were simulated by holding the first combustion temperature at 400°C and varying the after-burning one between 600 and 1100°C, the after-burning residence time between 1 and 3 s and the excess air between 30 and 160%. Results showed that the thermal failure mode is the most important factor controlling the number and concentration of emitted organics leading to the formation of over 70 compounds at after-burning temperature of 600°C. At higher after-burning temperatures emissions are controlled by the fuel-oxidant mixing failure mode and, only when the after-burning is 800°C and the oxygen is 160% over the stoichiometric value, by the temporal failure mode. Based on results obtained some suggestions for reducing emissions of toxic organics in full-scale incinerators are given.

In this report the effect of process failure modes on aromatic and polynuclear aromatic compounds during the lab-scale incineration of municipal sludge is examined. The thermal, temporal, and fueloxidant mixing failure modes were tested. They were respectively simulated by holding the primary thermal exposure at 400°C and varying the afterburning between 500 and 1100°C, the afterburning residence time between 1 and 3 s, and the combustion oxygen using almost stoichiometric (30%) and large excess (160%). Results indicate that the thermal failure mode is the most important factor controlling the number of compounds emitted at an afterburning temperature below 700°C. At higher temperatures the number and amounts of aromatic and polynuclear aromatic compounds emitted are controlled by the fuel-oxidant mixing failure. It was found that, when fuel oxidant mixing failure was active, the formation of 20 aromatic and polynuclear aromatic compounds was observed even at an afterburning temperature as high as 1100°C while, when it was not active, at 900°C no emission of organics was detected.

To demonstrate that sewage sludge incineration could be a safe operation if well conducted and that hazardous compounds could also be disposed of without problems in sludge incinerators plants, a research program has been set up by the Italian Water Research Institute on a demonstrative scale plant where a completely circulating fluidised bed furnace and a rotary kiln furnace can operate alternatively. In this paper results of tests performed by a fluidised bed furnace on sewage sludge spiked with highly chlorinated compounds under different operating conditions are presented. A deep investigation on micropollutants formation has been made at different sampling points: before and after bag house filter, and at the chimney. The experimental data show that concentrations of PCDDs+PCDFs (TE) and of PAHs do not strictly depend on the operation of the afterburning chamber and that a complete compliance with the European and Italian standards on the emissions were always achieved even in the most critical conditions investigated.

Chemical conditioning and dewatering models are utilized to determine optimal conditions in order to minimize total cost (amortization, operating and disposal) of sewage sludge filtration under pressure. The results of a parametric study are discussed in this paper. The influence on filtration time and chemicals dosage of operation variables, such as feed sludge solids concentration, specific resistance to filtration, unit disposal cost and total operating time, is investigated. Total costs range from 6,000 to 20,000 Lit/hd y and show high incidence of disposal and amortization costs (55-82%). The latter is influenced to a large extent by the total operating time, in that longer machine operation allows a great reduction in equipment size to be obtained. Results put in evidence the great importance of input sludge concentration and chemical conditioning, to obtain a sludge of better filterability.

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