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Energy Balance in a Novel Approach to Sludge Processing

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In the coming years, large European wastewater treatment plants (WWTPs) will need to dispose of sludge by transforming it into inert material by thermal processes. In fact, the present European landfill directive 99/31 has restricted this disposal route which was practically abandoned by countries, where the implementation of the directive criteria in national legislations was very stringent, either imposing a limit of the organic carbon to 4 - 5 % in the wastes to be disposed (Germany) or including a leachate test with the very stringent limit of dissolved organic carbon of 80 mg/L (Italy). Moreover, guidelines for agricultural utilization, will become progressively more stringent due to increasing health concerns about the widespread diffusion of pathogenic and organic micropollutants in the environment (European Commission, 2000). Regardless, agricultural utilization involving large amounts of sludge to be spread on land does not seem feasible for the following reasons:

need of large extensions of fields and therefore long distances to be covered from WWTPs to the site of spreading;

need of large storage volume required when sludge cannot be used (winter periods and when the fields are flooded);

large WWTPs are often polluted by non-controlled industrial discharges that might hinder agricultural utilization of resulting sludge.

A more sustainable sewage sludge management system might be attained through a separation of primary and secondary sludge before their treatment and disposal. It would thus be possible to maintain agricultural utilization for biological sludge (secondary) and to convert to inert material by incinerating (on- or off-site) only the primary sludge (Mininni et al. 2004).

In fact, characteristics of primary and secondary sludge are quite different in terms of quality (pollutants and nutrients) and in terms of suitability for thickening, digestion and dewatering. Secondary sludge is expected to be less polluted than primary sludge and should be segregated and treated separately from primary sludge, thus sustaining its agricultural utilisation.

A sampling campaign carried out in Italy to assess the quality difference between primary and secondary sludge has highlighted that enrichment factors of organic micropollutants in primary sludge increase with respect to those in secondary sludge in the order of hydrocarbons, extractable organic halogens (EOX), anionic surfactants measured as methylene blue active substances (MBAS).

Sludge separation may also give flexibility to sludge management, decreasing dependence on conventional disposal options (as required in the European Directive 2008/98), as sludge of good quality (biosolids) can be recovered for agricultural utilization while the remaining primary sludge can be treated by incineration. The challenge in the coming years will be assuring sludge management the greatest flexibility as well as maximizing the recovery of valuable products and energy sources while reducing disposal only to inert materials, which no longer contain useful compounds.

One of the main goals in secondary sludge processing is improving the performance of anaerobic digestion. Secondary sludge contains up to 70 % of bacteria, so the above objective can only be achieved by accelerating the hydrolysis, which is the limiting step in the anaerobic process. Sludge disintegration treatments are able to disrupt biomass flocs and cell walls and to cause the release of the intracellular organic material. This treatment results in acceleration in the biological breakdown of particulate organic material into soluble, readily biodegradable fractions. The subsequent increase in biodegradable material improves bacterial kinetics, resulting in lower sludge quantities and, in the case of anaerobic digestion, increased biogas production. The technologies already in use for sludge disintegration, such as ultrasounds, by means of shear forces, have shown that energy input can account for 1-2 kWh/kg TS (Khanal et al., 2007, Müller, 2001).

In this context, IRSA has carried out anaerobic digestion tests with sonicated secondary sludge, in batch and semi-continuous mode, investigating the effect of pre-treatment on digestion performances and on particle surface charge, affecting dewaterability. It is worth noting that ultrasound pre-treatment (specific energy 5,000 kJ/kg TS, i.e., about 1.4 kWh/kg TS) considerably accelerates the reaction rate (doubling the k values), and biogas production increases by about 30 % with respect to the control reactor without sonication (Braguglia et al., 2008).

A second way to increase performance of secondary sludge anaerobic digestion is by thermal pre-treatment. The Cambi process may give even better results due to the drastic conditions of this disintegration process (temperature of 160 - 170 °C, pressure of 6 - 9 bar, residence time of more than 20 minutes). The Cambi process combined with anaerobic digestion allows increased VS removal and biogas production of 40 - 50 % in comparison with conventional sludge anaerobic digestion without disintegration.

Certainly, disintegration and digestion technologies of secondary sludge need to be optimised, taking into account both energy balance and sludge quality (reduction of volatile solids, dewaterability).

In this paper, the energy balance of different treatment options will be presented. The conventional treatment routes with mixed (primary + secondary) sludge thickening, digestion, mechanical dewatering and on-site incineration or on-site drying and off-site co-incineration will be compared with advanced options where the primary and secondary sludge are kept separate. Primary sludge treatment is performed by the conventional route, whereas secondary sludge is thickened by dynamic thickening and then disintegrated by sonication or thermal treatment and then digested. This analysis points out that the advanced options are much more convenient than the conventional ones in terms of reduction of exhaust gas from the drying-incineration process and in terms of total energy requirements.
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Keywords: Energy; anaerobic digestion; incineration; primary sludge; secondary sludge; sewage sludge characteristics; sewage sludge processing; thermal drying

Document Type: Research Article

Publication date: 2010-01-01

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