Ozonation effects for excess sludge reduction on bacterial communities composition in a full-scale activated sludge plant for domestic wastewater treatment.

Activated sludge process is the most widely diffused system to treat wastewater to control the discharge of pollutants into the environment. Microorganisms are responsible for the removal of organic matter, nitrogen, phosphorous and other emerging contaminants. The environmental conditions of biological reactors significantly affects the ecology of the microbial community and, therefore, the performance of the treatment process. In the last years, ozone has been used to reduce excess sludge production by wastewater treatment plants (WWTPs), whose disposal represents one of the most relevant operational costs. The ozonation process has demonstrated to be a viable method to allow a consistent reduction in excess sludge. This study was carried out in a full-scale plant treating municipal wastewater in two parallel lines, one ozonated in the digestion tank and another used as a control. Bacterial communities of samples collected from both lines of digestion thanks were then compared to assess differences related to the ozonation treatment. Data were then analysed with terminal restriction fragment length polymorphism (T-RFLP) analysis on 16S rRNA gene. Differences between bacterial communities of both treated and untreated line appeared 2 weeks after the beginning of the treatment. Results demonstrated that ozonation treatment significantly affected the activated sludge in WWTP.

Leaching behaviour of bottom ash from RDF high-temperature gasification plants.

This study investigated the physical properties, the chemical composition and the leaching behaviour of two bottom ash (BA) samples from two different refuse derived fuel high-temperature gasification plants, as a function of particle size. The X-ray diffraction patterns showed that the materials contained large amounts of glass. This aspect was also confirmed by the results of availability and ANC leaching tests. Chemical composition indicated that Fe, Mn, Cu and Cr were the most abundant metals, with a slight enrichment in the finest fractions. Suitability of samples for inert waste landfilling and reuse was evaluated through the leaching test EN 12457-2. In one sample the concentration of all metals was below the limit set by law, while limits were exceeded for Cu, Cr and Ni in the other sample, where the finest fraction showed to give the main contribution to leaching of Cu and Ni. Preliminary results of physical and geotechnical characterisation indicated the suitability of vitrified BA for reuse in the field of civil engineering. The possible application of a size separation pre-treatment in order to improve the chemical characteristics of the materials was also discussed.

Effect of short-term natural weathering on MSWI and wood waste bottom ash leaching behaviour.

Short term natural weathering was applied on municipal solid waste (MSW) and wood waste incinerator bottom ash (BA). The materials were analysed at different steps of treatment and characterized for chemical and mineralogical composition. Both short and long term leaching behaviour of main elements and heavy metals were investigated as well. Lead, zinc and copper were the main heavy metals to be released. After 12 weeks of treatment the concentration of leached zinc decreased. Lead concentration was not found to be influenced by pH and decreased only for the biomass samples. Weathering did not have beneficial effects on copper leaching, which was well described by complexation processes with DOC. The findings from the experimental campaign indicated that weathering reactions improved the mineral stability of the analysed materials but, in contrast with previous works, the treatment was not sufficient to guarantee pH stability and to comply with leaching law limits.

Aerobic biodegradation of 3-chlorophenol in a sequencing batch reactor: effect of cometabolism.

The aim of the present study was to investigate how phenol modifies, through cometabolism, the biodegrading capability of 3-chlorophenol (3-CP) in a sequencing batch reactor seeded with a mixed culture obtained from a domestic sewage treatment plant. Two laboratory-scale SBRs, one fed 3-CP only and the other fed 3-CP and phenol in the same concentration, were seeded with the partially acclimated biomass. The removal capability in both reactors was measured for progressive increases in the feed organic loading. Cometabolism enhanced biodegradation of 3-CP by reducing both the initial lag period and the time required for the complete removal. 700 mg/L 3-CP was demonstrated to be the highest concentration, which could be completely degraded during the active phase (fill plus react) either in the presence or absence of phenol as the growth substrate even though the lag period was shorter when phenol was present. The operating strategy required modification for the complete removal of 800 mg/L 3-CP. An increase in the phenol to 3-CP ratio did, however, improve 3-CP degradation rate.

Physical and mechanical properties of cement-based products containing incineration bottom ash.

This paper presents the results of a wider experimental programme conducted in the framework of the NNAPICS ("Neural Network Analysis for Prediction of Interactions in Cement/Waste Systems") project funded by the European Commission and a number of industrial partners under Brite-EuRamIII. Based on the fact that bottom ashes from waste incineration are classified as non-hazardous wastes according to the European Waste Catalogue, the aim of the present work was to investigate the feasibility of addressing the potential use of such residues in cement-based mixtures. This issue was suggested by the analysis of the properties of different bottom ashes coming from Italian municipal and hospital solid waste incinerators, which showed a chemical composition potentially suitable for such applications. Different mixes were prepared by blending bottom ash with ordinary Portland cement in different proportions and at different water dosages. The solidified products were tested for setting time and bulk density, unconfined compressive strength and evaporable water content at different curing times. The results of the experimental campaign were analysed through a statistical procedure (analysis of variance), in order to investigate the effect of mixture composition (waste replacement level and water dosage) on the product properties.

Fractional factorial design to investigate the influence of heavy metals and anions on acid neutralization behavior of cement-based products.

A major concern of cement-based solidification/stabilization of hazardous wastes is the interaction of waste contaminants on cement properties. Literature contains many examples of studies on the interference of individual contaminants on cement properties. Conversely, little information is available on how the interactions between contaminants affectthe properties of cement/waste systems. This paper provides a discussion on the interference mechanisms exerted by seven contaminants, five heavy metals and two anions, on cement hydration. The seven contaminants were selected on the basis of the typical composition of municipal solid waste incineration (MSWI) fly ash. Spiking experiments using pure compounds were performed according to a 2IV(7-3) fractional factorial design to simulate addition of MSWI fly ash to ordinary Portland cement. The acid neutralization behavior of the laboratory cement-contaminant mixtures was studied to detect the presence of solid phases responsible for the buffering capacity of the solid matrix. The results from the experimental work showed that Zn, Cl-, and SO4(2-) were the major factors influencing, occasionally in combination with other contaminants, strength and acid neutralization capacity of the cementitious products. The release of Cd, Cr, Cu, and Pb in the eluates as a function of pH also suggested possible chemical immobilization mechanisms of such metals within the hardened matrix.

Properties of Portland cement--stabilised MSWI fly ashes.

In the present paper, the properties of Portland cement mixtures containing fly ashes (FA) collected at four different Italian municipal solid waste incineration (MSWI) plants were investigated. In particular, physical/mechanical characteristics (setting time, unconfined compressive strength (UCS) and shrinkage/expansion), as well as the acid neutralisation behaviour of the solidified products were considered. The FA composition, revealing enrichment in heavy metals, chlorides and sulphates, significantly altered the hydration behaviour of Portland cement. Consequently, for some of the investigated FA the maximum allowable content for the mixtures to achieve appreciable mechanical strength was 20 wt.%. Even at low FA dosages setting of cement was strongly delayed. In order to improve the properties of FA/cement mixtures, the use of additives was tested.Moreover, the acid neutralisation capacity (ANC) of the solidified products was evaluated in order to assess the ability of the matrix to resist acidification, and also to provide information on hydration progression, as well as on heavy metal release under different pH conditions. Comparison of the results from the present work with previous studies carried out on spiked mixtures lead to the conclusion that the mechanical properties of the stabilised FA could not be predicted based on the effect exerted by heavy metals and anions only, even when the dilution effect exerted on cement was taken into account. It was likely that a major role was also played by alkalis, which were present in the FA at much higher concentrations than in cement.

Fate of antimony in municipal solid waste incineration.

The average antimony concentration in municipal solid waste is estimated to be about 10-60 ppm. Thermodynamical models predict a volatile behavior for antimony compounds, yet literature mass balances show that about 50% of the antimony input remains in the grate ashes. This fact can be explained by the formation of thermally stable antimonates in the fuel bed due to interactions with alkali or earth-alkali metals. Thermogravimetric experiments revealed an increased thermal stability for antimony oxide in presence of oxygen and calcium oxide. Spiking experiments on the test incinerator TAMARA showed that chlorination processes have a strong effect on antimony volatilization whereas high fuel-bed temperatures and addition of antimony oxide only have a moderate effect. In the grate ashes, antimony shows a pH-depending leaching property, which is typical for anionic species. This fact supports the thesis that antimony is present in the grate ashes in an anionic speciation.

Optimization of the solidification/stabilization process of MSW fly ash in cementitious matrices.

The solidification/stabilization (S/S) process of municipal solid waste (MSW) fly ash in cementitious matrices was investigated in order to ascertain the feasibility of a washing pretreatment of fly ash with water as a means of maximizing the ash content of cementitious mixtures. Four types of fly ash resulting from different Italian MSW incineration plants and ASTM Type III Portland cement were used in this study. Ash-cement mixtures with different fly ash/cement (FA/C) ratios were made using untreated and washed fly ash. Washing of fly ash with water was realized by a two-stage treatment (liquid/solid=25; mixing time=15 min for each stage). The cementitious mixtures were characterized for water demand, setting time, mechanical strength, and heavy metals leachability. Comparison between the above properties of mixtures incorporating untreated and washed fly ash (particularly, setting characteristics), coupled with economical evaluation of the S/S process when applied to untreated and washed fly ash, proved the feasibility of washing pretreatment as a means of maximizing the incorporation of MSW fly ash in cementitious matrices (ash content up to 75%-90% by weight of total solid).