Research articleSewage sludge ash recovery as valuable raw material for chemical stabilization of leachable heavy metals
Introduction
Sewage sludge is an unavoidable by-product of all wastewater treatment processes. With a production of 50–70- g of Dry Matter per person equivalent per day (Vouk et al., 2016), continuous sewage sludge production is becoming one of the crucial issues for water industries. As a consequence, sewage sludge management represents a severe global challenge (Raheem et al., 2018).
In Europe the use of sewage sludge in agriculture is widely applied, because of its significant N, P and K content, that makes it a possible substitute of organic fertilizers (Donatello and Cheeseman, 2013). Fig. 1 shows that in 2015 Portugal, Ireland, United Kingdom, and Spain used more than 75% of the sludge as fertilizer, while more than 65% of sludge was composted in Hungary, Finland, and Estonia.
However, it was shown that the important elements in sewage sludge are slightly soluble and their release is very slow (Escudey et al., 2007). Additionally, several factors, mainly related to the presence of heavy metals and unstable pathogens are making land-spreading of sewage sludge practice not safe for the environment and human health (Krüger et al., 2014, Krüger and Adam, 2015, Wang et al., 2018c). This option is also considered unsuitable in the global waste management strategy, because it implies a lot of resources consumption. Additionally, it also can be a source of soil contamination and degradation of urban landscape (Barrera-Díaz et al., 2011; Ewa Krzywy-Gawrońska, 2010).
As a consequence, in the Netherlands, in the Flemish region of Belgium and in some regions of Germany, restrictive heavy metal limits for sewage sludge and sludge treated soils have been adopted, and sewage sludge land-spreading has been banned (Final Report Part III: Project Interim Reports RPA, 2010) (see Fig. 1, Eurostat, 2019).
However, data reported in Fig. 1 show that in other EU countries such as Greece, Italy, Malta, Bosnia-Herzegovina, Romania, and Croatia, landfill remains the major disposal route for sewage sludge. In non-EU country an alternative options is represented by sea disposal of sewage sludge (Husillos Rodríguez et al., 2012), that in some countries is done without prior purification (Rouse and D., 2013). This option has been banned in Europe since 1999, following the implementation of the EU Urban Wastewater Treatment Directive (Directive 91/271/EEC, 1991).
In Europe, the incineration is one of the few accepted alternatives to landfilling for this waste. Fig. 1 shows that The Netherlands, Germany, Slovenia, and Switzerland use incineration as the most common treatment.
Indeed, the thermal treatment of organic matter significantly reduces the amount of waste up to 90%, by exploiting its energetic potential (Vouk et al., 2017). However, the sewage sludge combustion is not always considered as a satisfactory disposal option as in some cases up to 30% of solid residues need further processing and are often landfilled (Cieślik et al., 2018). Recently several papers discussed the possible uses of SSA as a raw material in construction as sintered bricks (Anderson et al., 1996), tiles (Lin et al., 2008), zeolites (Zhang et al., 2018), blended cement (Pavlík et al., 2016), artificial lightweight aggregates, and other materials (Pacheco-Torgal et al., 2008). The uses of wastes in construction materialscan be an environmentally and economically good practice, since the concrete industry is today dealing with the decreased availability of the raw resources (Zacco et al., 2012). However, some of these recovery strategies need high energies processes (Zhang et al., 2018).
Moreover, in the case of cement production, the relatively high content of chlorides presents a limitation for SSA use (Pavlík et al., 2016).
The recovery of phosphate from SSA using acid leaching process was also proposed (Biswas et al., 2009), however in this case acid insoluble SSA wastes are generated (Li et al., 2017).
In this paper an alternative use of SSA is proposed, that requires low technologically sophisticated processing, and does not require any pre-treatment.
In the last years, a stabilization technology (COSMOS) was developed at the University of Brescia: it was designed to reduce heavy metals leachability in municipal solid waste incineration (MSWI) fly ash (Bontempi et al., 2010, Bosio et al., 2013). Contrarily to the commonly used technological solutions for heavy metals stabilization, using commercial chemicals, COSMOS technology was addressed to use waste and by-products source for heavy metals stabilization (Rodella et al., 2017). The process was based of MSWI fly ash, FGD residue, coal fly ash, and an amorphous silica source, such as colloidal silica (Struis et al., 2013), rice husk ash and silica fume (Benassi et al., 2016). The obtained stabilized material is an inert, that has several potentialities to be used as a safe (Guarienti et al., 2016, Guarienti et al., 2014), and green filler (Bontempi, 2017). The stabilization mechanism was explained by sorption of metal ions on silica, through the substitution of protons from silanol groups on the silica surface by the metal ions from the solution.
In this paper, we propose the use of SSA, instead of the amorphous silica. FGD residue, the main by-product of the desulphurization system used to remove SOx from coal combustion products (Benassi et al., 2017), and an inexpensive calcium hydroxide are the other two components used in the process.
The new method (patent number 102019000006651, 2019) takes advantage of the valuable raw materials contained in SSA, such as phosphate, silica, and alumina.
The phosphorus present in SSA exists in water-insoluble phases, mainly as iron, aluminum, and calcium phosphates (Rolewicz et al., 2018). Recent studies have shown that phosphate stabilization can effectively decrease the leaching of heavy metals (Wang et al., 2018a) present in MWSI fly ash. Indeed, phosphate combines with at least 38 elements to form over 300 naturally occurring insoluble phosphates (Eighmy and Dykstra Eusden, 2004). For example, in hydroxyapatites Ca can typically be substituted with cations with ionic radius between 0.69 and 1.35 Å and in chloroapatites it can be substituted with cations with ionic radius between 0.80 and 1.35 Å. These metal substitutions can be complete or partial as in the case of Pb and Zn, respectively (Eighmy and Dykstra Eusden, 2004).
In addition, silica and alumina can promote other reactions. In particular, it was reported that silica and alumina present in SSA can form calcium silicate hydrates (C–S–H) or aluminum silicate hydrates (C-A-H) during a polymerization process with calcium materials (Chen and Lin, 2009).
These reactions, that are due to the pozzolanic characteristic of SSA, are determining the cementitious materials hydration, and can be used for heavy metals stabilization (Guo et al., 2017).
Literature reports that he amorphous content of SSA ranged from 35 to 75%, which suggests that the SSA reactivity can be different depending on its origins (Lynn et al., 2015).
MSWI fly ash, can be a Ca source, together with the FGD residues that contain Ca(OH)2 (Bosio et al., 2014b), making possible the formation of C–S–H and C-A-H by SSA addition.
This approach allows saving significant resources and grants the possibility of internal processing and management of several streams of solid waste produced during sewage sludge and municipal solid waste thermal utilization and can easily be applied in several EU waste treatment plants.
The potential application of industrial by-products for remediation strategies (Benassi et al., 2017, Pasquali et al., 2018) has attracted extensive interests. This de-pollution strategy, that enables resources recycling by reducing environmental burden of waste management, was recently defined as a new chemistry approach: the Azure Chemistry (Zanoletti et al., 2018a, Zanoletti et al., 2018b).
Section snippets
Material
The MSWI fly ash used in this study were provided by an Italian incinerator located in the Lombardy region. The flue gas cleaning system consists of dry scrubbers, fabric filters, an activated carbon injection system, and a combination of selective non-catalytic reduction and selective catalytic reduction. FGD residues used in this work are by-products generated by air pollution control equipment in coal-fired power plants of the Italian energy-company. Silica fume powder was provided by
Results and discussion
MSWI fly ash, silica fume, and FGD residues compositions were reported in recent works (Benassi et al., 2017, Rodella et al., 2017). Silica fume mainly consist of amorphous silica particles. The crystalline phases present in MSWI fly ash are: calcium hydroxide, quartz, anhydride, calcite, sylvite, calcium chloride hydroxide and sodium chloride. FGD consists of calcium sulphite hemihydrate and calcium hydroxide. The SSA composition, derived from chemical analysis, is reported in Table 2. Despite
Conclusions
This paper proposes, for the first time, the use of a waste material (SSA) for the immobilization of leachable heavy metals contained in MSWI fly ash. This stabilization technology involves also the use of by-products (FGD residues).
Leaching tests, made on the synthesized materials, highlight that heavy metals concentration (mainly Pb and Zn) is reduced in respect to corresponding values in leachate of MSWI fly ash.
Due to the initial materials composition and the phase analysis (provided by
Acknowledgement
The authors thank Ministero dell'Ambiente e della Tutela del Territorio e del Mare for supporting this work through the RENDERING Project and Christian Adam (from BAM Federal Institute for Materials Research and Testing) to provide sewage sludge ash samples.
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