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Effects on Ni and Cd speciation in sewage sludge during composting and co-composting with steel slag Zheng-zhong Zeng, Xiao-li Wang, Jian-feng Gou, He-fei Zhang, Hou-cheng Wang and Zhong-ren Nan Waste Manag Res 2014 32: 179 DOI: 10.1177/0734242X14521682 The online version of this article can be found at: http://wmr.sagepub.com/content/32/3/179
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WMR0010.1177/0734242X14521682Waste Management and ResearchZheng et al.
Review article
Effects on Ni and Cd speciation in sewage sludge during composting and cocomposting with steel slag
Waste Management & Research 2014, Vol. 32(3) 179–185 © The Author(s) 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X14521682 wmr.sagepub.com
Zheng-zhong Zeng1, Xiao-li Wang1, Jian-feng Gou1, He-fei Zhang2, Hou-cheng Wang1 and Zhong-ren Nan1
Abstract Sewage sludge and industrial steel slag (SS) pose threats of serious pollution to the environment. The experiments aimed to improve the stabilizing effects of heavy metal Ni and Cd morphology in composting sludge. The total Ni and Cd species distribution and chemical forms in the compost sewage sludge were investigated with the use of compost and co-compost with SS, including degradation. The carbon/nitrogen ratio of piles was regulated with the use of sawdust prior to batch aerobic composting experiments. Results indicated that the co-composting with SS and organic matter humification can contribute to the formation of Fe and Mn hydroxides and that the humus colloid significantly changed Ni and Cd species distribution. The decreased content of Ni and Cd in an unstable state inhibited their biological activity. Conclusions were drawn that an SS amount equal to 7% of the dry sludge mass was optimal value to guarantee the lowest amount of Cd in an unstable state, whereas the amount was 14% for Ni. Keywords Chemical form, composting, heavy metals, sludge, steel slag
Introduction An average of 139 million m3 of municipal sewage is currently generated in China every day, with the emission of sludge ranging from 2.537×107 to 5.07×107 tonnes/year (80% moisture). The volume of sludge production has drastically increased as a result of rapid urbanization and industrialization as well as the need to meet ever-increasing demands on water quality. Dealing with urban sewage sludge has therefore become an urgent concern, with the speed of its production increasing between 10 and 15% annually (Wong, 2009). Compared with the landfill and incineration, the ‘biosolids’ characteristic of sludge is a good resource for agriculture and forestry. Sludge is rich in N, P, Mg, organic matters, essential trace elements, and nutrients for plant maintenance, so sludge can be widely used to maintain and restore forests and rangelands (Qiao et al., 2000). The physicochemical properties of soil can be improved through aeration and improving water-holding capacity (Wong et al., 2001), and the number of soil microorganisms can be also increased as well as the contents of nutrients of N, P, and Mg. In this regard, land application has become another attractive option for sludge treatment and disposal. In both Europe and the USA, land application has become a part of the main direction toward disposing urban sewage sludge (PRC Ministry of Housing and Urban-Rural Development, 2011). As the sludge is rich in organic matters and hydrophilic gelatinous structures, it is a good addition
to repair and improve the Chinese loess which is barren and vulnerable to soil erosion. However, heavy metals and several potentially harmful organic substances are major factors that restrict the agronomic and forestry applications of sewage sludge. Heavy metals can be removed from sewage sludge through source control or chemical methods. However, the source control method is costly, and determining the sources of heavy metal pollution is difficult. Chemical methods are also expensive, complicated to operate, and difficult to scale. Numerous studies have achieved good results in passivating heavy metals in sludge by using additives, such as fly ash and lime amendments, to change heavy metal chemical forms (Djedidia et al., 2009; Fang and Wong, 1999). However, no study has been conducted on the cocomposting of sewage sludge with steel slag (SS). 1College
of Earth and Environmental Sciences, Lanzhou University, Lanzhou, PR China 2Tianjin Environmental Sanitation Engineering Design Institute, Tianjin, PR China Corresponding author: Jian-feng Gou, College of Earth and Environmental Sciences. Lanzhou University, No. 222, South Tianshui Rd, Lanzhou 730000, Gansu Prov., China. E-mail: [email protected]
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Table 1. Distribution of the chemical forms of Ni in raw materials.
Sewage sludge Control sludge Sawdust Steel slage
EF
CBF
FMOF
OBF
RF
Total
1.51 (8.84) 82.13 (39.67) 0.88 (6.68) 0.34 (1.85)
1.46 (8.57) 56.21 (27.15) 1.20 (9.11) 1.775 (9.70)
4.635 (27.13) 44.29 (21.39) 1.75 (13.24) 2.575 (14.23)
4.97 (29.12) 13.59 (6.56) 0.86 (6.53) 4.225 (23.36)
4.50 (26.34) 10.82 (5.23) 8.49 (64.44) 9.2 (50.86)
25.73 207.04 13.18 18.09
Values are mg/kg (%). CBF, carbonate-bound fraction; EF, exchangeable fraction; FMOF, Fe-Mn oxide fraction; OBF, organic-bound fraction; RF, residual fraction.
Table 2. Distribution of the chemical forms of Cd in raw materials.
Sewage sludge Control sludge Sawdust Steel slage
EF
CBF
FMOF
OBF
RF
Total
0.04 (3.47) 8.52 (40.67) 0 0
0 6.95 (33.15) 0 0
0.59 (57.92) 3.77 (17.98) 0.03 (19.23) 0
0.1 (9.41 0.50 (2.39) 0.05 (38.46 0
0.32 (28.71) 1.22 (5.83) 0.06 (42.31) 0
1.01 20.95 0.13 0
Values are mg/kg (%). CBF, carbonate-bound fraction; EF, exchangeable fraction; FMOF, Fe-Mn oxide fraction; OBF, organic-bound fraction; RF, residual fraction.
In China, SS is the second largest source of solid waste and it accounts for 12–15% of raw steel production; the development of a method to efficiently dispose SS is therefore urgently needed. SS is a molten mixture of a variety of metal oxides with strong alkaline characteristics, including the main Ca, Fe, and Si oxides and small amounts of Mg, Al, Mn, and P. The main mineral components of SS are C2S, C3S, monticellite, Ca, Mg, rhodonite, and f-CaO. Besides being used as a metallurgical and construction raw material, SS can be used as slag phosphate, silicon, and acidic soil conditioner for agriculture (Ren et al., 2008). Currently, the widely accepted belief is that the total amount of heavy metals in sewage sludge cannot provide sufficient information on the potential hazardous effects of such metals on the environment; this is because the biological activity, mobility, and ecotoxicity of heavy metals depend strongly on their specific chemical forms (Obbard, 2001; Planquart et al., 1999; Wang et al., 2006). Chemical forms typically consist of the following: exchangeable fraction (EF), carbonate-bound fraction (CBF), Fe-Mn oxide fraction (FMOF), organic-bound fraction (OBF), and residual fraction (RF). The first two forms are appropriate for migration, easy for biological absorption, and highly harmful to crops and forest resources (Lake et al., 1984; Lin and Zhou, 2008). SS can be used as a sludge composting additive because of its alkalinity and richness in f-CaO, Fe, and Mn components. This study aims to examine the effect of heavy metals (Ni and Cd) in compost sludge by investigating the effects of SS and changes in organic matter, as well as the migration and transformation among the five chemical forms of the heavy metals. The study also determines the optimal amount of slag and identifies new ways to reduce and control sludge heavy metal activity. The findings are expected to help improve land utilization and to realize the potential of sludge for agriculture- and forestry-related applications.
Materials and methods Experimental materials Dewatered anaerobically digested sewage sludge was collected from Yanerwan wastewater treatment plant in Lanzhou, China. Sawdust and SS were respectively obtained from decorations in Lanzhou University and from Yuzhong steel plant in Lanzhou City. The size of sawdust particles was 1–20 mm, and sawdust moisture was about 5%. SS was crushed to 0.149 mm for experimental use. The initial municipal sewage sludge had a low concentration of heavy metals, in which Cd bound in CBF was not even detected. Nitrate of Ni and Cd were added into the original sludge to achieve full five chemical forms, with their concentration close to that prescribed by the China Construction Standard on the Disposal of Sludge from Municipal Wastewater Treatment Plant – Quality of Sludge Used in Gardens or Parks (GBT23486-2009, PRC Ministry of Housing and Urban-Rural Development, 2009). However, such a concentration is not beyond the maximum of 420 mg/kg (85 mg/ kg per pollution; regulation standard of the US Environmental Protection Agency, 2007). Detailed information on the amount of the five forms of Ni and Cd is shown in Tables 1 and 2 and the elements in the SS are listed in Table 3.
Composting facilities The composting equipment included six laboratory-scale simulated fermentation tanks with an effective volume of 31 l each. Two layers of gauze were covered on the porous sieve plate at the bottom of each tank. The sewage sludge was air-dried at ambient temperature. The moisture contents of the six well-mixed materials were maintained between 50 and 60% throughout the aerobic composting phase. Air was used to aerate the sludge from the gauze located at the bottom, which was supplied by a hot air blower. The six groups of composting sludge could reach a
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Zheng et al. Table 3. Elements in steel slag. Element
%
Mg Al Si S Cl K Ca Ti Mn Fe
0.72 1.16 8 0.62 0.3 0.26 56.1 1.28 1.75 29.81
pH=12.25.
Table 4. Experimental pilot programmes and components. Programme
Materials
Ratio (w/w dry weight)
SS0 SS7 SS14 SS21 SS28 SS35
Sludge/sawdust Sludge/sawdust/fly ash Sludge/sawdust/fly ash Sludge/sawdust/fly ash Sludge/sawdust/fly ash Sludge/sawdust/fly ash
14:2 14:2:0.98 14:2:1.96 14:2:2.94 14:2:3.92 14:2:4.90
SS, steel slag.
temperature as high as 50°C which lasted 3 days during the 54-day composting process. The dewatered sludge may lack sufficient porosity for adequate aeration and demonstrate an imbalanced carbon-to-nitrogen (C/N) ratio. Bulking agents are needed to provide structural support when the compost materials are too wet to maintain air spaces with the compost pile and to reduce moisture content or change the C/N ratio (Wang et al., 2008). The literature (Zheng et al., 2009; Li, 2012) and experimental results indicate that the best C/N ratio of raw composting materials is 25–35. The sludge ranged from 5–8, and the sawdust was approximately 200. Sawdust was mixed with the dewatered anaerobically digested sewage sludge as a bulking agent at 7:1 (w/w, dry weight) to obtain a C/N ratio of approximately 30. The sewage sludge–sawdust mixture was then mixed thoroughly with SS at 0, 7, 14, 21, 28, and 35% (w/w dry weight) with a concrete mixer. Enhanced heavy metal sludge without SS (SS0) was used as a control. Six pilot programmes and components were listed in Table 4.
Project determination and methods Six groups of the mixed materials of the sludge, sawdust, and SS were evenly stirred, and three sludge samples in each treatment were taken from three point of the fermentation tanks diagonally. Sludge samples were oven dried at 105°C, ground into powder, and sieved through a 0.149-mm sieve. Then the three sludge samples of each treatment were thoroughly mixed and homogeneous samples were remove, using the method of
quartering, which was used as the detection sample of each treatment (China Environment Protection Agency, 1997). The total heavy metal concentrations of Ni and Cd were digested first with 65% HNO3 and 40% HF, and then with 72% HClO4 (Walst, 1971) until white smoke completely disappeared, at 110, 120, and 210°C, respectively. The method of sequential extraction performed in this study followed that of Tessier et al. (1979); although this procedure is still plagued by some limitations, it is generally accepted (Zimmerman and Weindorf, 2010). The chemical reagents used as extractants for heavy metals in the sequential extraction are listed in Table 5. The EF, CBF, FMOF, OBF, and RF were analysed. Dry sewage sludge sample (1 g) was placed in a 50-ml centrifuge tube. Chemical reagents in different ratios were added to the sludge by steps, and different forms of extracted liquid were obtained. Each extraction step sample was then centrifuged at 2395×g for 10 min. The supernatants were transferred in a volumetric flask, and deionized water was added to adjust the volume to 50 ml. The solution was filtered through a 0.45-µm filter paper and analysed with a flame atomic absorption spectrophotometer (Themol-M6 American). Total organic carbon was determined by oxidation with K2Cr2O7 in a concentrated H2SO4 medium, and dichromate excess was measured with (NH4)2Fe(SO4)2 (Yeomans and Bremmer, 1989). The pH values of the sludge samples were identified with the extract at a sludge/ deionized water ratio of 1:5 (w/v) with the use of a digital pH meter.
Results and discussion Changes in the total amount of the heavy metals during composting The variation in the total amount of Ni and Cd during composting is shown in Figure 1. After high-temperature composting, the Ni content in the SS0 control test showed an obvious decline from 261.97 to 193.20 mg/kg, whereas an increment of 0.58 mg/kg for Cd was noted. These results agreed with the increase in tissue Cd content of plant as a result of the incremental rate change in sewage sludge; the tissue Ni content did not increase after the change in compost because of the low Ni content in the sewage sludge compost (Wong and Lai, 1996; Simeoni et al., 1984); however, both Ni and Cd total concentrations decreased in the other five treatments. In the six treatments, the total content in latter experiments were all less than that in the SS0 whether before or after the composting. This result may be attributed to the dilution effect of the slag and its strong alkalinity, which prompted the changes in sludge microbial community. As a result, Bacillus pasturii and Bacillus alcalophilus became metabolically active. They leached the heavy metals from the piles and hence decreased the total content.
Effects on Ni and Cd Speciation by cocomposting with the addition of steel slag Nickel. The sludge in different wastewater plants did not demonstrate a fixed distribution for Ni in the different chemical
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Table 5. Chemical reagents and procedures used in the sequential extraction steps of heavy metals. Steps
Fractions
Reagent added to sludge (ratio, v/w, with dry matter)
Duration of extraction (h)
1 2 3 4
Exchangeable Carbonates Fe-Mn oxides Organic
1 8 4 5.5
5
Residual
1 M MgCl2 (8:1) 1 M NaOAC/HOAC (8:1) 0.04 M NH2OH.HCl (20:1) 0.02 M HNO3 (3:1); 30% (v/v) H2O2 (10:1); 3.2 M NH4Ac (5:1) 65% (v/v) HNO3 [15:0]; 40% (v/v) HF [10:0]; 72% (v/v) HClO4 [5:0]
To end of reaction
Figure 1. Variation in the total amount of Ni and Cd in the test sludge during composting. SS, steel slag.
Figure 2. Ni speciation change and distribution during composting.
CBF, carbonate-bound fraction; EF, exchangeable fraction; FMOF, Fe-Mn oxide fraction; OBF, organic-bound fraction; RF, residual fraction; SS, steel slag.
forms. Wong and Lai (1996) reported that Ni was predominant in CBF, FMOF, and OBF. Dudka and Chopecka (1990) determined that Ni was a major component in FMOF. In this study, Ni was a major component in EF and CBF (Figure 2). Ni was easily absorbed, and it caused toxic effects on the plant. In the SS0, the EF and CBF proportions decreased from 40.04–7.75% and 25.77–17.22%, respectively. Increments of 24.79 and 17.32% for FMOF and OBF, respectively, indicated the low biological activity of Ni in the sludge after composting. A large effect on Ni chemical forms with the different amounts of SS was noted during the composting process. After cocomposting with SS, the effective state (EF and CBF) proportions of SS35, SS28, SS21, SS14, and SS7 were individually reduced by 41.87, 44.01, 39.97, 43.68, and 38.58%, respectively. The significant increase in FMOF and OBF fractions and the decrease in EF and CBF fractions suggested that the addition of SS could effectively prevent Ni mobility. With the effect of
composting and the proper use of SS resources considered, 14% of SS was determined as the appropriate amount for Ni. Cadmium. Cd is an active and hazardous heavy metal. Figure 3 indicates that the predominant species in the sludge before composting were EF and CBF, and ratios of EF and CBF in the six groups were all larger than 74%, a result indicating the high potential mobility and bioavailability of Cd. Therefore, sludge containing Cd should not be directly applied to agricultural fields prior to further treatment. After simple composting (SS0), the EF amount of Cd sharply declined and thus reduced Cd bioavailability. When composting disposed with SS, the FMOF ratios of SS7, SS14, SS21, SS28, and SS35 were significantly higher than that of SS0 which may be a result of the SS alkaline effect. This result prompted the EF and CBF amounts of Cd to synchronously decline, with the biological activity of Cd reduced clearly. After composting, the order of the effective state of the six sludge samples was as follows: SS0 > SS14 > SS35 > SS28 > SS21 > SS7.
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Zheng et al.
Figure 3. Cd speciation change and distribution during composting.
CBF, carbonate-bound fraction; EF, exchangeable fraction; FMOF, Fe-Mn oxide fraction; OBF, organic-bound fraction; RF, residual fraction; SS, steel slag.
In SS7, the effective state amount was reduced to a maximum of 66.28%, and the FMOF and OBF proportions increased to 66.6%, a result indicating the poor activity and mobility of SS7. Although the SS14, SS21, SS28, and SS35 proportions of EF and CBF clearly showed declining trends, the SS7 unit passivation effect was the best, and the value here was set as the suitable amount of SS to passivate Cd.
Impact mechanism Co-composting with SS can transform toxic EF and CBF into stable FMOF and OBF in heavy metals. SS is a porous material formed through high-temperature smelting, is rich in calcium oxide, iron, silicon, magnesium, aluminium, and manganese (Table 3), and appears to be alkaline in character. The characteristics of SS contributed to the formation of iron, aluminium, and manganese gender hydroxide colloid, and the pH value caused by f-CaO significantly affected the formation of FMOF. Colloid surface was positively charged when the pH value was less than the isoelectric pH value, but the specific absorption effect was weakened with the increased positive charges; as a result, the adsorption capacity of heavy metals was slowed down. When the pH value was above the isoelectric pH value, the adsorption capacity of the heavy metals dramatically increased because the colloid surface was negatively charged (Ding et al., 2001). Composting could form a large amount of humus, including carboxyl, alcoholic hydroxyl, phenolic hydroxyl, and carbonyl functional groups. Once the humus dissolves in the solution of the heap body, several acidic functional groups dissociate. Humic acid or fulvic acid is highly fragmented and forms organic–metal sludge complexes by the combination effect of bivalent and multivalent metals because of the mutual repulsion of charged groups. Therefore, EF was transformed to OBF. A single humus molecule is connected to other molecules to form a chain-like structure via polyvalent cations; coagulation then occurs (Chen et al., 2008; Stevensen, 1982), which is also conducive to the formation and stabilization of heavy metal OBF. Figure 4 indicates that the pH value increased gradually with an increase in the amount of SS, and the value changed between 7.68 and 8.6. In the SS0 test (Figures 2 and 3), the heavy metal RF ratios decreased after the composting because of the complex role of heavy metal characteristics and heap material humification.
However, the Ni and Cd residual fraction changed irregularly when composting with SS. Except for SS28, the RF amounts for Ni in the other four experiments all decreased. By contrast, except for SS7, the RF amounts for Cd clearly increased. Figures 2 and 3 also show that a large number of effective states were transformed into the stable state, a result indicating that the SS alkaline effect influences the chemical fraction distribution of metals. This finding is attributed to SS alkalinity, which neutralizes carbonic acids and provides reactant circumstance to form FMOF and OBF. Therefore, the change in SS was significant in reducing the mobile and easily available fractions of Ni and Cd. In the case of Cd, slagadded composting enabled the toxic EF to be transformed significantly into other stable states and thus significantly reduced the biological activity of Cd.
Influence of organic matter by SS-added composting The organic matter of sludge can improve soil structure, water retention capacity, and permeability. The organic matter influences the effect of fertilizers (Zhang and Pang, 2005). It also has a significant effect on heavy metal pollution and global carbon balance (Xu et al., 2007). Aerobic decomposition of organic matter is a complex process, which is shown in the following:
OM + O 2 + Nutrient Microorganism Cytoplasm + CO 2 + H 2O + NH3 + SO 4 2 − + ... + Heat
(formula 1)
where OM is ‘organic matter’. Most of the organic matter of sludge were degraded with the gradual deepening of compost, the large amount of humus formed, and the NH3 that was generated from biological decomposition in the maturity composting stage and that was transformed into nitrates by nitrification bacteria:
22NH 4 + + 37O 2 + 4CO 2 + HCO3 → 21NO3− + C5H 7 NO 2 + 20H 2O + 42H +
(formula 2)
After addition of the slag, the OH– of mineral H+ was found in the neutralization reaction. Formula 2 is moved to the right part of the equation, resulting in formula 1 also being moved to the right. The organic matter proportions decreased consistently with an
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Waste Management & Research 32(3) compost materials can affect heavy metal speciation, which can result in different evolutions of physicochemical parameters during composting. After the SS-added composting, the sludge exhibited high stability and low bioavailability. It can be applied to the loess as an organic soil fertilizer to enhance soil water retention capacity and improve the ecological environment of the loess areas in northwest China.
S+
%HIRUH
$IWHU
Conclusions
66
66
66
66
66
66
Figure 4. pH value changes of the heap materials before and after composting. SS, steel slag.
66 66
66 66
66 66
7G Figure 5. Organic matter changes during composting. SS, steel slag.
Composting or co-composting using SS reduced the total amount of heavy metals, as some heavy metals were leached out with the leachate. SS-added compost efficiently contributed to passivating the effective state of heavy metals in the sludge. The strong alkalinity of SS and the effect of degradation of the organic matters effectively transformed the Ni and Cd unstable state into a stable state. The humification generated from the degradation of organic matter also catalytically affected the conversion of the five heavy metal chemical forms. Cd reached the maximum effect of passivation when the slag-addition ratio was 7% of the dry sludge, whereas that for Ni was 14%. Provided that Cd is more toxic than Ni, the best optional amount of SS7 to control Ni and Cd biological activities during composting was determined. When applied into practice, sewage sludge that is used for agriculture or forest-related applications must be treated directly with composting or with the addition of SS. The addition ratio of SS can vary by 7 or 14% on the basis of the Ni and Cd contents, respectively, in the sludge to reduce the content of the unstable state and achieve passivation of the metals.
Declaration of conflicting interest The authors declare that there is no conflict of interest.
increase in the amount of SS. As a result, the organic matter of sludge was fully degraded. Six experimental sludge contents of organic matters decreased along with compost (Figure 5). The decreasing trend confirmed the dilution effect of the increased SS amount when the samples were measured simultaneously. Composting resulted in the decomposition of organic matters and the significant variation in properties, such as moisture, pH, ammonia, dissolved organic carbon, and humus in compost mixtures within a relatively short period (Amir et al., 2005; Hsu and Lo, 2001). The reduced organic matter was the inevitable outcome of microbial degradation. As mentioned previously, the significant transformation of Ni and Cd chemical forms after the 54-d composting was closely bound to the humification of organic matter. Humus increased, and the acid-base properties of sludge changed after the complex effects of sludge aerobic composting and organic matter degradation. Ni and Cd chemical forms were also recombined by factors that affect the ferromanganese element dissolution of the slag with strong alkali; mineral conversion (Ashworth and Alloway, 2008) transformed the efficient state to a stable state. Walter et al. (2006) and Hsu and Lo (2001) reported that different raw
Funding This work was financially supported by the National Natural Science Foundation of China (51178209).
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Effects on Ni and Cd speciation in sewage sludge during composting and co-composting with steel slag Zheng-zhong Zeng, Xiao-li Wang, Jian-feng Gou, He-fei Zhang, Hou-cheng Wang and Zhong-ren Nan Waste Manag Res 2014 32: 179 DOI: 10.1177/0734242X14521682 The online version of this article can be found at: http://wmr.sagepub.com/content/32/3/179
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research-article2014
WMR0010.1177/0734242X14521682Waste Management and ResearchZheng et al.
Review article
Effects on Ni and Cd speciation in sewage sludge during composting and cocomposting with steel slag
Waste Management & Research 2014, Vol. 32(3) 179–185 © The Author(s) 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X14521682 wmr.sagepub.com
Zheng-zhong Zeng1, Xiao-li Wang1, Jian-feng Gou1, He-fei Zhang2, Hou-cheng Wang1 and Zhong-ren Nan1
Abstract Sewage sludge and industrial steel slag (SS) pose threats of serious pollution to the environment. The experiments aimed to improve the stabilizing effects of heavy metal Ni and Cd morphology in composting sludge. The total Ni and Cd species distribution and chemical forms in the compost sewage sludge were investigated with the use of compost and co-compost with SS, including degradation. The carbon/nitrogen ratio of piles was regulated with the use of sawdust prior to batch aerobic composting experiments. Results indicated that the co-composting with SS and organic matter humification can contribute to the formation of Fe and Mn hydroxides and that the humus colloid significantly changed Ni and Cd species distribution. The decreased content of Ni and Cd in an unstable state inhibited their biological activity. Conclusions were drawn that an SS amount equal to 7% of the dry sludge mass was optimal value to guarantee the lowest amount of Cd in an unstable state, whereas the amount was 14% for Ni. Keywords Chemical form, composting, heavy metals, sludge, steel slag
Introduction An average of 139 million m3 of municipal sewage is currently generated in China every day, with the emission of sludge ranging from 2.537×107 to 5.07×107 tonnes/year (80% moisture). The volume of sludge production has drastically increased as a result of rapid urbanization and industrialization as well as the need to meet ever-increasing demands on water quality. Dealing with urban sewage sludge has therefore become an urgent concern, with the speed of its production increasing between 10 and 15% annually (Wong, 2009). Compared with the landfill and incineration, the ‘biosolids’ characteristic of sludge is a good resource for agriculture and forestry. Sludge is rich in N, P, Mg, organic matters, essential trace elements, and nutrients for plant maintenance, so sludge can be widely used to maintain and restore forests and rangelands (Qiao et al., 2000). The physicochemical properties of soil can be improved through aeration and improving water-holding capacity (Wong et al., 2001), and the number of soil microorganisms can be also increased as well as the contents of nutrients of N, P, and Mg. In this regard, land application has become another attractive option for sludge treatment and disposal. In both Europe and the USA, land application has become a part of the main direction toward disposing urban sewage sludge (PRC Ministry of Housing and Urban-Rural Development, 2011). As the sludge is rich in organic matters and hydrophilic gelatinous structures, it is a good addition
to repair and improve the Chinese loess which is barren and vulnerable to soil erosion. However, heavy metals and several potentially harmful organic substances are major factors that restrict the agronomic and forestry applications of sewage sludge. Heavy metals can be removed from sewage sludge through source control or chemical methods. However, the source control method is costly, and determining the sources of heavy metal pollution is difficult. Chemical methods are also expensive, complicated to operate, and difficult to scale. Numerous studies have achieved good results in passivating heavy metals in sludge by using additives, such as fly ash and lime amendments, to change heavy metal chemical forms (Djedidia et al., 2009; Fang and Wong, 1999). However, no study has been conducted on the cocomposting of sewage sludge with steel slag (SS). 1College
of Earth and Environmental Sciences, Lanzhou University, Lanzhou, PR China 2Tianjin Environmental Sanitation Engineering Design Institute, Tianjin, PR China Corresponding author: Jian-feng Gou, College of Earth and Environmental Sciences. Lanzhou University, No. 222, South Tianshui Rd, Lanzhou 730000, Gansu Prov., China. E-mail: [email protected]
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Table 1. Distribution of the chemical forms of Ni in raw materials.
Sewage sludge Control sludge Sawdust Steel slage
EF
CBF
FMOF
OBF
RF
Total
1.51 (8.84) 82.13 (39.67) 0.88 (6.68) 0.34 (1.85)
1.46 (8.57) 56.21 (27.15) 1.20 (9.11) 1.775 (9.70)
4.635 (27.13) 44.29 (21.39) 1.75 (13.24) 2.575 (14.23)
4.97 (29.12) 13.59 (6.56) 0.86 (6.53) 4.225 (23.36)
4.50 (26.34) 10.82 (5.23) 8.49 (64.44) 9.2 (50.86)
25.73 207.04 13.18 18.09
Values are mg/kg (%). CBF, carbonate-bound fraction; EF, exchangeable fraction; FMOF, Fe-Mn oxide fraction; OBF, organic-bound fraction; RF, residual fraction.
Table 2. Distribution of the chemical forms of Cd in raw materials.
Sewage sludge Control sludge Sawdust Steel slage
EF
CBF
FMOF
OBF
RF
Total
0.04 (3.47) 8.52 (40.67) 0 0
0 6.95 (33.15) 0 0
0.59 (57.92) 3.77 (17.98) 0.03 (19.23) 0
0.1 (9.41 0.50 (2.39) 0.05 (38.46 0
0.32 (28.71) 1.22 (5.83) 0.06 (42.31) 0
1.01 20.95 0.13 0
Values are mg/kg (%). CBF, carbonate-bound fraction; EF, exchangeable fraction; FMOF, Fe-Mn oxide fraction; OBF, organic-bound fraction; RF, residual fraction.
In China, SS is the second largest source of solid waste and it accounts for 12–15% of raw steel production; the development of a method to efficiently dispose SS is therefore urgently needed. SS is a molten mixture of a variety of metal oxides with strong alkaline characteristics, including the main Ca, Fe, and Si oxides and small amounts of Mg, Al, Mn, and P. The main mineral components of SS are C2S, C3S, monticellite, Ca, Mg, rhodonite, and f-CaO. Besides being used as a metallurgical and construction raw material, SS can be used as slag phosphate, silicon, and acidic soil conditioner for agriculture (Ren et al., 2008). Currently, the widely accepted belief is that the total amount of heavy metals in sewage sludge cannot provide sufficient information on the potential hazardous effects of such metals on the environment; this is because the biological activity, mobility, and ecotoxicity of heavy metals depend strongly on their specific chemical forms (Obbard, 2001; Planquart et al., 1999; Wang et al., 2006). Chemical forms typically consist of the following: exchangeable fraction (EF), carbonate-bound fraction (CBF), Fe-Mn oxide fraction (FMOF), organic-bound fraction (OBF), and residual fraction (RF). The first two forms are appropriate for migration, easy for biological absorption, and highly harmful to crops and forest resources (Lake et al., 1984; Lin and Zhou, 2008). SS can be used as a sludge composting additive because of its alkalinity and richness in f-CaO, Fe, and Mn components. This study aims to examine the effect of heavy metals (Ni and Cd) in compost sludge by investigating the effects of SS and changes in organic matter, as well as the migration and transformation among the five chemical forms of the heavy metals. The study also determines the optimal amount of slag and identifies new ways to reduce and control sludge heavy metal activity. The findings are expected to help improve land utilization and to realize the potential of sludge for agriculture- and forestry-related applications.
Materials and methods Experimental materials Dewatered anaerobically digested sewage sludge was collected from Yanerwan wastewater treatment plant in Lanzhou, China. Sawdust and SS were respectively obtained from decorations in Lanzhou University and from Yuzhong steel plant in Lanzhou City. The size of sawdust particles was 1–20 mm, and sawdust moisture was about 5%. SS was crushed to 0.149 mm for experimental use. The initial municipal sewage sludge had a low concentration of heavy metals, in which Cd bound in CBF was not even detected. Nitrate of Ni and Cd were added into the original sludge to achieve full five chemical forms, with their concentration close to that prescribed by the China Construction Standard on the Disposal of Sludge from Municipal Wastewater Treatment Plant – Quality of Sludge Used in Gardens or Parks (GBT23486-2009, PRC Ministry of Housing and Urban-Rural Development, 2009). However, such a concentration is not beyond the maximum of 420 mg/kg (85 mg/ kg per pollution; regulation standard of the US Environmental Protection Agency, 2007). Detailed information on the amount of the five forms of Ni and Cd is shown in Tables 1 and 2 and the elements in the SS are listed in Table 3.
Composting facilities The composting equipment included six laboratory-scale simulated fermentation tanks with an effective volume of 31 l each. Two layers of gauze were covered on the porous sieve plate at the bottom of each tank. The sewage sludge was air-dried at ambient temperature. The moisture contents of the six well-mixed materials were maintained between 50 and 60% throughout the aerobic composting phase. Air was used to aerate the sludge from the gauze located at the bottom, which was supplied by a hot air blower. The six groups of composting sludge could reach a
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Zheng et al. Table 3. Elements in steel slag. Element
%
Mg Al Si S Cl K Ca Ti Mn Fe
0.72 1.16 8 0.62 0.3 0.26 56.1 1.28 1.75 29.81
pH=12.25.
Table 4. Experimental pilot programmes and components. Programme
Materials
Ratio (w/w dry weight)
SS0 SS7 SS14 SS21 SS28 SS35
Sludge/sawdust Sludge/sawdust/fly ash Sludge/sawdust/fly ash Sludge/sawdust/fly ash Sludge/sawdust/fly ash Sludge/sawdust/fly ash
14:2 14:2:0.98 14:2:1.96 14:2:2.94 14:2:3.92 14:2:4.90
SS, steel slag.
temperature as high as 50°C which lasted 3 days during the 54-day composting process. The dewatered sludge may lack sufficient porosity for adequate aeration and demonstrate an imbalanced carbon-to-nitrogen (C/N) ratio. Bulking agents are needed to provide structural support when the compost materials are too wet to maintain air spaces with the compost pile and to reduce moisture content or change the C/N ratio (Wang et al., 2008). The literature (Zheng et al., 2009; Li, 2012) and experimental results indicate that the best C/N ratio of raw composting materials is 25–35. The sludge ranged from 5–8, and the sawdust was approximately 200. Sawdust was mixed with the dewatered anaerobically digested sewage sludge as a bulking agent at 7:1 (w/w, dry weight) to obtain a C/N ratio of approximately 30. The sewage sludge–sawdust mixture was then mixed thoroughly with SS at 0, 7, 14, 21, 28, and 35% (w/w dry weight) with a concrete mixer. Enhanced heavy metal sludge without SS (SS0) was used as a control. Six pilot programmes and components were listed in Table 4.
Project determination and methods Six groups of the mixed materials of the sludge, sawdust, and SS were evenly stirred, and three sludge samples in each treatment were taken from three point of the fermentation tanks diagonally. Sludge samples were oven dried at 105°C, ground into powder, and sieved through a 0.149-mm sieve. Then the three sludge samples of each treatment were thoroughly mixed and homogeneous samples were remove, using the method of
quartering, which was used as the detection sample of each treatment (China Environment Protection Agency, 1997). The total heavy metal concentrations of Ni and Cd were digested first with 65% HNO3 and 40% HF, and then with 72% HClO4 (Walst, 1971) until white smoke completely disappeared, at 110, 120, and 210°C, respectively. The method of sequential extraction performed in this study followed that of Tessier et al. (1979); although this procedure is still plagued by some limitations, it is generally accepted (Zimmerman and Weindorf, 2010). The chemical reagents used as extractants for heavy metals in the sequential extraction are listed in Table 5. The EF, CBF, FMOF, OBF, and RF were analysed. Dry sewage sludge sample (1 g) was placed in a 50-ml centrifuge tube. Chemical reagents in different ratios were added to the sludge by steps, and different forms of extracted liquid were obtained. Each extraction step sample was then centrifuged at 2395×g for 10 min. The supernatants were transferred in a volumetric flask, and deionized water was added to adjust the volume to 50 ml. The solution was filtered through a 0.45-µm filter paper and analysed with a flame atomic absorption spectrophotometer (Themol-M6 American). Total organic carbon was determined by oxidation with K2Cr2O7 in a concentrated H2SO4 medium, and dichromate excess was measured with (NH4)2Fe(SO4)2 (Yeomans and Bremmer, 1989). The pH values of the sludge samples were identified with the extract at a sludge/ deionized water ratio of 1:5 (w/v) with the use of a digital pH meter.
Results and discussion Changes in the total amount of the heavy metals during composting The variation in the total amount of Ni and Cd during composting is shown in Figure 1. After high-temperature composting, the Ni content in the SS0 control test showed an obvious decline from 261.97 to 193.20 mg/kg, whereas an increment of 0.58 mg/kg for Cd was noted. These results agreed with the increase in tissue Cd content of plant as a result of the incremental rate change in sewage sludge; the tissue Ni content did not increase after the change in compost because of the low Ni content in the sewage sludge compost (Wong and Lai, 1996; Simeoni et al., 1984); however, both Ni and Cd total concentrations decreased in the other five treatments. In the six treatments, the total content in latter experiments were all less than that in the SS0 whether before or after the composting. This result may be attributed to the dilution effect of the slag and its strong alkalinity, which prompted the changes in sludge microbial community. As a result, Bacillus pasturii and Bacillus alcalophilus became metabolically active. They leached the heavy metals from the piles and hence decreased the total content.
Effects on Ni and Cd Speciation by cocomposting with the addition of steel slag Nickel. The sludge in different wastewater plants did not demonstrate a fixed distribution for Ni in the different chemical
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Table 5. Chemical reagents and procedures used in the sequential extraction steps of heavy metals. Steps
Fractions
Reagent added to sludge (ratio, v/w, with dry matter)
Duration of extraction (h)
1 2 3 4
Exchangeable Carbonates Fe-Mn oxides Organic
1 8 4 5.5
5
Residual
1 M MgCl2 (8:1) 1 M NaOAC/HOAC (8:1) 0.04 M NH2OH.HCl (20:1) 0.02 M HNO3 (3:1); 30% (v/v) H2O2 (10:1); 3.2 M NH4Ac (5:1) 65% (v/v) HNO3 [15:0]; 40% (v/v) HF [10:0]; 72% (v/v) HClO4 [5:0]
To end of reaction
Figure 1. Variation in the total amount of Ni and Cd in the test sludge during composting. SS, steel slag.
Figure 2. Ni speciation change and distribution during composting.
CBF, carbonate-bound fraction; EF, exchangeable fraction; FMOF, Fe-Mn oxide fraction; OBF, organic-bound fraction; RF, residual fraction; SS, steel slag.
forms. Wong and Lai (1996) reported that Ni was predominant in CBF, FMOF, and OBF. Dudka and Chopecka (1990) determined that Ni was a major component in FMOF. In this study, Ni was a major component in EF and CBF (Figure 2). Ni was easily absorbed, and it caused toxic effects on the plant. In the SS0, the EF and CBF proportions decreased from 40.04–7.75% and 25.77–17.22%, respectively. Increments of 24.79 and 17.32% for FMOF and OBF, respectively, indicated the low biological activity of Ni in the sludge after composting. A large effect on Ni chemical forms with the different amounts of SS was noted during the composting process. After cocomposting with SS, the effective state (EF and CBF) proportions of SS35, SS28, SS21, SS14, and SS7 were individually reduced by 41.87, 44.01, 39.97, 43.68, and 38.58%, respectively. The significant increase in FMOF and OBF fractions and the decrease in EF and CBF fractions suggested that the addition of SS could effectively prevent Ni mobility. With the effect of
composting and the proper use of SS resources considered, 14% of SS was determined as the appropriate amount for Ni. Cadmium. Cd is an active and hazardous heavy metal. Figure 3 indicates that the predominant species in the sludge before composting were EF and CBF, and ratios of EF and CBF in the six groups were all larger than 74%, a result indicating the high potential mobility and bioavailability of Cd. Therefore, sludge containing Cd should not be directly applied to agricultural fields prior to further treatment. After simple composting (SS0), the EF amount of Cd sharply declined and thus reduced Cd bioavailability. When composting disposed with SS, the FMOF ratios of SS7, SS14, SS21, SS28, and SS35 were significantly higher than that of SS0 which may be a result of the SS alkaline effect. This result prompted the EF and CBF amounts of Cd to synchronously decline, with the biological activity of Cd reduced clearly. After composting, the order of the effective state of the six sludge samples was as follows: SS0 > SS14 > SS35 > SS28 > SS21 > SS7.
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Zheng et al.
Figure 3. Cd speciation change and distribution during composting.
CBF, carbonate-bound fraction; EF, exchangeable fraction; FMOF, Fe-Mn oxide fraction; OBF, organic-bound fraction; RF, residual fraction; SS, steel slag.
In SS7, the effective state amount was reduced to a maximum of 66.28%, and the FMOF and OBF proportions increased to 66.6%, a result indicating the poor activity and mobility of SS7. Although the SS14, SS21, SS28, and SS35 proportions of EF and CBF clearly showed declining trends, the SS7 unit passivation effect was the best, and the value here was set as the suitable amount of SS to passivate Cd.
Impact mechanism Co-composting with SS can transform toxic EF and CBF into stable FMOF and OBF in heavy metals. SS is a porous material formed through high-temperature smelting, is rich in calcium oxide, iron, silicon, magnesium, aluminium, and manganese (Table 3), and appears to be alkaline in character. The characteristics of SS contributed to the formation of iron, aluminium, and manganese gender hydroxide colloid, and the pH value caused by f-CaO significantly affected the formation of FMOF. Colloid surface was positively charged when the pH value was less than the isoelectric pH value, but the specific absorption effect was weakened with the increased positive charges; as a result, the adsorption capacity of heavy metals was slowed down. When the pH value was above the isoelectric pH value, the adsorption capacity of the heavy metals dramatically increased because the colloid surface was negatively charged (Ding et al., 2001). Composting could form a large amount of humus, including carboxyl, alcoholic hydroxyl, phenolic hydroxyl, and carbonyl functional groups. Once the humus dissolves in the solution of the heap body, several acidic functional groups dissociate. Humic acid or fulvic acid is highly fragmented and forms organic–metal sludge complexes by the combination effect of bivalent and multivalent metals because of the mutual repulsion of charged groups. Therefore, EF was transformed to OBF. A single humus molecule is connected to other molecules to form a chain-like structure via polyvalent cations; coagulation then occurs (Chen et al., 2008; Stevensen, 1982), which is also conducive to the formation and stabilization of heavy metal OBF. Figure 4 indicates that the pH value increased gradually with an increase in the amount of SS, and the value changed between 7.68 and 8.6. In the SS0 test (Figures 2 and 3), the heavy metal RF ratios decreased after the composting because of the complex role of heavy metal characteristics and heap material humification.
However, the Ni and Cd residual fraction changed irregularly when composting with SS. Except for SS28, the RF amounts for Ni in the other four experiments all decreased. By contrast, except for SS7, the RF amounts for Cd clearly increased. Figures 2 and 3 also show that a large number of effective states were transformed into the stable state, a result indicating that the SS alkaline effect influences the chemical fraction distribution of metals. This finding is attributed to SS alkalinity, which neutralizes carbonic acids and provides reactant circumstance to form FMOF and OBF. Therefore, the change in SS was significant in reducing the mobile and easily available fractions of Ni and Cd. In the case of Cd, slagadded composting enabled the toxic EF to be transformed significantly into other stable states and thus significantly reduced the biological activity of Cd.
Influence of organic matter by SS-added composting The organic matter of sludge can improve soil structure, water retention capacity, and permeability. The organic matter influences the effect of fertilizers (Zhang and Pang, 2005). It also has a significant effect on heavy metal pollution and global carbon balance (Xu et al., 2007). Aerobic decomposition of organic matter is a complex process, which is shown in the following:
OM + O 2 + Nutrient Microorganism Cytoplasm + CO 2 + H 2O + NH3 + SO 4 2 − + ... + Heat
(formula 1)
where OM is ‘organic matter’. Most of the organic matter of sludge were degraded with the gradual deepening of compost, the large amount of humus formed, and the NH3 that was generated from biological decomposition in the maturity composting stage and that was transformed into nitrates by nitrification bacteria:
22NH 4 + + 37O 2 + 4CO 2 + HCO3 → 21NO3− + C5H 7 NO 2 + 20H 2O + 42H +
(formula 2)
After addition of the slag, the OH– of mineral H+ was found in the neutralization reaction. Formula 2 is moved to the right part of the equation, resulting in formula 1 also being moved to the right. The organic matter proportions decreased consistently with an
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Waste Management & Research 32(3) compost materials can affect heavy metal speciation, which can result in different evolutions of physicochemical parameters during composting. After the SS-added composting, the sludge exhibited high stability and low bioavailability. It can be applied to the loess as an organic soil fertilizer to enhance soil water retention capacity and improve the ecological environment of the loess areas in northwest China.
S+
%HIRUH
$IWHU
Conclusions
66
66
66
66
66
66
Figure 4. pH value changes of the heap materials before and after composting. SS, steel slag.
66 66
66 66
66 66
7G Figure 5. Organic matter changes during composting. SS, steel slag.
Composting or co-composting using SS reduced the total amount of heavy metals, as some heavy metals were leached out with the leachate. SS-added compost efficiently contributed to passivating the effective state of heavy metals in the sludge. The strong alkalinity of SS and the effect of degradation of the organic matters effectively transformed the Ni and Cd unstable state into a stable state. The humification generated from the degradation of organic matter also catalytically affected the conversion of the five heavy metal chemical forms. Cd reached the maximum effect of passivation when the slag-addition ratio was 7% of the dry sludge, whereas that for Ni was 14%. Provided that Cd is more toxic than Ni, the best optional amount of SS7 to control Ni and Cd biological activities during composting was determined. When applied into practice, sewage sludge that is used for agriculture or forest-related applications must be treated directly with composting or with the addition of SS. The addition ratio of SS can vary by 7 or 14% on the basis of the Ni and Cd contents, respectively, in the sludge to reduce the content of the unstable state and achieve passivation of the metals.
Declaration of conflicting interest The authors declare that there is no conflict of interest.
increase in the amount of SS. As a result, the organic matter of sludge was fully degraded. Six experimental sludge contents of organic matters decreased along with compost (Figure 5). The decreasing trend confirmed the dilution effect of the increased SS amount when the samples were measured simultaneously. Composting resulted in the decomposition of organic matters and the significant variation in properties, such as moisture, pH, ammonia, dissolved organic carbon, and humus in compost mixtures within a relatively short period (Amir et al., 2005; Hsu and Lo, 2001). The reduced organic matter was the inevitable outcome of microbial degradation. As mentioned previously, the significant transformation of Ni and Cd chemical forms after the 54-d composting was closely bound to the humification of organic matter. Humus increased, and the acid-base properties of sludge changed after the complex effects of sludge aerobic composting and organic matter degradation. Ni and Cd chemical forms were also recombined by factors that affect the ferromanganese element dissolution of the slag with strong alkali; mineral conversion (Ashworth and Alloway, 2008) transformed the efficient state to a stable state. Walter et al. (2006) and Hsu and Lo (2001) reported that different raw
Funding This work was financially supported by the National Natural Science Foundation of China (51178209).
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