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Carbon and phosphorous removal from primary municipal wastewater sludge using recovered aluminum Tulip Chakraborty, Michelle Gabriel, Ali Amiri, Domenico Santoro, John Walton, D. Scott Smith, Madhumita B. Ray, and George Nakhla Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03405 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 16, 2017
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Carbon and phosphorous removal from primary
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municipal
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aluminum
wastewater
sludge
using
recovered
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Tulip Chakrabortya, Michelle Gabrielb, Ali Safarzadeh Amirib, Domenico Santoroa,b, John
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Waltonc, Scott Smithd, Madhumita Raya*, George Nakhlaa*. a
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- Department of Chemical and Biochemical Engineering. University of Western Ontario,
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London, Ontario, Canada. b
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- Trojan Technologies, London, Ontario, Canada. c
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d
- USP Technologies, Atlanta, Georgia, USA.
- Department of Chemistry and Biochemistry, Wilfrid Laurier University, Waterloo, Ontario,
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Canada.
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KEYWORDS: chemically enhanced primary treatment, coagulant recovery, phosphorous
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removal, primary sludge.
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ABSTRACT: In this work, recovery of aluminum from coagulated primary sludge and its reuse
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potential as secondary coagulant were investigated. The recovery process consisted of releasing
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the particle-bounded aluminum from primary sludge by acidification (HCl or H2SO4), followed
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by separation using centrifugation for dissolved coagulant recovery. The recovered coagulant
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was then reused for treating primary wastewater and overall coagulation efficiency was
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determined. While with fresh alum, the removal efficiencies of total suspended solids, chemical
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oxygen demand, total phosphorous, and total nitrogen were 85%, 65%, 80% and 33%,
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respectively, a drop in removal efficiency of total suspended solids and chemical oxygen demand
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was observed for recovered aluminum (85% to 60% and 65% to 50%, respectively). Nitrogen
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concentration remained almost constant with each cycle, while phosphorous in the effluent
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increased by 1 mg/L and 3 mg/L in the first and second cycle, respectively. Precipitation of
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various aluminum species was modelled for determining the recovery potential of aluminum at
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low pH. Preliminary cost analysis indicates that optimum recovery of aluminum occurred at a pH
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of 1.5 for both acids. Struvite precipitation effectively removed increased phosphorus solubilized
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by acidification at the end of second cycle, however, it also decreased the amount of aluminum
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available for recycle.
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GRAPHICAL ABSTRACT
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1. Introduction
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The worldwide production of municipal wastewater exceeds 330 billion m3 per annum [1,2,3].
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As municipal wastewater treatment plants focus on resource recovery and energy efficiency,
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chemically enhanced primary treatment (CEPT) is re-gaining importance due to its enhanced
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removal of organics, thus simultaneously increasing biogas production while reducing aeration
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energy and excess biological sludge production. The added coagulant in the CEPT process
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generates significant amounts of chemical sludge contributing up to 5% - 20% of the total solids
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with ferric coagulant and 15% - 40% with alum [4,5,6]. Thus, treating these solids or sludge to
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recover coagulants not only offsets the cost of disposal by decreasing chemical sludge volume
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but also the cost of dosing fresh coagulant in wastewater [7,8,9]. Different separation techniques
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such as acidification, basification, ion exchange and membrane processes have been tested to
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separate various coagulants from sludge [8]. While membrane processes (pressure filtration
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membranes) have been tested in purification of the recovered coagulant [10,11], their inability to
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adequately remove dissolved contaminants, coupled with membrane fouling is a significant
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limitation. Precipitation due to basification has shown recovery of aluminate salts at a pH of 11.4
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[12], however, the cost of sodium hydroxide, twice that of sulphuric acid coupled with recovery
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at high concentrations (950 mg/L Al) makes it a challenging process [13]. While other alternate
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base such as calcium hydroxide (cheaper than sodium hydroxide) was used, only 50% recovery
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of aluminum at a pH of 11.4 was possible due to the lower solubility of calcium salts compared
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to aluminate salts [14]. Ion-exchange processes require organic solvents for regeneration and
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their marginal cost benefit for low value coagulants make them unfavourable [13]. The most
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efficient and cost-effective method to recover coagulant from the chemical sludge has been
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found to be acidification, which involves neutralizing the flocs of hydroxide precipitant to
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release the coagulant salt into solution [15,16,17]. The reused coagulant was able to remove
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turbidity, chemical oxygen demand (COD), total suspended solids (TSS), and total nitrogen (TN)
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from wastewater [18,19,20,5,6]. Unfortunately, acidification is not selective and at low pH other
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contaminants such as organic matter, metals, nitrogen, and phosphorous can also be dissolved
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[15,8].
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Previous studies have focussed on coagulant recovery and their reuse in drinking water
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treatment [21,22]. However, dosing recycled coagulants into the drinking water process resulted
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in higher levels of disinfection by- products (DBP) formation due to higher dissolved organic
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carbon (DOC) at the final chlorination step. Hence, in the case of drinking water applications it is
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not advisable to use the recovered coagulant directly without subjecting the coagulant to further
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purification.
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Addition of the coagulated sludge without any further processing was evaluated on various
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types of waste streams. Chu [23] demonstrated the use of recycled alum sludge as a coagulant to
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remove dyes from textile dying wastewater. While the recycled sludge reduced the hydrophobic
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dye, addition of fresh coagulant to supplement the recycled coagulant was necessary to remove
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hydrophilic dye, which was detrimental for the recycled water quality.
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Published research on aluminum recovery from biosolids (generated from primary wastewater
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treatment) is limited [8,17,24] mostly due to the complex nature of wastewater as compared to
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drinking water and other effluents. While Xu et al [8] and Ishikawa et al [17] focussed on
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aluminum recovery from wastewater and clarifier sludge, they did not elucidate the effect of
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organics on subsequent cycles due to coagulant recovery. Jiménez et al [24] investigated the
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secondary sludge stabilization with sulphuric acid and the effects on various mixing aspects
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while focussing on inactivation of microorganisms. These studies mainly focussed on the
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recovery of coagulants and did not investigate the effect of the recycled coagulant on other water
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quality parameters such as dissolved organics and nutrients. Therefore, the main objective of this
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study was to assess the impact of recycled aluminum on the removal efficiency of the various
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water quality parameters like total phosphorous (TP), soluble phosphorous (sP), COD, soluble
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chemical oxygen demand (sCOD), TSS and TN and to address the organic loading. The optimum
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acidification pH for the maximum recovery of alum coagulant was established, and the impact of
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recycled aluminum on the fractionation of additional organic loading in terms of TSS, TP, sP,
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COD, sCOD and TN was characterized.
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[2] Methods and Materials:
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[2.1] Aluminium sulphate dose optimization
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The different water quality parameters such as TP, sP, TN, TSS, COD and sCOD of the PI
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collected from the Pottersburg water pollution control plant (WPCP) located in London, Ontario,
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Canada were measured. A 10% aluminium sulphate [Al2 (SO4)3.18H2O] stock solution in
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deionized water was used to treat 1L of PI with a phosphorous: aluminum molar ratio of 1:2.
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Using a jar test apparatus (Phipps & Bird PB-900), the solution was mixed at a fast rate (100
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rpm) for 1 minute followed by a slow rate (30 rpm) for 20 minutes. The flocs were allowed to
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settle for 30 minutes and the effluent was analyzed for TSS, TN, TP and COD.
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[2.2] Aluminum recovery and reuse:
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The coagulated water from the jar test was centrifuged at 3500 rpm for 5 min and the sludge
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was collected. The alum was recovered by acidifying the sludge to a pH of 0.5, 1.5 or 2.5 with
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concentrated H2SO4 (36N) or HCl (12N). The acidified sludge was mixed using a magnetic
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stirrer at 170 rpm for 60 min, and then centrifuged at 3700 rpm for 10 min. The supernatant was
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removed and was analyzed for aluminum concentration using ICP-MS (ICP-OES Vista Pro
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Axial, Varian, Australia). The recovered aluminum from the supernatant of the centrifuged
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sludge was then reused to dose a fresh sample of PI and the procedures described above were
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repeated. There was no significant effect on the pH of PI by the addition of recovered coagulant
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due to the large difference in volume of the recovered coagulant and PI, decreasing it slightly
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from 7.1 to 6.9.
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[2.1.1] Analytical Methods:
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Solution pH was measured with a digital pH-meter (HACH, HQ11d). COD (method 8000),
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sCOD (solution filtered through 0.45 µm) (method 8000), TP (method 10209), sP (method
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10209), TN (method 10242) were measured using HACH methods. TSS was measured
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according to standard APHA methods (method 2540d) [25]. ICP-MS (ICP OES Vista Pro Axial,
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Varian, Australia) was used to measure aluminum concentration.
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[2.1.2] Characterization of sludge:
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The sludge obtained from the centrifugation of PI dosed with alum was characterized for
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sludge volume index (SVI). To quantify the amount of residual aluminum (after acidification) in
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the sludge (denoted henceforth as waste sludge), it was digested with nitric acid for 2 hr and
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analyzed by ICP-MS.
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[3] Results and discussions:
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[3.1] Characteristics of the influent wastewater:
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PI was collected from the Pottersburg WPCP. The sampling was conducted on heavy rainy as
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well as bright sunny days; as a result, water quality varied considerably and the average values of
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TSS, TP, sP, COD, sCOD, TN and pH were 240 ± 87 mg/L, 6 ± 2 mg/L, 2.9 ± 0.64 mg/L, 480 ±
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37 mg/L, 230 ± 21 mg/L and 7.4 ± 0.5, respectively (Table S1).
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The phosphorous in the PI was measured and the aluminum-to-phosphorous molar ratio was
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set to 2:1 for dosing of aluminium sulphate in PI. For data analysis, the TP concentrations in the
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water were divided into 3 groups: 3.3 – 4.9 mg/L, 5.2 – 6.5 mg/L and 6.9 – 8.2 mg/L. The effects
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of coagulant dosing on the different water quality parameters i.e. TSS, COD, TN and TP were
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studied for each phosphorous concentration and are presented in Table 1. The percentage
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removal of suspended solids in the PI marginally increased (2% - 3%) in the different TP levels
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(Table 1). The particulate phosphorous (PP) was removed by enmeshment in the solids. COD
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removal at three different phosphorus concentrations showed no significant difference. Since on
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an average the soluble COD accounted for 50% of the total COD, removal efficiencies of ~ 65%
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for total COD are noticeably high when compared to an average of 85% TSS removal. This is
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possibly due to the removal of sCOD by adsorption on freshly formed aluminum hydroxide [26].
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Table 1. Removal of TSS, COD, sP, PP due to alum coagulation at different initial total
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phosphorous concentrations. TP (mg/L)
TSS removal (%)
COD removal (%)
sP removal (%)
PP removal (%)
TN removal (%)
3.3 – 4.9
84 ± 6.4
64 ± 7.2
82 ± 0.64
63 ± 7.1
15 ± 5.8
5.2 – 6.5
86 ± 1.1
64 ± 7.3
82 ± 0.4
68 ± 2.2
25 ± 5.6
6.9 – 8.2
86 ± 1.7
65 ± 2.7
94 ± 0.05
80 ± 0.16
20 ± 2.7
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Table 1 shows increasing removal of phosphorus with higher initial concentrations.
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Conducting ANOVA analysis (with a 95% confidence level) on particulate and soluble
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phosphorous, the differences in soluble phosphorous removal efficiencies at different initial TP
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concentrations were deemed as insignificant and the removal of the particulate fraction as
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significant. A slight variation in the removal of TN was observed probably due to the varying
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particulate nitrogen concentrations in the wastewater.
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After coagulation, the effluent was analyzed for different water quality parameters (aluminum,
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TP, sP, TN, COD, sCOD and TSS), and was centrifuged to separate the sludge from the effluent.
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The sludge was then treated with acid for repeated recovery of aluminum (refer to the graphical
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abstract).
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[3.2] Aluminum recovery at different pH
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In this research, concentrated HCl and H2SO4 were used separately to acidify the sludge to pH
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0.5, 1.5 and 2.5. The recovery of aluminum from the acidified sludge at three different pH values
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is shown in
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Figure 1.
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The recovery of aluminum shown in
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Figure 1 represents the aluminum present in acidified sludge (obtained from coagulation of PI,
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and as determined by ICP) and not inclusive of the aluminum present in the effluent and the
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spent sludge. The total mass balance on aluminum includes its concentration in the effluent (1.4
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± 0.1%) and the spent sludge (21 ± 3.1%).
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Figure 1. Recovery of aluminum from acidified sludge at different pH
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It is well-established that at low pH, solubility of metals in water increases [27]. It is also
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worth mentioning that addition of concentrated acid helped in the reduction of sludge volume
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considerably. Acidification with both HCl and H2SO4 achieved very similar recoveries as both
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acid anions, i.e. sulphate and chloride, complex weakly with aluminum. At a pH of 0.5, about 84
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± 2.2% of the aluminum from the sludge was released using H2SO4, while HCl was able to
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release about 86 ± 1.3% of the aluminum. At a pH of 1.5, aluminum recovery using H2SO4 and
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HCl was 77 ± 3.1% and 76 ±3 %, respectively, while at pH 2.5, recovery was only 48 ± 13% and
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55 ± 7%, respectively. As stated earlier, the addition of acid would not only help in releasing the
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bound aluminum but also various other metals present in the sludge. The concentrations of
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released metals from the sludge by acidification with either of the acids (HCl or H2SO4) were
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measured using ICP-MS (Figure S3).
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Xu et al [9] reported that while recovering aluminum from the sludge, there is a high
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possibility of recovering other metals likes iron, magnesium, manganese, copper, which is
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confirmed in this work. The average concentrations of copper, manganese and iron in the
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recovered coagulant were 1.2 mg/L, 1.8 mg/L, and 78 mg/L, respectively. The concentrations of
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heavy metals like arsenic, lead and cadmium was 0.1 mg/L, 0.7 mg/L, and 0.05 mg/L,
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respectively. Although there was a minor accumulation of the heavy metals, there was no
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significant increase (
