The WWTS used in this study was obtained from the Wen–chang Wastewater Treatment Plant, Harbin, China. The dewatering of WWTS was conducted by using a belt filter press, and cationic polymeric flocculants were used for the flocculation and dewatering of the activated sludge. The sludge cake generated from the activated sludge process is approximately 1.6×105 kg d–1 in wet weight with 24% solids, which is directly landfilled.
DWTS were collected from the chemical coagulation/flocculation unit of the 3rd drinking-water treatment plant in Harbin, China. The 3rd plant uses a conventional process with aluminum sulfate [Al2(SO4)3] as the primary coagulant and a small amount of activated silicic acid with no pH adjustment. Treatment includes mechanical lattice flocculation basins, settling basins, and fast filters. Sludge and backwash water are not discharged simultaneously.
The DWTS and WWTS were treated by air–dry method and were ground into sizes below 100 μm that were sufficiently fine to be mixed homogeneously. DWTS, WWTS, and water
glass [sodium silicate–Na2O·(SiO2)x·(H2O)y] were the raw materials for production of ceramsite [20]. The components of WWTS are shown in Table 1. In order to indicate the components of DWTS are similar to those of clay, and DWTS can be tested as a substitute for clay for production of ceramsite, the components of clay and DWTS are shown in Table 2 and Table 3. The modulus of water glass used in the study was 3.2 (Water glass is an important component for the pelletization of ceramsite and the eutectic point of the mixture can be reduced by the reaction of sodium silicate and air (Na2O·nSiO2·xH2O+CO2→Na2CO3+nSiO2+xH2O ), and by the dispersion of alkali metal oxide (i.e., Na2O et al.) originating from the decomposition of Na2CO3
(Na2CO3→Na2O+CO2 ) in the heating process [35]).
By simple calculation of each inorganic matter content according to the optimal parameters (DWTS/WWTS=45/55, water glass/(DWTS+WWTS)=20%, sintering temperature=1000
℃
, sintering time=35min), it can be obtained that the ratio of SiO2: Al2O3: Fe2O3: CaO in the mixture of DWTS, WWTS, and water glass for production of ceramsite is 27.2: 15.8: 6: 3.5 (SiO2 and Al2O3 are acidic oxides and Fe2O3 and CaO are basic oxides).The simulated contents (wt.%) of tested oxide (SiO2, Al2O3, Fe2O3 or CaO) are adjusted by adding the oxide or the other three oxides (Fe2O3, CaO, and SiO2 or Al2O3) to the raw materials. All the used oxides (SiO2, Al2O3, Fe2O3 and CaO) with particle sizes below 10 μm were of the highest purity and of analytical grade.
Table 1. Component analyses of dried WWTS (wt.%).
component analyses
SiO2 Al2O3 Fe2O3 CaO MgO P2O5 K2O Others Carbonaceous Matter 16.28 6.35 5.15 4.10 1.67 1.65 1.12 <0.89 <62.90
element analyses
Zn Fe Mn Si Cu Ca Mg O Cr
0.13 3.81 0.14 8.26 0.03 3.18 1.02 48.83 0.03
P Na K Al Ni C S N Others
0.84 0.34 0.95 3.36 0.10 27.41 0.04 1.03 <0.50 Table 2. Component analyses of dried clay (wt.%).
component analyses
SiO2 Al2O3 Fe2O3 TiO2 K2O MgO CaO Others Carbonaceous Matter 64.89 24.95 2.50 1.20 1.20 0.50 0.50 <0.5 <3.60
elements analyse
Zn Fe Mn Si Cu Ca Mg O Cr
0.06 1.83 0.08 32.62 0.02 0.37 0.31 44.56 0.02
P Na K Al Ti C Ba Zr Others
0. 11 0.26 0.98 15.81 0.65 1.88 0.03 0.01 <0.40
Table 3. Component analyses of dried DWTS (wt.%).
component analyses
SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O Others Carbonaceous Matter 40.61 27.36 6.99 2.62 1.89 1.28 1.05 <1.0 <17.20
element analyses
Zn Fe Mn Si Zr Ca Mg Sr Cr
0.02 4.47 0.57 20.55 0.02 1.95 1.10 0.02 0.01
Cl Na K Al Rb C Ti O Others
0.11 0.74 1.03 14.16 0.01 6.52 0.16 48.06 <0.50 Table 4. Average content of heavy metalsin WWTS at different
wastewater treatment plants (mg kg–1).
Site (China) Cd Cr6+ Cu Pb
Beijing 42.20 179.6 300 937.40
Shanghai 0.19–5.55 1.13–70 101–426 59.5–129
Shenyang 11.82 147.8 274.53 306.82
Changsha 8.1 436.5 331.3 151.8
Wuhan 0.70 72–78.6 148.4 55.1
Harbin 5.9–13.5 89–326.3 93–333.2 59.5–134 Table 5. Content of heavy metals in reference WWTS samples (mg kg–1).
NO Cd Cr Cu Pb
Minimum 1 100 100 50 Medium 25 500 250 500 Maximum 50 1000 500 1000
In leaching tests, the reference sludge samples were made with heavy metals by adding metal solutions [Cd(NO3)2, K2CrO4, Pb(NO3)2, and CuSO4 were of the analytical grade] into the dried WWTS, mixing and allowing them to react for 30 days. The contents of Cd, Cr, Cu and Pb were designed according to the basic data obtained through analysis of activated sludge at different places in China as shown in Table 4. The synthetic metal solution was prepared by dissolving 0.05 g L–1 of Cd2+, 0.1 g L–1 of Cr6+, 0.1 g L–1 of Pb2+ and 0.5 g L–1 of Cu2+ in deionized water. The simulated content of heavy metals was prepared by adding the tested heavy metal compounds into WWTS. The content of heavy metals added to the WWTS are shown in Table 5. The ceramsite prepared for leaching was made with this WWTS.
2.2. Methods
The WWTS containing heavy metals and DWTS was treated by air-dry method and was ground into sizes below 100 μm that are sufficiently fine to be homogeneously mixed. The ceramsite for determination of the stabilization of Cd, Cr, Cu and Pb was
made of DWTS, WWTS, and sodium silicate. The raw materials were mixed and pelletized to particle sizes of 5–8 mm and left at a room-temperature of about 20
℃
fora few days (about 3 days) and then the samples were dried at 110
℃
in a DHG-9070A blast roaster (China) for 24h. The heating of samples started at 20℃
, heated at a rate of 8℃
/min in a SX2-10-12 muffle furnace (China), and the samples were soaked at 200℃
, 600℃
, and 800℃
for a duration of 10min and at 1000℃
for a duration of 35min, and then these samples were naturally cooled until they reached room temperature.Bulk density which includes all voids and spaces in the volume, particle density which is also the apparent specific gravity of the aggregates includes all intraparticle voids, water absorption determined from the weight differences between the sintered and water saturated samples (immersed in water for 1h), and porosity ((1–bulk density/particle density) ×100%) were analyzed [34]. To achieve statistical soundness, at least three replicates were carried out for each sample. Thermal behaviors of samples were examined by thermodifferential and thermogravimetric analyses (DT–TG) using a ZRY–2P simultaneous DT–TG analyzer (China) while the samples were heated at a rate of 8
℃
/min from 20℃
to 1080℃
in air. Samples weighed from 4 to 10 mg in mass, and they were put into a Pt–Rh crucible with 20 taps. All curves were evaluated using the TA–instruments software. The second derivative differential thermal curve was used for determination of peak temperature. Scanning electron microscope (SEM) and energy dispersive spectrum (EDS) analyses were conducted by using S–570 scanning electron microscope and TN–5502 X–ray energy dispersive spectrum (Japan).Compressive strength of ceramsite was analyzed by using an INSTRON 5569 automatic material testing machine (USA) while the sintered ceramsite with diameter of 6–8 mm was placed vertically on the platform of the press and was pressed at a crosshead speed of 0.5 mm min–1 until it was crushed.
Toxicity of ceramsite samples was determined by using a revised method derived from toxicity characteristic leaching procedure (TCLP), a standard method used to determine waste leaching toxicity, updated on the basis of hazardous waste extraction procedure (EP) by USEPA [36]. By using this method, the leaching test was conducted with the solution prepared at a liquid–solid ratio of 1L/200g, and stirred at 110rpm for 24h or 30d. To achieve statistical soundness, at least three experiments were carried out with each sample. The supernatant was analyzed using Perkin–elmer Optima 5300DV Inductively Coupled Plasma Atomic Emission Spectrometer (ICP–AES, U.S.A). Total contents of heavy metals in WWTS or sintered ceramsite were extracted by acid digestion (using HNO3/HClO4/HF) according to USEPA SW3050 and were examined by ICP–AES. Ceramsite were ground in a small agate mortar and XRD patterns of powder ceramsite were recorded on a D/max–γ β X–ray diffractometer with 50mA and 40Kv, Cu Kα radiation (Japan). XRD analyses were conducted by using an XRD pattern database (International Centre for Diffraction Data, ICDD) and the samples were scanned for 2ζ ranging from 10 to 90°. Major components of raw materials and ceramsite containing heavy metals were analyzed by using Philips PW 4400 XR spectrometer (X–ray fluorescence–XRF, Netherlands).