Washing ratios of 1:10; 1:30 and 1:50

Based, on the results obtained in the washwater conductivity analysis the S:DW washing ratios of 1:10, 1:30 and 1:50 were separately analyzed with the collected dried sludge generated from the LLP at pH 12 with the mixed Na2CO3-NaOH reagents as precipitant. The washes performed at the different washing ratios are illustrated in figure 71.

Figure 71. Washing of collected dried sludge for recovery of CaCO3-rich solids

After the 24 h shaking period at 300 rpm, the washwater was separated from the washed solids by centrifugation-decantation and about 200 mL of the washwater was filtrated for further analysis. The average pH, T and conductivity parameters

0 5 10 15 20 25 30 35 40 45 50

10 20 30 40 50 60 70 80 90 100 110

Washwater-Cond. (mS/cm)

DW/Solids

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measured in the filtrated washwater were compared with the LLP as indicated in table 46. The results are given in table 47.

Table 47. pH , T & Cond. in washwater at different solids (S) to washwater (deionized water) ratios

Washing ratio

As seen in table 47, the pH values in the washwater at the different washing ratios were almost the same with values around 10. However, the conductivity decreased with the increased in the amount of washwater, which might indicated only dilution of the soluble fraction present in the collected dried sludge as part of the permeate that remained in the sludge as indicated in figure 64.

Furthermore, in the filtrated washwater chemical analysis were performed at the different washing ratios and were compared to the LLP concentration indicated in table 45. The results for Ca, Mg and hardness are given in figure 72.

Figure 72. Ca, Mg and Hardness conc. in washwater at different solids to washwater ratios (S:DW)

As illustrated in figure 72, the concentration of Ca with values of less than 5 mg/L in each of the analyzed washwater at the different washing ratios was negligible compared to the LLP concentration, which indicated a stable Ca in the formed solid state. On the other hand, the precipitated form of Mg was less stable where significant amounts got dissolved by the washing treatment. However, based on the solids to washwater ratio of 1:50 it seemed that the dissolved amount reached a relatively finite value since its concentration decreased, which showed sign of dilution. And, the hardness in the LLP was about 4 and 9 times larger than the

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washwater values in the 1:10, 1:30 and 1:50 washing ratios respectively. The results obtained in the chemical analysis performed in the different washwater ratios for TOC, TIC and TN are illustrated in figure 73.

Figure 73. TOC, TIC & TN conc. in washwater at different solids to washwater ratios (S:DW)

Figure 73 indicated that the washing process removed organic content from the generated precipitation at the different washing ratios and the TIC concentration for the washing ratios of 1:30 and 1:50 were around the TIC value of 323 mg/L found in the LLP. Furthermore, the concentrations of TN in the washwater at each of the evaluated washing ratios were very low with values lower than 60 mg/L compared to the value of 2208 mg/L found in the LLP. Additionally, SO4 was also measured in the washwater generated at the different solids to washwater ratios. The results given in figure 74 indicated that the washing process helped in removing the SO4 salts formed after the drying of the collected sludge.

Figure 74. SO4 conc. in washwater at different solids to washwater ratios (S:DW)

Furthermore, based on the average amounts of initial dried sludge and the recovered solids after the oven-drying step, the percentage losses of solids after the washing treatment were estimated, the results are given in figure 75.

0

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Figure 75. Percentage of solids loss after washing treatment at different wash ratios

As illustrated in figure 75, with the solids to washwater ratio of 1:10 the percentage of solids loss was about 40% and for the 1:30 and 1:50 washing ratios the loss percentage increased to about 50%, which might indicated further removal of soluble and/or undesired substance from the recovered solids. Furthermore, based on the amount of the dried solids recovered from the pH 12 precipitation treatment and washing ratio of 1:10; the amount of solids generated with respect to the permeate were estimated and the results were compared to the value obtained in the smaller LLP treated volume of 0,5 L. The results of the generated dried sludge and recovered solid obtained in both treated volumes of LLP are illustrated in figure 76.

Figure 76. CaCO3-rich solids recovery efficiency in 10 L precipitation vs. 0,5 L precipitation

As seen in figure 76, compared to the 0,5 L treated permeate the overall efficiency of the recovery of solids in the larger treated volume of permeate was lower with a decreased of about 35% with respect to the values obtained in the smaller precipitation volume. This decreased in the efficiency could be linked directly to the

0

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handling of larger amounts of permeate, generated precipitation and dried solids, which increased also the probability of losses.

As indicated in table 41, for the solids to washwater ratio of 1:10 generated from the LLP at pH 12, the concentration of substances known to form soluble salts such as Cl^- and S were found in the recovered solids with concentrations of 25000 and 2780 ppm respectively. Thus, in order to evaluate the removal of these substances by the evaluated washing ratios, the composition of the solids recovered after the washing ratios of 1:30 and 1:50 were analyzed through XRF. In the analysis, since the concentration of SO4 in the LLP was relatively high it was assumed that the S found in the recovered solids was in the form of SO42- salts and Cl^- was assumed to be as NaCl. The results obtained are given in figure 77.

Figure 77. NaCl and SO4 concentration in recovered solids after washing step at different solid to washwater (S:DW) ratios

Figure 77 indicated that the removal of soluble undesirable substance such as NaCl from the precipitation might be a function of the washing ratio. For instance, based on the solids to washwater ratio of 1:10 by increasing the washing ratio to 1:30 the estimated concentration of NaCl in the recovered solids decreased considerably from 41209 ppm to 2698 ppm. Moreover, the same was observed for the estimated SO4

whose concentrations for the recovered solids after the washing ratios of 1:30 and 1:50 were lower than the LOD of 1208 ppm corresponding to sulfur.

Furthermore, the concentration of heavy metals in the recovered solids regulated by the EU Directive for Sewage sludge used in agriculture were below the limit ranges indicated in the regulation for each of the solids recovered after the evaluated washing ratios. The results are given in table 48.

0 5000 10000 15000 20000 25000 30000 35000 40000 45000

(1:10) (1:30) (1:50)

Conc. (ppm)

NaCl SO4

<3600 <3600

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Table 48. Heavy metals in recovered solids regulated by EU Directive for Sewage sludge used in agriculture

Substance Sewage Sludge^‡

Solids recovered with Washing ratio (S:DW) 1:10 1:30 1:50 Hg (ppm) 16 - 25 <0,01 - - Cd (ppm) 20 - 40 <1 <60 <60 Pb (ppm) 750 - 1200 <2 <25 <25 Cr (ppm) 1000 - 1750 8,0 <54 <54 Cu (ppm) 1000 - 1750 <2 <50 <50 Ni (ppm) 300 - 400 <2 <30 <30 Zn (ppm) 2500 - 4000 - <30 <30

‡ EU Directive of 1986 “on the protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture”

Additionally the oxides of Al, Ca, Fe, K, Mg, Mn, P and Si were estimated based on the XRF results for the solids recovered from the washing ratios of 1:30 and 1:50.

The estimated oxides were compared with the oxides corresponding to the washing ratio of 1:10 already shown in table 38 and also with typical composition found in cement kiln fees. The results are given in table 49.

Table 49. Oxides composition in recovered solids after different wash ratios and found in cement kiln feed

Substance 1:10 1:30 1:50 Feed cement

kiln^‡

Al2O3 (%) 0,04 - - 3

CaO (%) 31 37 40 44

Fe2O3 (%) 0,05 0,32 0,32 1 K2O (%) 1,3 0,29 0,27 - MgO (%) 6,9 2,8 <1,2 <3 - 5 MnO (%) 0,03 0,04 <0,01 - P2O5 (%) 0,70 0,53 0,50 - SiO2 (%) 1,8 <1,6 <0,8 14

TiO2 (%) - 0,06 0,07 -

Sum (%) = 42 42 42 -

‡(Oates, 1998)

Table 49 indicated that as the washing ratio increased from 1:10 to 1:30 to 1:50 a further decreased of K2O and MgO was observed in the recovered solids and other substances such as Fe2O3 did not changed considerably with the increased of washing ratio from 1:30 to 1:50. However, for the case of CaO the mass percentage increased with increased of washing ratio approaching the common mass percentage value of 44% found in cement kiln feed and with respect to MgO the only recovered

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solids whose concentration was above the limit of 5% found in cement kiln feed was the value from the washing ratio of 1:10. The CaO mass percentages with respect to the sum of the estimated oxides are represented in figure 78.

Figure 78. CaO mass percentage in recovered solids from different washing ratios & cement kiln feed (Oates, 1998)

Figure 78 showed that as some of the evaluated substances were washed out from the solids, the CaO mass percentage increased with increased in washing ratio approaching more to the mass percentage found in cement kiln feed and reaching a value of 94% with respect to the sum percentage of the estimated oxides for the washing ratio of 1:50. This indicated that the generated CaCO3-rich solids were very stable independently of the washing ratio conditions.