Similar to the SWW, although the experimental data obtained from testing the SEWW fitted well to the first- and second-order kinetics (Table 4B and Fig. SI-3), the minimal differences between qexp and q demonstrate the second-order kinetic better fits than the first-order. Comparing with the SWW, both rate coefficients (k1 and k2) of iron-based sludge found to be decreased when the SEWW was tested (Table 4A and Table 4BA and 4B). In oppose to iron-based sludge, k1 and k2 values of alum-based sludge (Sludge-B and -C) marginally increased and a significant increment of both values were noted when Sludge-D was used (Table 4A and Table 4BA and 4B).
3.4.2. Influence of iron and aluminium concentrations on P removal rate
Fig. 3 clearly demonstrates an increased P removal rates with increasing Al to Fe mass ratio from both SWW and SEWW. A good correlation (R2 = 0.997 for k1 and R2 = 0.780 for k2) between reaction rate coefficients (k1 and k2) and Al to Fe mass ratio was noted when alum-based sludge were considered in removing P from SWW. Further, similar relation (R2 = 0.997 for k1 and R2 = 0.840 for k2) was noted when iron-based and alum-based sludge were analysed together. However, a poor correlation (R2 = 0.426 for k1 and R2 = 0.109 for k2) was observed when the test was conducted using SEWW (Fig. 3B). Although the metals ions concentrations were the highest in Sludge-A, P adsorption rates were the lowest for both types of wastewaters reporting P removal rates are irrespective to the ion concentrations in sludge.