Fig. 5 showed that at the same dosage (0.05gAl/gTSS) obvious distinctions in EPS distribution were observed for different aluminum salt coagulants. As shown in Fig. 5, concentrations of all sludge EPS fractions were decreased under chemical conditioning with different aluminum salt coagulants, indicating that both LB-EPS and TB-EPS were compressed. Therefore, the sludge flocs became more compact upon the addition of aluminum salt coagulants and floc strength was enhanced. This observation was in agreement with the report of Zhang et al., who found that EPS AZD2461 was the major mechanism of sludge conditioning with inorganic coagulants. From Fig. 5 can see that the concentration of extractable EPS (LB-EPS and TB-EPS) reduced when the sludge was treated with aluminum salt coagulants. SEPS concentrations conditioned by Al13 and PAC2.5 were lower than other aluminum salt coagulants, decreased from 8.85 mgDOC/gTSS to 4.06 and 5.99 mgDOC/gTSS respectively. This observation demonstrated that Alb and Alc showed better performance in compressing EPS structure than Ala. It was consistent with particle size analysis, the sludge flocs formed by Alb and Alc were more compact and less compressible. In addition, acidification was observed after addition of aluminum salt coagulants, it was more evident for aluminum salts with low basicity (see in Table S1). As mentioned above, this acidification might affect the subsequent sludge treatment and disposal. It was reported that acidic treatment significantly change the chemical structure of organic matters (e.g. protein-likes) and this might lead to the dissolution of sludge EPS and cations (e.g. Ca2+, Mg2+, Al3+ and Fe3+, etc.) (Zhang et al., 2015a). From Fig. 5 can see that the TB-EPS concentrations conditioned by Al13 and PAC2.5 increased again comparing to other aluminum salt coagulants, this also indicated that Alb and Alc showed better performance in compressing EPS structure than Ala, which made SEPS and LB-EPS transfer into TB-EPS, so TP-EPS conditioned by Al13 and PAC2.5 increased when SEPS and LB-EPS concentration conditioned by Al13 and PAC2.5 decrease, so their TB-EPS concentration was increased. Many studies have demonstrated that sludge filtration property was mainly dependent on properties of SEPS fraction, and the high content of protein in SEPS was always detrimental to sludge filterability (Zhang et?al., 2014a, Zhang et?al., 2014b and Zhang et?al., 2015b). In addition, reduction of SEPS concentration contributed to improvement of sludge filterability, it was obvious that PAC with high alkalinity and Al13 were more effective in removal of SEPS. In order to get insights into the changes in chemical composition, EPS samples were characterized by EEM and HPSEC later.
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