The Impact of Aluminium Salt Dosing for Chemical Phosphorus Removal on the Settleability of Activated Sludge

Abstract

The use of metal salts like aluminium in the precipitation of phosphorus in activated sludge plants has increased considerably in recent years due to the need to achieve tighter discharge consents for phosphorus in treated wastewater effluent. The impact of aluminium salt (Al3+) dosing on the settleability of activated sludge as a function of zone settling velocity (ZSV) and stirred specific volume index (SSVI) were investigated in batch settleability tests over a three-year period. The results showed that ZSV increased with increasing dose of aluminium salt as SSVI decreased. This trend was observed for dosing concentrations of less than 100 mg/L. At a dose concentration >100 mg/L, the trend was reversed as ZSV decreased and SSVI increased. At dose concentrations of <100 mg/L, Al3+ helped in the bioaggregation of dispersed activated sludge flocs, thereby improving settleability. The surface morphology from the scanning electron microscope (SEM) images indicated that the initial potential of interfloc bridging, open floc formation, and spindly bulking noticed in the undosed activated sludge flocs were remarkably reduced as the flocs became more compacted after Al3+ treatment. At >100 mg/L of Al3+, the sludge settleability started to disintegrate due mainly to surface charge reversal linked to the formation of aluminium hydroxides and the resultant disintegration of the activated sludge floc structure.

Effect of Aluminium Salt Dosing on Activated Sludge Settleability Indicators: A New Settleability Model Development

“…In an earlier study by Ojo and Ifelebuegu [19], the batch settling test measurements of zone settling curve (ZSC) of ZSV reveals ZSV as a linear representation ( Figure 1) of sludge height as a function of time (m/h) for 100 mg/L Al 3+ dosed sludge. Also, the results as previously reported [19] showed that the ZSV increased, while SSVI decreased with increasing concentrations of Al 3+ salt up to <100 mg/L. Beyond this concentration the sludge settleability disintegrated due mainly to surface charge reversal.…”

“…The mixed liquor samples (MLSS concentration of the samples ranged from 2800 to 3500 mg/L, with the pH ranging from 6.5 to 7.5 and a phosphate concentration of 7 to 12 mg/L) were sampled in 32.5 L sample containers from the distribution chambers and analysed in the laboratory within 24 h of sampling. Details of the methods have been previously described [19].…”

“…The MLSS samples were dosed with varying concentrations of alum ranging from 0 to 250 mg/L. The detailed processes have been recently described by the authors [19]. The ZSV and SSVI were computed from the results of the the batch settling tests.…”

“…Meanwhile, other researchers have utilised the empirical relationship with the sludge settleability parameters such as SVI, diluted sludge volume index (DSVI) and SSVI to obtain the Vesilind v 0 and k parameter [13,19,[40][41][42]. The research conducted by Pitman [43,44] involving an analysis of six years full scale studies resulted in the development of an empirical relationship between the constants of Vesilind's equation and SSVI.…”

“…Several factors affect the efficiency of FSTs ranging from hydraulic and loading factors to external factors like tank configuration, surface area, depth, flow distribution, and turbulence in conveyance structures [13,[16][17][18]. Also, several other factors affecting sludge settleability including; particle size, sludge particle shape, sludge floc porosity, flocculability tendency and extracellular polymeric substances (EPS) have been extensively investigated by various researchers [18][19][20]. However, the effects of chemical dosing on activated sludge settleability have been largely neglected.…”