Abstract
There has been a significant rise in the use of aluminium salts (Al3+) for the chemical precipitation of phosphates in wastewater treatment plants due to growing stricter regulatory requirements for wastewater effluent release to the environment. The modelling of the settleability of the resultant Al3+ sludge in present engineering practice for design and optimisation are still based on conventional sludge settleability models. This paper describes a novel activated sludge settleability model which is designed to analyse the effects of Al3+ dosing on activated sludge settleability indicators, zone settling velocity (ZSV), and stirred specific volume index (SSVI). The impact of Al3+ dosing concentrations on ZSV and SSVI of full scale activated sludge plant were analysed in the laboratory over a three years’ period and the exponential form of the Vesilind equation was optimised and validated to include alum chemical dosing parameters. The proposed model equation was found to effectively describe the settleability of Al3+ dosed sludge for dosing concentrations range of 0 to 100 mg/L.
Highlights
The increasing technological advancements and sustained growth in the human population have contributed to increased discharges of phosphorus into the surrounding water bodies causing eutrophication and leading to environmental pollution and adverse human health issues [1,2,3,4].there are stricter effluent quality requirements for wastewater treatment plants in Europe with
The findings showed agreement with Vesilind [24] assertion that as biomass concentration was increased in zone settling test, settling velocities decreased but a non-linear correlation between the settling velocity and the range of buoyant density was observed but investigations have not considered the impact of chemical dosing on activated sludge settleability
As earlier mentioned that there exists an exponential relationship between the zone settling velocity (ZSV) and Al dosing (Figures 5 and 7) but because the relationship is a non-linear relationship, calculating the value of the new empirical coefficient (ZSVO, Kd, CO, and CK ) of aluminium dosed activated sludge model can be evaluated by linearizing Equation (11) to become (14)
Summary
The increasing technological advancements and sustained growth in the human population have contributed to increased discharges of phosphorus into the surrounding water bodies causing eutrophication and leading to environmental pollution and adverse human health issues [1,2,3,4]. Since the mass flux theory was developed by Kynch [21] various studies have been conducted [6,7,22,23], to examine the relationship between zone settling velocity (ZSV) and activated sludge concentration (X) and to evaluate the efficiency of FSTs in an ASP, by a number of empirical models [24,25,26,27,28]. Among the empirical models proposed to relate the initial settling velocity to the sludge concentration is the Vesilind equation [24] It is the most acceptable empirical model for predicting the sludge settleability behaviour within the FST.
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