Optimisation of the Design and Operation of Chemically Dosed Activated Sludge Plants

• Main Author: Ojo, Peter
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2019
• Subjects: Biogas; Phosphorus; Sludge

The use of metal salts such as aluminium (Al3+), ferric (Fe3+)and calcium(Ca2+)in the precipitation of phosphorus in activated sludge plants has increased considerably in recent years to achieve tighter discharge consents for phosphorus in treated wastewater effluent. Despite this, the modelling of the settle ability of the resultant metal salt dosed sludge in current engineering practice for design and optimisation are still based on conventional sludge settleability models. Also, the full impacts of the metal dosing on other downstream processes including dewaterability and digestibility of the chemical dosed sludge are not fully understood. In this research, the impact of aluminium (Al3+), ferric (Fe3+)and calcium (Ca2+)salt dosing on the settleability of activated sludge as a function of zone settling velocity (ZSV) and stirred sludge volume index (SSVI) were investigated in batch settleability tests over a three years' period. The effect of chemical phosphorus removal (CPR) on sludge dewaterability and digestibility were investigated in a laboratory-scale experiments using sludge samples from a full-scale wastewater treatment plant. The results of the settleability tests showed that ZSV increased with increasing dose of aluminium, ferric and calciumsalt as SSVI decreased. This trend was observed for dosing concentrations ofless than 100mg/L. At a dose concentration >100mg/Lthe trend was reversed as ZSV decreased and SSVI increased. At dose concentrations of <100mg/L, Al3+, Fe3+and Ca2+helped in the bioaggregation of dispersed activated sludge flocs thereby improving settleability. At >100mg/Lof Al3+, Fe3+ and Ca2+ the sludge settleability started to disintegrate due mainly to surface charge reversal linked to the formation of aluminium, ferric and calciumhydroxides and the resultant disintegration of the activated sludge floc structure. Revised activated sludge settleability modelswhich is designed to predict the effects to Al3+, Fe3+ and Ca2+ salts dosing on activated sludge settleability indicators; zone settling velocity (ZSV) and stirred specific volume index (SSVI) were described. The exponential form of the Vesilind equation was optimised and validated to include aluminium, ferric and calcium chemical dosing parameters. The proposed model equation which is an extended exponential relationship was found to effectively describe the settleability of aluminium, ferric and calcium sludge for dosing concentrations of 0 to <100mg/Lof Al3+, Fe3+ and Ca2+ respectively. The results of the digestibility tests showed that there was a 21.3%and 35.7%reduction in the biogas volume during anaerobic digestion of surplus activated sludge (SAS) at 100mg/L doses of Al3+and Fe3+salts respectively. This demonstrates that Al3+has less of a reduction effect compared with Fe3+salts on biogas generation during anaerobic digestion of sludge. The dewaterability tests showed that primary sludge (PS) dewaterability was improved by up to 16.25%by Al3+, 25.10%by Fe3+and 26.77%by Ca2+,while that of surplus activated sludge (SAS) was reduced by 72.96%, 63.82%and 52.56%respectively, at a metal salt ( Al3+, Fe3+ and Ca2+ ) dose of 50mg/L .Consequently, a pre-precipitation process during CPR where phosphorus is removed in the primary tank would, therefore, enhance sludge dewaterability. The reason for capillary suction time (CST)reduction in primary sludge may be due to surface chemistry and disruption of extacellular polymeric substances (EPS) structure, causing the release of interstitial water into surrounding sludge matrix through the interaction between multivalent and divalent ions(Al3+, Fe3+ and Ca2+) and sludge particles. Therefore, the CST increase in secondary sludge may be due to the interaction of multivalent coagulant (Al3+and Fe3+) and divalent ion (Ca2+) with higher EPS content limiting the outflow of interstitial water from the sludge floc pore spaces.