Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Program Name/Specialization

Biological and Chemical Sciences

Faculty/School

Faculty of Science

First Advisor

D. Scott Smith

Advisor Role

Primary supervisor

Second Advisor

Wayne J. Parker

Advisor Role

Co-supervisor

Abstract

Phosphorus (P) is a fundamental element necessary for all life forms and a key component in the fertilizer industry. Meanwhile, the excessive load of P to water bodies due to human activities has the potential to promote eutrophication. Wastewater treatment plants remove P either biologically or chemically and produce P rich sludge which could be a potential renewable source for P. At present, commercial technologies exist for P recovery from biological wastewater sludge. However, P recovery from chemical sludge particularly iron(III)-phosphate (Fe-P) sludge generated in chemical P removal plants that use iron(III) salts remains a challenge.

This study explored, in lab bench scale, the influence of pH, competing anions (), and Fe redox chemistry using reducing agents like ascorbic acid (AA) on direct P and Fe(III) release and recovery from simulated Fe-P sludge. Initial tests were performed in the absence of organic matter interferences followed up by tests in more realistic matrices where the effect of organic matter (OM) (carbonaceous and nitrogenous constituents) on chemical phosphorus removal and recovery is investigated.

Fe-P sludge was prepared to mimic the inorganic fraction of real Fe-P sludge using ferric additions to synthetic wastewater. The impacts of acidic/basic wet treatment (pH effect) and competing anions () on P and soluble Fe release from Fe-P sludge were assessed. Factors influencing recovery, such as sludge age and Fe/P molar ratio were also investigated. Results revealed that alkaline treatment had minimal effectiveness in releasing iron (

The impact of reductive dissolution using AA under acidic conditions on P and Fe release from lab simulated Fe-P sludge was examined. To find the optimum conditions for Fe-P reductive solubilization and vivianite precipitation, factors like AA/Fe molar ratio, pH, and Fe-P sludge age were tested. Moreover, the reductive, chelating, and acidic effects of AA, were evaluated by comparing them with hydroxylamine (a reducing agent), oxalic acid (a chelating agent), and inorganic acids (to assess pH effects), including HNO3, HCl, and H2SO4. At pH values of 3 and 4, and with Fe/AA molar ratios of 1:2 and 1:4, complete solubilization of Fe-P sludge and reduction of Fe(III) were observed. The sludge age, up to 11 days, did not affect the reductive solubilization of Fe-P when AA was added. Hydroxylamine treatment resulted in negligible reductive dissolution of Fe-P sludge, while oxalic acid treatment at an Fe/oxalic acid molar ratio of 1:2 and pH 3 facilitated non-reductive dissolution, leading to solubilization of both phosphorus (95±2%) and Fe(III) (90±1%). Inorganic acids treatment at pH 3 resulted in very low P and Fe release (

The influence of OM on P removal performance was tested using different synthetic wastewater recipes containing model carbonaceous and nitrogenous constituents and combination of them. Carbonaceous constituents including meat extract (ME), potato starch, glycerol, and Luther Marsh concentrate (LM) represented proteins, carbohydrates, lipids, and natural organic matter respectively. Peptone, urea, and ammonium chloride represented nitrogenous constituents. Results showed insignificant effect of nitrogenous constituents, potato starch and glycerol on P removal performance. However, ME and LM possessed a remarkable reduction in P removal performance of (3.0±0.4%) and (23±1%) respectively. No change in P removal performance observed for carbonaceous constituents in the presence of nitrogen source except for LM which exhibited a substantial increase by (23%). The higher measurements for soluble Fe (SFe) residuals for ME (87±5%) and LM (51±1%) indicated occurrence of interactions between Fe(III) cations and the negatively charged functional groups like hydroxyl, carboxyl, and phenolic groups available in ME and LM. This suggested Fe solubilization as the mechanism responsible for reducing P removal. Wet alkaline (pH 10) and ascorbic acid treatments showed no influence of OM on P release from Fe-P sludge.

Convocation Year

2023

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