Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

Ray, Madhumita B

2nd Supervisor

Nakhla, George

Co-Supervisor

Abstract

Chemically enhanced primary treatment (CEPT) process is a promising method for carbon redirection and improving the performance and efficiency of wastewater treatment processes. CEPT is employed to precipitate colloidal and suspended matter from wastewater; however, this requires a significant amount of coagulant and produces large volumes of sludge. Coagulant recovery (CR) from the precipitated sludge has the potential to reduce sludge quantities, associated costs for disposal of sludge, cost of dosing fresh coagulant by regenerating and purifying the coagulant before reuse. This research was conducted to understand the feasibility and implications of CR in municipal wastewater.

In order to evaluate the use of CR in municipal wastewater, recovery of aluminum and iron, which are the two most widely used coagulants, from primary sludge originated from coagulated raw wastewater and their reuse potential as secondary coagulant was investigated. The recovered coagulant which was obtained through acidification of the primary sludge, reused for treating primary wastewater and overall coagulation efficiency was determined as a function of the recovery cycles (two in number). While with fresh aluminum sulphate, the removal efficiencies of total suspended solids (TSS), chemical oxygen demand (COD), total phosphorous (TP), and total nitrogen (TN) were 85%, 65%, 80% and 33%, respectively, a drop in removal efficiency of TSS and COD was observed with recovered aluminum (85% to 60% and 65% to 50%, respectively). With fresh ferric chloride, 90% TSS, 77% TP, 62% COD, and 18% TN were removed from primary effluent, while with the recovered coagulant a decline in the TSS, COD and TN removal efficiencies and increase in their concentrations in effluent by approximately 10% occurred. Recoveries of both aluminum and iron declined with each cycle. Phosphorous was the most affected parameter with recycled coagulant, however, this could be precipitated as struvite at the end of the second cycle. Equilibrium modeling of various aluminum and iron species was conducted to determine the recovery potential of aluminum and iron at low pH. The chemical equilibrium modes predicted the formation of complexes like jurbanite, gibbsite for aluminum and jarosite, strengite for iron, which reduced the recovery. The effects of recycling of coagulant on various water quality parameters in the effluent were also determined. A preliminary operational cost analysis conducted on the recovery process demonstrated that the increased cost of acidification can be offset by the reduction in costs of fresh coagulant, dewatering and sludge disposal.

Distribution of micropollutants (MPs) with fresh and recovered iron and aluminum in recovered coagulant and effluent was investigated. Based on their relative abundance in wastewater and range of octanol-water coefficients, 18 MPs from different groups such as antibiotics, food additives, surfactants were selected. The MPs were spiked into the influent from a primary stream collected from a local wastewater plant. The distribution of MPs in wastewater and the removal during coagulation were compound specific. MPs with log Kow ow >2.5 were sorbed on the coagulated sludge. The distribution ratio (Kd) of all the MPs with log Kow >2.5 was calculated and the extent of buildup on sludge due to repeated recycling was determined. Onlyow >2.5 was being recycled with the recovered coagulant. This study thus alleviates the concern of building up of the MPs during recycle of the coagulants.

Additionally, to assess the impact of CR on anaerobic digestion (AD), CEPT sludge, sludge obtained from use of recovered coagulants (recovered sludge), and the residual solids (spent sludge) after CR were subjected to AD at mesophilic conditions for 15 days. Approximately 52% destruction of volatile solids was observed for CEPT and recovered sludges, while for the spent sludge it was 47%. Both CEPT and the recovered sludge had similar methane formation potential reaching a maximum of 205 mL CH4/gCOD and the spent sludge could produce only 50 mL CH4/gCOD due to unavailability of organics. A chemical equilibrium model predicted the formation of vivianite and pyrite as iron-phosphorous (Fe-P) and iron-sulfur (Fe-S) compounds, respectively in the CEPT and recovered sludges formed during AD. This observation was key in determining that there was no difference in the Fe-P and Fe-S compounds formed in the CEPT and recovered sludges. The findings of this work demonstrated the potential of CR for wastewater and water treatment facilities for energy and cost saving.

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