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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

Dr. Madhumita B Ray

Abstract

Seasonal algal blooms in drinking water sources have increased significantly over the recent past as a result of increased temperature and nutrient loading in surface water due to agricultural and surface runoff. More than 95% of algal cells can be removed by coagulation and flocculation processes. However, algal organic matter (AOM) is not removed well during coagulation, thus causes several operational challenges in drinking water treatment. This research was conducted to investigate the effectiveness of coagulation, granular activated carbon adsorption, and filtration processes on AOM removal and to evaluate disinfection by-products formation potential with/without UV irradiation.

Initially, coagulation performance for the treatment of algae-laden raw water was investigated systematically by central composite design using response surface methodology. The main mechanism of algae and AOM removal was charge neutralization at an optimum pH of around 6.0. Thereafter, the optimum coagulation conditions using alum for AOM of six different algal and cyanobacterial species were determined. The AOM removal by coagulation correlated well with the hydrophobicity of the AOM solution. The disinfection by-product formation potential of the AOM due to chlorination was determined after coagulation.

The efficiency and mechanism of AOM removal by granular activated carbon (GAC) adsorption were determined by batch adsorption experiments. The adsorption equilibrium data followed both Langmuir and Freundlich models. The adsorption process followed a pseudo-second-order kinetic model, and the calculated thermodynamic parameters indicated that GAC adsorption for AOM removal was spontaneous and endothermic in nature.

The fouling behavior of the microfiltration membranes after GAC adsorption pre-treatment was investigated and the filtration resistance and AOM removal efficiency were determined. The GAC adsorption increased the removal of AOM, decreased membrane fouling, and identified intermediate blocking as the major fouling mechanism of the membrane.

The effects of combined low-pressure ultraviolet (LPUV) irradiation and chlorination on the disinfection byproducts (DBPs) formation from AOM was investigated for common algae existed in surface water, AOM degradation was likely promoted by photodegradation of aromatics, and chlorine oxidation/substitution. Insights obtained of this work will help in properly designing and operating the AOM removal and reducing DBPs formation during water treatment of algae-laden source water.

Summary for Lay Audience

Algal blooms frequently occur in surface water, such as rivers, lakes, reservoirs as a result of climate change and frequent eutrophication, causing deterioration of water quality in drinking water supplies. There are multiple barriers, including coagulation, clarification, filtration and disinfection processes applied in drinking water treatment plants to remove pathogens, viruses, and other contaminants and make the treated water safe enough for potable use. Although algal cells are removed well (> 95%) during conventional processes of drinking water treatment, algal organic matter (AOM), as the metabolites of algae cells, is not removed well during coagulation. AOM causes several challenges in drinking water treatment such as increased coagulant demand, blocking the activated carbon adsorption sites, growth of biofilm causing membrane fouling, and increased formation of disinfection by-products (DBP) during chlorination.

This research was conducted to investigate the effects of treatment including coagulation, granular activated carbon (GAC) adsorption, filtration processes for AOM removal and evaluation of disinfection by-products formation potential with/without UV irradiation for six different species of algae and cyanobacteria. The results indicated that an average of 47.4% of AOM in terms of dissolved organic carbon (DOC) can be removed at the optimum coagulation-flocculation condition. The specific ultraviolet absorbance (SUVA) and hydrophobicity of AOM can be used as surrogate parameters to predict coagulation-flocculation efficiency. GAC could remove AOM, mitigating the irreversible fouling of a microfiltration membrane. Commonly applied UV irradiation dose of 40 mJ/cm2 insignificantly affects DBP formation, more attention should be given to evaluate the feasibility of enhanced UV irradiation dosage on the degradation of AOM from some algal species. The experimental results obtained in this research are useful for determining the optimal coagulation, GAC adsorption and microfiltration conditions to be adopted and to minimize the DBP formation in treated drinking water.

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