Year

2019

Degree Name

Master of Philosophy

Department

School of Civil, Mining and Environmental Engineering

Abstract

Digested sludge centrate also known as sludge centrate is a by-product of the anaerobic digestion process obtained from sludge dewater. Sludge centrate contains most of the nutrients and some organics from the initial sewage sludge. Nutrients, namely ammonia and phosphorus can be extracted from sludge centrate in the forms of calcium phosphate or struvite precipitate. However, the concentration of organic matter and nutrients in raw sludge centrate is too low for for direct anaerobic digestion and precipitation. Thus, a novel approach of employing seawater-driven forward osmosis to pre-concentrate organic matter and nutrients in sludge centrate is vital to render sludge centrate anaerobically digestible and less chemical-intensively precipitable.

This thesis aimed to demonstrate for the first time the efficiency of organic matter enrichment in sludge centrate using a seawater-driven forward osmosis process. In particular, this research elucidated the impacts of membrane materials, prewetting procedures and draw solution on the performance of the seawater-driven forward osmosis process. The results indicated that the cellulose triacetate membrane (CTA) offered better performance than the polyamide membrane (PA) in terms of organic materials enrichment, fouling resistance and membrane cleaning efficiency. The prewetting protocol using 70% alcohol solution led to the significantly increased pure water flux of the PA membrane, while this step appeared to be ineffective to improve that of the CTA membrane. Membrane fouling decreased the enrichment efficiency of organic matter since the deposition of suspended particulate matter on the membrane surface caused fouling and loss of organic matter from the concentrated sludge centrate. The results showed that increasing the draw solution concentration increased flux but did not aggravate membrane fouling, however, it could reduce the efficiency of physical flushing to recover the flux. Seawater showed comparable forward osmosis performance to that of analytical grade NaCl as draw solutes in terms of flux and organic enrichment. The results also showed that seawater as the draw solution resulted in more membrane fouling and lower flux recovery compared to NaCl.

This study also proposed and evaluated techniques to mitigate membrane fouling during nutrient enrichment in sludge centrate by forward osmosis. Phosphorus precipitation in the bulk feed solution and on the membrane surface was systematically quantified and compared with respect to solution pH and operation time. The results indicated that low efficiency of nutrients enrichment when using seawater as the draw solution was attributed to formation of phosphorus precipitates. Moreover, increase in pH during the filtration also caused low efficiency of ammonium enrichment. Formation of inorganic precipitates (i.e. calcium phosphate and struvite) on the membrane surface led to severe fouling. The results also showed a relationship among pH, duration of filtration, fouling and phosphorus precipitation. This study demonstrated that membrane fouling could be minimized by encouraging phosphorus precipitation in the bulk solution (avoid feed solution mixing) and increasing the membrane surface area to shorten operation time. Furthermore, proper ratio of membrane area over permeate volume to enrich nutrients was 175 m-1. The reduction in membrane fouling also resulted in a proportional increase of phosphate and ammonia in the final concentrated feed solution.

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