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Evaluation and validation of passive aeration simultaneous nitrification and denitrification (PASND) in a biofilm reactor for Low-Energy wastewater treatment

Hossain, Md Iqbal (2017) Evaluation and validation of passive aeration simultaneous nitrification and denitrification (PASND) in a biofilm reactor for Low-Energy wastewater treatment. PhD thesis, Murdoch University.

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In today’s rapidly urbanized and industrialized society, there is an increasing need for cost-effective and environmentally sustainable technologies for the removal of nutrients from the polluted water. Wastewater treatment principally relies on microorganisms for their ability to take up organic substrate and nutrients (i.e., nitrogen) from wastewater. The need for active aeration of the bulk wastewater to provide oxidation power to micro-organisms makes the wastewater treatment process extremely energy intensive. However, a recent study suggests that by exposing the biomass directly to air (passive aeration), the cost associated with aeration could be reduced. This thesis aimed to establish such a pas-sively aerated biofilm (to remove organics and nitrogen from wastewater) from activated sludge in order to investigate its real-world feasibility.

To enable cost-effective removal of organic compounds, a biofilm enriched with glyco-gen accumulating organism (GAO) was developed by selective enrichment from acti-vated sludge using sequences of anaerobic flooding followed by aerobic exposure of the biofilm directly to the atmosphere. The transition of activated sludge to the GAO biofilm was completed within eight weeks of continuous selective operation. The GAO biofilm enabled anaerobic removal of organic carbon (biochemical oxygen demand or BOD) from wastewater which was stored intracellularly as poly-hydroxyalkanoate (PHA). The PHA was oxidized in the subsequent aerobic stage to regenerate the biofilm’s BOD stor-age capacity. With using acetate as synthetic BOD, the biofilm demonstrated efficient (>99%) and stable removal of organic carbon at an average rate of 256 mg BOD L-1 h-1.

Long-term operation of the established GAO biofilm leads to the very low amount of excess sludge produced. This is of particular interest since sludge disposal cost is the second greatest operational expense in traditional wastewater treatment facilities. The av-erage excess sludge (volatile suspended solids or VSS) production rate was found to be 0.05 g VSS g-1 BOD removed which is about 10-times lower than that of activated sludge process. Factors such as the high biomass content (21.41 g VSS L-1 of reactor) and the low growth yield of GAO were found to be associated with little sludge production. In addition, a high number of a predatory protozoan (Tetramitus) was found inhabiting the biofilm that minimized sludge production by effectively grazing on cells.

In order to allow next to organic carbon also nitrogen removal from wastewater, a hybrid biofilm system was developed by incorporating zeolite (an ion-exchange material) into the GAO biofilm and activated sludge as the sole source of nitrifying bacteria. During the anaerobic phase, zeolite adsorbed ammonium which was removed in the subsequent aer-obic stage by the combined action of nitrifying and GAO bacteria via simultaneous nitri-fication and denitrification (SND). The occurrence of SND under full atmospheric partial pressure was confirmed by trickling nitrate solution over the biofilm system which resulted in nitrate reduction in full atmospheric condition. Over four months of continu-ous operation, the biofilm reactor demonstrated sustained BOD (>90%) and nitrogen (about 70%) removal performance with a short hydraulic retention time (HRT) of 5 h (2 h anaerobic and 3 h aerobic phase). The inadequate nitrogen removal efficiency was attributed to the limited capacity (1.474 mg NH4+-N g-1) of the zeolite used in this study. However, a subsequent repeat treatment of the effluent in the same biofilm reactor re-sulted in about 96 % ammonium removal from wastewater.

The capability of the GAO biofilm to treat high-strength (up to 4-times) wastewater was evaluated while keeping the same anaerobic duration (2 h). The amount of ammonium adsorbed onto zeolite was found to increase proportionally with influent feed concentration. However, the aerobic time required for zeolite regeneration was longer. Compared to single (1x) and double (2x) strength wastewater, the quadruple (4x) strength synthetic wastewater resulted in nitrite accumulation which took about 5 h (aerobic phase) for complete reduction. Similarly, the BOD removal rate of the biofilm system increased from 543 to 2308 mg L-1 h-1 for 1x and 4x strength wastewater, respectively. The in-creased uptake of BOD by GAO biofilm resulted in the improved storage of PHA (5.02 and 18.6 mmol L-1 for 1x and 4x wastewater, respectively) which contributed to the effi-cient regeneration of zeolite. The biofilm system showed its stability for the treatment of different strength wastewater over a period of 2-months operation suggesting the feasi-bility of 4x or more concentrated wastewater treatment using the proposed biofilm tech-nology with low aeration energy input.

To further optimize nitrogen removal performance in the zeolite amended GAO biofilm, several anaerobic and aerobic phases were used while keeping the total treatment time the same (8 h). An increase in the treatment cycles from 2 to 8, increased the nitrogen removal efficiency from 79% to >99%. A simple numerical model was developed that could effectively explain the trends of nitrogen removal in multiple treatment cycles on the basis of the Langmuir ion-exchange isotherm.

The main conclusion drawn from the study is that passively aerated GAO biofilm system can be established from standard activated sludge within a reasonable time. The amend-ment of the GAO biofilm by the addition of zeolite as an ammonium adsorbent enables nitrogen removal from wastewater. The proposed biofilm technology has the potential to reduce the energy cost associated with aeration while significantly improving nitrogen removal from high-strength wastewater.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Engineering and Information Technology
United Nations SDGs: Goal 6: Clean Water and Sanitation
Goal 13: Climate Action
Supervisor(s): Cord-Ruwisch, Ralf
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