A novel aerobic process for carbon and nitrogen removal from wastewater using a biofilm with passive aeration
Flavigny, Raphael (2015) A novel aerobic process for carbon and nitrogen removal from wastewater using a biofilm with passive aeration. PhD thesis, Murdoch University.
Conventional municipal wastewater treatments use about 50 % of their energy for bulk liquid aeration to oxidise dissolved organic carbon (C) to CO2 and ammonium (NH4+) to nitrite (NO2-) and nitrate (NO3-). This thesis aims at reducing the energy requirement for bulk liquid aeration for the oxidation of dissolved carbon and ammonium in wastewater. This was done by developing two separate biofilm reactors that respectively oxidise C and NH4+ with passive aeration. Thereafter, the combination of the two processes was tested to achieve complete dissolved carbon and total Nitrogen (N) removal without aerating the bulk liquid.
The first biofilm reactor removed dissolved carbon with a sequencing batch mode. The biofilm was flooded with wastewater, and dissolved C was biologically stored as Poly-Hydroxyalkanoates (PHAs) under anaerobic conditions, followed by the oxidation of PHAs to CO2, under aerobic conditions. The aeration was achieved by draining the wastewater, resulting in mere exposure of the biofilm to atmospheric oxygen partial pressure. The storing biomass was developed in 9 weeks from Activated Sludge (AS) and biomass of a storage driven denitrification biofilm, with a strict oscillation of: anaerobic conditions with acetate in solution, and exposure to the atmosphere (i.e. aerobic conditions) without dissolved carbon. The DNA analysis along with the testing of metabolites in biomass and solution demonstrated that the oscillating conditions enriched the biomass with Candidatus Accumulibacter, a known Glycogen Accumulating Organism (GAO). The process was operated over 9 months and repeatedly stored acetate as PHAs under anaerobic conditions and oxidised it during air exposure. Overall, > 80 % of the acetate added to the biofilm was removed at a rate of 4 Cmmol/L/h (128 g/m3/h BOD) and the reactor’s Hydraulic Retention Time (HRT) was 3 h. Both the rate and HRT were faster than conventional AS processes.
The second reactor developed aimed at reducing the energy cost for oxygen supply for ammonium oxidation. To reduce the energy use for ammonium oxidation a two-step method was used. The first step was ammonia adsorption onto zeolite used as carrier for nitrifying biomass. The second step was the ammonia oxidation of the adsorbed ammonia using trickle method for oxygen transfer. The zeolite used in this study was an Australian Clinoptilolite zeolite (2 – 3.35 mm) with a maximum ammonium adsorption rate of 0.12 mmol-N/g/h (1.68 mg-N/g/h). Results showed that the nitrifying biomass was capable of oxidising 93 % of adsorbed ammonium on zeolite as nitrate when trickling the whole batch volume (1 bed volume) of wastewater, but the recovery reduced to < 34 % when only 20% of the liquid was recycled for reduced energy expense. To complete the total N removal, nitrate drained from this reactor was denitrified in the first reactor by GAO using PHA stored. The combination of the two reactors achieved 99 % of the acetate removal and 93 % of the nitrogen removal. However, liquid recirculation between two reactors was thought to be an energy cost that could be prevented.
To reduce energy consumption, a single zeolite amended biofilm was synthesised by adding the GAO, nitrifiers and zeolite powder together, with the objective to remove dissolved C and total N within a single biofilm reactor. The operating principle was to simply fill the reactor and keep it anaerobic, so as to let the wastewater in contact with biofilm to biologically store dissolved acetate and adsorb ammonium on zeolite (Stage 1). Then aerobic conditions (Stage 2) were provided by draining the liquid. The liquid was recirculated for mass transfer at 0.4 m3/m2/d, which is a fraction of that used in trickle filter reactor. The zeolite amended biofilm reactor treated wastewater with a total treatment time of 19 h. Removal efficiencies were > 94 % for C and 80 % for total N. The production of dinitrogen gas under atmospheric oxygen partial pressure demonstrated that Simultaneous Nitrification and Denitrification (SND) occurred in air. SND in air can be explained to be due to an oxygen gradient formed in the biofilm.
As the biofilm had been synthesised from different biomasses, it was tested for medium term sustainability. GAOs and nitrifiers were considered sustained in the synthesised biofilm over an operating period of 30 cycles (3 months), as their removal rates remained similar or improved over time. However, over this period the nitrite oxidizing bacteria (NOB) were washed out of the system, which is advantageous for effective nitrogen removal and known as SND over nitrite. It means that the zeolite amended biofilm reactor can effectively denitrify wastewater with low C/N ratio. On the contrary, conventional wastewater treatment plants are not effective at denitrifying low C/N wastewater.
To optimise the zeolite amended biofilm reactor, its treatment time was shortened from 19 to 5 h and by omitting liquid recirculation in Stage 2. Under these short treatment times the removal efficiencies were > 83 % and 75 % for C and total N respectively over the tested period of 18 cycles. The system operating with 5 h treatment times is promising as a pre-treatment process for C and total N removal with a minimum of energy expense. This work does not propose a new process, it is the novel combination of the processes that achieve the novelty in this thesis.
Overall, the system developed in this work is a novel combination of known biological and chemical steps for carbon and nitrogen removal from wastewater. In the first reactor, direct atmospheric contact between the carbon storage biomass and oxygen to oxidise PHAs was novel. In the zeolite amended biofilm reactor, the operation of SND in full atmospheric oxygen condition was novel and demonstrated zeolite bio-regeneration as well as PHAs oxidation. This novel biofilm reactor repeatedly removed soluble carbon and total nitrogen without liquid recirculation, over a medium term operation.
|Publication Type:||Thesis (PhD)|
|Murdoch Affiliation:||School of Engineering and Information Technology|
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