Inactivation of pathogens in urine nitrification reactors

Through nutrient recovery by urine separation, the VUNA project aims to develop an effective sanitation system that simultaneously helps address important issues such as environmental pollution and water scarcity. Refinement of the nutrient recovery process presents several challenges: at a technical level, as nitrifying bacteria are sensitive to several parameters; and from a public health perspective, as nutrient recovery from urine implies exposure to pathogens. As part of this problematic, the present study focused on four main objectives: operating nitrification reactors, characterising both virus and bacteria inactivation, and evaluating different inactivation mechanisms. Indicator organisms (bacteriophages and bacteria) were used as surrogates for human pathogens. In a first set of experiments, the bacteriophage MS2 was spiked continuously over 60 days in a continuous flow nitrification reactor. Nitrification did not affect the bacteriophage concentration within the reactor. The second set of experiments consisted of the operation of small scale batch and semi-batch reactors to test in total three different bacteriophages (X147, MS2, Qbeta) and two bacteria (Salmonella typhimerium, Enterococcus spp.) under varying parameters. Pathogens can be inactivated or affected by different mechanisms. Four possible inactivation mechanisms were further evaluated: 1) effect of biological activity in a nitrification treatment system, 2) effect of the air-water interface in a buffer (PBS) and nitrified urine, 3) effect of the complexity of solution, 4) effect of ambient temperature. Results indicated that nitrification caused bacterial inactivation but did not influence the bacteriophages concentration. Air-water interface did not affect bacteria and showed mixed results for bacteriophages. Ambient temperature was not an inactivating parameter for neither of the groups studied. Finally, the bacteriophages presented signs of resistance, possibly due to a protective effect of the complex solution in the nitrification reactor.

Optimization of Reactor Startup and Nitrogen Removal of Aerobic Granular Sludge Systems

Samuel Lochmatter

Aerobic granular sludge (AGS) in sequencing batch reactors (SBR) has recently made its proofs as full-scale technology for the biological treatment of urban wastewater. In contrast to conventional flocculent activated sludge, microorganisms aggregate to dense granular biofilm due to shear stress from aeration. Hence, AGS has much better settling characteristics and allows the construction of more compact wastewater treatment plants (WWTP) without secondary clarifiers. Due to diffusion limitations of oxygen through the biofilm, aerobic and anaerobic/anoxic zones coexist in AGS. Hence, AGS-SBR has the potential to treat organic matter (COD), nitrogen (N), and phosphorous (P) simultaneously in one single reactor. The focus of the present PhD thesis was on the startup of AGS-SBR and the optimization of biological nutrient removal (BNR). For the investigation of the optimal startup conditions, a study testing seven parameters in parallel was conducted. The main conditions identified for a rapid startup of AGS-SBRs with good nutrient removal performances were (i) the alternation of high and low dissolved oxygen (DO) phases during aeration, (ii) a settling strategy avoiding too high biomass washout, (iii) the adaptation of the pollution load in the early stage of the startup in order to ensure that all soluble COD was consumed before the aeration phase, (iv) higher temperature (20°C), and (v) a neutral pH. Under such conditions it took less than 30 days to produce granular sludge with high removal performances for COD, N and P. A control run of the best startup strategy led to very similar results, proving the reproducibility of the experimental approach. This control run was operated for 80 days without any problems concerning the stability of the granular sludge or the nutrient removal performances. Concerning the bacterial community composition during the startup phase, a general shift of the predominant populations from Intrasporangiaceae and Sphingobacteriales to Dechloromonas and Zoogloea was observed. This shift was mainly due to general conditions with lab-scale AGS-SBR, rather than the specific operation parameters tested in the different experimental runs. However, it has been observed that polyphosphate-accumulating organisms (PAO) and glycogen-accumulating organisms (GAO) related populations were favored by the adaptation of the pollution load in the early stage of the startup in order to ensure that all soluble COD was consumed before the aeration phase. Besides the pollution load, the temperature and the pH had a significant impact on the global bacterial community structure. The predominant PAO were presumably Dechloromonas. In order to optimize BNR by AGS-SBR, different aeration strategies were tested. It has been concluded that the N-removal efficiency of COD-limited systems can be considerably enhanced with improved aeration strategies. Strategies promoting alternating nitrification and denitrification (AND) were significantly more efficient than simultaneous nitrification and denitrification (SND) strategies. The introduction of low DO phases or even anoxic phases in an early stage of the total aeration period probably enhanced denitrifying P-removal and led to COD savings. Intermittent aeration, which is a realistic AND strategy for full scale applications, led to the highest N-removal efficiency. The short mixing times implemented with this strategy were not problematic for the stability of the granules. Finally, to further optimize N-removal in COD-limited systems, the potential of aeration control for the achievement of N-removal over nitrite was investigated. It was shown that aeration phase length control combined with intermittent aeration or alternating high-low DO, is an efficient way to achieve N-removal over nitrite. N-removal efficiencies of up to 95% were achieved with this way of reactor operation. At 20°C, N-removal over nitrite was achieved within 20 – 60 days and it was possible to switch from N-removal over nitrite to N-removal over nitrate and back again. At 15°C, the nitrite-oxidizing bacteria population could be reduced, but nitrite oxidation could not be completely repressed. However, the combination of aeration phase length control and high-low DO was successful to maintain the nitrite pathway at 15°C where the maximum growth rate of nitrite-oxidizing bacteria is clearly higher than the one of ammonium-oxidizing bacteria. In conclusion, this thesis showed the potential of the optimization of operation conditions for the startup of AGS-SBR and BNR. An efficient strategy for the startup with flocculent inoculum sludge has been developed, maintaining high BNR. Moreover, it has been shown that N-removal under COD-limited conditions can be improved by aeration control, either by enhancing denitrifying P-removal or achieving N-removal over nitrite.

EPFL2013

Optimization of Reactor Startup and Nitrogen Removal of Aerobic Granular Sludge Systems

Samuel Lochmatter

EPFL2013