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Determination of the zeolite optimal diameter for the settlement of nitrifying bacteria in an aerobic bed fluidized reactor to eliminate ammonia nitrogen.
Santiago Pozo-Antonio a. Received: June 13 th , Received in revised form: May 12 th , Accepted: June 4 th , Abstract In this work, the determination of the diameter of zeolite as a support for a microbial aerobic fluidized bed reactor is performed. The design of the reactor is recommended by Navarro and Palladino .
For the present study, the zeolite is crushed and classified granulometrically. Subsequently, the diameters of 0. After the study of adherence of nitrifying bacteria, the obtained adhesion values for each diameter are not significantly different from each other. However, 1 mm is chosen to achieve higher adhesion values. Subsequently the aerobic fluidized bed reactor proposed by Navarro and Palladino  is built with the 1 mm-diameter zeolite. This presented an inlet flow of 1.
Keywords : nitrification, Nitrosomonas , Nitrobacter , microbial adhesion, fluidization, water. These ions are naturally present in the aquatic environment as a result of atmospheric deposition, surface runoff and groundwater, dissolution of geological deposits rich in nitrogen, biological decomposition of organic matter and nitrogen fixation by certain organisms .
Further, humans severely alter the nitrogen cycle by increasing its availability in many regions of the planet as a result of point and diffuse sources of pollution. Widespread pollution produces problems such as toxic algae blooms that after intake through food or water can lead to various physiological disorders and symptoms of intoxication , eutrophication [6,7], acidification of rivers and lakes with low or reduced alkalinity , direct toxicity of nitrogenous compounds in aquatic animals  and adverse effects on human health .
In most countries, there are laws and legal regulations establishing limits for the concentration of ammoniacal nitrogen in the wastewater industry such as the Biological Oxygen Demand BOD and Chemical Oxygen Demand COD . Removal of nitrogen present in the wastewater resulting from domestic and industrial activities is usually carried out combining biological processes of nitrification and denitrification, since costs are lower compared to the physical and chemical processes.
Nitrification consists in the biological oxidation aerobic process of ammoniacal nitrogen in a first step to nitrite and its oxidation to nitrate, carried out by autotrophic ammonia-oxidizing bacteria Nitrosomonas and nitrite-oxidizing Nitrobacter , respectively .
According to the nitrification reaction:. Therefore, concentration of dissolved oxygen in water must be kept around 2 to 3 mg. L -1 for an adequate nitrification . Among the most common biological processes, we have fluidized activated sludge, where a granular bed within a column is used. In the column, the lower part has the input of effluent to be treated and recycled effluent and the top part has the treated effluent outlet.
In these systems, feed and recycle rates must sustain the fluidization of the bed [1,21] and degradation of organic matter by micro-organisms on the surface of the particles [1, ]. Air injection is essential to get a proper aeration in the system . In these systems, granular activated carbon, sand and clay are used as microbial support [1, ]. The constant levels of biomass must to be maintained. Excess biomass can be removed by friction between particles or transported to a device where the biomass is separated from the particles; these are incorporated back into the bed .
Navarro and Palladino proved the effectiveness of an aerobic fluidized bed reactor using as support: granular activated carbon . They determined that higher efficiencies correspond to low flow rates and high organic loads. In this work, we determine the optimal zeolite size for the correct physiological development of nitrifying bacteria to be used in a subsequent aerobic fluidized bed reactor to remove ammonia nitrogen existing in industrial wastewater. The building, design and dimensioning of the aerobic fluidized bed reactor have been based on previous works [1, 17].
The optimal size of the zeolite employed as a microbial support was determined by adhesion tests. The whole investigation takes three different stages:. To ensure a proper implementation of the fluidized bed aerobic reactor a number of biological, chemical and physical parameters such as temperature, pH, microbial support, volume, feeding rate, retention time, and the reactor dimensions and configuration must be taken into account .
Alkaline media are more favourable for nitrifying bacteria with optimal levels of pH between 7. The pH value affects the environmental conditions for a favourable physiological development of microorganisms, and also has a great influence on the inhibition degree of the nitrifying bacteria.
It was found that ammonium produces the inhibition of Nitrobacter . Nitrifying bacteria are sensitive to multiple substances heavy metals, organic compounds, free ammonia, etc. These substances can interfere with the cell metabolism, reducing the rate of formation of intermediates compounds . For these bacteria to develop physiologically, a proper support material should be chosen to take into account the biological process itself, the equipment size and experimental conditions that bacteria will find.
In this case, the preferred zeolite particle size for the fluidized bed reactor was between 0. Most of the materials, such as sand and clay with irregular shapes and sizes, were chosen to take into account that they were cheap and readily available.
One of the most important variables in reactor design is the density of the aerobic fluidized bed material, as it affects the bed hydrodynamics and has a direct effect on power consumption . It must be noted that particles should not be fragile as because of their continuous movement they can collide and become fragmented, changing their fluidizing characteristics and thus making even more difficult the control of the bed expansion.
In this sense, there are works that use active carbon as the support, with a density lower than that of the natural zeolite . One advantage of fluidized bed reactor is the large surface available for adhesion, where the biofilm can grow. It was determined that optimum pore size is about five times the dimension of the cells, that leave room for two adhered cells and two more cells growing by fusion and some free space between them for transferring substrates .
Materials and methods. Crushing and sieve analysis. Upon receipt of the zeolite from the supplier, the grinding was started to obtain the required diameters 0.
Microbial adhesion. For the microbial adhesion tests, assemble of 16 mini digesters were used with a volume of mL, for each one of the zeolite diameters Fig. Each mini-digester contained 40 g of zeolite, mL of liquid industrial waste and 72 mL of sludge. The industrial waste feeding liquid to the reactor corresponds to a synthetic solution, whose composition was obtained from previous works . Table 1 details the composition of the feeding liquid and Table 2 shows that of the saline solution used to prepare the synthetic industrial waste liquid.
The mini-reactors were inoculated with microorganisms nitrifying bacteria from an activated sludge plant, which was in a process of recovery. Prior to their employment in the mini-reactors, they were subjected to the following characterization Table 3 :. Once the mini-digesters were filled and the aeration pumps connected, the system was allowed to evolve for five days. After this initial time zeolite and liquid samples were taken every two or three days. Mini-digesters were grouped in pairs, for each zeolite diameter.
Each sampling day, the zeolite and liquid of both mini-digesters from one of the pair for the three different zeolite diameters were analyzed. Most assays were done three times in order to have well defined average values.
This method allows considering that the mass of the mini-digester was kept constant during the whole sampling time. The whole experiment ran for three weeks. Adhesion tests were performed with the zeolite extracted from each mini-reactor. The extracted zeolite was placed on a filter paper on a capsule and introduced into a hot oven for six hours to ensure complete drying of the support. To determine the mass of the adhered solids the following expression can be applied:.
AM: adhered solids mass g A: mass of the dry filter paper, where the zeolite is placed g B: mass of the supporting capsule g C: mass of zeolite added initially to the mini-digesters g D: dry mass of the capsule with the zeolite and filter after six hours into the heating oven g. Implementation of aerobic fluidized bed reactor. After the adhesion test, the construction of the aerobic fluidized bed reactor based on the construction of Navarro and Palladino was performed using the zeolite of the most adequate diameter .
The reactor consisted of an acrylic column 10 cm in diameter and 1. The final 20 cm corresponded to an inverted truncated cone with a 20 cm diameter base Fig. This design was chosen in order to prevent particles escaping from the reactor. The acrylic column had two side outlets located at 25 and 70 cm from the top. Both side outlets with a 2. The plate that support the filler was composed of a stainless steel mesh placed 10 cm above the base. Below the plate there were feeding inlets for the effluent to be treated and recycling.
Above these two inlets two cross-placed diffusers ensured an even air distribution. The installation also had a 20 cm high pyramidal settler with a weir at the top for discharge of treated effluent and a bottom outlet for recycling. A purge was also available for use if necessary. The supporting material used was zeolite of the chosen diameter.
This was loaded onto the column to achieve a fixed bed height of 15 cm. This height was set so that a significant liquid volume was above the bed of particles, thereby enabling work with different fluidizing speeds and thus different bed expansions. Two peristaltic pumps completed the system, for a constant feed rate and an optimum recirculation. The reactor was filled with g of zeolite, mL of liquid industrial waste and mL of sludge.
The liquid industrial waste had the same composition as that used in the mini-digesters Tables 1 and 2. Results are shown in Table 4. Tuning of the reactor. A nitrogen load speed NLS of 0. A first run was performed with water to verify the state of the seals and the general operation of each system component Fig.
Aeration equipment is critical for this type of reactors as to achieve the expected efficiency a proper air distribution is needed. Once confirmed that no leakage or conduction problems occurred, 8 L of the synthetic industrial waste liquid were introduced within the column. Then the structure was mounted and the system switched on. Firstly, with a total recirculation and later, when the recirculation rate was chosen, the feeding was started.
The feeding pump was connected to a timer to achieve the necessary inflow. The reactor was in operation for 21 days like the mini-digesters.