Submerged aerobic fixed-film reactors are mainly used for carbonaceous, and ammonia removal in the aerobic treatment of urban wastewaters. They are also used in secondary or in tertiary treatment. Today, these bioreactors have several applications in industry (Mendoza-Espinoza et al., 1999; Chaudhary et al., 2003). They combine compactness and high removal efficiencies in a large range of hydraulic and organic load. In secondary treatment, aerobic biological degradation of organic matters in submerged aerobic fixed-film reactor is made in the presence of bacteria according to the reactions in Figure 1.

 

 

This degradation involves three reactions in the same time: the first one (Reaction 1) corresponds to an oxidation of organic matter with H₂O and CO₂ as byproducts, it releases also energy, necessary to bacterial cells maintenance and growth; it uses external organic matter (Exogenous process). The second one (Reaction 2) is related to multiplication of bacterial cells using organic matters. The third one (Reaction 3) uses biomass when the dissolved substrate becomes rare (Endogenous process) and bacteria cells use their own reserves. The kinetics of the endogenous process reaction is relatively low as compared to the exogenous processes.

The packing filling the reactor allows the development of biofilm by its colonization over the available surface area, in which bacterial cells adhere to each other. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance (EPS) allowing the formation of biofilm (Horan, 2003; Harvey et al., 2011).

It is well known that the performances of bioreactors depends on biofilm growth, which could be influenced by several factors, among others: packing characteristics (Mendoza-Espinoza, 1999; Cheremisinoff, 2002; Prenafeta-Boldú et al., 2008), aeration conditions (Kassab et al., 2010; Jin et al., 2012; Albuquerque et al., 2012), substrate concentration and its nature in wastewater (Amrouche et al., 2011; Chen et al., 2012).

The submerged aerobic fixed-film reactor performances could be assessed by the biodegradation yield (Y). It is expressed as follows:

 

 

where: Y, Yield (%) removal of the substrate (in terms of BOD); L₀, Initial organic load of the substrate, expressed in BOD (MgO₂/L); L, final organic load of the substrate at time t, expressed in BOD (MgO₂/L).

The removal performances could also be assessed by calculating biodegradation kinetics constants of organic pollution. In batch reactor, the most significant period in the growth cycle is the exponential growth phase, when the population of biomass is perfectly adapted to the substrate. The first-order model, neglecting endogenous respiration, provides accurate simulations of biodegradation kinetics. It can be written as follows (Cheremisinoff, 2002; Mara, 2003):

 

 

where:

t: time (d); kt: first-order BOD biodegradation kinetics constant expressed as (d^-1) depending on the temperature according to the following expression:

 

 

where: first-order BOD biodegradation kinetics constant at T₀, expressed as (d^-1): temperature coefficient, equal to 1.032, this value is in the range of those given by Lesouef et al. (1992) and Queinnec et al. (2006).

T: Temperature of the synthetic wastewater expressed in °C; T₀: Standard temperature at 20°C.

The integration of equation 2, taking into account equation 3, provide the expression of substrate degradation in time:

 

 

The linearization of this equation allows the determination of the constant using semi-logarithmic coordinates according to Equation 5.

 

 

This equation was used in this study. Glycerol substrate used is pure. So the COD will be considered almost equal to the BOD (Mara, 2003). Other physicochemical parameters such as conductivity, pH, dissolved oxygen and turbidity could be linked to the assessment of submerged aerobic fixed-film reactors performances (Mendoza-Espinoza et al., 1999; Akin et al., 2005; Albuquerque et al., 2012) and could give some explanations about the efficiency of this bioreactor against wastewater treatment. This work aimed to study the effect of initial concentration of the synthetic wastewater on the adaptability of bacterial cells on a submerged aerobic fixed-film reactor packing on surface area and the biodegradation kinetics; four tests were made with concentrations: 330 MgO₂/L (S1), 480 MgO₂/L (S2), 860 MgO₂/L (S3) and 1120 MgO₂/L (S4). 

Experimental setup

 

Experiments were carried out in a reactor operating in batch mode (Figure 2). It was a cylindrical column, with height of 1 m and diameter of 0.125 m, made of opaque PVC.  It was filled with 5 L of the synthetic wastewater. The air was introduced at the bottom of the column through an air flow equal to 10 L/h, by a rectangular diffuser (8 × 1 cm).

The bioreactor was filled with Biocell® packing (Figure 2) with excellent physical characteristics (Table 1). It was supported by a grid placed just above the air sparger at the bottom of the column for maintaining the packing fixed. Another grid was used to fix this packing to avoid its flotation by the effect of crossing air bubbles.

 

 

 

 

Synthetic wastewaters preparation and experiments conduct

 

Synthetic wastewaters were prepared in COD/N/P ratio of 100/4/1 (EPA, 1997; Mara, 2003) (Table 2), using distilled water. Pure glycerol was the organic substrate. Salts of sodium nitrate (nitrogensource) and potassium phosphate (phosphorus source) and some oligoelements (FeSO₄, MgSO₄, MnSO₄ and CaSO₄) were also added to synthetic wastewaters prepared. Doses of oligoelements were low (Mara, 2003; Jin, 2012).

 

 

To initiate the adaptation, inoculation by bacterial flora was made in the reactor. This bacterium was extracted from soil (Mara, 2003; Horan, 2003). Table 3 illustrates the physicochemical characteristics of synthetic wastewaters.

 

 

 

Submerged aerobic fixed-film reactor performances monitoring

 

Chemical oxygen demand (COD) and turbidity were monitored using a photometer type Palintest 7000. pH and dissolved oxygen were measured using a Hach sensor 40d-HQ -Multi parameters, which can also measure the temperatures of water and air. The conductivity was measured by Orion model 125. For measuring the COD concentrations, samples from the bioreactor were filtered through filters having a pore size of 0.45 µm. For the other parameters, measurements were realized directly in samples.

 

Turbidity evolution

 

Turbidity results for the different synthetic solutions are presented in Figure 5. Results illustrated in Figure 4 shows that the turbidity followed the same trend for all initial concentrations tested, except for S1 (330 MgO₂/L).The turbidity decreased at the beginning for S2, S3 and S4, and then, it increased to reach a maximum. But for S1, turbidity increased probably because of the quick adaptation of bacteria as mentioned above, inducing an increasing in biomass formation (Reaction 2).

 

 

The different synthetic wastewaters have different initial turbidities, from 10 to 30 NTU. This was due to the quantity of matters, especially from soil, added to each synthetic wastewater, despite the filtration of synthetic wastewaters. On the adaptation phase, turbidity decreased probably by physical interception and the adsorption and flocculation of biofilm due to extracellular polymeric substances (EPS) secreted by bacteria (Zhang and Liu, 2005; Hongyuan et al., 2013).

The increase of turbidity coincided with the beginning of the degradation phase. These two processes occurred at the same time according to the Reactions 1 and 2. This increase may be due to some factors: augmentation of biomass quantity, a part of which can be detached by abrasion of biofilm due to the air flow or by gases (CH₄, N₂, CO₂, etc.) released by anaerobic/anoxic processes, generally produced when the thickness of biofilm became high. Turbidities reached a maximum, which depended on the initial concentration (Table 6).

 

 

Turbidity results confirm clearly those of COD, and allow the same conclusions concerning the behavior of batch bioreactor (Valentis, 1988; Kwok et al., 1998). So, the turbidity was a monitoring parameter which could characterize the stage of the biological process.

 

 

pH evolution

 

pH experimental results are presented in Figure 6. Figure 6 shows, at the beginning, an increase of pH for S1 and S2 tests, a slight decreased for S3 and a constant evolution for S4. After the adaptation phase, a pH increasing trend was observed for all tests. The pH behavior in test S1 could be probably explained by their rapid kinetics. It could be corroborated by COD and turbidity results of S1.

 

 

During biodegradation, the pH measured in synthetic wastewaters resulted from an equilibrium between bacterial cells catabolism process (CO₂ formation from Reaction 1) and from the denitrification process (increase of pH), as reported by some researchers (Fabregas, 2004; Akin et al., 2005). It could also be affected by the stripping of CO₂ (Cohen et al., 2004, Morales et al., 2013).

To explain pH behavior, the biofilm detachment caused a degradation of biomass protein. Akin et al. (2005) and Fabregas (2004) showed that this increase could be explained by the denitrification which occurred during anoxic phase in a SBR. The noted decrease of DO as shown above limited the oxygen transfer in the synthetic wastewater and promotes the conditions of the denitrification which occurred in the biological matter. The pH could be used to monitor the biological process. Its evolution depended on turbidity and the aeration of synthetic wastewater.

 

 

Dissolved oxygen evolution

 

DO results are illustrated in Figure 7. It has been clearly oxygen decreased continuously in S2 and S3 tests. Whereas, in S4 test, the observed initial DO was low (Table 7). DO increase before a decrease. This behavior can be explained by the viscosity of S4 synthetic wastewater, due to its high concentration (1120 MgO₂/L), which prevents an efficient oxygen transfer (Stemmet et al., 2008; Kherbeche et al., 2013). But, during the adaptation phase a little part of substrate was consumed, so a DO increase was observed. After that, the DO decreased because of the substrate consumption by bacterial cells during biodegradation phase (Reaction 1).

 

 

 

The oxygen transfer could also be decreased because of the biological suspended matter, as reported by some authors (Rosenberger, 2003; Germain et al., 2007). It controlled both kLa20 and α-factor. Moreover, biological suspended matter accounted for the effects of the viscosity (Garcia-ochoa et al., 2000; Jin et al., 2001; Rosenberger, 2003). On the other hand, the α-factor was affected by the surfactants through EPS secreted by biological matter. Jimenez et al. (2014) showed a strong depression in bubble rise velocity and mass transfer in the presence of surfactants.

When transferring, oxygen must penetrate through the soluble microbial products (SMP) and then diffuse through the flock matrix (EPS) (Jimenez et al., 2014).

However, during biological process, the substrate could be transformed to some bio products which acted as surfactant agents, and affected the oxygen transfer (Garcia-ochoa et al., 2005; Painmanakul, 2005). So, the DO was correlated with the process biodegradation through its diminution during the process. 

The main conclusions that could be drawn from this study are:

 

1. The various stages of the process of biological degradation occurring in a batch reactor are the adaptation, the degradation of the substrate and the endogenous respiration.

2. The biological kinetics presents two slopes, the first one is slight and corresponds to the adaptation phase, the second one is accentuated, it corresponds to the biodegradation phase.

3. When the substrate concentration is low, the duration of the adaptation phase is small and the elimination of the substrate is easy.

4. The tests gave excellent results in terms of the abatement rates: 94.2% (S1), 91.7% (S2), 91.8% (S3) and 79.5% (S4) which corroborated with calculated biological kinetics constants (0.407 d^-1 (S1), 0.346 d^-1 (S2), 0.341 d^-1(S3), and 0.232 d^-1(S4)). These results confirmed also the good adaptability at low concentrations of substrate.

5. The turbidity was revealed as a great indicator of bacteria metabolism. Its increase coincided with the beginning of the substrate degradation; several factors were acting in the same time: development of bacteria cells in which, a part was detached by abrasion due to the air flow or by the anaerobic/anoxic process occurring when the thickness of biofilm increased.

6. In the same time, the pH increased when the turbidity increased and dissolved oxygen decreased because of aerobic bacteria metabolism, striping of CO₂ and increasing in viscosity due to some metabolites produced during biological reaction.

The authors did not declare any conflict of interest.