Evaluation of the Performance of an Activated Sludge Bioreactor for the Sulfide Removal in the Presence of Nanoparticles: Kinetic Study and Respirometry Test

Document Type : Research Paper


1 Faculty of Chemical and Materials Engineering, Shahrood University of Technology, Shahrood, Iran

2 Department of Chemical Engineering, Isfahan University of Technology, Isfahan, Iran



In this project, experimental investigations were performed to measure the bio-kinetic parameters of the sulfide removal using an activated sludge bioreactor (ASBR) in the presence of nanoparticles. For this aim, silica nanoparticles (NPs) and exfoliated graphene oxide (GO) nanosheets were synthesized and characterized using FTIR, XRD, and TEM analysis. Then, three types of bioreactor systems, including activated sludge without nanoparticles (AS), in the presence of SiO2 NPs, and GO nanosheets, were utilized for different ranges of sulfide concentrations. The variation of pH values in the ASBR systems was investigated during the kinetic experiments. Respirometry tests were employed to calculate the maximum yield factor (Y) and endogenous decay coefficient (kd) in Monod’s model and the maximum cell growth rate during the sulfide removal. Moreover, different kinetic models, including Monod, first-order, second-order, Moser, and Contois, were compared. The values of the R2 coefficient (0.992, 0.927, and 0.976 for AS, SiO2, and GO bioreactor systems, respectively) showed that the Monod model was appropriately fitted with the experimental data, and the SiO2 sample had the best condition for both sulfide removal and biomass growth and GO sample had the worst performance. 


Nowadays, the presence of sulfurous compounds in aqueous media has been converted to one of the most serious risks due to their high toxicity. Hydrogen sulfide (H2S), as a volatile organic compound, is the most common form of sulfurous compounds and exists in both forms of bisulfide (HS-) and sulfide ions (S2-) in wastewater. Usually, two moles of O2 are consumed per mole of S2-, and therefore sulfide ions decrease the dissolved oxygen in water [1-5]. Moreover, sulfurous compounds are so toxic and count as a huge threat to the aqueous ecosystem. Thus, it is necessary to choose a suitable, economic, and environmentally friendly technique to eliminate this pollutant from wastewaters and waste gases [4-6]. Different physicochemical methods, including advanced oxidation processes (AOPs) and catalytic conversion, have been employed to remove sulfide ions from wastewaters by the oxidation of sulfide to either sulfate or elemental sulfur [7-9]. Since these methods operate with considerable energy consumption and need some chemicals, more cost-effective approaches are required. Nowadays, physicochemical processes, such as oxidation and direct air stripping, are commonly used for sulfide removal [10-13]. Nonetheless, biological methods have attracted great attention due to their more practical and economic features comparison to physicochemical methods under the same operational conditions [4]. It is well-known that the activated sludge process is the most commonly utilized technique for biological wastewater treatment [9].
Substrate removal occurs along with the growth of biomass during the biodegradation process. The rate of this process can be affected by various parameters involving microbial population, the chemical structure of the substrate, pH, oxygen concentration, temperature, and accessibility of nutrients [6, 14]. Also, the adsorption of pollutants directly relates to biomass concentration, and its performance improves with increasing the concentration of biomass in a bioreactor [15]. In addition, the performance of microorganisms has been dramatically affected by the presence of nanoparticles (NPs) by their development in different processes [16, 17]. Recently, various kinds of NPs have been explored in wastewater, which can be adsorbed onto activated sludge (AS) [18, 19]. Consequently, some researches were performed to determine the effect of NPs’ existence on biological wastewater treatment methods, especially the AS process. Many studies have been evaluated the toxicity of different NPs, such as SiO2, TiO2, Ag, Zn, and Cu nanoparticles on AS [17, 20-24]. Nevertheless, the literature survey showed that despite nanoparticles’ application in wastewater treatment [25-27], a few works have been reported the sulfide removal from wastewater using biological systems in the presence of nanoparticles. Rollemberg et al. [28] compared the performance of aerobic granular sludge (AGS) and activated sludge flocs (ASF) for eliminating organic contaminants during nitrification, denitrification, and dephosphatation processes. They found that the AGS system produced less sludge and a higher endogenous consumption rate than ASF, and the yield factors (Y) were obtained 0.36 and 0.55 g VSS/g COD for AGS and ASF, respectively. Akdemir et al. [29] studied the effect of carbon nanotubes (CNTs) on removing heavy metals in a continuous activated sludge process. They found that the presence of CNTs increased the amount of COD and also the removal efficiencies of nickel (79.99%), copper (99%), and chromium (99%). In another work, Akdemir et al. [30] studied the presence of CNTs on the performance of activated sludge treatment. The results indicated that adding CNTs from 10 to 30 mg/L led to enhancing nitrogen removal from 65% to 75% and decreasing ammonium concentration to 70%. Adding CNTs increased the saturation constant (Ks) from 1406 to 2355 mg/L, while the Y factor remained without significant change. Alizad Oghyanous et al. [31] employed four different submerged membrane bioreactors in a lab-scale to determine biokinetic coefficients and optimize their performance during eight months. The results revealed that yield factor, maximum growth rate (μmax), the half-velocity coefficient for the substrate (Ks), and endogenous decay coefficient (Kd) were achieved between 0.0733-0.310 mg/mg, 1.249-3.672 day-1, 1-3.156 mg/L, and 0.985-3.119 day-1, respectively. Moreover, the best operating condition for activated sludge was obtained at 15 days sludge residence time and COD loading of 0.1 (g COD/L.day). 
The daily development of nanoparticles in the different processes has caused remarkable effects on the performance of microorganisms. On the other hand, studying and evaluating the biological removal of sulfide pollutants by comprehensive comparison between the kinetic models of specific growth of biomass in different bioreactors can help the improvement of their performance. Therefore, the object of the present work is to investigate the effect of GO nanosheets and SiO2 nanoparticles on the reaction kinetic of the sulfide removal in the activated sludge bioreactor (ASBR) system using a respirometry test. The remaining sections of this study are outlined as follows: First, the GO and SiO2 nanomaterials are synthesized and then analyzed to investigate the physical and chemical characteristics. Then, the activated sludge system is employed to remove sulfide ions in the presence of the nanomaterials. Next, the bio-kinetic parameters are determined using a respirometry test, and different kinetic models are also compared. The paper ends with a conclusion, which recaps the main results of the investigation. 

Graphite powder (99.99 % purity), nitric acid (HNO3), sulfuric acid (H2SO4), hydrochloric acid (HCl), and potassium chlorate (KClO3) were purchased from Merck Company. Ammonia solution (25%), tetraethyl orthosilicate (TEOS), and ethanol were obtained to prepare silica nanoparticles from Merck Company. Sodium sulfide hydrate (Na2S.9H2O, 60% purity) was obtained as a substrate from Samchun Pure Chemical Company (SPCC). Sulfide ion concentration was measured using iodine, potassium bi-iodate (HI2KO6), sodium thiosulfate (Na2S2O3), and potassium iodide (KI) purchased from Merck Company.

Characteristics instruments
Fourier transform infrared spectroscopy (FT-IR) (Lambda-25, PerkinElmer, USA) was employed to determine the functional groups that exist on the surface of prepared materials. X-ray diffraction (XRD) (PW3710; Philips, Netherlands) was utilized to indicate the crystallinity of the prepared nanomaterials. The size and surface morphology of the exfoliated graphene oxide and silica nanoparticles were characterized by transmittance electron microscopy (TEM) (PHILIPS CM120, USA). DO-meter (DO-5510 Lutron Electronic Enterprise Company, Taiwan) was operated to measure dissolved oxygen concentration.

Synthesis of nanomaterials 
According to the literature [12, 32], the exfoliated graphene oxide (GO) nanosheets were prepared from graphite powder using the Hummer’s method. The summary of the synthesis procedure is shown in Fig. 1a. The amorphous silica (SS) nanoparticles as also synthesized in a simple way. As shown in Fig. 1b, firstly, 1 mol TEOS was dissolved in 6.3 mol ethanol. Then a mixture containing 1.8 mol aqueous ammonia (25%) and 6.3 mol ethanol was added to the previous solution and stirred for an hour. Eventually, the mixture was washed several times with distilled water and dried 24 h at 60 °C to obtain SiO2 nanoparticles as powders.

Measurement of biomass growth and sulfide removal
The mixture liquor volatile suspended solid (MLVSS) was measured to determine the biomass growth based on a standard method reported in the literature (2540 Solids#43) [33]. Sulfide ion concentration was also measured through the iodometric method [33]. 

Determination of bio-kinetic parameters 
A respirometry test was used to obtain bio-kinetic parameters in the first step. It is based on mass balance as the following [34]:
Where ΔX is the generated biomass, Δ0 is the consumed oxygen, and (-ΔS) is the total consumed substrate. Eq. (1) can be written as the following:           

The amount of produced biomass per unit of the consumed substrate is called the yield factor (Y):

Monod’s growth model is used to define two terms on the right-hand side of Eq. (2) as [35]:


where μmax, KS and kd are attributed to the maximum growth rate (day-1), the half-velocity coefficient for the substrate (gS/L) and endogenous decay coefficient (day-1), respectively.
Oxygen uptake rate (OUR) is an important parameter obtained by a respirometry test. Eq. (6) is obtained by substituting Eqs. (4) and (5) into Eq. (2):

In Eq. (6), the first term is the exogenous respiration related to substrate consumption [36]. The second term on the right-hand side is called endogenous respiration, which defines the oxygen consumption of bacteria for the hydrolyzation of dead cells, their movement, and cellular proliferation. 
The bio-kinetic parameters (kd and Y) can be obtained by measuring the oxygen consumption rate. These two parameters demonstrate the interaction between the biomass and the substrate. The endogenous contribution can be determined by operating a bioreactor without any substrate (fixing S=0). Therefore, Eq. (6) is transformed to the following equation