The effect of wall-shear stress on biofilm physical structure
Previous works have reported that higher shearing stresses led to thinner biofilms (Kwok et al. 1998; Laspidou and Rittmann 2004; Liu and Tay 2002). However, as shown in Fig. 2, the biofilm thickness obtained in this study increased from 2.1 to 2.7 mm while the wall-shear stress increased 0.8 and 1.29 Pa. This may because the range of wall-shear stresses and the structure of pilot sewers were different from which used in the previous research. In the study of Liu and Tay, the shear stresses were 6.5–9.0 Pa. In the study of Kwok, the biofilms were formed in 3-L internal loop airlift reactors. However, our results were supported by the study of Guzmán et al. (2007) which has demonstrated that the wall-shear stress within the range of 1.1–1.4 Pa was suitable for biofilm growth in sewers_ENREF_19. Biofilm density was impacted by some factors including hydraulic condition and microbial species (Christensen and Characklis 1990) (Laspidou and Rittmann 2004). The wall-shear stress affects the biofilms horizontally and vertically. The loose surface of biofilm would be washed away and the attached biofilms were compressed by the increase of wall-shear stress, leading to the increase of the biomass density (Xu et al. 2016). The higher density observed at higher wall-shear stress in this study was supported by others’ research (Liu and Tay 2002; Vieira et al. 1993), that a higher wall-shear stress resulted in a denser biofilm was founded in their studies.
Porosity and density of biofilm are both important to the transformation in biofilm. According to our previous research, the oxygen penetration depth in biofilm was higher in lower wall-shear stress (Xu et al. 2016). The greater wall-shear stress was, the greater the density of biofilm was. In other words, a denser biofilm under higher shear stress could lead to the decreased oxygen penetration depth. Previous studies have shown that slight shearing stress was favorable for the formation of the inattentive and porous structure of biofilm (van Loosdrecht et al. 1995, 2002). Because biofilm porosity decreased with the increase of wall-shear stress, the dissolved oxygen was minimum in the biofilm cultured at wall-shear stress of 2.0 Pa among these three wall-shear stresses. Oxygen played a significant role in the process of microbial growth and the different oxygen conditions inevitably had a major impact on the microbial community structure of the biofilms.
The effect of wall-shear stress on biofilm structure was in three aspects. Firstly, with the increase of wall-shear stress, biofilm became thinner. Then, with the increase of wall-shear stress, biofilm density became greater. Lastly, the microbial community in biofilm was affected by wall-shear stress (Cheng et al. 1997).
The effect of wall-shear stress on biofilm microbial structure
The microorganism amount
Biofilm is a complex micro-ecological structure composed of microorganisms and EPS. Flow rate, the diameter of sewer (Guzmán et al. 2007) and substrate concentration were important factors affecting biofilm growth in sewers. In this study, wall-shear stress was calculated by combining flow rate, slope and fullness, which can be seen as a comprehensive factor. Due to the use of synthetic sewage, the inorganic materials in the biofilms were few thus could be ignored. The average mass density of the biofilm (TS) which did not consist of Extracellular Polymeric Substances (EPS) could represent the microorganism quantity. The amount of microorganism was calculated according to the average biofilm density, thickness, the surface area of biofilm growth and the EPS. As show in Fig. 7, the amount of microorganism increased with the increase of wall-shear stress until the wall-shear stress reached 1.45 Pa. When wall-shear stress exceeded 1.45 Pa, although the biofilm density increased as well, the thickness of biofilm decreased at a greater degree, resulting the drop of the microorganism amount. When the wall-shear stress was 1.45 Pa, the average biofilm density was 74 ± 5 kg/m3, just 7.5% less than which obtained at wall-shear stress of 2.0 Pa, while the biofilm thickness was 2.4 ± 0.1 mm, 20.8% greater than which reached at wall-shear stress of 2.0 Pa.
According to the calculated data, a simple empirical model (Eq. 4) about microorganism and wall-shear stress was developed
$${\text{X}}\, = \, - \,0.0485F^{2} \, + \,0.161F\, - \,0.0142$$
(4)
where X is the amount of microorganism (kg); F is wall-shear stress (Pa).
As shown in Fig. 7, the R2 of Eq. (4) was 0.91, indicating that it is reasonable. In addition, in order to validate the model, the measured data was used when COD was 200 mg/L. As the Fig. 8 shown, the R2 was 0.90.
The effect of wall-shear stress on microbial composition
The wall-shear stress influenced the mass transfer in biofilm and it played an important role in the microbial composition of the biofilm. Previous research was mostly focused on the influence of wall-shear stress on the physical structure such as biofilm thickness and biofilm density (Kwok et al. 1998; Liu and Tay 2002). However, the impact of wall-shear stress on the biofilm microbial composition did not obtain enough attention (Rochex et al. 2008). As shown in Fig. 5, although Deep-Sea-Hydrothermal-Vent-Gp-6 (DHVEG-6)-norank6 and Methanospirillum were the dominant bacteria in three biofilms, their proportions were different. DHVEG-6 is known as haloarchaea, distantly related to halobacteriales, (Casamayor et al. 2013) and has been detected in marine environments, terrestrial soils and saline lakes including deep sea methane seep sediments (Nunoura et al. 2011). The distribution of DHVEG-6 indicated that it could produce methane in this study, though the physiological and metabolic functions of DHVEG-6 were not fully known (Kuroda et al. 2014). Methanospirillum was a fastidious anaerobic specie and could produce methane with H2–CO2, and its preferred living temperature and pH were 30.0–37.0 °C and 6.6–7.4, respectively (Ferry et al. 1974).
In fact, the proportion of Methanospirillum in sewer biofilm increased with the increase of wall-shear stress, but the amount was not, because that the microorganism amount did not increase with the increase of wall-shear stress. Although the proportion of Methanospirillum in sewer biofilm cultured at wall-shear stress of 1.45 Pa was not the highest, the amount of Methanospirillum was at the most. To be exact, the more Methanospirillum was, the greater methane production was and it implied that Methanospirillum played the crucial role in methane formation in gravity sewers in this study.
Model development and validation
Methane production is related directly to the amount of microorganism which is influenced by wall-shear stress in sewers. So the wall-shear stress could play an important role in methane production. The aims of model was to make certain of the role that wall-shear stress played in methane production (Ai et al. 2016; Chaosakul et al. 2014).
$${\text{X}}\, = \, - \,0.0485F^{2} \, + \,0.161F\, - \,0.0142$$
(4)
$$Q_{{CH_{4} }} \, = \,Y_{{CH_{4} /X}} \cdot X \cdot {\theta }^{T - 20} \cdot HRT .$$
(5)
$$\begin{aligned} {\text{F}}\, = \, & 0.21941\, + \,0.44146{\text{I}}\, + \,1.73331{\text{n}}\, - \,0.52041{\text{v}}\, + \,0.13167{\text{nI}} \\ & - \,0.24688{\text{vI}}\, - \,1.47281{\text{nv}}\, + \,0.23833I^{2} \, - \,0.12750n^{2} \, + \,1.88828\nu^{2} \\ \end{aligned}$$
(3)
where \(Q_{{CH_{4} }}\) is the methane production, mg/(L wastewater day); \(Y_{{CH_{4} /X}}\) is the yield coefficient, mg methane/kg biomass; X is the amount of microorganism, kg; \(\theta\) is the temperature coefficient = 1.05; T is the temperature of sewer wastewater, °C; HRT is the wastewater retention time, h; F is the wall-shear stress, Pa; I is the slope of sewers, ‰; n is the fullness degree of sewers; v is the velocity of flow, m/s.
Equation (5) was similar to the model previously proposed by Chaosakul et al. (2014) except that the X and \(Y_{{CH_{4} /X}}\) took the place of (A/V) and γ (the specific rate of CH4 emission), respectively and Eq. (3) was based on our previous study (Ai et al. 2016).
Figure 9 illustrated the relationship between the methane production and wall-shear stress. The R2 value of Eq. (5) was 0.95, indicating that the Eq. (5) was also reasonable.
In order to validate the model, the measured data was used when COD was 200 mg/L. The fitting results were showed in Fig. 10. Results showed the model’s predictions, agreed with the measurements well, the difference between measurement and simulation was found in the rage of 1.5–13.0%. In this model, substrate concentration and pipe size were not considered that leaded to some errors between measurement and simulation. And it would be revised in our future research.
Most of methanogen was strictly anaerobic and a small amount of methanogen was anoxic and aerobic. According to the laying of gravity sewer, there are manholes among the sewers. Usually, there is oxygen in gravity sewers. In this study, the methane production was less than which obtained in rising main sewers in previous research (Guisasola et al. 2008, 2009). An important reason may be that anaerobic environment in rising main sewers is more beneficial to methane production. In addition, rising main sewers were full of sewage so that biofilm could develop on the entire surface of sewers. There is an obvious difference between gravity sewer and rising main sewer. Gravity sewers are not full of sewage. The biofilm could not grow on the places which are not covered with sewage. This would lead to difference in microorganism amount and that affects the methane production.