There is no direct impact of salinity on membrane performance since the membranes used in MBR have big enough pores that can pass ions freely without being affected by osmotic pressure. However, salinity indirectly affects membrane performance by affecting not only the floc sizes, but also the quality and quantity of biopolymers.
1. Effect of short term salinity hike
The effect of short term salinity hike was investigated using a pilot scale setup (Reid, 2006). When chloride (Cl–) concentration was raised up to 4.5 g/L using NaCl, membrane permeability dropped almost immediately as shown in Fig. 1 (note: seawater contains 19.4 g/L chloride). The recovery of flux did not occur quickly after the salinity went back to the original level. Although particle size appeared not changed, protein portions of SMP and EPS were significantly increased as summarized in Table 1.
Fig. 1. Permeability drop due to a salinity shock. HRT = 72 hr, SRT = 54 days, flux = 8 LMH (Reid, 2006).
Table 1. Average fraction of SMP and EPS at low and high chloride level (Reid, 2006)
2. Effect of long-term high salinity
The mixed culture used in activated sludge process has a great adaptability that allows microorganisms to survive in varying environment, e.g. high temperature above 40 oC, high or low pH, high salinity, etc. Adaptation occurs either by changing metabolism of same microorganisms or by proliferation of most adaptable species in the given environment. Therefore, the effect of moderate salinity change can be absorbed by the mixed culture of activated sludge without leaving significant drawback in membrane performance as long as the change occurs gradually over a long period of time.
In general membrane performance decreases as salinity increases. There is no clear cut maximum allowable salinity, but it might be somewhere around 3.5-7.0 g/L as NaCl or 5-10 mS/cm as conductivity in long-term operation assuming a stable salinity. If it is above this range, MBR will be still operable, but more membrane surface areas than normal must be considered to treat the flow. According to literature, COD removal appeared intact even at a salinity of 160g/L as NaCl in a controlled environment (Lay, 2010), but it is highly doubtful that membrane performs properly at such high salinity.
High salinity negatively impact membrane performance by hampering floc formation, where the charges of particles are more effectively shielded by the counter ions so that charge induced flocculation can be largely inhibited. As shown in Fig. 2, mixed liquor from a MBR treating wastewater from a polymer synthesis process. Due to the extremely high salinity measured by conductivity (62 mS/cm at 20 oC vs 48 mS/cm for seawater at the same temperature), cells and debris hardly form floc. The flux of membranes in this application was no more than 5 LMH due to the severe membrane fouling by diffused mixed liquor.
Fig. 2. Microscopic image of MBR mixed liquor with a conductivity of 62 mS/cm (x400).
3. Other effect of high salinity (Lay, 2010)
Salinity influences the alpha factor indirectly by affecting the viscosity and the coalescence of air bubbles. When salinity increases from 0 g/L to 15 g/L, bubbles tend not to coalesce, which positively contribute to the alpha factor. However, the increased medium viscosity at high salinity negatively affect alpha factor.
Salinity has direct impact on the oxygen solubility in the form of the factor beta. As shown in Eq. 2 and Eq 3 here, solubility of oxygen decreases as salinity increases. Fig. 3 illustrate the relation among DO, salinity, and water temperature. For example, beta is at 0.94 for TDS = 10 g/L and 0.92 for TDS = 15 g/L, but reduces to 0.74 for TDS = 50 g/L.
Due to the complex relations among many environmental factors, the actual effect of salinity on oxygen dissolution can vary site by site.
Studies show high salinity is detrimental to biological phosphorous removal. In one study phosphorus removal efficiency decreased from 84% to 22% when the salt content increased from 9 to 60 g/L. However, it is not clear whether the phosphorous removal efficiency can be recovered if the experiment was sustained for longer term and microbial community adapts to the high salinity.
Nitrifiers can readily adapt to 30 g/L of salt, but they are inhibited at 40 g/L. However, as microbes often surprise us with their great adaptabilities, it might be possible to acclimate the nitrifiers, which are composed of diverse microorganisms, to a very high salinity, if enough acclimation time is given.
Fig. 3. Dissolved oxygen as functions of water temperature and salinity (Lay, 2010).
© Seong Hoon Yoon