Results

Effect of hydraulic retention time on COD removal
Figure 2 shows the diagram of COD removal efficiency versus different concentrations of TPH at two HRTs (18 & 24 hr) for each reactor.
Figure 2  COD removal efficiency at two HRTs of 18 and 24 hours versus ratios of TPH/COD molasses in (a) MBR and (b) RBC.
As shown in Figure 2 increasing the ratios of TPH/COD molasses has led to the reduction of COD removal efficiency in both reactors. This is resulted from the fact that increasing TPH/COD molasses makes microorganisms start to use oily hydrocarbons instead of nutrients produced by molasses.
On the other hand, in order to form bonds between water and oily pollutant molecules, some concentration of surfactant twin-80 was added to the system which helps the absorption of hydrocarbons by microorganisms.
As depicted in Figure 2 when the ratio of TPH/COD was greater than 0.6, the slope of the efficiency decrement was increased. This is due to inhibition caused by aromatics and hydrocarbons in oily wastewater. Furthermore, COD removal efficiency was increased for higher HRT. This is caused by the contact between nutrients and microorganisms for a longer retention time.

The effect of HRT on TPH removal
TPH removal efficiency versus different concentrations of TPH at two HRTs (18 & 24 hr) for each reactor is shown in Figure 3.
Figure 3  TPH removal efficiency at two HRTs of 18 and 24 hours versus ratios of TPH/COD molasses in (a) HMBR (b) RBC.
As shown in Figure 3 increasing HRT has led to increasing the TPH removal efficiency, because pollutants contacted microorganisms for a long hydraulic retention time.
Increasing the ratio of TPH/COD molasses to 0.6 has led to increasing the TPH removal efficiency in both reactors but when the ratios of TPH/COD molasses was greater than 0.6, the efficiency of both systems in removing the pollutant was reduced. This is due to the fact that the increase in the concentration of hydrocarbons on biofilm distorts the cellular metabolism of microorganisms and prevents them from using carbon molasses for their metabolism and reproduction. This will, in turn, reduce MLSS in system and the potential for removing the pollutant will be significantly reduced. Thus, in treating the oily wastewater in such reactors, it is recommended not to choose the ratio of TPH/COD molasses more than 0.6.

The effect of various ratios of TPH/COD molasses on TPH removal efficiency
Figure 4 shows the TPH removal efficiency for ratios of TPH/COD molasses at HRT of 24 hours in both reactors.
Figure 4  Comparing TPH removal efficiency in RBC and HMBR in different ratios of TPH/COD molasses at HRT of 24 hours.
This comparison shows that TPH removal efficiency for all concentrations of the oily pollutant used in this project has been higher in hybrid membrane than RBC.

The effect of various ratios of TPH/COD molasses on suspended solids removal efficiency
Figure 5 shows the suspended solid's removal efficiency by two reactors at various concentrations of the pollutant.
Figure 5  Comparing the suspended solids' removal efficiency in RBC and hybrid membrane bioreactor for various ratios of TPH/COD at HRT of 24 hours.
This comparison shows that as the concentration of the oily pollutant increases, the suspended solids removal efficiency is reduced in both reactors. The effluent suspended solids of the system was increased with increasing oily pollutant concentrations because the bio-film detached from the media due to the toxicity of oily pollutant [13].
Also the diagram shows the higher efficiency of HMBR than the RBC in removing the suspended solids because of the membrane performance.

Investigating the changes of permeate flux from membrane over time
Figure 6 shows the changes of permeate flux from membrane in a typical pressure of 1.2 bar.
Figure 6  Permeate flux of membrane versus time.
When the permeate flux of the membrane was about 30 L/m2.hr ( it takes 6 days for average MLSS of 3000mg/l and about 5 days for higher concentrations) chemical cleaning of the membrane is performed.
Higher permeate flux of the membrane at HRT of 24 hours than 18 hours proves higher efficiency of removing organic substances and suspended solids and thus reduction of the membrane fouling and higher permeate flux as well.