Investigation of organic pollution removal efficiency in saline wastewater using by hybrid microbial growth in sequencing batch reactor

Significant rise in concentration of saline wastewater entering the treatment plants has been resulting in many problems in the biological treatment processes. On the other hand, the specific conditions of physicochemical treatment methods for saline and hyper saline wastewater have limited their application on a large-scale. Over the past few decades, Sequencing Batch Reactor (SBR) process has been widely used as an efficient, well-designed and practical approach for treatment of domestic and industrial wastewater due to its cost-effectiveness and simplicity.

In this study, a lab-scale Hybrid Sequencing Batch Reactor (HSBR) was used to examine the effect of salinity (NaCl), increased from 0-6.7% (g NaCl/ L wastewater), on the biological treatment. The Pilot-scale Plexiglas HSBR system with circular cross section was used for this study had a working volume of 6.4 L with dimension of 18 cm for diameter and 30 cm for its height. Polyethylene moving carriers with an average specific surface area of 500 m2/m3 and density of 95 kg/m³ were used as media for attached growth of biofilm. Two identical peristaltic pumps and three analogue time switches (Theben Germany) were used for controlling influent and effluent wastewater. The HSBR was operated in 24 h working cycle including 1 h influent feeding, 20 h reaction (aeration) time, 1 h settling and 1 h decanting. The activated sludge seeds were obtained from Shahid Beheshti University (SBU) municipal wastewater treatment plant located in north of Tehran (Iran). System was introduced by 3250 mg/L of MLSS as the start-up seed. The dissolved oxygen (DO) concentration, pH and Temperature in the system were maintained between 2.6-6.8 mg/L, 7.2-8 and 18-26 °C respectively throughout the study. COD, MLSS, MLVSS and SVI parameters have been measured over a period of 7 months operation.

Results indicated that by increasing salt concentration from 0 to 67.7 g NaCl/L, the COD removal efficiency reduced from 94.22% to 53.69%. The adverse inhabitation effect of salinity causes significant decrease in COD removal efficiency. At 5 gNaCl/L, COD removal rate was increased to 94.22% which was the highest removal efficiency observed in this study. The reason for this little improvement was stimulatory effect of salt on activity of microorganisms. This finding that a small amount of salt (below 10 gNaCl/L) is in favor of bacterial growth and reproduction is in line with other studies. At the last salinity level of 67.7 gNaCl/L, after about 30 days to achieve the steady state conditions, the COD removal efficiency dropped at the rate of 41.76% compared to the highest efficiency at 5 gNaCl/L. The high concentration of salt causes loss of cellular activities, dehydration and inhabitation of many active enzymes in biological treatment. However, system showed a good performance for removal of organic matters compared to conventional SBR system even at 30 gNaCl/L (82.36%). The results of this study show that the removal efficiency of organic pollutant was inhibited less severely than those reported at equal levels of salinity. This might be result from the diversity of biological treatment process, variety of microorganism in biomass and influent wastewater. Moreover, it could be attributed to the biofilm special 3D-structure and capabilities for growth and reproduction of many kind of bacteria. Attached growth of microorganisms has a lot of distinct advantages over suspended growth. First, it could stimulate multi-cultural bacterial growth and consequently causes high active biomass. Second, biofilm unique structure could improve the interaction of substrate with microorganisms by proving a multiple reaction site. Third, as suggested by other articles, some protective substance like organic polymer which secreted by microorganism and acted as a defense mechanism in harsh environment, could grow more easily in biofilm than suspended sludge, and therefore salinity has more significant inhabitation on suspended sludge compared to biofilm

MLSS and MLVSS were measured to represent the mass of microbes that exist in bioreactor. MLSS values were slightly increased with increase of salinity to 20 gNaCl/L. by increasing salinity further to 67.7 gNaCl/L, MLSS reached to its highest level at 10,530 mg/L. it means that MLSS increased about 69% from the beginning of the study. By increasing salinity, various kinds of species started to suppressed and as a result, the number of dead cells increased at higher rate compared to non-saline environment. On the other hand, slat-resistant microorganisms like halotolerant and halophilic species, have a chance to grow more favorably. As a result, the total amount of these specific organic matters increased with increase of salinity. It was also seen that MLVSS was increased to 3240 mg/L at the second stage of salinity corresponding to 5 gNaCl/L, and then had a relatively constant values until the end of experiments at 67.7 gNaCl/L salinity. This is suggested that regardless of total mass of microbes whether live cells or dead cells, viable and active cells are remained constant. As mentioned before, some specific salt-resistant microorganisms which were not dominant species at first, could grow and survived in high saline environment and therefore the amount of viable biomass remained unchanged.

In this study, sludge settling performance was improved by increasing salinity. By increasing salinity from 0-67 gNaCl/L the SVI decreased from 156 mL/g to 27 mL/g and this result showed the improvement of sludge settling property with increase of salinity. In addition, sludge settling velocity was increased. By increasing salinity from 5 to 67.7 gNaCl/L, the differences in settled sludge at 10, 20 and 30-minute of SVI experiment became less and less, meaning that the suspended sludge was settled faster. Some factors could cause this better performance in settling property including a) inhabitation of filamentous bacteria in saline condition which led to better settling conditions b) selection of denser sludge which caused by combination of electrostatic and hydrophobic interactions and consequently reduction of repulsive force between particles c) with salinity activated sludge flocs become smaller and closer d) washout of lighter sludge flocs.