The Performance Of Step-fed Sequencing Batch Reactor Treating Swine Wastewater

With the expansion of livestock industry, pollution control of animal feedlots becomes a key factor for alleviating the agricultural non-point source pollution. The shortage of available carbon source for denitrification is the main limiting factor in the bio-treatment of livestock wastewater. The step-fed sequencing batch reactor (SFSBR) is a new type of the enhanced nitrogen removal system, characterized by a multiple anoxic feeding for supplying the organic carbon source to denitrifiers. To facilitate the application of SFSBR in livestock industry, a series of lab-scale experiments were conduced to investigate the performance of SFSBR treating swine wastewater under various operating conditions. The relationship between the pollutants removal efficiency and operating parameters in SFSBR, such as aeration intensity, loading rate, influent carbon nitrogen ratio, feeding volume ratio, was identified in aspects of pollutants evolution, microbial characteristics and on-line process parameters including oxidization reduction potential (ORP), pH and dissolved oxygen (DO). As such, an optimal combined operating condition was determined for the SFSBR, together with the profiles of ORP, pH and DO in the reactor. In addition, "the feasibility of sludge granulation in the SFSBR system was explored using swine wastewater as substrates. Main results obtained in this study are as follows:Provided the degradation of organic matter following the first order model, the removal of organic matter in SFSBR is correlated with the reaction time and feeding number. Prolonging the reaction time or multiplying the feeding number would benefit the organic matter removal in the SFSBR system. Given both nitrification and denitrification completed in each sub-cycle of non-aeration-aeration, the theoretical nitrogen removal efficiency is a function of the feeding volume ratio of the last sub-cycle to the total feeding volume (β) and the ratio of initial volume in the tank before feeding to the total feeding volume (V₀/V_F). The nitrogen removal efficiency of SFSBR would be improved by decreasingβor increasing V₀/V_F. The comparison of A/O-SBR with SFSBR in tank volume shows that the required tank volume of SFSBR is smaller than that of A/O-SBR at the same sludge age.The performance of SFSBR with decreasing feeding volume was influenced by the loading rate, aeration intensity, feeding volume ratio and influent carbon nitrogen ratio. For the twice-fed SBR with decreasing feeding volume, nitrogen removal was inhibited by the increase of organic loading rate (the influent carbon nitrogen ratio varied in random), while the increase of aeration intensity enhanced the nitrification rate, shortened the nitrification time, and improved the phosphorus removal during the aeration phase. When the feeding volume ratio (greater than 1:1) decreased in the twice-fed SBR, the phosphorus rather than nitrogen removal was significantly improved. In the triple-fed SBR with decreasing feeding volume, both nitrogen and phosphorus removals were inhibited due to the increase in the nitrogen loading rate (the influent carbon nitrogen ratio was fixed), while those were enhanced by promoting the influent carbon nitrogen ratio. The ORP decreasing range as well as pH decreasing range varied corresponding to the performance of SFSBR under conditions of overloading or enhanced denitrification.Sludge granulation was achieved in SFSBR treating swine wastewater. Granular sludge cultivated in this study is yellow brown and its diameter falls in the range of 0.5 to 1.0 mm, and rich in bacteria species. The loading ratio and influent carbon nitrogen ratio did have significant impacts on the extra-cellular polymers (ECPs) and structure of granular sludge. The results of polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) shows that the diversity of bacterial community in activated sludge increased as the influent carbon nitrogen ratio increased.According to the results of orthogonal experiment, the influence order of operating parameters on the performance of SFSBR treating swine wastewater is quite different. The nitrogen loading rate contributed most to the build-up of NO₂^--N, NO₃^--N and NH₄⁺-N in the effluent, while the aeration intensity had the greatest effect on the net nitrogen removal efficiency. The influent carbon nitrogen ratio has a profound influence on the sludge volume index (SVI) and the fraction of granular sludge with a diameter over 0.5 mm (f_0.5mm). In SFSBR, the increase of the feeding volume ratio from 3:2 to 3:1 did not affect the nitrogen removal performance remarkably. The optimal combined operating parameters for nitrogen removal include a carbon to nitrogen ratio of 7mg COD/mg NH₄⁺-N, a feeding volume ratio of 3:1, a nitrogen loading rate of 0.026 gNH₄⁺-N/(gVSS·d), and an aeration intensity of 4.2 L/(m³Â·s) in a twice-fed granular sludge sequencing batch reactor treating swine wastewater, at which the net nitrogen removal efficiency has reached 72% with an inorganic nitrogen concentration of 21 mg/L in the effluent. PCR-DGGE results show that optimization of nitrogen removal of SFSBR did have an impact on the bacterial community, and the microflora become less diversified in the granular and mixed sludge, although the bacterial community stay stable in the SFSBR system treating swine wastewater.The variation of ORP, pH and DO has a potential to function as indicators of nitrogen removal in the SFSBR with decreasing feeding volume. The ORP "nitrogen broken point" could indicate termination of nitrification when the initial ammonia nitrogen concentration and aeration time match the aeration intensity. In contrast, the ORP "nitrate knee" could act as an indicator for the termination of denitrification when endogenous denitrification plays an important role in the non-aeration phase. The pH "ammonia valley" also could serve as an indicator for the termination of nitrification, while its sensitivity is interfered with the initial ammonia nitrogen concentration and nitrification time. In addition, both the ORP decreasing range during the non-aeration phase and the pH decreasing range during nitrification have the potential to serve as the control parameter for feeding distribution. In addition, the variation rate of ORP and pH during nitrification could be used to estimate the nitrification rate. As the jump of DO coincides with the termination of nitrification, it could be used to control aeration.The biological phosphorus removal usually happened in the first non-aeraion/aeration sub-cycle of SFSBR in this study, but became insignificant in following sub-cycles. A further study needs to be done to improve the biological phosphorus removal in the SFSBR technique for swine wastewater treatment.