Nitrogen Removal Performance Via A Combined PN-Anammox Process In A Single-stage Reactor

It is a hot subject in nitrogen removal technology to comnine partial nitrification(PN)combined with anaerobic ammonia oxidation(Anammox)in a single reactor.It has attracted increasing interest in the field of wastewater treatment worldwide since in such operational process space,equipment investment and energy consumption can be reduced.A novel bioreactor was carried out in this study to achieve both higher nitrogen removal efficiency and lower biomass loss and at the same time simplify the operational process in a single reactor.There were two zones in the reactor:carriers K1 attached with aerobiotic biofilm were filled in the draft-tube which was called oxygen limited and riser zone to complete partial nitrification reaction;while the fiber carriers attached with anaerobic biofilm were fixed in the gap between draft-tube and outer tube which was called anaerobic and downer zone to carry out Anammox reaction.Aeration at the bottom of the draft-tube supplied dissolved oxygen(DO)for reaction and energy for the fluid circulation.Biofilms were cultured in two different experimental systems.Two kinds of carriers attached with different biofilms were put into the combined reactor for single-stage PN-Anammox nitrogen removal treatment to synthetic wastewater.The influence of aeration pattern and dissolved oxygen on nitrogen removal efficiency was investigated.The influence of various factors on the concentration of matrix and microbiology as well as the mass transfer in the reactor were analyzed based on the theatrical modelling.The main achievements of this research are as follows:(1)Biofilm culturing experiments included two various parts:the Anammox biofilm was trained and cultured in a fixed-bed reactor and effluent reflux was carried out to improve nitrogen removal efficiency and shorten the startup time.After operation for 230 days the nitrogen efficiency was higher than 80%as the total nitrogen loading rate(NLR)reached 3.8 kg-N/(m3·d).The biofilm on the fiber carriers showed compact structure under scanning electron microscope(SEM);a sequencing batch biofilm reactor(SBR)was set up to culture aerobiotic biofilm enriched by ammonia oxidation bacteria(AOB)on K1 carrier.Organic was added to accelerate the biofilm culture process in the startup phase.80%of ammonia in the influent was converted to nitrite after 60 days of operation and the biofilm with a thickness of 0.1-0.3 mm can be seen under SEM observation.(2)A novel nitrogen removal reactor combining PN and Anammox was developed whose geometrical parameters were determined in this study.Carriers K1 attached with aerobiotic biofilm were filled into draft-tube to form oxygen-limited in riser zone and completed PN reaction.Aeration was set on the bottom of the draft-tube to supply oxygen for PN reaction and energy for liquid circulation.Fiber carriers attached with anaerobic biofilm were fixed in the gap between the draft-tube and outer tube to form anaerobic in downer zone and Anammox reaction was carried out to promote Anammox reaction.The nitrogen removal efficiency was up to 75%on the 120th day under NLR 0.28 kg-N/(m3·d)which indicated aerobiotic bacteria and anaerobic bacteria can cooperate well in the reactor and the combined process was developed successfully.The influence of aeration rate on nitrogen removal performance was studied by experiments and the results showed that aeration rate and pattern had significant influence on nitrogen removal performance:the best nitrogen removal performance could be achieved at the aeration rate of 0.03 vvm when concentration of matrix feeding was kept constant;an interval aeration pattern was in favor of improving the performance of Anammox reaction by supplying DO for PN reaction and at the same time power for circulation.The experiments were operating under room temperature for 25 days showed that up to 30%of feeding ammonia nitrogen was removed and nitrogen removal efficiency rose obviously with an increase in temperature.(3)One-dimensional modeling approach for this study based on classic model of reaction-diffusion-convection mixing was developed and solved.Some basic law about mass transfer and microorganism growth inside biofilm was achieved.Diffusion coefficient,seepage velocity and feeding matrix concentration which have great influence on mass transfer and microorganism growth inside biofilm were investigated.The influences of the abovementioned three parameters on the concentration gradient of dissolved oxygen and microorganism growth were investigated and the results showed that diffusivity;Results show that seepage velocity and diffusivity directly affect the depth of penetration and the growth of microorganism:the higher the seepage velocity and diffusivity are,the deeper in biofilm dissolved oxygen can reach and the more significant the growth of microorganism becomes.Furthermore,feeding matrix concentration also have significant impact on oxygen concentration along the direction of the biofilm depth:the decline of DO became more notable with an increase in concentration of feeding matrix,meanwhile microorganism concentration remained insignificant.Oxygen concentration along the direction of the biofilm depth was tested by PA2000 oxygen microelectrode and experimental results agreed well with the model.(4)In order to reveal the reaction kinetics law in this work the mixing time,oxygen mass transfer and nitrogen removal performance in reactor combining PN/Anammox were investigated based on mixing mass transfer model and model of mass transfer coupled with biofilm.Aquasim and Matlab were used for numerical solutions.The results show that:it took 100 s for the liquid to reach complete mixing.The time would be shorter with an increase of liquid velocity and axial diffusivity coefficient;there was no obvious oxygen transfer between gas and liquid and no bubbles flowing into downer zone with liquid circulation judging by the similar DO concentration detected at the bottom and top area of the riser zone.Liquid circulation velocity,dissolved oxygen volume mass transfer coefficient,thickness of boundary layer,nitrogen loading rate and the biofilm area ratio of anaerobic to aerobiotic were studied by solving the model of mass transfer coupling with biofilm.The results show that the above parameters have great effect on the nitrogen removal performance.