Experimental Study On Removal Of Nitrogen By Symbiotic System Of Oedogonium-Brevicingulatum And Activated Sludge

Excessive discharges of nitrogen-containing industrial wastewater,agricultural wastewater,and domestic wastewater into waterbodies have caused a series of environmental problems,e.g.eutrophication,which seriously threaten aquatic ecological balance and drinking water safety.In view of this,the Ministry of Environmental Protection has raised the municipal sewage discharge standard(GB18918-2002),and implemented level 1 A standard for ammonium nitrogen(NH4+-N,8 mg/L)and total nitrogen(TN,15 mg/L).However,it was still a challenge for the municipal wastewater treatment plants(WWTPs)to deal with the high concentrations of nitrogen after conventional secondary treatment.Hence,it is urgent to develop an advanced wastewater treatment method aiming at denitrification.Algae-bacteria symbiotic system is an environmentally friendly technology due to its advantages of high nitrogen removal efficiency,low energy consumption and resource utilization.This paper selected Oedogonium brevicingulatum,the dominant filamentous alga in the secondary settlementation tank of a WWTP,as the source of algae.The activated sludge took from the aerobic tank of the WWTP was chose as the source of bacteria.Based on establishing a high efficient algae-bacteria symbiotic system,the effects of water quality conditions(pH,aeration,heavy metal Cd)on the nitrogen removal efficiency and nitrogen transformation were investigated.Meanwhile,the variations of biophases in the symbiotic system were observed.Furthermore,the effects of this symbiotic system on nitrogen removal efficiency in actual sewage was evaluated.The following results are obtained in this paper:(1)With the initial algae ratio(RAS)of 1:1 to 5:1,the symbiotic system can efficiently remove NH4+-N and TN in simulated sewage with initial RAS of 3:1.On the 5th day,the removal efficiency of NH4+-N and TN reached 70.90%and 69.20%,respectively.With the increase of RAS,NH4+-N was apt to transform into NO2+-N and NO3=-N in the simulated effluents.The maximum net conversion rate(NCT)of NO2=-N was 0.0013 mg/h(1st day),and the maximum NCT of NO3--N was 0.035 mg/h(1st day)in the symbiotic system with RAS of 3:1.On the 5th day,the nitrogen removal by biological assimilation was accounted for 77.68%.With the initial pH value of 6-9,the system showed the optimum nitrogen removal efficiency in pH 9.0.Due to the common effects of algal assimilation and chemical volatilization,the removal efficiency of NH4+-N and TN achieved 70.90%and 69.20%,respectively.The maximum NCT of NO2--N in simulated wastewater was 0.003 mg/h(1st day),and the maximum NCT of NO3--N was 0.038mg/h(2nd day).On the 5th day,the nitrogen removal by biological assimilation was accounted for 64.72%.Aeration was beneficial to improve the nitrogen removal efficiency of the symbiotic system in simulated effluents.With the aeration rate of 0.2 L/min,the system showed the optimum removal efficiency of nitrogen.The removal efficiency of NH4+-N and TN was 79.50%and 73.40%,respectively.When the aeration rate was 0.2 L/min,the maximum NCT of NO2--N was 0.0027 mg/L(1 st day)in simulated wastewater,and the maximum NCT of NO3--N was 0.018 mg/L(2nd day).On the 5th day,the nitrogen removal by biological assimilation was accounted for 82.51%.The presence of Cd2+ obviously inhibited the nitrogen removal efficiency in the symbiotic system.When the concentration of Cd2+ was 10 mg/L,the removal efficiency ofNH4+-N and TN was 36.06%and 32.90%,which was inhibited by 46.41%and 50.00%of the control group(free Cd2+),respectively.(2)In the process of treating simulated effluent by the symbiotic system,the variations of biophases in the system were related to the water quality condition(pH and Cd2+ concentration).When the pH of sewage was 7.1,Oedogonium brevicingulatum and activated sludge was well-symbiosis.Activated sludge microbial species was dominated in ciliate.Oedogonium brevicingulatum was arranged with branching filaments of single cells.The algal cells were green and prosperous.When the pH declined to 6.0,the chlorophyll contents of the algal cells decreased;the algal cells began to yellow and then decayed.Additionally,the activated sludge was loose with the disappearing of protozoan,and simultaneously appeared a large number of filaments.In the presence of Cd2+(10 mg/L),the biofilm of the algal-bacterial symbiotic system was damaged,while the algal surface was wrinkled and damaged.(3)Preliminary treatment and evaluation of the influent of raw water and secondary settlementation tank of a certain WWTP in Xiangtan City by the algal-bacterial symbiotic system were investigated.After 5 days treatment,the removal efficiency of NH4+-N,TN and COD in raw water was 91.90%,85.62%and 90.71%,respectively,while it was 64.10%and 60.00 and 49.42%in the secondary sedimentation tank water,respectively.In the process of sewage treatment,the pH was kept in 6-9,and the system was in aerobic state.The effluent NH4+-N,TN,COD,and pH all conformed to the level 1 A emission standard for municipal wastewater(GB 18918-2002).