Study On The Wastewater Treatment Of Recirculating Aquaculture Systems By PCL-SND System

High concentrations of ammonia derived from fish droppings and bait and nitrite(generated by the oxidation of ammonia) is toxic on fishes in recirculating aquaculture systems(RAS). Nitrates also has toxic effects on fish when accumulated to a certain concentration. Therefore, the removal of nitrogen compounds in RAS is the core of water treatment, The most commonly used method is the biological denitrification in RAS. Simultaneous Nitrification and Denitrification(SND) is a new biological denitrification technology,with a high nitrogen removal efficiency, short reaction time, cost, etc., has been more and more attention.In recent years, SND mainly focused on the treatment of domestic sewage and municipal wastewater, and achieved a good effect, few research on aquaculture wastewater treatment. This thesis is to use Polycaprolactone(PCL) as a carbon source and biofilm carrier to treatment aquaculture wastewater with low carbon and nitrogen ratio in Air-lift reactor by SND; determine the optimum HRT; further optimization the system to achieve efficient treatment of aquaculture wastewater, analyzed microbial community structure in the special period to understand the mechanism of biological nitrogen removal, provide a theoretical basis for future water treatment process optimization.1、Startup process of PCL-SND systemWith 10% fill(v/v) of PCL carrier, HRT of 24 h, the influent ammonia(NH4+-N) concentration of 10mg/L, nitrate(NO3--N) concentration of 50 mg/L, the PCL-SND system achieved a stable state at 45 d. At this time the removal efficiency of NH4+-N and total nitrogen(TN) was 76.55±0.98% and 56.85±2.21%, respectively. NO3--NE concentration was 21.36±1.60mg/L, there is no obvious accumulation of NO2--N. Biomass grew rapidly early, slower growth and gradually stabilized late. p H changed little during the trial, the average of p HE was 7.81 ± 0.22. Alkalinity relatively lower early, later increased due to denitrification rates increased. after reaching a relatively stable state. Scanning electron microscopy showed that with the reaction proceeds, PCL losses more serious. PCL can be used as biofilm carrier and carbon source treatment aquaculture wastewater with low C/N by SND.2、Effect of nitrogen removal by PCL-SND systemThe TN removal efficiency decreased with the decrease of HRT in a certain range(HRT<24h), and the effluent nitrate concentration increased with the decrease of HRT. However when HRT > 8h, the removal rate of NH4+-N was basically stable(85% ~ 89%). The best nitrogen removal was achieved at HRT of 24 h. The average removal efficiency of TN and NH4+-N was 68.56±1.64% and 87.75±2.78%, respectively, and the NO3--N concentration of effluent was 15.72±1.46mg/L. The p H and alkalinity decreased with the decrease of HRT. However, the biomass increased with HRT decreased.Community structure of microbial biofilms under different HRT conditions by high-throughput sequencing technology. The results showed that HRT has significant impact on microbial community structure of biofilm. 6 groups of samples, the Proteobacteria has an absolute advantage, relative abundance was 80.94%~96.41%, increasedwith the HRT increases. The relative abundance of Bacteroides gradually decreases with the HRT increases. Chloroflexi and GN02 had little difference. Betaproteobacteria was major microflora at the order level. Diaphorobacter of Betaproteobacteria was the most abundant genus in biofilms, it’s abundance increases with the HRT increases, up to 59.11% when HRT was 30 h.3、Optimization of PCL-SND system and the actual treatment efficiencyThe best effects for nitrogen removal was when PCL’s filling ratio is 15%. the removal efficiency of TN and NH4+-N was 94.38% and 76.51±4.35%, respectively. NO3--NE concentration was 0.6mg/L, there is no obvious accumulation of NO2--N. No significant change in effluent p H with the filling ratio increased, total alkalinity increases with increasing filling ratio. The effect of aquaculture wastewater treatment, The average removal rate of NH4+-N was 75.39±0.065%, TN removal efficiency was 85.90±0.014%, NO3--NE concentrations stabilized at 3mg/L, there is no obvious accumulation of NO2--N, the average of p H was 7.40±0.075, total alkalinity was 271.39±21.55 mg/L Ca CO3. PCL-SND system has good nitrogen removal ability.Community structure of microbial in biofilms under different filling ratio of PCL. Community structure quite different in the three samples. Proteobacteria has an absolute advantage(85.40%~87.67%) in the 3 groups. Diaphorobacter of Betaproteobacteria was the most abundant genus in biofilms, up to 65.46% when the filling ratio is 12.5%. Filling ratio is 15%, Paracoccus and Hydrogenophaga became the dominant genus.Changes in the microbial community structure when changing the influent water. High diversity of microbial communities in artificial wastewater biofilms. Difference was significant in microbial community structure between artificial wastewater and aquaculture wastewater. Relative abundance >1% is only 2 microflora in aquaculture wastewater, Proteobacteria(94.80%) and Bacteroidetes(4.41%),respectively. The relative abundance of Diaphorobacter up to 70.16%, Thermomonas and Alicycliphilus become dominant genus in aquaculture wastewater.