CFD modeling of dynamic airflow and particle transmission in an aircraft lavatory

Lavatories are frequently used facilities,especially on long-haul flights.Flushing a vacuum toilet in a lavatory can induce strong airflow,produce aerosols in the toilet bowl,and resuspend deposited particles from the floor.However,the exact particle transport routes and the fates of particle after toilet flushing are unclear so far.This investigation used computational fluid dynamics(CFD)to model the transient airflow and pollutant transport after a toilet flushing process in a lavatory of a commercial aircraft.The time-varying pressure profile measured in a laboratory was assigned to the drainage valve as boundary conditions.The aerosols generated inside the toilet bowl during flushing and the particles resuspended from the lavatory floor were used as particle sources.Lagrangian tracking of airborne particles in the lavatory was conducted.In addition,ammonia gas was used to examine odor perception.The multi-physics software program COMSOL 5.4 was employed for numerical solution after being validated.The results revealed that more than 70%of the generated particles in the toilet bowl are drained into sewage.A few particles may leak out of the toilet bowl and remain suspended in the air for more than five minutes when the toilet lid is open during flushing.Flushing the toilet with a closed lid can effectively reduce the particle leakage and the spread of odor gas,but it leads to greater deposition of particles on both the lid and seat.There is a slight inhalation exposure risk in the initial three minutes after flushing with a closed lid.

Characteristic of COD removal and sludge settleability in biological treatment of hypersaline wastewater

In order to investigate the feasibility of biological treatment of bypersaline wastewater produced from toilet flushing with seawater at low temperature, pilot-scale studies were established with plug-flow activated sludge process at low temperature （5-9℃） based on bench-scale experiments. The critical salinity concentration of 30 g/L, which resulted from the cooperation results of the non-halophilic bacteria and the halophilic bacteria, was drawn in bench-scale experiment. Pilot-scale studies showed that high COD removal efficiency, higher than 85 %, was obtained at low temperature when 30 percent seawater [ seawater/（seawater ＋ sewage） ] was introduced. The salinity improved the settleability of activated sludge, and average SV dropped down from 38% to 22. 5% after adding seawater. Sludge bulking could be forborne effectively because filamentous bacteria couldn＇t subsist under high salinity concentration.

Nitrification-denitrification via nitrite pathway in biological treatment of hypersaline wastewater

Pilot-scale studies on biological treatment of hypersaline wastewater at low temperature were conducted and results showed that seawater salinity had a strong negative effect on notrouomonas and nitrobaeter growth, but much more on the nitrobaeter. The nitrification action was mainly accomplished by nitrosomonas. Bench-scale experiments using two SBRs were carried out for further investigation under different conditions of salinities, ammonia loadings and temperatures. Biological nitrogen removal via nitrite pathway from wastewater containing 30 percent seawater was achieved, but the ammonia removal efficiency was strongly related not only to the influent ammonia loading at different salinities but also to temperatures. When the ratio of seawater to wastewater was 30 percent, and the ammonia loading was below the critical value of 0. 15 kgNH4 ^＋ -N/（ kgMLSS · d） , the ammonia removal efficiency via nitrite pathway was above 90 %. The critical level of ammonia loading was 0. 15, 0. 08 and 0. 03 kgNH4 ^＋ -N/（ kgMLSS · d） respectively at different temperatures of 30℃, 25℃ and 20℃ when the influent ammonia concentration was 60 - 80 mg/L and pH was 7.5 - 8.0.