In order to investigate the feasibility of biological treatment of hypersaline 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 degrees
C) 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 experiments. Pilot-scale studies showed
that high COD removal efficiency, higher than 80%, was obtained at low temperature
when 30 percent seawater was introduced. The salinity improved the settleability of
activated sludge, and average sludge value dropped down from 38% to 22.5% after adding
seawater. Seawater salinity had a strong negative effect on notronomonas and nitrobacter
growth, but much more on the nitrobacter. 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 temperature.
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 the different temperature 30 degrees
C, 25 degrees C and 20 degrees C when the influent ammonia concentration was 60-80
mg/L and pH was 7.5-8.0.