The Performance Of Nutrients Removal From Seawater Toilet Flushing Sewage By Spirulina Platensis

Through rapid urbanization and increasing pollution,water scarcity and has become leading issues.However,in most cities toilets are flushed with high quality water which has been reported to account for 20 to 35%of typical household freshwater demand.To tackle these growing global issues,more economic and environmentally friendly alternative water should be taken in to consideration.In coastal cities seawater with minimal treatment can be used for toilet flushing,which could minimizing the health risk associated with cross-connections since it can be easily detected from the taste.Currently,it is practiced on a large scale only in Hong Kong,which has applied a dual water supply system providing freshwater for potable uses and seawater for toilet flushing since 1950s.And seawater flushing has covered 80%of the population,and fulfills about 20%of the city’s total water demand till now.Although the use of seawater for toilet flushing has relived water scarcity stress,the salts originating from the saline water will eventually end up in the wastewater and reach the wastewater treatment plants,where they may affect the biological wastewater treatment process.The saline sewage resulting from seawater flushing is mixed with wastewater from freshwater uses,treated with conventional wastewater treatment methods,then released to the sea.The mixed sewage has a salinity that is about a fifth the salinity of raw seawater.Besides,the saline sewage appears to have sacrificed some major potential water reuse options such as irrigation.Hong Kong International Airport has applied Triple Water System for seawater and grey water reuse integration to maximize alternative water.And as an economic microalgae,Spirulina platensis can grow under high salinity(up to 70g/L NaCl)which made it protential in saline wastewater treatment.To achieve nutrient removal and biomass harvest simultaneously,in this study we adopted the case of Hong Kong International Airport,applying Spirulina platensis in saline wastewater treatment,which was combined of black water and grey water with blending ratios of 1:0,7:3,5:5 and 3:7.We clarified the possibility of applying Spirulina platensis in seawater toilet flushing sewage treatment through hierarchical cluster analysis via SPSS.The correlation analysis showed that the main factor affacting nutrient removal and the growth of S.platensis was salinity.It proved that microalgae could grow in all tests and harvested biomass and growth rate declined as the proportion of black water increased.And removal efficiencies of nitrogen and phosphorus had similar tendencies with algal growth,organic carbon reduced contrarily.We figured out that S.platensis in the test 7:3 had the best performance in nutrient removal indicating it could remove more nutrient(especially phosphorus and organic carbon)under higher salinity compared to Hong Kong who had a large scale application of seawater toilet flushing since 1950s.We used kinetic model and FTIR to illuminate nutrient removal performance in wastewater and metabolic mechanism under salt stress.It demonstrated that when exposed to higher salinity,photosynthesis was inhibited by detaching phycobillin from PS I with utilizing fewer ammonium for protein synthesis,but respiration was enhanced with reducing more organic carbon and producing more osmotica--carbohydrate in biomass and even in EPS.