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Contaminants in septic tank effluent (STE) are expected to be removed by the soil system before
discharging to the environment. However, potential contaminants such as
phosphorus (P), caffeine and artificial sweeteners do find their way to watercourses impacting aquatic eco systems. In
this study, the attenuation of STE P, caffeine and saccharin were investigated
in untreated soil and in soil with reduced microbial activity, in aqueous
solutions and in the complex matrix of STE. Time series sorption and desorption
experiments using batch equilibrium and a column experiment of STE P attenuation
were conducted. The results revealed that the soil distribution coefficients (Kd) were: P 81.57 > caffeine 22.16 > saccharin 5.98 cm3/g, suggesting greater soil affinity to P adsorption. The data revealed that 80% of saccharin
and 33% of caffeine attenuation was associated with microbial activities rather
than adsorption processes. However, a complete removal of saccharin and
caffeine did not occur during the equilibration period, suggesting their
leaching potential. The dominant mechanism of P attenuation was adsorption
(chemical and physical), yielding P retention of >73% and 35% for P in aqueous solution and in STE matrix,
respectively, for batch equilibrium. The soil in the column acted as effluent P sink retaining 125 mg P/g soil of effluent P. The attenuation of P, caffeine and saccharin in the
aqueous solution was greater than in STE, suggesting that the complex
composition of STE reduced soil adsorption ability, and that other substances
present in STE may be competing for soil binding sites. The data revealed that caffeine
and P had similarities in the interaction with soils and thus caffeine may be
considered as a STE tracer of anthropogenic source of P in receiving waters.