Assessing Domestic Wastewater Effluent with a Battery of Bioassays after Treatment with a Specific Consortium of Microalgae and Different Flocculation Methods

Abstract

Wastewater stabilization ponds (WSP) are a common method to treat wastewater before release. The purpose of the current study was to find the most inexpensive coagulation and flocculation reagents to use during harvesting of WSP algae from effluent so that water without algae can be generated for irrigation purposes. In the study, the toxicity of effluent from the algae-based WSP system after treatment with alum and chitosan was investigated. The chemical composition of the resultant supernatants was tested in conjunction with a battery of bioassays (Daphnia magna, Allium cepa, Lactuca sativa and Triticum aestivum). The bioindicator organisms were exposed to varying concentrations of the supernatant resulting from harvesting the algae biomass with chitosan (0.368 g/L chitosan) and aluminium sulphate (9.96 g/L powder). Alum was lethal to D. magna, while chitosan did not seem to affect the Daphnia test specimens. Undiluted chitosan supernatant and 100% alum resulted in minimal inhibition of the rate of T. aestivum seed germination, while both resulted in significant inhibition of the L. sativa seed germination rates. Similarly, minimal effects were found for the root growth in T. aestivum, with significant root growth inhibition observed in L. sativa and A. cepa. In A. cepa, clear differences were observed in the inhibition of root growth when the two reagents were compared. The alum and chitosan supernatants had a total carbon, iron and sulphate concentration of 327.78 mg/L, 0.12 mg/L and 68.44 mg/L and 182.04 mg/L, 0.16 mg/L and 166.80 mg/L, respectively. Based on the test specimens’ response, chitosan as flocculation method was less toxic when compared with alum coagulation, even though both algae harvesting methods changed the chemical composition of the algae-treated water.