Physico-chemical characterization of domestic wastewaters in the "Vie-Nouvelle" agglomeration, Cotonou, Benin

Abstract

Wastewaters are characterized by a high variability of flow and composition according to its origin. The objectives of this study were to evaluate the daily variation of the pollutants load from domestic wastewater in "Vie-Nouvelle" agglomeration in the city of Cotonou. In practice, samples were collected over 24 hours with one sample every hour, at the entrance of wastewater treatment plant of the agglomeration. Various parameters were measured at laboratory. Determination of methods specific to each parameter was used. The results show that these wastewaters are characterized by a great pollution variability during the diurnal period of day. The pollutant load showed the following variations : pH values ranging: 7.1 – 7.3 (7.2 ± 0.1 for average); conductivity: 555 - 953 (804 ± 112) μS/cm; turbidity:139 - 238 (192 ± 27) NTU; COD: 675 - 1,983 (1,250 ± 350) mg O₂/l; BOD5: 261 - 440 (1,257 ± 350) mg O2/l; NH4+: 18 - 34 (24 ± 5) mg/l; NTK: 36 – 78 (57 ± 12) mg/l; PO₄^3-: 6 – 18 (13 ± 4) mg/l; PT: 6 – 18 (13 ± 4) mg/l; Al: 3,998 – 8,508 (5,990 ± 1,338) μg/l; Cd: 0.5 – 6 (2 ±1 ) μg/l; Co: 1 – 3 (1 ± 1) μg/l; Cr: 19 – 110 (40 ± 25) μg/l; Cu: 113 – 615 (224 ± 135) μg/l; Fe: 3,232 – 16,842 (6,905 ± 3,846) μg/l; Mn: 114 – 452 (211 ± 90) μg/l; Ni: 18 – 85 (36 ± 17) μg/l; Pb: 52 – 243 (101 ± 50) μg/l; and Zn: 411 – 2,490 (908 ± 538) μg/l. These wastewaters do not comply with the discharge standards. They are moderately biodegradable and required denitrification and dephosphatation treatment processes before their release into aquatic ecosystems.

Keywords: Domestic wastewater, wastewater variation, daily variation, pollution load, physico-chemical parameters.