Abstract:
Pollution of surface water resources especially rivers from point and non-point sources is a major environmental concern. While rivers are a major source of water for reuse in aquaculture, the occurrence of pollutants in river water including nutrients, organic matter total suspended solids (TSS) etc. Therefore, to remediate, this study aimed to investigate the use of high rate algal pond (HRAP) system to treat polluted river water to standards for reuse in aquaculture.
The study employed two pilot scale HRAP designed with and without a paddle wheel, constructed and operated at the Faculty of Natural Resources and Environmental Sciences, Namasagali campus, Busitema University. Physico-chemical water quality parameters and nutrients including: Temperature, pH, dissolved oxygen (DO), electrical conductivity (EC), total dissolved solids (TDS), nutrients (i.e. N & P species) and TSS were monitored for six weeks from 25/01/2023 to 02/03/2023 insitu and in the laboratory following standard methods of water and wastewater treatment by APHA (1998).
Results revealed that the paddle when driven HRAP 1 performed better than the HRAP 2 that had no paddle wheel both in terms of effluent quality and pollutant removal efficiency. The effluent quality from HRAP 1 and 2 were: Temperature: 25.9±0.6 °C & 26.0±0.7 °C, pH:9.31±0.30 & 10.02±0.09, DO: 3.19±0.35 mg/L& 3.29±0.41 mg/L, EC: 136.06±8.76 μS/cm & 141.89±9.58 μS/cm, TDS: 68.9±6.5 mg/L & 71.4±3.9 mg/L, NO2--N: 0.198±0.03 mg/L & 0.202±0.02 mg/L, NO3--N: 0.31±0.14 mg/L & 0.27±0.14 mg/L, NH4 --N: 0.16±0.01 mg/L & 0.16±0.03 mg/L, PO43--P: 9.26±1.83 mg/L & 9.68±2.45 mg/L and TSS: 22.22±6.20 mg/L & 10±2.98 mg/L respectively. However, the pollutant removal efficiencies of HRAP 1 and 2 for different nutrients were: NO2--N: 3.41 & 2.88%, NO3--N: 10.34 & 10.60%, PO43--P: 18.49 & 19.67%, NH4--N: 20.00 & 11.11% and TSS: -36.99 & -5.82% respectively. The higher effluent quality produced from HRAP 1 than 2 could mainly be attributed to the presence of a paddle wheel that constantly mixes and exposes algae to sunlight for optimal algal production and consequently better removal of nutrients through uptake and assimilation.
In conclusion, the paddle wheel driven HRAP 1 revealed a better effluent water quality and higher removal efficiency for all parameters than HRAP 2 (i.e. without a paddle wheel), generating an effluent within the general standards suitable for reuse in aquaculture for the commonly cultured fish species: Oreochromis niloticus and Clarias gariepinus.
