Wastewater treatment effectiveness of the decommissioned and current dissolved air flotation (DAF) plant in a fish factory, Walvis bay, Namibia

Abstract

The fish processing industries are of great concern worldwide as they produce significant volumes of wastewater which is produced during various stages of the fish canning process such as transportation, brine procedure, removal of unwanted fish parts (head and guts), cooking, canning, sterilization and cleaning of the equipment. This wastewater may contain physicochemical and microbiological pollutants as a result of the various fish canning stages that release contaminants which may not be easily removed before this wastewater is discharged into aquatic ecosystems. Pollution has been reported from fish factories that release wastewater in the Walvis Bay marine area in Namibia. This wastewater has often not met the mandatory water quality requirements, therefore, the fish factory had to determine the effectiveness of the decommissioned (old) and current (new) DAF plant in treating wastewater from their fishing operations. Wastewater from fish processing industries is highly contentious and thus poses treatment challenges due to the presence of organic matter, FOG and salt. Even though, there is a potential to reduce waste and solids in the wastewater, there is a lack of decision-making and monitoring methods to address problems of water quality caused by fish processing. There is also a big gap between the literature and wastewater management implementation methods due to the different fish processing procedures at various fish factories. Therefore, this study aimed at comparing the concentrations of parameters in the wastewater discharged from the old and new DAF plant at the fish factory. The study determined the relationship between the measured parameters at the different discharge sites and at the DAF plant as well as the differences in the wastewater discharged during different seasons. Wastewater samples were collected from three sites (old and new) namely, the DAF plant, Backwash (BW) and the Cooling Tower (CT) during both autumn and winter seasons and these were analysed using physicochemical and microbiological parameters. The results revealed that the mean concentration of most parameters was higher in the wastewater at the old sites when compared to the new sites. The new plant was found to be efficient in the removal of pollutants from the wastewater before it is discharged into the surrounding environment. Despite the better performance of the new plant, parameters such as FOG (0.3 ± 0.1 mg/l), (0.1 ± 0.03 mg/l), (0.2 ± 0.1 mg/l);DO (1 ± 0.3 mg/l),(13 ± 10 mg/l), (2 ± 0.3 mg/l), N (41 ± 15 mg/l), (11 ± 4 mg/l),(0.9 ± 0.1 mg/l), P (8 ± 2 mg/l), (4 ± 0.8 mg/l),(0.5 ± 0.1 mg/l) and NO3 (0.9 ± 0.5 mg/l),(0.5 ± 0.0 mg/l), (3 ± 3 mg/l) at the new DAF, new BW and new CT, respectively, were not significantly different (P > 0.05) from those of the old DAF plant at all discharging sites. Therefore, even though the new plant seems to be working better than the old plant, the wastewater concentration being discharged into the ocean was still not acceptable. The comparison of parameters analysed between autumn and winter indicated that there were no significant differences (P > 0.05) of parameters in the wastewater between these seasons. The PCA multivariate tool identified a strong relationship between TSS, BOD, COD, FOG, N, P, NH3 and FC at the old site. The relationship of parameters may signify the presence of solids, organic matter, blood, proteins and oil as well as nutrients and bacteria within the discharged wastewater. The DO levels in the discharged wastewater from the fish factory was low (> 2.0 mg/l) (below the WHO acceptable limits), implying that the final wastewater from the factory may contribute towards the increased seasonal events of local algal blooms production as a result of increased nutrients which favour low oxygen conditions for the bloom biomass decays in autumn. It is therefore recommended that the fish factory segregates waste by removing solids from wastewater to improve the quality of the wastewater before the treatment. Additionally, the application of a jar test to determine the best dosage as well as the type of chemicals to be used has proven to be successful in fish canning. The use of alternative coagulants to assist the pollutant load removal efficiency of each parameter from the wastewater will also be effective and it is important to ensure that the DAF plant operates at the correct capacity pressure to carry out its function properly. The aerobic biological treatment method is an alternative method of treating fish canning wastewater that should be considered while more research should be conducted to determine its success in removing pollutants from wastewater.