BIOLOGICAL TREATMENT OF DAIRY WASTEWATER

Two wastewater streams from the dairy parlour are relevant: (1) wastewater originating from cleaning, disinfection and sanitation of milking equipment, containing water, milk and detergents; and (2) wastewater from floor washing, containing manure, urine, waste milk and dirt. These are usually flooded into wastewater settling ponds which, depending on design, are variably effective in terms of utilisation of irrigation and settled solids thereafter. They do, however, increase the potential risk of surface and ground water contamination, since dairy wastewater contains a variety of suspended and dissolved solids, nutrients, fats, sulphates, chlorides, and trace and soluble organic compounds, and is characterised by a high biological (BOD) and chemical (COD) oxygen demand. Depending on system and operation the contents are highly variable which requires standardization as some of the nutrients are useful to save on fertilizer costs and otherwise appropriately treated before being discharged into any aquatic environment. To study this further the objective of the project referenced aimed to conduct a physico-chemical analysis of wastewater produced by dairy farms and assess the feasibility of using low-cost biological wastewater treatment methods for mitigating the environmental effects thereof.

Wastewater contents were sampled from ponds of two dairy farms in KZN and analysed for all dissolved and suspended constituents. Thereafter the contents were mimicked in 2L laboratory sampling bottles: Large solid particles were removed by sedimentation, followed by filtration through filter papers with different pore sizes. The samples were then autoclaved for 15 minutes at 121^o C and stored at 4^o C before use. The (mimicked) synthetic waste water (SDWW) was compiled to contain 2.8 g/L NH₄Cl, 0.1 g/l MgSO₄.7H₂O, 0.076 g/L CaCl₂.H₂O, 2 g/L KH₂PO₄ and 4 g/L NaHCO₃, to which 2 g/L dry skim milk powder was added after autoclaving. Micro algal strains of Chlorella, Spirulina and Scenedesmus spp were then added based on previous studies showing their potential to remediate dairy wastewater.

Preliminary results showed specific growth rates of 0.103, 0.083 and 0.101 per day and doubling times of 0.148, 0.119 and 0.145 for respectively Chlorella, Spirulina and Scenedesmus. Thereafter, the algal strains were cultivated to select the best performing strains and assessed for growth and nutrient removal potential, and the best performing strains were further cultivated in actual/real dairy wastewater. The specific growth rates of the selected strains in 10% SDWW increased to 0.305, 0.132 and 0,258 per day and the doubling times to 0.440, 0.372 and 0.190 for respectively Chlorella, Spirulina and Scenedesmus.  From an initial concentration of 59.81 mg/L NH₄⁺-N, 56.78 mg/L PO₄^-2 and 269 mg/L COD, the selected strains of Chlorella, Spirulina and Scenedesmus removed respectively about 79, 58 and 67% of NH₄⁺-N; 85, 90 and 88% of PO₄^-2; and 72, 44 and 71% of COD.

Conclusion: The preliminary laboratory results showed that the selected strains may be effective in reducing nutrient and COD levels in simulated dairy wastewater and can be further investigated.