Investigating the effects of bacterial decay rate on dilution and diffuser design

The time for bacterial concentration to decrease by 90% (T₉₀ values) can dramatically vary from a few hours to a few days, depending on environmental conditions. However, the diffuser design of outfall projects is typically calculated based on a constant T₉₀ value. This study was designed to determine the initial dilution necessary to ensure low levels of coliform bacteria when using diffusers that are not designed to adapt to varying T₉₀ values. Dilution rates are calculated using several T₉₀ values. The results showed a strong inverse correlation between dilution rate and T₉₀ value. To maximize the dilution rate and decrease T₉₀ values, efficient diffuser design is essential. Various diffuser configurations were investigated to meet the dilution criteria. Suggested diffuser scenarios increased the near-field dilution and therefore the desired value of total dilution. The results presented may contribute to an improved understanding and selection of T₉₀ values for the management of marine outfalls.