Flow Dynamics in the Equalization and Retention
Units of a Textile Company's Effluent Treatment
Plant (ETP): Implications for Removal Efficiencies
and Biodegradability Index

Radioactive tracer (RT) residence time distribution (RTD) technology and conventional effluent quality techniques have been respectively applied to assess the performance (flow dynamics) of the effluent treatment plant (ETP) and effluent quality [Removal Efficiencies (RE) and Biodegradability Index (BI)] of a textile printing company in Ghana. The RTD investigation focused on the process flow anomalies (dead volumes, stagnant zones, channeling, and short-circuiting or by-passing of waste-water flow) in two (2) sedimentation tanks (T1 & T2) of the Equalization and Retention Unit (ERU) of the ETP. RTD established an experimental Mean Residence Time (MRT) of 60.7 mins (T1) and 122.7 mins (T2) respectively; these were higher than the design MRTs of 46.3 mins (T1) and 35.7 mins (T2), signifying the existence of dead volumes in the tanks. The fluid dynamics in T1 and T2 modelled with an International Atomic Energy Agency (IAEA) developed RTD analysis software revealed that the Perfect Mixers in Series with Exchange (PMSE) model best described the flow regime in T1. PMSE model description equations conceptualized T1 as consisting of two perfectly mixed tank systems in series with exchange having an active volume of 46%, stagnant volume of 26% with a complete dead volume of 28%. The Perfect Mixers in Parallel (PMP) model best described the flow structure in T2. Flow model parameters analysis conceptualized T2 was as consisting of four (4) compartments arranged in parallel with total active volume of 93.1% and dead volume of 6.9%. The mixing efficiency in T2 (estimated variance of 9.02469×105 ) was better than TI (estimated variance of 4.77730×105 ). Effluent quality parameters assessed generally compared favorably with the World Health Organization (WHO) and Ghana's Environmental Protection Agency (EPAGhana) recommended values, though color and turbidity were above recommended levels. The RE for effluent quality parameters were comparatively (with literature data) low except total suspended solids (TSS). Consequently, there is a need to improve influent treatment to enhance effluent quality. Calculated BI of 0.33 indicates a slowly biodegradable influent; necessitating the incorporation of a biological treatment unit into the ETP to boost biological degradation. 1