Investigation of hydrodynamic non-idealities in a pilot-scale pond bioreactor

Abstract

Pond-type reactors are widely used in biological wastewater treatment in view of their simplicity and low cost of construction. The knowledge of an approximate hydrodynamic behavior can contribute to the rational design of such operation. Thus, the aim of this work is to investigate, in a pilot scale pond bioreactor, the non-ideality through residence time distribution (RTD) technique and validation of a model for the homogeneous phase material balance. Experimental runs were carried out using the pulse injection technique of methylene blue tracer at constant water flow rate. The pilot-scale reactor constructed with a typical geometry design of those employed in animal waste treatment in southern Brazil. Residence time of 3, 6, and 9 days and a height/width ratio geometry parameter (H/D) of 0.25 and 0.18 were adopted. For each run normalized RTD functions and their moments (average residence time, variance and asymmetry) were evaluated. Variance analysis shows equivalent hydrodynamic behavior in the parameter range employed. To describe the non-idealities, we theorized several dimensionless RTD function models using a series of CSTRs (classified as conventional, bypass and recycle).The models were adjusted to the experimental data by non-linear regression and evaluated by the successive calculation method using the moments of RTD. The best model found is the representation of the pond reactor as two CSTRs in series with different volumes (first reactor with 91.5% of useful volume) and presence of dead zones (useful volume 84.9% of real volume).