Chemical dosing optimization in primary treatment units receiving waste water from denim washing plant

Abstract

The ready-made garment (RMG) sector has become one of the largest manufacturing sectors in Bangladesh. Among various RMG industries, denim-washing factories are water intensive and utilize a wide variety of chemicals and dyes. In this study an assessment of chemical dosing optimization in the primary treatment unit for a denim garment washing plant has been done for various water quality parameters.After finding the optimum dosing for different chemicals a sensitivity analysis has been conducted to find the effective combined dosing both for removal efficiency of selected water quality parameters and economic point of view. For this, chemical dosing optimization has been done for alum, ferrous sulfate and polymer with controlled pH condition.This is because pH controlling increases the removal efficiency. A comparison between the optimum dosing and half of the optimum dosing has been conducted to find the sensitivity for removing selected water quality parameters. It was observed that, even after reduction of chemical dosing by 50%, removal efficiencies for color, turbidity, TSS, COD, and BOD5 did not seem to decrease significantly. Based on the observations from the above mentioned analysis,combined dosing of different concentartions for alum, ferrous sulfate and polymer were carried out to find the removal efficiencies. It was found that combination of chemicals provided better removal efficiencies than individual chemical dosing. Polymer and alum dosing provided better removal efficiencies for different water quality parameters. Dosing with combination of chemicals, having polymer and alum also provided better removal results. Ferrous sulfate provided least removal efficiencies for most of the water quality parameters when compared to other individual chemicals. Combinations having ferrous sulfate in them also provided relatively less removal of various water quality parameters. A cost analysis for that different combined dosing was also carried out to determine the possible effective dosings. The cost analysis will facilitate respective ETP management of denim washing factories with selection of combined dosing of chemicals, which is function of the overall design of ETP (i.e. primary, secondary and tertiary units) and willingness to pay for primary treatment stage.