Biodegradation kinetics of the soluble slowly biodegradable substrate in polyamide carpet finishing wastewater

Background: Carpet manufacturing and finishing with purely synthetic fibers has received relatively little attention, compared to other textile processing types. This study evaluates the biodegradation kinetics of organic compounds generated from polyamide-based carpet manufacturing. Results: Experiments were conducted on pre-washing and dyeing/softening wastewater effluents. Model evaluation of oxygen uptake rate profiles with dual hydrolysis kinetics revealed that the soluble slowly hydrolysable chemical oxygen demand (COD) was the major fraction, constituting nearly 97% of the biodegradable COD and 78% of total COD content. Degradation of the slowly hydrolysable COD fraction was characterized with a rate coefficient of 0.72 day^-1, a significant rate limiting step for substrate utilization. Model simulation of system performance indicated that an unusually long hydraulic retention time was required for an activated sludge system to reduce the effluent COD concentration. Conclusion: Compared to domestic wastewater, two additional hydrolysable COD fractions with different degradation kinetics were characterized. The dyeing and softening step had the highest slowly biodegradable organic matter content, with the lowest degradation rate. Simulation results showed that soluble slowly hydrolysable COD degradation did not cause any problem in terms of effluent quality. With the system operated under reduced solids retention time, the effluent COD quality was significantly influenced by the slow hydrolysis rate of soluble hydrolysable matter.