Chemical components of bleached eucalypt kraft pulp effluent COD and treatment removal efficiency during normal mill operation and maintenance shutdowns

摘要

In order to meet increasingly strict Brazilian COD emissions limits, mills must understand thencomponents that contribute to effluent COD, how these vary between normal mill operationnand maintenance shutdowns, and how this variation affects treatment efficiency. To this end,nprimary and secondary effluents from a Brazilian bleached eucalypt kraft pulp mill activatednsludge system were analyzed for COD, lignin, extractives, carbohydrates and AOX over a sixthnmonth period that included two general maintenance shutdowns and four months of normalnoperation. Primary effluent presented significantly different compositions during periods ofnnormal operation and mill shutdowns. During normal operation, the main components of effluentnCOD (909 mg/l average) were carbohydrates, followed by lignin. However, the lignin fraction wasnthe main component of secondary effluent COD during both normal operation and millnshutdowns. Higher removal efficiencies for COD carbohydrates and AOX were observed duringnnormal operation compared to shutdowns, while no difference in removal efficiencies of ligninnand extractives was observed. Carbohydrate removal efficiency was significantly lower in onenof the parallel treatment lines. The different removal efficiencies reflect not only variationsnin effluent composition, but possibly differences in system operational control which shouldnbe explored in greater detail