A thesis s ubmitt ed in partial fulfillment of the requirements for the degree of Master of Science

Thesis (M.Sc.) - Asian Institute of Technology, 1993

Two batch and one continuous laboratory scale reactors of anaerobic-oxic activated sludge process were carried out to treat modified tapioca starch wastewater. The batch reactors #1 and #2 were operated at different Food/Microorganism (F/M) ratio ranging from 0.2 - 0.7 and 0.3 - 1.0, respectively. While the continuous reactor was operated with F/M ranging from 0.3-1.0. The batch reactors achieved around 90% COD removal efficiency. However when chemica1 pretreatment was app1ied, the COD removal efficiency increased to around 92%. The SVI values obtained from those batch studies were ranging from 47-180 ml/g which indicated good sludge settleability. Although the microscopic observations showed the existence of short filamentous microorganism in the batch reactors it did not affect the settleability of sludge. Further, the continuous reactor yielded an average COD removal efficiency of 93% which was higher than that of batch reactors. In this study very good sludge settleability was also obtained which was evidenced by the SVI values ranging from 40-120 mL /g. The microscopic observations did not show existence of any filamentous microorganism in the reactor. However, the effluent quality obtained from this study in terms of COD was around 150 mg/L, which was higher than the normally expected value of around 100 mg/L. Therefore a series of studies were carried out to reduce the COD value to lower than 100 mg/L. Powdered activated carbon dose of 1g for 1L of effluent with a contact time of 1 hr. produced a final effluent having COD value less than 100 mg/L.