Thesis (Ph.D.) - Asian Institute of Technology, 1993

A doctoral dissertation submitted in partial fulfillment of the requirements for the degree of Doctoral of Engineering.

It has been established that biological reactions occur in sewer systems, and decompose organic matter and other constituents in wastewater. Thus, it appears worthwhile to utilize the self-purifying capacity of sewer systems. This study deals with recirculation of wastewater so that a portion of the conveyance conduit works as a circulating biological reactor (referred to as Sewage Circulating Reactor). Experiments were performed with a pilot scale unit made of a 87 m long galvanized iron channel (8 cm wide) in which wastewater was recirculated back to the starting point. Surface reaeration was the only source of oxygen. The unit was first fed with synthetic wastewater at Chemical Oxygen Demand (COD) and Total Nitrogen (TN) loading rates of 16.81 - 62.02 gCOD/m2/d and 1.8 - 7.84 gTN/m2/d respectively. An optimal TN removal of 0.77 gTN/m2/d was obtained at a TN loading rate of 3.78 gTN/m2 /d. The corresponding COD loading and removal rates were 27.45 and 22.0 gCOD/m2/d respectively. The biofilm areal density was higher and more fluctuating in the beginning of the channel while it was lower and more uniform towards the end. Later campus wastewater was fed to the unit at loading rates of 15.73 - 19.06 gCOD/m2/d and 4.66 - 5.44 gTN/m2/d. In spite of zero dissolved oxygen (DO) in the influent, the system showed COD removal rates close to 9.5 gCOD/m2/d (40 - 60%), and TN removal rates about 1.6 gTN/m2/d (23 - 26%). The biofilm in the experiments with campus wastewater was thicker than in the experiments with synthetic wastewater. The ratio of total volatile solids (TVS) to total solids (TS) in experiments with campus wastewater was approximately half of that for experiments with synthetic wastewater. A distinct decrease in the DO level in the system was observed with increasing organic and TN loading rates in experiments with synthetic and campus wastewater. This resulted in a decrease in nitrification rates at higher loadings. Thus, the overall system performance at high loadings was limited by hampered nitrification due to lack of oxygen.