Abstract
In this study, the relationship between flowing up of settled sludge and water velocity in a primary treatment tank of on-site wastewater systems (johkasou) was examined. The water velocity was measured by an acoustic Doppler velocimeter (ADV). Settled sludge flowed up easily when the inflow rate and the height of the settled sludge increased. At the same time, average-velocity on the surface of the sludge increased, indicating that water velocity affected the up flow of the sludge. On the other hand, the more settled sludge was compressed, the less the sludge flowed up. In the 10,000 mg/L settled sludge concentration, the sludge flowed up massively when average-velocity on the surface of the settled sludge was greater than 1.5 cm/sec. Moreover, the peaks of time-series velocity data by moving-average of 2.5 - 5.0 seconds corresponded approximately to flowing up. The faster peak-velocity, the more massively the sludge flowed up. Decrease of both average- and peak-velocities is necessary to decrease flowing up of the settled sludge.
