It is an object of the invention to provide an effective technique for
detecting the quality of water with high accuracy. A representative water
treatment apparatus includes an aerobic treatment region that treats
water aerobically, a downstream region in which the water aerobically
treated in the aerobic treatment region flows, a water quality sensor
that is submerged in the downstream region and detects the water quality
and a sensor washing arrangement that washes the water quality sensor by
supplying the water of the downstream region to the water quality sensor
at a flow rate higher than water flowing around the water quality sensor.
As a result, sludge generated due to aerobic treatment can be prevented
from being deposited on the water quality sensor and/or deposited sludge
can be removed, so that the accuracy of water quality detection of the
water quality sensor can be enhanced.