Abstract: A sewage treatment plant for use aboard ship comprises a vacuum assisted fresh water flush system delivering sewage to a collection tank. Sewage is collected from the collection tank in batches for treatment. The sewage first enters a settlement tank and is then transferred to an aeration tank. After aeration, treated sewage is then transferred out in batches. The batch processing overcomes the difficulties of operating a continuous flow arrangement on a moving platform where liquid can move back and forth, which presents difficulties in controlling the level of biological activity. A controlled amount of sewage transferred to the aeration tank is diluted with grey water. A sludge storage tank is provided and suspended solids probes are provided in the aeration and settlement tanks such that sludge is automatically withdrawn from these tanks to the storage tank to maintain the level of suspended solids within predetermined limits.