WATER TREATMENT SYSTEM

PROBLEM TO BE SOLVED: To provide a water treatment system that prevents backflow of raw water to a circulation water drawing pipe and that enhances adaptability to a variation of raw water flow rate, especially a rapid increase of the raw water flow rate even when installing the water treatment system in a place where infrastructure is not built such as in a developing country.SOLUTION: A water treatment system includes: a reactor by which raw water to be treated is subjected to anaerobic treatment by an anaerobic microorganism group; and a separator for separating treated water obtained in the reactor from biogas. An adjustment tank mixes the raw water with circulation water which is part of the treated water treated in the reactor to supply it as adjustment water to the reactor. Here, the adjustment tank is partitioned by a partition wall and has a raw water storage part in which the adjustment water is stored and a circulation water storage part in which the circulation water is stored. The circulation water is configured to unilaterally flow into the raw water storage part by overflowing over the partition wall.