<p>A wastewater treatment method in an embodiment includes: mixing primary aggregates made of magnetic substances and a dispersion medium to make a suspension; and passing the suspension through a filter to allow the primary aggregates to remain on the filter and to form a secondary aggregate made of an aggregate of the primary aggregates. Further, the wastewater treatment method in the embodiment includes; passing wastewater through the secondary aggregate to remove suspended solids and oil in the wastewater; dispersing the secondary aggregate in a dispersion medium to decompose the secondary aggregate into the primary aggregates and to wash the primary aggregates; and collecting the primary aggregates with magnetic separation.</p>
