When colloidal particles bear a same sign electric charge, they mutually repel each other and cannot aggregate together.
To facilitate the filtration, the colloidal suspension must be first flocculated by adding a concentrated solution of salt to the system.
More correctly speaking, the electrical double layer existing at the surface of the particles is compressed by the added electrolyte and collapses at high ionic strength.
The electrical repulsion no longer hinders the aggregation of particles and they can then coalesce to form a flocculent precipitate that is easy to filter.
[2] The efficacy of adsorbates or suspension modification in imparting interparticle electrostatic repulsion is most commonly studied in terms of zeta potential.