A novel technique utilizing the gelation reaction of alginate has been proposed for the separation of solid from liquid in difficult-to-filter colloidal suspensions. Colloidal suspensions are mixed with a sodium alginate solution, and this mixture is added to a calcium chloride solution, resulting in the entrapment of colloidal particles by the calcium alginate gel. Gel suspensions are then drained gravitationally, followed by mechanical expression of gel particles. This technique is especially effective in the treatment of colloidal muddy water of high solid concentration, which is often produced as wastes of certain construction processes. Fundamental aspects of this process are investigated using sodium bentonite/PMMA as experimental materials. The alginatebentonite/PMMA mixture is added dropwise to the calcium solution. When an alginate concentration in the droplets is above 1000 ppm, submicron particles are perfectly entrapped in calcium-alginate gels. Decreasing the droplet size of the mixture expedites gelation since the diffusion of calcium ions into droplets determines the rate of gelation reactions. Reducing the alginate content expedites expression of the bentonite/PMMA gel.