Over the past decades, the pulp and paper industry has made considerable efforts to reduce fresh water usage and minimize effluent discharges. Several effluent free BCTMP mills using traditional evaporation technology are already in operation in various parts of the world with increased load to evaporator. Paper industry initialized application of freeze concentration by using ice as a separating agent in the Louisiana-Pacific mill. Several drawbacks were discovered during their commercialization, including washing water freezing at 0degreesC, ice formation and plugging in the transportation pipe, difficulties with crystal size control, and high concentration of fiber fines. This process was proven to be successful in the lab and pilot scale, however, it has difficulties in the actual mill application. This work studied the possibility of concentration of pulp mill effluent through gas hydrates formation. Gas hydrates are solid solutions which gas/gases molecules are resided inside the crystal structure formed by water molecules. This process was designed to specifically address these problems associated with previous practice. A continuous gas hydrates formation unit that can perform crystallization, washing and separation functions is built. However, a batch reactor operated at constant volume and temperature were used extensively in this research. CO2 and i-C4H10 are selected to form gas hydrates. Specific problem such as possibility of formation, effectiveness of washing and final water purity were investigated. All of these data and experience can be very useful in the design of commercial unit. The reduction of effluent TSS and TDS via gas hydrate freezing are very effective to reach a level more than 99% removal. However, the reduction of COD removal is only about 60%. Through recrystallization gas hydrate experiments, the efficiency of COD removal can be improved to 80%. Therefore a gas hydrate modeling is developed to simulate multiple recrystallization for predicting improved effluent COD reduction further.