for slickwater fluids the use of sweeps is common. sweeps are temporary reductions in the proppant concentration, which help ensure that the well is not overwhelmed with proppant. as the fracturing process proceeds, viscosity-reducing agents such as oxidizers and enzyme breakers are sometimes added to the fracturing fluid to deactivate the gelling agents and encourage flowback. such oxidizers react with and break down the gel, reducing the fluid's viscosity and ensuring that no proppant is pulled from the formation. an enzyme acts as a catalyst for breaking down the gel. sometimes ph modifiers are used to break down the crosslink at the end of a hydraulic fracturing job, since many require a ph buffer system to stay viscous. at the end of the job, the well is commonly flushed with water under pressure (sometimes blended with a friction reducing chemical.) some (but not all) injected fluid is recovered. this fluid is managed by several methods, including underground injection control, treatment, discharge, recycling, and temporary storage in pits or containers. new technology is continually developing to better handle waste water and improve re-usability.