To control hydrostatic backpressure of disposal wells connected to a disposal plant surface network, density of water to be injected from a water disposal plant system into disposal wells formed in a subterranean zone, a density of a hydrostatic backpressure-modifying additive to modify a density of the water, a target total injection flow rate of the water, and a vertical depth of a portion of the subterranean zone through which the water is to be injected are identified. Injected water flow rate upstream of an injection point into the multiple disposal wells, wellhead injection pressure needed to achieve the target total injection flow rate and a total injection rate are periodically received. Based on these parameters, dosage rate of the additive to maintain the target total injection flow rate is periodically determined. An additive quantity is injected into the water and periodically modified based on the periodically determined dosage rate.