Abstract: A method to establish, maintain and control pH using carbon dioxide in aqueous dyeing processes applicable to dyeing a wide range of substrates with an aqueous dyeing solution incorporating either a water soluble or insoluble, natural or synthetic type of dye in batch or continuous processes, and at atmospheric pressure or under pressure.

Abstract: Efficient control of the pH of a flowing alkaline wastewater stream is accomplished by monitoring the pH of the flowing stream, generating a signal in response to the pH monitored, and injecting CO.sub.2 as necessary to reduce the pH. High pressure liquid CO.sub.2 is supplied to a proportional control valve which is operated in response to the signal to change the high pressure CO.sub.2 fluid to an intermediate pressure CO.sub.2 fluid in a manner so as to significantly reduce its density by creating a significant gaseous fraction. This lower density CO.sub.2 fluid is supplied to a spring-loaded injector having an orifice in communication with the flowing stream. The intermediate pressure is above the pressure at which the injector opens whereby the lower density CO.sub.2 fluid is injected into the flowing stream, allowing particularly precise control to be achieved.