I don't know the details, but I assume that this is industrial wastewater, not human waste. Any waste that is discharged directly or indirectly into the navigable waters of the State of Texas is subject to regulatory approval and oversight by TCEQ. TCEQ has a model of the current and desired conditions of the segment of the Colorado to which the wastewater will be discharged. This model determines the amount of each pollutant that can be added to the river to maintain the desired water quality for its assessed level of use. To my knowledge almost all of the Colorado is assessed as high use/high contact, so the permitted limits will be set at some of the strictest levels indicated by the model.
It is impossible to economically discharge perfectly clean water from a wastewater plant, but it is quite possible to discharge water that does not pollute the receiving stream to the point that it is hazardous to human health or the environment. The company will be issued a permit regulating the quality of the wastewater to meet the requirements shown by the model. The company will have to sample their effluent regularly and report the results to TCEQ monthly. Periodically a TCEQ inspector will inspect the facility and its records.
142,500 gallons per day of properly treated wastewater will not pose any hazard to human health or the environment. That may sound like a lot of water to you, but it really isn't. The City of Austin's two domestic wastewater treatment plants are permitted for 150,000,000 gallons per day. 142,500 gallons per day is the maximum amount that they are allowed to average over a month. In practice they will discharge less than that. If they operate within their permit, this wastewater doesn't present a danger.
That doesn't mean that there won't be fuckery and the State won't turn a blind eye to said fuckery. It means that if the rules are followed and enforced, there won't be a significant danger from this discharge.