# PRINCIPLES OF ENVIRONMENTAL ENGINEERING

1. A river has a TDS of 400 mg/L. An olive factory wants to discharge a brine (concentrated salt solution) with 10,000 mg/L TDS into the river. The Regional Water Quality Control Board says that the TDS of the river must not exceed 500 mg/L to protect irrigation uses downstream. If the river has a flow of 100 m3/s, what flowrate of brine can the olive factory discharge? Answer: 1.05 m3/s

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2. In the winter, a stream flows at 10 m3/s and receives discharge from a pipe that contains road runoff. The pipe has a flow of 5 m3/s. The stream’s chloride concentration just upstream of the pipe’s discharge is 12 mg/L, and the runoff pipe’s discharge has a chloride concentration of 40 mg/L. Chloride is a conservative substance. (a) Does wintertime salt usage on the road elevate the downstream chloride concentration above 20 mg/L? (b) What is the maximum daily mass of chloride (metric tons/day) that can be discharged through the road runoff pipe without exceeding the water quality standard? Answer: yes (21 mg/L), 16 mton/d

3. Engineers do a treatment study on an industrial waste containing Chemical X. In a controlled system, the concentration of Chemical X drops from 150 mg/L to 75 mg/L in 14 hours. (a) What is the decay constant if the reaction is zero order? (b) What is the decay constant if the reaction is first order?

4. A first-order reaction that results in the destruction of a pollutant has a rate constant of 0.1/day. (a) How many days will it take for 90% of the chemical to be destroyed? (b) How long will it take for 99% of the chemical to be destroyed? (c) How long will it take for 99.9% of the chemical to be destroyed?

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