Homework Set #6

1. In Spring when temperatures are on the rise, it is common for conventional activated sludge wastewater treatment plants to start nitrifying. When this happens, the pH drops due to the following overall stoichiometry:

Where represents bacterial cells.

What is the rate of nitrification in moles of ammonia per minute, if the pH drops from 7.3 to 6.9 in 4 hours? (hint: conventional activated sludge systems are intensively aerated or mixed with air)

2. You are treating an alkaline groundwater by alum coagulation followed by base addition for corrosion control. Your target pH for the water in the distribution system is 8.5. Analysis of the finished water prior to base addition shows a pH of 7.2, and an alkalinity of 250 mg/L as CaCO₃. How much of either one of the following two bases would you have to add (in mg/L) to reach your target pH.

	a. Caustic Soda (NaOH)
	b. Soda Ash (Na2CO3)

3. Repeat #2, but this time consider that your water treatment system has a large uncovered finished-water reservoir. Make the extreme-case assumption that the water will reach equilibrium with CO₂ in the atmosphere, after base addition, but before entry into the distribution system.

Assigned: 26 Oct 2011

Due: 2 Nov 2011