Environmental Engineering

Question 1: The concentration of silver in seawater is 50 parts per trillion, and the total volume of seawater in the oceans of the world is approximately 1370 x 10⁶ km³.

(a) Determine the total mass (kg) of silver in the oceans. 

(b) What volume of seawater would have to be processed to recover one kilogram of silver, assuming an extraction process with 100% efficiency?

Question 2: A solution contains 45 mg/L Ba²⁺. A consultant has suggested that the Ba²⁺ could be removed from solution by converting it to solid barium sulfate [BaSO₄(s)]. To accomplish this, she proposes adding enough Na₂SO₄ to the solution to convert all the Ba²⁺ to BaSO₄(s), plus an extra 5 mg/L SO₄ ^2- as a safety factor. What dose (mg/L) of Na₂SO₄ should be added,and what would be the concentration of solids (in ppm), if essentially all the Ba²⁺ precipitated?

Question 3: When domestic wastewater undergoes a biological treatment processes, conditions are sometimes established that lead the microbes to convert ammonium ion, NH₄ ⁺, to nitrate ion, NO₃ ^-, in a process referred to as nitrification. Then, in a subsequent step known as denitrification, other microbes convert the nitrate to molecular nitrogen, N₂. The N₂ can then escape from the solution as nitrogen gas, N₂(g).

A wastewater containing 30 mg/L NH₄ ⁺ and 5 mg/L NO₃ ^- is to be treated by sequential nitrification and denitrification. What volume of N₂(g) will be generated per liter of water treated? Assume that the gas phase is at 25^oC and 1 atm total pressure.

Question 4: Ozone gas (O₃) is commonly injected into drinking water (also pools and hot tubs) as a disinfectant. Pure ozone decays rapidly, so it cannot be stored for long periods; as a result, it is typically generated onsite by exposing dried air to an electrical discharge that causes gaseous oxygen to be converted to ozone by the reaction: 3O₂(g) ---> 2O₃(g).

A water treatment plant for a city the size of Seattle must produce on the order of 400,000m³ (about 106 million gallons) per day of clean water. If ozone is injected into the water at a dose of 1 mg/L, how much air (1.0 atm total pressure, 15^oC, 21% O₂) must be processed per day to provide the ozone?

Question 5: All aqueous solutions contain both positively and negatively charged dissolved species,called ions. Positively and negatively charged ions are called cations and anions, respectively. Any overall solution must be electrically neutral, so the total concentration of charge on the cations in the solution must equal the total concentration of charge on the anions, where both concentrations have units of moles of charge per liter of solution. 

When a water sample is analyzed, one way to test whether the results are reasonable is to carry out a “charge balance”; i.e., to see whether the results satisfy the criterion for an electrically neutral solution. If the total positive charge differs from the total negative charge by more than 5%, the analysis is judged to be incomplete or incorrect.

A groundwater is reported to have the following ionic composition. Carry out a charge balance analysis. Do the data satisfy the criterion of <5% deviation from a perfect balance?

Ion                        Conc’n (mg/L)

Ca²⁺                       90

Cl^-                          122

HCO₃^-                     317

Mg²⁺                       48

Na⁺                       120

NO₃^-                     11.1

SO42–                     190