Prepare clean and clear solutions to the following problems. Highlight the final answer.

The BOD of a water sample is estimated to be $180m\frac{g}{L}.$ What volume of undiluted sample should be used in a standard BOD$-5$

test? Assume that the initial DO is $10m\frac{g}{L}.$

A town discharges $18,000\frac{m^3}{d}$ of treated wastewater into a stream. The treated wastewater has a BOD$-5$ of $12m\frac{g}{L}$ and a

deoxygenation constant of $\frac{0\mathrm{.}12}{d}$ at $20C.$ The stream has a flow rate of $0\mathrm{.}4\frac{m^3}{s}$ and an ultimate BOD of $5m\frac{g}{L}.$ The DO of the

stream is $6\mathrm{.}5m\frac{g}{L}$ and that of the wastewater is $1\mathrm{.}0m\frac{g}{L}.$ Determine the $DO$ and ultimate BOD after mixing.

Refer to the previous problem. Determine the DO concentration $5km$ downstream from the city discharge. Use a stream

temperature and a wastewater temperature of $10C.$ The reoxygenation constant is $\frac{0\mathrm{.}06}{d}.$ The stream velocity is $0\mathrm{.}85\frac{m}{s}.$