Practice Problems for Biological Treatment

1. A small wastewater plant has a flow of 200,000 Gallons per day. The reaction is first order and can be assumed to be:

and the reaction rate is then:

where A represents the BOD. If the concentration of A is 250 mg/L and k is 0.15/day, what detention time is required to obtain a removal of 90 percent in a CSTR lagoon? Would the removal be the same if that lagoon was divided into two parts, each of volume V/2? If not, what is the new removal?

Answers: 60 days; No, the removal would be much better and the effluent concentration would be reduced to only 8.3 mg/L.

2. A much different waste, with an initial concentration of 220 mg/L, is being treated in a CSTR. The waste has growth conforming to

What is the hydraulic detention time required to obtain a final effluent concentration of 10 mg/L? The growth characteristics of the system are:

Answer: First note that

then the applicable equation for the HRT is;

The HRT is 2.1 days. The influent concentration is not used.