A storm occurs with rain of varying intensity falling over a commercial area. The area has a combined sewer system. Two interceptor sewers direct flow to the wastewater treatment plant (WWTP). The hourly flow volumes measured in the two interceptor sewers immediately upstream of the WWTP are listed in the table below. The WWTP has a peak flow capacity of 12.5 MG/hr. Using the data generated for this storm:

1) Design an equalization basin with a safety factor of 1.2 to store the excess flow for later treatment, thus preventing a combined sewer overflow of untreated wastewater to the environment;

2) Determine the hour during which the peak MG above the plant capacity occurs.

Solution:

1) Plotting the straight, constant sloped cumulative capacity line, and the curved total interceptor volume line results in data producing the last column in the table "Net + or - MG". This data represents the position of the cumulative interceptor MG, either above or below the cumulative plant capacity line. The greatest variance in cumulative hourly data (net MG) occurs between hour 1and hour 3 (- 0.5 and + 1.2). The equalization basin volume needed to prevent a combined sewer overflow during this storm is 1.7 MG. With a safety factor of 1.2, design for (1.7 MG) (1.2) = 2.0 MG basin.

2) Peak MG above the plant capacity occurs at hour 2 (14.0 MG vs. 12.5 MG).