156.0o- 40o= 116.0oCThus, 240oF = 116.0oC

Convert 22oC to Fahrenheit.Example:

FLOWFlow is expressed in many different terms in the English system of measurement. The most commonly used flow terms are as follows: •gpm–gallons per minute•cfs–cubic feet per second•gpd–gallons per day•MGD –million gallons per dayIn converting flow rates, the most common flow conversions are 1 cfs = 448 gpm and 1 gpm = 1440 gpd. To convert gallons per day to MGD, divide the gpdby 1,000,000. For example, convert 150,000 gallons to MGD:150,000 gpd = 0.150 MGD1,000,000

In some instances, flow is given in MGD but is needed in gpm. To make the conversion (MGD to gpm), two steps are required.•Step 1:convert the gpd by multiplying by 1,000,000.•Step 2:convert to gpm by dividing by the number of minutes ina day (1440 min/day).

ProblemConvert 0.135 MGD to gpm.Example:

In determining flow through a pipeline, channel, or stream, we use the following equation:whereQ = cubic feet per second (cfs)V = velocity in feet per second (ft/second)A = area in square feet (ft2)

ProblemFind the flow in cubic feet per second (cfs) in an 8-inch line if the velocity is 3 feet per second.Example:

ProblemFind the flow in gpm when the total flow for the day is 75,000 gpd.Example:

ProblemFind the flow in gpm when the flow is 0.45 cfs.Example:

DETENTION TIMEDetention timeis the length of time water is retained in a vessel or the period from the time the water enters a settling basin until it flows out the other end. When calculating unit process detention times, we are calculating the length of time it takes the water to flow through that unit process. Detention times are normally calculated for the following basins or tanks: •Flash mix chambers (seconds)•Flocculation basins (minutes)•Sedimentation tanks or clarifiers (hours)•Wastewater ponds (days)•Oxidation ditch (hours)

To calculate the detention period of a basin, the volume of the basin must first be obtained. Using a basin 70 ft long (L), 25 ft wide (W), and 12 ft deep (D), the volume (V) would be:V = L x W x DV = 70 ft x 25 ft x 12 ftV = 21,000 ft3Gallons = V x 7.48 gal/f2Gallons = 21,000 x 7.48 = 157,080 gallons If we assume that the plant filters 300 gpm, then we have 157,080 / 300 = 524 minutes, or roughly 9 hours, of detention time. Stated another way, the detention time is the length of time theoretically required for the coagulated water to flow through the basin.

Key point:If the detention time is desired in minutes, then the flow rate used in the calculation should have the same time frame (cfm or gpm, depending on whether tank volume is expressed as cubic feet or gallons). If detention time is desired in hours, then the flow rate used in the calculation should be cfh or gph.If chlorine is added to the water as it enters the basin, the chlorine contact time (CT) would be 9 hours. That is, to determine the CT (concentration of free chlorine residual x disinfectant contact time [in minutes] used to determine the effectiveness of chlorine), we must calculate detention time.

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