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Lesson 22:
Wastewater Preliminary Treatment Calculations
Objective
In this lesson we will learn the following calculations:
Lecture
We have covered basic calculations such as area, volume, pressure, etc, which will be needed in this portion of the course. We have also covered feed rates, dosage calculations, etc. You know the concepts, so we'll carry that concept to this portion: wastewater treatment.
Let's begin by watching a video explaining what happens to wastewater.
Preliminary Treatment
The initial stage in wastewater treatment, after collection and pumping it to the plant, is preliminary treatment. During this process raw influent enters the treatment plant containing all kinds of materials (trash), and preliminary treatment protects downstream plant equipment by removing these materials. If the larger objects weren't removed it could cause clogs, jams, or excessive wear in plant machinery. Preliminary treatment also saves valuable space within the treatment plant.
Two of the processes used in preliminary treatment include screening and grit removal, although other preliminary treatment processes may be included, depending upon any materials that may present potential problems during the treatment process. We will not get into the other preliminary processes that a plant may need, those are location specific. We will focus on the basics that every plant needs: shredding and grit removal.
Screening
Screening removes large solids, such as rags, bottles, rocks, leaves and branches, and other objects from the flow before it moves on to the downstream processes.
Let's watch an animation showing how screening is accomplished.
You can also watch a video of this part of process at a plant.
Screening Removal Calculations
Wastewater operators responsible for screenings disposal are typically required to keep a record of the amount of screenings removed from the flow. To keep and maintain accurate screening records, the volume of screenings withdrawn must be determined. Two methods are commonly used to calculate the volume of screenings withdrawn:
or
Example:
A total of 72 gallons of screenings is removed from the wastewater flow during a 24-hour period. What is the screening removal reported as cubic feet per day (ft3/day)?
First, convert gallon screenings to ft3:
72 gal x (1 ft3/7.48 gal) = 9.63 ft3 screenings
Next, calculate the screenings removed during the 24-hour period:
Example:
A total of 1480 gallons of screenings were removed from wastewater screens during the month of May. What is the average screening removal reported as cubic feet per day (ft3/day)?
First, convert gallon screenings to ft3:
1480 gal x (1 ft3/7.48 gal) = 197.86 ft3 screenings
Next, calculate the screening removal. Remember May has 31 days!
Screening Pit Capacity Calculations
Recall that detention time may be considered the time required for flow to pass through a basin or tank, or the time required to fill a basin or tank at a given flow rate. In screening pit capacity problems, the time required to fill a screening pit is calculated with the following formula:
Example:
A screening pit has a capacity of 650 ft3. If an average of 5.2 ft3 of screenings are removed daily from the wastewater flow, how many days will it take for the pit to fill?
Grit Removal
The purpose of grit removal is to remove inorganic solids, such as sand, gravel, coffee grounds, and other similar materials, that could cause excessive mechanical wear. Several processes or devices are used for grit removal, all based on the fact that grit is heavier than the organic solids, which should be kept in suspension for treatment in following unit processes. Grit removal may be accomplished in grit chambers or by the centrifugal separation of biosolids. Processes use gravity/velocity, aeration, or centrifugal force to separate the solids from the wastewater.
Grit Removal Calculations
Wastewater systems typically average 1 to 15 cubic feet of grit per million gallons of flow (sanitary systems average 1 to 4 ft3/MG; combined wastewater systems average 4 to 15 ft3/MG of flow), with higher ranges during storm events. Generally, grit is disposed of in sanitary landfills, so, for planning purposes, operators must keep accurate records of grit removal. Most often the data are reported as ft3 grit removed/MG flow and is determined with the following equation:
Over a given period of time, the average grit removal rate at a plant (at least a seasonal average) can be determined and used for planning purposes. Typically, grit removal is calculated as cubic yards, because excavation is normally expressed in terms of cubic yards:
Example:
A treatment plant removes 12 ft3 of grit in 1 day. How may ft3 of grit are removed per million gallons if the plant flow is 4.3 MGD?
Example:
The monthly average grit removal is 3.2 ft3/MG. If the monthly average flow is 5,550,000 gpd, how many yd3 must be available for grit disposal if the disposal pit is to have a 90-day capacity?
First, calculate the grit generated each day:
(3.2 ft3/MG) x 5.55 MGD = 17.76 ft3/day
This means that the grit generated for 90 days would be:
(17.76 ft3/day) x 90 days = 1598.4 ft3
Now convert ft3 to yd3:
Grit Channel Velocity Calculations
The optimum velocity in sewers in approximately 2 feet per second (fps) at peak flow, since this velocity normally prevents solids from settling from the lines. When the flow reaches the grit chamber, however, the velocity should decrease to about 1 fps to permit the heavier, inorganic solids to settle. Velocity can be determined by using the float and stopwatch method and by channel dimensions.
Velocity by Float and Stopwatch Method
Example:
It takes a float 45 seconds to travel 52 feet in a grit channel. What is the velocity of the flow in the channel?
Velocity by Flow and Channel Dimensions
This calculation can be used for a single channel or tank or for multiple channels or tanks with the same dimensions and equal flow. If the flow through each unit is unequal, the velocity for each channel or tank must be computed individually. Velocity can be determined by the flow and channel dimensions using the following formula:
or
Example:
A plant is currently using two grit channels. Each channel is 4 ft wide and has a water depth of 1.8 ft. What is the velocity when the influent flow rate is 3.8 MGD?
First you need to calculate the area of the channels. Keep in mind there are 2 channels:
Area, ft2 = 4 ft x 1.8 ft x 2 (channels)
Area, ft2 = 14.4 ft2
Next, you need to convert the flow from MGD to ft3/sec using the common conversion: 1 MGD = 1.55 ft3/sec:
Now determine the velocity in the grit channels when the flow rate is 3.8 MGD (or 5.89 ft3/sec):
Example:
What is the velocity, in meters per second, of water flowing through a channel that is 3.5 meters wide and 2 meters deep if the rate of flow is 0.72 m3/sec?
Area = L x D
Area = 3.5 m x 2 m
Area = 7 m2
Now plug the values into the formula:
Summary
The initial stage in wastewater treatment, after collection and pumping it to the plant, is preliminary treatment. During this process raw influent enters the treatment plant containing all kinds of materials (trash), and preliminary treatment protects downstream plant equipment by removing these materials. If the larger objects weren't removed it could cause clogs, jams, or excessive wear in plant machinery. Preliminary treatment also saves valuable space within the treatment plant. Two of the processes used in preliminary treatment include screening and grit removal, although other preliminary treatment processes may be included, depending upon any materials that may present potential problems during the treatment process. Screening removes large solids, such as rags, bottles, rocks, leaves and branches, and other objects from the flow before it moves on to the downstream processes. Wastewater operators responsible for screenings disposal are typically required to keep a record of the amount of screenings removed from the flow. To keep and maintain accurate screening records, the volume of screenings withdrawn must be determined. The purpose of grit removal is to remove inorganic solids, such as sand, gravel, coffee grounds, and other similar materials, that could cause excessive mechanical wear. Several processes or devices are used for grit removal, all based on the fact that grit is heavier than the organic solids, which should be kept in suspension for treatment in following unit processes. Grit removal may be accomplished in grit chambers or by the centrifugal separation of biosolids. Processes use gravity/velocity, aeration, or centrifugal force to separate the solids from the wastewater. Wastewater systems typically average 1 to 15 cubic feet of grit per million gallons of flow (sanitary systems average 1 to 4 ft3/MG; combined wastewater systems average 4 to 15 ft3/MG of flow), with higher ranges during storm events. Generally, grit is disposed of in sanitary landfills, so, for planning purposes, operators must keep accurate records of grit removal. Most often the data are reported as ft3 grit removed/MG flow. The optimum velocity in sewers in approximately 2 feet per second (fps) at peak flow, since this velocity normally prevents solids from settling from the lines. When the flow reaches the grit chamber, however, the velocity should decrease to about 1 fps to permit the heavier, inorganic solids to settle. Velocity can be determined by using the float and stopwatch method and by channel dimensions.
Assignment
Complete the math worksheet for this lesson and return to instructor via email, fax or mail.
