Pondering Wastewater Management: Is Your Sedimentation Tank Up to Snuff?

For a circular primary sedimentation tank with a diameter of 50 ft. and a depth of 15 ft. treating peak flows of 2.5 mgd, is it sized appropriately?

• A. No, it is too small
• B. Yes, it meets the required detention time
• C. Yes, but only during low flows
• D. No, it is oversized

Answer: (B)

To determine if the circular primary sedimentation tank is sized appropriately, one must evaluate whether it provides sufficient detention time for the peak flow of 2.5 million gallons per day (mgd). To assess this, you first calculate the volume of the tank. The formula for the volume of a cylindrical tank is given by: \[ Volume = \pi \times \left(\frac{diameter}{2}\right)^2 \times depth \] For a tank with a diameter of 50 feet and a depth of 15 feet, the calculation would be: 1. Calculate the radius: 50 ft / 2 = 25 ft 2. Calculate the area of the tank's base: \( \pi \times (25 ft)^2 \approx 1963.5 ft^2 \) 3. Calculate the volume: \( 1963.5 ft^2 \times 15 ft \approx 29452.5 ft^3 \) Next, convert cubic feet to gallons using the conversion factor that 1 cubic foot = 7.48 gallons: \[ Volume = 29452.5 ft^3 \times 7.48 gal/ft^3 \approx 220,000

When it comes to wastewater treatment, one of the most crucial components is the sedimentation tank. Specifically, you might find yourself staring at the specs of a circular primary sedimentation tank, looking at its measurements—50 feet in diameter and 15 feet deep—and wondering, “Does this thing actually work for peak flows of 2.5 million gallons per day (mgd)?” Well, let’s break this down together!

First off, why is sizing so important? Picture your favorite coffee shop. If they try to make 100 lattes in a tiny espresso machine, they’re going to be in a world of hurt—flowing coffee and frustrated customers everywhere! Similarly, wastewater treatment systems need to be sized properly to handle peak flows without spilling over or causing issues downstream. So, how do we figure out if our tank is just right?

Let’s Crunch Some Numbers

We start with the volume calculation for our tank. The volume of a cylindrical tank is calculated with this neat little formula:

[

Volume = \pi \times \left(\frac{diameter}{2}\right)^2 \times depth

]

For our tank, it boils down to these steps:

1. Find the Radius:

• Half of the diameter gives us (50 \text{ ft} / 2 = 25 \text{ ft}).

2. Calculate Area of the Base:

• Plugging that radius back into our formula, we get ( \pi \times (25 \text{ ft})^2 ), which works out to about (1963.5 \text{ ft}^2).

3. Volume Calculation:

• Now we take that area and multiply it by the depth: (1963.5 \text{ ft}^2 \times 15 \text{ ft} \approx 29452.5 \text{ ft}^3).

But hold on, we’re not done yet! We need to convert that volume into gallons to see how it measures up against our peak flow. Did you know that 1 cubic foot equals about 7.48 gallons? So, we take our volume:

[

Volume = 29452.5 \text{ ft}^3 \times 7.48 \text{ gal/ft}^3 \approx 220,000 \text{ gallons}

]

So, Is the Tank Appropriately Sized?

Now that we’ve got about 220,000 gallons of space, we need to determine if that’s enough for the peak flow we mentioned earlier—2.5 mgd. Here’s where it gets interesting; we have to figure out the required detention time.

The required detention time is significant because it directly influences how well the tank can separate solids from the water. For a tank receiving peak flows of 2.5 mgd, you’d ideally want a detention time that allows all incoming wastewater enough time to settle before being released downstream. Here’s a general breakdown:

• Detention Time Calculation:

• To find out how long the wastewater stays in the tank, you can utilize the formula:

[

Detention Time = \frac{Volume}{Flow Rate}

]

• In our case, that means using our tank volume of 220,000 gallons and dividing it by 2.5 mgd (or 2,500,000 gallons/day).

Plugging in those numbers gives us a pretty decent estimate of about 0.88 days, or roughly 21 hours. Now considering that most sedimentation processes thrive with a detention time between 1 to 2 hours, our tank is looking pretty good!

What’s the Verdict?

Based on these calculations, we can confidently say the answer is B: Yes, it meets the required detention time. Understanding if your tank can handle the peak flow isn’t just math—it’s about giving the wastewater the right chance to settle down and become cleaner before it rejoins the water system. It’s like a well-planned party—there’s a flow, a rhythm, and sometimes it’s all about making space for your guests!

And as you look ahead to your study sessions and exams, remember this scenario. Solid understanding of sedimentation tank design not only gets you through tests, but it equips you with real-world solutions for keeping our water clean and safe. Who wouldn’t want to be part of that mission? You’re moving towards a future where your efforts could lead to cleaner waterways, better environmental policies, and, ultimately, healthier communities.