FRC has two main DAF system designs: the PCL High Rate DAF System and the PWL Open Style DAF System. How is it that we decide to use one DAF system design over the other for a given wastewater application? That’s a good question.
DAF systems are designed on two key calculations – solids loading rate and hydraulic loading rate. Solids loading rate determines how much free surface area a DAF system should provide to separate solids. Hydraulic loading rate determines how much effective area is required to maintain laminar flow.
Generally speaking, a plate pack DAF unit is suited for high hydraulic and low solids loading rates. Open style DAF tanks are better suited for high solids loading rates.
But before we go and place plate pack DAFs and open tank DAFs into specific applications or industries, let’s understand one concept: anywhere you can use a plate pack DAF unit, you can also use an open tank DAF unit, but the opposite does not hold true. Similar to the rule that says, “a square is a rectangle, but a rectangle is not a square.” The tradeoff for always going with an open style tank is the amount floor space they occupy, and their overall cost, especially as flow rates rise above a few hundred gallons per minute.
First, let’s discuss solids and hydraulic loading rates, and explore them with some simple examples.
1. Solids Loading Rate
When discussing solids loading rate, what we’re really asking is how many pounds of solids can we separate in one square foot of free area? 2 lbs/sqft is a low solids loading rate, while 15 lbs/sqft is pretty high.
Let’s say we’ve calculated that an incoming stream of wastewater carries 1000 pounds of solids per hour. If we use a solids loading rate of 5 lbs/sqft/hr, that means our DAF system needs to provide 200 sqft of free area. If we use a solids loading rate of 10 lbs/sqft, then our DAF only requires 100 sqft of free surface area.
When to use a specific solids loading rate is largely a function of process experience. Most DAF system designers know that poultry wastewater contains solids that separate very quickly and easily, so they can use higher solids loading rates. Bio-mass separation, on the other hand, occurs much more slowly and the DAF designer should use a lower solids loading rate.
2. Hydraulic Loading Rate
Hydraulic loading rate answers the question – over one hour, how many gallons of wastewater flow over one square foot of effective separation area? In other words, it's the amount of water that can be applied per unit of area with time, and not cause turbulence or re-entrainment of solids into the moving water. Where 0.5 gpm/sqft/hr is a low hydraulic loading rate, 2 gpm/sqft/hr is quite high. Let’s consider an example.
An open style DAF tank has 48 sqft of effective surface area. If you feed 65 gpm into the DAF unit and consider a recycle flow of 22 gpm, our calculation would be: (65 gpm + 22 gpm)/48 sqft = 1.81 gpm/sqft
If the same volume of water is fed into a plate pack DAF unit with 65 sqft of effective area, our hydraulic loading rate would be:
(65 gpm + 22 gpm) / 65 sqft = 1.34 gpm/sqft
The difference in hydraulic loading rates seems small, and it is, but when we consider the size of the DAF unit, we begin to see why plate pack configurations often make sense. The open tank DAF measures roughly 13’L x 8’W x 8’H, while the plate pack unit is only 7’L x 4’W x 8’H. That’s a quarter the size of the open tank unit.
By understanding the solids loading rate and hydraulic loading rate we’ve got almost everything we need to appropriately design and size a DAF system. There’s just one final question.
3. How much floor space do we have for wastewater process equipment?
A seemingly obvious factor to consider in DAF system design is how much floor space is available for the equipment. If we have essentially limitless space, then the footprint of the DAF unit has little bearing on system design.But when we’re dealing with limited floor space, having the ability to build tall DAF tanks (plate pack style), rather than wide & long DAF tanks (open style), can make all the difference. Let’s consider one more practice problem:
A pork processor produces 250,000 gpd of wastewater, loaded with 3,500 lbs/day of solids. Size an open tank DAF and a plate pack DAF to remove 100% of the solids.
If we make a solids loading rate assumption of 7.5 lbs/sqft based on the application – pork processing, the amount of separation area required is calculated as:
(3,500 lbs / ? area ) = 7.5 lbs/sqft => 467 sqft.
Knowing we need 467 sqft feet of separation area, an open tank DAF unit would need to be approximately 47’L x 10’W x 8’H. A plate pack configuration could provide the same surface area in a unit that’s only 13’ L x 8’ W x 10’ H. Again, the plate pack DAF configuration is about a quarter the size of the open tank unit.
In the end, you want a DAF system to perform its function, while incurring the lowest capital and operational costs possible.
That’s FRC’s design approach. Sometimes that’s achieved with an open tank DAF and others with a plate pack DAF. If you’re considering a dissolved air flotation system in your wastewater treatment process, contact us and we’ll find the best solution for you.
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