At FRC, we offer two primary Dissolved Air Flotation (DAF) system designs: the Plate Pack or High Rate DAF System and the Open Style DAF System. Deciding which system to use for a specific wastewater application depends on several factors, including solids loading rate, hydraulic loading rate, and available floor space. Understanding these considerations is key to selecting the most efficient and cost-effective solution for your wastewater treatment needs.
Key Design Factors
DAF systems are designed based on two fundamental parameters:
Solids Loading Rate: Determines the free surface area required to separate solids effectively. On both a Plate Pack DAF and Open Style DAF the free area is typically considered to be the sludge skimming surface area.
Hydraulic Loading Rate: Determines the effective separation area needed to maintain laminar flow and prevent turbulence or re-entrainment of solids. On a Plate Pack DAF, the effective area is the projection of the total plate pack area on the horizontal plane (typically ½ of the total installed plate area for plates installed at 60 degrees), while on an Open Style DAF the effective area is the same as the free area (sludge skimming surface area).
In general, Plate Pack DAF systems are ideal for applications with high hydraulic loading and low solids loading, while Open Style DAF systems are better suited for applications with high solids loading. Plate Pack DAF systems have a large effective area relative to their free area offering both cost saving and space-saving benefits. This makes them advantageous in facilities with limited floor space when the application allows for their use.
1. Solids Loading Rate
The solids loading rate measures the mass of dry solids that can be separated per unit of free surface area per hour (typically lbs/sqft/hr or kgs/m2/hr). This rate varies by application and is influenced by the characteristics of the solids being treated. The application may dictate a solids loading rate as low as 2 lbs/sqft/hr or allow for as high as 15 lbs/sqft/hr.
Applications with fast-separable solids, such as primary poultry wastewater, allow for higher solids loading rates. Conversely, slower-separating solids, like biomass, require lower loading rates for effective separation.
For example, if a wastewater stream contains 1,000 pounds of solids per hour, a DAF system designed for:
2 lbs/sqft/hr would require 500 sqft of free surface area.
10 lbs/sqft/hr would require 100 sqft of free surface area.
This is a large difference in the size of the DAF. Not understanding how the application affects the Solids Loading Rate could lead to major error in DAF sizing.
2. Hydraulic Loading Rate
The hydraulic loading rate evaluates the volume of wastewater flow over a unit of effective separation area per unit time (typically gpm/sqft or m3 per hr/m2), ensuring the flow remains laminar and prevents turbulence. Typically, this falls in the range of 1 to 3 gpm/sqft for most industrial applications but can go as high as 10 gpm/sqft in some applications. The DAF’s internal recycle rate should always be included in this calculation.
Example:An open style DAF with 48 sqft of free area, processing 65 gpm of low solids loaded wastewater with a 22 gpm recycle flow:
(65gpm+22gpm)/48sqft=1.81gpm/sqft
The same flow through a plate pack DAF with 7 sqft of free area and 65 sqft of effective separation area:
(65gpm+22gpm)/65sqft=1.34gpm/sqft
Although the hydraulic loading rates are similar, the size difference between the two systems becomes apparent:
Open tank DAF: 13' L x 8' W x 8' H
Plate pack DAF: 7' L x 4' W x 8' H
The plate pack system occupies roughly one-quarter the footprint of the open tank design, making it an attractive option when space is limited and the solids loading allows for it.
3. Other Considerations
The available building size and floor space is often a critical factor in DAF system design. While an open style DAF can handle a wider range of applications, its larger footprint and higher cost may be impractical in facilities with space constraints, and a plate pack DAF may be more suitable.
Example Application:
A pork processing facility generates 450 gallons per min (gpm) of wastewater containing 570 lbs/hr of dry solids.
Using a solids loading rate of 7.5 lbs/hr/sqft, the required free separation area is:
Free Area Requirement – (570 lbs/hr) / (7.5 lbs/sqft/hr) = 76 sqft
Using a hydraulic loading rate of 2 gpm/sqft and 100 gpm recycle flow, the required effective separation area is:
Effective Area Requirement – (410 gpm + 100 gpm) / (2 gpm/sqft) = 255 sqft
In this application the Plate Pack DAF system provides the same treatment capability while occupying significantly less space and at a lower cost.
DAF Size:
Open Style DAF: ~31' L x 10' W x 8' H
Plate Pack DAF: ~16' L x 8' W x 10' H
Selecting the Right DAF System
Ultimately, the choice between a Plate Pack and Open Tank DAF systems depends on the application’s specific requirements: solids and hydraulic loading rates, space constraints, and budget considerations. Plate pack systems can offer a smaller and more cost-effective footprint in situations with lower solids loading and high hydraulic loading, while open style systems are effective at handling high solids loading.
At FRC, we tailor DAF system design to balance performance, capital investment, and operational costs. Whether the application allows for a compact, high-rate solution or dictates a larger open style design, we can help identify the ideal system for your wastewater treatment process.
Contact us today to explore your options and design the most effective DAF solution for your needs.
Commentaires