Wastewater Calculations - Mathematics and Statistics for the Environment - page 730

24.14.2 p ressure F filtration, C alCulations
In pressure filtration, the liquid is forced through the filter media by a positive pressure. Several
types of presses are available, but the most commonly used types are plate and frame presses and
belt filter presses.

24.14.2.1 Plate and Frame Press
The plate and frame press consists of vertical plates that are held in a frame and that are pressed
together between a fixed and moving end. A cloth filter medium is mounted on the face of each
individual plate. The press is closed, and the biosolids are pumped into the press at pressures up to
225 psi. They then pass through feed holes in the trays along the length of the press. Filter presses
usually required a precoat material, such as incinerator ash or diatomaceous earth, to aid in solids
retention on the cloth and to allow easier release of the cake. Performance factors for plate and
frame presses include feed biosolids characteristics, type and amount of chemical conditioning,
operating pressures, and the type and amount of precoat. Filter press calculations (and other dewa-
tering calculations) typically used in wastewater solids handling operations include solids loading
rate, net filter yield, hydraulic loading rate, biosolids feed rate, solids loading rate, flocculant feed
rate, flocculant dosage, total suspended solids, and percent solids recovery.

24.14.2.1.1 Solids Loading Rate
The solids loading rate is a measure of the lb/hr solids applied per square foot of plate area, as
shown in Equation 24.148:

Biosolids( gph) × 8.34 lb/gal ×

tt)

(%Solids 100)

2 =

Solids loadingrate(lb/hr/ft ) (24.148)

2

Platearea(f

■ EXAMPLE 24.114
Problem : A filter press used to dewater digested primary biosolids receives a flow of 710 gal during
a 2-hr period. The biosolids has a solids content of 3.3%. If the plate surface area is 120 ft 2 , what is
the solids loading rate in lb/hr/ft 2 ?

Solution: The flow rate is given as gallons per 2 hours. First express this flow rate as gallons per hour:

710 gal/2 hr = 355 gal/hr

Then,

Biosolids(gph) × 8.34 lb /gal × (%Solids 100)

Solids loadingrate =

2

Platearea (ft )

355 gph × 8.34 lb/gal × (3.3 100)

=

2

120 ft

2

= 0.81 lb/hr/ft

Key Point: The solids loading rate measures the lb/hr of solids applied to each sq ft of plate surface
area. However, this does not reflect the time when biosolids feed to the press is stopped.

24.14.2.1.2 Net Filter Yield
Operated in the batch mode, biosolids are fed to the plate and frame filter press until the space
between the plates is completely filled with solids. The flow of biosolids to the press is then stopped
and the plates are separated, allowing the biosolids cake to fall into a hopper or conveyor below. The