Wastewater Calculations - Mathematics and Statistics for the Environment - page 736

■ EXAMPLE 24.125
Problem: A total of 4000 lb/day primary biosolids solids is to be processed by a vacuum filter. The
vacuum filter yield is 2.2 lb/hr/ft 2 . The solids recovery is 95%. If the area of the filter is 210 ft 2 , how
many hours per day must the vacuum filter remain in operation to process these solids?

Solution:

(lb/day)
Filter operation(lb/day)
Fil

Solidstofilter

%Recovery
100

2 =

Filteryield (lb/hr/ft) ×

2

terarea (ft )

= 4000 lb/day
hr/day

1
210 ft

95
100

2

2.2 lb/hr/ft × ×

2

x

4000 1×
××

× 95

x =

22 . 210 100

x = 8.2 hr/day

24.14.3.4 Percent Solids Recovery
As mentioned, the function of the vacuum filtration process is to separate the solids from the liquids
in the biosolids being processed; therefore, the percent of feed solids recovered (sometimes referred
to as the percent solids capture ) is a measure of the efficiency of the process. Equation 24.158 is
used to determine percent solids recovery:

Wet cakeflow(lb/hr) × Solids in cake 100)

Biosolidsfeed (lb/hr)× %Solidsinfeed 100)

(%

%Solidsrecovery =

×100

(24.158)

■ EXAMPLE 24.126
Problem: The biosolids feed to a vacuum is 3400 lb/day, with a solids content of 5.1%. If the wet
cake flow is 600 lb/hr with a 25% solids content, what is the percent solids recovery?

Solution:

Wet cakeflow(lb/hr) × solids in cake 100)

Biosolidsfeed (lb/hr)× %solidsinfeed 100)

(%

%Solidsrecovery =

×

100

25/100)
3400 lb/hr 5.1/100)

600 lb/hr( × 150 lb/hr

1

= ×= 100 ×= 100 87%

× 73 lb/hr

24.14.4 s and d rying b ed C alCulations
Drying beds are generally used for dewatering well-digested biosolids. Biosolids drying beds con-
sist of a perforated or open joint drainage system in a support media, usually gravel or wire mesh.
Drying beds are usually separated into workable sections by wood, concrete, or other materials.
Drying beds may be enclosed or opened to the weather. They may rely entirely on natural drain-
age and evaporation processes or may use a vacuum to assist the operation. Sand drying beds are
the oldest biosolids dewatering technique. They consist of 6 to 12 in. of coarse sand underlain by
layers of graded gravel ranging from 1/8 to 1/4 in. at the top and 3/4 to 1-1/2 in. at the bottom. The
total gravel thickness is typically about 1 ft. Graded natural earth (4 to 6 in.) usually makes up the