DIRECT FILTRATION OF STEEL MILL SCALE AND
METAL FINISHING RINSEWATERS WITH A CONTINUOUS FILTER
Ingolf V. Janerus, Technical Manager
Ray M. Shimokubo, Engingeer
Parkson Corporation
Fort Lauderdale, Florida 33334
Conventional granular media filters are used as polishing filters for many applications,
e.g., wastewaters containing mill scale from hot finishing of steel and metal finishing rinse-
waters. These fdters have to be taken off stream to be backwashed periodically. The associated equipment (pumps, valves, holding tanks, etc) and controls are expensive and complicated. At higher feed suspended solids concentrations the filters have to be backwashed
excessively, which is impractical.
The DynaSand Filter is a unique granular media fdtration system developed by The
Axel Johnson Institute for Industrial Research in Sweden. The media is continuously
cleaned while the filter is in operation. The dirtiest media is transported to the top of the
filter through an internal air lift and washed before being returned onto the bed. A smad
continuous stream of dirty reject water is produced. The filter wdl therefore be in continuous
operation and can fdter suspensions with high suspended solids concentrations at a low
pressure drop. This paper describes the filtration process and summarizes the results from
several demonstration projects and commercial installations to highlight the advantages of
the new concept.
CONTINUOUS SAND FILTRATION
Figure 1 depicts one module of a DynaSand Filter. This is an upflow gravity fdter. The
feed enters the filter through a number of riser tubes which discharge under a distribution
hood (A). The feed is introduced into the sand bed from the open bottom of the distribution
hood. There is as much cross sectional fdter area outside the hood as there is inside it. Water
flows upward through the sand bed (B) which is moving downward. The clear fdtrate exits
the sand bed, overflows a weir (C) and is discharged from the filter (D).
The used sand is transported to the top of the fdter (I) by an air lift (E) which extends
through the sand bed. The air is injected at the bottom end of the air lift. The flow of air,
water and sand in the air lift is very turbulent and the impurities are scoured loose from the
sand so that when the mixture exits the air lift (F) it consists of clean sand and very dirty
water.
The sand is returned to the sand bed through a gravity washer/separator (G) which allows
the fast-settling sand to penetrate but not the dirty liquid. The washer/separator is placed
concentrically around the upper part of the air lift and consists of several stages to eliminate
any risk of short-circuiting. In each stage, the sand is changing direction at least two to three
times. A liquid barrier ascertains that no dirty liquid accompanies the clean sand into the fdtrate compartment. The liquid barrier consists of a small amount of filtrate which is forced
upwards through the washer/separator counter-current to the sand.
The driving force for this transportation is the level difference between the fdtrate
weir (C) and the reject weir (J), which is adjustable. The clean sand is returned to the top of
the sand bed by means of a cone shaped distributor (H). The dirty reject water leaves the
filter as a continuous stream so there is no need for holding tanks for backwash water, etc.
The filtration is only taking place in the part of the sand bed above the distribution hood
(A). This part is generally about 40 in. deep to ensure deep bed fdtration. The sand is slowly
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