AFS Fiber Filter Paper
May 1, 2001
Following three years of testing, the Fiber Filter is being introduced to food processing plants. The machine is a new patented wastewater filtration machine that fills a niche between traditional filtration equipment.
The Fiber Filter works by introducing a flow containing suspended solids into a cylindrical fabric sleeve. The sleeves offered have opening sizes ranging from 20 to 190 microns (0.001 to 0.007 inches). Normally a fabric with holes this small would rapidly become blinded by the solids being filtered from the liquid stream. This problem is overcome in the Fiber Filter by vibrating the fabric at high frequency. The vibration is induced by (a) tensioning the filter sleeves with springs and (b) spinning a high speed (800 rpm) rotor inside the sleeve to induce pulse waves in the fluid being filtered. The rotor has axial paddles that pulse the liquid so as to make the filter sleeve vibrate. The pulsing draws filtered liquid through the fabric; this action has been described as capillary suction.
Small 6" sleeve diameter machines feature a single sleeve, 27" in length. The larger 12" sleeve machines come with either three or four sleeves mounted in series, depending on the capacity and degree of filtration required.
The sleeves in the larger machine are mounted on holders that are free to slide on four tie rods. The holder at the inlet end of the machine is mounted rigid to the tie rods, while the holder at the discharge end is mounted to a sliding member. In this manner the sleeves are stretched taught by springs mounted on the sludge discharge head of the machine.
In addition to axial paddles, the whirling rotor inside the sleeves has helicoid flights. These ribbon flights push the solids toward the sludge discharge end of the machine.
Where heavy concentrations (over 2%) of suspended solids are present in the feed flow, a tight two thirds pitch ribbon flight is used. This rapidly pushes a heavy flow of sludge solids to the discharge. In contrast, thin concentrations (measured in parts per million) of feed solids require a long double pitch ribbon flight. This arrangement removes the solids at a lower rate, resulting in thicker sludge at the discharge and preventing liquid from being ejected from the solids port of the machine.
The fabric used in the Fiber Filter sleeves is a monofilament tweed woven polymer. It is calendared flat on one side, which results in a very smooth surface. This calendared surface is positioned on the inside. This design detail minimizes any tendency for solids to plug or blind the fabric.
If the flow to a Fiber Filter contains excessive solids, these will load up the fabric sleeves. This loading is normally addressed by using a rotor with a helicoid ribbon of tighter pitch. However, if inbound solids are in excess of 4% it is possible for a sleeve to load with solids to the point where machine vibration becomes excessive.
A key application for this machine is at food processing plants with excessive suspended solids in their wastewater. Typically these processors either face a sewer surcharge or have spray field problems. While the dissolved solids cannot be captured, the Fiber Filter does an excellent job of removing suspended fiber.
Fiber Filters have been tested where solids content is extremely low. In one series of tests, capture rates of 75% were measured with feed solids of 1,970 ppm; 50% capture was achieved with 620 ppm feed; and a 25% capture rate with a feed of 204 ppm. This testing was conducted on effluent at a recycle (secondary fiber) paper mill.
A ready for market Fiber Filter exists where the flow to be filtered blinds existing static or rotary drum screens. These traditional screening devices are very economical to acquire and operate. However often they have been put into applications where they are only marginal performers. The Fiber Filter not only features non-blinding characteristics but it also offers finer filtration.
Increased capacity is obtained from a Fiber Filter by using existing sidehill and rotary drum screens as scalpers. The scalping action reduces the tonnage of solids going into the Fiber Filter, thus allowing either greater capacity or finer filtration.
The Fiber Filter was originally developed as a juice finisher. These machines are used to remove pulp from fruit juices prior to concentration in an evaporator. Removing the pulp reduces evaporator fouling. The Fiber Filter has gained its best acceptance as a juice finisher where it is able to replace a centrifuge or decanter.
Vincent Corporation, the designer and manufacturer of the Fiber Filter, is predominantly a producer of dewatering screw presses. Their interest in developing the machine arose primarily from a need to filter press liquor from a screw press. In many cases the press liquor is concentrated in evaporators. The principal products of this process are molasses and juice concentrate. The Fiber Filter has proven superior by offering finer filtration, reduced blinding, and/or reduced operating (maintenance) expense. Notable reductions in evaporator fouling have been achieved with its use.
It is noteworthy that sometimes a screw press is used to further dewater the sludge that is produce by the Fiber Filter. Generally this occurs when the Fiber Filter is used to separate suspended solids from a wastewater flow.
The solids sludge separated by a Fiber Filter will have a consistency in the range of 4% to 10%. When the liquid being filtered is press liquor from a screw press, the Fiber Filter sludge is generally re-admitted to the feed going to the screw press.
The primary adjustment of the machine is made by changing the inclination angle of the sleeve assembly. To filter either low gpm flows or feed flows with higher concentrations of suspended solids, the angle is kept low over the horizontal. In extreme cases the assembly in angled downward.
In contrast, for high gpm flow of dilute solids the sleeve assembly is angled upward as much as 30Âº. Increasing the elevation angle generally reduces flow capacity and results in higher solids content of the discharge sludge.
In almost all cases a good operating condition can be established by adjusting the elevation angle. In some cases a VFD (Variable Frequency Drive) is used to change the rotor speed. Changing the rotor rpm produces performance changes similar to those achieved by changing the elevation angle.
The Fiber Filter works best with constant gpm feed flow with constant suspended solids content. If the feed or solids vary, the machine is generally set for the most difficult condition. That way satisfactory performance is achieved over a range of flows and consistencies.
The liquid feed need not be pumped under pressure into the Fiber Filter. Gravity feed of liquid from an overhead tank or screw press has proven very satisfactory.
The Fiber Filter is supplied with a backflush system. Backflush is sometimes required to remove deposits from the outside of the filter sleeves. The system is actuated when the solids sludge discharge starts to become watery.
The backflush system consists of spray rings that periodically spray water (or caustic solution or filtrate) against the outside of the sleeves. This is done with the machine in operation. An air cylinder is used to cause the spray ring(s) to travel the full length of the filter sleeves. A timer panel is supplied which allows adjustment of the frequency of backflush. Frequently a booster pump is used to increase flush water pressure to 200 to 250 psi. Such pumps are turned on and off by the timer panel.
Because of the uniqueness of this machine, on-site testing is required of new applications. Laboratory testing is not practical because of the large amounts of feed liquid that are required. A fleet of demonstrator machines is maintained for field trials.
It is expected that the Fiber Filter will gain acceptance in many industries as its performance characteristics become better known and proven.