Potato Peel, Improved

August 1, 2008

A Model KP-10 screw press was recently put into service dewatering wastewater at a plant processing sweet potatoes and Russet (white) potatoes. All of the waste peel, starch, and water collects in a pit and then is pumped to the wastewater treatment area.

The waste first flows through a rotary drum screen, 24" in diameter by 48" long, with 0.020" diameter perforated screen. The helicoid flights attached to the inside of the screen are 2" high with 11" continuous pitch. The drum is angled upward at 15 degrees. Hot water flows through the backflush which runs continuously while the drum turns. The screen is also scrubbed with bristles, and it is washed once daily with a degreaser.

The sludge drops out of the rotary drum screen into the Vincent reaction conveyor. The reaction conveyor is essentially a trough with a special design ribbon-flighted auger that mixes hydrated lime [calcium hydroxide, Ca(OH)2] from the Vincent Doser with the sludge. The sludge and lime have a one minute reaction time at default speed.

During the reaction stage the sloppy, starchy sludge releases free water and turns into a wet, very press-able cake. Without proper reaction the waste peel is a sloppy, slimy mess that simply CANNOT be mechanically dewatered. When squeezed in one's fist, it oozes between one's fingers like raw egg yolk or mashed potatoes. The change in body is truly remarkable to observe. Note: Only the Russet potato waste required treatment with lime. The sweet potato waste dewatered fine by itself.

After reaction, the wet cake drops into the screw press where it is easily dewatered. Press cake from a recent start-up was measured as low as 65% moisture. This cake crumbled into a powdery pile as it dropped out of the press. Discharge cone pressure was left at 15 psi because at higher pressures the material would jam inside the press. At higher pressures the material continued reacting with the lime inside the press and would harden like cement. The spider bushing at the C-plate in this particular press created a bridging point where the over-reacted potato peel would jam.

Vincent sent three VFD's to individually control the lime doser, reaction conveyor, and screw press. One of Vincent's engineers was on-site to experiment with different lime dosages, mixing cycles, and press cycles. Lime dosage was critical for success. Without sufficient lime the material simply would not gain enough body to be pressed, even with a long (two minute) mixing time. Screen blinding was never an issue, and the reacted potato waste pressed very well at 60 Hz (18 rpm). Lime content seemed to have the biggest effect on how well the press dewatered. 3% lime by weight ended up being the magic number.

During trials the press was fed 1,500 pph reacted potato waste. It generated 500 pph press cake and 1000 pph press liquor. It was judged that the KP-10 press was running at no more than 20% capacity. (Two thirds of the spider bushing was plugged with cemented solids, and the press was definitely running under capacity.)

It is important to note that the press liquor was very high in starch, too high to be dumped straight to sewer. A hydrocyclone was used to pull the starches from the press liquor. This waste could be added back to the reaction conveyor.

The press cake will be offered to local farmers as feed.

Issue 201