June 24, 2016
Factories which produce soluble or instant coffee process their spent coffee grounds waste through screw presses. In most cases this is done so that the press cake can be used as boiler fuel.
Disposing of the press liquor presents a challenge. Not only are there fine particles which want to settle out, but there are trace amounts of oil which can cause upsets in the wastewater treatment plant (WWTP). We have seen a variety of ways in which these problems are solved.
The simplest process we have seen has a tank under the screw press, with a screened basket hanging over the inlet. Fines are caught in the basket, which operators empty periodically. The screened press liquor overflows from the tank into a floor drain.
Another simple system drains the press liquor into a tank mounted under the screw press. Natural gravity decanting takes place, allowing the coffee particles to settle to the bottom of the tank. There is an overflow drain at the top of the tank which allows clarified liquid to flow to a floor drain. The tank itself is mounted on a cart so that the tank can be pulled out from under the press once a week. The fines collected in the tank are shoveled out by hand.
We also see outdoor pits being used. The press liquor is allowed to flow into a pit, where the coffee particles settle out. Sometimes there are several pits in series, with clarified liquid flowing sequentially from one to another. The pits are shoveled out periodically.
Several processors in the States have made use of DAF (Dissolved Air Flotation) systems. The press liquor is filtered with juice finishers, static screens, or even decanters. The filtered liquid is then mixed with the normal plant effluent for DAF treatment.
In larger coffee plants a far more sophisticated system is used. The press liquor flows into a tank with an air agitator. The liquor is pumped from this tank up to a bank of drum screens. The particles of coffee separated by these screens are allowed to fall back down into the screw presses. The liquid from the screens is pumped to tricanters. The tricanters separate the flow into blackwater, coffee fines, and oil.
A modified system we recommended recently has the press liquor falling into a first tank where gravity decanting separates a lot of the coffee fines. A small progressive cavity pump is used to draw the fines from the bottom of this tank and pump them back to the inlet of the screw press. The clarified liquid from this first tank overflows into a second tank. The liquid from this second tank is pumped to an existing decanter which separates remaining solids before the liquid is sent to the wastewater treatment plant.
These same concepts can be used in a number of other applications where the fines which come through the screen sink because they are heavy.