Wastewater Treatment via Dry Filter Aid/Flocculant Addition to Automatic Pressure Filters (APFs)

Conventional wastewater treatment (WWT) generally uses a three-step process of clarification (i.e. gravity settling), overflow polishing by sand filters (and in some cases bog or cartridge filters), and underflow sludge dewatering by either conventional filter presses or automatic pressure filters. However when flows are low (i.e. about 200 gpm or less, depending on solids leading) direct APF filtration in one-step is far more practical. In-line injection of filter aids and polymer flocculants have been proven successful, but polymer dosage has been problematic, (i.e. control by Streaming Current has been useful but troublesome because of probe fouling and operator inattention to cleaning).  As a result, direct batch treatment/flocculation with a dry mixture has been successful because dry weight feeders can accurately control the dosage.

A brief discussion of APFs and how they compare to traditional dewatering methods is necessary at this juncture to familiarize the reader as to exactly what an APF is and does.

Horizontal filter presses have been used for fine pharmaceutical, pigment, food, beverage, mining, etc. dewatering for over 100 years due their versatility excellent cake washing capability, high solids cakes, and efficient capture (or excellent filtrate quality).  However, despite this versatility, many cakes tend to be sticky and 100% cake release cannot be assured.  As a result, practically all filter press applications require operator attention during cake discharge.

APF Concept

Automatic pressure filter (APF) dewatering consists of horizontal plate(s) between which is sandwiched a filter medium; pump pressure is used to force the slurry into the plates and through the medium upon which the cake collects.  In concept, it is similar to the vertical filter presses (or automatic tower presses (ATPs) by Hoesch and Larox) with one key distinction; namely APFs can use disposable filter media much like a roll of toilet paper fed between the plates, whereas the ATPs use a reusable cloth belt that is wound around rollers, must be tracked, and must be washed on its return.  In addition because of their design, ATPs filter on both sides of the belt whereas APFs filter in one direction. ATPs also operate up to 225 psig whereas APFs primarily operate at 30-100 psig.  As a consequence, ATPs are mainly used to dewater/wash products whereas APFs are used primarily for wastes, machine tool coolants, aluminum can manufacturing, aluminum foil manufacturing etc.

APFs are quite simple, especially when used with disposable filter media, since the accumulated cake and media is disposed of each cycle.  As a result, belt washing is not required, cloth blinding is eliminated and cake release is assured.  Repeatable high filtration rates are also assured.  The spent media can be rewound for separate disposal, dumped along with the cake and can even be doctored, brushed or washed to remove residual cake debris.  A variety of media can be used depending on the application, but most importantly, its strength is unimportant since a base belt can be used for conveyance.  Extreme flexibility in filter media choice is basically why APFs can be used in a wide variety of applications.

Polymer flocculants and filter aids are traditionally used via in-line injection to these APFs used in WWT applications.  However, in-line polymer dosage control has been problematic, (i.e., control by Streaming Current has been troublesome because of probe fouling and operator inattention to cleaning).  As a result, direct batch treatment flocculation with a dry mixture has been successful because dry weight feeders can accurately control the dosage.

Five of these dry treatment systems have been successfully implemented within DuPont and some in other industries.  The only drawback of the new treatment technology is where the wastewater is totally recycled in the process, which can cause excessive flocculant buildup.

To learn more about this new technology, please attend AFSS’s Annual Technical Conference in Louisville, KY, May 9-12, 2011.