Filter Press Pilot Testing Guidelines




The goal of filter press pilot testing is to collect the data required to accurately size a full-scale filter press system.


Typical data collected includes cake solids, cake density, total processing time, if desired filtrate suspended solids, slurry feed solids, slurry pH, actual chemical conditioning dosages, and maximum operating pressure.  Depending on the process requirements the total processing time may be further subdivided into steps such as filtration, cake wash, final squeeze, and cake air blow.


Description of Equipment and Outline of the Test


There are two pilot filter presses used for pilot testing the HPL300 and HPL470/500.  Both are “sidebar” filter presses with a manual hydraulic pump.  The filter press consists of a frame, hydraulic system, filter plates with filter cloths.  The HPL300 uses 300 mm x 300 mm filter plates while the HPL470/500 can use either 470 mm x 470 mm filter plates (using an adapter plate) or 500 mm x 500 mm filter plates.  (The 470 mm x 470 mm plate size is more common in the US while the 500 mm x 500 mm is more common in Europe.)  Other equipment needed for a test includes pumps, and mixing tanks with mixers.


There are two basic filter press configurations: recessed chamber and membrane.  Both pilot presses are suitable for recessed chamber testing with choice of which press to use based on the amount of sludge available and number of tests required.  A recessed chamber test requires the filter press, feed pump(s), and mixing tank with mixer.  Only the HPL470/500 press is suitable for membrane tests because the HPL300 membrane tests do not scale up very accurately.  A membrane test requires all of the equipment needed for recessed chamber test plus a source of compressed gas for membrane squeeze.


The basic test starts with preparing the sludge or slurry as required for the test and preparing the filter press for the test (installing the correct plates and filter cloth).  Once the press and sample are ready the pump is turned on and the press is filled.  When the press is filled filtrate will be seen and we start timing and taking filtrate samples.  The test is continued until both the low flow and maximum pressure conditions have been reached for filtration.  For a recessed chamber test the test is ended.  For a membrane test it is continued with the optional cake wash, membrane squeeze, and optional air blow down.  Once the test is finished the press is opened and the filter cake is removed and cake samples are taken.  During the cake release it is important to note how the cake released and the overall surface condition of the filter cloth.  Cake release is normally somewhat better on a larger press but if there is poor release on the test unit the release on the larger unit will be fairly poor.


Filter Cloths


There are numerous filter cloths available on the market.  Each cloth “style” has a characteristic weave pattern, particular type of yarn (thread), surface finish, porosity, thread count in each direction, weight, and material of construction.  Our preference to use as “open” (higher porosity) a filter cloth as we can since the bulk of the filtration is done by the cake and not the cloth.  On most sludges, we prefer relatively open monofilament fabrics because of their better cake release characteristics.  On other applications we will use “tighter” less porous fabrics.  Cloth tightness increases in this general order: monofilament, mono/multifilament fabrics, multifilament fabrics, multi/spun fabrics, felts, and special fabrics.  The cloth material will determine the maximum tightness for the cloth; nylon cloths can not be made as tight as a polypropylene cloth of the same type.


The two criteria for selecting a cloth are the initial quality of the filtrate, assuming that the sludge is properly conditioned, and cake release.  On process applications it is common to accept somewhat poor cake release for improve initial filtrate quality.  For most waste applications it is common to accept slightly dirty initial filtrate for improved cake release.  It should be noted that both experience and trial and error are used to select cloth.


If one is uncertain what the proper cloth for the application is, a cloth manufacturer should be consulted for suggestions to try.




The chemicals used for chemical conditioning are determined by the Baroid and CST tests.


Ferric chloride is usually obtained as a solution.

Lime is usually slurried at about 10% w/w concentration.

Polymer solutions usually have concentrations of about 0.1 – 0.25%


Other chemicals are usually added as solutions or slurries.



Detailed Procedure


Slurry and Sludge Preparation


  1. If testing a slurry, attempt to mimic the expected conditions as closely as possible.
  2. If adding ferric chloride and lime to sludge, use the dosages determined by the Baroid test.  Add the ferric chloride first and mix well then add the lime and mix well.
  3. If using polymer first make up the polymer solution and allow to age per the manufacturer’s recommendations.  If batch addition is used add the polymer and gently mix to avoid shearing the floc and occasionally gently mix to prevent excessive settling.  If inline injection is used the polymer will be added on the discharge side of the feed pump using another pump.  With inline injection the pump flow rates of both pumps must be calibrated to maintain proper dosing.


Recessed Chamber Test


  1. Install the cloth on the filter plates and put the plates in the filter press frame then close the press and pressurize the hydraulic cylinder to the proper pressure.  (300 bar for the HPL300 and 420 bar for the HPL470/500)
  2. Calibrate the pumps as required per manufacturer’s instructions.
  3. Close the press and make sure the closing hydraulic pressure was reached before tightening the locking ring.  Close the bottom filtrate valves and open the top filtrate valves.  The air blow inlet valves and wash water inlet valves are closed.
  4. Prepare the slurry as required with chemical conditioners or body feeds.
  5. Start the slurry/sludge feed pump
  6. After the press has filled, approximately 3 – 5 minutes, filtrate will be seen coming out the filtrate header.  Start timing the run and collecting the filtrate.
  7. At the designated intervals note the filtrate volume collected during the interval and pressure.
  8. Depending on the slurry concentration open the bottom filtrate valves from 0 – 6 minutes into the run.  Very concentrated slurries (above 50%) should be opened immediately and slurries and sludges with concentrations up to about 8 – 10% should wait till 6 minutes.  This is to insure that there is a good cake layer on all the filter cloth.
  9. Continue the run until the maximum pressure has been reached and the flow rate has reached 10 – 15 l/m2-hr.  At this point the press is full and the run is over.  The actual flow rate is determined by the total filtration area of the filter press.
  10. Turn off the pumps and shut all pump suction valves to prevent siphoning through the pump.  Open the slurry drain valve slowly to vent the pressure on the press.  Wait until the pressure is 0 psig before opening the press.
  11. When the pressure is 0 psig and with the slurry drain valve still open, open the press and drop the cake.  Normally when dropping the cake each cake is removed to weighed and samples take for cake solids, cake density, cake thickness and any other analysis required by the customer.


While removing the cake observe the cake release and describe as follows:


Drop               Cake drops unassisted from the cloth.

Excellent        Cake does not stick to the cloth but the dewatered core prevents unassisted release

Good              Cake releases with minor assistance

Fair                 Cake releases with assistance

Poor                Cake must be either partially or totally scrapped off the cloth.


Membrane Test


  1. The membrane test initially follows the Recessed Chamber Test procedure steps 1 – 8 when using the center feed membrane plates.  If using the corner feed membrane plates, none of the filtrate valves are closed and the slurry feed is on the upper right hand corner of the headstand, facing the headstand.  Note: the membrane hoses are not attached until later.
  2. Filtrate is continued until the maximum filtration pressure is reached and the flow rate has reached 30 – 40 l/m2-hr.  At this point the feed pumps are shut off and the isolation valve is closed.
  3. The membrane hoses are attached to the plates and the plates are inflated at rate of about 50 psig/min.
  4. If cake washing is to be done the membranes are inflated to pressure of 50 – 60 psig and all the filtrate valves except the upper left valve are closed and the wash water valve is opened.  Washing is done until for several minutes with filtrate samples being taken to determine when the cake is properly washed.  Note the wash water pressure should be about 70 psig or higher.
  5. With all the filtrate valves open, continue membrane inflation until the maximum squeeze pressure is reached and maintain the pressure until the filtrate flow has reached 5 – 8 l/m2-hr.
  6. If an air blow is done, all the filtrate valves, except the lower right, are closed and the air inlet valve is opened.
  7. Once the cycle is over, turn of the gas used to inflate the membranes, vent the membranes, and as described for the Recessed Chamber Test shut any pump suction valves to prevent siphoning, open the drain and isolation valves and allow the slurry/sludge to drain.
  8. Once the pressure has been relieved for both the membranes and the slurry inlet, the press is ready to open and for cake release to proceed as described in Step 13 of the Recessed Chamber Test.