Precoating of a pressure filter is accomplished by feeding a relatively small quantity of an appropriate material to the filter at a relatively high rate prior to initiating a filtration cycle.  The result is a very thin layer of the precoat material against the filter media so as to be between the media and the sludge being dewatered.


In a few very isolated cases, precoating of a filter is needed for satisfactory operation of the system.  In some cases the particle size of the sludge is extremely fine, averaging about 0.6 microns and excessive carry through of solids into the filtrate would occur in the absence of precoating even with the filter equipped with an extremely fine multi-filament filter media.


For the most part, however, precoating of a filter is done for the convenience of operating personnel.  This manifests itself in two different ways:

  1. By placing a layer of non-adhering type material between the filter cake and the plate, free discharge, that is discharge of the cake without outside assistance, is facilitated. If a filter will not release the cake rapidly at the end of the cycle, this not only creates an annoyance for the operator who must nudge the “stickers” loose, but also is very time consuming.  It does not take very many “stickers” in a large filter to extend the turn-around time between filtration cycles by 5 to 10 minutes and it does not take very many of the incidents before an operating cycle is lost from the operating day.


  1. By placing such a layer between the sludge and the filter media, a protection is provided to that media against grease and other materials that will blind the media and require its washing. Under normal circumstances, with most municipal sewage sludges, and where precoating is not practiced, it will be necessary to wash the filter every 50 to 60 operating cycles.  If a plant is operated one 24 hour per day basis, this is equivalent to a requirement for washing once per week.  Under the circumstances, it should not be necessary to wash a filter which has been properly precoated more than once every 2 to 3 months.  Washing of a filter, even with the best of automated washing systems, is a time consuming proposition requiring 6 to 12 hours for a large filter and this must be taken into consideration in operations schedules for the plant.




Precoat Material


Virtually any material which falls in the right particle size range and which is stable at the pressure and pH range involved may be used as a precoat material.  Some materials which have been used include diatomaceous earth, incinerator ash, ash from the coal fired boilers, cement kiln dust and leather buffing dust.  The required particle size for this service is not less than 10 microns or more than 200 microns with the majority of the particles in the 20 to 100 micron range.  Caution should be used where the sludge being dewatered is from a potable water treatment plant and where filtrate will be returned to the treatment plant for reprocessing as any material used for precoating must meet FDA requirements.


Precoat Dosage


The dosage of precoat material required to precoat a filter will vary depending on the particle density of the material being used.  For example, if ash from an incinerator is being used, the required dosage will be 10 lbs. per 100 square feet of filtration area.  If diatomaceous earth is being used, this will decrease to 5 lbs. per 10 square feet.


Precoat Rate


The minimum rate at which proper distribution of precoat material can be achieved is 0.35gpm/square foot of filtration area.  Keeping in mind that the filter will sometimes be in a dirty condition, the pump selected for precoat purposes should be capable of delivering this flow at a TDH of 160-180’.  This pump should have a curve that will not overload when the filter is clean, that is when head loss through the filter is only 40-60’.


Precoat Water


Water used for precoating a filter can normally be filtrate from the dewatering process or from another relatively clean source.  If taken from another source, it is imperative that the water not contain grease, oil or other similar material which might tend to blind the filter media.  The quantity of water required for precoating would be a minimum of 0.35gpm/square foot for a period of 6-7 minutes.  Minimum pressure required would be 70psig plus any piping losses and static head required to get it to the filter.  If supplied at a higher pressure, not requiring re-pumping to the filter, it is necessary to install a rate-of-flow controller ahead of the filter to throttle the flow to the desired level when the filter is clean.

The primary advantage for using filtrate for precoating purposes is that this does not increase the quantity of filtrate (water) that must be recycled to the treatment process or be otherwise disposed.  The principal disadvantage is that this water (filtrate) can be contaminated to the point where it cannot be used if a hole should occur in the filter media.  When this occurs, it becomes necessary to drain and clean out the filtrate tank and to refill this with water from another source.


Dry System-Description


Referring to Process Schematic Drawing #07-010538-00, operation of the precoat system is as follows:

  1. At the conclusion of the previous precoat cycle, valves PT1V5 (vent) and PT1V4 (drain) were opened. Inasmuch as the connection in the precoat tank drain valve PT1V4 is located at a predetermined level as required to provide the proper relationship of water to precoat material in that tank, opening of the vent valve and drain valve causes the level in the precoat tank to drain to the correct level.
  2. To initiate a precoat cycle, it is first necessary to feed the proper amount of precoat material into the precoat tank. This is accomplished as follows:
  3. Valve PT1V4 is closed, valve PT1V3 is opened, and the bin-activator, feeder and conveyor are started. The feeder is normally of the volumetric type and operation for a predetermined time as controlled by a timer provides the predetermined quantity of precoat material to the conveyor hence to the precoat tank.
  4. Upon conclusion of the supply cycle to the precoat tank as determined by the timer, the bin-activator and feeder shut off. Following a brief time delay to allow the conveyor to clear itself, the conveyor is shut off and valves PT1V3 and PT1V5 are closed, buttoning up the tank which now contains the proper amount of precoat material.
  5. The filter is then filled with water (filtrate). This is accomplished as follows:

The precoat pump is started, valves PT1V2 and PF1V4 are opened and valve PF1V8 is closed.  The function of valve PF1V8 is to prevent water from exiting below the elevation of the top filtrate eye (s) causing water to rise in the filter, displacing air from that unit.  At this point, water (filtrate) is pumped into the filter until it overflows from the top of the unit back into the filtrate tank and is recycled through the unit for a minute or two to insure that all air has been displaced.  This step is also normally controlled by a timer.

  1. Precoat material is now introduced into the filter as follows:

Valve PT1V1 is opened and valve PT1V2 is closed, diverting the flow being delivered by the precoat pump through the precoat tank.  It will be noted that, although the major portion of the flow is introduced into the bottom of the tank, a portion of this flow is diverted into the bottom of the tank.  A portion of this flow is diverted into the top of the tank through the drowning line.  Most precoat materials tend to be difficult to wet and introduction of a shower of water at the top of the tank is helpful in driving this material into suspension.  At the high rate of flow through the precoat tank, the contents of this tank are rapidly displaced into the filter.

  1. Flow is continued to be maintained through the filter as described in Step 4 above until the filter is ready to be put into filtration cycle. This is essential to keep the precoat material “plastered” against the filter media until it can be frozen into that position by a subsequent layer of sludge with its inherent pressure drop.  In the case of a fully automated system, steps 4 and 5 are considered as a single step and are controlled by a timer.
  2. As the filter goes into the filtration cycle, the precoat system is shut down by closing valve PF1V4, shutting down the precoat pump, closing valve PT1V1 and opening valves PT1V2, PT1V4 and PT1V5. Shortly after the filtration cycle has started, valve PF1V8 is opened.


On very long filters it is frequently necessary to balance the filter during the initial start-up of the system to achieve uniform distribution of precoat material on all plates of the filter.  This is accomplished using manual valves installed in the top filtrate exits at both ends of the filter, throttling them as required and leaving them in a set position to serve in effect as permanent orifices.