Crown Press Overview
Updated: Sep 18, 2021
Benchtop Belt Filter Press
As I've discussed elsewhere in this blog, I have two key pieces of laboratory/field equipment that I use to evaluate a wastewater treatment system: 1. Oxygen uptake rate (OUR) and 2. Adenosine triphosphate (ATP) testing. And as great as these tests are, their focus is specifically on the evaluation of the biological system in the wastewater treatment plant. For sludge dewatering, using a belt filter press, I can now add another unique and impressive piece of equipment which is the Crown Press shown in the image below. You can download the user manual at the bottom of this page as a PDF file. And for step-by-step instructions on how to use this device go to Crown Press In Use.
The Crown Press is a laboratory scale (benchtop) belt filter press (BFP). With this press you obtain sludge feed (primary sludge, primary + waste activated sludge, or digested sludge) from a treatment plant which you then use to produce cake in the lab. In doing this testing you can evaluate different chemical conditioning approaches (typically polymer), different types of belts, and, in my first use of the Crown Press, for evaluating odor control strategies with a dewatered, post-lime stabilized, sludge.
True Benchtop Belt Filter Press
In the next image you can get a better sense of the Crown Press size. The unit is small, solidly-constructed, and heavier than you might guess. This is a rugged device! But, and this is a very important "but," you need to keep the filter cloths (there are two) or filter fabric, between which the conditioned sludge is squeezed, CLEAN!! I will tell you this is easier said than done but if you don't take the time the filter fabric will begin to foul and the pore plug, significantly reducing the validity and consistency of your results. Warm water with dish soap are key here, along with some bleach every now and then.
The following text is from the Crown Press Owner's Manual:
The Crown Press is an instrument designed to simulate the action of a sludge dewatering belt filter press. The Crown Press was developed for designers of press belts, operators of wastewater plants, and manufacturers of sludge dewatering conditioners. It permits rapid evaluation of conditioners and belt materials for a given application.
The press features a pair of filtration belts between which a pretreated, gravity drained sludge cake is placed. Tension force is applied to the belts, drawing the belts and the sludge cake around a static, curved surface, or crown, which exerts pressure on the sludge cake, much as the rollers do in a working press. The user records the filtrate volume as a function of tension applied to the belt.
Other unique features of the Crown Press include:
Tension applied to the belt material is measured in pounds. When divided by the belt width, the force on the sludge is in units of pounds per inch, the unit of measure used in the adjustment of belt tension on full-scale presses.
The tension force applied pulls the belt material and the sludge cake around a curved surface. In addition to creating pressure on the sludge cake, bending action shears the cake and opens micro-fissures along its surface. Bending the belt fabric opens capillaries that allow the filtrate to wick away from the sludge and further simulates the action of a full-scale press.
Tension on the belt may be controlled in a timed fashion, simulating the process seen with the series of rollers in a full-scale press.
The sludge cake, located between the pair of filter belts in the Crown Press is free to migrate at the outer edges as pressure is applied. The migration causes the sludge to contact open belt material that allows further dewatering of the cake. The degree of migration can be measured; important information which cannot be collected with other laboratory presses.
Pressure generated filtrate is free to flow across both the top and bottom belt faces; a feature no other laboratory press offers.
The Crown Press permits easy exchange of belt fabric so a wide range of fabrics may be readily tested.
The graphic below is an illustration highlighting the major features of the Crown Press.
The maximum tension that may be read is 300 pounds. The nominal width of the Crown Press belt is six inches such that the nominal maximum tension is 50 pounds per inch.
The Crown Press Gravity Drainage and Plow Simulator kit is shown in the illustration below, scanned from the Crown Press Owner's Manual. The plow simulator, shown to the right in the illustration, works very well, allowing you to turn the sludge over for better gravity drainage, just as the plows on the gravity belt of a three-belt belt filter press would do. While you are slowly rotating the plow, gradually lowering it further into the sludge, you can easily monitor if you apply too much energy by observing the filtrate that is being collected. Any increase in turbidity lets you know you need to slow down the rotation and/or immersion of the plow.
Four Critical BFP Operating Zones
In a belt filter press there are four critical process or operating zones, each of which can be a limiting factor in producing the best (driest) sludge cake. The illustration below is a scan from the Crown Press Owner's Manual. It is up to the user of the press how the sludge feed is to be conditioned. Any combination of chemistries can be applied. In our first use of the Crown Press we used a high charge, high density, crosslinked polymer to condition the primary + waste activated feed sludge for use in the press.
The goal in Zone 1 is to condition the feed sludge to achieve maximum gravity drainage. A failure in Zone 1, due to improper chemical conditioning, is indicated by poor gravity drainage, leaving a lot of water entrained in the sludge, easily monitored by observing the flow of filtrate through the funnel (see illustration directly above).
In Zone 2, failure is indicated when the sludge is applied to the belts containing an excess of free water, following the gravity drainage step in Zone 1. The extra water increases the height of the developing cake, lubricating the sludge, which can lead to migration of the sludge off the edge of the belts (soft migration) creating housekeeping problems and inefficient solids capture.
In Zone 3, the compression section of the Crown Press, the applied pressure further squeezes the sludge to the edge of the belts, referred to as hard migration, representing another failure point. Hard migration, which results in reduced capture efficiency, is generally considered to be less severe than soft migration. But with hard migration the sludge cake formed at the outer edge of the belts will be wetter than the cake produced at the center of the belts, producing a wetter sludge than is optimal for the dewatering device.
In Zone 3, the high-pressure section of the Crown Press, the sludge is undergoing compression, similar to when it is being squeezed by the rollers on a belt filter press. A properly conditioned sludge, applied without overloading the press, allows the sludge to migrate to the sides of the belts, but not past the edges of the belts, maximizing solids capture and producing the driest possible sludge cake.
In Zone 4, pressure is taken off the belts and you are now evaluating cake release. Failure in this zone can be either by extrusion of the sludge into the weave of the belts or failure of the cake to release from the belts. Extruded solids will result in dirty belts that will reduce gravity drainage during the Zone 2 operation of the press.
You can download the user manual here. I had to read this three times before I felt comfortable using the press for the first time.