DEWATERING FOR BIG AND SMALL

Here is my long-awaited post on dewatering.

To paraphrase Alfred P Sloan, former head of Government Motors, there are dewatering devices "for every purse and purpose"  There are dewatering devices that are useful at package plants, and others that are useful at plants as big as 1 BGD (billion gallons per day).  Lets look at some.

I have yet to receive any figures for vendors.  I will do what I can.


GEOTUBES

A geotube is a special material that passes free water and leaves the solids behind.  It is a tube (go figure) most often used to dewater silt from lagoons and as an erosion barrier, but it has sludge dewatering uses too.  The tube is closed at both ends.

Sludge is pumped to the tube.  polymer is injected just before entering the tube.  The polymer coagulates the solids and separates them from the water.  free water drains through the tube.  Sludge is pumped in until the tube is full.  Water continues to drain away.  When drainage is complete, further dewatering occurs by leaving the tube at the site for dewatering by evaporation.  At some sites geotubes are stacked together for air drying.
When it is time to remove the sludge the tube is slit and the dewatered sludge removed by a front end loader.  In some cases the tube is placed in a roll-off container prior to use and the sludge, tube and all taken away to a landfill.

The dewatered sludge can be landfilled or land applied.  Most of the material is landfilled.  Cake solids range from mid teens to mid 20s percent. 
The advantages of geotubes are that they are

• relatively inexpensive
• can be used at small, medium and large sized plants
• are easy to use

They are a bit labor intensive for sludge removal and cake solids can be limited.  Collecting all the run off water can be a problem and the EPA does not want this water flowing into the waters of the state.
My friend Doug Brookhart likes these for small and package plants and says that they have been a godsend at some places.

IMPROVED SAND DRYING BED TECHNIQUES

Although sand drying beds are severely limited by the weather, a few simple and inexpensive improvement, and a different operating technique (free) can increase greatly the number of times the bed can be cycled. These improvements and techniques are:

Stir up the Sand Before Each Pour

Drying beds remove water by percolation (drainage) and evaporation. Percolation is rapid. Evaporation is slow and absolutely not controlled by the operator. Increasing percolation greatly speeds up sludge drying.

Stirring the drying bed before pouring a bed :

• Breaks up any crusts in the sand that would blind the bed
• Fluffs up the bed. This increases air voids in the sand, which increases the filtration rate

If the maintenance man or someone at the city garage is handy, a rake that will fit on the forks or toothed bucket of a tractor or bobcat loader that will rake the sand to a depth of 1-2 feet can be made from scrap. The tines should be 2-3 inches apart

Pour Shallow Beds

Evaporation only occurs at sludge surfaces. Sludge in the middle of the pour does not dry until air reaches it when the sludge cracks. The sooner air can get to the sludge the quicker it can dry. By pouring a shallow (10 inch) bed, the bed will crack sooner and allow all the sludge to dry.

Take it Out Wet if You Must. That’s Why Those Concrete Strips are in the Bed

Today’s drying beds usually have concrete strips for the tractor or bobcat tires so the bed can be mechanically cleaned. The operator is not shoveling the wet sludge into a wheelbarrow as in the bad old days. The wet sludge can be stockpiled, and the bed fluffed up with a rake

Don’t Be Afraid to Take Out Some Sand

Some operators wait until the sludge is very dry to avoid losing sand. DRYING BED SAND COSTS $10/TON (ABOUT $27/CUBIC YARD). Get the sludge off the bed and put on some new sand. Sand is cheap.

How Well Does It Work?

These small improvements work quite well, At a small plant in northeast Indiana aerobically digested sludge at about 1.5% TS is poured onto a prepared bed about 10” deep. The sludge will crack within 4 days if the weather is dry.

WEDGEWIRE BEDS

Wedgewire beds are made of plastic tiles with small slotted openings (figure 1). Sludge, conditioned with polymer is poured onto the beds. The polymer binds the sludge solids together, releasing free water, which drains through the tiles. When the bed is full, the sludge is left to set for a few days to continue dewatering. The dried sludge is taken off the tiles with a small tractor and taken to a stockpile. The cake solids depend on the sludge, but range from 15-22% TS.

The beds can be loaded at 2 lbs/sq.ft. for aerobically digested sludge, and 4 lbs/sq. ft. for anaerobically digested sludge.

The drying beds come as a package and include the tiles, a lightweight building to cover the beds (sometimes the building is translucent to speed drying), and a polymer makeup and dosing system. The beds are priced at $50-55/square foot, plus $7500 for the polymer feed system.

The advantages of wedgewire beds are:

• Simple technology. Not much can go wrong.
• Easy to size, price and engineer
• Fair to good solids capture.
• Relatively inexpensive to build. NOTE: the cost depends upon how often the beds are to be cleaned. For example, if the beds are sized to be used twice/week, the cost is about half that of beds sized to be cleaned once/week
• Cake that can be handled with a tractor and a bucket. NOTE: bobcat loaders are not recommended, as the steering system of braking the back wheels is too stressful for the tiles and tends to pop them out

The disadvantages of the wedgewire beds are:

Polymer costs may be high

• Labor intensive. The superintendent at Ada Ohio estimates that is takes 8 man-hours to pour and clean a 70’ x 20’ bed that dewaters 20,000 gallons of sludge for each pour. (But at Upper Sandusky, OH, it only takes 4 hours).

LIGHT DUTY BELT PRESS

The light duty belt press is a one-belt press with an extended gravity zone and one or two nip rollers to provide the pressure for dewatering (figure 3). The units are generally sold skid mounted and ready to operate with a polymer system, flocculation tank, sludge feed pump, washwater pump, sludge conveyor or cake pump and a control panel. 

The press will produce cake solids at 14-21% TS depending upon the type of sludge dewatered. Anaerobically digested sludge generally dewaters to higher cake solids content, but may not be as easy to handle in the field.

The advantages of a light-duty belt press are:

• Comes ready to use. No assembly required except connection to sludge, water and power
• Modest capital cost ($80,000 for 1.5 meter unit and 33 gpm capacity)
• Easy to use. NOT labor intensive
• Compact: Can fit in a space of a 1-car garage.
• Good cake solids: 13-21% depending upon the sludge
• Good solids capture
• Fair polymer consumption

The disadvantages of a light-duty belt press are:

• Higher capital cost, especially when the building cost is added in
• Noisy
• Lower cake solids that a 2-belt filter press
• Low output: about 33 gpm for 1.5-meter unit (One manufacturer has a unit with a drum pre-thickener ahead of the gravity drainage zone who claims an output of 100 gpm, but there are not many of these units sold).

REGULAR (2 BELT) BELT PRESSES

A regular belt press has 2 belts and come in widths ranging from 0.5 mdters to 3 meters.  It is possible to get these machines skid mounted, with the machine, polymer system, control panel and sludge feed pump on one skid.  The most common size is the 2.2 meter width.  Machine throughput ranges from 50 gpm to 300 gpm through these size ranges.  Throughput for the common 2.2 meter machine can range from 50 gpm to 180 gpm depending upon sludge type, condition and operator philosophy.

Belt press cake solids depend upon the type of sludge dewatered.  Aerobically digested sludges will range from 14-22%  Anaerobically digested sludge ranges from 18 to 30%, and raw sludge ranges from 20 to 32%

The advantages of a belt press are:

• Moderate price and fair ot good cake solids
• lower polymer dose
• easy to clean
• the operator can watch the sludge on the machine and make adjustments based on his observations
• relatively low maintenance requirements and costs
• low power costs

The disadvantages of the belt press are:

•  Odors, especially with anaerobically digested sludge and raw sludge.  Sometimes the machine is completely enclosed due to odors.
• Cake solids are not as high as a centrifuge
• High water rrequirements 
• Operators tend to turn down the sludge flow and turn up the polymer so they don't have to do as much work I have over 20 years experience observing this so don't complain that I am a meanie

SCREW PRESSES

A screw press is usually an auger inside a taperd screen.  sludge mixed iwth polymer is fed into the machine.  As the sludge passes thru the machinewater drains out through the screen.  The screen's taper provides osme pressure to aid dewatering.  This type of machine is very common for dewatering cow manure, both raw and digested, and works well bacause there is much undigested vegetable matter to provide structure and prevent solids from leading out through the screen.  New polymers called cross linked polymers have allowed these machines to be used for municipal sludges.

The advantages of these machines are

• Moderate equipment cost
• Low operator attention
• cake solids comparable to a belt press
• no washwater required
• less odor than a belt press
• Low power requirements

The disadvantgages are:

• lower throughputs than some othr methods
• higher polymer cost than a belt press, about double the cost

North Baltimore has a type of screw press if you want to go look at one


ROTARY PRESS

Rotary presses are interesting to my feeble mind.  They are sort of a cross bwtween a hose pump and a screw press and a geotube.The press is hose that allows water to pass.  The hose is wrapped in a u-shape sround a cam.  Sludge and polymer are pumped into the machine.  The can revolves slowly, pressing sludge into the hose, and flushing water out of the sludge and moving it through the hose, where it is discharged at the end

Photo Thanks to Fournier

These presses are getting popular, especially beause the manufaturers have convinced owners that they can be run unattended 24 hrs/day

The advantages of this machine are

• Fair to great cake solds.   great for ATAD and primary sludge, about like a belt press fro WAS and aerobically digested sludge
• Low power requirements
• quiet
• Low maintenance
• Medium polymer requirements
• Can be run round the clock without attention

the disadvantages are

• Low throughput/dollar cost
• Each channel can dewater about 50 gpm For large plants the presses canbe ganged, but the equipment costs are high

CENTRIFUGES

A centrifuge is a spinning cylindrical bowl that uses centrifugal force to separate water and solids.  Sludege mixed with polymer is injected into the bowl.  The solids migrate to the outside, where they are removes by a screw called a scroll.  The liquid passes through a weir and leave the bowl.

The centrifuge is the machine to have if maximum cake solids is important.  Centrifuge cake solids will be 5-10% drier than a belt press. The centrifuge has several advantages over other mahcines, including

• The driest cake solids
• No odors or aerosols
• unattended operation,including start-up and shutdown
• vey good solids capture, up to 99.5% (this may require and excessive polymer dose)

And of course some disadvantages

• Higher capital cost
• Polymer dose 150-200% of a belt press, but equal to some other forms of dewatering
• Noise - the machines rotate at 2000+ rpm
• Very high wear when dewatering gritty sludge
• For some applications, like lime stabilization and multiple hearth furnace operation, it is possible to get the sludge too dry unless the operator watches the process.

CONCLUSION

I suppose I should make up a table  I'llge tto that later

Which type of dewatering device is best for you?  It all depends upon the type of sludge you have, how much money you  have and how dry you deen it to be and how often you have to (want to) dewater.