The Deskins process is based on principles of affordable technologies that exceed the performance outputs of other sludge drying techniques.  The patented “Quick-Dry” process provides a unique and inexpensive way to solve the old sludge drying bed problems.  With simple and easy modifications of existing beds, old non-productive beds can be upgraded to excellent dewatering filters.  The Deskins “Quick-Dry” Filter Bed System can be installed into new sewage and water treatment plan projects and expansions.  It can also be installed cost-effectively in existing sand drying beds.  While the Deskins Process is a suitable cost effective alternative process to conventional sand drying beds, it is also a highly cost effective solution when used instead of mechanical dewatering.

 The process focuses on the concept of rapid drainage of free water with the assistance of a dewatering polymer.  The free water is rapidly removed by gravity while further water is removed by natural evaporative processes.  Similar to the oldest dewatering concept (sand beds), the Deskins “Quick-Dry” Filter Bed embodies new, appropriate technologies, while changing the performance dramatically.

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 “Quick-Dry” Filter Beds

Historically, the use of drying beds to dewater wastewater sludge has been limited to small or medium sized plants of less than about 5 ML/d or about 25,000 ep.  The Deskins process pushes beyond these barriers due to the increased loading rates and improved sludge drying times.  More of the bed’s surface is available with the Deskins process due to the elimination of the need for concrete “driveways” while the bed is still able to support the weight of retrieval equipment.  Sewage treatment plants up to 32 ML/d capacity have been successfully fitted with the Deskins process and the process is not limited to this size if the site conditions are adequate.

 “Quick-Dry” Filter Bed consist of four major components:

1. The Deskins process utilizes an in-line polymer preparation system that “injects” the polymer into the flocculation device.  This technique eliminates the need for batching tanks, mixers and polymer transfer pumps.  The in-line system uses only the polymer needed and activates the sludge to the maximum performance.  The polymer accelerates the particle agglomeration, increasing the total amount of water that can be drained and reducing the amount of water that needs to be evaporated.

2. In-line liquid-solid separation is achieved with a new flocculation system (RapidFloc Mixer).  This new self-contained technology has produced savings of up to 40% in polymer usage.

3. The “Quick-Dry” Filter Bed typically consists of a layer of sand placed over a substratum of differing grades of gravel.  Using the Deskins “Quick-Dry” Media allows for 100 percent bed saturation within 10 minutes from the start of a pour.  If the Filter Bed is pre-loaded with effluent, saturation occurs immediately.  The bed’s ‘layering’ imitates the Earth’s natural geological processes of filtering of liquids.  Different layers of aggregates including sand and stone are stratified to produce the maximum efficiency of the filtering process. 
   Within this stratification, a stabilization layer consisting of a polyurethane or stainless steel cellular confinement system allows for physical stabilization of the media as well as the weight distribution needed when heavy automated sludge retrieval equipment such as the Deskins Skimmer/Loader is used.  This use o equipment to remove the sludge eliminates the need for manual sludge removal.  Normal sand drying beds without concrete “Running Strips” could not support such.  Heavy equipment and all sludge material must be removed with shovels or similar equipment.  The stabilization media also prevents compaction of the filtering media which often leads to poor drainage and binding of the sand.  Conventional sand bed designs do not address the issue of compaction. An underdrain system is used beneath the gravel layers.   This is a lateral network of perforated pipes that is used to collect the filtrate for recycle to the head of the plant.  The perimeter of the Filter Bed usually consists of concrete walls that retain the sludge.  The concrete walls need only be about 600mm in height (more with low concentration sludge’s).  Earthen walls in place of concrete walls may also be used.

4. The Deskins process utilizes a specialized, self-contained retrieval unit.  The sludge retriever sweeps the entire bed with adjustable metal tines with virtually no sand loss.  It also levels and aerates the bed for the next pour.

 Normal Operation

In order to facilitate even application of the sludge to the bed, with minimal disturbance of the bed’s surface, the Deskins “Quick-Dry” Filter Beds can be “pre-loaded” or saturated.  When saturating the bed before the sludge is applied, two additional important benefits occur;

1. Saturation allows the sludge to flow evenly across the Filter Bed surface achieving maximum distribution.  A manifold system is typically recommended to facilitate the even distribution.

2. Saturation of the Filter Bed forces the trapped air out of the filter media.  When the underdrain system is re-opened, a natural vacuum, or siphoning effect, facilitates the dewatering of the sludge.  With the assistance of pumps, the dewatering process can be accelerated.  Decreased drying time occurs, allowing for faster retrieval of the sludge.  Conventional sand drying beds are not pre-saturated.  Regardless of the size and type of substratum filter media, and its subsequent “voids,” filtrate moves more rapidly through the system (however, size and type of media will cause drain times to vary).  Liquids will move more quickly through a saturated system than an unsaturated one.  For instance, a sponge when saturated will allow liquid to pass through it quickly whereas a dry sponge will not allow water to pass though until it reaches its holding capacity and all or most voids are filled with liquid.

 An automated valve that is closed during the pour cycle should be located in a wet well.  After the pour cycle has been completed the valve should be opened allowing for rapid exiting of water contained in the underdrain and substratum.  About 90 percent of the freed water exits within 12 hours.  Sludge continues to dewater so that the percent solids expected should be 18 to 22 percent in 24 hours, 22 to 30 in 72 hours and up to 60 percent in 7 days, depending on the season.  Solids can normally be removed within 48 hours.

In a conventional sand drying bed, following the removal of the cake solids, the surface of the sand must be worked by hand to smooth it and prepare it for the next application of sludge.  The Deskins “Quick-Dry” Filter Bed eliminates this problem, through the use of its retrieval equipment.  Minimal disturbance of the surface is obtained through the use of the hydraulic leveling system.  The Deskins Skimmer/Loader can be set at a certain depth to remove a precise amount of sand to prepare the bed for resurfacing.  The Deskins Skimmer/Loader can be fitted with manual or laser accuracy devices for the leveling process.

 Operating Guidelines

In general, the physical size of the Filter Beds is based on the amount of sludge to be dried each year.  Conventional sand drying beds are loaded at rates of 50 to 270 kg/m² per year and primary sludge in the upper end of the range for conventional beds.  The loading rate for Deskins “Quick-Dry” Filter Beds is significantly higher than for sand beds.  The Deskins Process is designed for a nominal 520 kg/m² per year for all types of sludge.  Pump sizing for sludge transfer to the Filter Beds should be at least 12 litres per second for beds less than 200 m² and at least 25 litres per second for larger Filter Beds.

 With conventional sand drying beds many extra precautions have to be taken in order to obtain optimal operation.  Sludge retrieval should be done so as to remove as little of the sand media as possible and care should be taken to avoid compaction of the sand bed.  Heavy equipment should not be allowed on the bed.  Compaction of the bed will result in reduced drainage rates, longer drying times and may increase the potential for plugging.  The Deskins Process addresses all of these concerns in order to optimize the system and obtain better performance.

Using the Deskins process sludge retrieval is accomplished by the use of automated skimming/loading retrieval equipment.  The filter media doubles as a stabilizer and weight distribution system.  This allows for the use of equipment on the bed itself without disturbing the effectiveness of the bed.  A skimming/loading unit will skim sludge at a precise depth controlled by hydraulic variable speed conveyor and transfers the removed material to an attached hopper.  After the hopper is full, the unit is driven to a solids storage area where the hopper hydraulically clumps the solids.  Skimmer/loaders can remove up to 2,000 square metres per hour.  Filter Beds can be designed so the solids can be skimmed and conveyored to the side of the bed for storage.

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 Performance Factors

Design criteria, use and performance of “Quick-Dry” Filter Beds are affected by many different factors including sludge type, conditioning, sludge application rates and depths, dewatered sludge removal techniques, and climatic factors.

The type of sludge applied and the effectiveness of chemical conditioners will play a part in the degree of dewatering of the sludge and the operation of the filter beds.  The sludge must be adequately conditioned in order to flocculate the sludge solids and release the free water, since the majority of water is removed through the filter media and the drainage system.

The Deskins Process utilizes an in-line polymer preparation system that injects the polymer into a special flocculation device.  The typical problems of standard sand drying beds where polymer is used include, chemical overdosing, sand binding with unattached polymer, and large floc particulate matter that can settle too rapidly and also bind the media.  The Deskins flocculation system utilizes equipment that provides more complete and precise polymer contact with the sludge helping to eliminate many of these problems.

Treatment plants have little or no control over the characteristics of sludge, and no control over climatic conditions.  The do, however, have some control over the depth of sludge application and the removal techniques of dewatered sludge.

 Typical Performance

The final solids concentration varies based upon the climatic conditions and the time the sludge remains in the bed after the free water has drained through the sand.  The following table shows varying loading rates between conventional sand drying beds and the Deskins “Quick-Dry’ Filter Beds, assuming that both systems are using polymer conditioned sludge.

Perhaps the greatest benefit of the Deskins “Quick-Dry” Filter Bed is the reduced amount of time that the sludge remains on the bed itself.  This translates directly into reduced bed area needed to process the sludge.  The Deskins “Quick-Dry” Filter Bed requires about one-fourth of the space of a conventional sand drying bed.  The following tables show the differences in the retrieval times (the interval between application of sludge onto the bed and when it can be removed from the bed) and their corresponding cake percent solids after that specified amount of time, assuming both systems are using polymer conditioned sludge.  The table refers to US conditions.

Where polymer is not used to condition the sludge, most of the water is released from the sludge through evaporation.  This can increase the retrieval time significantly, lowering the annual loading rate achievable.

 It can be observed in the tables that due the shorter retrieval cycles, the annual loading rate on the Deskins Filter Bed will be much higher than on the conventional bed.  The loading rate per application of sludge on the Deskins Filter Bed (10 kg/m² per application) is similar to the loading rate per application for conventional drying beds (5-15 kg/m² per application).  It is the ability of the Deskins system to achieve so many cycles per year (more than one per week) that results in the attractively high annual loading rate and small footprint achievable by the Deskins Filter Beds.  The annual loading rate (kg/m² per year) is equal to loading rate per bed application multiplied by the annual number of applications of sludge as shown in the table below. 

Another important consideration is that on conventional sand drying bed, only about 60% of the bed’s surface area is available for sludge dewatering.  The other 40% is used up by concrete runners for the retrieval equipment to drive on.  The Deskins Filter Bed utilizes the entire surface area for sludge dewatering and allows sludge retrieval equipment to be driven on the entire Filter Bed surface.

19-Apr-1999 Technical Bulletin