2.4 Dewatering

The removal of water content is a crucial step in sludge processing. It is accomplished through two stages: thickening and dewatering.

The primary objective of sludge dewatering is to

• Reduce transportation costs to the final disposal site,
• Improve sludge handling conditions regarding reuse and resource recovery,
• Increase the calorific value of the sludge by removing excess water before incineration,
• Reduce the volume of sludge for landfill disposal or land application,
• Minimise leachate production for land application and disposal.

Different types of intermolecular forces are responsible for water bonding to sludge solids, which can be classified into four distinct categories: free water, adsorbed water, capillary water, and cellular water.

Free water or gravitational water: This is the water that is loosely held in the sludge and can be easily removed through gravity or flotation methods. Gravity thickeners or drying beds are another example of free water removal, where water is lost rapidly through percolation.

Capillary water: This is the water that is held in the micropores of the sludge by capillary action, which is less than atmospheric pressure.

Adsorbed water or hygroscopic water: This is the water that forms a thin film around the soil particles and is tightly bound by electrostatic attraction.

Adsorbed and capillary water are more challenging to remove from sludge because they require greater forces. Chemical or mechanical methods can be used to separate water from the solids in sludge.

Cellular water is water inside cells in the sludge and can only be removed through thermal treatment or freezing. However, thermal drying is currently the most efficient method of eliminating cellular water.

Dewatering systems are used to process stabilised sludge. Let us look at the various dewatering methods.

Sludge Drying Beds

The most basic dewatering method is using drying beds, also known as sludge drying beds. These beds can remove liquid through evaporation and filtration of the liquid. However, one of the downsides of using drying beds is that they need a large land area to operate effectively. In cases with limited space, mechanical dewatering systems like centrifuges can be a viable alternative.

Sludge drying beds use a combination of draining and evaporation to remove water from sludge. The beds are constructed on the ground and can be permeable if there is no risk of infecting the underground water aquifers. However, if there is a risk of groundwater pollution, then a waterproof bed is built.

The suitability of drying beds depends on the climatic conditions and geographical location. For example, regions with low precipitation and short freezing periods are ideal for using drying beds throughout the year. In areas with high precipitation, it is recommended to have covered drying beds to protect against rainfall. The drying process is performed in batches, with the sludge being routed sequentially to multiple drying beds.

After dewatering, the sludge concentration is expected to range from 20 to 60% TS, depending on the residence time. Sludge conditioning is not mandatory, but adding lime can enhance the dewatering performance.

Sludge drying beds can be classified as sand, paved, artificial media, or vacuum-assisted drying.

Sand drying beds are rectangular and a conventional way of drying. A 23 – 38 cm sand layer (⌀0.3 – 0.75 mm; uniformity coefficient: < 4.0) is placed over 20 – 46 cm of gravel (⌀3 – 25 mm).

Paved drying beds are rectangular and constructed using pavement stones on the top of a 20 to 30-centimeter-thick sand or gravel base. These beds can improve the drying rate, simplify biosolids removal, and minimise bed maintenance. However, they usually require a larger area when compared to conventional sand drying beds.

Artificial media drying beds are typically made of stainless steel wedge wire or high-density polyurethane panels. The wedge wire bed is a narrow rectangular basin with a watertight structure. Sludge dewatering is achieved by introducing the sludge to the bed surface, allowing the sludge to float across the wedge wire surface. Once the bed fills up with sludge, the water enables the sludge to settle down and compress against the screen, serving as a filtration media. The water is now allowed to seep at a controlled rate. After the water drains, the sludge becomes more concentrated through drainage and evaporation, making removing it easier. The artificial media drying beds have several advantages, including no clogging, continuous and rapid drainage, and easy maintenance. Furthermore, polyurethane panel drying beds have the added benefit of low solids concentration in the filtrate, allowing for the dewatering of dilute sludge and smooth removal of sludge cakes.

Vacuum-assisted drying beds are a type of bed that enhances sludge dewatering by applying a vacuum at the bottom of porous filter plates. These beds are rectangular and have a concrete base covered with several millimetre-thick aggregate layers, which support a multimedia absorbent filter at the top. The aggregate layer is a vacuum chamber connected to a vacuum pump. To operate, polymer-preconditioned sludge is placed onto the bed and allowed to drain for approximately 1 hour. After that, the vacuum system starts working with a pressure of 34 – 84 kPa until the cake cracks and the vacuum is gone. Finally, the vacuum-dried sludge is kept for air drying for up to 2 days. The key advantages of vacuum-assisted drying beds are their short drying period, independence from weather, and their requirement for less area.

Planted Drying Beds

The reed beds are a modified version of conventional drying beds. They have an aquatic plantation at the bottom of the bed, making them more effective. The beds are rectangular with a top layer of sand 10 – 15 cm thick. This is followed by a layer of ⌀4-6 mm gravel that is 25 cm thick and a bottom layer of ⌀2 cm gravel that is 25 cm thick, overlaid with an under-drain system. A freeboard (the vertical distance between the surface of the sludge or wastewater in the drying bed and the top edge or rim of the bed) of 100 cm is recommended above the top sand layer.

The aquatic plantation helps with constant water drainage from the sludge. The roots of the plants provide a habitat for rich micro-flora, which effectively feeds on the organic matter present in sludge and transforms 97% of the organics to water and carbon dioxide, significantly reducing the volume of sludge. The solids loading rate designed for reed beds is typically 30-60 kg per meter square per year. The planted drying beds can operate for up to 10 years without removing accumulated residues.

Sludge Drying Lagoons

Sludge drying lagoons are an alternative option to drying beds for dewatering digested sludge. However, sludge is placed at depths three to four times greater than in a drying bed. It’s important to note that lagoons are unsuitable for dewatering untreated sludges due to their strong odour and potential to cause a nuisance.

Usually, drying lagoons are rectangular and enclosed by earthen dikes measuring 0.6 to 1.2 meters in height. The equipment includes sludge feed lines, supernatant decant lines, and mechanical sludge removal equipment. Unconditioned digested sludge is discharged to the lagoon in a way that accomplishes even sludge distribution. The sludge depths generally range from 0.75 to 1.5 meters. Dewatering occurs primarily through evaporation, with transpiration being a secondary factor.

Facilities are provided for decanting the supernatant, and the liquid is recycled to the treatment facility. The sludge is mechanically removed at a moisture content of approximately 70%. The time required for dewatering to a final solids content of 20% to 40% can take 3 to 12 months, depending on the climate and sludge depth.

Geotextiles

Geotextiles are highly durable and made from polypropylene fabric. They are designed to hold polymer-conditioned sludge, allowing water to filter through the tube wall while retaining fine-particle material. Due to the volume reduction within the container, these tubes are ideal for repetitive filling and dewatering cycles. Once the final cycle is complete, the retained fine particle materials consolidate by drying. This is because the remaining water vapour flows out through the geotextile. The dried sludge can be easily removed from the tube once the retained solids have achieved the desired dryness level. Geotextile offers numerous benefits, including enhanced dewatering, effective odour control, low suspended solids in effluent, and economical operation.

The sludge TS concentration is expected to be 20% or higher, depending on the residence time. Sludge conditioning is required.

Mechanical Methods

Raw or a mixture of raw and digested sludge can be dewatered using mechanical methods before thermochemical treatment (incineration or pyrolysis), burial or disposal in a landfill. Before mechanical dewatering, sludge conditioning is usually necessary. Following mechanical dewatering methods do exist:

Vacuum Filtration: The function of vacuum filtration is to decrease the water content of sludge, whether untreated, digested or elutriated so that the solid proportion increases. A vacuum filter comprises a cylindrical drum with a filtering medium of wool, cloth or felt, synthetic fibre, plastic or stainless-steel mesh or coil springs. The most used vacuum filters are rotary drum and spring coil types.

The sludge TS concentration is expected to range between 13 – 35%. The conditioning of sludge is required.

Pressure Filter Press: Dewatering of sludge in a pressure filter press is achieved by applying high pressure to force water out of the sludge. This was the first mechanical process developed to dewater digested sludge artificially, and it is becoming popular again due to automation. Filter presses can produce sludge cakes with a lower moisture content than vacuum filters.

Sludge dewatering has been achieved through various filter press designs. One popular design employs rectangular plates recessed on both sides to create chambers. These plates are then arranged face-to-face in a vertical position on a frame, with a movable and fixed head. Each plate is draped or fitted with a filter cloth. The plates are held together with enough force to withstand pressure during the filtration process, and this is achieved using hydraulic rams or powered screws.

Filter presses require several auxiliary equipment in addition to the filter press itself. These include feed pumps, a chemical sludge conditioning tank, and devices for handling the cake and cleaning the cloth. Periodic cleaning of the cloth is done with an acid.

The sludge TS concentration is expected to range between 15 – 50%, depending on the type of press being used. Sludge pre-conditioning is required and the polymer dosage can range between 2 – 4.5 g/kg of dry solids within the sludge.

Screw Press: Incoming sludge with a solid concentration of 1 – 2% is first sent to a flocculation tank, where a polymer is added to the feed sludge and then mixed in a static inline mixer. The flocculated sludge then spills over into an inclined screw, which rotates inside a stainless-steel wedge wire screen with a mesh size of 200 microns. As the sludge moves forward, the filtrate flows out through the screen, while the frictional pressure at the sludge/screen interface, combined with the improved pressure produced by the outlet restriction, forms the sludge cake. This dewatered sludge with a solid concentration of 20-25% is discharged onto a conveyor or directly into a collection bin. The screw press setup is made of stainless steel and is fully covered, which protects the unit from corrosion, keeps odour under control and provides good working condition. Moreover, the unit operation is fully automated, reducing operational costs compared to conventional methods.

The sludge TS concentration is expected to range between 18 – 20%. Sludge conditioning is required, and polymer dosage is considered as high as 15 g/kg of dry solids within the sludge.

Centrifugal Dewatering: The centrifugation process is widely used in various industries to separate liquids of different densities, thicken slurries, or remove solids. It is also used to reduce and dewater sludges. The efficiency of centrifuging is determined by its effect on solid recovery and cake moisture content for a given liquid feed rate or stable rate. Sludge dewatering can be achieved using solid-bowl conveyors and basket centrifuges.

Basket centrifuges are often preferred for partial dewatering at small plants and can concentrate and dewater the activated sludge without any chemical conditioning.

The sludge TS concentration is expected to be 20%. The sludge conditioning is required, and the polymer dosage is as low as 2 – 3 g/kg of dry solids within the sludge.

Mobile and Containerised Dewatering Systems

In recent years, there has been an increasing interest in using mobile dewatering trucks and containerised systems for managing small quantities of sludge for dewatering purposes.

Mobile systems operate by pumping septage from one septic tank into the truck where it is pushed through a filter that captures the solids and allows the liquids to pass through. A pump sends the liquids back into the septic tank through the vacuum hose. The sludge cake is also stored on the truck and is generally disposed of at the end of the day or the following morning.

Moos-KSA trucks are one such example of mobile dewatering systems and comprise two main parts: a truck chassis and a rectangular container. The container houses several tanks, including a storage tank for unconditioned sludge, a polymer tank, a dewatering tank, and a filtrate tank. The rectangular dewatering tank has a drainage system on both sidewalls, covered by a filter fabric. Additionally, the tank features a double-wall construction with filter cloth running along the centre of the tank.

Sludge that has been conditioned with polymer is pumped into the dewatering tank. The solid particles settle at the bottom of the tank, while the remaining liquid is filtered by gravity through the walls of the filter. The sludge cake can be removed by tilting the entire container and opening a gate in the back wall.

To dewater sludge to about 15% dry solids, a polymer dosage between 3 – 5 grams per kilogram of dry solids is required. The filtrate will contain low levels of suspended solids (100 – 300 mg/l).

Please visit the “Materials” tab for further information on the mentioned dewatering systems.