DEGRITTING
DEWATERING
Degritting is a crucial preliminary process in wastewater treatment that involves the removal of heavy, inorganic materials, often referred to as “grit”, from sewage or sludge. This grit typically includes:
- Sand
- Gravel
- Cinders
- Eggshells
- Other non-biodegradable, heavy particles
The primary goal of degritting is to protect downstream equipment, such as pumps, pipes, and other mechanical machinery, from abrasion and excessive wear caused by these abrasive materials. It also prevents the accumulation of inert solids in tanks and digesters, which would reduce their effective volume and efficiency, requiring costly cleaning.
Degritting usually takes place in specialized structures called grit chambers, or grit channels (sometimes using vortex systems or cyclones). These chambers are designed to slow down the flow of the wastewater just enough to allow the heavier grit particles to settle to the bottom, while lighter organic solids remain suspended and continue through the treatment process.
Municipal and industrial wastewater treatment plants use degritting as a preliminary step to remove inorganic solids that enter the system.
Factories that produce wastewater with high sand or gravel content, such as those in the food and beverage or pulp and paper industries, use degritting systems to treat their effluent.
The mining and mineral processing industry was the first to use hydro-cyclones and screw classifiers for degritting. Mineral processing, especially gold mining, relies on degritting spirals to separate inorganic minerals from slurries, which improves processing efficiency and reduces operational costs.
The construction industry reuses processed grit, particularly from municipal wastewater treatment. The mineral-rich grit is washed and dewatered, with the reclaimed sand and gravel sold for construction or landfilling.
Street and sewer maintenance removes grit and mineral wastes collected from road sweepings, sewer cleaning, and gully emptying. These degritting systems remove inorganic solids from the waste, which can then be properly disposed of or reused.
In the oil and gas industry, degritting is a crucial part of the broader solids control and processing systems. Sand, silt and other solid particles must be removed to protect downstream equipment and maintain the quality of the fluids.
- Solids Control in Drilling Fluids (Mud): During drilling operations, the drilling fluid (mud) brings rock cuttings, sand, and silt to the surface. Degritting equipment (like desanders and desilters, which use hydro-cyclones) removes these solids to:
- Protect Drilling Equipment: Minimize abrasive wear and tear on pumps, pipes, and other circulating system components.
- Recycle Drilling Fluid: Maintain the properties and usable life of the expensive drilling fluid, reducing operational costs and waste.
- Protecting Production Equipment: As oil and gas are produced from a reservoir, they often bring along sand and other sediments, particularly from wells with unconsolidated formations. Sand separators are used to remove these particulates from the well stream
- Treating Produced Water and Sludge: Solids must be removed from produced water (the water brought up with the oil and gas) before it can be re-injected or disposed of. Furthermore, in refineries and processing plants, degritting helps in sludge treatment processes to recover valuable products and prevent damage to dewatering and filtration equipment.
Degritting in the wastewater treatment process, is a significant source of odor. Grit contains high organic matter, and the settled grit is never perfectly clean. It could contain trapped and absorbed putrescible organic solids from the raw sewage. The materials in raw sewage, particularly in the initial screening and degritting stages, are highly prone to anaerobic decomposition (breaking down in the absence of oxygen). This decomposition releases highly odorous gases, most notably Hydrogen Sulfide (H2S), as well as mercaptans and amines. Degritting equipment and the disposal areas for the removed grit (unless the grit is thoroughly washed) are typically located at the headworks of the treatment plant, often near the facility boundary or nearby communities. This proximity makes odor control essential to prevent public complaints. Modern sludge and grit removal systems are often designed to be enclosed and may include features like continuous grit washing to reduce the organic content of the final grit product. This cleaner grit has a lower volatile solids (VS) content, which significantly reduces the potential for odor compared to unwashed grit. Even with cleaner grit however, the process area itself often requires odor management.
Many larger wastewater treatment plants produce biogas (a mixture primarily of methane and CO2) through anaerobic digestion of sludge. This biogas is often used as a fuel source (like natural gas) for combined heat and power (CHP) units or boilers at the plant. In a small number of odor control applications, a highly concentrated odorous air stream (or gas stream like raw biogas) can be treated by thermal oxidation (incineration) to destroy the odorous compounds. This process would require a fuel source, which could be the plant’s own biogas or, conceivably natural gas, but this is less common for general degritting air compared to other vapor-phase methods.
Dewatering is the process of removing water from a material. It is a solid-liquid separation process used across various industries, including municipal wastewater treatment, construction, mining, oil and gas, industrial manufacturing and food and beverage industries.
In wastewater treatment, mining, and other industrial processes, dewatering involves separating residual liquid from a solid material, such as sludge, filter cake, or mineral concentrate. The purpose is to reduce volume and the weight, making disposal transport more cost-effective. Municipal dewatering is driven by the need to manage and process sludge from public wastewater treatment plants, which requires robust solutions for managing process water, tailings, and dewatering mine shafts for safety and operational efficiency. In the food and beverage sector, liquids are separated from waste or byproducts like spent grains or fruit pulp.
Common methods of dewatering in the industrial and wastewater industries include centrifugation, filtration and drying beds.
- Centrifugation: Using high-speed rotation to separate solids from liquid based on density.
- Belt Filter Presses: Uses recessed-chamber filter plates and high pressure to squeeze liquid out through a filter cloth, leaving a filter cake.
- Plate and Frame Filter Presses: Using pressure to squeeze liquid out of a slurry, leaving a solid cake.
- Screw Presses: Uses a helical screw with decreasing pitch to compress the material and squeeze out liquid.
- Drying Beds: A simpler, passive method where sludge is allowed to dry by evaporation and drainage.
- Stringent Environmental Regulations: Governments globally are implementing stricter rules regarding the disposal of wastewater, industrial effluent, and sludge. This necessitates the use of efficient dewatering to minimize waste volume and meet disposal standards.
- Urbanization and Population Growth: Rapid urban expansion leads to increased municipal wastewater and sludge production, requiring upgrades and expansion of sewage treatment plants.
- Industrial Activity: Growth in key sectors like mining, oil and gas, food processing, chemicals, and pharmaceuticals generates significant wastewater and sludge that requires dewatering for processing and compliance.
Key technological innovations in dewatering are focusing on enhancing efficiency, sustainability, and control, particularly in sludge, slurry, and construction dewatering. Major advancements include smart and automated systems, advanced mechanical separation equipment, novel and hybrid dewatering techniques and a focus on sustainability and resource recovery.
- Smart and Automated Systems
- IoT-Enabled Monitoring and Control: The integration of sensors and Internet of Thing (IoT) technology allows for real-time performance, and energy consumption.
- Predictive Maintenance: Smart systems use this data to detect anomalies and predict potential equipment failures, enabling proactive maintenance and maximizing uptime.
- Automated Adjustments: Systems can automatically adjust pump speeds (e.g., via Variable Frequency Drive, or VFD pumps) or press operations to optimize efficiency and energy use based on conditions.
- Advanced Mechanical Separation Equipment
- High-Capacity and Energy-Efficient Centrifuges: Innovations in centrifuge design, including disc stack centrifuges for increased separation area, and high-speed operation (over 100,000 rpm in some cases), are leading to faster processing times and dryer solid cakes.
- Optimized Filter Presses: Continual improvements in belt filter presses and screw presses focus on cost-effective, continuous, and high-volume processing with simpler designs and lower maintenance.
- High Pressure Dewatering Rolls: Advanced versions are being developed to effectively handle fine and ultra-fine particles, particularly in mineral processing tailings, which are traditionally difficult to dewater.
- Novel and Hybrid Dewatering Techniques
- Thermally Assisted Mechanical Dewatering (TAMD): This novel approach uses controlled heat and pressure (below 100°C and 3000 kPa) to enhance moisture removal efficiency without the high energy consumption associated with full thermal drying.
- Electro-Assisted Dewatering: Techniques like electro-osmotic flow use an electrical field to mobilize water molecules toward a cathode, overcoming the binding forces that retain water in filter cakes, resulting in a drier solid product.
- Membrane Separation: The use of advanced membrane filtration systems (microfiltration, ultrafiltration, reverse osmosis) provides a sustainable and energy-efficient alternative to conventional methods, with ongoing innovations focusing on anti-fouling properties through surface functionalization (like hydrogels) and electric-field assistance.
- Hybrid Systems: Combining different methods, such as membrane filtration with centrifugal systems, leverages the strengths of multiple techniques for complex separation challenges.
- Focus on Sustainability and Resource Recovery
- Energy-Efficient Motors: The adoption of high-efficiency motors (like IE3 and IE4 levels) for pumps and other equipment significantly reduces power consumption.
- Water and Resource Reuse: Dewatering facilities the reuse of the separated water (filtrate/contrate) for non-potable applications and supports the recovery of valuable materials (e.g., nutrients, biogas, metals) from the dewatered solids, aligning with circular economy principals.
- Eco-Friendly Construction Dewatering
- Systems like recharge and recirculation systems redirect treated water back into the aquifer to minimize environmental disruption.
Deodorization is critically important to dewatering, particularly in wastewater treatment, because the process is a major source of concentrated odors at a treatment facility. Effective dewatering often requires an integrated odor control (deodorization) strategy. Dewatering is the final step in reducing the volume of sludge (or biosolids) before disposal or reuse. By mechanically removing a significant amount of water, it concentrates the solids. However, the solids (sludge cakes) contain high concentrations of organic matter and microbial byproducts, especially after anaerobic digestion. When this material is agitated, pumped and squeezed during dewatering (e.g., belt presses, screw presses, or centrifuges), volatile organic compounds (VOCs) are released into the air. The resulting sludge cake itself is a concentrated odor source during storage and transport. The strong unpleasant odors associated with wastewater dewatering are mainly caused by ammonia and organic nitrogen compounds (like hydrogen cyanide), hydrogen sulfide (H2S) organic sulfur compounds (such as mercaptans) and that are gaseous at ambient temperature.
How SulfaTrapTM Can Help
SulfaTrapTM offers a wide range of sorbents that can effectively remove all VOCs and odorous gases from the dewatering process. SulfaTrap’s custom designed systems, including the sorbent vessels, ventilation/gathering pumps and water management systems, have been installed in numerous applications in North America and Europe. Our systems are built to globally accepted standards and customer specifications. SulfaTrapTM also provides field services and supervision during installation, commissioning, and sorbent replacement to ensure hassle-free operation.
Features
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Inexpensive expendable sorbent materials provide simple, cost-effective operation
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Organic sulfur compounds are strongly adsorbed meaning there will be no odor released during sorbent replacement
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The sorbents are non-toxic, non-flammable and non-pyrophoric so they are safe to handle and easy to dispose
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Highly tolerant to air, water vapor and carbon dioxide
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Proprietary formulations enable customized solutions to any dewatering deodorization application.
| Compound | Odor | SulfaTrapTM Product | Application |
|---|---|---|---|
| Hydrogen Sulfide (H2S) | Rotten Eggs | SulfaTrapTM-R7Q/J | Biogas desulfurization, bulk H2S |
| Toluene, Benzene, Xylene, Naphthalene | Gasoline/paint thinner | SulfaTrapTM-R8HB | Biogas desulfurization, polishing bed |
| Siloxanes | "Rubber-like" chemical | SulfaTrapTM-Si | Siloxane removal from biogas |
| Mercaptans/Organic Sulfides | Garlic/rotten cabbage | SulfaTrapTM-R6 | Universal sorbent for organic sulfur removal |
| Ammonia (NH3) | Pungent/sharp | SulfaTrapTM-R8N | NH3, HCN Removal |
| Amines (e.g., Trimethylamine) | Fishy | SulfaTrapTM-R8HB | Pipeline Ultra-purification, LNG, HDS, Flare gas, Biodiesel, Biogas |
| Mercury + arsenic and selenium | Odorless | SulfaTrapTM-MT1 / SulfatrapTM-MT2 | Universal sorbent for removal of metals |
| Halides, Organo-halides | Pungent | SulfaTrapTM-A2 | Removal of hydrogen chloride (HCl), chloroform, tetrachloromethane |
| Oxygenates (e.g., aldehydes, alcohols, ketones) | Pungent/sharp | SulfaTrapTM-R8HB | Removal of oxygenates |
The choice of dewatering equipment and chemical conditioning can directly affect the intensity of odors. Processes involving high shear (like centrifuges) can break up sludge floc particles, potentially releasing more odorous compounds than low-shear methods. The type of polymer or chemical used to condition the sludge before dewatering can contribute to the odor profile as well. For example, some cationic polyelectrolytes can particularly degrade and generate amine odors like trimethylamine (TMA).
Deodorization techniques are the necessary countermeasure to the odors generated during dewatering to ensure a safe and tolerable working environment for the facility staff and well as the surrounding public. Odor complains from surrounding communities are one of the most significant challenges for wastewater treatment and food processing plants. Controlling odors from the dewatering step is vital for the facility’s social and legal license to operate. Additionally, minimizing odors from biosolids byproducts used in agriculture is crucial for their acceptance.
Common Deodorization Solutions for Dewatering Areas:
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Air Scrubbers (Chemical) – Foul air is passed through a packed tower, sprayed with liquid (e.g., alkaline or oxidizing chemicals) to absorb and neutralize odorous gases like H2S and ammonia.
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Biofilters – Foul air passes through a moist organic or synthetic media bed (e.g., peat, wood chips) where naturally occurring microorganisms consume and convert the odorous compounds into harmless, non-odorous products.
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Activated Carbon Adsorption – Odor molecules physically adsorb onto the porous surface of activated carbon or other sorbents as air passes through a bed.
Example of a SulfaTrapTM Solution for Deodorization
Figure 1 shows a simplified schematic of the gas deodorization configuration. In this example shown, the sorbent vessels house the SulfaTrapTM-R7H, SulfaTrapTM-R8HB and SulfaTrapTM-R8N sorbents to reduce the concentration of H2S, mercaptans, VOCs and NH3 to sub ppmv concentrations.
