1. Introduction
Solids dewatering represents one of the most critical unit operations in the water and wastewater treatment flowsheet. In the context of municipal wastewater treatment plants (WWTPs), biosolids management can account for 30% to 50% of the facility’s total operating budget. Consequently, the performance of dewatering equipment—specifically its ability to maximize cake solids concentration and capture rates—has a direct, linear impact on the lifecycle cost of the utility. As regulations regarding land application, landfilling, and incineration become increasingly stringent, the engineering selection of dewatering technologies has shifted from a simple capital cost decision to a complex analysis of total cost of ownership (TCO), energy efficiency, and reliable automation.
The primary objective of solids dewatering is to separate the liquid phase (filtrate or centrate) from the solid phase (cake) to reduce the volume of residual material requiring disposal or beneficial reuse. By transitioning sludge from a liquid state (typically 1–5% solids) to a semi-solid cake (typically 18–35% solids, depending on technology and sludge characteristics), utilities significantly reduce hauling volumes and associated tipping fees. Furthermore, for facilities utilizing thermal processes such as incineration or drying, minimizing moisture content is thermodynamically essential to reduce fuel consumption and maintain autogenous combustion.
This equipment category encompasses a variety of mechanical separation technologies, including belt filter presses (BFP), decanter centrifuges, screw presses, rotary presses, and plate-and-frame filter presses. Each technology applies different physical principles—pressure, vacuum, centrifugal force, or gravity drainage—to achieve separation. The selection process is heavily influenced by the upstream treatment processes (e.g., anaerobic digestion vs. aerobic digestion, thermal hydrolysis, chemical conditioning) and the rheological properties of the sludge.
For municipal consulting engineers and plant superintendents, the “one-size-fits-all” approach is obsolete. Modern selection requires a deep understanding of polymer chemistry interaction, hydraulic throughput versus solids loading rates, and the trade-offs between continuous high-energy operations (centrifuges) versus low-energy, lower-throughput operations (screw presses). Additionally, the regulatory environment driving Class A and Class B biosolids production places a premium on equipment hygiene, odor containment, and process stability.
OEM selection is equally critical. The market is populated by manufacturers with varying degrees of process expertise, fabrication quality, and support infrastructure. An OEM’s ability to provide accurate sizing data based on pilot testing, robust materials of construction suitable for corrosive environments (chlorides, H2S), and long-term service support for rotating assemblies is paramount. This article examines the engineering criteria for selecting dewatering systems and evaluates the capabilities of the major established OEMs in this sector.
2. How to Select Solids Dewatering Equipment
Selecting the appropriate dewatering technology involves a multi-variable analysis that balances process performance goals with site-specific constraints. Engineers must evaluate the following technical parameters to develop a robust specification.
Process Function and Performance Requirements
The two primary performance metrics for any dewatering device are cake dryness (measured in % Total Solids, TS) and solids capture rate (measured as a percentage of incoming solids retained in the cake).
Cake Dryness: This is the dominant economic driver. For every 1% increase in cake solids, the volume of sludge to be hauled decreases significantly. For example, increasing cake solids from 20% to 22% reduces wet hauling mass by approximately 9%. Engineers must define the minimum acceptable cake dryness based on the disposal method. Landfills often require a “paint filter test” pass, while incinerators require much higher dryness (often >25-28%) to minimize auxiliary fuel use.
Capture Rate: High capture rates (>95%) are essential to prevent the recirculation of solids back to the head of the plant (sidestreams). Poor capture rates increase the solids loading on the biological process, potentially leading to inhibition or permit violations. Centrifuges and filter presses typically offer higher capture rates compared to older belt press designs, although modern designs have narrowed this gap.
Hydraulic vs. Solids Loading Considerations
Dewatering equipment is sized based on two distinct limiting factors: hydraulic flow (gallons per minute, GPM) and solids mass loading (dry pounds per hour, lbs/hr).
Hydraulic Limits: This is determined by the volume of liquid the machine can physically process. In dilute sludges (e.g., waste activated sludge, WAS, at 0.5-1.0%), the hydraulic capacity is often the bottleneck.
Solids Loading Limits: For thicker sludges (e.g., anaerobically digested sludge at 2.5-4.0%), the limiting factor is the mass of solids the machine can convey and discharge. Engineers must specify equipment that can handle the maximum month solids production while operating within a reasonable schedule (e.g., 5 days/week, 8 hours/day for smaller plants; 24/7 for large plants).
Materials of Construction and Corrosion Resistance
Dewatering environments are aggressive. The presence of hydrogen sulfide (H2S), chlorides (especially in coastal areas or plants receiving industrial brine), and abrasive grit necessitates careful material selection.
Stainless Steel: 304L stainless steel is the standard for wetted parts in most municipal applications. However, 316L or Duplex stainless steel (e.g., 2205) is required for high-chloride environments or high-temperature applications to prevent stress corrosion cracking and pitting.
Abrasion Protection: For centrifuges, scroll wear protection is critical. Sintered tungsten carbide tiles or flame-sprayed hard surfacing must be specified for the conveyor flights. Replaceable tiles are generally preferred for ease of maintenance. For screw presses, the auger edges and the basket screen must be hardened to withstand friction from grit.
Polymer and Chemical Integration
Mechanical dewatering rarely works without chemical conditioning. High-molecular-weight cationic polymers are used to flocculate the sludge, releasing bound water.
Mixing Energy: The method of introducing polymer is critical. Centrifuges inherently provide high shear mixing, whereas belt presses and screw presses require upstream flocculation tanks or in-line static mixers with variable energy input.
Consumption Rates: Engineers must evaluate the “active pounds of polymer per dry ton of solids” (lbs/DT). Centrifuges typically consume more polymer than belt presses due to the high shear forces that can shear flocs. Screw presses are generally efficient but require precise polymer dosage control. Life cycle cost analysis must include projected annual polymer spend.
Energy Efficiency and Utility Consumption
The energy profile varies drastically between technologies.
Centrifuges: These are high-energy consumers due to the main drive motor required to spin the bowl at high RPMs (generating 2000-3500 x G). However, they offer the smallest footprint and high throughput.
Screw Presses and Rotary Presses: These operate at very low rotational speeds (< 1 rpm to 20 rpm), resulting in extremely low electrical consumption. They are ideal for plants seeking to minimize electrical peak demand.
Belt Filter Presses: These consume moderate energy (mostly for hydraulic power units and washwater pumps) but require significant volumes of washwater to keep the belts clean. The cost of pumping and treating this washwater must be factored into the selection.
Operations and Maintenance (O&M) Impacts
Operator Attention: Centrifuges and screw presses are enclosed systems that lend themselves well to full automation. They can often run unattended for long periods. Belt filter presses are open systems that require more frequent operator walkthroughs to monitor belt tracking and floc quality.
Cleaning cycles: Belt presses create aerosols and odors, requiring effective HVAC and odor control in the dewatering building. Screw presses and centrifuges are enclosed, improving the working environment.
Maintenance Complexity: Centrifuge maintenance requires specialized technicians for balancing and rotating assembly repairs. Screw presses have fewer moving parts and slow movement, leading to lower wear rates. Belt presses require regular belt and bearing replacements, which can be labor-intensive.
3. Comparison Table
The following table compares the locked list of OEMs based on their primary technology offerings in the solids dewatering sector. Engineers should use this to align project requirements (e.g., high cake solids vs. low energy) with the specific strengths of each manufacturer.
| OEM Name | Core Technologies | Engineering Strengths | Limitations / Considerations | Best-Fit Application |
|---|---|---|---|---|
| Andritz | Decanter Centrifuges, Filter Presses, Screw Presses, Belt Presses | Extensive global portfolio; high-capacity machines; advanced automation and drive systems; capability for extreme heavy-duty industrial and large municipal applications. | Equipment is often over-engineered for very small applications; higher initial capital cost for premium features. | Large municipal WWTPs, industrial mining/chemicals, plants requiring maximum throughput and automation. |
| Alfa Laval | Decanter Centrifuges, Drum Thickeners | Market leader in centrifugal separation technology; highly efficient G-force generation; innovative plate technology often paired for heating/cooling; excellent global service network. | Primary focus is centrifuges; less variety in low-energy pressing technologies compared to competitors. High-speed tech implies higher energy use. | Space-constrained plants, facilities prioritizing cake dryness above all else, complex industrial sludges. |
| Evoqua | Belt Filter Presses (J-Press), Screw Presses, Filter Presses | Deep legacy knowledge (legacy brands like USFilter/Winkle); strong support for municipal standards; broad range of dewatering and filtration options. | Large corporate structure can sometimes slow down custom engineering responses; navigating the vast product portfolio requires clear specifications. | General municipal wastewater, water treatment alum sludge, retrofit projects replacing legacy belt presses. |
| BDP Industries | Belt Filter Presses, Screw Presses, Gravity Belt Thickeners | US-based manufacturing; robust heavy-duty frame designs; excellent customization for retrofits; high reliability in belt press technology. | Does not manufacture centrifuges; focus is strictly on pressing technologies. | Mid-to-large municipalities preferring belt or screw presses; projects prioritizing domestic manufacturing and long service life. |
| Huber | Screw Presses (Q-PRESS), Disc Thickeners | Pioneers in modern screw press efficiency; high-quality stainless steel fabrication; extremely low energy consumption designs; enclosed, odor-free operation. | Technology portfolio is narrower (focused on screw press and thickeners); screw presses have hydraulic throughput limitations compared to large centrifuges. | Small to medium WWTPs, facilities prioritizing green/low-energy operation, plants with strict odor control requirements. |
4. Top OEM Manufacturers
The following manufacturers represent the established tier of suppliers for solids dewatering equipment. Selection among these OEMs should be based on the specific technology fit, service capabilities, and lifecycle cost analysis relevant to the specific project.
Andritz
Overview: Andritz is a global technology group with a massive footprint in separation technology. Their separation division is one of the few OEMs that manufactures virtually every type of mechanical dewatering equipment, ranging from decanter centrifuges to filter presses and screw presses. This allows them to offer technology-neutral advice, as they are not pigeonholed into a single solution.
Technical Differentiators: Andritz decanter centrifuges are renowned for their robust gearboxes and high-torque capabilities, allowing them to handle fluctuations in feed solids without stalling. They utilize advanced control algorithms to adjust the differential speed between the bowl and the scroll automatically, optimizing cake dryness in real-time. Their filter presses (overhead beam and side beam) are industry standards for mining and industrial applications but are also heavily used in water treatment for alum sludge dewatering due to the high capture rates and extreme cake dryness achievable.
Engineering considerations: Specifiers often look to Andritz for “mega-projects” or difficult industrial effluents (pulp and paper, chemical) where standard municipal equipment might fail. Their equipment is typically characterized by heavy structural steel and high-grade materials.
Alfa Laval
Overview: Alfa Laval is synonymous with centrifugal separation. With over a century of experience in high-speed rotation technology, they are a primary driver of innovation in the decanter centrifuge market. Their focus is heavily tilted towards high-efficiency separation for both liquid/solid and liquid/liquid/solid phases.
Technical Differentiators: The distinguishing feature of Alfa Laval dewatering centrifuges is the “Power Plate” and similar energy-recovery innovations that reduce the specific energy consumption (kW/ton) despite the high G-forces. They offer advanced abrasion protection systems that are field-replaceable, reducing downtime. Their control systems (often proprietary) provide deep analytics on vibration, bearing temperature, and relative torque, enabling predictive maintenance. Additionally, Alfa Laval frequently integrates their heat exchanger expertise with dewatering, offering solutions for sludge heating prior to dewatering or digestion.
Engineering considerations: Alfa Laval is a strong candidate when footprint is highly constrained and cake dryness is the paramount performance guarantee. Their units are compact and have high specific throughputs.
Evoqua
Overview: Evoqua (now part of Xylem) holds a vast portfolio of legacy water and wastewater brands. In the dewatering space, they are widely recognized for their belt filter presses and plate-and-frame presses (J-Press). They serve a massive installed base in the North American municipal market.
Technical Differentiators: Evoqua’s strength lies in the breadth of their pressing technologies. The J-Press filter press is a standard in the industry for dewatering metal hydroxides and water treatment plant lime/alum sludges, capable of producing very high solids cakes (>40-50% in some applications). Their belt filter presses feature robust frame designs and reliable tensioning systems. They have also expanded into screw press technology to meet the market demand for enclosed, low-energy dewatering.
Engineering considerations: Evoqua is often the preferred choice for retrofits where existing belt presses are being replaced, or for water treatment plants requiring batch dewatering via filter presses. Their extensive service network and parts availability for legacy equipment is a significant logistical advantage for utilities.
BDP Industries
Overview: BDP Industries is a specialized manufacturer focusing primarily on filtration and dewatering. Based in the United States, BDP has built a reputation for high-quality, heavy-duty belt filter presses and gravity belt thickeners. They have successfully transitioned into the screw press market as well.
Technical Differentiators: BDP distinguishes itself with the structural integrity of its machines. Their belt presses often feature vertical pressure zones and independent gravity zones that allow for superior hydraulic throughput. BDP uses 304 or 316 stainless steel standard for frames, avoiding the corrosion issues associated with painted carbon steel. Their screw press design features a unique basket and auger geometry designed to minimize blinding and maximize water release. BDP is particularly noted for its willingness to customize dimensions to fit existing building constraints.
Engineering considerations: Engineers specify BDP when looking for long-term durability and ease of maintenance. Their equipment is designed with the operator in mind, featuring accessible lubrication points and open frame designs (for belt presses) that make washing and belt changing less arduous.
Huber
Overview: Huber Technology is a German-headquartered company known for high-precision stainless steel manufacturing. They were early movers in popularizing the screw press technology (Q-PRESS) in the municipal market, shifting the industry paradigm away from belt presses for small-to-medium plants.
Technical Differentiators: The Huber Q-PRESS is characterized by its inclined installation and slow rotational speed, which ensures minimal wear and quiet operation. The design includes a self-cleaning mechanism for the cylindrical operational operational screen, utilizing a wiper system that prevents clogging—a common failure mode in screw presses. Huber’s equipment is fully enclosed, offering superior odor containment and hygiene. Their “sludge logs” technology maximizes residence time under pressure to achieve competing cake dryness levels.
Engineering considerations: Huber is an excellent fit for facilities prioritizing low operational complexity and energy efficiency. While hydraulic capacity per unit is lower than a large centrifuge, the reduction in noise, vibration, and energy often justifies the use of multiple units. They are highly specified in greenfield projects targeting sustainable/net-zero energy goals.
5. Application Fit Guidance
Not all dewatering technologies fit every application. Engineers must match the OEM strengths to the specific plant profile.
Municipal Wastewater (Small to Medium Plants)
For plants under 5-10 MGD, Screw Presses (Huber, Evoqua, BDP) are increasingly the standard of choice. The low operating speed reduces wear, and the automated operation allows for “lights-out” dewatering, where the machine runs overnight without an operator present. The energy savings are substantial compared to centrifuges, and the maintenance is far less specialized.
Municipal Wastewater (Large Plants)
For plants >20 MGD, Centrifuges (Andritz, Alfa Laval) typically dominate. The sheer volume of sludge requires the massive throughput density that only centrifugal force can provide. While energy costs are higher, the footprint savings are critical in urban environments. Large belt presses (BDP, Evoqua, Andritz) are also viable here if space permits and energy reduction is a primary driver, though odor control becomes more expensive with open belt presses.
Water Treatment Plant Residuals
Alum and ferric sludges are notoriously difficult to dewater because they are gelatinous. Filter Presses (Evoqua, Andritz) are often the best technology here, capable of exerting high pressures to produce a very dry cake suitable for landfill. Centrifuges are also used but can struggle with the abrasive nature of silt often found in water treatment residuals.
Industrial Wastewater
Industrial applications vary wildly.
- Food & Beverage: Often high in oil and grease (FOG). Centrifuges (3-phase) or DAF systems are common, but for dewatering, screw presses are gaining ground due to hygiene (enclosed).
- Mining/Aggregates: Requires extreme durability. Belt presses and Filter presses from Andritz or BDP are common due to their ability to handle inorganic slurries.
Retrofits vs. Greenfield
In retrofit scenarios, the existing building defines the technology. Replacing a belt press with a screw press is common, but screw presses are long and narrow, whereas belt presses are wide. Centrifuges are compact but require heavy structural foundations to handle vibration. OEMs like BDP and Evoqua often excel in retrofits due to customizable frame dimensions.
6. Engineer & Operator Considerations
Beyond the catalog specifications, the reality of living with this equipment dictates the long-term success of the installation.
Installation and Commissioning
Commissioning dewatering equipment is an art form involving the “triangle” of variables: Feed Rate, Polymer Dosage, and Machine Speed (Torque/RPM/Belt Speed). Engineers must write specifications that require the OEM to provide qualified process engineers—not just mechanical start-up technicians—for a minimum of 5-10 days to optimize these variables. Failure to optimize polymer dosage at startup can cost the utility tens of thousands of dollars annually in wasted chemicals.
Maintenance Access
Centrifuges: Require an overhead bridge crane or monorail capable of lifting the rotating assembly (often several tons). The layout must provide clearance for removing the scroll from the bowl.
Screw Presses: The screen baskets and augers eventually need removal. Sufficient clear space must be provided at the discharge end of the press (often equal to the length of the press itself) to pull the shaft.
Belt Presses: Requires 360-degree access for washdowns and belt changing. If placed too close to walls, operators will neglect maintenance on the “blind” side, leading to bearing failures.
Spare Parts and Reliability
Centrifuges rely on precision-balanced rotating assemblies. If a bowl is damaged, it often must be shipped to the OEM’s service center (e.g., Alfa Laval or Andritz service hubs) for repair, which can take weeks. Redundancy (N+1) is mandatory for centrifuge installations. Belt presses and screw presses utilize more off-the-shelf components (bearings, motors) and fabrication repairs can often be done locally or on-site, providing slightly more resilience for remote facilities.
Operational Lessons Learned
A common operator complaint regarding screw presses is the blinding of screens if the washwater pressure is inadequate. Engineers must specify booster pumps dedicated to the dewatering equipment to ensure 80-100 PSI washwater availability. For centrifuges, struvite formation is a major risk in anaerobically digested sludge lines. Glass-lined pipe or special coatings, along with ferric chloride dosing, may be required to prevent the centrate lines from clogging.
7. Conclusion
Selecting the right solids dewatering equipment is a pivotal decision that affects a treatment plant’s budget and operational ease for decades. There is no single “best” OEM; rather, there are optimized matches between plant constraints and technology strengths.
For large-scale, high-throughput needs where cake dryness is king, Andritz and Alfa Laval offer superior centrifuge technologies. For facilities prioritizing rugged, American-made pressing technology and retrofit flexibility, BDP Industries and Evoqua provide robust belt and screw press solutions. For energy-conscious, odor-sensitive, and automated small-to-medium applications, Huber and Evoqua’s screw press portfolios offer excellent lifecycle value.
Engineers must move beyond simple capital cost comparisons. A comprehensive evaluation must quantify the Net Present Value (NPV) of polymer consumption, energy usage, maintenance labor, and, most importantly, the hauling cost savings generated by achieving drier cake solids. By engaging these top OEMs early in the design phase and demanding rigorous pilot testing, utilities can ensure a dewatering solution that is reliable, cost-effective, and operationally sustainable.
source https://www.waterandwastewater.com/top-oems-for-solids-dewatering-equipment/
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