Separation Technologies for Sludge Dewatering

Particles in sludge feeds interact strongly one with another to prevent settling and offer significant resistance to filtration and compression. This leads to the need for dewatering forces to be compressive ones applied directly to the networked solid phase; sometimes shear forces can be an assist dewatering. Designs of filtration equipment most suitable for sludge dewatering have evolved to meet the intrinsic characteristics of sludges, the most important of which is their compressibility and fine particle sizes, which lead to cakes with extraordinarily high solids contents close to the filter medium. Hence, the membrane plate press, the belt filter, and the decanter centrifuge have become most widely accepted machines for sludge dewatering. Filter presses tend to yield a drier solids discharge, but the level of dryness depends on the sludge properties. The same feed properties dictate the need for chemical pre-treatment to ensure the highest rates of dewatering and best clarity of filtrate, and the correct choice of filter cloth is also crucial in these respects. Commonplace requirements in many processing plants are to minimize the amount of wastewater generated or to reduce the concentration of contaminants in the wastewater, and there are often underlying problems related to dewatering and handling of the sludge. A number of methods are used to reduce the amounts of wastewater discharged and the concentrations of contaminants in the discharge. These include source reduction technologies that minimize the amount of wastewater generated in the plant and treatment technologies that treat wastewater to reduce contamination levels. Contaminant level reduction is primarily either to make the water available for recycling or to reduce costs of treatment. In-plant treatment of wastewater is often a key strategy as a precursor to recycling, and a wide range of treatment options is available. These include careful consideration of alternative uses for the wastewater before it is sent to the treatment plant, technologies to stabilize the wastes (for example, wet oxidation), and separation/concentration technologies (including screens, settlers, filters, centrifuges, and membrane (bio-)processes), as well as thermal processes (for example, evaporation). For dewatering, economic considerations determine that mechanical processes are preferred over thermal ones. Wet oxidation is used to stabilize municipal and industrial wastewater sludges; at lower temperatures and pressures the sludge is conditioned to improve dewatering, but at higher temperatures and pressures biological sludge can be destroyed (as an alternative to incineration). The oxidation process is able to convert oxidisable constituents in the sludge, but still leaves a slurry that has to be dewatered. Hence, dewatering technologies are often key downstream operations in wet oxidation processes as well as in bioprocesses. we will focus on the separation technologies most suitable for sludge dewatering. These are primarily pressure filters, rotary drum filters, and centrifuges.

1. Dewatering-related properties of wastewater sludges

No two wastewaters are alike although, in summary, the general effects on filtration of variations in their characteristics are:

• feed compositions are complex mixtures of organic and mineral particles, biosolids, and molecular and ionic substances;
• feed composition is significant in controlling cake resistance, rate of filtration, and cake moisture content;
• feeds invariably require flocculation to “reduce” their fines content, and the negative effect of the fines on filtration;
• due to their higher biosolids content secondary sludges tend to form wetter cakes and cake form rates are slower when compared with filtration of primary sludges (under the same conditions of filtration);
• the filtering properties of many types of wastewater feeds are dependent on sludge age;
• formed filter cakes tend to vary from moderately to highly compressible.

During filtration, the compressible nature of a filter cake leads to the formation of a solids concentration variation through the cake that decreases from a maximum at the cake–cloth interface. The existence of compressibility in a cake suggests that further liquid can be removed from the cake by applying a compressive force to its surface—the so-called expression process.

2. Filters

The characteristics noted in above Table lead to a preference for pressure filters for sludge dewatering; in the case of some industrial sludges the rotary drum filter can sometimes be considered an option. To make sludge feeds more amenable to mechanical dewatering the feed is more often than not pretreated by flocculants or coagulants, agglomerating the feed particles to increase their effective size.

3. Filter presses

Plate and frame filter presses, recessed plate presses, and membrane plate presses are all used to dewater sludges. Filter plates are supported on side beams or suspended from an overhead beam; filter plates of 1.5 m × 1.5 m are typical, but 2 m × 2 m plates are increasingly common—and larger plates are being developed. For wastewater applications, 80 chambers in a recessed plate press or 60 chambers in a membrane plate press is not uncommon. The ability of the membrane plate presses to utilize the compressible nature of the sludge makes them particularly useful for sludge dewatering applications. A typical filtration cycle for dewatering is: (i) slurry feeding; (ii) cake squeezing by inflating the membranes; (iii) air blow through the cake; (iv) core wash and/or blow. The cake squeeze is affected by diaphragms that are pressurized up to 16 bar in order to lower cake moisture content (or, increase the volume of liquid recovered from the feed). Cake moisture content reductions are dependent on its compressibility properties but moisture contents of 25% more than can be achieved on a conventional filter press are not uncommon. Some operating data for recessed plate filter presses are given in the next Table:   Developments incorporated into modern filter presses to increase filter capacity, reduce cake discharge times, and reduce labor intensity include: (i) automation and mechanization of plate pack opening and plate shifting; (ii) use of long-travel hydraulic cylinders to move the pressure head to reduce press opening times (very large presses may have two moving pressure heads); (iii) cloth shaking or lifting mechanisms to promote cake discharge; (iv) cloth flushing or washing systems, which range from simple spray nozzles mounted above the plates to moving spray bars that are lowered and raised between plates singly or in groups, to remove adhering or penetrating particles (a limitation of most cloth washing systems is that only one side of the cloth is washed); (v) placement of the filter onto load cells to indicate if the filter fails to reach its tare weight (for filter control and/or throughput measurement); (vi) use of “bomb bay” doors to cover discharge chutes to prevent water entry into the dry cake handling facilities; (vii) light curtains and/or protective screens to prevent operator access. Although membrane presses are significantly more expensive than conventional filter presses, the additional capital and operating costs are often justified by shorter cycle times (and hence greater sludge throughput) and the more easily handled cake that is produced.

4. Belt presses

Belt filters are characterized by two continuous, tensioned filter cloths. Flocculated sludge is fed to the lower cloth (belt); initial dewatering is under gravity as the belt carries the sludge into a consolidation zone where it is progressively squeezed under pressure by the upper and lower belts moving towards each other to form a closed “envelope”. The cake is then squeezed under increasing pressure as the cloths move over a sequence of successively smaller diameter rollers. As the two belts pass over the rollers there is a relative movement of the belts, causing the liquid to be removed by a combination of expression and shearing to produce a dry, crumbly cake.     A key to successful operation of a belt press is that the feed must be flocculated, to avoid blinding of the filter belt and facilitate gravity drainage when it is initially fed to the belt. Conditioning is carried out by polyelectrolytes immediately before the drainage zone; some results for different sludge types are given in the next table. Special care must be taken with belt washing, carried out on the belt return cycle with rinse water flow rates as high as 50–200% of that of the sludge. For good machine operation, a feed sludge concentration >3–4% has been recommended.

5. Rotary drum filters

Vacuum filters have operational and process limitations that can be most important when choosing a filter for sludge dewatering. By definition, the driving force for dewatering is limited by the vacuum that can be applied; in practice, a vacuum of not more than 0.25 bar absolute (−0.75 bar g) can be applied. For this reason vacuum filters are not usually employed in systems where most of the particle sizes are smaller than about 5 μm; in turn, vacuum filters are rarely used to dewater municipal sludges but are more often suitable for some types of industrial ones. When vacuum filters are used, rotary drum filters are the preferred choice (Fig. 6) and their continuous operation and virtually no operator intervention during the normal operating cycle can be used to advantage.

6. Decanter centrifuges

High solids decanters are used to mechanically dewater environmental and biosolids sludges and are often a preferred choice of equipment due to

• the high forces of 2000–4000 g applied directly to the feed solids, enabling lower solids moisture contents (the “ultimate” cake dryness depends on the given sludge);
• its ability to handle higher solids content feeds;
• its continuous operation, with solids throughputs up to about 90 te h⁻¹;
• the solids handling capabilities intrinsic through its design, with solids conveyed co-currently along the walls of the bowl by a helical screw.

  The centrifuge can be over-torqued due to the flow properties of the thickened solids, or due to plugging by the accumulation of unconveyed solids in the bowl. Wear problems on the screw can also be caused by more abrasive particles. To improve centrate clarity, flocculants or coagulants are frequently added to the feed to agglomerate finer particles. Examples of the expected performance of decanters dewatering different types of municipal sludges are given in the next table:  

8. Filter media developments

Surface coatings applied to filter fabrics can enhance one or more of its filtration properties; microporous polymer coatings are a relatively new development used to provide a smoother and finer aperture size to the fabric surface and to facilitate easier detachment of the cake and prolong the lifetime of the medium. A polyurethane coating on a woven polyester substrate is the basis for Madison’s Primapor fabric for use on process filters such as rotary drums and filter presses. The “second generation” treatment developed by Madison, Azurtex, has the coating pushed farther into the body of the fabric so that the surface finish is less prone to mechanical damage from external forces. Both treatments give better particle retention with improved cake release.

9. Conclusions

Designs of filtration equipment most suitable for sludge dewatering have evolved to meet the intrinsic characteristics of sludges, the most important of which is their compressibility and fine particle sizes, which lead to cakes with extraordinarily high solids contents close to the filter medium. The sludge feed tends to be networked, that is particles interact strongly one with another to prevent settling and offer significant resistance to compression, which requires that the forces applied for dewatering be compressive ones applied directly to the networked solids phase. Hence, the membrane plate press, the belt filter and the decanter centrifuge have become most widely accepted machines for sludge dewatering. However, usually the same feed properties dictate the need for chemical pre-treatment to ensure the highest rates of dewatering and best clarity of filtrate, and correct choice of filter cloth is also crucial in these respects. The filter press tends to yield drier solids, but the choice of equipment depends not only on the cake dryness but also the process duty requirements and costs. [Ref.]