Getting the Best out of Dewatering Equipment

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Has the Dewatering Performance Declined over the years?

Sludge Dewatering and thickening is one of the most common processes in Water and Wastewater treatment plants. Belt presses, gravity tables and centrifuges are the usual equipment items used. In the case of belt presses and gravity tables, the simplicity of the equipment can lull operators and maintenance staff into a slow process of allowing the machines and settings to deteriorate.

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A recent survey of press installations ranging in age from 3 to 18 years has shown that many are being operated at 10 to 30% below the original design capacity. There are many reasons, but generally reduced performance appears to be as a result of a combination of factors, including staff turnover, maintenance and purchasing decisions, as well as a desire to reduce the amount of time spent adjusting the equipment.

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A de-tuned system can be more stable, and will tolerate more variability in the feed conditions. Extra available plant capacity can also encourage this approach.

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Performance factors

In restoring performance there are two process factors to consider. The first is that sludge dewatering is a combined physical /chemical process of flocculation, followed by a second process, the mechanical step of the actual dewatering. Efficient operation of both steps is important for successful dewatering.

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Flocculation

Flocculation of sludges depends strongly on the nature of the sludge. During initial

commissioning the equipment supplier and the flocculant supplier will normally

work together to select the best flocculation regime. This includes not only the

choice of the correct flocculant (usually a cationic polymer), but also the means

of dosing and mixing, as well as the dosage rate. If, over successive years,

the treatment process, or the treatment plant influent, changes, these selections

may need to  be revisited. The dosing settings, such as final dilution and

mixing adjustments also need to be changed, and these settings can be crucial

to good performance.

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Flocculant suppliers have an on-going source of revenue from the sale of

chemicals, and so tend to be the main source of information and service in

this regard. However, they may not have an in-depth knowledge of the mechanical

equipment adjustments required to suit the new conditions. In addition, they

have a vested interest in promoting their particular products.

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The polymer flocculant is the heart and soul of a biosolids dewatering

system. Whilst doctorate theses abound regarding the performance and selection

of the correct polymer for an application, the truth is it is a trial and error process,

with luck and skill involved.

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Commercial Factors

A common problem is Water Authorities with more than one treatment plant using polymer flocculants calling for tenders for supply of polymer flocculants for a number applications in one large contract. This works against selecting the best polymer for each application, as sometimes one suppliers’ polymer range does not suit every case.

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As a result of this approach, it is not uncommon to find poor performing polymers being used, with corresponding reduced dewatering performance of the equipment. This policy also results in a flurry of testing activity at tender time, followed by a long period during which the polymer supplier focuses his attention elsewhere.

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A major occasional problem is a world –wide shortage of raw materials, which has increases the price, and alternatively encourages polymer manufacturers to reduce the active content per kilogram of chemical.

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Mechanical Issues

In addition to the process considerations there are mechanical maintenance issues. Belt presses, like a car, have wear parts which need to be replaced, as well as requiring occasional adjustment. The main wear parts are the filter belts, cake scraper “doctor’ blades, and the seals for the belt spray wash boxes and side skirts, and the spray nozzles.

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Filter Belts

The filter belts have a direct effect on the process performance, and even the side skirt seals can affect process performance. Unfortunately many organizations leave the purchasing of these spares to procurement departments or mechanical maintenance staff, who might buy purely on apparent price, and the process performance can suffer as a consequence.

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The selection of the best filter belt is a compromise between an open weave to encourage drainage, the need to support the cake and reduce losses through the cloth, and the ability to wash the belt clean. Belt washing can be critical, and a worn water pressure booster pump, or worn nozzles, in combination with the wrong choice of belt, is a typical recipe for poor process performance.

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A further factor is the mechanical strength required, particularly the resistance of a filter belt to creasing and distortion of the seam (Which results in a curved seam, which some call a “smiley face” but those in the know call a “sad face”). A high quality belt press produces minimal stresses in the belt, whereas some compromise presses create more stress. The wrong choice of belt can drastically reduce throughput and capture.

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Seals

Simple items such as wash spray seals and enclosures are often

neglected, resulting in aerosol emissions. Whilst some operators

have reported that they have tolerated these aerosols for many

years without any ill effects on their staff, it is far better to carry

out the minimum maintenance required, and the benefits will include

a happier workforce. A happy operator will take more care of his

equipment and tend to produce a better process result.

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Spray Washbox

Seals need regular inspection and replacement where necessary. It is important that they be set up correctly in order to seal efficiently. A common installation mistake is to provide insufficient flexibility in the piping to the spray headers so that correct adjustment is impossible.

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Doctor Blades

Other minor mechanical areas where mistakes are made, are cake release “doctor” blades and gravity drainage zone skirt seals. There is a perception that if the thickness of material is increased, it will work better. However, in engineering, sometimes bigger is not always better, and the additional stiffness of doctor blades and side skirt sealing rubbers can result in mechanical problems such as poor belt tracking, and increased motor power consumption, and shorter belt life.

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Tuning

Like a motor vehicle, sludge dewatering systems should be tuned by specialist personnel at regular intervals. A plant with little changes in operating staff and process conditions might only need re-tuning every 3 or 4 years, whereas annual tuning and re­training might be appropriate for wastewater plants in high growth areas and with rapid staff turnover.

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Changes to sludge age can change the characteristics of the sludge, and require a different polymer and press settings.

In tuning, the specialist will review the system from the biosolids conditioning chemicals through the mixing and flocculation regime, and correlate this with the belt speeds, type of filter belt weave, belt pressures, and characteristics of the particular belt press. Some presses for instance are very rough on the formed floccs, whereas others might have a more gentle action. The specialist can also spot mechanically created process problems, such as poor belt washing or selection.

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Centrifuges

Most of these factors are also true with regard to centrifuge dewatering equipment, but made more complicated by the fact that the visual clues provided by the belt press are not available with the centrifuge. It is also easier to ignore poor performance of a centrifuge system, as the process are all happening inside the machine, under a cover. Off-tune centrifuges will consume a lot more polymer, or may produce much deteriorated capture rates, as well as reduced throughput.

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Conclusion

A properly adjusted dewatering system will provide cost savings through reduced polymer consumption, manpower for operation, and power consumption. A further benefit can be reduced disposal costs from a drier cake. As a high proportion of the operating cost of a treatment plant can be attributed to the sludge dewatering and handling, a small investment in maintaining and tuning this equipment can result in significant savings.

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