A simplified and typical flow-chart for wastewater applications is shown below although not all process steps are present, particularly for industrial wastewater.

Thickening of Activated Sludge

Many effluent treatment plants use a biological treatment step to reduce the COD and BOD of a wastewater before discharging into the environment via rivers and streams. During the operation, the colonies of bacteria develop and the growing biological body produces a solid waste as it feeds on the chemicals and nutrients.

It is necessary to continuously purge the growing biological mass to control its activity and efficiency. The purge stream is usually in the form of dilute slurry, 0.5% to 3% ds, which is easier to handle if it is first thickened. Some of the thickened slurry can be returned to the biomass to maintain an optimum concentration of bacteria. The remaining waste activated sludge is for disposal.

The US Centrifuge Systems CQ range of decanter centrifuges is well designed for these duties. Controlling the decanter operating parameters can optimize the consistency of the discharged sludge.

Sludge consistency can be chosen to match the needs for disposal. Slurry for land application should be thickened to reduce the volume of water but still needs to be thin enough to be spread evenly on the land. Slurry for feed to a digester can be thicker but often still needs to be pumped. When the disposed slurry is for landfill, dewatering the feed to a stackable consistency will reduce weight, volume and disposal costs.

A simple decanter installation (see the following page) provides the flexibility to ‘dial in’ operating parameters to produce a choice of thickened sludge concentrations. The installation allows for the use of a polymer to help the separation but a polymer is not an essential.

Typical Decanter Installation (click for larger image)

The decanter will thicken an activated sludge without polymer producing solids at 6%-ds ideal for land spraying. However, the liquid centrate remains cloudy, 0.2%-ds, and can still pose a disposal problem. Many companies use a low concentration of polymer to clear up the centrate to only 200-ppm ds.

By increasing the polymer concentration and adjusting the bowl speed and conveyor speed of the decanter, the system will provide clear centrate and solids discharged at 8%-ds. Once the system is set up, the hydraulic conveyor drive automatically adjusts its speed to control the consistency of the discharged solids. These are the parameters most used to produce solids as feed to a digester.

Increasing the concentration of polymer further and again adjusting the decanter operating parameters will thicken the feed and produce stackable solids at 12%-ds. As before, once the conditions are selected, the decanter will control the solids discharge by an automatic adjustment of conveyor speed. You can clearly see the difference in the consistency of the solids in the photograph below.

(A plant thickening 800,000 gallons per day of activated sludge at 1%-ds in the feed to an 8% pumpable slurry with a clear centrate, would require 66 to 100-lbs per day of polymer).

The decanter can both thicken and dewater, switching between the two in a matter of minutes without interruption to the feed.

It is not possible to state a single figure for cake dryness but the centrifuge will usually produce a cake that is touch dry and sometimes friable. Slurries from the primary clarifier are typically 30 to 35%-ds. Waste activated sludge can also appear friable but is typically only 15 to 20%-ds. Even though moisture concentration can vary widely, by optimizing the system operating parameters, solids will be discharged from the decanter with the typical consistency as shown in the photograph.

Dewatering Activated Sludge

In all thickening and dewatering applications when a polymer is used to aid separation the centrate clarity will typically look like the photograph with less than 200-ppm ds.

Note: The CQ decanter centrifuge will often produce better results than other items of dewatering equipment when secondary or biological sludges are present in the material to be processed. The primary reason for this is that there is no filter media present, which can become blocked or blinded by the biological sludge.

Thickening and Dewatering Digested Sludge

As with activated sludge, decanter-operating parameters can be adjusted to thicken or dewater digested sludge. Typical performance for dewatered digested sludge:

It is not always necessary to fully dewater digested sludge unless the goal is to reduce the total volume minimizing transport and landfill costs. Some companies choose only to thicken the sludge to meet their specific handling requirements. (Land spreading for example)

CQ decanter centrifuge units are fitted with a hydraulic drive on the decanter conveyor. They are easily configured to produce a wide range of results:

To produce runny solids, speed up the conveyor drive and turn down the polymer feed rate to between 1 to 2-lbs/1,000-lbs ds. Solids will typically look like the photograph below @ approximately 8%-ds.

To produce a thicker solids consistency, slow down the conveyor drive and increase the polymer feed rate again. At 3-4 lbs/1000 lbs ds, thickened solids will look something like the photograph below @ approximately 12%-ds. As you can see, at this consistency there is still a visible sign of free water but as the rate of polymer is increased, the solids become more homogenous.

By increasing the use of polymer dosage and controlling the decanter-operating parameters, the discharged solids become progressively drier. Per the photograph below @ approximately 18%-ds the solids can be handled satisfactorily with a mechanical shovel.

Per the photograph below @ approximately 23%-ds the solids have a ‘balling’ consistency.

Per the photograph below @ approximately 30%-ds the solids are quite friable.

For each site, it is important to optimize the use of polymer to be cost effective and still reach an acceptable goal for solids dryness.

Worldwide CQ Decanter Centrifuge Reference List Water Companies