Sludge Thickening Concentration

Further sludge concentration is first accomplished by the use of thickening equipment that will increase the solids content to between 2% and 5%.

Sludge thickening is achieved through one of two means: flotation and settling to the bottom by gravity or centrifugal force. The thickening operation separates water from the sludge as much as possible and is cost-effective because the cost involved in the process is well offset by the savings gained through the reduction of sludge volume, which decreases the capital and operating costs of subsequent sludge processing steps. In addition to reduction in volume that needs to be handled mechanically, sludge thickening is beneficial to the stabilization process (e.g., anaerobic digestion process) because it reduces biomass volume, tank size, and heating requirements.

The flotation process in sludge thickening is in principle similar to the flotation for pretreatment of wastewater. In the flotation process, air is injected into the sludge tank from the bottom under pressure and a large amount of air bubbles disperse into the sludge, attach themselves to the sludge solids and float them to the surface of the thickener. The layer that is formed by the floated sludge particles-air bubbles is removed from the surface by a skimming mechanism for further processing. The flotation method is particularly suitable for activated sludge because of the low specific gravity of the solids, which makes it difficult for gravity-based thickening to remove or separate solids from water. The common parameters for dissolved air flotation process are tabulated in Table 7.1.

Gravity thickeners are essentially circular primary settling tanks with or without mechanical thickening devices. The dilute sludge is fed into the settling tank where solids are allowed to settle over a few days. The thickened sludge is withdrawn from the bottom of the tank and pumped to the digesters or dewatering equipment. Gravity thickening can be achieved in a separate tank or within the clarifier, if it is so designed. Thickening within the clarifier is achieved at the lowest part of the clarifier within a sludge storage zone or hopper. Within the hopper, the sludge is slowly mixed with a motorized rack to enhance the release of water. The mechanical stirrer rack is made of a rotating set of vertical blades or rods, which make the stir-rer appear to be a picket fence (this appearance gives rise to the name, picket fence thickener) (Fig. 7.1). Often, a secondary tank is used to supplement or replace the thickening zone of the clarifier, especially when extended thickening times are required. These units are typically designed to store the accumulated solids for at least 24 hours. Chemicals are sometimes added to aid the thickening process (e.g., iron and aluminum salts, poly-electrolytes) in a process called sludge conditioning. Plain settling tanks can produce solids contents in sludges of up to 8.0% for primary sludges and up to 2.2% for activated sludge. Activated sludge can also be concen-

Table 7.1. Air flotation parameters for DAF.

Parameter Typical Value

Air pressure, psig 40-70

Effluent recycle ratio, % of influent flow 30-150 Detention time, hours 3 Air:solids ratio, lb air/lb solids 0.02

Solid loading, lb/ft2day 10-50 Polymer addition, lb/ton dry solids 10

trated by resettling in primary settling tanks. Gravity thickener design is very similar to that of primary settling basins. The mechanism of the process is also similar. Due to relatively higher solid content in sludge than that in wastewater, a heavy-duty scraper is often called for in sludge gravity thickening in order to move sludge to a hopper from which it is withdrawn and further processed. Gravity sludge thickening is often used for

Figure 7.1. A schematic diagram of a picket fence gravity thickener.


Figure 7.1. A schematic diagram of a picket fence gravity thickener.

Table 7.2. Mass loading for designing sludge thickeners.

Type of Sludge

Mass Loading, lb/ft2day

Primary sludge


Primary and trickling filter sludge


Primary and waste activated sludge


Waste activated sludge


primary sludge thickening, and flotation thickening is frequently employed for activated sludge thickening. A gravity thickener will be designed on the basis of hydraulic surface loading and solids loading. The design principles are to be the same as those for sedimentation tanks. The use of chemical additives (lime or polyelectrolytes) allows higher loading rates. The minimum detention time and the sludge volume divided by sludge removed per day (which represents the time sludge is held in the sludge blanket) is usually less than two days. The design parameters of gravity thickeners for different types of sludges are listed in Table 7.2.

Centrifuges are used both in thickening and dewatering of sludge. Centrifugal thickening involves the settling of particles in watery sludge under the influence of centrifugal forces. Application of centrifuges is limited to the sludge from activated sludge. The advantage of centrifugal thickening is its ability to thicken some difficult sludge. The downside of the centrifuges for sludge thickening are costs associated with power, maintenance, and skilled operators. Other types of sludge thickening equipment are the rotary drum thickener and gravity belt thickener; they are less common in sludge thickening operations.

The main design variables to be considered in selecting a thickening process are

• Solids concentration and volumetric flow rate of the feed system

• Chemical demand and cost if chemicals are employed

• Suspended and dissolved solids concentration and volumetric flow rate of the clarified stream

• Solids concentration and volumetric flow rate of the thickened sludge

Other variables that impact the selection of a thickening process are: subsequent processing steps; operation and maintenance (O/M) cost; and the reliability required for meeting successful operational requirements.

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