Sludge Processing and Disposal

Sludge Processing and Disposal

It should be appreciated that the sludges consist mainly of water and that dewatering is the first and most important requirement in sludge processing.  The cost of treating the sludges, particularly for wastewaters, is a major component of the total cost of treatment, and the effects of the final disposal methods and return flows from sludge treatment can have significant implications for the preceding processes.

The main objective of wastewater treatment is to reduce the pollution load on receiving waters.  The treatment processes concentrate some of the impurities in a sludge along with the microbial excess biomass.  Water treatment also produces a sludge from the chemical coagulation and separation of impurities.  The treatment and disposal of these sludges should be considered as an integral part of the treatment process.  

Objectives

The main objectives of sludge treatment are:

1. Reduction in the volume of sludge for disposal by removing some of the water

2. Collection of by-products which may be used or sold to off-set some of the costs of sludge                   treatment

3. Disposal of the sludge in a safe and aesthetically acceptable manner

4. Destruction of pathogenic organisms

5. Stabilization of the organic matter contained in the sludge.

Methods

Sludge treatment and disposal at any particular location may comprise any or all of the steps as outlined below.

1. Concentration - reduction in the volume of sludge to be treated by encouraging the sludge to compact to a higher solids content

2. Treatment - to stabilise organic matter, destroy pathogens and/or yield by-products

3. Dewatering and drying - removal of water, thus reducing sludge volume. Sludges with less than        80 per cent moisture content are usually spadeable

4. Disposal - the only places where sludge can be disposed of are into the air, onto land or water.            Whether or not the impact on the receiving environment is legally, aesthetically and ecologically        acceptable depend on both the degree of treatment provided and the method of dispersing the              sludge into the environment.

Table 1. Treatment and disposal options for wastewater sludges

Thickening	Stabilization	Dewatering	Partial disposal	Ultimate disposal
Gravity	Anaerobic digestion	Drying beds	Incineration	Sanitary landfill
Flotation	Aerobic digestion	Filter press	Pyrolysis	Crop land
Centrifuge	Lagooning	Centrifuge	Wet air  oxidation	Ocean
Elutriation	Heat treatment	Vacuum filter Belt press Lagooning	Composting

Here is only describing the Thickening process by Gravity, Flotation dan Centrifuge.

GRAVITY THICKENING

A typical waste sludge thickener is illustrated in Figure 1.  The tank resembles a circular clarifier except that the depth/diameter ratio is greater and the hoppered bottom has a steeper slope.  A bridge fastened to the tank walls supports a truss-type scraper arm mounted on a central shaft.  Sludge enters at the centre behind a circular baffle that directs it downward, and supernatant overflows a peripheral weir.  Settled solids are gently agitated by slow rotation of the scraper to dislodge gas bubbles, prevent bridging of the solids, and move slurry toward a central well for withdrawal.  Feed is provided continuously while the underflow may be extracted intermittently for further processing.

Figure 1. Gravity Thickener

FLOTATION THICKENING

Air flotation is most applicable in concentration waste-activated sludges and pretreatment of industrial wastes to separate grease or fine particulate matter.  Fine bubbles to buoy up particles may be generated by air dispersed through a porous medium, by air drawn from the liquid under vacuum, gases released  by electrolysis, or by air forced into solution under elevated pressure followed by pressure release.  The latter, called dissolved-air flotation, is the process employed most frequently in thickening sludges because of its reliable performance.

The major components of a typical flotation system are sludge pumps, chemical feed equipment to apply polymers, an air compressor, a control panel, and a flotation unit.  Figure 2 is a schematic diagram of a dissolved-air system.  Influent enters near the tank bottom and exits from the base at the opposite end.  Float is continuously swept from the liquid surface and discharged over the end wall of the tank.  Effluent is recycled at a rate of 30-150% of the influent flow through an air dissolution tank to the feed inlet.  In this manner, compressed air at 700-1000 kPa is dissolved in the return flow.  After pressure release, minute bubbles with a diameter about 80mm form and attach to solid particles and become enmeshed in sludge flocs, floating them to the surface.  The sludge blanket, varying from 200-600mm thick, is skimmed from the surface.  Flotation aids are introduced in a mixing chamber at the tank inlet.

Figure 2 Dissolved Air System

CENTRIFUGATION

Centrifuges are employed for both dewatering sludges and thickening waste slurries for further processing.  Applications include sludges difficult to dewater by gravity separation, such as alum coagulation residues and waste-activated sludge and lime-softening precipitates prior to recalcining.

All centrifuges have the same basic operating principle.  Solids are removed from the waste stream flowing through the machine under the influence of a centrifugal field of 100-600 times the force of gravity.  Particles are deposited against the spinning solid bowl while the overflow is a clear liquid supernatant.  The fundamental difference in centrifuges is the manner of solids collection and discharge - the method of discharge determines the size and nature of the particles removed by a particular unit.  Material encountered in wastes includes a broad range of granular, fibrous, flocculent, and gelatinous solids that differ in settling and compaction characteristics.  Therefore, the type of centrifuge adopted is determined by the particular waste as well as discharge requirements of supernatant clarity and cake dryness.  The two most popular types for handling sanitary wastes are the scroll or conveyor centrifuge and the imperforate basket.

Scroll centrifuges can handle large quantities of fairly coarse solids (Fig. 10).  Two principal elements are a rotating solid bowl in the shape of a cylinder with a cone section on one end and an interior rotating-screw conveyor.  Feed slurry enters at the centre and is spun against the bowl wall.Settled solids are moved by the conveyor to one end of the bowl and out of the liquid for drainage before discharge while clarified effluent discharges at the other end over a dam plate.  This system is best suited for separating solids that compact to a firm cake and can be conveyed easily out of the water pool.  

Figure 3. Centrifuge