Sedimentation is the process of allowing particles in suspension in water to settle out of the suspension under the effect of gravity. The particles that settle out from the suspension become sediment, and in water treatment is known as sludge. When a thick layer of sediment continues to settle, this is known as consolidation. When consolidation of sediment, or sludge, is assisted by mechanical means then this is known as thickening.
In water treatment sedimentation might be used to reduce the concentration of particles in suspension before the application of coagulation, to reduce the amount of coagulating chemicals needed, or after coagulation and, possibly, flocculation. When sedimentation is applied after coagulation, its purpose is usually to reduce the concentration of solids in suspension so that the subsequent filtration can function most effectively.
Sedimentation is one of several methods for application prior to filtration: other options include dissolved air flotation and some methods of filtration. Generically, such solids-liquid separation processes are sometimes referred to as clarification processes.
There is a variety of methods for applying sedimentation and include: horizontal flow, radial flow, inclined plate, ballasted floc and floc blanket sedimentation.
Standard texts should be referred to (for the time being) for the theory of sedimentation. It is relevant to note that development of the theory is based on the initial assumption the particles to be removed from suspension are spherical and solid, whereas in practice they are irregular in shape, exist as a wide range in size and can be permeable to flow of water. The challenge for researchers is to develop the theory to take account of these realities. The theory reflects two basic forms of sedimentation: unhindered and hindered settling.
When the concentration of particles in suspension is small, approx < 1% v/v, the particles settle with negligible impact on each other. As particle concentration increases beyond about 1% then adjacent particles progressively affect, i.e. hinder, the behaviour of each other. When particle concentration exceeds about 8% v/v the suspension is in the fully hindered regime (see method of concentration measurement below). The properties of the suspension are now a function of the volumetric concentration of the particles in the suspension, as well as dependent on other properties of the particles. With increasing particle concentration the degree of contact between particles progressively increases
When the particle concentration in the suspension is about 18% v/v then there is a transition in how the suspension appears. For concentrations greater than about 18% v/v, the upper interface between the water and suspension should be distinct. For concentrations less than about 18%, the interface is diffuse, increasingly so with smaller concentrations.
When the particle concentration is about 26% v/v adjacent particles become in permanent contact with each other such that the suspension enters the thickening regime.
Hindered settling in water treatment exploits suspensions with concentrations between about 12 and 22% v/v. A suspension in this range is called a floc blanket, which is actually a fluidised bed.
Method of particle volume concentration: In water treatment the particles, flocs, produced by coagulation are readily deformed. Therefore measurement of volumetric concentration has to be a compromise. A method used for more than 40 years is to collect a sample of the suspension to the top mark in a graduated cylinder, e.g. 1L, which is then left undisturbed for 30 minutes. The proportion of the liquid volume occupied by the settled particles after 30 minutes is taken as the particle concentration in the suspension.
Types of Sedimentation Tanks
Horizontal flow tanks
The simplest form of sedimentation is to fill a jar or tank with water, leave alone for a long enough time for particles to settle and then decant off the resulting water without the sediment. In practice this is rarely viable in treating water for townships, and therefore sedimentation tanks are operated continuously.
The simplest method of sedimentation is to use rectangular tanks with horizontal flow through them. The water with the particles in suspension is introduced at one end of the tank, then as the water flows to the other end of the tank settlement of particles in the water occurs. The aim is that a large proportion of the settling particles manage to reach the tank floor before the water is drawn out of the tank at the outlet end. Such horizontal flow tanks are usually built with a floor that slopes gently down to the inlet end to a hopper. The tank is fitted with a mechanism to scrape the sediment from the outlet end back to the inlet end and into the hopper from where it can be discharged hydraulically. In the design of such tanks detailed attention has to be given to the inlet and outlet ends so that the water flows from one end to the end as uniformly as possible.
Partly because rectangular tanks have a large footprint, multi-layer tanks (i.e. two or three decks) have been built. These tanks are usually multi-pass in that the water flows along the length of one layer before returning along the next.
Radial flow tanks
Radial flow tanks are circular with the inlet for the water at the centre and a peripheral outlet. Attention has to be paid to the design of inlet to support uniform distribution of flow to the whole of the tank. The sediment is scraped to a central hopper for its discharge.
Some circular tanks include additional features in the centre for flocculation (i.e. premix designs) and even recirculation of settled particles (i.e. premix-recirculation designs).
In unhindered settling tank size is governed by the time to be allowed for particles to settle through the depth of water. The simple theory shows that efficiency of removal of particles is governed by the area available for settlement. An approach to providing a large are with a small footprint is to use inclined plates or tubes. These are usually constructed with lightweight material in modular form which can be easily positioned in a concrete or steel tank.
Flow between such inclined plates can be co-current, counter-current or cross-flow. In the co-current arrangement, the water flows downward between the plates n the direction of particle settlement. In the counter-current arrangement, the water flows upward between the plates against the direction of particle settlement. In cross-flow the water flows across the plates, i.e. horizontal, at right angles to the direction of particle settlement. A design challenge for inclined settling is to maximise distribution of flow of water within and between plates and thereby maximise efficiency of particle removal. Inclined tubes can be used in either co-current or counter-current modes, but in most instances in the latter. Tube modules can be constructed in a variety of ways such that the cross-sectional shape of tubes can take various forms.
The density difference between water and the particles produced in water treatment by coagulation, flocs, in general is small. Therefore they settle slowly. Methods of plain sedimentation (i.e. horizontal, radial and inclined sedimentation) are preceded by a slow mixing process called flocculation. The purpose of flocculation is to assist coagulated particles to collide and adhere so as to grow into larger particles that might settle faster, and for the particle size distribution to be as small as possible. Flocculation can be assisted by the application of high molecular weight polymers called polyelectrolytes. In the first instance flocculation does not increase particle density – a property of flocs is that their density decreases with increase in particle size.
The density of floc particles can be increased by application of a ballasting agent such as Bentonite or fine sand. In the case of fine sand (80-200 µm), it is possible to recover it for recycling by means of passing the sludge collected from the sedimentation tank through hydrocyclones. The settling rate of floc ballasted with fine sand can be three or more times faster than floc that is not ballasted, and is usually settled in a tank with counter-current inclined plate system placed over a hopper in which the settled sludge is collected.
Floc blanket sedimentation
The first floc blanket tanks had an inverted pyramidal shape topped by a short vertical section. (‘Floc blanket’ is used in preference to ‘sludge blanket’. This is because the view is taken that floc in the suspension is still functioning as floc should, helping to remove the particles in the incoming water. Once floc becomes part of sediment and become surplus to the process, the sediment is sludge. Therefore, when sediment is allowed to accumulate on the floor of a tank that might appropriately be referred to as a sludge blanket, as is typical in sewage treatment.) The incoming suspension was fed downward into the apex of the pyramidal hopper by a single pipe. The blanket occupied most of the pyramidal hopper. The comparative success of floc blanket settling resulted in adaptation of premix-recirculation tanks being developed to include floc blanket zones. However, more effective developments and cheaper designs led to use of flat-bottomed rectangular tanks fed by an arrangement of multiple inlet pipes – candelabra like, or horizontal pipes with multiple orifices close to the tank floor. Further developments have led to the use of inclined plate or tube modules in such tanks.
An alternative to using a clarification process based on coagulation and flocculation for treating waters containing little mineral turbidity, is use the Sirofloc® process. Finely divided magnetite prepared by washing with a solution of sodium hydroxide at high pH is applied to the raw water together with a small concentration of a cationic polyelectrolyte . The magnetite particles absorb colour-producing dissolved organic substances and attract fine colloidal particles. The suspension is then passed through a magnetic field that causes the magnetite particles to be attracted by each other so forming into larger clumps. The magnetised suspension then passes into a radial flow sedimentation tank in which the clumps of magnetite settle. The settled magnetite is recovered by passing through hydrocyclones before being reconditioned with sodium hydroxide solution.
Gregory R. and Edzwald J.(2010) Sedimentation & Flotation, Chapt.9 in Water Quality & Treatment, 6th Edtn., AWWA & McGrawHill.
Bache D.H. and Gregory R. (2007) Flocs in Water Treatment, IWA Publishing