A thickener is an equipment structure used for the continuous gravity settling (sedimentation) of solids in suspensions. Suspension is fed into one or more basins or chambers and, whilst it is passing through, the solids settle out. The thickened solids are removed together with a portion of the liquid as thickened "underflow". The liquid, ideally containing no solids, forms the "overflow" from the thickener. Thickeners vary widely in size and configuration, but they all comprise: a. a vessel to provide volume and area needed for thickening, with the area being large enough to allow the solids to settle at a velocity faster than the upward velocity of the liquid; b. a system for introducing the feed and directing it into the flow paths that best utilize the vessel volume and area; c. an overflow system for collecting clarified liquid; d. a mechanism to convey settled solids to a discharge point.
Input suspension (carrying settleable solids) passes through supernatant layers (in the clarification zone) and spreads out as a feed layer at a depth where the solids concentration corresponds to that of the feed. The major part of the liquid component in the feed rises, carrying with it the finer solids (those particles whose terminal settling velocity is less than the rise velocity of the liquid). Flocculant is often added to the feed to assist separation of the fine solids; if it is, the solids continue to flocculate and in a well sized system will be removed in the clarification zone. Solids settling out of or through the feed layer pass into a critical zone. The solids then pass into the compression zone, where they are subjected to an increasing solids stress (arising from the weight of solids above) as they move deeper into the compressing layer. They are thus compacted or thickened.
A critical settling zone will only exist when the feed flux of solids exceeds that which can be transmitted through some limiting or critical concentration, i.e., when the thickener is fed and solids overload. The critical concentration therefore constitutes an upper boundary for solids loading, and zone settling imposes an area demand on thickener design. When the feed flux of solids is below the critical value, solids pass down through the zone faster than they are replenished at the top and the zone disappears. Hence, zone settling places no depth demand on thickener design. The general design of thickeners does not follow strict geometric proportions; the relationship between depth and diameter is important only to the extent that the tank volume will provide sufficient retention time, taking operating efficiency and mechanical design into account.
Fitch, E. B. and Stevenson, D. G. (1986) Solid/Liquid Separation Equipment Scale-up, Ch. 4 D. B. Purchas and R. J. Wakeman Eds., Uplands Press and Filtration Specialists, London.
Osbornie, D. G. (1990) Solid-Liquid Separation, Ch. 5, L. Svarovsky, Edn., Butterworths, London.