One of the largest applications for the decanter centrifuge, certainly in terms of machines sold, is the dewatering of waste slurries arising from industrial processing, from raw water treatment, and from the treatment of municipal sewage. Almost all industrial manufacturing processes generate liquid wastes containing suspended solids; even an industry as apparently unconnected with liquid processes in its operations as machinery manufacture. Machining uses complex liquids for cooling and lubricating machine tools, which then become contaminated with dirt and metal scrap.
Machine tool fluids are usually valuable enough to need recycling, and so they must be cleaned before re-use. This is also true of many industrial wastes, that the liquid, once cleaned, or the separated solids, once dewatered (and, perhaps, washed) can be recycled profitably. However, the main bulk of waste slurries have to be dewatered in order that the suspending liquid can be discharged to a river or lake without polluting it, and the solids can be sent for final disposal in as small a bulk and as safe a condition as possible.
Waste sludges, once adequately dewatered, can be sent to landfill, or for soil improvement or fertiliser use, or to incineration. For most of these final destinations, a high degree of water removal is beneficial, and the decanter’s ability to achieve high drynesses has led to its wide acceptance in the treatment of waste slurries.
The wastes produced in manufacturing processes vary in composition as widely as does the nature of the slurry-producing process. The range of modifications to the basic decanter structure enables it to cope with this wide variation in feed compositions.
Whether the ultimate destiny of the final sludge is to be recycled within the factory, or to be sent away for landfill, there is usually sound argument in favour of discharging it from the decanter in as dry a state as possible. The materials from which the decanter is made will have to be chosen well to match the likely corrosive or abrasive nature of the slurry components.
The variety in the nature of sludges from industrial sources can be seen from the following list of manufacturing and other processes creating waste slurries in need of treatment:
manufacture of acetylene
manufacture of aniline
steel works‘ blast furnace operation
processing of cellulose
coal processing wastes
extraction of coffee
decortication (debarking) of wood
flotation sludges from the de-inking of recycled waste paper
dye and pigment processing wastes
sludges from electrolytic and electrochemical processes
fish transport water
wet processing for the desulphurization of flue gases
iron ore scrubber slurries
paper mill operations
nuclear fuel and spent fuel processing
manufacture of TV tubes
all of which, and more besides, use decanters to dewater the slurries. In particular, it should be noted that, like the dust scrubbers from iron making, an increasing demand for better air pollution prevention is increasing the number of wet scrubbers in operation. Each scrubber installation produces a slurry of the removed dust, which becomes a potential decanter application.-Decanter centrifuge using in Industrial wastes
For any waste which is produced carrying an oily liquid as well as water, then the oil can be recovered separately from the water and the suspended solids, by the use of the three-phase decanter. This will occur especially in the oil refining and blending processes, but also in several food industry applications. The treatment of refinery slop oils is an important decanter application.
The increasing need to treat sludges of all kinds, especially toxic ones, is leading to the development of total sludge treatment systems, which aim to destroy all organic matter and leave a neutral inorganic sludge. Wet air oxidation was the first of these, and this has recently been joined by Kemira’s “
Water treatment sludges
The boundary between what is a direct-line-of-process slurry, and what is a “utility” application, is not easily drawn in the case of the treatment of raw water, surface or ground, to produce drinking water, or water for process use. Raw water is allowed time to settle out most of its suspended dirt, but coagulants may be used to hasten the process. The resultant slurry is, without doubt, a waste material, especially if the dirt content is high. If a high load of coagulant like alum is used in the treatment, then there may be a case for the recycling of the separated solids, for re-use, after suitable processing.
As well as the removal of sediment, waterworks may also soften the water, and may use lime for that purpose (as opposed to fixed bed ion exchange), again producing a sludge needing dewatering in a decanter.
For waterworks applications, decanters normally will be operated with polymer addition facilities, at alternative admission points. They will need full erosion protection for the flights, using tiles. They will have some kind of cake baffle, possibly a cone. and a variable speed back-drive, with good differential and torque control. They will be operated with deep neutral ponds, with the
Municipal sewage treatment
Probably the fastest growing, in market size terms, of all decanter applications, the treatment of sewage sludges is a vital part of the developed world’s attempts to improve its relationship with its environment. Now that dumping of sewage sludges at sea has finally stopped, the sewage from 15- 20% of the world’s population is now fully treated, giving rise to vast quantities of sludges.
The full sewage treatment process has three main stages:
- primary sludge, possibly mixed with screenings, which is a comparatively easy task;
- secondary (biological) sludge, which is not easily dewatered, and so is usually mixed with primary sludge, to give a third variety;
- mixed sludge;
- digester sludge.
The decanter can handle all of these sludges, with varying degrees of separation efficiency.
The use of coagulants and flocculants in the treatment of municipal sludges is now commonplace, and, indeed, the decanter would not be the efficient dewatering machine that it is today in the absence of modern, synthetic flocculants.
Secondary sludges are soft, and not easily dewatered, so that the role of the decanter in the processing of such sludges is more as a thickener, than as a dewatering tool, probably ahead of digestion. For this duty, the decanter will need full erosion protection, preferably with tiles on the conveyor flights. The decanter will operate at maximum speed with axial flow, with a deep neutral pond, and a pond setting just shallower than neutral. It should have a backdrive with good differential control, and a standard ratio, low torque gear box. Wash-out prevention and a cake baffle are necessary. An inflatable weir dam is a good idea, and can prevent wash-out. Sigma enhancement (i.e. the use of vanes, similar to the disc stack of a disc centrifuge) can be advantageous, but polymer addition may not be necessary, depending upon the degree of dryness required.
If digestion is not an option, then the secondary sludge could be mixed with the primary sludge and screenings, and this mixed sludge (or primary on its own) can then be efficiently dewatered by the decanter. A resultant dryness of 18-22% can readily be achieved. With dry solids operation, this can be increased to 25-30%, or even 30-35% solids (an appearance of being solid). The relative proportions of primary to secondary sludge, and of municipal to industrial, will affect the final figures. Uncommonly, figures of 40-50% have been experienced.
The specification for a decanter to handle primary sludge alone, or mixed primary and secondary, or digester sludges, would be as follows:
- polymer addition essential, with the ability to add internally;
- full erosion protection essential, using flight tiles;
- cake baffle useful;
- back-drive required, with good differential control;
- standard bowl speed sufficient (g levels up to 2500 depending upon precise design);
- wash-out prevention necessary on start-up (such as notched weir plates);
- deep neutral pond (better performance than shallow pond);
- axial flow.
For the more abrasive sludges, a casing wear liner will probably be needed, while for the most abrasive sludges, hard surfacing should be added to back and front of the flights around the feed entry into the pond.
For dry solids operation, the above applies, plus:
- negative pond operation;
- high-ratio (three-stage if necessary) high torque gearbox, with good torque control;
- a cake baffle or restriction of some form, or its equivalent, is essential.
The processes, such as those already mentioned (in Section 3.3.1), which treat whole waste slurries and destroy all organic matter, will find a place in municipal sludge treatment once they become established, considerably extending the decanter market.