The best way co determine che performance a particular centrifuge can achieve on a particular sludge is co carry out full-scale trials on-site. Optimally these tests should be carried ou c wi ch a cen crifuge of che same type, model and size as considered for the fi nal installation. Alcernarively, an on-sire rest with a centrifuge of the same type and model, bur different size, may be undertaken , though the size difference should nor be coo large. Of course, chis procedure would have co be carried out with each manufaccurer being considered for a specific project. Results from laboratory rests should be used with caution. Typically, laboratory tests are carried out with small bench centrifuges or other batch-wise working devices. Dynamic effects caking place in a full-scale centrifuge which are important for its performance do not rake place in such laboratory tests. Therefore, such cescs can be used co determine the suitability of decanter centrifuges for a certain product, and also co some extent che approximate determination of suitable polymer, polymer consumption and cake dryness. For a comparison between bids however, these tests are not suitable. As for other key equ ipment, the value of thoroughly checking installation references cannot be stressed enough fo r decanter centrifuges. Speaking to users about their operating experience, performance achieved (and how that compares co the performance that was originally guaranteed), as well as the operators' experience regarding service frie ndliness, maintenance costs, and support from the manufact urer or his representative - all th is is probably the most valuable information of all.
Determining Suitable Centrifuge Duties While centrifuges may be operated at the manufacturer's maximum quoted hydraulic throughput, for some situations the cake dryness may be sub-optimal and the polymer dose rate may have co be high co achieve good solids capture. For this reason it is recommended that in the absence of test or full-scale operating experience on the sludge co be processed, a hydraulic loading rate of approximately 50-70% of che maximum throughput be used for sizing centri fuges as this will likely achieve a good balance between capital cost of the centrifuge cake dryness and polymer consumption. It is also desirable that che normal solids loading race not exceed 5070% of the manufacturer's maximum loading race for good cake dryness and low polymer consumption. The maximum raced solids load usually relates co che capacity of the sl udge conveyor rather than the capacity
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of the machine co perform optimally in terms of cake dryness and polymer consump tion. In che design of centrifuge installations, it is important that the right balance be set between being overly optimistic in the capacity of the machines and not being coo cautious. The degree of conservatism used in the selection of centrifuge selection is of course also dependent on che availability of on-site testing information, previous experience with the particular sludge co be dewacered, the budget available, as well as the relative importance of capital cost versus operating cost for a specific project. In any case, it is uttermost important co consider downtimes, planned and unplanned , in the design of a centrifuge plant, e.g. by installin g stand-by capacity, or by holding a spare rotating assembly in stock, or by over-sizing the decanters such, chat in case of one being out of service, the ocher units can cover.
Example 1 Three duty machines sized co operate 140 hours per week (20h per day) at the maximum weekly so lids production rate and conservatively loaded at about 50% of the maximum load ing race, also have che capacity co process che maximum weekly solids production with two duty machines operati ng 154 hours per week (22 hours per day) at 70% of che maxim um loading rate. During periods when the centrifuges are heavily loaded, the polymer consumption may be higher than normal and cake dryness may also deteriorate, but not to a significant degree. In chis case, it will likely be beneficial to operate all centrifuges when available co achieve che lowest polymer consumption, albeit at the expense of slightly higher energy consumption.
Example 2 Two machines sized co operate on a duty/standby basis 40 hours per week (8 hours per day) 5 days per week at che maximum weekly solids production race and loaded at 70% of the maximum loading rate, also have the capacity co process the maximum weekly solids production with two duty machines in 7 hours per day/4 days per week at 50% of the maximum loading race. Alternatively, one duty machine would have the capacity co process the maximum weekly solids production in 8 hours per day/7 days per week, at 50% of the maximum loading rate. This will provide a very flexible and robust dewacering facility where the preferred operating mode may be established by
Journal of the Australian Water Association
optimising the trade-off between operating times and polymer consumption.
Summary Several parameters for the assessment of a decanter centrifuge's capacity or fo r comparison of several decanter centrifuges have been introduced. A closer look at those parameters reveals that the best picture is obtained if not just one, but several parameters are analysed in the course of a bid evaluation, as several aspects of centrifuge physics are important for its performance. No single parameter covers all these aspects. When comparing the relative capacity of different centrifuges for wastewater sludge dewacering, the authors recommend chat che greatest weight be given co g-volu me with equivalent clarification area, g-force and operating speed used as seco ndary indicators of centrifu ge capacity and performance. In spite of these performance parameters, performance guarantees can only be assessed on che basis of truly comparable on-site tests. As these are often not available, the im portance of a thorough and diligent check of installed references is stressed. In the absence of on-site testing, or previous experience, a conservative approach is recommended, wherever the budget allows it. In any case, the management of planned and unplanned downtimes is a key aspect in the design of a dewaceri ng fac ility and should be carefully considered as pare of the design.
The Authors Keith Cadee is General Manager Water Tech nologies Division, Water Corporation, Leederville, Western Australia 6007. Email: keich.cadee@watercorporation.com.au; Arnim Hertle is Senior Process Engineer and Sales Manager of Hiller GmbH , Schwalbenholzscr. 2, D-84137 Vilsbiburg, Germany. Email: hercle@hillerzentri. de. He is also working as an independent process engineer, providing consulting services for aeration systems and sludge treatment systems. Email: arnim.hercle@gmx.net References Ambler, C. M. ( 1952) The Evaluation of Centrifuge Performance. Chem. Eng. Prog.
48:3 Vesiland, P. A. ( 1979) Treatment and disposal of Wastewater Sludges. Revised Edition, Ann Arbor Science Publishers, Inc. Perry, R.H. and Chilton, C.H. (1973) Chemical Engineers' Handbook, Fifth Edition; New York, New York. McGraw-Hill Records, A. and Sutherland, K (2001) Decanter Centrifuge Handbook, First Edition, Elsevier Advanced Technology, Oxford (UK)