Sample flocculation assessment

When a sample of sludge arrives in the laboratory for flocculant assessment, there are a number of common tests which are conducted. Its settling rate under gravity is observed and its pH is measured. A small sample, about 50 ml, is spun in a laboratory test-tube centrifuge for a fixed time, say 5 to 10 minutes. The g-level at the tip of the centrifuge will be of the order of 1500 to 2000 g. The volume of sediment will be recorded. 

One can also examine the dispersion with a microscope one must exercise caution in preparing the samples for examination. In general, it is necessary to dilnte the sample. If the sample is diluted with solvent, there is a possibility of floccnlation. Electron microscope studies of the surfaces of etched dry coating films can be nseful for assessing variations in dispersion (242). Flocculation gradient... 

Filtration of flocculated suspensions can be assessed using the filterability test method described in chapter 11. Using the filterabihty number F against the product Gt an optimum flocculation can be determined for filtration through a sample of porous granular material as exemplified in Figure 4.16. 

In all the above methods, care should be taken in sampling the suspension, which should cause as little disturbance as possible for the structure to be investigated. For example, when one investigates the flocculation of a concentrated suspension, dilution of the system for microscopic investigation may lead to break down of the floes and a false assessment is obtained. The same applies to examinations of the rheology of a concentrated suspension, since transfer of the system from its container to the rheometer may lead to a break down of the structure. 

From measurements of some of the eight process parameters mentioned, it is required to assess the performance of the decanter. It is normal to monitor the feed rate, Qf. and the additive rate. Qp, with flow meters. Periodically, gravimetric analyses are conducted on samples of feed, cake, centrate and, if necessary, the additive. Performance is judged by how high is the solids recovery, R, and how low is the flocculant dose, Pd, when this is used. Recovery is the percentage of solids in the feed that reports to the cake discharge. Flocculant dose, sometime referred to as polymer dose, is the amount of dry polymer used per unit dry solids in the feed, usually expressed as kg/t db (kilograms per tonne dry basis). 

In most applications, the amount of polymer used is just sufficient to flocculate a sample of the feed. The amount necessary, as assessed in the laboratory, is generally the amount used in practice on the centrifuge, plus or minus a small fraction. However, recently there has been considerable development in decanters and their use in obtaining extra-dry cakes from compressible sludges, particularly effluents. In these instances, the consumption of polymer has increased considerably. 

The analyses mentioned so far all help to form a picture of the sludge to compare with past experience. This experience will enable the choice of a range of likely polymers that might be effective. A selection of anything from 10 to 20 polymers would be made, and preferably fresh samples of the flocculant will be made. Alternatively, already prepared concentrated solutions will be diluted to provide, say, 100 ml samples at 0.1% concentration. Alternatively, for expediency and economy, old (but not too old) samples will be used initially, and then a narrower range of fresh polymer samples will be made up on the basis of the initial assessment. The initial assessment is made by adding aliquots of each polymer to separate measured quantities of sludge. 

Further laboratory tests may be instituted to assess the dewaterability of the floes produced. There are a number of techniques employed for this. A flocculated sample probably would be recentrifuged in the laboratory bottle spinner to observe any change in the settled volume. 

A flocculation test for the detection of increased levels of 7-globulins in serum. It consists of addition of the serum sample to zinc sulphate solution and assessing the degree of turbidity, which is proportional to the increase in the 7-globulin fraction. 

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