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DEWATERABILITY OF ACTIVATED BIOSOLIDS

Summary

Activated biosolids arise from many aerobic treatment processes, and the results in terms of dewatered cake solids from installations around Australia have shown a great deal of variability. By analysing the dewatered cake solids against the percentage of non-volatile solids (ash) in the dewatered cake, a strong correlation between the two has been established. This allows specifiers and suppliers of dewatering equipment to characterise the biosolids, in order to form a basis for agreed performance of the equipment.


A simple observation method is also proposed, which allows a quick evaluation of whether the dewatering device is close to the optimum performance on a particular biosolids mass. The data is based on conventional belt press performance, however it is also applicable to conventional centrifuges. A new generation of belt presses and centrifuges offer higher performance, but this is still within the expected performance band of the data.
 

Introduction

Activated biosolids processes are currently in fashion around the world, and particularly in Australia, due to our relatively low energy costs. These processes include intermittently decantered aeration, continuous aeration, secondary activated biosolids processes, and biological nutrient removal. All of these processes result in excess biosolids, which need to be dewatered for disposal or further processing. A wide variation in the solids content of the biosolids cakes, ranging from 11% solids to 20% solids, has been encountered at plants of similar process design, even using identical dewatering equipment, at different installations. The physical nature and appearance of the cake can be the same for very different solids contents.


It is proposed to use the non-volatile (ash) solids content of the dewatered cake as an indicator of the attainable dewatered solids content of a particular biosolids population. We use ash content of the filter cake, rather than the feed suspension to the dewatering plant, as dissolved solids in the liquid phase can provide variable values through the treatment process. The variety of treatment processes in use today, as well as variable influent, can result in varying ash contents of the cake, and this value is often not stated or known at the pre-construction phase.

Discussion

Physical Appearance

A factor which is often not realised by engineers working purely on a theoretical basis, is that even though the solids contents of the filter cake might range through a wide set of values, the appearance and the handling characteristics of a well dewatered biosolids cake are often virtually identical for a 11% solids cake compared to an 18% solids cake. This is particularly so in the case of the belt press, where the shearing action and the compressive processes, produce a cake which is easily handled and of a dry appearance.

Simple Test
A simple but reliable test of the dewatering efficiency of a particular system can be made by observing the cake as it discharges from a belt press. If it is of a dry, crumbly appearance, and a sample when compressed in the hand only produces a few drops of moisture, then it is well dewatered.

High Solids Contents

Anecdotal evidence suggests that in some countries, such as China, the attainable cake solids in an activated biosolids process is in excess of 20% solids, however this data has not yet been confirmed by the author. It is however, believed that the high proportion of industrial wastes in the influent to the treatment works, can raise the ash content of the cake, and therefore the solids content. Also, some plants dosed with metal ions, or lime, can have a higher biosolids ash, and a few such plants exist in Australia, particularly in Sydney. Biological Neutrient Removal plants also appear to produce higher cake solids.

Getting More Predictable Results

In the past there have been instances where dewatering equipment suppliers have been penalised due to the actual plant dewatered biosolids cake solids content falling below an arbitrarily fixed design figure. The results of this survey, show that if the ash content is known prior to installation, the results will fall within a narrower band, and the performance of the equipment can be predicted more accurately.

Table 1 above shows a summary of the data at the individual plants. The majority of the results were with one manufacturer's range of belt presses, however other manufacturers are included. In addition, some centrifuge results are also shown. The results are a combination of data taken from grab samples on operating plants which are not necessarily optimised, as well as data from acceptance tests, where the equipment has been optimised by skilled operators.
 

The results expressed in graphical form, shows a good correlation between the non-volatile solids (ash) content and the attainable cake solids. The scatter in results can be attributed to the fact that some of the plants were not optimised (as described above) and variations in the selection of the equipment, as well as the design of the individual piece of equipment. Most biosolids dewatering equipment does not have an absolute maximum capacity, and the operating point for the equipment could be above or below the nominal capacity of the dewatering device. However, the normal range of commercial loadings usually results in a variation of 1 or 2 percentage points in cake solids when the throughput is reduced, unless the dewatering plant is heavily overloaded.

In general, the processes employed in the treatment plant do not appear to have a direct measurable affect on the attainable cake solids, except that when dosing with metals is part of the treatment process this can result in a higher ash content of the cake. Similarly, as discussed above, a high incidence of industrial influent can have a similar affect. Biological Nutrient Removal processes appear to produce quite a good cake, as do some intermittent processes with a secondary aerobic digestion.


Dewatering of anaerobically digested biosolids is a well established technology, and results from European experience are shown in figure 2. This shows a similar affect and correlation between the cake solids and the ash content.

 

Polymer dosage

A secondary factor in the influence of the ash content of the cake, is that a higher ash cake, will produce an apparently lower polymer dosage rate. This is due to the fact that the non-volatile solids generally have a lower polymer demand, and when the rate is expressed as a dosage rate per tonne of dry solids, then the numerical result is a lower figure.
 

New Generation devices

The latest designs of high solids belt presses and centrifuges can achieve higher solids contents than as indicated by the regression line of the graph of cake solids v ash content. The diaphragm type filter press can achieve similar results. This type of equipment is generally of the order of twice as expensive as conventional equipment, and would only be used in Australia in instances where the small difference in water removal is significant in terms of disposal costs.


In Australia, where landfill and further treatment such as dosing with lime for stabilisation is currently used, and land costs are in order of magnitude less than those in Europe and the Americas, the small difference in water content has to date not justified the cost of the new generation of dewatering equipment.

 

Conclusions

A correlation between ash content and dewaterability of activated biosolids from aerobic processes has been established, and can now be used as a benchmark for performance of dewatering equipment. The characterisation of the nature of the activated biosolids, can therefore be enhanced by a nomination of the anticipated volatile solids and non volatile solids of the dewatered cake. Treatment plants with a high proportion of industrial waste in the influent can produce a higher ash content biosolids cake, and hence a higher solids cake.


If a higher solids content than the regression line is desired, the new generation of dewatering equipment is required, but the cost will be much greater.

References

"Belt Press and Gravity Table Dewatering Process Performance-Myth and Reality"- Ian H Bane -AWWA Qld Regional Conference Cabarita Lakes 1990.
"Solid Bowl Centrifuges for Wastewater Sludge Treatment"- E.A.Retter and R Schilp-Filtration and Separation - June 1994
"Pilot Trials at C.O.R.D.A.R. (Italy) Comparison between BPF WR new generation Belt press and Noxon and Humbolt high performance Centrifuges". (Unpublished)
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