PhD Research Program

University of Strathclyde, Glasgow (UK)

Oct. 1995- Sep. 1998


On the Role of Aluminium Hydroxide in the Conditioning of an Alum Sludge

by

Eleftherios N. Papavasilopoulos

Supervised by Dr D. H. Bache BSc PhD DSc MCIWEM, Reader in the Dept. of Civil Engineering at the University of Strathclyde.

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Contents

Useful Background Reading

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Problem definition

Alum sludges arise from the treatment of raw waters based on aluminium sulphate as the primary coagulant. The resultant alum sludge floc forms a three-dimensional network and is a two \endash phase mixture of solids and water. The water content varies between 95 to 99 %, this being typical of the levels found in such sludges before and after thickening. The solids can be thought as forming the skeleton o f the floc and arise from two sources: (a) the entrapment of impurities in the water (e.g. colour, turbidity, algae, hardness) and (b) the coagulant byproducts.

There is a wealth of research work providing insight into the mechanisms for colour and turbidity removal using alum as a coagulant. In the case of colored \endash upland waters, important mechanisms for colour removal are: (a) charge neutralisation \endash precipitation, (b) adsorption and (c) simultaneous precipitation. Often, waterworks operate within a pH range of 6 - 8 in conjunction with dosage rates of alum providing conditions for the formation of amorphous, Al(OH)3(s). In this instance, colour removal occurs by adsorption of the colour inducing substances (mainly humic and fulvic acids) onto the gelatinous precipitate of Al(OH)3 (s) through bonds which can include van der Waals interactions, H-bonding. The gelatinous precipitate can represent up to 40 - 50 % of the dry floc mass and is expected to have a major b earing on the overall sludge characteristics.

Of the various sludge handling methods in association with sludges from low in turbidity, coloured waters, dewatering by means of plate filter presses is generally a favoured option. Alum sludges are often regarded as "difficult to dewater". Following the general pattern of water release in a flocculated suspension, it is a characteristic feature of alum sludge that even at very large pressures the final cake is characterised by a 40 % moisture content. In the process of dewatering, work needs to be done in order to overcome the water binding forces (related to the chemical nature of the sludge particles) and the friction within the sludge cake as water percolates through the system (tied t o particle size). An added problem in dewatering of alum sludges stems from their compressible nature.

From the above it appears that the composition of the floc, its structure and the particle size distribution are all of major concern in dewatering. Evidence in literature indicates that difficulties in alum sludge dewatering are associated with the behaviour of the gelatinous aluminium hydroxide but this aspect has not been thoroughly explored. This suggests that dewatering is closely linked to coagulation practices. With regard to particle size there is a difficulty in defining what is meant by the unit size for sludges which tend to be flocculent by nature i.e. is it a fundamental particle size or is it tightly bound aggregat e s within the floc. Generally, sludge containing a large fraction of fine material dewaters poorly, this being largely attributed to the blinding of sludge filter cakes from the migration of fines. The remedy for this type of problem rests with the control of fines, either removing them (i.e. elutriation in wastewater sludges) or in the case of alum sludges, tying them together with polymers to form larger subunits as an aid to water release.

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Alum sludge conditioning and the concept of optimum dose

Alum sludge is often dosed with synthetic organic polymers in order to enhance dewatering. The process of conditioning has become an essential component of sludge management and can represent up to 50 % of the total costs. There appear to be a variety of polymers available to the water industry, the main difference between the various products being in their molecular weight and ionicity. The most recent synthetic polymers have molecular weights in the order of millions and are frequently used as conditioning agents.

It is generally agreed that polymer dosing causes a shift into larger size aggregates/groupings of flocs. The sludge attains a more open and porous structure and the dewatering rate increases. Despite the dramatic effect of polymers on the dewatering rates, views expressed in literature suggest that polymer conditioning is not matched by changes in the extent of water release. Full- scale investigations generally indicate that conventional conditioning and dewatering processes yield cakes, which are usually, characterised by a 18 - 25 % wt DS.

Considering the costs of conditioning and the potential health risks associated with overdosing (toxicity of residual monomers discharged to waterways), it is important to establish the appropriate dose (termed as optimum dose) during conditioning. The theme of optimum polymer conditions has been a target of much research over the last 25 years. However, from a n appraisal of the available literature, it appears that there is no universal criterion for determining the optimum polymer dosage. It is widely accepted that each sludge behaves in a different manner in terms of the particular polymer demand, this mainly arising from differences in the nature of the sludge. Given the variety of polymeric flocculants available to the water industry, the identification of the optimum conditions depends on a series of factors related to the sludge nature, the polymer propert ies, costs and disposal route and the dewatering efficiency. Thus, it is prudent to note that an optimal polymer dose actually depends on the specific requirements of the treatment process.

Generally, the symptoms of polymer dosing can be gauged by an a rray of tests. Standard tests such as the Capillary Suction Time (CST) and the Specific Resistance to Filtration (SRF) have featured for decades as means of selecting an optimum dose, the optimum often interpreted at the minimum CST and SRF. These standar d tests have been criticised over the years, limitations have been identified and several modifications have been suggested. New approaches include the investigation of the rheological properties of conditioned sludge, the viscosity of the liquid stream (centrate / filtrate) and the charge status of the conditioned sludge and / or the liquid stream. Despite the wealth of studies following the theme of optimum dosage conditions, little is known on the precise factors that control the optimum dosage as identified in the individual tests and how these should be interpreted.

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Main Aims of the study

The main aims of this study were:

(1) to provide insight into the factors that control the optimum dosage of a nonionic polymer when dosed in an alum sludge derived from treatment of coloured water - low in turbidity and

(2) to identify which of the tests (if any) provide the best guide of the optimum dose in practice.

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Conclusions

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The contributions of the research work

The main contributions of this research work can be listed as follows:

  1. the development of an SEC methodology for measuring polymer residuals following alum sludge conditioning can provide a valuable pathway for understanding polymer usage and its mechanisms;
  2. greater insight into the role of the hydroxide precipitate in controlling polymer demand and dewatering behaviour;
  3. insight into the mechanisms controlling the centrate viscosity behaviour;
  4. insight into the efficacy of laboratory - based test methods vis a vis the appropriate dosages used in practice.

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Useful reading

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