Lene Haugaard Mikkelsen

Physico-chemical characteristics of full scale sewage sludges with implications to dewatering

An investigation was carried out for a variety of different sewage sludges in order to establish correlations between sludge composition, structure and dewatering properties. Results indicated that the fraction of extracellular polymeric substances (EPS) in sludges was the most important parameter with respect to sludge structure. With high EPS contents, sludges had a lower shear sensitivity and lower degree of dispersion. This in turn lead to better filterability in terms of low resistance to filtration (SRF). The floc stabilising role of EPS components was not consistent with DLVO-theory, as the zeta-potential increased with increased EPS content due to increased EPS charge content. This indicates that polymer entanglement is a key factor to stable floc structure. This does not rule out the possible change in dispersion due to changed electrostatic repulsion for a given EPS content. While EPS had a good effect on floc stability and filterability, the cake dry matter content decreased with large EPS contents. This could be due to an osmotic pressure related to the polymer charge quantity, or it could be caused by water entrapment in the floc structures. A high degree of sludge dispersion increased the cake dry matter content in filtration. This mechanism is, however, impractical due to high SRF and not important to conditioned sludge. In practice, dewatering also includes sludge expression. Taking this into account, osmosis related to EPS charges is likely to be increasingly important (increasing the negative effect of EPS content on cake dry matter).

The shear sensitivity of activated sludge: relations to filterability, rheology and surface chemistry

Abstract Many solid/liquid separation properties are dependent upon the number of fine (micron sized) particles present in the suspension, which in turn is a consequence of the flocculation/break-up balance of the sludge. The present study revealed strong correlations between the dispersed mass concentration, resistance to filtering (capillary suction time) and rheological parameters for activated sludge suspensions of varying solids content. It seems likely that the restriction of flow caused by development of inter-particle networks causes increased surface erosion of primary particles. The increased dispersed mass concentration in turn causes deterioration of filterability. The shear sensitivity parameter quantifies the degree of dispersion in response to turbulent shear and is thus expected to be of prime importance for solid/liquid separation processes. The parameter reflects changes of the surface chemistry, and is therefore suitable as a means of assessing variations of surface chemistry for full scale activated sludge plants. Differences in interaction energies of different sludges may be estimated from shear sensitivity estimates. The observed changes in interaction energy upon addition of sulphide or detergent, changed pH or lowering of the ionic strength were in qualitatively agreement with DLVO-theory.

The shear sensitivity of activated sludge: an evaluation of the possibility for a standardised floc strength test

The presence of micron-sized primary particles is critical in many solid/liquid separation processes, and characterisation of stability with respect to the degree of dispersion is therefore relevant. Characterisation of floc strength so far has mainly dealt with the size of flocs and not addressed quantification with respect to primary particles. In the present work floc strength was quantified in terms of the shear sensitivity (kss) as a standardised parameter based on the recent adhesion-erosion model (AE-model). The shear sensitivity quantifies the degree of dispersion for low total solids contents and intermediate turbulent shear rates. It reflects the affinity of adhesion/erosion of primary particles to and from flocs and thus provides a means of characterising the interaction energy between the sludge colloids.

Applications and limitations of the colloid titration method for measuring activated sludge surface charges.

Surface charge quantification of polymer solutions and sludge suspensions were investigated by the colloid titration technique and compared to charge densities obtained by pH-titration. The colloid titration technique worked well for polymer samples. The charge quantity of humic acid and activated sludge extracellular polymers (EPS) was estimated to be -1.51 and -0.42 meq/g, respectively. These values are reasonable when compared to pH-titration results. The surface charge of activated sludge particles appears to be below the limit of detection. However, surface charge estimates are obtained, when the reactant doses and sample concentration are increased. It is suggested that such estimates are not correct, but artefacts of the non-stoichiometric precipitation of the polymeric reactants at high doses. It appears that the colloid titration method is limited to conditions of low reactant doses and valid for charge determination of extracted sludge polymers, whereas the method is not valid for charge determination of whole sludge.

Variations in microcolony strength of probe-defined bacteria in activated sludge flocs

The strength of activated sludge flocs is important for the flocculation, settling and dewatering properties of activated sludge and thus the performance of wastewater treatment plants. Little is known about how different bacteria affect the floc properties, so in this study it was investigated whether the strength and other characteristics of large microcolonies within activated sludge flocs from a full-scale nutrient removal plant varied significantly between different phylogenetic groups of bacteria. The investigation was carried out by using a shear method for deflocculation of activated sludge flocs, combined with different chemical manipulations under defined conditions. The identification and quantification of the microcolony-forming bacteria were conducted with group-specific gene probes and fluorescence in situ hybridization. The focus was on the microcolonies and not on the entire sludge flocs. In general, the results showed large difference in the strength and colloid-chemical properties of the different probe-defined microcolonies. By applying extensive shear to the system, less than 12% of the microcolony biovolume of the Beta-, Gamma- and Deltaproteobacteria and Actinobacteria could be disrupted, thus forming strong microcolonies. Alphaproteobacteria and Firmicutes formed weaker microcolonies (42-61% could be disrupted by shear). For most groups, several intermolecular forces determined the strength of the microcolonies: hydrophobic interactions, cross-linking by multivalent cations and perhaps entanglements of extracellular polymeric substances. However, the dominant force varied between the various phylogenetic groups. The large difference between the different phylogenetic groups indicated that only a few species were present within each group, rather than many different bacterial species within each phylogenetic group had similar floc properties.

Precipitation of humic acid with divalent ions

The aim of this study is to investigate precipitation properties of humic acid (HA). This is done by studying a commercial available humic acid salt (HA) from which a phase diagram is established by adding various amounts of BaCl2 to different concentrations of HA at pH 5.5. The phase diagram shows tree characteristic regions with markedly different precipitation courses: region I at HA concentration below 0.15g/1, region II at HA concentration between 0.15 and ~2g/1 and region IV at HA concentration above 3.5g/1. Furthermore, a forth intermediate region III is observed between ~2 and 3.5g/1.In order to understand these measurements and precipitation behaviour a phase diagram model is constructed using a simple thermodynamic approach. The result of this analysis shows that precipitation in region I and II can be explained by a simple equilibrium reaction between the HA and Ba2+ .The phase diagram model can not account for the observed precipitation in region III and IV. However, in region IV the HA appears to posses colloidal properties, which is supported by precipitation taking place at a constantξ-potential-21mV with the same amount of added barium ion regardless HA concentration. With respect to region ill, it is not exclusively determined whether precipitation is caused by HA behaving as a polyelectrolyte or possessing colloidal properties. The general observation throughout is that HA appears to behave as a polyelectrolyte at low concentrations and exhibits colloidal properties at high concentrations.