Dewatering of disintegrated excess sewage sludge

Sewage purification results in a large amount of organic sewage sludge, which consists mainly of micro-organisms. The cell membranes of the micro-organisms can be destroyed by mechanical disruption. The research work concentrates on the improvement of anaerobic digestion (Kunz et al., 1994; Müller, 1996; Kopp et al., 1997; Dichtl et al., 1997). One objective of the disintegration of sewage sludge is the improvement of dewaterability. By destroying the cell walls the intra-cellular water is set free and can be separated mechanically when dewatering the sludge.

Mechanical disintegration is a well established process for obtaining intra-cellular products such as proteins or enzymes from biotechnological applications (Schwedes and Bunge, 1992). The disintegration methods investigated in this research work have all been proven to be suitable for breaking up micro-organisms. For continuous operation stirred ball mills and high-pressure homogenizers are applied. The impact of disintegration on sewage sludge characteristics and dewaterability have been examined. Mechanical disintegration of sludge destroys the floc structure and increases the amount of colloidal particles (Kopp et al., 1996). The amount of colloidal particles has a great effect on polymer-demand and dewaterability.

The excess sludge was disintegrated by a stirred ball mill (Netzsch, type LME 4) and a high-pressure homogenizer (APV-Gaulin, LAB 60). For examination of cell disruption rates the excess sludge was treated with different operational parameters. The duration of grinding, the agitator speed and the size of the grinding beads of the stirred ball mill and the fluid pressure (D p) of the high-pressure homogenizer were varied.

The rate of cell disruption can be measured using two biochemical parameters. To determine the disintegration rate DRo the oxygen consumption must be measured. Then the defined specific oxygen consumption OCd of the disintegrated sludge has to be related to the specific oxygen consumption of the untreated sludge OCo.

DRo = 1 - (OCd/OCo) [%] (1)

Using the chemical oxygen demand (COD) a maximum release has to be determined by an alcalic total fusion process (CODa).

The disintegration rate can be described by the following equation using the CODo of the untreated and the CODd of the disintegrated sludge (COD values were analysed in filtrated samples):

DRCOD = (CODd - CODo) / (CODa - CODo) [%] (2)

After mechanical treatment the excess sludge was dewatered using a beaker centrifuge at centrifugal accelerations (a) of 1500 to 27000 * g, a decanter at a centrifugal acceleration of 900 * g and a chamber filter press at a filtration pressure of 13 bar. Additionally a high-pressure (150 bar) filter (Hess et al., 1991) was used for the post-treatment of the filter cake. During the experiments with the beaker centrifuge no conditioning of the sludge took place. Using the decanter and the filterpress conditioning was achieved with organic polymers or ferric salts. An optimal conditioning (g conditioning agent/kg SS) was achieved by measuring the streaming-potential. During this conditioning agents were added untill the isoelectric point was reached, where no electrostatic forces affect the particles. The dry solids content (DS) of the sludge cake was determined under deduction of the conditioning agent-mass.

RESULTS AND DISCUSSION

The dewatering properties depend on the characteristics of the suspension and on the used dewatering machine, because the structure of the sewage sludge is changed significantly by the mechanical treatment. Using a beaker centrifuge fundamental knowledge of the dewatering properties of disintegrated excess sewage sludges were to be gathered. Within these experiments no conditioning agents were added in order not to overlap the effect of the disintegration by the flocculation. For the sludge dewatering in technical scale the conditioning is essential. Because the disintegration influences the conditioning properties this topic was investigated especially. To determine the optimum conditioning agent dosage the streaming potential was measured. A decanter was used for the dewatering in the centrifugal field. For the filtration experiments a chamber filter press was employed. The investigation of the compressibility of the filter cake was carried through with a high pressure filter.

BEAKER-CENTRIFUGE

When operating a beaker centrifuge at high centrifugal accelerations, sludge which has been disintegrated with a high-pressure-homogenizer shows a significant increase of the dry solids content in the sediment compared to the untreated sewage sludge. Fig. 1 shows the results of treated sludge - at 5 different homogenizer pressures - in comparison to those of untreated sludge. It can be seen that higher contents of dry solids in the sediment (DSS) are obtained when applying centrifugal accelerations of more than 3000 * g.

Fig. 1: Content of dry solids in the sediment after disintegration in a high-pressure-homogenizer

The reason for this effect is the intensive disintegration of the sludge caused by the application of the high-pressure-homogenizer. The centrate contains a large portion of organic components whereas a high degree of the inorganic components can be found in the sediment. The increase of solids content caused by the disintegration can be attributed to the fact that the ratio of the inorganic to the organic portion in the sediment increases. The correlations, which are presented in Fig 2, can be shown by measuring the content of volatile solids in the sediment (VSS). At centrifugal accelerations of 1500 g the content of volatile solids is lower for the disintegrated sludges than it is for the untreated ones. Disintegration setsfree organic particles of small diameters, which can be separated only by higher centrifugal accelerations. The higher the disintegration rates, the more the centrifugal forces have to be increased in order to separate the smaller particles.

Results of the suspended solids content in the centrate confirm this statement. With increasing disintegration rate the amount of organic components that remain in the centrate - as dissolved or colloidal particles - increases. Even at high centrifugal accelerations they cannot be removed from the centrate.

Fig. 2: Amount of volatile suspendid solids in the sediment after disintegration in a high- pressure-homogenizer

The separation of organic components can be described by referring the mass of volatile solids in the sediment of the disintegrated sludge (mVSd) to the one of an untreated sludge (mVSo). Doing so it becomes apparent that the disintegration rate as well as the centrifugal acceleration have an influence on the separation of the organic components. After a nearly complete disintegration at a homogenisation pressure of 1000 bar only 25 to 35 % of the dry organic components are separated into the sediment, compared to the amount separated from the untreated sample (Fig. 3). This portion corresponds to the quantity of cellwall contained in the entire cell mass. It can be assumed that after a complete disintegration only the fragments of the cell wall will be separated. All the other intra-cellulare components are completely disrupted and will remain in the centrate even at very high centrifugal accelerations.

The disintegration leads to a theoretical separateability of the intra-cellular water. Calculations have shown that the portion of water that is attached to the organic components is not influenced by the disintegration (Müller, 1996). The dewatering performance is thereby defined by the portion of organic components in the dry solids which have to be separated. A high content of dry solids in the sediment can be achieved by a low portion of organic components.

Disintegration with the stirred ball mill leads to a slight increase of the dry solids content in the sediment after dewatering in the beaker centrifuge. This is mainly a result of the abrasion of the grinding beads. Although this abrasion was subtracted from the measured dry solids content it increases the density of the separated particle-agglomerates, thus influencing the sedimentation properties and causing a compression of the sediment.

Fig. 3: Portion of the separated volatile solids content in the sediment after disintegration

SLUDGE CONDITIONING

To achieve a high content of dry solids in the sediment during sludge dewatering a sludge-conditioning has to be realized beforehand. Prior to the investigations concerning the influence of disintegration on dewaterability in technical machines, conditioning had to be optimised in several experiments.

Conditioning of disintegrated excess sewage sludge requires an increase in specific conditioning agent mass from 10 up to 15 to 20 gram per kilogram dry mass because of the higher content of fine particles produced by the disintegration. Using a combined conditioning with ferric chloride sulfate (FeClSO₄) and a cationic polymer, the excess consumption of conditioning agent caused by the disintegration could be reduced. This combined conditioning lead to the best dewatering results with all the investigated machines. Measuring the streaming potential the optimum amount of conditioner could easily be detected for disintegrated and untreated sludges.

DECANTER

Practical dewatering experiments in the centrifugal field could be carried out with a bench-scale decanter. The results were quite similar to the ones obtained with the beaker centrifuge. After disintegration with a high-pressure-homogenizer the dry solids content in the sediment could be increased from 9 % for the untreated sludge to more than 12 % for the treated sludge (Fig. 4). A linear correlation between the solids content and the disintegration rate could be observed. In contrast to the experiments with the beaker centrifuge even the organic particles were separated from the centrate into the sediment by conditioning agents. Whether the release of the intra-cellulare components is responsible for the increase of the dry solids content cannot be ascertained, because the sediment still has a high water content of more than 80 %.

Investigations with excess sewage sludges from different wastewater purification plants confirm the observed improvement of the dewaterability caused by a disintegration with a high-pressure-homogenizer. The excess consumption of conditioning agent for the disintegrated sludges was around 50 to 100 % of the one for untreated sludge.

Fig. 4: Correlation between the disintegration rate after high-pressure-homogenisation and the dry solids content in the sediment of the decanter

CHAMBER-FILTERPRESS AND PRESS-FILTRATION

The improvement of dewaterability encountered when using a lab-scale filter couldn't be confirmed in experiments with a chamber-filter-press. As a result of disintegration a decline of the dry solids content in the filter cake was measured. Nevertheless the achieved contents of dry components of up to 24 % are very good results for excess sewage sludge. They were obtained by the combined conditioning with ferric chloride sulfate and a polymer. Because of the poor results of disintegrated sludges, further investigations concentrated on the post-treatment of the filter cake using a high-pressure-filter.

The pre-dewatered sludge from the chamber-filter-press was then pressed for 15 minutes at a pressure of up to 100 bar. Results are described in Fig. 5. The possible pressure is influenced by the choice of the conditioning agent. In exceeding the shear strength of the cake a breaking-through of solid components occurs. The pure polymer conditioning allows only a relatively low pressure of 12 bar because the polymerflocs are of poor stability. Even though the flocs are of great compressibility, only a negligible increase in dry solids content can be measured because the gel-like flocs quickly block the filtermedium. The permeability of the filtercake decreases and a further flow of filtrate is inhibited.

Conditioning with ferric chloride sulfate makes the use of higher pressures possible, because of the increased strength of the flocs, but at pressures above 50 bar a breaking-through of solid components occurred again. Because of the great strength of the flocs the compressibility of the cake was low and only a marginal increase in dry solids content could be observed.

The compression behaviour could be optimised by the combined conditioning with ferric chloride sulfate and a polymer. First of all the inorganic conditioning agent produces small, pressure-resistant agglomerates, which prevent the premature blocking of the filter. The polymer cross-links the agglomerates to bigger flocs, which can't break through. Combined flocculation allows pressures of 100 bar. The dosage of lime lowers the compressibility because of the enlarged portion of inorganic and stabile components.

As a result of disintegration the compressibility of the sludge at a pressure of 100 bar increases significantly compared to that of the untreated sludge. Because the untreated sludge does have a better pre-dewatering performance than the disintegrated sludge the results are nearly the same at the end.

Fig. 5: Dry solids content in the filter cake after filtration (DS_F) and following press-filtration (DS_P)

The achieved dry solids content of the press-filter-cake had a maximum of 220 g DS/kg. Together with the conditioning agent the measured dry solids content had a maximum of 260 g DS/kg or 26 % DS. That prooves clearly that the presscake still consists mainly of water. The separatability of the intra-cellular water can only have a positive effect on the obtained dry solids content, if an extensive dewatering of the sludge has taken place. As long as the sediment-structure still consists of a large amount of water bound to the organic components, the release of intra-cellular water seems to have little benefits.