Iwa international Specialist Conference

64.

Everybody gets the sludge that he deserves - Influences of Sludge Treatment Dewaterability of Sewage Sludge

Dr.-Ing. Julia B. Kopp, Kläranlagen Beratung Kopp, Oberger Weg 1, D-38268 Lengede, Germany
Tel:[phone], Fax: [phone]; Mail: [e-mail]
"Everybody gets the sludge that he deserves", which means that the dewatering characteristics of a sewage sludge are essentially the product of the previously applied treatment procedures. Sludge characteristics are of course influenced by the consistency of the incoming wastewater, but also by the applied treatment procedures. The impacts of the treatment procedures vary widely. Sludge characteristics can, for example, be influenced by mechanical stress during pumping procedures, by mechanical treatment (e.g. the amount of substances retained by the screens), by biological treatment (influence of sludge age and sludge loading), and by sludge treatment (storage periods in holding tanks, type of sludge stabilization etc.). The real problem lies in the quantification of these influences, and it is therefore necessary to closely examine treatment procedures on location, in order to evaluate their influences on sludge characteristics.

Dewatering is a physical/mechanical unit operation used to achieve the highest possible dried solids content, reduce sludge volume and improve stability of the sludge. Dewatering is the basic requirement for reducing costs for transportation, disposal and possible thermal treatment of the sludges. The amount of water that can be separated during dewatering depends on the distribution of the water fractions. The water distribution can be measured by thermo-gravimetric and dilatometric tests. The measuring instruments have to be adjusted and calibrated, so that a direct statement can be made concerning the maximum suspended solids content in the sludge cake after mechanical dewatering [1]. Only the free water content can be separated by centrifugation and filtration. The dewatering results described by the parameter DS(A), that is DS after separation of the free water fraction. In this paper the following factors will be discussed:

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  influences specific to the respective wastewater treatment plant
  
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  influence of sludge types
  
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  influence of volatile suspended solids
  
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  influence of sludge stabilization
  
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  influence of particle size distribution
  
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  influence of exopolymeric substances
  
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  influence of sludge conditioning
  
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  correlation between capillary suction time and dewatering results
  
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  influence of mechanical stress and disintegration
  
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  influence of thermal treatment
  
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  influence of precipitant over-dosage
  
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  further influences and evaluation
  

An important factor that affects sewage sludge dewaterability is the percentage of waste activated sludge in the total sludge mass. Usually primary sludges are easy to dewater. The amount of free water is high, because little water is bound by capillary forces as interstitial water. In contrast, waste activated sludges are difficult to dewater, since the percentage of free water is low. As the percentage of waste activated sludge in the total sludge mass increases, the amount of free water decreases, thus decreasing the quality of the dewatered product.

Due to the application of advanced wastewater treatment technologies, pre-sedimentation time decreased, because more carbon compounds were needed for biological phosphorus removal and for denitrification in the aeration plant. Shorter pre-sedimentation periods lead to a lower percentage of primary sludge and an increased percentage of waste activated sludge. All in all this has a negative influence on sludge dewaterability.

Biological stabilization changes the organic fraction and particle size distribution in a sewage sludge thus changing its dewatering behavior. Especially at small wastewater treatment plants, sludge is stabilized aerobically simultaneously, which results in a sludge age of minimum 25 days. In addition, these plants normally have no pre-sedimentation process, and therefore only produce waste activated sludge for dewatering. Standard degrees of degradation for an aerobic simultaneous sludge stabilization amount to ca. 30 % degradation of the volatile suspended solids. Sludge digestion is in Europe a standard method for anaerobic sludge stabilization, and is applied mainly at medium size and large wastewater treatment plants. Generally sludge is stabilized anaerobically in the mesophilic temperature range (35 - 37°C) for at least 20 days. Degrees of degradation for the volatile suspended solids lie at about 45 %. No unequivocal statement can be found in literature how sludge digestion changes dewaterability. It is a general opinion that digestion improves dewatering results, because part of the organic mass is degraded. Operation of mechanical dewatering equipment can be regulated more easily, because sludge characteristics become more stable due to long retention times in the digestion process.

In order to determine the influence of sludge stabilization on the free water content, extensive batch tests with various sludges were conducted. Besides the parameter DS(A) also the parameters mean particle size, the range of the particle size distribution and polymer demand are shown as mean values in at least three independent repetitions of the batch tests.

Concerning water bindings of a sewage sludge, two contradictory effects can be observed as a consequence of sludge stabilization. On the one hand the amount of volatile suspended solids decreases during digestion; this affects the dewatering result positively. On the other hand the particle size distribution and mean particle size change due to degradation of organic substances and due to mechanical stress during stirring. A decrease in mean particle size is accompanied by an increase in the sum of particle surface, which leads to an increase in adsorption water quantity. At the same time a decrease in particle size is followed by an increase in the number of pores thus causing an increase of capillary bound interstitial water. A particle size reduction therefore always leads to a deterioration of the achievable dewatering results.

Another parameter often used to describe a suspension is the range of the particle size distribution (PSD). Here the 90 % cumulative frequency (c90) is related to the 10 % cumulative frequency (c10). For a monodisperse suspension, this ratio is one and the value c90/c10 increases with the increase of the range of particle size distribution. It can be shown that the achievable dewatering result DS(A) decreases for smaller values of c90/c10, since this has a negative influence on the free water content. The range of particle size distribution influences the pore volume thus influencing the fraction of capillary bound interstitial water in the sludge cake. If one imagines the dewatered sludge cake as a pile of monodisperse particles, than the pore space of random particle packing is 40 %, for compact storage it is 30 %. If the particles are distributed polydispersely, smaller particles can fill out the free pore space, thus decreasing interstitial volume. Little pore volume leads to small interstitial volume and to higher fractions of free water thus improving the dewatering results. This interpretation is corroborated by the measuring values of the sludge suspensions. Sludge stabilization leads to a decrease in mean particle size and to a small range of particle size distribution. Both effects cause a deterioration of the dewatering result.

The distribution range can also be used as an approach, to explain the different dewatering results of primary and waste activated sludge. Since waste activated sludges consist mainly of bacteria, which are in comparison to other particulate sludge contents very homogenous, they bind much more water in their sludge particles and flocs. Porosity of waste activated sludge particles in comparison to primary sludge particle should therefore be much higher. Due to the more inhomogeneous consistency of primary sludge a tighter packing of the individual particles in the sludge floc must be assumed. Then the fraction of capillary bound interstitial water is also smaller and as a consequence more free water can be separated during mechanical dewatering.
[1] J. Kopp, N. Dichtl, Prediction of full-scale dewatering results by determining the water distribution of sewage sludges, Water Science and technologie Vol 42, No 9, pp 141-149, 2001
If there is the interrest on more details on further influencees on the dewaterability of sewage sludges it is a pleasure to me to submit a full paper at the conference. Best regards – J. Kopp

65.

THERMAL DRYING OF SEWAGE SLUDGE - example how a new technology was introduced in Finland
Timo Holmberg
SUMMARY

In Finland an increased interest has arisen to find out some new solutions to handle sewage sludge. In central Europe thermal drying has been a normal way for sludge handling for a long time and it has been proposed to be a potential technology in Finland as well. In thermal drying dewatered sludge is dried up to 90 % (TS). Pathogenic bacteria die and final product can be stored for a long period without health bacterial risk.

In Finland, composting has been the major way in sewage sludge handling and it was composted in open windrows. By the 90’s, composting was carried out in the composting plants and the open windrows has been made forbidden gradually.
However, the use of composting plants does not eliminate problems related in composting. The most likely are a long post-composting time, odour problems and high final product amount. Utilisation of the final product has been difficult and prices are low if any.
Preliminary study of the thermal drying was made by the support of the Finnish Government. It contained Environmental Impact Assessment of the utilisation of the dried sludge and feasibility study.

The results of the feasibility study showed that investment cost is comparable to the cost of composting reactor. Operating costs are related to the oil price if oil is used as an energy source. Other energy sources could be eg. biogas and landfill gas. Taking into consideration the very high oil price at the moment in Finland thermal drying is a competitive possibility for sewage sludge handling.

The idea of the Environmental Impact Assessment was to compare environmental impacts of different utilisation alternatives such as using fertilizer in agriculture, in forest or utilisation as energy source.
EIA expressed that utilisation as fertilizer in agriculture or in forest has low harmful impacts. Burning of sludge may cause in some cases problems because of the heavy metals.
Based on the preliminary study a thermal drying plant was built in Joensuu. The capacity of the process is 8000 tons of sludge per year and water evaporation capacity 630 kg/h. Dry matter content of the input product is 25 % and the final product is 85 %.

66.

Mechanical sludge disruption with increased energy and reduced sewage sludge productions at a waste water treatment plant