Dewatering and processing sewage sludge is a major cost item for water authorities. However, research to improve this is difficult because the current lab scale dewatering methods are very different from those used at wastewater treatment plants. Now a simple lab method is available with two advantages: both its mechanism and the results are comparable with dewatering installations at sewage plants, which makes the method predictive of practice.
In the Netherlands, the water authorities produce 1.3 million tons of dewatered sewage sludge annually. The dewatering and handling of sewage sludge are expensive and can account for as much as 20-30% of the operational cost yearly. Transportation of dewatered sludge can cost 5 to 10 euro per ton of sludge and the costs of final disposal can be as high as 75 to 100 euros per ton of sludge. Doing experimental tests with a full-scale dewatering installation to improve sludge dewatering is not easy, because the equipment is needed to process the sludge continuously and fast switching of settings is not possible. Moreover, laboratory research on dewatering was often debatable because of the discrepancy with the situation at wastewater treatment plants. Therefore, there was a need for a new method which makes laboratory research valid again.
Laboratory and practice
There are different laboratory-scale methods to evaluate dewaterability and the effect of sludge pre-treatment on dewaterability. Examples are the CST-test (‘capillary suction time’) and the SRF-test (‘specific resistance to filtration’); the binding energy test (determination of the required energy to remove the water from sludge floc). However, with these methods it is not possible to predict the dry matter content (%DS) of the sludge cake of full-scale dewatering installations. Moreover, all these tests work in a totally different way compared to full scale plants. For example, the commonly used CST and SRF tests are based on gravity and mild vacuum respectively, while in the full scale a mechanical force removes the water from the sludge structure. Another drawback is that these tests can only report the filterability of the sludge in terms of time: the higher the filtration time, the less the dewaterability of the sludge. However: in practice, there is no relation between the time and dry matter content of the sludge cake.
Discrepancies
The results of studies by different laboratories are often not very comparable. For example, Zhang et al. (2019) determined the bond energy of sludge, which is the required energy to remove water from sludge matrix, to investigate the effect of anaerobic digestion on sludge dewaterability. The researchers concluded that dewaterability improved with anaerobic digestion, based on the measured lower binding energy values. However, Liu et al. (2021) came to an opposite conclusion based on CST values. They observed that anaerobic digestion increased the CST values of sludge from 70 s to 1400 s and concluded that the dewaterability deteriorated after anaerobic digestion. The results of these studies are clearly contradictory and therefore have no predictive value for full-scale wastewater treatment plants.
In the past, testing of dewaterability in laboratories was often done with a mini-filter press, for example, a Mareco mini filter press. However, in recent years this method has proven less reliable due to changes in sludge composition as a result of bio-P sludge and/or the application of heat pre-treatment. Due to the finer particles in the sludge and the consequent quicker clogging of the filter cloth, the mini-filter press is no longer sufficed. The result lagged far behind compared to what was achieved in practice. Moreover, the operating principle (and thus the results) of a mini-filter press cannot be compared to those of full-scale installations (separation in a centrifugal field versus cloth and cake filtration in a filter press).
Material and methods
In this study, a more realistic dewatering method for the laboratory is presented. Such a method ideally meets two criteria: its operation is comparable to the dewatering equipment on wastewater treatment plants, and the possibility to predict the DS concentration of the sludge cake in the full-scale installations.
Based on a practical laboratory-scale centrifuge dewatering method created by Weij (2018) and a previous study by To et al. (2016), the laboratory method was developed further. Before testing with several sludge types from the field, tests were performed with different rotation speeds (G-forces) and different centrifugation times to calibrate the method for the specific setup. Sludge was used from a typical medium-sized Dutch wastewater treatment plant with pre-sedimentation and an activated sludge system, where sludge from another wastewater treatment plants is also digested (WWTP A). This gave different DS contents of the sludge cake. The choice was made for the combination of rotation speed and centrifugation time at which the DS content was comparable to the practical results of WWTP A. We used these settings in further research.
Next, we tested the method on sludge from 3 other wastewater treatment plants in the Netherlands: B, C, and D. Sludge samples A and D were thermally pretreated (THP). THP pre-treatment is the process of boiling the sludge for 30 minutes at a temperature of 145°C to 165°C prior to anaerobic digestion. The sludge sample from WWTP B is the normal surplus sludge from an activated sludge plant. The sludge sample from WWTP C is a mixture of secondary and primary sludge without any pre-treatment (70% secondary and 30% primary sludge based on DS concentration).
Figure 1. Steps of laboratory scale centrifugation dewatering method
Figure 1 shows the different stages of the dewatering procedure. Firstly, a polymer solution (0.3% active PE, w/v %) is added to a beaker containing 100 grams of sludge. This mixture is carefully poured between two beakers several times until the sludge floc appears and clear water is visible between flocs. The sludge floc settled and is manually separated from the water as best as possible.
Then water in the sample is squeezed out with a belt filter or a hand press (step 2). In step 3, the sample is ‘packed’ in two layers of Dispolab filters (glass fiber GF/C) and placed in a mesh bag. That package is then put in a centrifuge tube with a holder to keep the package at some distance from the bottom of the tube. Subsequently, the package is centrifuged, in two steps: 5 minutes at 1040 × G, then after decanting another 15 minutes at 1040 × G. To determine the dry matter content of the resulting sludge cake, the sample is dried in an oven at 105 °C according to the applicable NEN standard.
Figure 2 and 3. Comparison of full-scale and laboratory scale centrifugation dewatering results in terms of DS concentration of the sludge cake
Results and discussion
The results of the laboratory dewatering method are in line with full-scale dewatering results (see Illustration 2). The maximum deviation in the results was 3% for WWTP A. For WWTP B the deviation was 0.4%, for WWTP C 0.6% and for WWTP D 1.8%. The error bars in the graph show that the variation between the duplicates in all the samples from the different locations was small. This indicates that the lab method used is very reliable.
Also, this method showed the positive effect of THP pretreatment on sludge dewaterability in terms of higher DS concentration, which is in line with the results from full-scale. Therefore, this method can be useful to evaluate and predict the effect of pretreatments on sludge dewaterability. This can help in making decisions about scaling up pretreatments to improve dewaterability.
Other advantages of this method are its relative simplicity and high reliability, and that it can be applied in most laboratories as long as there is a large lab centrifuge.
Conclusion
The aim of this study was to validate and further develop a practical reproducible laboratory scale dewatering method with predictive value for full scale installations. The results of the lab-method had to be comparable with the full-scale dewatering results.
The presented centrifuge dewatering method gave a small deviation in dry matter (DS) concentration of the sludge cake compared to dewatering by field plants, in other words, the results were comparable. In addition, the effect of THP pretreatment on sludge dewaterability was demonstrated, with again similar values for lab method and full-scale plants. In other tests also different pre-treatments techniques were successfully evaluated.
The new method was tested with sludge samples from different wastewater treatment plants in the Netherlands. The differences between lab results and full-scale results were so small that the reproducibility and thus the reliability of the procedure is high. The method is simple and applicable in most laboratories, both in industry and in research institutes.
SUMMARY
Research on sludge dewatering in the laboratory is often difficult because the usual lab-methods are very different from the way sludge is dewatered in installations at wastewater treatment plants. Now a new simple lab method is available that is comparable to the full-scale dewatering installations and has predictive value for dewatering on wastewater treatment plants with and without some kind of pre-treatment.
References
Liu Q, Li Y, Yang F, Liu X, Wang D, Xu Q, Zhang Y, Yang Q. 2021. Understanding the mechanism of how anaerobic fermentation deteriorates sludge dewaterability. Chemical Engineering Journal. 404:127026.
To VHP, Nguyen TV, Vigneswaran S, Duc Nghiem L, Murthy S, Bustamante H, Higgins M. 2016. Modified centrifugal technique for determining polymer demand and achievable dry solids content in the dewatering of anaerobically digested sludge. Desalination and Water Treatment. 57(53):25509-25519.
Weij P 2018. Sludge dewatering with lab centrifuge (Internal work instruction), Delfluent Services / Delft Blue Innovations.
Zhang W, Dong B, Dai X. 2019b. Mechanism analysis to improve sludge dewaterability during anaerobic digestion based on moisture distribution. Chemosphere. 227:247-255.
