Grantee Research Project Results
Final Report: Influence of Biocolloids and Biocolloidal Structure on the Dewaterability of Anaerobically Digested Sludge
EPA Grant Number: R823486Title: Influence of Biocolloids and Biocolloidal Structure on the Dewaterability of Anaerobically Digested Sludge
Investigators: Darby, Jeannie L. , Poxon, Theresa L. , Tsuchihashi, Ryujiro
Institution: University of California - Davis
EPA Project Officer: Aja, Hayley
Project Period: October 1, 1995 through September 1, 1997
Project Amount: $132,678
RFA: Exploratory Research - Engineering (1995) RFA Text | Recipients Lists
Research Category: Safer Chemicals , Land and Waste Management
Objective:
Sludge produced in the anaerobic digestion process has high water content, typically 94 to 98 percent by weight. In spite of this high water content, digested sludges are difficult to dewater by mechanical processes alone. The difficulty arises from two sources: the rate at which water can be removed from digested sludge is extremely slow, and the total proportion of water that can be removed is too low to be practically significant. Use of the proper sludge conditioning process is critical to the management ofsludge due to high unit cost and the large amount of conditioning agents required.
Research Hypotheses. The research reported herein was based on the premise that the current method of modeling sludge dewaterability (particle size model), while useful, has had the practical consequence of limiting the development of improved sludge dewatering technologies and alternative conditioning processes. The limit imposed by the current model arises from its implicit conceptualization of sludge as discrete particles in water?a conceptualization that significantly oversimplifies the realities of complex microbial systems. The approach of this work is to extend the conceptual model of solid-water interactions in digested sludge to the next logical level of complexity by including the effects of extracellular biopolymers. Specifically, the following hypotheses were made:
- The quantitative significance of the extracellular biopolymeric component of anaerobically digested sludge has been significantly underestimated in previous work as a result of methodological limitations.
- The soluble colloidal sludge biopolymers (biocolloids) are present in sufficient concentrations to form a space-filling structure (matrix) that is gel-like in its ability to reduce the mobility and, hence, the separability of sludge water; as a consequence, the biopolymers are of primary importance in determining the dewatering properties of digested sludge.
- Effective conditioning is achieved by alteration or disruption of the polymeric matrix with consequent freeing of trapped water.
- Distributions of particle size are influenced by the biopolymeric structure of the sludge, and therefore represent a secondary rather than a fundamental sludge characteristic.
Project Objectives. The primary objective of this research was to gain a better understanding of the influence of biocolloids and biocolloidal structure on sludge dewatering characteristics and conditioning mechanisms, and to investigate the practical significance of a new conceptual model. Specific research objectives were to: (1) determine the existence and extent of biocolloidal structure in anaerobically digested sludges; (2) define correlations between structure, particle size, and dewaterability in digested sludges generated under differing conditions; and (3) determine the effect of various conditioning agents on sludge structure.
Experimental Design and Methods. Anaerobically digested sludge samples were collected from four wastewater treatment plants (WWTP): (1) Fairfield-Suisun Subregional (FSSWTP), (2) Sacramento Regional (SRWTP), (3) East Bay Municipal Urban District (EBMUD), and (4) City of Phoenix (Phoenix). Sludge samples from each WWTP were examined twice every 3 months, eight times in a year.
Physical Characteristics of Digested Sludge Samples. Physical characteristics of the sludge samples were evaluated by total solids (TS), volatile solids (VS), pH, and filterability. Measurements of TS, VS, and pH were carried out according to standard methods (American Public Health Association, 1997). Filterability was measured as suggested by Vesilind (1988).
In Situ Measurement of Extracellular Polyanions in Digested Sludge. Extracellular polyanions in digested sludge were quantified as alginic acid equivalent using in situ measurement as suggested by Poxon and Darby ( 1997).
Observation of Sludge Structure Through Scanning Electron Microscopy. Development of a conceptual model based on the structure of sludge requires visual evidence. Scanning electron microscopy was used to examine the structural characteristics of sludge. Two dehydration methods for visualization of the sludge structure?critical point drying and freeze drying?were compared.
Evaluation of the Effect of Biocolloidal and Polyanion Precipitation on the Structure and Dewaterability of Digested Sludge. Ammonium sulfate was used for biocolloidal precipitation. Filterabilities and scanning electron micrographs of the sludge before and after precipitation were compared. Ruthenium red was used for precipitation of polyanions. Sodium chloride, ferrous sulfate, and ferric chloride were used to compare the effect of charge neutralization.
Summary/Accomplishments (Outputs/Outcomes):
Physical Characteristics of the Sludge Samples. Physical characteristics of the sludge samples are reported in Table 1. Samples from EBMUD tended to have higher TS (average TS 2.5 percent, SD 0.41), whereas filterability tended to be low in the samples from FSSWTP. There was no obvious correlation between TS and filterability. Average values of pH were 7.14, 7.27, 7.52, and 7.44 for FSSWTP, SRWTP, EBMUD, and Phoenix, respectively.Quantification of Extracellular Polyanions in Digested Sludge. The average ECPA estimated as an alginic acid equivalent were 0.195, 0.211, 0.185, and 0.244 (mg as alginic acid/mg sludge) for FSSWTP, SRWTP, EBMUD, and Phoenix, respectively. As discussed by Poxon and Darby (1997), there were separate correlations between filterability and the amount of ECPA for each data set in spite of a fairly large degree of scatter. Seasonal changes in the sludge characteristics, a change in operating condition or digester feed composition, or a combination of those factors might have caused those exceptions as the two data points were taken in the end of the winter season. Varying trends in the relationships between filterability and ECPA concentrations suggest that specific biochemical properties due to different feed composition or digester operating conditions would be of greater importance than ECPA concentrations themselves in their influence on dewaterability.
Table 1. Physical characteristics of digested sludge.
spr 1 |
spr 2 |
sum 1 |
sum 2 |
fall 1 |
fall 2 |
win 1 |
win 2 |
avg |
st dev | |
pH | ||||||||||
FSSWTP |
7.2 |
7.06 |
7.08 |
7.38 |
7.1 |
7.1 |
7.08 |
7.08 |
7.1375 |
0.106066 |
SRWTP |
7.1 |
7.16 |
7.43 |
7.37 |
7.31 |
7.17 |
7.31 |
7.34 |
7.27375 |
0.116243 |
EBMUD |
7.68 |
7.3 |
7.5 |
7.73 |
7.4 |
7.44 |
7.48 |
7.6 |
7.51625 |
0.145006 |
Phoenix |
7.4 |
7.29 |
7.63 |
7.41 |
7.68 |
7.43 |
7.36 |
7.3 |
7.4375 |
0.1438 |
TS (%) | ||||||||||
FSSWTP |
2.01 |
2.15 |
2.13 |
2.14 |
2.02 |
2.13 |
2.3 |
2.17 |
2.13125 |
0.09062 |
SRWTP |
1.76 |
1.68 |
1.69 |
1.76 |
1.72 |
1.49 |
1.72 |
0.65 |
1.55875 |
0.377111 |
EBMUD |
2.67 |
3.16 |
2.89 |
2.01 |
2.09 |
2.26 |
2.74 |
2.22 |
2.505 |
0.417167 |
Phoenix |
2.33 |
2.03 |
2.06 |
2.13 |
2.25 |
2.05 |
2.15 |
2.13 |
2.14125 |
0.10357 |
VS (% of TS) | ||||||||||
FSSWTP |
64 |
63 |
59 |
57 |
57 |
59 |
60 |
60 |
59.876 |
2.531939 |
SRWTP |
63 |
65 |
65 |
62 |
64 |
66 |
52 |
63 |
62.6 |
4.440077 |
EBMUD |
58 |
53 |
63 |
62 |
66 |
63 |
54 |
60 |
59.875 |
4.580627 |
Phoenix |
66 |
67 |
66 |
64 |
64 |
65 |
67 |
66 |
65.625 |
1.187735 |
Filterability
(kg2/s2m4) | ||||||||||
FSSWTP |
0.00549 |
0.0061 |
0.00794 |
0.0063 |
0.0045 |
0.0035 |
0.004 |
0.0049 |
0.0054 |
0.0014 |
SRWTP |
0.00262 |
0.00306 |
0.0021 |
0.0017 |
0.0015 |
0.002 |
0.001 |
0.0019 |
0.001 | |
EBMUD |
0.00138 |
0.00215 |
0.0026 |
0.0024 |
0.002 |
0.0025 |
0.002 |
0.0015 |
0.0021 |
0 |
Phoenix |
0.00317 |
0.00372 |
0.00484 |
0.0044 |
0.0041 |
0.0028 |
0.002 |
0.0033 |
0.0036 |
0.001 |
Visualization of Sludge Structure. Filamentous structure of extracellular materials was observed in the SEM images when the sample was freeze-dried. Individual particles were seen to be not discrete, but surrounded by and embedded in a fibrous matrix structure of ECM. These findings are of particular interest in light of the fact that anaerobically digested sludge is generally considered to be a nonflocculent system, and is believed to contain significantly less ECM than does activated sludge. The SEM results for freeze-dried samples of digested sludge, however, clearly indicate a floc-like ECM structure.
Effect of Biocolloidal and Polyanion Precipitation on the Structure and Dewaterability of Digested Sludge. Conditioning of digested sludge by ruthenium red did not drastically change the filamentous structure of extracellular materials, although aggregation of ECM is observed in the SEM images when viewed in a low magnification. Large flocs were observed through eye observation when ruthenium red was added to the digested sludge samples. The interpretation of this result is that ruthenium red neutralized the negative charge in the digested sludge to cause aggregation of sludge flocs, while the structure of ECM was not changed. After conditioning with ammonium sulfate, however, the filamentous structure was not clearly seen, indicating precipitation of extracellular polyanion and deformation of sludge structure. This result was consistent in the digested sludge from four different wastewater treatment plants. Relative filterability tended to increase with an increase in the amount of ECPA. Although the data exhibit a fairly large degree of scatter, relative filterability also tends to increase with an increase in the amount of ECPA, except for the samples from Phoenix in the summer season. These results can be interpreted as evidence of the effect of biocolloidal and polyanion precipitation on the improved dewaterability of digested sludge. Varying degree of effects on improvement of dewaterability is probably due to varying biochemical composition of the sludge.
The effects of different conditioning agents on dewaterability were compared. The maximum filterability was achieved when about 0.1 (N) of ferric chloride was added, and the filterability was lowered as more ferric chloride was added. The result indicates that the maximum filterability was achieved at the point where the aggregation of sludge particles was maximized by charge neutralization. On the other hand, a larger amount of ammonium sulfate is needed to achieve about the same filterability as that with ferric chloride. Although the precipitation of ECM, or deformation of sludge structure, was suggested to affect the sludge dewaterability, the efficiency of ammonium sulfate for the sludge conditioning is not as high as widely used conditioning agents. Conditioning processes to take the ECM precipitation or deformation into account would be of interest in future research.
Conclusions. The following conclusions were formulated:
- In determination of the existence and extent of biocolloidal structure in anaerobically digested sludges, a fibrous matrix structure of extracellular materials and sludge particles trapped or embedded in the structure was found through scanning electron microscopy. The freeze-drying method should be used for dehydration of the sample to visualize the structure of digested sludge.
- In determination of the existence and extent of biocolloidal structure in anaerobically digested sludges, separate correlations were found between the amount of ECM and filterability, in spite of a fairly large degree of scatter.
- In determination of the effect of various conditioning agents on sludge structure, ammonium sulfate was found to deform, or precipitate, the structure of ECM in the digested sludge. A correlation was found between the amount of ECM and change in filterability with ammonium sulfate addition.
The results support the hypothesis that the alteration or disruption of the polymeric matrix structure of digested sludge would affect the filterability of digested sludge. The effectiveness of the conditioning with ammonium sulfate, however, was lower than that of widely used conditioning agents. Conditioning processes to take the ECM precipitation or deformation into account would be of interest in future research.
References:
American Public Health Association. Standard Methods for the Examination of
Water and
Wastewater. 17th edition. American Public Health Association,
Washington, DC, 1989.
Poxon TL, Darby JL. Extracellular polyanions in
digested sludge: measurement and
relationship to sludge dewaterability. Water Research 1997;31(4):749-758.
Vesilind PA. Capillary suction time as a fundamental measure of sludge dewaterability. Journal of the Water Pollution Control Federation 1988;60(2):215-220.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 2 publications | 1 publications in selected types | All 1 journal articles |
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Type | Citation | ||
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Poxon TL, Darby JL. Extracellular polyanions in digested sludge: measurement and relationship to sludge dewaterability. Water Research 1997;31(4):749-758. |
R823486 (Final) |
not available |
Supplemental Keywords:
anaerobically digested sludge, extracellular polyanions, scanning electron microscopy, biocolloidal structure., Scientific Discipline, Waste, Municipal, Environmental Chemistry, Chemistry, Engineering, Environmental Engineering, municipal sludge, particle size, ammonium sulfate, sludge dewatering, municipal waste, anaerobic digestion, wastewater treatment plants, biocolloids, water mobilityProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.