Dewatering in biological wastewater treatment: A review
Graphical abstract
Introduction
Municipal and industrial wastewater contain high amounts of COD, nitrogen, and phosphorus, which are usually degraded or removed by biological wastewater treatment (Lindrea and Seviour, 2002). The activated sludge process is by far the most common process, but alternative processes such as biofilm systems or granules systems also exist (de Bruin et al., 2004). An integrated part of the biological wastewater treatment is the solid–liquid separation, where the treated water is separated from the activated sludge. In the conventional activated sludge process, this is done by clarifiers, but there is an alternative: membrane bioreactors, where a membrane is used instead of the clarifier (Brindle and Stephenson, 1996, Lindrea and Seviour, 2002). The outcome of the process is treated wastewater (effluent), return sludge, and excess sludge.
In some cases, excess sludge is transported to digesters for sludge reduction and bioenergy production. However, in many cases, other types of sludge handling takes place, e.g. transportation to agricultural fields or drying and incineration. Since the water content of excess sludge is high, it must be dew before further handling, typically by belt filters, filter press, decanter centrifuges, and sludge mineralization beds (Sørensen and Sørensen, 1997). Thus, several solid–liquid separation processes are involved in wastewater treatment for separating sludge from the treated wastewater as well as for sludge dewatering. The dewatering process is costly, and the composition and properties of the sludge are important for the separation process (Bruus et al., 1992, Sørensen and Sørensen, 1997, Chu et al., 2005).
This paper reviews the existing literature on sludge dewaterability, i.e. sludge filtration and consolidation. Fig. 1 summarizes the key parameters that affect various sludge properties such ad dewaterability. Sludge contains flocs, and sludge properties are mainly determined by the size, shape, density and strength of the sludge flocs. Thus, an understanding of the sludge flocs is crucial for a more general understanding of sludge dewatering. Flocs, on the other hand, consist of microorganisms, extracellular polymeric substances (EPS), organic debris and inorganic particles. Some of the components are produced during the biological process and some of the components come from the influent. Further, floc density and strength are influenced the content of e.g. catons and inorganic particle and also by shear forces and thereby indirectly by the design and operation of the plant.
The floc properties not only influence sludge filtration and consolidation but also other processes such as flocculation, settling and membrane fouling, i.e, literature data show that sludge components that cause problems in filtration and consolidation also cause problems in other types of separation processes (e.g. sedimentation, centrifugation, sludge mineralization bed, and membrane bioreactors). Thus, many of the conclusions from this paper are of generic value for all solid–liquid separation processes for biological sludges.
Section snippets
Sludge composition
Biological activated sludge consists primarily of biological flocs that are formed by growth of microorganisms and by adsorption of particles from the influent. The flocs consist of microorganisms, either as single cells, filamentous bacteria or microcolonies, organic fibers, inorganic particles (salt and sand), and extracellular polymeric substances (EPS). The typical size of the flocs is 129 ± 109 μm (Mikkelsen and Keiding, 2002) – see sketch of a typical sludge floc in Fig. 2.
Sludge flocs
Specific filtration flow rate
Several methods exist for comparing dewaterability of different types of sludge such as capillary suction time, sludge volume index, average specific resistance of the cake, and the specific filtration flow rate. The specific filtration flow rate (SFF) is a useful term especially for filtration and consolidation processes. Dewatering often involves both filtration (cake formation) and consolidation (cake compression), and for biological sludge it is difficult to distinguish between the two
Sludge cake compressibility and blinding
When the sludge cake is compressible, it means that the cake porosity, ε, decreases with increasing pressure, resulting in higher resistance. This can be modeled by the following constitutive equations (Tiller and Yeh, 1987):where β is an empirical parameter and ε0 is the porosity of an uncompressed cake. The equation is similar to the one used for the average specific cake resistance, and combining the two equations gives αav = k(1−ε)m, where k is the ratio between α0 and
Conductivity and water hardness
The composition of inlet wastewater varies from plant to plant, e.g. due to different industries, rainfall, etc. This affects both the biological wastewater treatment and the dewaterability of the biological sludge produced. Several studies have shown that the wastewater conductivity, water hardness, and pH vary; i.e. ionic composition and concentrations vary. This strongly affects the dewaterability of the biological sludge.
Both floc structure and strength strongly depend on ionic composition
Sludge pH
Activated sludge flocs contain a lot of EPS which contain titratable groups and are negatively charged at neutral pH. The EPS components are almost non-charged at pH around 2.6–3.6 (Liao et al., 2002), whereas the charge increases with pH (Raynaud et al., 2012). As EPS components and electrostatic forces play a central role in floc structure, sludge pH indirectly affects the floc structure and sludge dewaterability. At low pH, the bulk suspension only contains few colloidal particles, and the
Biological process
The solid–liquid characteristics of the sludge is influenced by the wastewater composition and the way the sludge is produced, e.g. by the conventional activated sludge process, membrane filtration in MBR, biofilms, or by mesophilic and thermophilic digestion.
Table 3 summarizes sludge characteristics and filtration properties from two surveys of sludge filtration properties in terms of total EPS content, mean floc size, shear sensitivity for the release of particles during shear treatment, kSS,
Sludge storage
Sludge is often stored before dewatering. However, the biological processes do not stop during storage; thus, floc structure and composition change, which in turn affects and often reduces dewaterability (Bruus et al., 1993). Several factors are involved in these changes such as hydrolysis of EPS components, reduction of Fe(III) to Fe(II), which is a poorer flocculant, and production of sulfide by microbial sulfate reduction that subsequently precipitates and removes Fe (III) and Fe(II) (
Pumping and stirring of sludge
Sludge flocs can be destroyed due to high shear levels which reduce sludge dewaterability. Particles and sludge flocs aggregate under low shear rates and break up at elevated shear rates (Mikkelsen and Keiding, 1999, Mikkelsen and Keiding, 2002). Break-up of sludge flocs (fragmentation) lower the mean size of the flocs (Jarvis et al., 2005). At higher shear rates, smaller particles (e.g. single cells) are desorbed from the floc surface due to erosion (Mikkelsen and Keiding, 2002; Biggs et al.,
Summery of factors that influence sludge quality
Thus, several parameters affect the dewaterability of sludge: the physico-chemical properties of the feed, the biological treatment, and the handling of the sludge before and during dewatering. Table 4 summarize the conclusions from the text. The composition of the incoming wastewater affects the properties of the sludge produced, especially the organic compounds, pH, and the ion composition. The biological process and the plant design as well as the further sludge handling (pumping, mixing,
Improvement of sludge filterability by flocculation
Sludge dewatering is an expensive operation in wastewater treatment plants. It is not possible to improve the dewatering process by applying higher pressure in filtration processes due to the high compressibility of sludge cakes. Instead, sludge can be pre-treated by adding coagulants e.g. polyaluminium chloride (PAC) or ferric salts (FeSO4Cl), followed by addition of flocculants or by adding flocculants alone. This improves sludge dewaterability significantly and reduces the costs of the
Conclusion
Great variation in sludge dewaterability is observed among wastewater treatment plants; hence the floc and sludge properties have a high impact on the specific filtrate flow rate. The best dewaterability is observed for sludge that contains strong compact flocs and low concentrations of single cells as well as dissolved EPS. This gives the best sedimentation in the clarifier, the highest permeate flux in MBR systems, the highest filterability (belt filters and sludge mineralization bed), the
References (88)
- et al.
Effect of mixing on the rheological characteristics of conditioned sludge: full-scale studies
Water Sci. Tech.
(1997) Physical and chemical properties of activated sludge floc
Water Res.
(1993)- et al.
On the stability of activated sludge flocs with implications to dewatering
Water Res.
(1992) - et al.
Filtration properties of activated sludge in municipal MBR wastewater treatment plants are related to microbial community structure
Water Res.
(2013) - et al.
Rheological characteristics of the cationic polyelectrolyte flocculated wastewater sludge
Water Res.
(2005) - et al.
The activated sludge and microbial substances influences on membrane fouling in submerged membrane bioreactor: unstirred batch cell test
Desalination
(2005) - et al.
The influence of creep on cake solid volume fraction during filtration of core–shell particles
Colloid Surf. A: Physicochem. Eng. Asp.
(2008) - et al.
Creep effects in activated sludge filter cakes
Powder Technol.
(2007) - et al.
Gravitational sedimentation of flocculated waste activated sludge
Water Res.
(2003) - et al.
Energy demand in sludge dewatering
Water Res.
(2005)
Porosity and interior structure of flocculated activated sludge flocs
J. Colloid Interface Sci.
Relations between extraction protocols for activated sludge extracellular polymeric substances (EPS) and EPS complexation properties Part I. Comparison of the efficiency of eight EPS extraction methods
Enzyme Microb. Technol.
Moisture distribution in sludges based on different testing methods
J. Environ. Sci.
Effect of different extraction methods on bound EPS from MBR sludges. Part 2: influence of extraction methods over molecular weight distribution
Desalination
Sludge quality aspects of full-scale reed bed drainage
Water Res.
Gravity drainage of activated sludge: new experimental method and considerations of settling velocity, specific cake resistance and cake compressibility
Water Res.
A comparative study of fouling-related properties of sludge from conventional and membrane enhanced biological phosphorus removal processes
Water Res.
Dynamic analysis of cake properties in microfiltration of soft colloids
J. Membr. Sci.
A review of floc strength and breakage
Water Res.
Bound water content of biological sludges in relation to filtration and dewatering
Water Res.
Osmotic effects in sludge dewatering
Adv. Env. Res.
Variations in microcolony-strength of probe-defined bacteria in activated sludge
FEMS Microbiol. Ecol.
Filter cake formation from structured suspensions
Trans. IChemE
Adhesion characteristics of nitrifying bacteria in activated sludge
Water Res.
Enhanced sludge dewatering by dual polyelectrolytes conditioning
Water Res.
Interparticle interactions affecting the stability of sludge flocs
J. Colloid Interface Sci.
Filtration of core-shell colloids in studying the dewatering properties of water-swollen materials
Chem. Eng. Sci.
Comparison of sludge characteristics and performance of a submerged membrane bioreactor and an activated sludge process at high solids retention time
Water Res.
Physico-chemical characteristics of full scale sewage sludges with implication to dewatering
Water Res.
Effects of colloidal stability on clarification and dewatering of activated sludge
Water Sci. Tech.
Disintegration of activated sludge flocs in the presence of sulfide
Water Res.
A conceptual ecosystem model of microbial communities in enhanced biological phosphorus removal plants
Water Res.
Microbial communities involved in enhanced biological phosphorus removal from wastewater – a model system in environmental biotechnology
Curr. Opin. Biotech.
Correlation of physicochemical properties and sludge dewaterability under chemical conditioning using inorganic coagulants
Bioresour. Technol.
Quantification of the exchangeable calcium in activated sludge flocs and its implication to sludge settleability
Sep. Purif. Technol.
The use of ferric chloride and anionic polymer in the chemically assisted primary sedimentation process
Chemosphere
Observations on dewaterability and physical, chemical and microbiological changes in anaerobically stored activated-sludge from a nutrient removal plant
Water Res.
Compression dewatering of municipal activated sludge: effects of salt and pH
Water Res.
The Terzaghi-Voigt combined model for constant-pressure consolidation of filter cakes and homogeneous semi-solid materials
Chem. Eng. Sci.
Influence of the charge density of cationic polyelectrolytes on sludge conditioning
Water Res.
The impact of deflocculation-reflocculation on fouling in membrane bioreactors
Sep. Sci. Technol.
The influence of solids retention time on activated sludge bioflocculation and membrane fouling in a membrane bioreactor (MBR)
J. Membr. Sci.
Moisture distribution in activated sludges: a review
Water Res.
Anaerobic deflocculation and aerobic reflocculation of activated sludge
Water Res.
Cited by (247)
Triclocarban transformation and removal in sludge conditioning using chalcopyrite–triggered percarbonate treatment
2024, Journal of Hazardous MaterialsCharacterization of the dewaterability of excess aerobic granular sludge
2024, Environmental Technology and InnovationAluminum speciation in polymerized aluminum chloride: Roles and chloride ion migration in sludge dewatering
2024, Journal of Environmental Chemical EngineeringResearch progress in improving sludge dewaterability: sludge characteristics, chemical conditioning and influencing factors
2024, Journal of Environmental Management