Elsevier

Water Research

Volume 82, 1 October 2015, Pages 14-24
Water Research

Dewatering in biological wastewater treatment: A review

https://doi.org/10.1016/j.watres.2015.04.019Get rights and content

Highlights

  • Strong compact sludge flocs give the best dewaterability of biological sludge.

  • Water hardness improves floc strength and sludge dewaterability.

  • Variation in conductivity and pH reduce dewaterability due to flocs disintegration.

  • Anaerobic storage lowers sludge dewaterability.

  • Shear and pumping destroy flocs and reduce sludge dewaterability.

Abstract

Biological wastewater treatment removes organic materials, nitrogen, and phosphorus from wastewater using microbial biomass (activated sludge, biofilm, granules) which is separated from the liquid in a clarifier or by a membrane. Part of this biomass (excess sludge) is transported to digesters for bioenergy production and then dewatered, it is dewatered directly, often by using belt filters or decanter centrifuges before further handling, or it is dewatered by sludge mineralization beds. Sludge is generally difficult to dewater, but great variations in dewaterability are observed for sludges from different wastewater treatment plants as a consequence of differences in plant design and physical-chemical factors. This review gives an overview of key parameters affecting sludge dewatering, i.e. filtration and consolidation. The best dewaterability is observed for activated sludge that contains strong, compact flocs without single cells and dissolved extracellular polymeric substances. Polyvalent ions such as calcium ions improve floc strength and dewaterability, whereas sodium ions (e.g. from road salt, sea water intrusion, and industry) reduce dewaterability because flocs disintegrate at high conductivity. Dewaterability dramatically decreases at high pH due to floc disintegration. Storage under anaerobic conditions lowers dewaterability. High shear levels destroy the flocs and reduce dewaterability. Thus, pumping and mixing should be gentle and in pipes without sharp bends.

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):(1ε)=(1ε0)(1+pps)βwhere 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

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