Elsevier

Water Research

Volume 46, Issue 4, 15 March 2012, Pages 1045-1051
Water Research

Polymer flocculation mechanism in animal slurry established by charge neutralization

https://doi.org/10.1016/j.watres.2011.11.078Get rights and content

Abstract

Flocculation and filtration of animal manure is practically and environmentally beneficial. However, the flocculation mechanism in manure need to be clarified to use the technique efficiently rather than relying on trial-and-error. Manures were flocculated with polyacrylamides. Floc size, dewaterability, dry matter and turbidity were measured. At optimal polymer volume, the charge neutralization was determined, i.e. amount of negative manure particle charge neutralized by positive polymer charge. The optimal cationic polymer properties were linear and very high molecular weight, which caused efficient particle catching. And it had medium charge density, which caused efficient particle attachment. The required charge neutralization was 5–23% (15% for the optimal polymer). Polymer bridging proved the dominant flocculation mechanism; patch flocculation may be slightly significant for some polymers, while coagulation proved insignificant. Manure’s high ionic strength, high dry matter content and highly charged small molecules caused bridging to be more dominant in manure than in other typically flocculated media.

Graphical abstract

Highlights

► Mechanism upon manure flocculation is identified by observing charge neutralization. ► Proved: necessary to neutralize 5–23% of manure particle charges by polymer charges. ► Mechanism: Polymer bridging dominant, while patch flocculation slightly. ► Polymer bridging proved more dominant than in previously observed media. ► Contrast: manure’s high ionic strength, dry matter and charge on small particles.

Introduction

Solid–liquid separation of slurry produces a small nutrient- and energy-rich solid fraction. The other separation product is a liquid that contains most of the ammonia, which is useful as a local nitrogen and water source. The separation of animal manure has proved useful on many farms, and about 3% of all Danish manure is separated (Birkmose and Thygesen, 2010). For the majority of separation purposes, polymer flocculation has been found to be the best technique (Hjorth et al., 2010).

Polymer flocculation is typically done with the use of polyacrylamides. Because manure particles are negatively charged (Christensen et al., 2009), the applied polymer is cationic (Hjorth et al., 2010). The molecular size, structure and charge density of the polymer affects the effectiveness of the flocculation process (Ashmore and Hearn, 2000, Bouyer et al., 2001, Larsson et al., 1999). In the past, polymers of medium to very high molecular weight, both branched and linear, with charge densities between 5% and 75% have been used for manure (Dao and Daniel, 2002, Hjorth et al., 2008, Masse et al., 2010a, Vanotti et al., 2005).

Three different flocculation mechanisms can be expected to occur during flocculation of animal manure (Gregory, 1973).

  • 1.

    Coagulation is the process in which a polyelectrolyte is used to reduce or neutralize the electrical repulsion between particles, causing aggregation of the particles.

  • 2.

    Electrostatic patch flocculation is the adsorption of polymer onto local sites of a particle, causing patches of local charge reversal, resulting in a positive–negative attraction between the particles.

  • 3.

    Polymer bridging is the adsorption of segments of each polymer chain onto the surface of more than one particle, causing them to aggregate.

Depending on the flocculation mechanism, different amounts of negative manure particle charge need to be neutralized by the positive polymer charge; here termed the degree of charge neutralization. In other media than manure, an amount of charge neutralization of approximately 20–200% has been observed (Besra et al., 2002, Borget et al., 2005, Eriksson et al., 1993, Gill and Herrington, 1987, Mende et al., 2007, von Homeyer et al., 1999, Walker and Grant, 1996).

Animal manure is different from other typical flocculation media such as wastewater and paper pulp, due to its high ionic strength, i.e. a conductivity of 15–30 mS/cm (Masse et al., 2010b, Mondor et al., 2009). This causes reduced efficiency of the polymer due to (i) the reduced range of electrostatic interaction between manure particles and the polymer, and (ii) polymer curling (Dahlgren, 1994, Eriksson et al., 1993). In addition, the concentration of particles in manure is high, i.e. 3–7% dry matter, and the small particles are highly charged – about 60% of the total charge is related to particles <50 μm in size (Christensen et al., 2009). Thus, the relative influence of the three flocculation mechanisms in manure cannot be expected to be the same as their relative influence in other media.

The optimal method for flocculation varies greatly between different farming operations due to the large variation between manures. At present, identification of the best method is performed by carrying out multiple trial-and-error flocculations. However, gaining an understanding of the mechanism behind the flocculation of manure using polymers should help to make the trial-and-error approach unnecessary. However, there is little literature on this topic.

The aim of this study is therefore to explore the mechanisms involved in flocculation of animal manures, based on separation performance and the degree of charge neutralization. Three finishing pig slurries were flocculated using seven polyacrylamides with various characteristics. Four important separation performance criteria were measured (Hjorth and Christensen, 2008): floc size, dewaterability, solid dry matter content, and liquid turbidity. The degree of charge neutralization was also determined.

Section snippets

Manure and polymer properties

Three manures were collected from three typically run commercial finishing pig farms. Samples of 50–100 L were taken from agitated pre-tanks. Treatments were carried out after 1–2 months storage at 5–10 °C. Analyses were performed immediately after the treatments.

Seven cationic polyacrylamide polymers with different properties were used (Table 1).

Treatments

The three manure samples were flocculated using the polymers (Table 1). Samples of 0.5 L were placed in 1-L beakers and flocculated using one of nine

Results and discussion

To examine the mechanism of manure flocculation, three finishing pig manures was flocculated with seven different polymers. No correlation was observed between the dry matter content of the manures and the average surface charge of the manure particles (Table 2). This is in accordance with the previously observed absence of correlation between manure dry matter content and volume of polymer required for optimal flocculation (Masse et al., 2010a).

The average optimum polymer volumes required for

Conclusions

The optimal cationic polymer was very large, linear and had medium charge density, causing efficient particle catching, respectively, attachment. The required charge neutralization, i.e. negative manure particle charge neutralized by positive polymer charge, was 5–23%, and 15% of the optimal polymer. Polymer bridging was the dominant flocculation mechanism, patch flocculation was possibly slightly significant for some polymers, and coagulation was insignificant. Manure’s high ionic strength,

Acknowledgments

The authors thank Peter V. Christensen and Morten L. Christensen (Aalborg University, Denmark) for valuable discussions. Financial support for this study was provided by the Danish Council for Strategic Research as part of the project Cleanwaste. Further, the authors thanks Kemira Water for the supply and valuable discussions.

References (30)

Cited by (0)

View full text