Factors Affecting Polymer Demand.

8/12/2019 Factors Affecting Polymer Demand.

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FACTORS AFFECTING POLYMER DEMAND

Matthew J. Higgins1, Sudhir N. Murthy

2, Yen-Chih Chen

1, Nicholas Maas

1

1Dept. of Civil and Env. Engineering, Bucknell University, Lewisburg, PA

2Washington DC Water and Sewer Authority

INTRODUCTION

Synthetic organic polymers are widely used by industry and municipalities to facilitate

dewatering of sludges. Polymer is needed to produce a floc that is strong enough to remain

intact and large enough to dewater well during the mechanical dewatering process. The amountof polymer demand needed for good floc properties varies from plant to plant, depending on

digestion process, polymer type, and dewatering process. In fact, two plants with the same

digestion type and same polymer may exhibit different polymer demand. In addition, differentpolymer types will also have different optimum doses for even the same sludge type. The

reasons for the differences are not well understood. It was hypothesized that biocolloids, madeup primarily of protein and polysaccharide, created the majority of polymer demand in digested

biosolids. As a result factors affecting biocolloid concentration and the coagulation of thebiocolloids by polymer would impact polymer demand.

RESEARCH OBJECTIVES

The objectives of this research were to better understand the factors which affect polymerdemand in order to better predict polymer demand and also develop strategies to reduce polymer

demand. To achieve these objectives, the research focused on the following topics:

1) Role of biocolloids on polymer demand;

2) Impact of polymer characteristics on polymer demand;3) Impact of shear on polymer demand;

4) Quantification of shear for full-scale dewatering equipment (centrifuges, belts, etc..);

5) Implications for utilities and practitioners.

RESEARCH BENEFITS

The benefits of the research include useful tools for understanding the factors which impact

polymer demand, how plants can utilize this information to reduce polymer demand, and also

predict how process changes such as changes in digestion or dewatering equipment may impactpolymer requirements for conditioning.

RESEARCH SCOPE AND METHODOLOGY

Joint Residuals and Biosolids Management Conference 2005

Copyright 2005 Water Environment Federation. All Rights Reserved


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Samples from a number of different treatment plants were collected and analyzed for a number

of different parameters. The samples included mixed liquor, waste activated sludge (WAS),conventional aerobically and anaerobically digested, thermophilic anaerobically digested,

temperature phased anaerobically digested (TPAD), and an auto-thermal aerobically digested

sample (ATAD). The samples were analyzed for biocolloid concentration (both protein and

polysaccharides) for different size fractions, optimum polymer dose, dewatering properties, andthe mesophilic anaerobically digested samples were analyzed for metal content.

To examine the effect of polymer characteristics, the same sludges were dewatered usingpolymer with differing charge density, molecular weight, branching, and functional groups.

RESULTS AND DISCUSSION

A brief summary of the results is provided in the following sections.

Role of Biocolloids. A linear correlation was found between the concentration of biocolloids (asmeasured by protein and polysaccharides less than 4.2 m) and the optimum polymer dose(OPD) as shown in Figure 1. All samples were dewatered using the same polymer and same

procedure. Interestingly, the thermophilic samples tested typically had the highest biocolloid

concentration and highest polymer dose, and mesophilic anaerobic digested biosolids had lowerbiocolloids, while MLSS and aerobically digested samples had the lowest OPD and biocolloid

concentration.

Optimum Polymer Dose (meq/kg)

806040200

Protein+Polysaccharide

(mg/L)

3000

2000

1000

0

WM

An

An

AnAn

T

M

AnA

Figure 1. OPD vs 4.2 m size fraction of protein and polysaccharide showing different sampletypes (A=aerobically digested, An=anaerobically digested, M=MLSS, T=TPAD, W=waste

activated sludge).




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These results demonstrate that the small floc particles made up primarily of protein and

polysaccharides create a sink for polymer demand. The results also suggest that decreasingbiocolloid concentration will decrease polymer demand.

Impact of Shear on Polymer Demand. Research has shown that shear can significantly impactpolymer demand, in that greater amounts of shear results in greater polymer demand. Research

was performed in this study to better understand how different biosolids and polymer types

responded to shear during dewatering. The shear was measured using the unitless parameter Gt,where G is the velocity gradient (related to the power/shear imparted to the sludge, with units of

1/time) and t is the time that the shear is applied. Figure 2 demonstrates the response of different

sludge types to shear. In these tests, the optimum polymer dose was determined for different Gtvalues. The slope of the lines are related to how shear sensitive the sample was, with greater

slopes meaning greater shear sensitivity. A greater shear sensitivity relates to more polymer

demand during dewatering, especially in high shear dewatering devices. The results showed thatWAS samples were the most sensitive to shear followed by thermophilic, TPAD, and then

anaerobic and aerobic digestion were approximately the same.

0

20

40

60

80

100

120

0 20000 40000 60000 80000 100000

Aerobic

Anaerobic

TPAD

Thermophilic

WAS

OptimumPolymerDose(meq/kg)

Gt

Figure 2. Comparison of shear response for four different digested sludge types using the samelinear polymer.

Additional testing demonstrated that different polymers respond differently to shear, and someare much better at forming flocs under high shear than others. For example, high molecular

weight polymers (greater than about 1-2 million) are needed for effective conditioning under

high shear. In addition, for high molecular weight polymers, branched polymers appear to




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provide the best conditioning over a greater range of shear. Therefore, a plant with a shear

sensitive sludge (such as thermophilic or WAS) and dewatering equipment that impartssignificant shear, could consider a high molecular weight, branched polymer for effective

conditioning.

Shear in full-scale dewatering equipment. Using techniques previously reported, theequivalent shear of full-scale dewatering equipment can be determined. A set of tests were

performed in the field, and along with some existing data in the literature, the equivalent shear of

several different dewatering devices was determined. The results are shown in Figure 3. Beltfilter presses had the lowest equivalent shear, and the two high solids centrifuges that were tested

had the greatest equivalent shear. Using this information along with previously determined

relationships, the estimated polymer dose for a mesophilic anaerobically digested biosolids wascalculated. These calculated values are also shown in Figure 3. This analysis shows that the

polymer demand for this biosolids can vary by a factor of about 5 depending on the dewatering

equipment. This factor is even greater for a thermophilic or WAS sample since they are moresensitive to shear.

0

20000

40000

60000

80000

1 105

1.2 105

1.4 105

BeltFilterPress

ScrewPress

LSCen

trifuge

MSCentrifuge

Recessed

Cham

b.

HSCen

trifuge

HSCentrifuge2

EquivalentGt

Estimated Polymer Dose (g/kg)

10

20

30

Figure 3. Comparison of Gt values for dewatering equipment and estimated OPD for each device

assuming an anaerobically digested sludge and same polymer type.




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Engineering Significance. The results allow practitioners to understand how process changes,

such as different digestion or dewatering equipment, will impact the polymer demand. Inaddition, it provides tools to estimate the polymer demand that may occur as a result of these

process changes. In addition, the results can be applied to develop methods to reduce polymer

demand, thereby reducing costs.

Reducing polymer demand. The results demonstrated a clear relationship between biocolloid

concentration and polymer demand. The more biocolloids in solution, the greater the polymer

demand for conditioning. Therefore, reducing the biocolloid concentration would also reducethe polymer demand and the associated costs. Several strategies could be used to reduce the

biocolloid concentration. For example, a direct inverse correlation was found between the Fe

content of the anaerobically digested sludges and the biocolloid concentration. In other words,as the Fe content of the sludge increased, the biocolloid concentration decreased. This suggests

that addition of Fe would reduce the biocolloid concentration.

Greater amounts of shear have been shown to increase the biocolloid concentration, therefore,

reducing the shear would also reduce the biocolloid concentration. This could be accomplishedby minimizing the distance that sludges are piped and pumped prior to dewatering or choosing

low-shear dewatering equipment.

Predicting polymer demand. The research has shown how different digestion and dewatering

processes impact polymer demand. The main findings are that thermophilic digestion greatlyincreased polymer demand due to greater concentrations of biocolloids, and high solids

centrifuges generally increased polymer demand due to greater shear exerted on the sludges

during the dewatering process. If a plant is considering upgrading to thermophilic digestion, theresults from this research can be applied to estimate how this change will impact polymer

demand. Similarly, when upgrading to a high solids centrifuge, the increased polymer demandcan be estimated using the relative shear associated with the device.

More detailed information on the research results and their application can be found in theWERF 01-CTS-1 Report.

SUMMARY AND CONCLUSIONS

1. Biocolloids, or particles less than 4.2 m which are made up primarily of protein andpolysaccharides, created a significant amount of the polymer demand for digested

biosolids. A good linear correlation was found between this biocolloid concentration and

the optimum polymer dose of the samples. Digestion processes that used highertemperatures, such as thermophilic, ATAD, and TPAD had significantly greater amounts

of biocolloids and polymer demand than mesophilically digested sludges.

2. For the 34 different polymers examined, the polymer charge density is a key factor inexplaining differences in polymer demand for different polymers. The different cationic

polymers studied have different charge densities and characteristics such as linear or

branched configurations, and hydrophobic moieties. In addition, a second set of

polymers with similar varying properties including a wide range of molecular weights




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(MW) was examined. Lower MW polymer had a significantly greater polymer demand

than higher MW polymers (greater than about 1 million).3. Shear during dewatering is an important factor in affecting the optimum polymer dose,

with greater shear requiring more polymer for conditioning. Lower MW polymers were

not as resistant to shear as higher MW polymers. In addition, branched polymers appear

to have better resistance to shear than linear polymers.4. As the temperature of digestion increased, the sludges became less resistant to shear, and

were more susceptible to floc breakup. This means for increasing amounts of shear, the

polymer demand increased more for thermophilic sludges compared to mesophilicsludges. Undigested sludges were also very susceptible to shear.

5. The results demonstrate that both charge neutralization and polymer bridging are key

mechanism for conditioning with polymer.6. The shear associated with several full-scale dewatering devices was measured and

quantified using the unitless Gt term. The shear of two high solids centrifuges was

measured in the range of 75,000-120,000, medium solids centrifuges 25,000-35,000, anda belt filter press was 10,000. This means, for the same sludge type, a greater polymer

demand would be encountered for high shear devices such as the high solids centrifuge ascompared to a relatively low shear belt filter press.

Acknowledgements

This project was funded by the Water Environment Research Foundation, project 01-CTS-1.

The authors would like to thank the Project Subcommittee for their valuable review and

contributions to this work.



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