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Procedia Environmental Sciences 16 ( 2012 ) 357 362

1878-0296 2012 Selection and/or peer-review under responsibility of Basel Convention Coordinating Centre for Asia and the Pacifi c and National Center of Solid Waste Management, Ministry of Environmental Protection of Chinadoi: 10.1016/j.proenv.2012.10.051

Available online at www.sciencedirect.com

The 7th International Conference on Waste Management and Technology

Research on drying effect of different additives on sewage sludge

Fenfen Zhua,*, Huimin Jianga, Zhaolong Zhangb, Luyao Zhaoa, Jiawei Wangc , Jun Huc, Hui Zhangc

a School of Environment & Natural Resources, Renmin University of China, Beijing, China b School of Resource Environment and Tourism, Capital Normal University, Beijing, China

c Beijing Drainage Group, No. 1 Gaobeidian, Chaoyang District, Beijing, China

Abstract

With the increasing amount of sewage sludge produced in China, it has been a serious environmental problem because of its containing heavy metal, virus and some organic pollutant. However, high water content is a critical problem to treat sewage further. Deep dehydration of sludge (moisture content 60%) has significant relationship with subsequent treatment and disposal. We should optimize process technology, aiming at improving drying effect and reducing economic cost. To add some chemicals to dewater sewage sludge is one of the cost-saving technologies. This study examined the influence of additives on dewatering characteristics of sewage sludge, and the additives includes lime, coal ash, diatomite, sodium dodecyl sulfate (SDS) and fly ash from municipal solid waste incinerator. As to each additive we tried different additional ratio from 5% to 30% based on mass. It is found that lime (analytically pure) is the most efficient to dewater sewage sludge, then the next is coal ash, diatomite, sodiumdodecylsulfate (SDS) and fly ash. The result demonstrates that as to lime the addition dose of 25% is cost-effective. Whats more, we performed experiments of adding two kinds of additives to find out if there is any interaction between additives so that the dewatering function can be strengthened or reduced. As lime has the best result in solely addition and because it can disinfect the sewage sludge, it is added in every combination experiments with different addition ratio, for example, combination of lime (5%) and coal ash (20%), or combination of lime (10%) and diatomite (10%). We found that the dewatering function is seldom strengthened or reduced, and it is just like adding the two additives respectively. 2012 The Authors. Published by Elsevier B.V. Selection and/or peer-review under responsibility of Basel Convention Coordinating Centre for Asia and the Pacific and National Center of Solid Waste Management, Ministry of Environmental Protection of China. Key words: Sewage sludge, Deep-dehydration, Single additives, Combination additives

* Corresponding author. Tel.: +86-10-82502694; fax: +86-10-82502694; E-mail address: [email protected]

Available online at www.sciencedirect.com

2012 Selection and/or peer-review under responsibility of Basel Convention Coordinating Centre for Asia and the Pacific and National Center of Solid Waste Management, Ministry of Environmental Protection of China.Open access under CC BY-NC-ND license.

Open access under CC BY-NC-ND license.

http://creativecommons.org/licenses/by-nc-nd/3.0/http://creativecommons.org/licenses/by-nc-nd/3.0/

358 Fenfen Zhu et al. / Procedia Environmental Sciences 16 ( 2012 ) 357 362

1. Introduction

In 2010, the total disposal capacity of sewage daily in China is about 133.93 million tons [1]. Normally, 7.5 tons dewatered sewage sludge (moisture content is 80%) would be produced per ten thousand tons sewage. So there would be 1.004 million tons of sludge produced (moisture content is 80%) everyday in China.

Besides the large amount of sludge, it can cause serious environmental problems if the sludge was not treated properly, because sewage sludge contains heavy metals, virus, and some organic compounds [2,3]. The virus, heavy metals, poisonous and harmful organic materials restrict the resource utilization of the sewage sludge [4]. The traditional or mature treatment and disposal methods for sewage sludge are landfill, incineration, compost, land utilization, building materials use, and so on [5]. However, the efficiency and cost of all those technologies are strongly affected by the water content of sewage sludge. For example, high moisture content will hurt compost and digestion [6]. It is also inconvenient to transport when the sludge contains high moisture content. And landfill requires that the moisture content is below 60%. However, water content of 60% or even lower seldom can hardly be reached by traditional thicken and dehydration in waste water treatment plant. And it is normally about 80% water content [7]. As a result, deep-dehydration technology is necessary.

The deep-dehydration technology includes thermo-drying process, reagent dehydration and so on. According to the heat resource, thermo-dehydration includes flue gas drying and steam heat drying [8]. The thermo-drying costs high not only the set-up fee, but also the operation fee. As to the deep-dehydration technologies, adding some medicine to change the characteristics of sewage sludge is one of the popularly used methods [9], because most of its cost depends on the price of reagent and the characteristics of the reagent, though some reagent dehydration process will lead the subsequent processing to some trouble [10]. The additives can improve the sludge particles structure and promote the dehydration efficiency [9].

Certainly, the medicine used is the key point of this process. Now the most widely used and mature reagent is lime. However, lime drying process has it own shortcomings, for example, because of adding CaO, the pH of the product can easily reach to 14 which prevent the wide use of its product. And whats more, the product of it has better be sent to cement industry so that the CO2 emission will not be added to the sludge treatment because of the production of lime [11]. While, adding lime is only in favor of building materials use [12]. So we are now testing other reagent to see if they can substitute lime.

Now we would choose from additives as follows: lime, coal ash, diatomite, sodium dodecyl sulfate (SDS), fly ash from municipal solid waste incinerator and the combined additives of lime coal ash and lime diatomite. These additives are economic and easy to get. Some of them are used in the previous studies but needed to be optimized.

2. Materials and methods

2.1. Materials

Municipal sewage sludge is obtained from some sewage treatment plant in Beijing. The moisture content is about 82%1%.

The experimental instruments we used to check the sludge component were listed in Table 1. And the typical element component of this kind of sewage sludge was shown in Table 2.

Table 1. Experimental instruments

MSIHighlight


Items Instruments

Metallic element Atomic Absorption Spectroscopy, HITACHI, Z-2000

TC JENA multi N/C 2100S and YENA HT1300 Solid module

Cl- Ion Chromatography, DIONEX, ICS-900

Table 2. Sewage sludge component

Items VSS(%) TC(mg/g) Cl(mg/g) K(mg/g) Fe(mg/g) Cu(mg/g) Zn(mg/g) Mn(mg/g) Ca(mg/g) Pb(mg/g)Contents 52.5 385.4 9.75 1.71 28.26 0.137 1.26 0.255 28.4 0.032

And there are six additives such as lime (analytical pure), industrial lime, coal ash, diatomite, sodium

dodecyl sulfate (SDS) and fly ash from municipal solid waste incinerator (half dry dust removal).

2.2. Methods

In this study, we tried single additive experiments and combined additives experiments. (1) Single additive experiments We tried different additional ratio from 5% to 30% based on mass (5%, 10%, 15%, 20%, 25%, 30%)

for each reagent except diatomite (30%). And we used the set-up made by ourselves to stir mixture of the additives and sludge quickly and uniformly. The experiment and operation details were shown in Table.3.

Table3. Experimental conditions for single additives experiments

Addition dose (Mass) Operation details Additives 5% 10% 15% 20% 25% 30% Operation revolution/(r min)

Mixing time/ min

lime

Industry lime Coal ash Diatomite Sodium dodecyl sulfate (SDS)

Fly ash

180 20 4

(2) Combined additives experiments Whats more, we performed experiments of adding two kinds of additives to find out if there is any

interaction between additives so that the dewatering function can be strengthened or reduced. The operation steps were similar as that of single additive experiments besides that the combination of two kinds of additives was got by the following design. Take lime and diatomite for example as shown in the Table.4.

Table 4. Experimental conditions for combined additives experiments

lime\diatomite 5% 10% 15% 20% 25% 5% M11 M12 M13 M14 M15 10% M21 M22 M23 M24 15% M31 M32 M33


20% M41 M42 25% M51

Notice: M11 means lime would be added by the amount of 5% sewage sludge with the same added amount of diatomite.

3. Results and discussion

3.1. Single additive experiments

The moisture content of the product in single additive experiments was shown in Fig.2. Among all of these experiments, it can be seen that lime has the best function in reducing the moisture, and the next is industry lime. The SDS (C12H25-OSO3Na) and diatomite had almost the similar effectiveness.

Fig. 1. Moisture content of products from single additives experiments

For the coal ash, because its original sludge had much higher moisture content than the others, the water content in the product of coal ash experiments seems a little high. But if we check the slope of the lines, we may find that it would be between industry lime and diatomite. The moisture content decreased more quickly by adding coal ash than diatomite and SDS (C12H25-OSO3Na).

As to lime, we think that lime had much higher content of alkaline oxide than industry lime, fly ash and coal ash, which would release heat, OH- and react with water when it meets water, so that the addition of it can reduce the water content of sludge. However, adding SDS (C12H25-OSO3Na) or diatomite has just increased the solid content of the product without any other function. .

3.2. Combined additives experiments

Based on single additive experiment the order of drying effectiveness of the additives is: limeindustrial lime coal ash diatomite SDS fly ash. Considering experimental error, we think that diatomite has the similar drying effectiveness as SDS. In the subsequent experiment we combined lime separately with diatomite and coal ash to perform the combined additives experiments. Because coal ash has relative better drying function, we did more experiments than diatomite.

We included the experimental results of combined lime and diatomite, and some experimental results of combined lime and coal ash as well as some single additives experiments results in Figure 2. From Figure 2 it can be seen that the average moisture content decreased with the increasing of diatomite content, and combination of 20% lime with 10% diatomite could reach the moisture content of 60%. Comparing with the same amount of lime addition, the drying function of the combined additives was


worse. Or we could say that the diatomite did not promote the additives drying effectiveness. And the drying effect of lime + coal ash is between that of lime + diatomite and that of lime.

10%lime 10%lime 10%lime 10%lime

20%lime 20%lime15%lime 20%lime25%lime

30%lime 25%lime30%lime

Fig. 2. Drying effectiveness of combined additive (lime and diatomite, lime and coal ash)

Notice: L1 and L2 means different doses of the lime additive. D means diatomite additive. C means coal ash additive. The pillar of L+C means lime + coal ash at the same dose with lime + diatomite. For example, in the first pillar, it is

10%lime + 5%coal ash. From Figure 2 it can also be concluded that 10%lime+20%coal ash, 20%lime+10%coal ash can reach

the moisture content of 60%. 30%lime reached the moisture content 56.7%, when the original moisture content is 81.5%. Using coal ash substitute lime was an economic choice from the view of reducing water content.

4. Conclusions

Firstly, it can be concluded that the order of drying effectiveness is: lime industrial lime coal ashdiatomite SDS fly ash. And we think diatomite has the similar drying effectiveness as SDS. Diatomite can be used as a standard to find out if there is other drying function of some reagent besides dilution because of adding solid.

In the combination additives experiments, 20%lime+10%diatomite can reach an almost satisfied result. And 10%lime+20%coal ash, 20%lime+10%coal ash, can reach the moisture content of 60%.

Acknowledgements

This research is supported by the Basic Research funds in Renmin University of China from the central government (12XNLJ02).

References

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