J. Cent. South Univ. (2012) 19: 1383-1387 

DOI: 10.1007/s11771-012-1154-7

Multi-objective model of fast-solidification sludge

ZHOU Jian(周健), LIU Jie(刘杰), LI Xiao-pin(李晓品), ZHANG Yong-sheng(张永胜), LIU Yi(刘轶)

Key Laboratory of The Three Gorges Reservoir Region’s Eco-Environment of Ministry of Education,

Chongqing University, Chongqing 400045, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: Many sludge curing technologies often have problems like long curing time, high cost, and low efficiency in the condition of low temperature. The compressive strength, moisture content and temperature are defined as the constraint conditions, and solidified cost, pH, COD, NH₄⁺-N concentration are defined as the objective functions. The response surface analysis is used to obtain a variety of response expressions of factors, and the multi-objective optimization model of fast-solidification sludge is established. Then, the curing agent formulas are optimized. After three-day conserving, the curing sludge could meet the landfill conditions.

Key words: solidified sludge; multi-objective model; sewage sludge; response surface methodology

1 Introduction

In recent years, people pay more and more attention on the sludge treatment and disposal technology. Especially, with a large number of small-scale small-town sewage treatment plant being built up and running, it produces a large amount of small, scattered, difficult-to-centralize disposal sludges. The sludge solidification technology is simple, and has low cost, good stability of pollution, good mechanical property and so on, which is suitable to apply in small town. Curing dewatering sludge to meet the landfill standards becomes a research focus [1-9]. CHAO et al [10] used lime soil to solidify dewatering sludge, and the result shows that in 20-day conserving the optimal ratio can meet the landfill standards. ZHAO et al [11-12] used lime soil to solidify dewatering sludge, and the curing sludges all need 14-20 days conserving to meet the landfill standards. CHANG and TU [13] used clay and cement, clay and lime to solidify dewatering sludge, and the curing sludge can meet landfill standards until 14-day conserving. JIN [14] used cement, aluminum sulfate, calcium chloride and water reducing agent to solidify dewatering sludge, and the study shows that after 7-day conservation the curing sludge can meet the mechanical strength of landfill requirements. CHE et al [15] used cement fly ash and cinder to solidify dewatering sludge, and the study shows that after 7-day conservation the curing sludge can meet the mechanical strength of landfill requirements, and leachate concentrations of pollutants can meet national standards.

Because of the long curing time, the problems of high cost, low efficiency in the condition of low temperature, large area occupation, etc, are still not resolved. In this work, compressive strength, moisture content and temperature are defined as the constraint conditions, solidified cost, pH, COD, NH₄⁺-N concentration are defined as the objective functions. Using response surface analysis, the expressions of a variety of factors are obtained, and the multi-objective optimization model of fast-solidification sludge is established. Through multi-objective model, the process parameters and proportion of curing agent dosage are optimized.

2 Experimental

2.1 Materials

Dewatering sludge came from a sewage treatment plant in Chongqing. The sludge was saved in polyethylene barrels, which was sealed, shading, air free, and in low temperature (4 °C) in case of dehydration and degeneration. Physical and chemical indicators of test sludge are listed in Table 1.

According to earlier studies, considering economical factor and curing effect, lime (A), addition agent (B), Portland cement (C), and fly ash (D) were chosen as curing agents.

2.2 Methods

By means of response surface analysis method, the function expressions of response values for a variety of factors were obtained, and the optimization model of rapid sludge solidification technology was established based on constraints of moisture content (c_M), compressive strength (σ), cost of curing, pH and concentration of leaching pollutants. The values of design variables are listed in Table 2 with three levels for each variable. Design of response surface test is presented in Table 3.

Table 1 Physical and chemical indicators of test sludge

Table 2 Values of design variables

3 Results and discussion

The strength, moisture content, pH and extraction concentration of solidified products according to the design experiment are evaluated, and the test results are given in Table 3.

3.1 Regression analysis

Using Design Expert to analyze the data in Table 3, the quadratic polynomial regression equation models of related factors are obtained. The equations are

σ/kPa=126.79+21.39×A+11.51×B+4.68×C+2.86×D+

4.38×E                               (1)

c_M/%=85.75-1.56×A-1.07×B-0.84×C-1.01×D-

0.42×E                               (2)

pH=9.865+0.79×A-0.18×B+0.056×C-0.03×D-

0.003×E                                (3)

COD/(mg·L^-1)=6 752.21-329.75×A-237.5×B-

87.40×C+1.59×D-27.76×E                 (4)

ρ(NH₄⁺-N)/(mg·L^-1)=72.09-9.16×A-0.71×B-0.85×C+

0.88×D-0.57×E                          (5)

Multiple correlation coefficient R² of the equations are 0.9703, 0.9822, 0.9704, 0.9785 and 0.9649, respectively, which are all higher than 0.95. This shows that the model can respond to the changes in each index better.

3.2 Multi-objective model

“Disposal of sludge from municipal wastewater treatment plant. Quality of sludge for co-landfilling” (GB/T 23485-2009) requires that co-landfilling sludge must reach the following conditions: transverse    shear strength>25 kN/m², moisture content<60%, and 5

1) Objective function 1: Cost

The price of curing agent is the key factor which determines the cost of sludge solidification. So, the cost of curing agent should be as low as possible under the premise of meeting the landfill standards. The equation is as

                             (6)

where C_i≥0 (i=1, 2, …, n), is the price of the i-th curing agent; x_i≥0 (i=1, 2, …, n), is the dosing proportion of the i-th curing agent.

Market price of addition agent is 870 RMB Yuan/t, that of Portland cement is 390 RMB Yuan/t, that of Lime is 400 RMB Yuan/t, and that of Fly ash is 60 RMB Yuan/t.

2) Objective function 2: pH

Lime mixed with dewatering sludge could increase the intensity and lower the moisture content of curing. pH should between 5 and 10 according to GB/T 23485—2009. However, pH of tested curing sludge is between 9.3 and 12. The pH should be minimized to ensure the landfilling operation.

min pH=h(x₁, x₂, …, x_n, t, T_i)                    (7)

3) Objective function 3: Concentration of extract

To prevent secondary pollution, it should minimize the concentration of pollutants:

min COD=f(x₁, x₂, …, x_n, t, T_i)            (8)

min ρ(NH₄⁺)=g(x₁, x₂, …, x_n, t, T_i)               (9)

4) Constraint 1: Compressive strength

The indicator of compressive strength could reflect whether curing sludge reaches the requirement of GB/T 23485—2009 or not. Thus, the strength is defined as a constraint condition here. When the sludge was co-landfilled, its shear strength should reach 25 kPa, and the unconfined compressive strength should reach 50 kPa [16]. The design requirement of curing sludge treatment is as follows:

                (10)

where q_u is the unconfined compressive strength, q₀ is the minimum strength needed to meet, and T is the maximum time to achieve required strength.

5) Constraint 2: Moisture content

The moisture content should be less than 60% according to GB/T 23485—2009:

              (11)

Table 3 Design of response surface test           

6) Constraint 3: Temperature

The temperature is set at 10, 20 and 30 °C, considering the effect of different temperatures on the curing efficient and cost.

Combining the above equations, sludge multi-objective model for fast curing sludge can be expressed as

                  (12)

Using Design Expert and Matlab to solve Eq. (12), the results are listed in Table 4.

Table 4 shows that under the conditions of different temperatures and the dosage of lime between 1.2% and 1.9%, the desired curing effect and cost saving could be achieved. Under the condition of low temperatures, evaporation and hydration are slow, and the strength grows slowly. Thus, increasing addition agent dosage to 6.1% and increasing the amount of fly ash to 6% could reach the strength requirements in 3 d. With the temperature rising, water evaporation increases, and hydration is accelerated, then the dosage of curing agent could be reduced to meet the landfill standards. Especially, under the condition of high temperature, the contents of addition agent and Portland cement all decrease to 3% and the intensity still grows rapidly.

3.3 Verification of multi-objective model

To verify the correctness of multi-objective model, the confirmation test results are listed in Table 5. The relative error values between predicted value and actual value are listed in Table 6.

Table 6 shows that the MC (NH₄⁺) and pH have small relative errors, indicating that the model can accurately predict the results and multi-objective optimization model is reasonable and reliable. All of the curing solution determined by the model can meet the requirements of GB/T 23485—2009. Compared with dewatering sludge, leaching pollutants of curing sludge are significantly reduced. COD removal rates reach 82.11%, 80.57% and 80.28%, and NH₄⁺-N removal rates reach 92.19%, 92.37% and 92.22%, respectively. The costs of curing program at different temperatures are 68.6, 48.6 and 39.1 RMB Yuan in 2009, and are 78.7, 56.0 and 45.4 RMB Yuan in 2011.

Table 4 Results of multi-objective optimization model at different temperatures

Table 5 Results of validation test             

Table 6 Relative error values between predicted value and actual value

4 Conclusions

1) Using variety of factor functions, the optimization model of rapid sludge solidification technology is established. The confirmation test shows that multi-objective model is reasonable and reliable.

2) Trough model calculation, best solidification formula corresponding to temperature is as follows; At 10 °C, the mass ratio of sludge solidification agent components is dewatering sludge:lime:addition agent:P-cement:fly ash=100:1.9:6.1:3.7:6.0; At 20 °C, the mass ratio of sludge solidification agent components is dewatering sludge:lime:addition agent:P-cement:fly ash= 100:1.7:3.9:3.0:6.0; At 30 °C, the mass ratio of sludge solidification agent components is dewatering sludge:lime:addition agent:P-cement:fly ash=100:1.2:3.0: 3.0:4.7.

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(Edited by YANG Bing)

Foundation item: Project(2009ZX07315-005) supported by the National Water Pollution Controlled and Treatment Great Special Fund of China

Received date: 2011-07-26; Accepted date: 2011-11-14

Corresponding author: ZHOU Jian, PhD, Professor; Tel: +86-15523829081; E-mail: zhoujiantt@126.com