Elsevier

Bioresource Technology

Volume 102, Issue 21, November 2011, Pages 9843-9851
Bioresource Technology

Optimization of operating parameters for sludge process reduction under alternating aerobic/oxygen-limited conditions by response surface methodology

https://doi.org/10.1016/j.biortech.2011.07.079Get rights and content

Abstract

Batch tests were employed to estimate the optimal conditions for excess sludge reduction under an alternating aerobic/oxygen-limited environment using response surface methodology. Three key operating parameters, initial mixed liquor suspended solids (initial MLSS), HRT (hydraulic retention time) and reaction temperature (T), were selected, and their interrelationships studied by the Box–Behnken design. The experimental data and ANOVA analysis showed that the coefficient of determination (R2) was 0.9956 and the adjR2 was 0.9912, which demonstrates that the modified model was significant. The optimum conditions were predicted to give a maximal ΔMLSS yield of 226 mg/L at an initial MLSS of 10,021 ± 50 mg/L, an HRT of 9.1 h and a reaction temperature of 29 °C. The prediction was tested by triplicate experiments, where a ΔMLSS yield of 233 mg/L was achieved under the chosen optimal conditions. This excellent correlation between the predicted and measured values provides confidence in the model.

Highlights

► Alternating aerobic/oxygen-limited condition was the first time applied to the excess sludge reduction. ► The initial MLSS, HRT and Temperature were used as the three key parameters in oxygen-limited tank. ► We model the reciprocal relationships of the key parameters using RSM. ► The optimum condition was under the initial MLSS of 10,195 ± 50 mg/L, HRT of 9.8 h with T of 20.4 °C. ► HRT was a relative important parameter and more concentration on HRT should be concerned in continuous-flow experiment.

Introduction

Activated sludge technology has taken a predominant role in municipal and industrial sewage treatment, both domestically and abroad. The major problem with this biological technology is the generation of a large amount of excess sludge (Ye and Li, 2010), the management and disposal costs for which take up 50–60% of the total operational costs in sewage treatment plants (Campos et al., 2009). The final treatments for excess sludge commonly include land application, land filling and incineration (Ginestet, 2006), yet these create environmental challenges because there are inadequate locations for land filling or incineration in densely populated urban areas (Chen et al., 2002). Therefore, efficient methods to cope with this problem are urgently required. Excess sludge disposal methods that are widely used today may be classified into two broad approaches (Mahmood and Elliott, 2006): (1) post-treatment methods, which are used to decrease the excess sludge produced (Liu et al., 2009, Papadimitriou et al., 2010), which have disadvantages in that they require extra expense and, most importantly, do not tackle the problem at the root cause; (2) process reduction methods, which lower sludge production by modified or improved biological treatment systems, such as extended aeration, membranes and so on (Abbassi et al., 1999, Jiang et al., 2009). The advantage of the latter approach is that it may minimize excess sludge yield through processing and avoid affecting the efficiency of sewage treatment (Rajesh Banu et al., 2009). Thus, currently, processing treatment may be the most promising and radical approach to solving the excess sludge problem.

In recent years, more and more studies have confirmed that the oxygen-limited environment had many advantages. It is well known that the growth rate of aerobic heterotrophs is hampered when oxygen concentrations are low (Alagappan and Cowan, 2004). Researchers have already noted special phenomena under oxygen-limited conditions (Chen et al., 2010, Chuang et al., 2007) and recent research shows that microorganisms exhibit some new characteristics under low dissolved oxygen (DO) conditions (Hu et al., 2005, Peng et al., 1999). It is of great importance to study the operational characteristics of sewage treatment facilities under low DO concentrations (Alagappan and Cowan, 2004). Hence, it is valuable to examine the phenomenon of sludge reduction under an oxygen-limited environment. In this study, microorganisms were exposed to an aerobic environment with adequate nutrition and then transferred into an oxygen-limited environment with no substrate supply, in order to study excess sludge reduction under alternating aerobic/oxygen-limited conditions.

The operating parameters used in excess sludge reduction technology are important and different combinations of these parameters will affect the effectiveness of the excess sludge reduction. Researchers have pointed out that an appropriate hydraulic retention time (HRT) in the excess sludge holding tank is essential for cost-effectiveness and at the same time may reduce negative impacts on process performance and sludge characteristics (Ye et al., 2008). Wang et al. (2008) found that a reasonable HRT in an anaerobic tank is 8–12 h. Low and Chase (1999) studied the effect of the mixed liquor suspended solids (MLSS) on excess sludge reduction, showing that when the MLSS increased from 3000 to 6000 mg/L, the production of excess sludge decreased by 12%, while increasing MLSS from 1700 to 10,300 mg/L, would decrease excess sludge by 44%. Thus, it was demonstrated that the control of MLSS could reduce excess sludge production. Chen et al. (2003) also found that various combinations of HRT and reaction temperature caused different reduction effects and altered the dominant factor in excess sludge reduction. Therefore, it might be supposed that excess sludge reduction might be impacted by control of the key operating parameters, such as the initial MLSS value, HRT and reaction temperature in the oxygen-limited tank. These were therefore regarded as the important parameters in this study and their optimal combination was applied to continuous-flow experiments.

To examine sludge reduction, having an appropriate response to measure is essential. Biomass production usually decreases by an equivalent percentage to the reduction in sludge production (Liu and Tay, 2001) so ΔMLSS (the initial minus the final MLSS values) should change by different degrees with respect to different operating conditions (Chen et al., 2003). In this paper, ΔMLSS was regarded as a function of the initial MLSS, the HRT and the reaction temperature in the oxygen-limited tank and was therefore measured during the batch tests as the most important response for examining the sludge reduction effect.

The original single factor optimization method cannot account for the mutual effects of all the factors involved and also requires a large number of experiments (Uma Maheswar Rao and Satyanarayana, 2007). Response surface methodology (RSM) is a useful statistical technique for researching complex variable processes based on the Box–Behnken design (BBD). This method can effectively describe the interactions between independent experimental factors and response parameters (Guo et al., 2009, Guo et al., 2011). Therefore, both the single factor method and RSM were employed in this paper to design and determine the optimal conditions for excess sludge reduction.

To the authors’ knowledge, there have been few studies on using a combination of operating parameters through a Box–Behnken design and RSM to achieve improved sludge reduction. Therefore, the objective of this paper was to study the mutual relationships between the three evaluated operating parameters, initial MLSS, HRT and reaction temperature, and sludge reduction under alternating aerobic/oxygen-limited conditions. It is expected that the optimal operating conditions resulting from the batch tests will offer important reference values for later continuous flow experiments.

Section snippets

Sludge cultivation and seed microorganisms

Before the batch tests, an anaerobic/anoxic/aerobic (A2/O) reactor equipped with five compartments, comprising an anaerobic tank, an anoxic tank, three aerobic tanks and a sludge settling tank was employed to cultivate the sludge. A microbial seed, taken from the Harbin Wenchang sewage treatment plant, was inoculated into the A2/O process. The HRT was controlled at 8 h, the DO concentration in the aerobic tank was controlled between 2 and 4 mg/L, and the influent chemical oxygen demand (COD) of

Analysis of single factor test results

The results of the single factor tests shown in Fig. 2a and b indicate that the sludge reduction yield, SCOD, TOC, protein and glucose content in the supernatant were altered by variation in HRT (Gao et al., 2009). In Fig. 2a and b, the nutrient contents in the supernatant increased from 4.5 to 9 h, then decreased from 9 h to the designed 12 h. Clearly, the nutrient content in Fig. 2a was higher than that in Fig. 2b, and Table 2 also shows that the maximum ΔMLSS occurred when the HRT was around 9 

Conclusion

RSM was adopted to estimate the effect of the three key factors on process sludge reduction under alternating aerobic/oxygen-limited conditions. According to experiments and statistical analysis, the three key parameters all had significant mutual effects on ΔMLSS reduction yield. A ΔMLSS reduction yield of 233 mg/L was obtained experimentally under the optimal conditions comprising an initial MLSS of 10,021 ± 50 mg/L, a reaction time (HRT) of 9.1 h and a reaction temperature of 29 °C, which closely

Acknowledgements

This research was supported by the National Nature Science Foundation of China (Grant Nos. 50821002 and 51008105). The authors also gratefully acknowledge the financial support by the State Key Laboratory of Urban Water Resource and Environment 2010QNo2, the China Postdoctoral Science Foundation (Grant No. 20100471068) and the Heilongjiang Nature Science Foundation (Grant No. QC2010105).

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