Minimization of excess sludge production by in-situ ...

Biotechnology Advances 31 (2013) 1386–1396

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Research review paper

Minimization of excess sludge production by in-situ activated sludge treatment processes — A comprehensive review Wan-Qian Guo a, Shan-Shan Yang a,⁎, Wen-Sheng Xiang b, Xiang-Jing Wang b, Nan-Qi Ren a,⁎⁎ a b

State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, PR China Research Center of Life Science and Biotechnology, Northeast Agricultural University, Harbin 150030, China

a r t i c l e

i n f o

Article history: Received 1 April 2013 Received in revised form 31 May 2013 Accepted 10 June 2013 Available online 19 June 2013 Keywords: In-situ excess sludge reduction Lysis-cryptic growth Uncoupling metabolism Worms' predation Improved/novel sludge reduction processes Review

a b s t r a c t The widespread application of conventional activated sludge treatment process has been employed to deal with a variety of municipal and industrial sewage. While the generation of waste activated sludge (WAS) was considerably huge, the management and disposal expenses were substantially costly. A promising process aimed for WAS reduction during the operation process is urgently needed. Thus, increasing attentions emphasizing on the improved or novel sludge reduction processes should be intensively recommended in the future. This review presents the current and emerging technologies for excess sludge minimization within the process of sewage treatment. The ultimate purpose of this paper is to guide or inspire researchers who are seeking feasible and promising technologies (or processes) to tackle the severe WAS problem. © 2013 Elsevier Inc. All rights reserved.

Contents 1. 2.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . In-situ activated sludge reduction processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Sludge reduction by lysis-cryptic growth . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Activated sludge process combining chemical oxidation . . . . . . . . . . . . . . . . 2.1.2. Activated sludge process combining ultrasound . . . . . . . . . . . . . . . . . . . . 2.2. Sludge reduction by uncoupling metabolism . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Oxic-settling-anaerobic (OSA) process . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Repeatedly coupling of aerobic/anaerobic process . . . . . . . . . . . . . . . . . . . 2.2.3. Uncouplers-induced sludge reduction . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Sludge reduction by worms' predation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Two-stage sludge predation systems . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Oligochaeta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3. New concept for sludge reduction by worms' predation . . . . . . . . . . . . . . . . 2.4. Sludge reduction by improved/novel processes . . . . . . . . . . . . . . . . . . . . . . . . Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Research prospects for lysis-cryptic growth technology . . . . . . . . . . . . . . . . . . . . 3.2. Research prospects for uncoupling metabolism technologies . . . . . . . . . . . . . . . . . . 3.2.1. OSA system and novel systems based on repeatedly coupling of aerobic/anaerobic process 3.2.2. Uncouplers-induced uncoupling metabolism . . . . . . . . . . . . . . . . . . . . . 3.3. Research prospects for worms' predation technology . . . . . . . . . . . . . . . . . . . . .

⁎ Corresponding author. Tel./fax: +86 451 86283008. ⁎⁎ Corresponding author. Tel./fax: +86 451 86282008. E-mail addresses: [email protected] (S.-S. Yang), [email protected] (N.-Q. Ren). 0734-9750/$ – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.biotechadv.2013.06.003

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4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1395 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1395 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1395

1. Introduction Widespread application of the conventional activated sludge (CAS) process has been employed to deal with a variety of municipal and industrial sewage. This process has taken an irreplaceable role in wastewater treatment plants (WWTPs) domestic and abroad. Of all the possible ways that remove organic pollutants by microorganism metabolism, waste activated sludge (WAS) is the unpleasant by-product for its large volume and offensive odor. Moreover, the annual produced WAS has been and will continue to increase in the foreseeable future. On the other hand, the expense for WAS treatment is costly, which has been estimated to take up 50–60% of the total operational costs in WWTPs (Campos et al., 2009). However, the prohibitions of conventional sludge treatment and disposal methods including land application, incineration and land filling have been proposed by the economic, environment and legal regulations (Yang et al., 2011). Hence, considering the environmental burden and the enormously high expense, the management and disposal of this increasing WAS have become one of the most stringent challenges in biological sewage treatment field (Tian et al., 2013). According to the previous researches, the WAS reduction approaches were mainly classified into two categories (Mahmood and Elliott, 2006): (1) sludge reduction trough post treatment; and (2) in-situ activated sludge reduction in the process of sewage treatment. Post treatments are referred to the sludge reduction methods, which reduce excess sludge that has already been produced during sewage treatment process. Various approaches, relying on either single or a combination of physical, chemical and biological methods have been employed to reduce WAS (Lou et al., 2011; Saby et al., 2002; Wang et al., 2011; Zhang et al., 2007). However, what should be aware of these post treatments are the high operational complexity and expenses, together with a large amount of energy consumption. Comparing with

the post treatment, in-situ excess sludge reduction process is of prominent advantages. The significant characteristic of in-situ excess sludge reduction is that it can minimize the sludge yield from the source of sewage treatment process itself. Some researchers have already demonstrated the great application potential of this in-situ excess sludge reduction process (Guo et al., 2007; Wei et al., 2009; Xing et al., 2008; Yu et al., 2006). Hence, the developments of novel and feasible sludge reduction technologies that emphasize the in-situ sludge reduction will be a considerable impetus in the future investigation. Sustainable sludge handling may be defined as a method that meets requirements of efficient recycling of resources, without supply of harmful substances to humans or the environment (Commission of European Communities, 1998). In China, the selected/preferred sludge reduction processes/technologies included chemical or/and physical lysis-cryptic methods combining with the activated sludge processes (He and Wei, 2010; Ma et al., 2012; Wang et al., 2011); sludge reduction technologies based on uncoupling metabolism (Feng et al., 2012; Tang et al., 2011; Wang et al., 2008; Xing et al., 2008); sludge reduction by worms' predation technology (Lou et al., 2011; Tian and Lu, 2010; Tian et al., 2010; Wei et al., 2009); and other improved/novel sludge reduction processes (Lin et al., 2009; Xiao et al., 2011) (Fig. 1). For the selected sludge reduction processes/technologies, the effluent quality is also required to meet the more stringent effluent regulation and standard worldwide (Saktaywin et al., 2005; Chinese CJ3025-93; Chinese GB18918-2002; Chinese 12th Five-Year Plan). Aiming at efficient sludge process reduction systems development, there are two priorities to obey: (1) the WAS yield is minimized within the sewage treatment process itself; (2) the selected process will not affect or deteriorate the effluent quality especially the nutrient removal efficiency. Although some summarizations on sludge reduction have already been documented, still the review concentrated on in-situ sludge reduction is far from sufficient. The objective of this paper is thereby

Fig. 1. Outline of in-situ activated sludge reduction technologies.

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aimed to provide a state-of-the-art review of current and emerging approaches that were applied to minimize sludge yield during the sewage treatment process. 2. In-situ activated sludge reduction processes 2.1. Sludge reduction by lysis-cryptic growth Sludge reduction can be achieved by lysis-cryptic growth of microorganisms. The biomass grows on the lysates that dissolved from its own. When microbial cells are disintegrated, the microorganism cell contents would be released into the liquid, and these organic autochthonous substrates are reused by itself for metabolism (Chu et al., 2009). Sludge disintegration technologies, such as ozonation, chlorine dioxide, ultrasound treatments, and so on forth, have been developed and are now widely applied to in-situ activated sludge reduction field (Cui and Jahng, 2004; Gallard and von Gunten, 2002; He et al., 2006; Huysmans et al., 2001; Park, 2001; Saktaywin et al., 2005). Although numbers of papers have already reviewed on the chemical or/and physical lysis-cryptic growth methods, here, a totally new perspective is given to the chemical or/and physical lysis-cryptic methods combining with the existing biological sewage treatment systems (BSTSs) from an in-situ activated sludge reduction viewpoint (Fig. 2). 2.1.1. Activated sludge process combining chemical oxidation In chemical oxidation cell lysis techniques, ozone, with strong cell lysis ability, is reported as the most powerful oxidant and disinfectant. During sludge ozonation process, microorganisms were killed and the released organic substances were oxidized (Cui and Jahng, 2004). It was well documented that the sludge ozonation pretreatment could effectively improve the sludge settling properties, facilitate the sludge biodegradability and reduce bulking and scumming (Chu et al., 2009). In order to lower the excess sludge yield in BSTSs, Yasui and Shibata (1994) developed an activated sludge process coupled with ozonation process. In recent years, this sludge ozonation system has been widely studied by many researchers (He et al., 2006; Saktaywin et al., 2005; Vergine et al., 2007; Yan et al., 2009). Summarizing the combined sludge ozonation process with different structured BSTSs in Table 1, sludge ozonation is proved an efficient and promising sludge disintegration technique. However, there are still some limits hindered its wide usage in full-scale sewage treatment plants. As a strong oxidant, ozone reacts with microorganism or organic materials indiscriminately; this may lower the

oxidation efficiency of activated sludge and increase the operation cost since more than needed ozone has to be added to the system. In order to lower the input costs, other cost-effective technologies are regarded as the alternatives for excess sludge minimization. As compared to ozone, the oxidizability of chlorination (Cl2), chlorine dioxide (ClO2), or hydrogen peroxide (H2O2) is weaker. Saby et al. (2002) achieved a 65% sludge reduction by adding 0.133 g Cl2/g mixed liquid suspended solids (MLSS). A combined sequencing batch reactor (SBR) process with the ClO2 system had successfully reduced up to 58% sludge yield (Wang et al., 2011). According to the study of Fu et al. (2008), the addition of 10 mg ClO2/g DS (dry solids) into the activated sludge process would achieve zero excess sludge discharge. He and Wei (2010) reported using membrane bioreactor (MBR) with H2O2 process, the average sludge yield from 0.15 to 0.006 g MLSS/g COD was obtained. Moreover, the application of this MBR system with the sludge Fenton oxidation process showed a better performance for the enhancement of TN removal. For the reason of different reactor configurations and diverse operation conditions, the optimal dosage of the oxidizer is hard to reach coherence. Moreover, a strong disinfectant and residential ozone (or other oxidants) in the recycled sludge might damage the bioactivity in the BSTSs. Meanwhile, the formation of undesirable chlorinated by-products such as trihalomethanes (THMs) would occur in the chlorination-activated sludge process (Gallard and von Gunten, 2002; Park, 2001), which is harmful to human beings. Therefore, the practical application of chemical oxidants-activated sludge process would impose serious challenges in full-scale sewage treatment plants. 2.1.2. Activated sludge process combining ultrasound Ultrasound treatment could avoid the above-stated disadvantages caused by chemical treatment technologies. Reported positive effects, including improving the sludge biodegradability, dewaterability, and biosolids quality (Khanal et al., 2007; Weemaes and Verstraete, 1998), shortening the retention time (Tiehm et al., 1997), and even inducing the subsequent clean energy recovery (Yang et al., 2012), ultrasound pretreatment was demonstrated a promising physical lysis-cryptic growth technology. Owing to the effectiveness and operational simplicity, this “ultrasonic lysis-cryptic growth” technology combining with BSTSs have been thoroughly studied in laboratory, pilot and full-scale experiments (Table 2). However, Xu et al. (2010) proposed that when applied individual ultrasound pretreatment to WAS, the liquid would absorb a majority of ultrasonic energy. As a result it will weaken the ultrasonic energy imposed on WAS. Thus, the

Fig. 2. Schematic diagram of lysis-cryptic growth process.


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Table 1 Summarized experiences of activated sludge process combining chemical oxidation. References

Sludge process reduction systems

Highlights

Lee et al. (2005) Cui and Jahng (2004)

CAS + O3 process A/O + O3 process

Suzuki et al. (2006)

A/A/O + O3 process

Huysmans et al. (2001)

SBR + O3 process

Dytczak et al. (2007)

SBR + O3 process

He et al. (2003)

MBR + O3 process

He et al. (2006)

MBR + O3 process

Wang et al. (2008)

MBR + O3 process

Song et al. (2003)

MLE type MBR + O3 process

Hwang et al. (2010)

MBR + turbulent jet flow O3 contactor process

Results showed that there is no excess sludge discharged from this proposed system during 112 days operation. A combination of AO with O3 process was developed to realize both the excess sludge reduction and the nitrogen-control in the effluent. Results demonstrated that the solubilized excess sludge by sludge ozonation would act as a reducing power for denitrification and a source of nitrogen. A novel biological simultaneously sludge reduction and nutrient removal process, the A/A/O + O3 process with phosphorus adsorption column was established. This combined system successfully achieved 34–127% of sludge reduction and around 80% of phosphorus recovery during 92 days operation. A lab-scale SBR system with intermittent recycled (two-thirds of the sludge twice a week) ozonated sludge was developed. An average recycled 0.019 g O3/g SSozonated ozonated sludge would result in a 50% sludge reduction yield. There were no adverse effects on the nutrient removal efficiency in the effluent. By combining the ozonation stage, 20% returned ozonated sludge had no negative impact on the effluent quality of the SBR process. Combining anaerobic biofilm-aerobic membrane process with O3 process, a zero sludge yield was obtained during 2-month operation, furthermore, there was no effect on the organic matters removal. In a MBR + O3 process, the optimized ozone dosage of 0.16 kg O3/kg MLSS was achieved in batch experiment. By conducting the continuous operation experiments, a 4% returned ozonated sludge flow rate would achieved only 0.039 kg MLSS/kg COD excess sludge during 120 days. Economic analysis also demonstrated that this MBR + O3 process was a cost-efficient process. Mathematical models were developed to investigate the controlled parameter in this combined MBR + O3 process. The ratio of flow-rate draining to ozonation unit (q) to influent wastewater flow-rate (Q) was proved the main operational parameter in MBR + O3 process. In this MBR + O3 process, 0.0067 q/Q ratio and 0.72 sludge lysis ratio in ozonation unit (ξ) would result in a zero excess sludge discharge, and excellent effluent quality except for TP removal could be achieved in MBR. The modified Ludzack–Ettinger (MLE) type MBR system combined with O3 process resulted in zero excess sludge discharge. This combined process proved good performance of nutrient removal efficiency. The synergic effects of mechanical disintegration (hydrodynamic cavitation) and chemical oxidation (O3) on sludge reduction were demonstrated by combined a turbulent jet flow ozone contactor (TJC) with the MBR process. This combined process effectively improved the sludge reduction effect with less energy consumption.

solution to enhance the utilization efficiency of ultrasonic energy in WAS treatment was urgent needed. According to previous literatures, higher ultrasonic utilization efficiency could be obtained by combining the ultrasound treatment with other chemical/physical/biological methods (Zawieja et al., 2008). Ma et al. (2012) proposed a combination of ultrasonic and alkaline method. The continuous operation indicated that 56.5% excess sludge reduction could be achieved in this pilot-scale lysis-cryptic growth system. According to economical analysis, this combined ultrasonic/alkaline technology was demonstrated effectively lower 11.4% of the total operation cost than that of the conventional CAS system without sludge pretreatment. Yang et al. (2013) also testified that when combining the ozone with ultrasound pretreatments, the disintegration of excess sludge was enhanced, and consequently, resulting in a high ratio of sludge reduction. 2.2. Sludge reduction by uncoupling metabolism Microbial metabolism is a sum of biochemical transformation that includes by the interrelation of the catabolic and anabolic reactions.

Under normal conditions, the catabolism of microbe is coupled with the anabolism through the transfer of the generated energy (Aragón et al., 2009). Uncoupling metabolism in the microorganism has been demonstrated to disassociate with the energy coupling between catabolism and anabolism. If the uncoupling phenomenon appeared, the synthesis of ATP is inhibited. Then, part of the energy is consumed to non-growth-associated reactions. The energy generated from the oxidation of organic substrate is partially used for anabolism. Thus, when maintaining the microorganism under the energy uncoupling metabolism condition, a decrease in the microbial synthesis is induced. 2.2.1. Oxic-settling-anaerobic (OSA) process Uncoupling metabolism phenomenon can be induced under some abnormal circumstances, such as high initial substrate concentration to the initial biomass concentration (So/Xo as COD/biomass) (Liu, 2000; Liu et al., 1998), chemical uncouplers (Qiao et al., 2011; Ye and Li, 2010), oxic-settling-anaerobic (OSA) process (Chen et al., 2003; Jin et al., 2008; Wang et al., 2008) and repeatedly coupling of aerobes/anaerobes environment (Feng et al., 2012; Xing et al., 2008;

Table 2 Summarized experiences of activated sludge process combining ultrasound. References

Sludge process reduction systems

Yoon (2003)

MBR + UV process

Highlights

In order to elaborate the relationships between the operation parameters in the combined MBR + UV process, a mathematical model was developed. A new definition of F/M ratio for the MBR-SD system was suggested to evaluate the actual organic loading rate. Yoon et al. (2004) MBR + UV process This combined MBR + UV system could completely prevent the sludge production. The effluent quality of the MBR + UV system slightly deteriorated. Zhang et al. (2007) SBR + UV process Results proved that 91.1% excess sludge could be reduced in this combined SBR + UV process, and there was no impact on the organic content and settleability of sludge in the SBR. However, considering the effluent quality, a high phosphorus concentration in the effluent was observed. Ahmad et al. (2011) SBR + UV process With a maximum 30% lysed excess sludge in the combined system, a pilot-scale SBR system combining ultrasonic waves achieved an approximately 78% reduction in excess sludge yield. However, the effluent quality was deteriorated. Lin et al. (2012) SBR + UV-ClO2 process By combined the UV-ClO2 technology with SBR, there was 55% excess sludge reduced by recycling 70% UV-ClO2 disrupted sludge. However, the same disadvantages induced by UV-ClO2 technology are the increase in effluent TP and TN concentration.

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Table 3 Evaluation of advantages and disadvantages on in-situ activated sludge reduction processes. ♦ Sludge reduction by lysis-cryptic growth ➢ 1. 2. 3. 4.

Advantages Improve the sludge settling properties and dewaterability Enhance the biodegradability Shorten the retention time Reduce the bulking and scumming

➢ 1. 2. 3. 4. 5. 6. 7.

Disadvantages Disintegrate the microorganism and organic materials without selection Corrode the reactor and system Produce undesirable hazardous by-products High energy and operational cost Complicated operation/control process Difficult to optimize the chemical oxidation reagent dosage Deteriorate the effluent quality, especially TP or TN concentrations

♦ Sludge reduction by uncoupling metabolism OSA system and novel systems based on repeatedly coupling of aerobic/anaerobic process ➢ Advantages ➢ Disadvantages 1. No extra-chemical or physical addition 1. Lack of practical application experience 2. Improve the sedimentation ability 3. Capable of treating complex components or high strength organic pollutants 4. Flexible to operate and easy to be meliorated 5. Economical efficiency and environmental friendliness Uncouplers-induced uncoupling metabolism ➢ Advantages 1. No significant change in the configuration of the CAS processes

➢ Disadvantages 1. Little is known about the fates and the potential hazards of the metabolic uncouplers for eco-environment, sludge ecosystem, and even the human 2. The application of single metabolic uncouplers might result in microbial acclimatization during a long-term running period 3. The selection and optimization of appropriate metabolic uncouplers need a mass of fussy and reduplicative experimental inputs that always reach inconsistent results 4. Deteriorate the effluent nutrient removal efficiency

♦ Sludge reduction by worms' predation ➢ Advantages 1. Low cost 2. No secondary pollution to the environment and human health.

Yu et al., 2006). Oxic-settling-anaerobic (OSA) process is a modification of CAS process by introducing an anaerobic tank in the sludge return line. Westgarth et al. (1964), probably for the first time, proposed the concept of uncoupling mechanism under an alternative anaerobic/aerobic condition. This created fasting/feasting condition would eventually achieve a remarkable biosynthesis reduction to the microorganism. In the past twenty years, the investigation of OSA process has already attracted a great number of attentions, and achieved many encouraging results (Chen et al., 2003; Jin et al., 2008; Tang et al., 2011; Wang et al., 2008). Chudoba et al. (1991) found that when compared with the CAS process, 20–65% reduction in the specific sludge production was obtained in OSA process. Furthermore, the sludge settleability was improved. Therefore, the simultaneous sludge disposal and the organic pollution removal in the improved constructed OSA process will be encouraged in the further studies. For a better understanding of OSA process, the mechanism of this process has already been investigated (Chen et al., 2003; Jin et al., 2008; Tang et al., 2011; Wang et al., 2008). One theory is the energy uncoupling theory, which speculated that when microorganisms stayed in an oxygen-deficient and starved condition, cell energy in the form of ATP or food storage might be depleted (Dawes and Sutherland, 1992). After the starved microorganisms returned back to the aerobic condition with nutrition supplied, the microorganisms would re-synthesize necessary energy reserves (ATP storage) prior to biosynthesis (Chudoba et al., 1991). Therefore, this cycling alternative environment will dissociate catabolism from anabolism so that energy uncoupling occurs to regulate cell metabolism. Another theory is

➢ 1. 2. 3.

Disadvantages Difficult to control the predators' species and quantities Risky to prey on some slow-growth functional microorganism Unknown the relationship between the operational controlled conditions and the worm growth 4. Serious nitrate and phosphate release in the effluent

the sludge decay theory. According to the study of Chen et al. (2003), 58% sludge reduction yield was induced by sludge decay. Jin et al. (2008) also found that the microbe cell decay would result in 66.7% sludge reduction, while the contribution of uncoupling mechanism is only 7.5%. Wang et al. (2008) reported that an ORP lower than − 250 mV in the anaerobic tank would benefit both the sludge decay and energy uncoupling. Besides these two main theories, other theories such as slow growth bacteria domination, sludge lysis and lysis-cryptic growth were all studied (Jin et al., 2008; Tang et al., 2011). Although the vital factors that induced the sludge reduction were still under discussion, the positive effect of in-situ sludge reduction by OSA process is obvious (Table 3). 2.2.2. Repeatedly coupling of aerobic/anaerobic process According to the aforementioned energy uncoupling theory, it is suspected that a system constructed by decoupling anabolism and catabolism of microbe might induce a reduction of biomass. Based on the viewpoint of decoupling aerobes/anaerobes treatment, Yu et al. (2006) developed a repeatedly coupling of aerobic and anaerobic process (rCAA) fixed-bed reactor. A long-term operation of 450 days demonstrated that there was no accumulation of excess sludge in the reactor. Xing et al. (2008) found that this rCAA system installed with the macroporous microbial carriers would induce about 30– 50% sludge reduction as compared with CAS process. Results showed that this simulative coupling of aerobes/anaerobes conditions in the rCAA reactor could cause energy loss among the microorganisms. However, the energy uncoupling was not the only cause for sludge reduction. Researchers speculated that sludge cryptic growth might


be another influencing factor. Feng et al. (2012) proposed that in this repeated biotransformation process. This alternative aerobic compartment/the subsequent anaerobic compartment would make the sludge to be solubilized into low micro-molecule compounds, and the solubilized organic compounds would be utilized to synthesize new cells through lysis-cryptic growth. Although the mechanism of sludge reduction in the rCAA system was not fully investigated, this system provided a bright idea for the sewage treatment system. Thus, repeatedly coupling of aerobes/anaerobes treatment was demonstrated a good performance of sludge reduction (Fig. 3). Yang et al. (2011) for the first time introduced the oxygen-limited environment into the sludge reduction field and achieved a significant result. It demonstrated that when exposing WAS to an alternating aerobic with adequate nutrition/oxygen-limited without substrate supply environment, biosynthesis reduction would be detected. Studies also found that the key operating parameters control (Yang et al., 2011), including initial MLSS, hydraulic retention time (HRT) and reaction temperature (T), would achieve a better sludge reduction effect. Therefore, this novel method (exchanged aerobic/oxygen-limited environment) will be promising for the continuous or full-application process at a low operational cost. 2.2.3. Uncouplers-induced sludge reduction Chemical metabolic uncouplers such as nitrophenol, chlorophenol, 2,4-dinitrophenol (dNP), para-nitrophenol (pNP), pentachlorophenol, 3,3′,4′,5-tetrachlorosalicylanilide (TCS), 2,4,5-trichlorophenol (TCP), cresol, aminophenol and etc., are all lipophilic weak acids (Aragón et al., 2009; Qiao et al., 2011; Yang et al., 2003). It was reported that the addition of chemical metabolic uncouplers was able to reduce excess sludge in diverse sewage treatment systems, such as continuous stirred-tank reactor (CSTR), sequencing batch reactor (SBR), membrane bioreactor (MBR) and full-scale activated sludge systems (Chen et al., 2006, 2008; Chong et al., 2011; Henriques et al., 2005). Many of which had already been proved to effectively lower the sludge yield during the sewage treatment process (Low et al., 2000; Yang et al., 2003). Through the study of 12 metabolic uncouplers affecting sludge reduction, Strand et al. (1999) found that the effect of TCP was the most obvious. The similar conclusion was also drawn by Aragón et al. (2009). Qiao et al. (2011) applied TCP as a model to study the fate and residual toxicity of chemical uncoupler in SBR system. Results showed that more than 4 mg/L TCP (2.5 mg/L residual concentration in the effluent) would lead to acute toxicity. Referred to the biotoxicity, other metabolism uncouplers including 2,4-dinitro-phenol, 3,3′,4′,5-tetrachlorosalicylanilide, 2,4-dichlorophenol p-nitrophenol, are all proved the xenobiotic organics matters, and even toxic to the microorganism and environment (Chen et al., 2002; Chong et al., 2011). From a safety and environmental friendly viewpoint, non-toxic chemical uncouplers should be chosen for a long-

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term practical application. Among the chemical uncouplers, TCS was reported more environmentally sound (Liu and Tay, 2001). Chen et al. (2002) reported a 0.8 mg/L TCS dosage would reduce as high as 78% excess sludge. Ye and Li (2005) added 40 mg TCS/d into a CAS process, 30% sludge reduction was achieved. According to the subsequent study by Ye and Li (2010), the combination of OSA process with TCS also achieved 56% sludge reduction. However, the effluent quality of this combined process has been affected. This deteriorative effluent quality might because of the chemical stress induced by the uncouplers. This chemical stress caused by chemical uncouplers might alter the metabolism of microorganism, as a result of impacting the natural and engineered ecological habitat (Ray and Peters, 2008). Hence, optimizing the dosages of chemical uncouplers and exploiting environmental friendly and safe uncouplers need in-depth investigation and longterm evaluation. Although chemical uncouplers showed excellent effects on excess sludge reduction in the lab-scale experiments, in the practical application, it should be realized that metabolic uncouplers still have their own defects: (1) the optimization dosage of a chosen metabolic uncoupler needs a mass of reduplicative experimental inputs, while the results were always inconsistent; (2) the long-term application of single metabolic uncouplers might result in biological acclimation phenomenon, this biological acclimation might lead to lower sludge reduction capability; (3) most of the metabolic uncouplers are xenobiotic and potentially harmful to environment, little is known about the fates and the potential hazards of the residual metabolic uncouplers would threaten the eco-environment, sludge ecosystem, and even the human health; (4) the application of metabolic uncouplers was reported to deteriorate the effluent nutrient removal efficiency. 2.3. Sludge reduction by worms' predation Sludge reduction by worms' predation is a method that using microfauna to prey on microorganism (activated sludge). The principle of using miniature animals' predation comes from the food chain theory. According to the food chain theory, sludge reduction by worms' predation is due to the metabolic maintained needs and higher living organisms' formation. When the microfauna preyed on the microorganisms, there is only 1/10 energy to be transferred into the next nutrition level; the predator in the conversion process consumed 9/10 energy. That is, the energy would loss when it converted from low nutrition level (bacteria) to high trophic level (microfauna) (Lee and Welander, 1996a). Therefore, the worms' predation accompanies by a decrease in the biological solid production. 2.3.1. Two-stage sludge predation systems Researchers found that although use worms as the predators in the wastewater treatment system could effectively lower the sludge

Fig. 3. Schematic diagram of repeatedly coupling of aerobic and anaerobic process fixed-bed reactor.

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yield, the application and distribution of predators are uncontrollable in biological sewage treatment systems (Khursheed and Kazmi, 2011). In order to solve this problem, a two-stage system is proposed. Two-stage sludge predation system is a pattern that the first stage is referred to the bacterial stage, a completely mixed reactor without biomass retention (for the purpose of stimulating the growth of the dispersed bacterial); and the second stage is designed as a predator stage (activated sludge system) with a long SRT for the growth of protozoa and metazoan (Ratsak, 1994). Results obtained in this pattern of process proved that, about 60–80% sludge yield was reduced in the second stage when compared with the sludge yield produced in the first stage (Lee and Welander, 1996a). However, their subsequent study showed that this two-stage type resulted in a significant release of nitrate and phosphate in the effluent (Lee and Welander, 1996b). Ghyoot and Verstraete (2000) modified the second stage to be the MBR system. A higher predator's amount in the second stage could be maintained in the MBR configuration, and the sludge production by the MBR system was 20–30% lower than that produced in the CAS system. Zhai et al. (2000) employed a two-stage MBR system to cultivate the sludge enriched in protozoa. Increased sludge reduction yields and enhanced nutrient removal efficiency could be simultaneously achieved in this proposed system. Based on the mechanism of sludge reduction by this two-stage system, a novel integrated system consists of an integrated oxidation ditch with vertical circle (IODVC) and a Tubificidae reactor (dominant worm was Branchnria Sowerbyi) were established by Guo et al. (2007). The excess sludge reduction ratio of 46.4% was achieved in the first mode, while the average sludge yield of the integrated system was 6.19 × 10−5 kg SS/kg COD in the second mode. Wei et al. (2009) integrated IODVC with a down flow type of worm reactor. In this integrated system, the average sludge yield and sludge volume index (SVI) in the IODVC were 0.33 kg · SS/kg · CODremoved and 78 mL/g, respectively. However, considering the effect on effluent quality, the presence of the worm has caused phosphorus release into the effluent, which may hold back its further full-scale application.

2.3.2. Oligochaeta Besides the protozoa and metazoa' predation, the studies of oligochaete's predation on activated sludge and their impacts on

sewage treatment efficiency were carried out. Representative oligochaetes, such as Nais, Aeolosoma, Tubificidae, Pristina, have attracted increasing attentions and interests (Huang et al., 2007; Liang et al., 2006; Lou et al., 2011). A dynamic energy-budget model underlying Nais growth was proposed to facilitate insight into the reaction processes by Ratsak et al. (1993). The parameters of the model including maintenance rate coefficient and energy conductance were extended to describe the growth of Nais elinguis in sewage treatment plants. Liang et al. (2006) inoculated Aeolosoma hemprichi in a CAS process, and their results showed that this system would achieve about 39–65% sludge reduction yield. Simultaneously, the presence of A. hemprichi enhanced the stability of the sludge settleability and total phosphorus (TP) removal rate in the process. Wei et al. (2003) operated a pilot-scale experiment to compare sludge reduction effect by inoculated oligochaete in both the MBR and CAS reactors, respectively. Results indicated that the average worm density of the aeration tank in the MBR reactor (10 total worms/mg of volatile suspended solids (VSS)) was much less than that in CAS reactor (71 total worms/ mg of VSS). Hence, it was concluded that the worm growth in the CAS reactor was much better than that in the MBR. However, rapid worm growth would have a severe impact on the effluent quality in CAS reactor, and the microorganism species and quantities are difficult to control. Another successfully operated full-scale WWTP was a Tubificidae and microorganisms symbiotic ecosystem proposed by Lou et al. (2011). During the long-term experiment, the excess sludge production rate was reduced from 0.21 to 0.051 kg/m3. Results showed that in this symbiotic ecosystem, improvements of both sludge reduction and nutrient removal were observed.

2.3.3. New concept for sludge reduction by worms' predation When considering the full-scale application, unstable worm growth in the worm reactor is the major disadvantage and bottleneck (Guo et al., 2007; Hendrickx et al., 2009, 2011; Huang et al., 2007) (Fig. 4). Researchers have brought up some possible solutions to this problem. Elissen et al. (2006) set up a novel reactor with Lumbriculus variegatus immobilization. A 75% decrease in the amount of TSS was observed. Hendrickx et al. (2009) demonstrated that when immobilized L. variegatus in a continuously operated reactor; the worms would contribute 41–71% total VSS reduction. Tian and Lu (2010) developed a novel

Fig. 4. A new reactor concept for sludge reduction set-up for the experiment with the large continuous worm reactor.


structured perforated panels reactor, a static sequencing batch worm reactor (SSBWR) combined with the aeration system and the cycle operation. This reactor achieved a 33.6% sludge reduction yield, with simultaneously nitrification-denitrification for nitrogen removal. Tian et al. (2010) also stated that the good performance of this worms' predation process was strongly dependent on process operation conditions, such as the immobilization of the worms, the type of sludge, oxygen concentration, etc. Hence, in spite of a higher apparent sludge reduction efficiency achieving in bench and lab-scale experiments, the impact factors of worms' predation on the wastewater treatment process, including effluent quality and sludge characteristics, still require further investigation. 2.4. Sludge reduction by improved/novel processes Novel sludge reduction systems of different configurations have already become the current research trend. The novel gravel contact oxidation reactor (GCOR) which filled a normal activated sludge reactor (ASR) with crushed stone globular aggregates was reported to be able to reduce WAS in some pilot and small scale engineering studies (Lin et al., 2009). Xing et al. (2006) exploited an inclined-plate membrane bioreactor (iPMBR) for excess sludge reduction. During 123 days pilot-scale operation, there was no excess sludge being discharged. An upflow anaerobic sludge blanket (UASB) was employed as the pretreatment for organic loading reduction in MBR, and this system succeeded in 90% excess sludge production decrease (Hossein and Jalal, 2011). Ichinari et al. (2008) combined a household wastewater treatment system with an aerobic sludge digester. Results showed that approximately 35% sludge was reduced in the combination system. In consideration of simultaneous sludge reduction and organic pollutants removal, a new two-stage bioreactor system constructed by combining three-phase fluidized-bed (TFB) and sludge-reduction fixed-bed bioreactor (SFB) processes with different structured porous carriers was proposed by Feng et al. (2008). During 470 days continuous operation, simultaneous removal of organic carbon and nitrogen with in-situ excess sludge reduction were achieved. Xiao et al. (2011) also achieved a simultaneous electricity production and sludge reduction in two-chamber MFC. 3. Discussion Owing to the rapid development of industry and economy, the generation of industrial and domestic wastewater has been increasing year by year. In 2009, the generation of excess sludge yields amounted to 25 million tons in China. A most recent investigation, published in February 2013, has shown that there are 3340 municipal sewage treatment plants located in China by the end of 2012. The capacity of sewage treatment is about 142 million m3/d, an increase of more than 6 million m3/d compared to 2011 (Housing and urban–rural development of the People's Republic of China, 2012). Detailed information can be found at the website http://www. mohurd.gov.cn/zcfg/jsbwj_0/jsbwjcsjs/201303/t20130301_213010. html. Thus, the increasingly generated excess sludge has become an unpreventable burden in the practice of WWTPs. In China, stringent environmental legislation (GB18918-2002) and 12th Five-Year Plan, which began in 2011 in China, have already put forward new policies on excess sludge disposal and management. Furthermore, this excess sludge issue has already received much more public attention and awareness in many nations in the world than ever before. Therefore, owing to the above stated technical and economical superiority, expectantly, the exploitation and development of efficient in-situ excess sludge reduction facilities will arouse more and more concerns by the governments, the public and scientists. The development and deep investigation of cost-effective and easily controlled in-situ sludge reduction technologies will be strongly recommended in the future studies. Additionally, a critical and objective evaluation of the

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advantages and disadvantages for the aforementioned BSTSs could provide more valuable information in practical application (Table 3).

3.1. Research prospects for lysis-cryptic growth technology Although assisted lysis-cryptic growth technologies combined with BSTSs have already been extensively studied (Ahmad et al., 2011; Hwang et al., 2010; Lin et al., 2012), it might not be an economicefficiency and convenient technology (Ma et al. 2012). Taking sludge ozonation pretreatment for example, when the costs of sludge dewatering and disposal were considered, economical analysis estimated that the operational costs of the combined BSTSs were lower than that of the CAS system (Yasui et al., 1996). Ahn et al. (2002) also reported that the ozonation process might be more economical than sludge incineration at small and medium-sized WWTP. When it comes to economical analysis, it was acknowledged that these economic progresses had achieved only in some small or pilot-scale experiments, whereas, the utilization of this technology in large and full-scale WWTPs was hindered, not to mentioned other operational drawbacks, such as toxic by-products generation, complicated system operation, reactor corrosion, and even effluent quality deterioration (Chu et al., 2009; Gallard and von Gunten, 2002; Liu, 2003). Therefore, in order to realize the practical application of this technology, some substantial problems are taken into consideration:

(1) Sludge lysis-cryptic growth technologies would result in an increase in the organic matters, including SCOD and phosphorus concentration in the effluent. These increased organic matters might exasperate the effluent quality in the BSTSs during long-term operation. ➢ Recommendation: More efficient and secure lysis-cryptic growth technologies are to be developed based on the diverse reactor configuration and pattern. Combining different ENPR-BSTSs (enhanced nitrogen and phosphorus removal biological sewage treatment systems) with lysis-cryptic growth technologies might be a brilliant solution. Mechanism study on microfauna populations and microbial community in the bioreactor could be considered as another research direction, strengthen the metabolism of the functional microorganism (such as ammoniaoxidizing bacteria (AOB), nitrite-oxidizing bacteria (NOB), phosphate-accumulating Organisms (PAO) and other functional bacteria) in the system would effectively enhance the organic removal efficiency in the BSTSs. (2) For the existing chemical and physical lysis-cryptic growth technologies, considerable energy requirements and capital inputs limited the practical application of these pretreatment technologies. ➢ Recommendation: Significant attention should be paid to the economic analysis and evaluation of these applied lysis-cryptic growth technologies. Taking into account the high-energy requirements and capital inputs of these lysis-cryptic growth techniques, the exploitation of cost-effective or combined technologies are regarded as the alternative in the future study. In-depth mechanism research for the synergistic effects of the combined pretreatments is expected to obtain higher WAS disintegration capability. ➢ Recommendation: Besides, more attention could be paid to the use of pretreated WAS as the inexpensive substrates for hydrogen or methane energy production. Research progresses have already achieved in this aspect (Kim et al., 2010; Xu et al., 2010; Yang et al., 2012). The high cost requirements for the proposed lysis-cryptic growth techniques might be offset by the generation of environmentally friendly, clean, recyclable and renewable energy (such as hydrogen, methane, and etc.).

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3.2. Research prospects for uncoupling metabolism technologies 3.2.1. OSA system and novel systems based on repeatedly coupling of aerobic/anaerobic process OSA system and novel systems based on repeatedly coupling of aerobic/anaerobic process possess remarkable advantages including non-chemically or physically assistance, flexible operation, easy improvement, economical efficiency, environmental friendliness. These processes have already been regarded as the noticeable and promising processes for future investigation (Jin et al., 2008; Tang et al., 2011; Wang et al., 2008; Ye and Li, 2010). However, the experience of full-scale practical application for the OSA process is insufficient. Therefore, encouraged by the full-scale application, the forthcoming investigations are expected to solve the obstacles that blocked its practical application: (1) The nutrient removal performance of the OSA process, especially the nitrogen and phosphorus removal efficiency need to satisfy the effluent quality standard. ➢ Recommendation: A combined process, i.e. OSA with ENPR processes, or modified/improved OSA system will be encouraged to meet the more stringent effluent regulation and standard in further investigation. Applying high performance liquid chromatographic (HPLC), gel-permeating chromatography (GPC), three-dimensional excitation–emission matrix (EEM) fluorescence spectroscopy, Fourier transform infrared spectroscopy (FTIR) and other advanced technologies to detect the characteristics and formation of the effluent of organic matters (EfOM) and the organic contaminant degradation mechanism are to be tapped. (2) A detailed life cycle assessment (LCA) on the OSA system or the repeatedly coupling of aerobes and anaerobes systems is recommended for the future study. In order to provide an ideal choice for satisfying the overall sustainability of WWTPs, it is expected that the introduction of LCA will identify which processes or coupling patterns have a positive impact on the simultaneous sludge reduction and organic matters removal. (3) Recommendation: Activated sludge models (ASMs) such as ASM1, ASM2, ASM2D, ASM3 and etc., could be introduced to predict the complicated reaction mechanism and to optimize the process operation parameters. The development of mathematic models to explain the relationship between the operational conditions and the nutrient removal efficiency need special concern. However, despite it is the most widely used models for activated sludge systems, the calibration of the ASM models still remains to be answered (Jukka and Kauko, 2012), because it is well-known that the parameters of activated sludge models are poorly identifiable (Brun et al., 2002). Hence in the future research work, a description of complete and systematic calibration on new treatment processes/operational parameter values is required, and needs all our great effort. 3.2.2. Uncouplers-induced uncoupling metabolism As for uncouplers-induced uncoupling metabolism, the advantage is that little configuration change has to make of the existing BSTS (Chen et al., 2008; Chong et al., 2011; Henriques et al., 2005). Due to its higher efficiency and easier operation, the application of chemical uncouplers became an efficient method which could lower the excess sludge yield during in-situ activated sludge reduction process. Nevertheless, some uncouplers, including chlorinated and nitrated phenols uncouplers, are difficult to be biodegradable (Morville et al., 2006; Qiao et al., 2011). The residual and intermediate products of these metabolic uncouplers also present risk to the human health and the ecosystems environment. Moreover, it may require secondary treatment to remove the chemical residuals in the effluent (Tian et al., 2013). Thus, long-term evaluation of the metabolic uncouplers

on low-price, non-hazardous, and environment friendly seems to be an arduous and significant task, the choice of optimum metabolic uncouplers in sewage treatment practice must be very prudent. For the purpose of accomplishing the desired sludge reduction and required effluent quality, some feasible ways to go are discussed based on the main current shortcomings: (1) According to the present researches, little is known about the fates and the potential hazards of the existing residual metabolic uncouplers for eco-environment, sludge ecosystem, and even the human health. ➢ Recommendation: For the purpose of providing valuable suggestions in operating and improving activated sludge process combing uncouplers, in-depth investigations could be conducted to study the fate and the degradation process of the metabolic uncouplers by using Liquid ChromatographyMass Spectrometry (LC-MS), High Performance Liquid Chromatography (HPLC) and other advanced detecting techniques; (2) In most of the previous researches, single metabolic uncoupler was invariably continuously dosed in the activated sludge system. However, the application of single metabolic uncoupler for a long-term application might cause the microbial acclimatization and eventually negate the effects of sludge reduction performance. ➢ Recommendation: Further research should still be focused on the development of novel high-efficient, non-toxic, environmentally friendly metabolic uncouplers. The dosage of intermittent pattern or the application of combined uncouplers might be considered in further investigation. It is expected that certain synergetic effect of the combined uncouplers might enable more efficient reduction in excess sludge yields compared with the single uncoupler. (3) Another technical problem associated with the practical application of metabolic uncouplers is that the selection and optimization of appropriate metabolic uncouplers need a mass of fussy and reduplicative experimental inputs while always achieve inconsistent results. ➢ Recommendation: Mathematical tools were recommended to simulate and model the real reduplicative experiments on simultaneous sewage treatment and in-situ sludge reduction. Applying mathematical models to determine and optimize the dosage of a chosen chemical uncoupler should be encouraged in further research work. 3.3. Research prospects for worms' predation technology The application of worms' predation approach to reduce excess sludge yields has already attracted more and more attention and interests since it possesses numerous advantages, including convenient operation, little energy requirement, no secondary pollution and so on forth. Although many remarkable progresses have already been achieved by previous researchers, however, there are still some challenges restricted its development in practical application (Tian and Lu, 2010): (1) the unstable species and quantities of the worm growth in the worm reactor are difficult to control; (2) the nutrients release worsens the effluent quality. Some recommendations proposed in the following are expected to provide directional guidance for practical applications of this worms' predation technique: (1) The unstable species and quantities of the worm growth in the worm reactor are difficult to control. ➢ Recommendation: As is mentioned in Section 2.3.3, the novel immobilized worm pattern reactors are proved good performance of stabilizing the worms in the system. However, the effect of essential operating conditions such as temperature, the aeration condition, DO concentration, pH value,


MLSS concentration and other influential parameters in this novel system are not yet well established. Therefore, in-depth studies, emphasized on investigating the vital impact factors and their interactions of sludge reduction, are to be deeply discussed in the future study. (2) The nutrients release worsens the effluent quality. ➢ Recommendation: For enhancing the biological nitrogen and phosphorus removal efficiency, the spatially compartmentalized anaerobic, anoxic and aerobic conditions in the configuration of the BSTSs were recommended. A creative thinking is to exploit novel or integrated processes combining worms' predation technique. ➢ Recommendation: Applying clone library, proteomics, genomics, fluorescence in situ hybridization (FISH), and ribosomal RNA pyrosequencing techniques to investigate the interaction of the functional microorganism community, the enhanced sewage treatment efficiency and in-site sludge reduction molecular mechanism are to be explored. The examination of the reaction mechanism of the nitrifiers, PAO, and other functional microorganism in these proposed systems is also recommended in the future study. 4. Conclusion From a comprehensive standpoint, the search for appropriate methods that can remove the nutrients and simultaneously produce less excess sludge from the source of sewage treatment process should be advocated in the future study. Solely pursuit of low sludge production at the expenses of ignoring the sewage treatment efficiency is not advisable. Additionally, well-developed activated sludge mathematical models will provide scientific guidance for the establishment of new wastewater treatment processes and the investigation of function mechanism. Therefore, introducing activated sludge ASM mathematical models and MATLAB simulation platform as modeling method to reveal the organic matter degradation and in-situ sludge reduction mechanism are recommended in the future study. For the purpose of exploiting and choosing practical novel processes for the municipal and industrial sewage treatment with simultaneous excess sludge yield minimization, some criterions that should be taken into consideration are summarized as follows: • The improved/combined process can be easily modified based on the existing sewage treatment processes; • The chosen process should have economic-effectively engineering feasibility; • The selected sludge reduction process would not deteriorate the nutrient removal efficiency in the effluent. • The extra chemical or physical addition should avoid the formation of undesirable hazardous and detrimental by-products to environment or human health; • The proposed modified or novel processes should be paid attention to the sustainability for long-term application. • The development of mathematical models could be introduced to predict the complicated reaction mechanism and to optimize the process operation parameters. The criterions presented above are expected as guidance for future process design and operation of sewage and in-situ WAS minimizing integrated systems. Acknowledgments This research was supported by the National Nature Science Foundation of China (Grant Nos. 51008105 and 51121062). The authors also gratefully acknowledge the financial support by the State Key Laboratory of Urban Water Resource and Environment (Grant No. QA201211), Heilongjiang Nature Science Foundation (Grant No.

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