Zhaoxu Peng, Zhongyuan Zhao, Tianyu Lou, Kun Jiang and Lei Li

(1. School of Water Conservancy Engineering, Zhengzhou University, Zhengzhou 450001, China; 2.State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China)

Abstract: Viscous sludge bulking is a rare phenomenon in activated sludge process. The performances of nutrients removal were investigated with normal sludge and viscous bulking sludge. The results showed that when COD loading and C/N ratio were around 0.13 mg COD/(mg MLSS·d) and 7.67, the effect of viscous sludge bulking on the maximum specific oxidation rates of was very little, while the maximum specific oxidation rates ofdecreased from 24.69 mg/(g·h) to 1.20 mg/(g·h). Compared with normal sludge, viscous bulking sludge had bigger particle size and more extracellular polymeric substances (EPS). The mass transfer resistance in sludge flocs might be the main cause of the difference inoxidation rates. Therefore, this study demonstrates that viscous sludge bulking is beneficial to enhance simultaneous nitrification and denitrification (SND), and excessive EPS will exhibit storage function during phosphorus removal process.

Keywords: activated sludge; nitrogen removal; viscous sludge bulking; nitrification kinetics; phosphorus uptake

0 Introduction

Sludge bulking is a frequent phenomenon in the activated sludge system with severe environmental and economic consequences[1]. It is mainly associated with excessive growth of filamentous bacteria. The commonest factors to induce “filamentous bulking” are long sludge retention time (SRT), low dissolved oxygen (DO), low organic load, and so on[2-3]. Except for filamentous bulking, there is another type of sludge bulking termed “viscous bulking” , which is determined by sludge bulking without filamentous bacteria. Viscous bulking is usually formed by excessive extracellular polymeric substances (EPS)[4], and often occurs when the nutrients is deficient or the organic load is high[5]. When viscous bulking occurs, the viscosity of sludge flocs increases significantly[6]. Unlike filamentous bulking, viscous bulking does not occur frequently, and adding flocculant is considered to be an effective method to prevent viscous bulking[7].

So far, most research about sludge bulking is focused on how to eliminate it but not to make use of it. Recently, Guo et al.[8]established a low energy-saving wastewater treatment system. In the system, limited or slight filamentous bacteria were stimulated by the low DO. By utilizing the characteristics of filamentous bacteria such as larger specific surface area and stronger ability to degrade low concentration substrates, this system can improve treatment performance while saving energy consumption. It presented a novel pathway to treat filamentous sludge bulking. In some special cases, sludge bulking can be utilized. However, few researches have been reported in this new area, especially for viscous bulking.

The aim of this research was to investigate the characteristics of nutrients removal performance in viscous bulking state. The kinetics of nitrification between normal sludge and viscous bulking sludge were compared. It is also hoped this research can provide a new perspective on viscous bulking.

1 Material and Methods

1.1 Experimental Design

The experiments were operated using synthetic wastewater which contained (per liter)CH3COONa·3H2O (663.80 mg), NH4Cl (166.90 mg), KH2PO4(18.80 mg), CaCl2·2H2O (40 mg), NaHCO3(375 mg), MgSO4(80 mg), and micro nutrients solution[9](0.30 mL). The corresponding water quality was 330 mg/L COD, 45 mg/L NH4+-N, and 4.30 mg/L PO43--P.

The study was carried out using one sequencing batch reactor (SBR) with a height of 700 mm, diameter of 200 mm, and working volume of 12 L. Sampling ports with a 10 cm interval were set in the vertical direction of the reactor, as shown in Fig.1. Each cycle contained feed period, anaerobic period, aerobic period, and settle/discharge/decant period (Table 1). At the end of each aerobic period, excess sludge was discharged to control SRT. At the end of settle period, supernatant was discharged to control hydraulic retention time (HRT). Temperature was controlled at 21 ± 1 ℃ by temperature controller. Inoculated sludge was obtained from an anaerobic/anoxic/aerobic (AAO) reactor in our laboratory (SVI and MLSS were around 90 mL/g and 2000 mg/L, respectively), and synthetic wastewater was treated with the same components as above. This research consisted of a sequence of three experimental phases. In the first phase, inoculated sludge was acclimated to the synthetic wastewater under low COD load; in the second phase, COD load increased with the increase of volume exchange ratio to induce viscous sludge bulking; in the third phase, activated sludge under viscous bulking was operated under low COD load. Operation procedures are highlighted in Table 1.

Fig. 1 Schematic diagram of experimental equipment

Table 1 Operation procedures in each experimental phase

1.2 Kinetic Analysis

The nitrification rates are fitted by the Monod equation[10]:

(1)

Otherwise, the nitrification rate could also be expressed in the following form:

(2)

wherenis the reaction order;kis the reaction rate constant. Actually, many nitrification kinetics occurring in biological nitrogen removal systems is close to a first-order model (n=1)[10].

1.3 Calculatory Procedures

The SND efficiency and NO2--N accumulation ratio (NAR) were calculated using Eq.(3) and Eq.(4)[11]:

But before she went to sleep the girl turned to her husband, and said: If in your dreams you fancy that you hear strange noises, be sure you do not stir, or get up to see what it is

(3)

(4)

1.4 Analytical Methods

2 Results and Discussion

2.1 Variations of Activated Sludge Characteristics

EPS, MLSS, and sludge settleability fluctuated during the experiment. In this experiment, EPS mainly consisted of polysaccharides and proteins. The ratio of EPS to MLSS can be used to measure the amount of EPS secreted per unit mass of activated sludge. It correlated well with SVI (r2=0.92), as shown in Fig.2. In the first experimental phase, the average SVI and EPS/MLSS were 98.30 mL/g and 18.68 mg/g, respectively. In the third experimental phase, they changed to 345.36 mL/g and 27.76 mg/g, respectively. Meanwhile, the average viscosity and MLVSS/MLSS increased from 1.21 mPa/s and 0.82 to 1.51 mPa/s and 0.88, respectively. The morphology of activated sludge in each experimental phase was observed by microscope. Only little filamentous bacteria were present. Therefore, the viscosity sludge bulking was confirmed[16]. Particle size distribution was tested in the 100thand the 250thcycle, and the results are shown in Fig.3. Compared with normal sludge, the viscous bulking sludge had a larger particle size. The polysaccharides and proteins contained in EPS are all hydrophilic substances. Researches have demonstrated that EPS could contribute to aggregate sludge flocs, so the viscous bulking sludge with more EPS are prone to form larger particle size[17].

Fig.2 Variations in sludge characteristics

Fig.3 Particle diameter distribution

2.2 Nutrients Removal Performance

2.2.1Variationsinnutrientconcentrations

2.2.2Cyclicanalysis

Fig.4 Nitrogen removal performance

Fig.6 Variations in concentrations of nutrients during a typical cycle (the 123th cycle)

Fig.7 Variations in concentrations of nutrients during a typical cycle (the 381th cycle)

2.2.3Nutrientsremovalcharacteristics

Fig.8 Nitrogen removal characteristics

Fig. 9 Phosphorus removal characteristics

2.3 Nitrification Kinetics

Table 2 Parameter values of nitrification kinetics in different activated sludge states

2.4 Evaluation of Nutrients Removal Characteristics in Viscous Bulking

3 Conclusions