Research on sludge-fly ash ceramic particles (SFCP) for synthetic and municipal wastewater treatment in biological aerated filter (BAF)

Bioresource Technology 100 (2009) 4955–4962

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Research on sludge-fly ash ceramic particles (SFCP) for synthetic and municipal wastewater treatment in biological aerated filter (BAF) Yaqin Zhao a, Qinyan Yue a,*, Renbo Li b, Min Yue a, Shuxin Han a, Baoyu Gao a, Qian Li a, Hui Yu a a b

Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan 250100, China Shandong Center for Disease Control and Prevention, Jinan 250100, China

a r t i c l e

i n f o

Article history: Received 2 February 2009 Received in revised form 6 May 2009 Accepted 17 May 2009 Available online 21 June 2009 Keywords: Sludge-fly ash ceramic particles Clay ceramic particles Biological aerated filter Synthetic and municipal wastewater

a b s t r a c t Sludge-fly ash ceramic particles (SFCP) and clay ceramic particles (CCP) were employed in two lab-scale up-flow biological aerated filters (BAF) for wastewater treatment to investigate the availability of SFCP used as biofilm support compared with CCP. For synthetic wastewater, under the selected hydraulic retention times (HRT) of 1.5, 0.75 and 0.37 h, respectively, the removal efficiencies of chemical oxygen demand (CODCr) and ammonium nitrogen (NHþ 4 –N) in SFCP reactor were all higher than those of CCP reactor all through the media height. Moreover, better capabilities responding to loading shock and faster recovery after short intermittence were observed in the SFCP reactor compared with the CCP reactor. For municipal wastewater treatment, which was carried out under HRT of 0.75 h, air–liquid ratio of 7.5 and backwashing period of 48 h, the SFCP reactor also performed better than the CCP reactor, especially for the removal of NHþ 4 –N. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Sewage sludge, which is a by-product in the process of wastewater treatment, usually contains complex components. It is not only rich in organic matter, nitrogen, phosphorus and other nutrients, but also contains harmful substances such as heavy metals, organic pollutants and pathogens (Wang et al., 2005). At present, the common treatment methods of sewage sludge include landfilling, ocean disposal, incineration and composting (Bridle and Skrypski-Mantele, 2000). However, many problems exist in the above methods more or less, such as high disposal costs, gigantic energy consumption and secondary pollution to the environment (Lundin et al., 2004). Landfilling for instance, which is being commonly used nowadays, can induce the hazardous substances in sewage sludge to be transmitted to plants, livestock and humans, and this will obviously lead to long-term risks for public health (Spinosa and Veslind, 2001). Furthermore, as a potential resource, sewage sludge is wasted to some extent by those treatment methods (Khwairakpam and Bhargava, 2009). Therefore, searching for an economical and effective way to reuse the sludge has become one of the increasingly concerned issues in the environmental protection field. Many researchers are devoting themselves to the explorations about recycling of sewage sludge currently. Their researches have included composting (Schmidt et al., 2006), generating energy

* Corresponding author. Tel.: +86 531 88365258; fax: +86 531 88364513. E-mail address: [email protected] (Q. Yue). 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.05.025

(such as producing methane through fermentation or low-temperature pyrolysis) (Shen and Zhang, 2003), making building materials (such as brick, glass, cement, ceramic products, melting materials and degradable plastics) (Fytili and Zabaniotou, 2008), making active carbon (Martin et al., 2003), etc. Through ameliorating the sewage sludge to make this kind of waste useful, their attempts have played important roles in reducing environmental pollution and ecological damage. Among them, making ceramic particles from sewage sludge reaps extremely significant benefits to both environment and economy, which attracts much attention of the community currently. The ceramic particles, which are generally used as materials in building industries as well as filter media in water treatment (Yang et al., 2005), are using clay as the main raw materials (that is clay ceramic particles, CCP). The production process of them has a huge demand for clay exploited from farmland mostly, which will lower the quality of cultivated field and threaten food production in the long run (Han et al., 2009). In this research, for the purpose of reusing sewage sludge and approaching the object of treating waste with waste at the same time, sewage sludge is used as a potential substitute of clay to produce ceramic particles serving as filter media. Rich in organics, sewage sludge can produce tiny aperture inside the ceramic particles after being sintered. Additionally, a large quantity of fly ash, which is mainly generated from coal as a result of the incomplete combustion in many industrial processes, has caused many environmental problems. Considering that it has similar mineral contents with clay, fly ash can be used as cementing agent to substitute a part of clay (Ramamurthy and Harikrishnan, 2006).

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In this paper, a new kind of ceramic particles – sludge-fly ash ceramic particles (SFCP), was developed by employing dewatered sludge, clay, and fly ash at high temperature (above 1100 °C) with a mass ratio of 1:1:1. Because the SFCP must be sintered at temperatures above 1100 °C, pathogens existing in the sewage sludge can be destroyed and organics can be decomposed and gasified, so the products will not generate odors. In addition, the heavy metal elements (e.g. Hg) in the sludge which have low boiling points, will volatilize in the sintering process with exhausted air (Cusidó et al., 2003); other heavy metal elements such as Cu, Mn, Pb, Cr, Cd, and so on, will exist mostly in the form of ion crystal after high-temperature sintering and can be solidified stably in the ceramic particles (Xu et al., 2008). Thus, it is not easy for heavy metals to discharge from SFCP into the environment and cause secondary pollution. In this research, the characteristics of SFCP employed as biofilm media were compared with those of CCP in two biological aerated filters (BAFs) by treating synthetic and municipal wastewater. The effect of hydraulic retention time (HRT) and media height on the removals of chemical oxygen demand (CODCr) and ammonium nitrogen (NHþ 4 –N) was investigated. In addition, the capabilities responding to loading shock and recovery after short intermittence of the SFCP reactor were discussed in comparison with those of the CCP reactor. 2. Experimental

injection; then the particles were diverted to the front of a rotary kiln to finish desiccation and fired at high temperature of 1100 °C for 45 min; at the end of the rotary kiln, products were separated by screen and particles with diameters of 3–5 mm were chosen for filling BAFs. The characteristics of CCP and SFCP, which were evaluated by the instrument, Poremaster60 (Quantachrome, USA), are given in Table 2 (Han et al., 2009). It can be seen that SFCP have higher total porosity, larger total surface area and lower bulk and apparent density compared to CCP, and these characteristics were the essences of feasibility for filter materials feasible being used as BAF media (Kent et al., 1996). To inspect the safety of SFCP for water treatment, lixivium of SFCP was also examined for several metal elements (Mun, 2007). According to HJ/T299-2007, China (Solid waste – Extraction procedure for leaching toxicity – sulphuric acid and nitric acid method), the leachate test was done as follows: 150 g SFCP was extracted with 1.5 L sulphuric acid and nitric acid solution (solid:liquid = 1:10, pH 3.20) shaking for 18 h at 30 r/min. The concentrations are as shown in Table 3 (Han et al., 2009), where it can be seen that all heavy metal contents in lixivium were much lower than thresholds determined by GB 5085.3-2007, China (Hazardous Wastes Distinction Standard-Leaching Toxicity Distinction). As lixivium of SFCP contents little toxic heavy metals, its secondary pollution can be ignored. Thus, it is considered suitable and safe for SFCP to be applied in wastewater treatment. But if used in drinking water treatment, it should be considered very carefully.

2.1. Materials 2.2. Experimental set-up Sludge ceramic particles, which were put forward by Nakouzi et al. (1998) firstly, are made from sludge as the main raw material and some other appropriate additives. In this research, CCP were made by clay as the sole raw material. SFCP were made by mixed raw materials with dewatered sludge, fly ash, and clay at a mass ratio of 1:1:1. The composition of three raw materials is as shown in Table 1 (Han et al., 2009). The dewatered sludge, which had not been digested, was taken from No. 2 Wastewater Treatment Plant in the city of Jinan, Shandong Province, China. Both CCP and SFCP were made in the Filter Media Plant in Jinan under the same manufacture process: the raw materials were mixed in a muller and transported into a rotational disk; the powdered materials were conglutinated to particles with diameters of 2–8 mm due to water

The experimental system is shown in Fig. 1. Two lab-scale BAFs made from polymethyl methacrylate were filled with CCP and SFCP, respectively, and both of them were designed in the same form. The cylindrical reactors had a diameter of 10 cm and a height of 160 cm. In order to support the biofilm media and determine well-distributed air supply from the diffusers, 0.75 L pebbles were filled in the support zone and a flange was located at 10 cm from the bottom of each column. 8.25 L filter media (CCP or SFCP) with a height of 105 cm was filled in each column. Along the filter media zone, five sampling ports were placed at 25, 45, 65, 85, and 105 cm, respectively. After being filled with filter media, each reactor had a capacity of 6 L liquid, 2.5 L of which was the effective reaction vol-

Table 1 The composition of raw materials (%). Raw materials

SiO2

Al2O3

MgO

CaO

Fe2O3, Na2O, K2O

Loss on ignition

Dewatered sludge Clay Fly ash

31.3 69.3 57.5

0.13 14.3 24.4

1.3 2.69 1.60

4.03 1.99 6.00

– 2.47 7.12

63.2 9.25 3.38

Table 2 The comparison on characteristics between SFCP and CCP. Filter material

Total porosity (%)

Pore size distribution (lm)

Particle diameter (mm)

Bulk density (g cm3)

Apparent density (g cm3)

Total surface area (m2 g1)

CCP SFCP

24.7 37.7

0.5–1.0 0.5–1.0

3–5 3–5

1.89 1.32

2.51 2.11

3.05 8.99

Table 3 Concentrations of heavy metal elements in the leachate (SFCP:liquid = 1:10) (mg L1). Heavy metal

Cu

Zn

Cd

Pb

Cr

Hg

Ba

Ni

As

Concentrations Thresholds

0.042 100

0.017 100

0.001 1

0.073 5

0.006 15

0 0.1

0.019 100

0 5

0.011 5

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Fig. 1. Schematic diagram of experimental system (dimensioning unit: cm). A: column filled with CCP; B: column filled with SFCP.

ume that soaked filter particles, and the remanent liquid was in the buffer zone, which was set at the top of the column to stop media from flowing out when backwashing. The effluent port was located at 5 cm to the top. The raw wastewater (synthetic wastewater or municipal wastewater), which was stored in the feed water tank, was injected to the bottom of two columns, respectively, by feed pumps controlled at the same flow rate. The wastewater flowed upward and the effluent was collected in an effluent tank to be used as backwashing water. Two air diffusers, which were linked to air compressor, were located at the bottom of each column – one supplied air during regular treating process and the other supplied backwashing gas during the backwashing operation. As biofilm grew thick during water treatment, the BAF should be backwashed periodically, otherwise it may get blocked and water and air would not flow smoothly. The backwashing period was chosen as 48 h generally, and it was changed according to the concentration of pollutants in the influent (e.g. in the experiment with high pollutant concentrations, the chosen backwashing period was 24 h) (Fdz-Polanco, 2000). A backwashing sequence included three phases – air scour, air–water scour and water scour, and it was executed as follows: firstly, when the feed pump was turned off, the valve connected to air scour diffuser was opened up and the air rotameter was adjusted to a velocity of 6 L min1 for 5 min; then, the valve linked to the backwashing pump was started and the liquid rotameter was adjusted to a velocity of 4 L min1, while clean water mixed with gas were injected for 10 min; finally, the valve connected to air scour diffuser was turned off and clean water was pumped into the reactors for another 5 min. As the backwash would cause a dilute of the concentrations of pollutants in the effluent, it was carried out after the samples were taken.

2.3. Analytical methods and wastewater characteristics Samples were taken from different sampling ports and the sampling time was fixed precisely with various HRT, respectively. Parameters, such as dissolved oxygen (DO) in buffer zone, temperature, pH, CODCr and NHþ 4 –N for raw wastewater and effluent of each reactor, were monitored regularly. The measuring and analyzing of samples were based on standard methods (State Environmental Protection Administration of China, 2002a). Experiments of treating synthetic wastewater and municipal wastewater were done in the two BAF reactors. The synthetic wastewater was made up to simulate the characteristics of raw domestic wastewater in China with the preparations as shown in Table 4, and the municipal wastewater was taken from the effluent of primary sedimentation tank in Jinan Wastewater Treatment Plant, China. The characters of synthetic wastewater and municipal wastewater are listed in Table 5. 2.4. Inoculation and start-up The parameters of inoculated sludge, which was taken from the oxidation ditch process of No.1 Wastewater Treatment Plant in the city of Jinan, were as follows: SV = 40%, MLSS = 4 g L1, SVI = 100 mL g1. Two lab-scale BAFs were both inoculated with that activated sludge and aerated for 24 h. Then, sludge was drained out and the synthetic wastewater was injected in continuously. The basic operation parameters were chosen with HRT of 3 h, air–liquid ratio (A/L) of 7.5, total flow rate of 0.84 L h1 and backwashing period of 48 h (since the 6th day). In the first 5 days, the reactors were not backwashed for the purpose that microorganisms could attach well on the media particles. Concentrations 1 and of CODCr and NHþ 4 –N in the influent were around 300 mg L

Table 4 Concentrations of the preparations for synthetic wastewater (mg L1). Preparations

Glucose

Soluble starch

Sodium acetate trihydrate

Ammonium sulphate

Peptone

Potassium biphosphate

Chemical formulas Concentrations

C6H12O6 88–220

(CH2O)n 100–250

CH3COONa3H2O 160–400

(NH4)2SO4 83–208

– 24–60

KH2PO4 18.4–46

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Table 5 Characters of synthetic wastewater (SW) and municipal wastewater (MW) (mg L1). Wastewater

CODCr

BOD5

NHþ 4 –N

TP

Temperature (°C)

pH

SW SWHa MW

150–250 300–800 70–170

65–110 130–350 24–58

15–20 30–40 18–25

4–4.5 10–11 2–2.2

15–19

6.5–7.7

11

7.3–7.8

a

Synthetic wastewater for experiment with high pollutant concentrations.

25 mg L1, respectively. The pH of the influent ranged from 6.52 to 7.71, corresponding to the effluent pH of 7.12–7.81 for CCP column and 7.14–7.85 for SFCP column. As shown in Table 1, raw materials such as CaO, Al2O3, K2O, Na2O, Fe2O3 and MgO may be lixiviated out slowly from CCP and SFCP by H+ in the liquid. As a result, the effluent pH was higher than that of the influent. During the whole operation process, the system was operated at room temperature ranging from 15 to 19 °C. DO was above 1.5 mg L1 throughout the experiment process, so sufficient oxygen could be supplied for the aerobic microorganisms. The acclimatization period lasted for 20 days until both reactors reached steady state. Henceforth, the system turned to normal operation and investigation on the effects of various operation parameters (such as HRT and media height) for wastewater treatment, was carried out.

the CODCr removal efficiency fluctuated significantly, which might be caused by the falling off of biofilm from the particles which were not so steady in the beginning of experiment operation. However, as interception and absorption turned out to be saturated, the NHþ 4 –N removal efficiency fell henceforth. Since the 4th day, CODCr removal increased because of the growing of the heterotrophic bacteria, while NHþ 4 –N removal efficiency was still low because the nitrifying bacteria were autotrophic and have a long growth cycle (Mmwndoza-Espinosa, 2001). Since the 6th day, backwashing operation was carried out, and NHþ 4 –N removal efficiency in the SFCP column fluctuated up and down as a result according to the backwashing period (2 days), which may attribute to the growing nitrifying bacteria that attached on the particles infirmly. On the 8th day, NHþ 4 –N removal efficiency in the CCP column even rose to 97% but soon came to a sharp drop as it did in the SFCP column 6 days before, which represented that the microorganisms adapted to the environment in SFCP faster than that in CCP. On the 11th day, the CODCr removal dropped while NHþ 4 –N removal rose, and this indicated that nitrifying bacteria took precedence. For about 16 days after inoculation, the removal efficiencies of CODCr and NHþ 4 –N in both reactors began to increase progressively, which implied that a balance of competition between heterotrophic bacteria and nitrifying bacteria was achieved, and start-up period was complete as removal efficiencies reached steady state (Mann et al., 1999) on the 20th day.

3. Results and discussion 3.1. Reactor performance during start-up As shown in Fig. 2, during the period of start-up, removal efficiencies of CODCr and NHþ 4 –N changed irregularly. It took about 20 days until the removal efficiencies of CODCr and NHþ 4 –N reached around 90% and 80%, respectively. In the last several days, SFCP column had larger removal efficiencies than CCP column, so it was considered that the microorganisms adapted to the environment in SFCP better than that in CCP at the end of start-up period. Because SFCP had higher total porosity than CCP, microorganisms could attach on them quickly. As shown in Fig. 2a, the removal efficiency of CODCr was really high in the first 2 days and dropped quickly afterwards, and it was even obvious for that of NHþ 4 –N shown in Fig. 2b. Tiny apertures and pores in the ceramic particles could intercept some substance with large molecule structure in the effluent, such as soluble starch and peptone, and microorganisms also absorbed nutrition when they began to grow. Because SFCP had more apertures and pores than CCP, pollutants can be intercepted by them more rapidly and microorganisms can also attach on them more quickly, that was why SFCP column performed relatively better than CCP column on NHþ 4 –N removal soon after the inoculating operation. But

As HRT is an important parameter for the BAF, HRTs of 1.5, 0.75 and 0.37 h were selected based on the effective volume of the column (2.5 L) and the reactors were operated for one week under each HRT. During the whole experiments, A/L was kept at 7.5:1, backwashing period was 48 h, and flow rates of raw wastewater were 28 mL min1, 56 mL min1and 112 mL min1 for the three HRTs, corresponding to air influx of 0.2 L min1, 0.4 L min1and 0.8 L min1, respectively. Fig. 3 shows the influence of HRT on CODCr and NHþ 4 –N removals. As can be seen, the effluent quality from SFCP column was better than that from CCP column. Because more surface area was supplied to microorganisms for contacting and digesting pol-

b 100

Removal efficiency of NH 4 -N (%)

90

+

Removal efficiency of CODCr (%)

a 100

3.2. Influence of HRT on CODCr and NHþ 4 –N removals

80 70

CCP SFCP

60 50 40

0

3

6

9

12

Time (d)

15

18

21

80 60 40

CCP SFCP

20 0

0

3

6

9

12

Time (d)

Fig. 2. Removal efficiencies of CODCr (a) and NHþ 4 –N (b) during start-up.

15

18

21

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a

250

b

III

II

I

I

II

III

20

NH4 -N (mg/L)

150

influent effluent of CCP effluent of SFCP

100

15 influent effluent of CCP effluent of SFCP

10

+

CODCr (mg/L)

200

5 50

0

0

7

14

7

21

14

CCP SFCP

100

150

50

50 0

0 0

25

50

75

CCP SFCP

100

100

0

15

CCP SFCP

10

75

0

25

50

75

100

Filter media height (cm)

0

0

CCP SFCP

50

75

100

HRT = 0.37 h CCP SFCP

15 10

+

5 0

25

Filter media height (cm)

20

+

+

0

50

b-III

15 10

5

CCP SFCP

100

100

HRT = 0.75 h

20

NH 4 -N (mg/L)

NH 4 -N (mg/L)

b-II

HRT = 1.5 h

20

50

150

Filter media height (cm)

Filter media height (cm)

b-I

25

HRT = 0.37 h

200

CODCr (mg/L)

150

a-III

HRT = 0.75 h

200

NH 4 -N (mg/L)

CODCr (mg/L)

a-II

HRT= 1.5 h

200

CODCr (mg/L)

a-I

21

Time (d)

Time (d)

5 0

0

25

50

75

100

Filter media height (cm)

0

25

50

75

100

Filter media height (cm)

þ Fig. 3. Influence of HRT on CODCr (a) and NHþ 4 –N (b) removals (HRT: I – 1.5 h; II – 0.75 h; III – 0.37 h) and variation of concentrations of CODCr (a-I; a-II; a-III) and NH4 –N (b-I; b-II; b-III) with the filter media height.

lutants by SFCP than CCP, SFCP reactor had larger removal efficiencies than CCP reactor. There were slight decreases on removals of CODCr (Fig. 3a) with reducing HRTs: the average removal efficiencies were 86.6%, 84.6% and 80.1% in CCP, and 88.6%, 87.4% and 85.3% in SFCP column, corresponding to HRTs of 1.5, 0.75 and 0.37 h, respectively. Under the three conditions, the concentrations of CODCr in the effluent from both reactors reached the national discharge standard of level 1-A (GB 18918-2002) (State Environmental Protection Administration of China, 2002a) and a slight ascendancy was found in the SFCP column. Removals of NHþ 4 –N (see Fig. 3b) dropped greatly in both columns as HRTs decreased, but it was obviously that SFCP column performed much better than CCP column, especially under short HRT (0.37 h). The average removal efficiencies for NHþ 4 –N under three HRTs were 86.0%, 83.3% and 39.1% in CCP column, compared with 88.9%, 87.2% and 63.2% in SFCP column, respectively. Under HRTs of 1.5 and 0.75 h, the concentration of NHþ 4 –N in the effluent from both reactors met the national discharge standard of level 1-A (GB 189182002). However, only SFCP column met the national discharge standard of level 1-B (GB 18918-2002) under HRT of 0.37 h. Therefore, the SFCP reactor had a potential larger treatment capacity than CCP reactor. It was indicated when HRT decreased from 1.5 to 0.75 h, nitrification was not affected obviously in both reactors. But the shorter HRT (0.37 h), corresponding to larger organic loads, resulted in a faster multiplication rate of heterotrophic bacteria, so autotrophies like nitrifying bacteria could not predominate in the competition of

obtaining living space. In addition, microorganisms were highly affected under a faster flow rate of water and air, and it was even serious to the nitrifying bacteria that had a longer generation period as well as a slower multiplication rate. Moreover, backwashing period of 48 h was somewhat long for the short HRT and it could not comply with the need of washing out extra biofilm of heterotrophic bacteria. Therefore, although microorganisms could attach well on the SFCP, the removal of NHþ 4 –N still subsided remarkably in the last 3 days in the SFCP column. Based on the results of Fig. 3, HRT of 0.75 h was chosen in order to satisfy national discharge standard of level 1-A (GB 18918-2002) (State Environmental Protection Administration of China, 2002a), and a suitable treatment capacity could also be achieved at the same time. 3.3. Variation of concentrations of CODCr and NHþ 4 –N with the filter media height The influence of filter media height under various HRTs on CODCr (a-I; a-II; a-III) and NHþ 4 –N (b-I; b-II; b-III) removals is shown in Fig. 3. It can be seen that in an up-flow BAF, degradation of CODCr mostly took place in the lower section, while NHþ 4 –N was mostly removed in the upper section. Under HRTs of 1.5, 0.75 and 0.37 h, the CODCr concentrations of samples taken from 5 sampling ports in SFCP reactor were all lower than those in CCP reactor (see Fig. 3 a-I, a-II, and a-III). CODCr was mostly degraded within the first 25 cm media height in both reac-

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tors. For example, under HRTs of 1.5 h, the removal efficiency of CODCr in this section (0–25 cm) was 50.5% in CCP column and 62.1% in SFCP column, respectively. Fig. 3b-I, b-II, and b-III represented that the SFCP column also had an advantage over CCP for NHþ 4 –N removal all through the media height under the three HRTs. The concentration of NHþ 4 –N dropped quickly within the first 25 cm media height, and this might be attributed to biosorption. As a kind of cation, ammonium could be adsorbed by extracellular polymeric substances which were electronegative (Vijayaraghavan and Yun, 2008), then it was transferred inside the cells and assimilated into organic amino groups (Van Rijn et al., 2006). Under long HRT (1.5 h), the lowest 1 NHþ 4 –N concentration was found at 85 cm abnormally (1.34 mg L 1 for CCP reactor and 0.40 mg L for SFCP reactor). While at 105 cm 1 in contrast, the concentration of NHþ 4 –N raised a little (2.76 mg L for CCP reactor and 1.34 mg L1 for SFCP reactor). The reason for this phenomenon was speculated as follows: on one hand, biofilm that fell off from the media particles could be taken upwards by the current, and accumulated at the top layer; on the other hand, air bubble normally followed the path of the least resistance, and at the top of filter media layer, it flew along the internal wall of reactor more often, which caused a lack of oxygen inside the media particles. As a result, microorganism decomposed itself and organic nitrogen was converted to NHþ 4 –N. However, as HRT decreased, the water rate and air influx raised, and these effects were eliminated. As HRTs decreased, the section with the largest removal of NHþ 4 –N moved upwards along the media height: for HRTs of 1.5 h and 0.75 h, the section was 45–65 cm in both reactors, corresponding to 65–85 cm for HRT of 0.37 h. It could be indicated that the heterotrophic bacteria which degraded CODCr were the dominant groups in the lower section while the nitrifying bacteria which degraded NHþ 4 –N mainly existed in the upper section (Fdz-Polanco, 2000). The boundary between them moved upwards with decreasing HRTs and the moving rate of it was faster in CCP column than in SFCP column, which revealed that the distribution

a

3.4. Reactor performance at high pollutant concentrations For the purpose of investigating capabilities responding to loading shock for the two reactors, an experiment with high pollutant concentrations was executed for 8 days. Measured parameters were as follows: HRT of 0.75 h, A/L of 7.5:1, corresponding to wastewater flow rates of 56 mL min1 and air influx of 0.4 L min1. The organic loads changed with CODCr concentrations, and backwashing operation was carried out every 24 h. CODCr and NHþ 4 –N removals at high pollutant concentrations are shown in Fig. 4a and b, from which we can see that SFCP column performed much better than CCP column throughout the 8 days. During the first 6 days, with CODCr of 288.35– 1 in the influent, 535.33 mg L1 and NHþ 4 –N of 31.01–32.52 mg L respectively, the effluent from SFCP column could meet the national discharge standard of level 2 (GB 18918-2002) (State Environmental Protection Administration of China, 2002a). If a requirement of national discharge standard of level 1-A should be achieved for SFCP reactor in this situation, the approximate concentrations of CODCr and NHþ 4 –N in the influent should not be higher than 320 mg L1 (see Fig. 4a and b, the 3rd day) and 20 mg L1 (see Fig. 3b-II), respectively. While for CCP reactor, the thresholds were even lower. However, on the 7th and the 8th days, as organic loads, which were more than twice as usual, were somewhat too high for both reactors, the removal efficiency of CODCr dropped for about 10–14%. Further more, because the high organic loads

b

750

influent effluent of CCP effluent of SFCP

600

35 30

NH 4 -N (mg/L)

450

+

CODCr (mg/L)

of microorganisms in CCP column was affected more seriously with the changes of HRT. Thus, in order to achieve much better efficiency under short HRT, more filter media should be filled in the CCP column than in the SFCP column. So it can be concluded that to achieve the same removal efficiency, less filter media was required for BAF of SFCP than that of CCP. Moreover, BAF of SFCP requires smaller reactor, leading to less powerful pump and assuming less electricity power.

300

25 20 15 influent effluent of CCP effluent of SFCP

10 150

5

0

2

4

6

8

0

2

4

Time (d)

c 350

d

300

8

33 30

250 CCP SFCP

+

200

NH 4 -N (mg/L)

CODCr (mg/L)

6

Time (d)

150

CCP SFCP

27 24 21

100

18

50 0

25

50 75 Filter media height (cm)

100

0

25

50 75 Filter media height (cm)

100

þ Fig. 4. CODCr (a) and NHþ 4 –N (b) removals at high pollutant concentrations and variation of concentrations of CODCr (c) and NH4 –N (d) with the filter media height on the 6th day (HRT = 0.75 h, A/L = 7.5:1, backwashing period = 24 h).

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column were higher than those from CCP column in the first 2 days, but since the 3rd day, SFCP reactor behaved better than CCP reactor. Though the influent concentration ranged considerably, the effluent concentration of CODCr from both reactors did not vary greatly. However, it can be seen that NHþ 4 –N removal was affected violently after the pause of air supply. In the first 4 days, the average removal efficiency of NHþ 4 –N was 44.3% in CCP column and 46.4% in SFCP column, much lower than that of 83.3% and 87.2% for two reactors which were operated under normal condition, respectively (see Fig. 3b-II, HRT = 0.75 h). Because nitrifying bacteria which oxidized NHþ 4 –N in the reactors were aerobic, they were sensitive to the lack of dissolved oxygen; while heterotrophic bacteria which degraded CODCr were facultative aerobic and some were even anaerobic. That could explain why there were such great differences between NHþ 4 –N and CODCr removals after the short intermittence. In order to recover the removal efficiency of NHþ 4 –N, low organic loads was adopted on the 5th day: concentration of CODCr in the influent was 46.18 mg L1, corresponding to biological oxygen demand (BOD5) of 20 mg L1. It was until the 10th day that the concentration of NHþ 4 –N in the effluent did not fluctuate up and down with that in the influent, when the NHþ 4 –N removal increased steadily to 65.7% and 83.3% for CCP column and SFCP column, respectively. It could be concluded that it took more time for the removal of NHþ 4 –N to recover in CCP reactor than in SFCP reactor.

led to a fast growing of heterotrophic bacteria, the nitrifying bacteria was inhibited (Elenter et al., 2007), and the removal efficiency of NHþ 4 –N kept low during this experiment. For SFCP column, it was around 50%, but still a little higher than that for CCP column. Fig. 4c and d illustrates variation of concentrations of CODCr and NHþ 4 –N with the filter media height on the 6th day since the reactors were operated under high pollutant concentrations. As is shown, it was obviously that SFCP column also performed better than CCP all through the media height. Compared with Fig. 3a-II and b-II, it could be seen that as organic loads increased, the role of the upper media became more important. The boundary between heterotrophic bacteria and nitrifying bacteria moved upwards with the increasing organic loads. As the concentrations of CODCr and NHþ 4 –N were still high in the effluent, more filter media should be filled in the reactor in order to achieve much better removal efficiency. If the two reactors were used to treat wastewater with higher organic loads, more filter media should be added in CCP column than in SFCP column. The results above all revealed that SFCP reactor could cope with loading shock much better than CCP reactor. Due to higher total porosity and larger total surface area, more biomass could attach on the surface of SFCP, thus the degradation of pollutants was more active in SFCP reactor, which contributed to mitigating the loading shock. 3.5. Recovery after short intermittence

3.6. Reactor performance for municipal wastewater treatment In order to simulate some accidents occurred in the practical operation such as power failure, water and air were resupplied to both columns after a 1-day stop. Normal operation was carried out with HRT of 0.75 h, A/L of 7.5:1 and backwashing period of 48 h. CODCr and NHþ 4 –N removals in the following 10 days after short intermittence are shown in Fig. 5a and b, from which it can be seen that concentrations of CODCr and NHþ 4 –N in the effluent from SFCP

An investigation was executed for municipal wastewater treatment between CCP and SFCP reactors. The wastewater, which was taken from the effluent of primary sedimentation tank in Jinan Wastewater Treatment Plant, China, had a temperature of 11 °C and pH of 7.3–7.8. The average concentration of CODCr, BOD5 and 1 , 35 mg L1 and 21.7 mg L1, NHþ 4 –N were around 103 mg L respectively.

a 300

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þ Fig. 5. CODCr (a) and NHþ 4 –N (b) removals after short intermittence; CODCr (c) and NH4 –N (d) removals for municipal wastewater (HRT = 0.75 h, A/L = 7.5:1, backwashing period = 48 h).

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Performance of the two reactors during this period is shown in Fig. 5c and d. Average removal efficiencies of CODCr and NHþ 4 –N were 60.45% and 54.49% for SFCP column, compared to those of 54.35% and 41.98% for CCP column, respectively. However, the removal efficiencies were all lower than those in the former experiments (e.g. in Fig. 3a and b, for synthetic wastewater in SFCP column under HRT of 0.75 h, average removal efficiencies of CODCr and NHþ 4 –N were 87.4% and 87.2%, respectively). The activities of microorganisms in both reactors were affected greatly by the lower degradable CODCr concentration (71.2–162.4 mg L1) and the lower temperature (11 °C) of practical wastewater. Especially, nitrifying bacteria were sensitive to temperature and had a suitable living temperature of 20–30 °C, thus NHþ 4 –N removal was influenced more significantly than CODCr removal by low temperature (He et al., 2007). Generally, with a water temperature of 11 °C, the concentration of CODCr (23.2–48.8 mg L1) and NHþ 4 –N (8.0– 11.5 mg L1) in the effluent from SFCP reactor could meet the national discharge standard of level 1-A and 2 (GB 18918-2002) (State Environmental Protection Administration of China, 2002a), respectively. Consequently, SFCP has a promising prospect utilized as filter media in wastewater treatment. 4. Conclusions SFCP reactor performed better than CCP reactor for both synthetic and municipal wastewater treatment. The microorganisms adapted to the environment in SFCP faster than that in CCP as SFCP had higher total porosity. The influence of HRT on synthetic wastewater treatment in the SFCP column was smaller than that in the CCP column, and SFCP reactor had significantly better capability responding to loading shock than CCP reactor. Moreover, although NHþ 4 –N removal in both reactors was affected more greatly compared to CODCr removal after a short intermittence (24 h), it took less time for SFCP reactor to recover than CCP reactor. Acknowledgements This research was supported by National Technological Support Plan 2006BAJ08B05-2, Shandong High-tech Project 2007GG 20006003 and Technological Progress Plan 2006061073 of Jinan, Shandong Province of China. References Bridle, T., Skrypski-Mantele, S., 2000. Assessment of sludge reuse options: a life cycle approach. Water Sci. Technol. 41 (8), 131–135. Cusidó, J.A., Cremades, L.V., González, M., 2003. Gaseous emissions from ceramics manufactured with urban. Waste Manage. 23, 273–280. Elenter, D., Milferstedt, K., Zhang, W., et al., 2007. Influence of detachment on substrate removal and microbial ecology in a heterotrophic/autotrophic biofilm. Water Res. 41, 4657–4671.

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