Short communication

Simultaneous removal of carbon and nitrogen from

municipal-type synthetic wastewater using net-like

rotating biological contactor (NRBC)

Zhiqiang Chen a,*, Qinxue Wen b, Jianlong Wang b, Fang Li a

a School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, Chinab Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China

Received 21 September 2005; received in revised form 2 June 2006; accepted 5 June 2006

Abstract

The treatment of municipal-type synthetic wastewater was carried out using a three stages net-like rotating biological contactor (NRBC). The

results indicated that, compared with conventional rotating biological contactor (RBC), NRBC have several advantages, such as quick start-up,

high biomass concentration and can handle high organic loading rates. The COD and total nitrogen removal rates achieved were 78.8–89.7% and

40.2–61.4%, respectively, in aerobic treatment of low COD municipal-type wastewater at hydraulic retention times (HRT) from 5 to 9 h. The COD

removal rate achieved 80–95% when organic loading varied between 16 and 40 gCOD/m2 d. A large amount of nematodes were found in the

NRBC system, which made the NRBC system produce relatively low amounts of waste sludge, due to their grazing.

# 2006 Elsevier Ltd. All rights reserved.

Keywords: Net-like rotating biological contactor (NRBC); Municipal-type synthetic wastewater; Organic loading; Carbon removal; Nitrogen removal

www.elsevier.com/locate/procbio

Process Biochemistry 41 (2006) 2468–2472

1. Introduction

Biological wastewater treatment processes are classified as

either attached growth or suspended growth. In attached growth

process, an active thin layer of microorganisms known as

biofilm is developed on the solid support. Organic matter,

nutrients and oxygen diffuse into the biofilm where they are

consumed and reacted by the living microorganisms, while the

products diffuse out from the biofilm. Attached growth

processes seem to be more stable than suspended growth

processes, especially important when the wastewater has

considerable fluctuations in flow rate and concentrations.

The RBC biofilm process was introduced in the 1960s and

obtained substantial popularity in the 1970s. Today, RBC is still

an important biofilm process in existing treatment facilities

for sewage and industry wastewater treatment because of

its advantages of easy operation and low energy consumption

[1–4].

Conventional RBC discs have the disadvantage of low

specific surface area and therefore low biomass content.

* Corresponding author.

E-mail address: czq0521@tom.com (Z. Chen).

1359-5113/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.procbio.2006.06.003

Accordingly, conventional RBCs have the disadvantages of

long start-up time and low capacity. Gupta AB and Gupta SK

[5] used a three stage conventional RBC to treat synthetic

domestic sewage and the organic loading of the first stage was

only 6.9–20.7 gCOD/m2 d. Francis and Evans [6] found that the

DO concentration in RBC systems could be inadequate when

the organic loading was up to 17.6 gBOD/m2 d. Research on

RBCs to find out how to improve their ability to handle high

organic loading rates is therefore undertaken.

RBCs have been used extensively to remove nitrogen from

wastewater [7–10]. Pilar Teixeira and Rosa’rio Oliveira [11]

combined anaerobic and aerobic RBCs to achieve nitrogen

removal, partially submersed disks were used for nitrification

while completely submersed disks were used for denitrification.

For conventional RBC, the effect of various operating parameters

like turbulence, disc rotation speed, hydraulic conditions and

recirculation on nitrifying RBC biofilms had been studied in

detail [12–15]. Simultaneous organic and nitrogen removal had

been reported in a micro-aerobic environment [16]. Gupta AB

and Gupta SK [5,17] employed RBCs to treat domestic

wastewater in fully aerobic condition when water temperature

was 26 8C and influent COD was 250 mg/L, TN removal rate

achieved 49.23%, 45.17% and 43.81% at HRTof 24, 18 and 15 h,

respectively.

Z. Chen et al. / Process Biochemistry 41 (2006) 2468–2472 2469

Table 1

Composition of the influent

Component Concentration (mg/L)

Glucose 300–1000 as total COD

NH4Cl 25–35 as N

TN 30–40

K2HPO4 1/2 of total phosphorusa

KH2PO4 1/2 of total phosphorusb

CaCl2 5

MgSO4�7H2O 100

FeCl3 0.1

NaHCO3 Given pH 7.0–8.0

a TP 1–3 mg/L.b No Org-N in the synthetic substrate.

A new form of RBC was developed when the net-like

structure material was used as discs, giving the reactor a higher

biomass. The objectives of this study were to investigate the

simultaneous organic and nitrogen removal and analyze the

performance with high organic loading in an aerobic net-like

rotating biological contactor (NRBC) system.

2. Material and methods

2.1. NRBC experimental

The schematic diagram of the experimental system is shown in Fig. 1. The

working volume of bio-reactor and secondary settling tanks were 69 and 30 L.

The NRBC was composed of three stages and each stage had nine 30 cm

diameter net-like structural discs. The interspacing of the discs was 5.5 cm. The

discs were mounted on one stainless steel shaft, geared with different rotation

speed by an electric motor with variable power input. The discs were made of

PVC and covered with 2 cm thick net-like structural rubber material with 97%

porosity. The average specific surface area per stage was 1.27 m2 and the

submerged percentages of discs were 45%.

2.2. Synthetic wastewater

The substrate composition employed is given in Table 1. Tap water was used

as dilution water. Glucose, NH4Cl and phosphate were provided as the carbon,

nitrogen and phosphorus source, respectively. NaHCO3 was added as alkalinity

source and to give an influent pH of 7.0–8.0.

2.3. Analytical methods

The location of sampling points In, E1, E2, E3 are marked in Fig. 1 and

samples were collected by injector. Collected samples were settled for 30 min to

analyze COD, NH4+-N, NO2

�-N, NO3�-N, TN and biomass concentration. The

analysis for these items was performed using standard methods [18]. COD

concentration was detected by the open reflux method and NH4+-N was detected

by the Nesslerization method. Nitrate was determined by UV absorption at 220

and 275 nm using spectrophotometer (752-UV, Shanghai, China) and nitrite by

the sulphanilamide acid reaction. Biofilm weight was determined by a sus-

pended solids methodology: a piece of the net-like material was cut off, the

biomass flushed off this sample by water, and the water with the biomass was

filtered with 0.45 mm membrane and dried for at least 24 h at 105 8C. DO was

measured by an YSI DO200 meter, pH by Hanna HI9024 pH meter. The reactor

was maintained at room temperature.

A piece of the net-like material was also cut and flushed with tap water to

count the number of nematodes in the mixed liquid. After counting the number

of nematodes with a microscope (JGX-1, Shanghai, China), the mixed liquid

was filtered using 0.45 mm membrane. The filtered biomass was dried to

measure the solids weight in order to determine the number of nematodes

per mg biomass in the biofilm.

Fig. 1. Schematic diagram of the experimental system: (1) water tank; (2) inlet pump

structural discs; (6) secondary settling tank.

3. Results and discussion

3.1. Start-up of the NRBC

Sludge with MLSS 3000 mg/L was collected from a

wastewater treatment plant and the NRBC reactor was

inoculated with 10 L of the sludge. For the initial formation

and accumulation of biofilm, the reactor was fed synthetic

wastewater of COD 600 mg/L with rotating speed = 5 rpm and

HRT = 3 h.

Biofilm on the net-like discs was observed on the third day and

fast biofilm accumulation was observed after that. The COD and

ammonia removal rate achieved 80.2% and 84.5% on the seventh

days, which suggested that the start-up of the NRBC was

completed, The biofilm mass on net-like discs at the three stages

were 351.4, 228.1 and 115.8 g/m2 during the following operating

period. Photos of NRBC discs can be seen in Fig. 2.

3.2. COD and nitrogen removal in NRBC

In order to investigate simultaneous carbon and nitrogen

removal in NRBC system, low COD municipal-type waste-

water influent was simulated. The NRBC system was operated

for 450 days and the operation period was divided into five

stages. Five different HRT were investigated and each HRT was

kept for 90 days. The initial 10 days of each stage were

considered interim-state and the following 80 days of each

stage were considered to be at steady-state. Three samples were

; (3) electric motor with variable power input; (4) stainless steel shaft; (5) net-like

Z. Chen et al. / Process Biochemistry 41 (2006) 2468–24722470

Fig. 2. The photo of the net-like disc. (A) Before start-up; (B) after start-up.

Table 2

COD and nitrogen removal in NRBC under different HRT

Sample Feed rate

(L/h)

HRT

(h)

NH4+-N

(mg/L)

NO2�-N

(mg/L)

NO3�-N

(mg/L)

In 13.8 5 31.2 � 2.0

E1 19.4 � 1.3 1.5 � 0.2 5.2 � 0.2

E2 12.7 � 0.8 4.3 � 0.3 7.7 � 0.5

E3 7.4 � 1.4 3.7 � 0.2 8.8 � 0.3

In 11.5 6 32.4 � 1.5

E1 16.7 � 3.4 1.6 � 0.1 7.2 � 0.3

E2 10.3 � 0.7 3.4 � 0.1 8.3 � 1.2

E3 5.3 � 1.5 3.6 � 0.7 9.2 � 0.6

In 9.9 7 31.4 � 0.9

E1 13.3 � 0.3 2.4 � 0.5 6.1 � 1.0

E2 7.5 � 0.8 4.5 � 0.2 7.9 � 0.4

E3 3.8 � 0.8 3.2 � 0.3 9.2 � 1.7

In 8.6 8 30.5 � 4.8

E1 12.1 � 1.5 3.6 � 0.5 6.5 � 0.6

E2 5.5 � 0.8 4.2 � 0.3 8.8 � 0.8

E3 2.4 � 0.0 3.0 � 0.2 9.2 � 1.5

In 7.7 9 31.5 � 3.9

E1 10.1 � 1.3 3.6 � 0.6 5.7 � 0.4

E2 5.9 � 0.5 3.7 � 0.8 6.4 � 0.8

E3 2.2 � 0.7 2.2 � 0.3 8.3 � 0.6

In: influent; E1, E2 and E3: effluent of three stages; each value was averaged by samp

speed was 5 rpm.

collected at 8:00, 13:00 and 18:00 each day at steady-state

stage. Table 2 gives average steady-state values of 240 samples

at each HRT stage.

The effluent concentrations of NH4+-N, NO2

�-N, and

NO3�-N varied as HRT increased from 5 to 9 h. Most effluent

TN was NO2�-N and NO3

�-N, which meant nitrification did

well in the NRBC system. Most of the carbon was consumed in

the first stage and the COD removal percentage increased from

47.4% to 66.0% as HRT increased from 5 to 9 h. DO

concentration increased from 2.8 to 4.9 mg/L in the first stage

as the HRT changed from 5 to 9 h. The DO concentration in the

last two stages varied from 4.1 to 6.5 mg/L throughout the

operation. The pH in the reactor varied between 7.3 and 7.8

while the wastewater temperature during the study ranged from

19.8 to 22.5 8C. The effluent phosphate was also measured

occasionally but no significant difference was observed.

Simultaneous removal of carbon and nitrogen was observed in

the NRBC system. The TN concentration was reduced through

the three stages for all the HRTs tested, which meant that

simultaneous nitrification and denitrification (SND) occurred in

NRBC system even though there was significant DO in the bulk

liquid in all cases. The effluent COD concentration was reduced

from 66.9 to 32.5 mg/L and the COD removal rate increased from

78.8% to 90.0% as the HRT increased from 5 to 9 h. The effluent

TN concentration was reduced from 20.1 to 12.5 mg/L and the

TN removal rate increased from 40.2% to 61.4% as the HRT

increased from 5 to 9 h.

Nitrogen removal involves in the two independent processes

of nitrification and denitrification. Nitrification and denitrifica-

tion processes often occur in different spaces because they

require different habitats, especially in terms of availability of

TN

(mg/L)

% TN

removal

COD

(mg/L)

% COD

removal

DO

(mg/L)

33.6 � 1.1 315.3 � 9.4

27.2 � 1.3 165.8 � 11.0 47.4 2.8 � 0.1

25.3 � 1.4 113.6 � 2.3 4.1 � 0.2

20.1 � 0.7 40.2 66.9 � 10.9 78.8 5.7 � 0.1

32.6 � 2.2 316.5 � 6.8

25.2 � 0.6 150.6 � 3.5 52.4 3.9 � 0.2

22.3 � 1.3 89.7 � 4.0 5.2 � 0.1

18.4 � 0.7 43.6 47.9 � 1.3 84.9 6.0 � 0.1

31.6 � 1.4 309.9 � 13.5

21.2 � 1.3 140.3 � 1.4 54.7 4.1 � 0.0

19.0 � 1.6 61.4 � 10.0 5.7 � 0.1

16.4 � 1.7 48.1 36.7 � 7.5 88.17 6.2 � 0.3

30.6 � 2.1 315.3 � 11.6

20.8 � 1.9 119.6 � 13.0 62.1 4.3 � 0.1

17.3 � 0.7 56.7 � 13.4 6.0 � 0.1

14.4 � 1.6 53.0 38.7 � 9.9 87.7 6.3 � 0.2

32.4 � 0.7 314.7 � 3.1

19.2 � 1.5 106.8 � 2.1 66.1 4.9 � 0.1

15.7 � 1.1 51.1 � 1.4 6.1 � 0.1

12.5 � 1.3 61.4 32.5 � 4.0 89.7 6.5 � 0.3

les in steady-state days; HRT are based on total reactor volume; shaft rotational

Z. Chen et al. / Process Biochemistry 41 (2006) 2468–2472 2471

electron acceptors. Simultaneous nitrification and denitrifica-

tion (SND) within biofilms can occur only under the following

conditions: (1) nitrifers and denitrifiers must be present in the

biofilm and (2) suitable growth conditions for each of the

responsible strains of bacteria must be created somewhere in

the biofilm.

The observed denitrification imply that anoxic conditions

must have formed somewhere in the NRBC system. The

relatively high bulk liquid DO levels imply that the anoxic

environments must be within the thick biofilm on the net-like

structural material. Biofilm DO gradients supply adequate

growth conditions for nitrifying bacteria in the surface layers

and for denitrifying bacteria in the deeper layers. Compared

with biofilms in conventional RBC, channels and pores of the

NRBC net-like material make substrate transfer efficient,

which increased the performance of the SND process.

An increase of HRT resulted in a lower organic loading and

the competition for oxygen shifted to the advantage of the

autotrophic nitrifiers, thus improving the autotrophic nitrifica-

tion contribution (Table 2). Many facilities have not been

designed for both nitrification and denitrification in China.

NRBC system, combining denitrification and nitrification

processes, will save investment and running costs when

nitrogen removal is required.

Some municipal and industrial wastewater has high COD

concentration. It will be a good choice if NRBC can handle high

organic loading and treat high COD concentration wastewater.

The effect of organic loading (in terms of COD) on the NRBC

system performance (indicated by COD removal rate) was

investigated. The results in Fig. 3 indicated that NRBC could

withstand high organic loading and the COD removal rate

achieved 80–95% at the organic removal loading of 16–

40 gCOD/m2 d. The COD removal rate decreased, however,

from 80% to 35% as the organic removal loading increased

from 40 to 70 gCOD/m2 d, implying that 40 gCOD/m2 d is the

upper load limit for NRBC. The relatively high load capacity

can be explained by the large specific surface area of the net-

like material. This allows for the accumulation of high reactor

biofilm biomass. The net-like structural discs also induce

turbulence near the interface, facilitating efficient mass transfer

Fig. 3. COD removal rate under different organic loading: (*) influent COD;

(&) Nv removal rate; (~) COD removal. The operation condition of NRBC

was: HRT 5h, temperature 20–23 8C and rotation speed 5 rpm.

(oxygen, substrate, nutrients, etc.). The failure at loads above

40 gCOD/m2 d can be explained by excess biofilm accumula-

tion, filling in pores and reducing the mass transfer capabilities.

In order to improve nitrogen removal rate in NRBC system,

effluent from secondary sedimentation tank was pumped into

the first stage. Results showed that TN removal in the NRBC

can be increased to 75% in this way, when the recycle ratio of

nitrified water to influent wastewater flow was 2 and HRT was

5 h.

3.3. Nematode in NRBC system

Nematodes are important for the decomposition of organic

material in edaphic environments. It also plays an important

role in aerobic biological treatment of wastewater. Nematodes,

which keep biofilm porous and facilitate oxygen diffusion

based on their burrowing and feeding activities, play an

important role in biofilm systems [19]. Kinner and Curds [20]

reported that nematodes were the most abundant grazers in

RBC system.

Nematode abundance in activated-sludge system generally

represents less than 1% of the micro-fauna; their presence is

limited by the short retention time of the biomass in the system.

Consequently, their abundance increases with the mean cell

retention time. In RBC system, where the biomass retention

time is much longer, nematodes are more abundant. In NRBC

system, we found that there were 700–800, 400–600, 200–

300 individuals/mg in the three stages, respectively, and most

of these nematodes belong to rhabditis.

3.4. Waste sludge in NRBC system

As the environmental and legislative requirements on the

discharge of excess sludge produced during the biological

treatment of wastewater have been strengthened, the cost for

disposal of excess sludge has become higher and higher. This

has given an impetus to the search for new strategies for excess

sludge reduction in biological wastewater treatment processes.

Nematodes were abundance in NRBC system, which may be

helpful for sludge reduction.The yield of dry waste sludge was

9.92 g/d in the NRBC system when influent COD and HRT

were 400 mg/L and 5 h, respectively, which was equivalent to

0.075 g (sludge)/g(COD). This waste sludge yield of the NRBC

was lower than typical RBC yields (0.5–0.6 g(sludge)/g(BOD))

[21], which may be the result of the low F/M and high nematode

grazing.

4. Conclusions

The NRBC performance with high organic loading and

simultaneous removal of organic matter and nitrogen was

investigated under well-aerated operation, and the following

results were obtained.

1. N

RBC, with net-like material pasted on the RBC discs, can

solve the problem of slow start-up, low reactor biomass

content and low capacity to handle high organic loading of

Z. Chen et al. / Process Biochemistry 41 (2006) 2468–24722472

conventional rotating biological contactors (RBC) when

treating wastewater. The net-like material pasted on the discs

has 97% porosity, which supply large areas for biomass

accumulation and it causes turbulence near the interface bulk

liquid interface to facilitate efficient mass transfer.

2. S

imultaneous nitrification and denitrification (SND)

occurred in the NRBC system. The COD and total nitrogen

removal achieved were 78.8–89.7% and 40.2–61.4%,

respectively, when low COD municipal-type wastewater

was treated and hydraulic retention time (HRT) varied

between 5 and 9 h.

3. T

he yield of dry waste sludge was 9.92 g/d in the NRBC

system when influent COD and HRT were 400 mg/L and 5 h,

respectively, which was equivalent to 0.075 g(sludge)/

g(COD).

Acknowledgement

We would like to thank Professor Rune Bakke for his help

with language modification.

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