Research on UNITANK technology and its application in municipal wastewater treatment plant

Research on UNITANK Technology and Its Application in Municipal
Wastewater Treatment Plant
Dongjie Bao
1, a
, Zhanmeng Liu
2,b
and Jing Li
3,c

1
Xingtai Polytechnic College, Xingtai, Hebei, 054035, China
2
School of Civil Engineering and Architecture, East China Jiaotong University, Nanchang, Jiangxi,
330013, China
3
Xingtai Polytechnic College, Xingtai, Hebei, 054035, China
a
,
b
,
c

Keywords: UNITANK, Wastewater Treatment, Technological Process, Operation Mode

Abstract. Land treatment process was chosen as the first-stage project in Xingtai Wastewater
Treatment Plant located in Xingtai city, Hebei Province, PR China. UNITANK was chosen as the
treatment technology in the secondary project in order to ameliorate the drawbacks of large land
occupation and inconvenient management. In this paper, the operation modes of sewage treatment
system, flush water system and sludge discharging system of UNITANK were discussed in detail on
the basis of introduction of the basic structure and the technological process. According to the
analysis of the project, it is suggested that UNITANK is more suitable for small or middle wastewater

treatment plants with the advantages of simple structure, less land occupation, cost-efficient and
reliable operation.
Introduction of UNITANK Technology
As a new wastewater treatment technology，UNITANK was developed and changed from SBR
(Sequencing Batch Reactor), combining the advantages of SBR, normal aeration tank, and 3-ditch
oxidation treatment systems. UNITANK makes full use of the advantages of them and avoids their
drawbacks such as the intermittent influent in SBR, the large land occupation of 3-ditch oxidation
treatment system, and extra facilities in normal aeration tank system[1-3].

Fig.1 Structural Representation of UNITANK
The tanks of UNITANK are also called alternating biological treatment tank. The basic unit of a
typical UNITANK system has 3 rectangular tanks(A,B,C tank), which hydraulic connects with each
other through the punches in the public wall or the public channel at the bottom. There is a set aeration
system in each tank, which can be a microporous aeration head, surface aerator or submersible
aerator. The outside two tanks (A,C tank) include fixed effluent weirs and sludge slot, they are used
as the aeration tank and the settling tank alternately, while the middle tank (B) is only used as a
Applied Mechanics and Materials Vols. 361-363 (2013) pp 640-643
Online available since 2013/Aug/08 at www.scientific.net
© (2013) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/AMM.361-363.640
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,
www.ttp.net. (ID: 129.128.216.34, University of Alberta, Edmonton, Canada-06/10/14,03:39:28)
aeration tank[4-6]. In addition, wastewater enters any tank of these three ones under the automatic
control of the electric gates. It is a continuous influent and alternate working system, which is shown
in Fig.1.
Engineering Application of UNITANK Technology
Overview of the Project. Land treatment process was chosen as the first-stage project in Xingtai
Wastewater Treatment Plant located in Hebei Province of China. The technology has drawbacks in
large communities because it requires much greater area of land than the more compact trickling filter
and activated sludge process. UNITANK process was chosen as the treatment technology in the

secondary project. The scale of the design is 10xl0
4
m
3
/d. The designed quality of influent and effluent
is shown in Table 1, which achieves the secondary standard of "Discharge Standard of Pollutants for
Municipal Wastewater Treatment Plant " (GB18918-2002,China).
Table 1 The Designed Quality of Influent and Effluent
items BOD
5
COD SS NH
3
-N T-P
influent [mg/L] 180 400 200 30 4
effluent [mg/L] ≤30 ≤100 ≤30 ≤25 ≤3.0
Technological Process. The technological Process is shown in Fig.2.

Fig.2 The Technological Process
Basic Structure and the Main Designing Parameters of UNITANK Reactor. There are four
groups of UNITANK reactor, each group contains three rectangular tanks marked A, B, C. The tanks
share the public wall and have hydraulic connection with each other through the public channel at the
bottom. Outlets of the connected channel were installed at the center of the outside tanks. A
stabilizing cover was set above the outlet to prevent water disturbance on the sludge. The outside
tanks (A, C )have a size of 32x32x5 (depth) m, the middle tank (B tank) has the size of 32x15x5
(depth) m, the tanks’ effective depth is 4m.
The designed processing capacity of the four groups is 10 x 10
4
m
3
/d, that is, each group’ design

treatment capacity is 2.5xl0
4
m
3
/d (0.29m
3
/s). Volumetric load is 0.33kgBOD/ (m
3
·d), sludge load is
0.104kgBOD/(kgMLVSS·d), sedimentation tank surface load is 1.02m
3
/(m
2
·h), weir load is
1.55L/(s·m), sludge concentration is 4kg/m
3
, aerobic sludge age is 9d, hydraulic retention time is
9.7h.
Operation Mode. The operation mode includes three forms, namely, the operation mode of sewage
treatment system, the operation mode of flush water system and the operation mode of sludge
discharging system[7].
The Operation Mode of Sewage Treatment System. The UNITANK system has 3 rectangular
tanks(A,B,C tank), which hydraulically connect with each other through the public channel at the
bottom. Wastewater enters any tank of these three ones under the program control. There is a set
Applied Mechanics and Materials Vols. 361-363 641
aeration system independent in each tank. During the running process, water flows into the tanks in
turn, while the middle tank is always in the aeration status, the two edge tanks with intermittent
aeration are used as the aeration tank and the settling tank alternately. The two edge tanks with larger
volume have a water outlet weir and surplus sludge pump. This technology has advantages of time
control in influent, aeration, precipitation, mud and effluent and has a continuous influent, effluent

and alternating work mode.
Each cycle comprises two main operation stage with the same operation process. They are
mutually symmetrical and connected through the transition period. The operation mode of Xingtai
Wastewater Treatment Plant is 8 hours per cycle, which is shown in Table 2.
Table 2 Operation Mode of Xingtai Wastewater Treatment Plant
stages time [h] tank A tank B tank C
main
stage
1
stage 1 2.5
influent, aeration
aeration settling, effluent
stage 2 0.5 aeration influent, aeration settling, effluent
stage 3 1.0 settling influent, aeration settling, effluent
main
stage
2
stage 4 2.5 settling, effluent aeration influent, aeration

stage 5 0.5 settling, effluent influent, aeration aeration
stage 6 1.0 settling, effluent influent, aeration settling

stage 1: Raw water first flows into tank A in which there is a great quantity of activated sludge with
high concentration because of the operation as a sedimentation tank in the last stage of the cycle.
Organic matter is adsorbed and partially degraded after the mix of the influent and the activated
sludge. With continuous aeration, organic matter has been further degraded when the mixture flows
into tank B. At the same time, the activated sludge of tank A enters into tank B and tank C in turn.
Redistribution of sludge has been completed in the tanks during the process of pushing flow. Finally,
the mud-water separation puts into effect in tank C. Water is discharged through the water outlet weir
and excess sludge is discharged by the submersible sewage pump after the separation.

stage 2: Wastewater enters into tank B. Organic matter is removed by aeration in this tank, then
flows into tank C. The effluent of sedimentation flows out of tank C. While aeration is still in progress
in tank A to remove the organic matter adsorbed in the previous stage.
stage 3: Aeration stops and sedimentation begins in tank A. The sewage flows into tank B and out
of tank C.
In the main stage 2, the sewage flows into tank C, then flows into tank B. Effluent flows out of tank
A after sedimentation. Compared with the main stage 1, flowing direction in this stage is on the
contrary.

The Operation Mode of Flush Water System. Initial effluent in the edge tanks can’t be
discharged until it flushes into the processing system and is clarified because of mixture of mud and
water being full of the effluent trough under aeration condition. Generally, there are two kinds of
washout water discharging systems. First, controlled by the electric gate, flush water flows into
pumping station through trench. This method has merits such as it is easy to operate and manage, it is
not necessary to add extra equipment. But it has shortcomings of hydraulic shock load on the water
pumping station. If UNITANK’ running series increase and its operation timing is changed, the
impact load is relatively low and impact on the system reduce. Second, controlled by the electric gate,
flush water flows into the wastewater disposal basin, then is elevated into the intermediate tank by the
submersible sewage pump inside the disposal basin. This method has merits of less influence on the
water pumping station, but has shortcomings of complex operation and management, extra
equipments of wastewater disposal basin and sewage submersible pump.
Only the second operation form can be used in Xingtai Wastewater Treatment Plant because its
water pumping station is far from the processing factory. The gate of outlet trench is off and the gate
of wastewater trench is open in the early stage of precipitation. The flush water is discharged into the
642 Sustainable Cities Development and Environment Protection
wastewater disposal basin, then is elevated into the intermediate tank by the submersible sewage
pump. The effluent during the early period of 15 minutes is used as backwash water. Effluent is
discharged through outlet trench as soon as the water quality is normal.
The Operation Mode of Sludge Discharging System. The concentration of residual sludge for
suction is low because the bottom of the tanks is flat and the single point method is chosen as the

operation mode of sludge pump. Usually there are two ways for discharging sludge, namely,
continuous discharge and intermittent discharge. The method of continuous discharge adopts a
continuous discharge of the mixture of sludge and water. It basically does not need to be controlled
and the excess sludge pump’s capacity is low. While the subsequent load of sludge concentrated and
dehydrated will increase because of the residual sludge concentration is low. The method of
intermittent discharge adopts the discharge of the mixture of sludge and water at a fixed time period,
as in the end of the mud sedimentation. It is necessary to control the sludge discharge time and the
capacity of excess sludge pump is higher. The method of intermittent discharge was adopted in
Xingtai Wastewater Treatment Plant, and the time of sludge discharge was set in static settling stage.
The Actual Effect of Operation. The UNITANK system of Xingtai Wastewater Treatment Plant
was put into operation in 2008. The treatment effect is good and the operation is in a stable condition
for more than 4 years. The actual quality of influent and effluent is shown in Table 3.
Table 3 The Actual Quality of Influent and Effluent
items BOD
5
COD SS NH
3
-N T-P
influent [mg/L]
93～179 227～352 87～195 22～41 1.72～3.62
average of influent [mg/L] 146 315 162 33 3.2
effluent [mg/L] 24 72 25 18 2.0
Conclusion
UNITANK is a new wastewater treatment technology with the advantages of simple structure, less
land occupation, cost-efficient and reliable operation. It has been considered as an attractive pathway
to be applied in small or middle wastewater treatment plants, especially as one of the selections for
project expansion or reconstruction. As a promising technology, it could draw much more attention in
the future.
Acknowledgements
This work was financially supported by the Constructional Science and Technology Foundation of

Hebei Province (2012-127), Xingtai Science and Technology Support Program (2012ZC152, 2013),
Science and Technology Support Program of Hebei Province (2013).
References
[1]J.Jeffrey Peirce, Ruth F. Weiner, and P. Aarne Vesilind:Environmental Pollution and Control
(Butterworth-Heinemann, 1997).
[2]V. N. Samokhin:Design Handbook of Wastewater Systems (Allerton Press, Inc., New York 1997).
[3]Jeffrey A. Nathanso:Basic Environmental Technology:Water Supply,Waste management, and
Pollution Control (Pearson Education., publishing as Prentice Hall 2003).
[4]Vriens L et al.,in:Water,1990, p.52.
[5]Henze M,Grady C P L,Gujer Jr et al.,in:IAWPRC Scientific and Technical Report No.3,London,
UK,1987.
[6]Henze M, Gujer W, Mino T et al.,in:Water Sci Technol,Issue1,Vol.39 (1999), p. 165-182.
[7]Fagen Zhang, Junxin Liu,in: Journal of Environmental Sciences, Issue2,Vol.19 (2007), p. 142.
Applied Mechanics and Materials Vols. 361-363 643
Sustainable Cities Development and Environment Protection
10.4028/www.scientific.net/AMM.361-363


Research on UNITANK Technology and its Application in Municipal Wastewater Treatment Plant
10.4028/www.scientific.net/AMM.361-363.640

DOI References
[6] Henze M, Gujer W, Mino T et al., in: Water Sci Technol, Issue1, Vol. 39 (1999), pp.165-182.