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

Land treatment process was chosen as the firststage 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, costefficient and reliable operation.

Research on UNITANK Technology and Its Application in Municipal

Wastewater Treatment Plant

Dongjie Bao 1, a, Zhanmeng Liu2,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

bdjer330@163.com, bhoverliu@yahoo.com, c1391369317@qq.com

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

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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 10xl04m3/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

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 104m3/d, that is, each group’ design treatment capacity is 2.5xl04m3/d (0.29m3/s) Volumetric load is 0.33kgBOD/ (m3·d), sludge load is 0.104kgBOD/(kgMLVSS·d), sedimentation tank surface load is 1.02m3/(m2·h), weir load is 1.55L/(s·m), sludge concentration is 4kg/m3, 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

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

main

stage

1

main

stage

2

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

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

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

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[4]Vriens L et al.,in:Water,1990, p.52

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UK,1987

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

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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

http://dx.doi.org/10.1016/S0273-1223(98)00829-4