Underground structure pattern and multi AO reaction with step feed concept for upgrading an large wastewater treatment plant

A large wastewater treatment plant (WWTP) could not meet the new demand of urban environment and the need of reclaimed water in China, using a US treatment technology. Thus a multi AO reaction process (Anaerobic/oxic/anoxic/oxic/anoxic/oxic) WWTP with underground structure was proposed to carry out the upgrade project. Four main new technologies were applied: (1) multi AO reaction with step feed technology; (2) deodorization; (3) new energy-saving technology such as water resource heat pump and optical fiber lighting system; (4) dependable old WWTP’s water quality support measurement during new WWTP’s construction. After construction, upgrading WWTP had saved two thirds land occupation, increased 80% treatment capacity and improved effluent standard by more than two times. Moreover, it had become a benchmark of an ecological negative capital changing to a positive capital.


Background
Bishui Wastewater Treatment Plant (WWTP) was the largest WWTP in Beijing's Tongzhou district, only 2.2 km away from the core region of the sub-urban area. It was first constructed in 2002, using an American treatment technology (deep pond aeration process), with capacity of 100,000 m 3 /d. The WWTP's land occupation is 22 hectare, and with discharge standard of Chinese National Class B. It now treats 84% of wastewater loading in Tongzhou district.
As the rapid development of Tongzhou, the WWTP could not meet the new demand of the suburban environment and the need of reclaimed water. Odor and noise also brought uncomfortableness to the public. In this context, the WWTP launched an upgrading project and selected underground structure scheme as the construction pattern to carry out the upgrading [1].

Sewerage system
The whole sewerage system within the scope of the WWTP was incomplete, in addition to the new district use of a distributed sewerage piping system, the vast majority of areas used a combined sewerage piping system. As the current capacity of the WWTP couldn't fulfil the requirement of the peak discharge of Tongzhou district, so the overflow of sewage discharged into the North Canal, causing serious pollution of the river. In 2016, the water quality of North Canal was under the class V quality standards for surface water. Flow measurements carried out at the Bishui WWTP indicating that the peak discharge reaching to 120,000 m 3 /d. Refer to Tab. 1 for the water quality indices of North Canal.

Design capacity
The design scale of the upgrading WWTP is 180,000 m 3 /d, and the variation coefficient to adjust the inflow amount and the peak flow in dry season and rain season is 1.3, with land occupation of only 7 hectare.

Effluent water quality and treatment efficiency
Refer to Tab. 2 for the influent and effluent design quality indices and the treatment requirements of the upgrading WWTP project and as a comparison, effluent design quality of old WWTP is also listed. It is shown that the treatment process mainly involves the removal of organic substances and SS. At the same time, there is a relatively high requirement on denitrification, dephosphorization and the levels of ammonia nitrogen. In order to reach the required standard for effluent water quality, consideration must be given to the relations between different pollutants and the impact, upon which the design effluent water quality of the plant is based [2].

Wastewater treatment process
Sewerage piping system will sent influent wastewater water into the WWTP by gravity. After treated, the effluent will be sent out by pump. The usage of the effluent is mainly as follow: first as resource for reclaimed water system of Tongzhou district, the other is send to North canal as ecological water compensation. Mainstream process is multi AO reaction system, excess sludge is treated by centrifugal dewater and disinfection is choosing UV sterilization process. For details, the wastewater treatment process is showed in Fig.1 sludge loading rate is 0.08kgBOD5/(kgMLSS.d); the air/liquid ratio is 8.6:1. Civil engineering dimension is LxBxH= 141.0x60x (lower layer 9.7+upper layer 5.5) m. Internal return pumps has been installed as an emergence facility to enhance denitrification ability once influent contains too much ammonia.   Design scale: 180,000 m3/d. Two units are set up, thirteen sets for each unit. Design water flux is 32 l/m 2 .h. Typical water quality of UF system is as follow: SS ≤ 1 mg / l , Turbidity ≤ 0.1NTU. The function of submerge UF basin is to minimize suspended solids concentration into 0 mg/L, which can also decrease organic matter concentration in effluent, for the most difficult part in the whole treatment process is to guarantee COD<30mg/L.

Application of new technology
With the rapid development of global economy and the increasing prominence of environmental issues, how to balance the economic development and the environmental pollution has gradually attracted attention around the world. In particular, the conference on reduction of carbon emission held in Copenhagen recently has shown the necessity and urgency for international action to handle the climate changes. In this context, the upgrading WWTP in the design process has adopted the advanced green energy-saving technology, thus handling the environmental protection and resources utilizations of the sewage treatment under the premise to fully standardize discharge.

Muti AO reaction technology
Compared to the anaerobic/anoxic/oxic (A/A/O) process, the Multi AO reaction process has plenty of advantages, such as greater carbon flexibility for denitrification, reduced bioreactor volumes due to higher sludge concentration, and has been successfully applied in many full-scale WWTPs. In this WWTP, wastewater is divided into three parallel lines, which is delivered to A/O tanks. Fig.5 provides a detailed layout of multi AO reaction process in this WWTP. From perspective of space, three pairs of A/O layout in series. Wastewater will pass through a sequence of different reaction environment (anaerobic-oxic-anoxic-oxic-anoxic-oxic). Advantages of such kind of arrangement are as follow: 1) organic distribute along the whole reactor equably, which is benefit for organic degradation of activated sludge; 2) anoxic part and oxic part have changed alternately, which is benefit to inhibit appearance of sludge bulking; 3) this method is simply to be apply to upgrading established WWTP, what only need to do is separate effluent into some parts and change parts of oxic unit into anoxic unit [3] [4].  ammonia nitrogen (NH4 + -N) of 0.5±0.2 mg/L and total phosphate (TP) of 0.2±0.1 mg/L. The step feed strategy was judged to be an important factor for nitrogen removal via nitrite in the multi AO reaction system [5] [6].

Deodorization
As Bishui WWTP is an urban facility, deodorization becomes an imperative necessity. It is crucial to design an integrated deodorization scheme for odor-generating structures in WWTP structures by giving consideration to treatment processes and structural design. WWTP has three sources of odor: the pre-treatment area, the bio-treatment area and the sludge treatment area. Based on the ventilation schemes for the treatment facilities and the relevant international standards, the air volume of the deodorization system should be respectively as follows: pre-treatment area: 30,000 m 3 /hr; biotreatment area: 128,000 m 3 /hr; sludge treatment area: 22,000 m 3 /hr. The whole odor treatment capacity is 180,000 m 3 /hr.

New energy-saving technology
In addition to applying high-efficiency wastewater treatment technology, the upgrading WWTP has adopted the following advanced green energy-saving technologies: 1) The power generation by reclaimed water and heat pump of water source has made it feasible for the world-class sewage treatment plant. Energy recovery system can provide enough heat and cooling water necessary within the WWTP, and also provide to an extra 200 thousand m 2 municipal building around.
2) Optical fiber lighting system can replace lighting illumination during the daytime, which is one of the largest power consumption in the WWTP operation. Optical fiber lighting system can save 50% of the illumination power consumption for the underground workshops.

Dependable old WWTP's water quality support measurement during new WWTP's construction
During the construction of WWTP, the following support measurements have been adopted to ensure the sewerage treatment to standardized discharge (Chinese National Class B): new style microbe carrier, genetically engineered bacteria, surface aeration device, and advanced ammonia removal technology are used to ensure the discharge standard during the upgrade when half of the reactor volume is occupied.

Significance
With innovation of technology, this project will become a world top level green, low carbon, and intelligent WWTP. Refer to Tab. 3 Comparison between old WWTP and upgrading WWTP.

High land utilization
Under the concept of underground structure and advanced treatment technology, the upgrading WWTP cut land occupation for a great proportion and 14 hectare of land occupation has been saved, which means the saved land can supply tremendous value of social and commercial.

Environmental friendly
All treatment structures have been built underground and odor has been organized to deodorization facility, which can resolve all environment conflictions of noise and odor between urban facility and surrounding citizen. Fig.7 provides a detailed layout of comparison between bird's eye views of the WWTP.

Full-scale resource utilization
After the construction, the surface will become an ecological complex with the functions of landscape (Fig 8), scientific research, scientific education, sports and leisure. It has become a benchmark of an ecological negative capital changing to a positive capital.

Outlook for future development
Underground structure for WWTP is becoming a trend across the world, especially for upgrading project in urban area. Here are two types of outlook for this technology's application in the future: 1) Establishment of standards: It is urgent to establish a series of standards and codes for underground WWTP, especially for firefighting aspects. For lack of standard's support, engineer always divide fire compartment on the basis of standard for underground garage, which leads to waste and inconvenience.
2) Utilization of land: for now, all underground WWTPs in china choose to build the surface space as a leisure park without other options, since of land-use type is strict supervised by the government. If land-use type for underground WWTP becomes flexible in the future, perhaps more and more comprehensive utilization projects will appear, such as underground for WWTP and surface for Maker Space.

Conclusion
A large wastewater treatment plant (WWTP) was built with a multi AO reaction process (Anaerobic/oxic/anoxic/oxic/anoxic/oxic) and an underground structure. Four main new technologies have been applied: (1) Multi AO reaction with step feed technology; (2) Deodorization; (3) New energy-saving technology such as water resource heat pump and optical fiber lighting system; (4) Dependable old WWTP's water quality support measurement during new WWTP's construction.
After construction, upgrading WWTP had saved two thirds land occupation, increased 80% treatment capacity and improved effluent standard by more than two times. Moreover, a smelly and noisy aeration pond an ecological complex with the functions of landscape, scientific research, scientific education, sports and leisure. Moreover it had become a benchmark of an ecological negative capital changing to a positive capital.