Introduction
China has water shortage problems, especially in its northern region. The accelerated industrialization
in the country further compounds the issue. A drastic and costly corrective measure is the South-North
Water Diversion Project, which will transfer 40 billion m
3/year of water from the south to the north.
But other, less dramatic and less expensive abatement measures also are being utilized.
Water pollution control, wastewater recycling and reuse, water usage minimization and groundwater
exploitation are basic necessities to protect the already limited available water resources in China.
Wastewater recycling and reuse is an important, economically efficient way to conserve water. The
practice not only lessens the impact of wastewater discharges on the environment, but also preserves
the "virgin" water for other water uses that require higher quality.
In one recent project in the City of Dalian, a fixed-film biological treatment technology known as
Submerged
Aerated Filter (SAF™) and a deep bed sand filter (DeepBed™) were utilized to
upgrade a municipal secondary effluent for various industrial reuses.
Dalian
Wastewater Reuse Project
Dalian is located in Liaoning, a northern province of China. With a population of 2 million,
its daily water consumption is about 1 million m
3/day. It is a city with limited water resources,
with the majority of city water coming from two reservoirs located 100 to 150 miles away.
To address their water shortage problems, Dalian investigated various potential solutions such as
seawater desalination, water conservation and importation from other regions. However, wastewater
reuse was designated as the top priority solution potential for several reasons:
- Reusable wastewater is readily available within the city, and the treatment and reuse of
the wastewater will not only abate the pollution problem but will also alleviate the water
shortage situation.
- There are many large industrial complexes, enterprise zones and commercial and public
institutions that are adjacent to the wastewater treatment plants that can utilize large
quantities of reuse wastewater in place of potable water.
- Dalian is a progressive city with a close relationship to the central government.
Oftentimes, the government will implement new policies in Dalian before such policies are
carried out nationwide.
In 2000, Dalian Ji Company began to research the most suitable wastewater treatment and
recovery solution. It first located a municipal wastewater treatment plant in the Dalian
Development Zone (DDZ) that had several potential big industrial water users in the adjacent
area and secured the exclusive right to the usage of the wastewater.
The company first considered upgrades to existing treatment systems. However, space for additional treatment
equipment was limited. Therefore, the evaluation of fixed-film biological treatment systems relied heavily
on footprint size and high treatment capacity. The TETRA® SAF technology and DeepBed provided by Severn Trent
Services were selected over other fixed-film biological processes such as BAF (Biological Aerated Filter),
MBBR (Moving Bed Bio-Reactor) and RBCs (Rotating Biological Contactors) because of their simplicity, reliability
and cost advantages.
SAF is an upflow, fixed-film biological reactor that uses a very coarse mineral media (~20-40 mm ES) compared
to a conventional BAF (~2-6 mm ES). By avoiding the use of fine media, the system eliminates the high head
loss, air and water distribution problems and plugging potential typically associated with BAF. Since it has
no nozzles, no small openings, and no moving parts, pluggage potential is further minimized. The SAF reactor
requires less instrumentation and valving than the competitive BAF system, since backwash is not required and
there is no differential head loss between the reactors and the distribution of influent wastewater and process air.
In August 2001, Heng Ji and Severn Trent Services entered into an agreement to start the first phase of the
wastewater recovery project to treat 5,000 m3/day of secondary effluent from the DDZ municipal wastewater
treatment plant for reuse.
Two main treatment systems are utilized to remove biochemical oxygen demand, ammonia-nitrogen (NH4-N), and
total suspended solids (TSS) from secondary effluent generated by the existing treatment process. The Dalian
reuse plant currently produces a high quality reusable effluent with NH4-N at < 1 mg/L, TSS < 1 mg/L and
chemical oxygen demand (COD) < 40 mg/L.
In the scheme, a DeepBed gravity filter follows a SAF reactor. After the solids removal step, the final plant effluent is disinfected.
The capital investment for the 20,000 m3/day wastewater recovery plant is estimated at US $2.5 million. The land area it occupies is 1,200 m2. The total operating, maintenance and capital depreciation cost is about US $0.12/m3 including an operating cost of US $0.04/m3. The market price for this high quality reusable wastewater is about US $0.25/m3 which is significantly below the fresh water rate for industrial use at US $0.40/m3. That is one of the incentives for the industrial customers to practice wastewater reuse. Another motivating factor is that the city government will lay the pipeline to convey the tertiary treated wastewater to the customers' site for their utilization. The industrial customers will have to use it or risk having the water cut-off. Heng Ji, with the encouragement from the city government, will have a good return from its capital investment after only 3 years of payback. The city government will alleviate the water shortage dilemma and save the fresh water for attracting foreign investment to Dalian.
Conclusion
To help alleviate the water shortage and pollution problems in China, tertiary treatment of municipal sewage and industrial wastewater for reuse have proven effective. The Dalian wastewater reuse project has demonstrated that the SAF fixed-film biological process and DeepBed filtration are technically efficient and cost effective to upgrade secondary effluent for non-potable reuses. The treatment owner, reuse water end users, and the city government all benefit from the project. The practice proves to be a win-win policy.
For more information, email info@severntrentservices.com