In its northern region, China has water shortage problems and accelerated industrialization in the country further compounds the issue. As a corrective measure, the South-North Water Diversion Project will transfer 40 billion m3/year of water from the south to the north. Other less dramatic abatement measures are water pollution control and wastewater reuse practices.
Water pollution control, wastewater recycle and reuse, water usage minimization and saving 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. After making significant investments in pollution control plants, it is additionally cost effective to further treat the effluent for various reuses. This will not only lessen the impact of wastewater discharges on polluting the environment, it will also preserve the virgin water for other water users that require higher quality.
Reuse projects
Severn Trent Services has participated in three projects in China that involve tertiary treatment of wastewater for reuse.
One project in Dalian uses a fixed-film biological treatment technology (SAF) and a deep bed sand filter (DeepBed) for upgrading a municipal secondary effluent for various industrial reuses. The other two projects employ the same treatment technologies for potential recycle of wastewater for internal reuses in two petrochemical complexes.
The SAF treatment technology and DeepBed filter system were proven to be technically efficient and cost effective for upgrading secondary effluent for reuse.
The Dalian reuse plant with a capacity of 20,000 m3/day flow is producing a high quality reusable effluent with NH4-N at <1 mg/L, TSS < 1 mg/L and COD < 40 mg/L. The treatment plant owner is satisfied with an investment return of less than 3 years payback period. The end users realize about 60% cost savings by using the tertiary treated wastewater in place of potable water.
The city of Dalian preserves the precious drinking water for domestic consumption and has sufficient fresh water to entice foreign investors to Dalian.
Treatment technologies
The three water reuse projects in northern China that Severn Trent Services participated in had limited space but upgrades to treatment equipment were considered nonetheless. Therefore the evaluation and eventual selection of a fixed-film biological treatment system relied heavily on footprint size and high treatment capacity. The TETRA SAF (Submerged Aerated Filter) technology provided by Severn Trent Services was 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 its 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. Because 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, because backwash is not required and there is no differential head loss between the reactors and the distribution of influent wastewater and process air.
Dalian Heng Ji wastewater reuse project
Dalian is located in Liaoning, a northern province of China with a population of 2 million. The daily water consumption is about 1 million m3/day and consists of 350,000 m3/day for domestic consumption, 350,000 m3/day for industrial uses and 300,000 m3/day for commercial and public institutions uses. Dalian has limited water resources and the majority of city water comes from two reservoirs located 100 to 150 miles away.
Dalian investigated various potential solutions such as seawater desalination, water conservation and importation from other regions, to address their water shortage problems.
However, wastewater reuse was designated as the top priority solutions potential for the following reasons:
Reusable wastewater is readily available within the city boundary. The treatment and reuse of the wastewater will not only abate the pollution problem but will also alleviate the water shortage situation. In effect, the polluting wastewater is now becoming a water resource;
There are many large industrial complexes, enterprise zones, and commercial and public institutions that are adjacent to the wastewater treatment plants that can use reuse wastewater in place of potable water. This will save the potable water for other users that require higher water quality; and
Dalian is a progressive city with a close relationship to the central government. Oftentimes, the central government will implement new policies in Dalian before such policies are carried out nationwide. A vivid example of this is the current privatization of wastewater reuse being encouraged and demonstrated as policy in Dalian.
Dalian Heng Ji Co. anticipated a great business opportunity in wastewater reuse in Dalian city.
In 2000, the company began to look into various wastewater treatment and recovery technologies. 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. Then after a vigorous technical evaluation and cost comparison, it selected the SAF fixed-film biological treatment technology and DeepBed filtration process by Severn Trent Services.
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.
Currently, both parties are proceeding to the second phase of the project for another 15,000 m3/day effluent recovery.
Two main treatment systems are used to remove Biological Oxygen Demand (BOD), Ammonia-Nitrogen (NH4-N), and Total Suspended Solids (TSS) from secondary effluent generated by the existing treatment process. 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 three years of payback. The city government will alleviate the water shortage dilemma and save the fresh water for attracting foreign investment to Dalian.
To help alleviate the water shortage and pollution problems in China, tertiary treatment of municipal sewage and industrial wastewater for reuse should be further evaluated. Of particular interest and importance is the persistent drought situation in northern China where chronic water shortage warrants all creative measures to prevent polluting the existing limited water resources and create new water sources. The reuse of wastewater fits this scenario well.
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.
