Tertiary treatment of wastewater for reuse in China

Wastewater reuse is an important and economically efficient way to save water in water deprived regions of northern China. Jeffrey J Chen, Pacific Rim sales manager at Severn Trent Services, reports on a project that involves tertiary treatment of wastewater for reuse.

Dalian is located in Liaoning, a northern province of China where the 2 million population consume about 1 million m3/d water: about 350,000m3/d for domestic consumption, 350,000 m3/d for industry and 300,000m3/d for commerce and public institutions. Dalian has limited water resources; the majority of city water comes from two reservoirs located 160 to 240km away.

After investigating various potential solutions, such as seawater desalination, water conservation and importation from other regions, to address Dalian’s water shortage problems, wastewater reuse was designated as the best solution for a number of reasons:

· Reusable wastewater is readily available within the city boundary so 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 use reuse wastewater in place of potable water, preserving the potable water for users that require higher quality water.

· Dalian is a progressive city with a close relationship to the central government. Often, the central government use Dalian as testing ground before carrying out policy nationwide. Hence, privatisation of wastewater reuse

is currently being endorsed

in Dalian.

Dalian Heng Ji Company anticipated the increase in the uptake of wastewater reuse as a business opportunity in Dalian city and 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.

After a vigorous technical evaluation and cost comparison, it selected the SAFTM fixed-film biological treatment technology and DeepBedTM filtration processes developed 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 5000m3/d 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 15,000m3/d 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 20,000m3/d wastewater recovery plant is estimated at US$2.5 million. The land area it occupies is 1200m2. The total operating, maintenance and capital depreciation cost is about US$0.12/m3 including an operating cost of US$0.04/m3.

The 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. Industrial customers will have to use it or risk having their water cut off.

Heng Ji, with backing 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.

The Dalian wastewater reuse project has demonstrated that the SAFTM fixed-film biological process and DeepBedTM filtration are technically efficient and cost effective to upgrade secondary effluent for non-potable reuses. The Dalian reuse plant with a capacity of 20,000m3/d is producing a high quality reusable effluent with NH4-N at <1mg/L, TSS <1mg/L and COD <40mg/L. The treatment plant owner is satisfied with an investment return of less than three years payback period. The end users realise about 60% cost savings by using the tertiary treated wastewater in place of potable water. The city of Dalian can now preserve precious drinking water for domestic consumption and has sufficient fresh water to entice foreign investors to Dalian. To help alleviate water shortage and pollution problems in China, tertiary treatment of municipal sewage and industrial wastewater for reuse needs further evaluation. Persistent drought in northern China, has exacerbated chronic water shortage and warrants all creative measures to prevent polluting the existing limited water resources and create new water sources.

Treatment technologies

The evaluation and eventual selection of a fixed-film biological treatment system relied heavily on footprint size and high treatment capacity. The TETRA SAFTM (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 (20-40mm ES) compared to a conventional BAF (2-6mm 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 minimised. 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.

Contact: Severn Trent Services

Tel: +44 121 313 4622

Email: [email protected]

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