- 23 November 2006 -

Raising the standard of industrial wastewater

Upgrading a plant to meet new discharge standards while working in limited site space presents a unique challenge. A carbon treatment system combined with wet air regeneration proved to be an effective solution, say John Meidl and CC Lu from Siemens Water Technologies.

In the early 1990s, engineers at Renda Industrial Park (RIP) in TaSheh, Taiwan, determined that its existing activated sludge system was unable to meet new discharge standards. They needed to find a way to expand its wastewater treatment plant and adequately treat chemical and petrochemical wastewaters to meet new effluent standards of <80mg/l COD and <50 mg/l nitrate nitrogen, which would enable the industrial park to continue to discharge directly to a nearby stream.

RIP also needed a system that would control volatile hydrocarbon emission and bubble from the aeration zone. The major challenges for the activated sludge system were that the wastewater had a very low BOD/ COD ratio (less than 0.1) and low organic carbon/nitrogen ratio (near 1), and that site space was very limited.

Integrated technology
After evaluating and pilot testing several different methods of treatment, the PACT system combined with Wet Air Regeneration (WAR) was chosen to meet the stringent effluent standards of the business park since the wastewater treatment plant could be upgraded and capacity increased without much change in footprint.

The PACT (powdered activated carbon treatment) system combines biological treatment and carbon adsorption into a single, synergistic treatment step. With a PACT system, far less carbon is used than in a granular system to achieve better treatment than a combined activated sludge/GAC system could achieve. Also, powdered carbon costs less than granular carbon, and it can be regenerated in a slurry form by using the WAR system.

WAR systems are used in municipal and industrial wastewater treatment applications to regenerate spent powdered activated carbon from PACT systems. WAR systems regenerate the spent carbon as slurry, so there are no dewatering steps required. Carbon recovery rates of 90% or more are common. At the same time, the process oxidises the organic sludge associated with the spent carbon, reducing the sludge to a small amount of sterile ash for disposal. This can eliminate the expense and liability of sludge disposal.

On-site pilot testing saved time and money because performance was tested and the effluent quality was guaranteed by Siemens Water Technologies and Jinn Wei Engineering & Construction, a NT$50,000,000 engineering film based in Kaohsiung, Taiwan. The effluents meet all requirements for COD and effluent discharge standards.

PACT systems have a small footprint compared to the large area required by biological treatment and add-on alternatives systems. Relative to the costs associated with biological and chemical/physical processes, the operation costs of this system are much lower. Also, the PACT/WAR system has a fully enclosed solids destruction system with minimal offgas stream that can be fully treated within the PACT system itself. There is also a minimal amount of sludge disposal compared to biological and physical/chemical processes. The system also meets Taiwan's governmental criteria for Innovative Technology Qualification (i.e. low cost versus competitive wastewater treatment systems, high removal efficiencies, recovery of resources, and low generation of secondary pollutants). Colour and odour can also be controlled, as well as residuals disposed of with minimal costs. No spent carbon dewatering is required when a WAR system is used, and a 90% carbon recovery rate can be achieved.

Meeting effluent requirements
Implementation of the PACT/WAR process began in TaSheh, Taiwan in 1996. This was the first installation of its kind in Taiwan. Since then, and based on the success at TaSheh, PACT/WAR systems have been installed at Industrial Parks in Ta Yuan, Fang Yuan, and Kue San.

PACT system operation
The PACT system operates as a conventional single stage system. The aeration system and clarifiers are designed to handle high levels of mixed liquor solids (>15,000 mg/l) to ensure that sufficient carbon is maintained therein to enable meeting the stringent effluent COD and colour limits imposed upon the plant. Table 1 is a summary of averages for the years 2004 and 2005. The MLSS is made up of approximately 15% biomass, 55% carbon, and 30% ash.

WAR system operation
Gravity-thickened spent carbon slurry fed to the WAR system is in the range of 10~15% solids. The spent carbon is composed of approximately 15% biomass and 55% carbon. Hardness in the spent carbon varies between 550 mg/l and 900 mg/l as CaCO3. Na2CO3 is added to the control scale in the WAR system's heat exchangers.

WAR operation flow rate is kept at about 7 CMH. Rx temperature is kept in range of 240~250oC. The system operates autothermally, requiring no outside source of fuel other than that supplied by the biomass/adsorbed organics in the spent carbon. The regenerated carbon slurry is colourless; slurry of ash is the only material wasted to a drying bed and dried solids are land filled.

Siemens Water Technologies and Jinn Wei Engineering and Construction employed a wide range of services and technologies to make the PACT/WAR installation possible, including pilot testing and process design, major equipment supply, project management support, installation support and training and start up services. .


John Meidl is product manager for PACT® Systems Sales for Siemens Water Technologies and CC Lu is Far East regional manager for Siemens Water Technologies.