Chemical facilities are under mounting pressure to process ever larger quantities of wastewater to increasingly higher standards while staying within a variety of cost constraints. Plant operators face the dilemma of how to maintain treatment throughput at reasonable cost even when the plant reaches design capacity. Potential changes to production mixes can compound the challenge. Fortunately, adopting the latest wastewater-treatment technology can inject new life into a plant, extending useful asset life without heavy upfront capital investment. For instance, adding a unit to feed pure oxygen into a biological treatment system can boost its overall performance.
This article showcases a highly successful installation of such technology in March 2011 at the Daesan, Korea, complex of LG Chemicals, one of the largest Korean chemical companies. The site manufactures a variety of products, including vinyl chloride monomer, polyvinyl chloride, polypropylene, and low- and high-density polyethylene.
When the opportunity arose to increase production capacity at Daesan, the existing air-based treatment system was judged incapable of meeting future demand for dissolved oxygen in the bioreactors comprising the secondary treatment stage. Without an effective intervention, the performance of this stage would deteriorate. Such a dropoff might lead to environmental excurions that could expose the company to regulatory penalties or even to the suspension of the plant's operating license, and also might adversely affect the downstream tertiary-treatment stage.
Potential upgrades had to be able to deal with challenges that included high salinity wastewater and secondary-treatment-stage operation at temperatures as high as 48°C. Another stipulation for the upgrade was that installation of all equipment mustn't interfere with current plant operation.
When approached to propose solutions to increase the oxygenation capacity of the existing wastewater-treatment plant, Linde suggested its proprietary Solvox-V process. This technology, which is particularly suitable for oxygen dissolution into basins with low water depths, would boost the dissolved oxygen in the secondary treatment stage. Because of its high driving force of pure oxygen and efficient oxygen transfer, a Solvox-V unit can achieve dissolved oxygen levels exceeding 2 mg/l, which enhances secondary treatment.
In basic terms, the unit consists of submersible pumps with very high hydraulic efficiency factor, a distributor for the division of water with low oxygen content into separate streams, gas/liquid-contact venturi pipes for efficient oxygen transfer in the form of very fine bubbles, plus mixing nozzles for the even distribution and dispersion of oxygen-enriched water. High velocity jets generated at the outlet of the unit recirculate the oxygenated wastewater at the bottom of the basin. Signals from dissolved oxygen probes at each aeration basin automatically control oxygen injection. The oxygen input can be adjusted for seasonal needs or production campaigns.
The Solvox-V process combines excellent oxygen utilization and high oxygenation efficiency rates with intensive wastewater agitation. It is more economical than other methods and offers high flexibility for oxygen dissolution and process mixing, especially when integrated with existing aeration equipment. Assembly requirements are minimal, with no construction work required; the system is ready to operate within a very short time. Assembly is possible in fully operational tanks, thus avoiding the costs and inconvenience associated with a process shutdown.
To ensure the equipment could handle the highly saline, high-temperature operating environment at Daesan, Linde harnessed a nanoparticle ceramic-based coating and used sacrificial zinc anodes. We also provided a frame that allowed for lowering the equipment into the wastewater without damaging the existing aeration tubes and facilitated withdrawal for planned maintenance.
The necessary equipment was delivered to the site two weeks ahead of schedule, and installed and commissioned in two days. No shutdown to normal operations was required because the equipment was set on purpose-built steel frames (Figure 1) and mounted above the existing air-based diffuser system at the base of the tank. Within two days, performance testing confirmed the process was working optimally; almost immediately LG Chemicals experienced an increase in dissolved oxygen. It was possible to reduce the existing aeration system by more than 40% and still achieve the desired operation from the secondary treatment stage. The performance of the secondary treatment stage was improved to 70% from around 60%.
Linde's intervention focused on the biological treatment phase, removing up to 70% of the pollution load as measured by chemical oxygen demand. The improvement in this secondary treatment stage reduces the work required for the downstream processes, including a Fenton's reagent tertiary stage, as well as the total cost of treatment.
The system implemented at LG Chemicals achieved the required outcome without the need to physically build any extra treatment volume capacity. Using the plant's existing assets and retaining everything on the same footprint, a technologically advanced but straightforward upgrade was achieved in less than a week.
DARREN GURNEY is a senior process engineer, water and aquaculture, for Linde Gases, Guildford, U.K. E-mail him at email@example.com.