| Background |
With rapid industrialization and urbanization, discharge control of hazardous wastes has drawn ever-growing attention from both the public and the industries. As Singapore is a city country with high population density and concentrated manufacturing industries, the proper treatment of hazardous waste is especially important.
Since
hazardous wastes are generated from very diversified sources, it is extremely
difficult or even impossible to develop a universal process, which can cover
all hazardous waste treatment requirements. Within various industries, however,
hazardous wastes can still be classified as a relatively common variety of
specific operations. These can be divided into four very common sources, which
may be described as:
1. Off specification products and excess raw
materials;
2. Spent catalysts and purification residues;
3. Sludge and other residue from waste treatment
operations within the manufacturing facility;
4. Contaminated and spent organic (solvents)
as well as solvent residues.
Some simple and/or well-developed processes such as solidification/fixation and distillation/extraction etc with monitoring and control systems have been applied to hazardous wastes Categories 2-4. For Category 1, even though the most common source of hazardous waste by all industries falls within this category such as cleaning and plating industries etc, and the process has been well defined and developed, there is no satisfactory product for on-site monitoring, analysis and control. As a consequence, all hazardous waste treatment process for Category 1, especially for those companies whose business is to collect and process various kind of hazardous wastes from different sources, are almost all human monitored with very little or no automatic control. Therefore, a fully automated system with on-site monitoring of multivariable environmental parameters, automatic and intelligent analysis, and automatic control the treatment condition is highly desirable.
Many studies have been reported on monitoring, analysis and control problems and possible solutions suggested for the process. However, these results are derived under very simple assumption such that they are only suitable for pH and Oxidation/Reduction Potential (ORP) analysis and demonstrated by simulation and lab experiments. Other important variables such as Total Dissolved Solid (TDS), Suspended Solid (SS), Chemical Oxygen Demand (COD), Heavy Metals (HMs) and Some of Anions (SAIs) are all analyzed off line, and it takes hours or even days to obtain the results. No comprehensive and effective real time solutions for industrial application have been suggested so far. This project considers a general hazardous waste chemical treatment process and aims at improving both product quality and quantity by optimizing the whole system based on performance criteria, which incorporates both analytical chemistry and process control. It will also provide a unique opportunity to follow a group of workers monitoring simultaneously the exposure and adverse effects of the process for a period of time, and develop a composite system model from individual components by considering couplings between different variables, different phase and different parts in the system. With the composite model, the optimization techniques are used to find the optimal operation conditions, which minimize the operation cost while meeting all the requirements on discharge quality. The optimal conditions are then realized by large-scale control system which must be able to provide tight multivariable control performance and ability to adapt to load and chemical content changes.
