Waste Audit - To Measure is to Know
Updated: Aug 7, 2021
“To measure is to know. If you cannot measure it, you cannot improve it. When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind.” These are the words of Lord Kelvin, a successful innovator, and the most important physicist of 19th century. His words are applicable to all the STEM projects irrespective of timeline.
The optimisation efforts in industry are typically focused and limited to maximizing the yield of major products as that is the main profit-making stream for the business. The waste generated during processing is taken care of only from the environmental compliance standpoint. The most polluting component in the waste defines its character and also defines the disposal pattern. The quantitative analysis of waste streams can identify the opportunities for industrial symbiosis and more so profits that can supplement the primary business.
The quantitative analysis of waste is called waste audit and involves six stages of think-action activities. The first stage involves understanding the processes in the manufacturing facility and constructing a flow diagram. The second and third stages involve analysis of input and output streams to quantify current level of waste reuse and water wastage along with documentation of waste hauled to off-site or disposal. The fourth stage involves identifying the material imbalances in the process. The data generated in the first four stages enables us to identify the waste reduction and valorisation opportunities. The waste reduction can happen by making changes into the process by changing the catalyst or adopting latest separation techniques or with even simple modifications in the process control loop. The waste valorisation options can be by further processing of the waste or by preventing mixing of the waste streams at the point of their generation and ensuring that the waste is actually a saleable by-product. The ultimate stage in waste audit involves performing a cost-benefit analysis for all the identified options for waste reduction or waste valorisation and preparing an implementation plan.
Consider the example of a waste audit of mining waste generated during aluminum production. Aluminum is produced by Bayer’s process that involves separating alumina from bauxite by dissolving alumina selectively in alkali. The isolated alumina is then smelted to yield aluminum metal. Bauxite contains 45% alumina and the remaining 55% is composed of iron oxide, silica, titania, scandium oxide, vanadium oxide and even rare earth metals. Only alumina is the indented intermediate leading to production of aluminum. The inherent design of process involving solubilizing alumina in alkali tenders 55% of the starting material (bauxite) as waste which is termed as red mud. The problem is that this red mud is alkaline, thereby creating ecological issues on open dumping. Each metric tonne of aluminum product generates twice the amount of red mud waste that is dumped in slurry form near the manufacturing facilities. If the Bayer process of refining alumina from bauxite is studied systematically, it leads to three options of reducing and valorizing waste. The first option is to valorize the red mud by forming composites with various polymers and enable its usage into building materials like panels, tiles, bricks or even EMF shielding panels. While this is quickly doable, adding more virgin material into waste can go wrong from a life cycle assessment standpoint if the emissions from the process of making construction materials fail to supersede the emissions caused by allowing the red mud to be dumped. The second option is to segregate the components in red mud by using mechanical, chemical and biotechnological techniques. The red mud slurry is thixotropic and alkaline. It is difficult to have more than 85% solid contents in red mud and that makes alkali reclamation a tough job. Even after the reclaiming, the solid red mud must be subjected to thermal treatments to be able to recover the iron oxide. After reclaiming the iron oxide, the leftover mud must be subjected to chemical or biotechnological processes to recover high value metal oxides of titanium, vanadium and scandium. Though, segregation is a tough job, it is an attractive option as millions of tonnes of red mud is lying to be valorised. The advantage of this method is that each fraction is routed to an existing supply chain and ensures guaranteed economic benefits from utilisation of red mud. The third option is to modify Bayer's process itself to ensure that red mud is not created. Bauxite can be subjected to acid leaching to separate the titanium oxide and silica before solubilizing alumina in alkali. After the removal of silica and titania, the standard Bayer’s process can be used and iron oxide can be easily separated even in alkaline conditions by filtration. The modified Bayer’s process ensures that there is no alkaline mud generated for disposal and the process has tremendous economic and ecological advantage. While this third option is futuristic and of immense academic interest, it is unviable to address the issue of already accumulated red mud. The cost benefit analysis of these three options varies with respect to the location of industry and existing and upcoming environmental protection laws, the current environmental compliance standards of the industry, the economic status and R&D investment capability of the concerned industry.
Waste audit thus gives an evidence-based determination of process improvement options, better understanding of the risks of disposal of hazardous waste and an enhanced alignment of operations with strategy. It won’t be long when waste audits will become a new normal in industry and environmental compliance agencies. It’s time to buckle up and start treating waste as a potential resource to be rejuvenated!
Dr. Saurabh C Patankar &
Mr. Kunal K Godambe