Chemical Fixation or better known as solidification process is a type of treatment method applied to stabilized/conditioned sludge, done in order to immobilize the contaminant with pozzolanic compounds (such as fine silicate), lime or cement so that it can be safely disposed at landfills or reuse back in land reclamation projects. The technology is widely used in different industries from plating to oil refinery, especially those that generate waste sludge that contains high amount of hazardous materials and chemicals like pesticides, heavy metals including toxic compounds like arsenic, cyanide and lead.
Usually the fixation process involves a two-stage remediation approach, first is through the use of carbon (dosed in at certain percentage based on fixed weight quantity) that acts based on absorption principle to contain the organic compound, before moving on to the use of stabilizing agents to react or encapsulates the sludge. The result of the process is a product that is high in pH enough to kill the pathogenic bacteria so that it can be used for secondary application. The main purpose of using fixation technique is not only to sterilize and convert the contaminants to non-reactive, stable compound but also to prevent harmful chemicals from leaching underground and creating wastewater leachate which will be very hard to deal with later and potentially causing contamination to underground water sources. Nowadays, even patented binders are being developed to replace conventional stabilizing agents that can be used in wide range of applications with more effective binding and encapsulating properties.
A basic conventional setup for a typical chemical fixation system can be represented by the diagram shown above. From here, wastewater or industrial sludge which can either exist in liquid or dewatered form is transferred to a pre-mixing tank that also acts as buffer storage. Cements are fed from the silo together with the sodium silicate going to a hopper with a mechanical mixer that allows homogenous mixing to take place. The stabilized treated waste can then be filled into drums or moved by truckloads to send it over to landfill for disposal. In terms of cost calculation, the whole operation can vary considerably depending on various factors and most of the times it can often fluctuate towards the high side mainly due to the addition of fixation chemicals. Some will cost more compared to the others, (especially proprietary chemical agents) but overall this method still offers a very safe and viable long term solution.
Usually cost can be lowered down by ensuring sludge dewatering process is done properly to take out as much water as possible so that less fixation chemical is needed. Other aspects like end solid quality depending whether it is monolithic or granular (which determines the degree of leaching) can also play a part that affects costing. In general monolithic solids are preferred as it will leach less compared to granular form.