Overview
Before being dispatched for dewatering or thickening in a wastewater treatment system, produced sludge must go through a thorough pre-treatment process. As a result, both stabilisation and conditioning are important processes in the proper handling and disposal of wastewater sludge. Depending on the size of the treatment system, the process usually needs protracted storage in a specially built large process buffer tank or even clarification basins. Certain larger plants will also use an aerobic digester or a facultative lagoon to extend the detention period for storing and conditioning the sludge.
What are the Role of chemicals in Sludge Conditioning?
Chemicals are widely used in wastewater treatments to precipitate and remove phosphorus and in some cases to improve the efficiency of suspended solids removal. Chemicals can be added to raw wastewater, to a secondary biological process, or to secondary effluent, in which case tertiary filters or tertiary clarifiers are used to eliminate the chemical precipitates.
Although theoretical rates of chemical sludge production can be estimated from the anticipated chemical reactions, competing reactions can make the estimation difficult.
For example, ferric chloride will form ferric hydroxide, which in turn will react with phosphate to form ferric phosphate. Classical jar tests are favoured as a means of estimating chemical sludge quantities. Quantities of precipitates in chemical sludge are influenced by such conditions as pH, mixing, reaction time, and opportunity for flocculation.
Sludge conditioning chemicals and their action
Sludge conditioning is done to help with the removal of water from the solids, and it's usually done with chemical dosing. Conditioning causes the smaller sludge particles to coalesce (or agglomerate) into larger ones, lowering the sludge colloidal (macromolecular size) concentration. This improves particle settling as well as the permeability of the product cake, which improves the efficiency of the downstream thickening and dewatering operations.
Chemical conditioning is achieved through the dosing of sludge with
1: Organic flocculants and/or inorganic (or mineral) coagulants (such as iron or aluminium) or polymers.
2: Mineral reagents are more suited to operations that require a robust floc rather than a big, compressible one, such as mechanical dewatering (using a belt or filter press). Mineral coagulants include the following:
-FeCl3, Fe2(SO4)3, AlCl3 or Al2(SO4)3
-Ferric chloro sulphate (FeClSO4)
-Salts of polyaluminium or polyferric
-In alkaline stabilisation, lime {slaked lime, Ca(OH)2} or quick lime (CaO) is utilised.
Ferric salts, particularly ferric chloride, are frequently selected for their cost-efficiency and effectiveness. Coagulants work by lowering the repellent surface charge of sludge particles, allowing them to clump together due to the destabilisation effect. Lime dosage can be used in conjunction with coagulant dosing to improve cake filterability and retain more hazardous metals with the solids. Lime dosing also helps to stabilise sludge by lowering putrescence, odour, and pathogenic microorganism content.
What is Sludge Stabilization?
Lime stabilisation of wastewater sludge is a typical procedure used to prevent pathogen growth and remove unpleasant odours before dewatering and disposal. In the stabilisation process, there are two sorts of procedures: one involves adding lime right before dewatering, and the other involves adding lime after dewatering, which is referred to as "post lime-treatment" stages.
When it comes to lime, there are two types: hydrated lime {Ca(OH)2} and quicklime (CaO), often known as dry lime. Quicklime is commonly used in the treatment process because its exothermic reaction, when mixed with water, disinfects and rids the sludge of microorganisms.
When comparing the pre-treatment and post-treatment processes during lime addition, the latter always offers greater advantages and is frequently the favoured option in wastewater treatment plants.
First and foremost, it permits the use of quicklime, which, because it is dry, does not return water to the sludge after it has been dried.
Second, scale issues and other maintenance concerns can be avoided.
The disadvantage is that post-treatment stabilisation necessitates enough mixing to avoid the production of putrescible components. As a result, having a good mixer and a reliable dry lime feed system is essential for maximum efficiency. A well-mixed sludge will have a soft texture that will make it easier to handle, especially when it comes to disposal part.
Conclusion
Several factors must be taken into account while designing the sludge stabilisation process, particularly local regulatory supervision. With a large volume of sludge expected, precautions must be taken to ensure that a good lime dosing system with a mixer is in place to treat the sludge and considerably reduce pathogen count before disposal. Sludge stabilisation was frequently combined with other processes in most wastewater systems, such as the heat treatment system, anaerobic digester, and aerobic digester.
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