2.3 Conditioning
Sludge conditioning is an essential process that aims to prepare the sludge for dewatering by improving its dewatering properties. This can be achieved using inorganic coagulants like ferric hydroxide, lime (calcium hydroxide), or alum (aluminium sulphate) for chemical conditioning. Organic polymers like cationic, ionic, or non-ionic can also be used. Sludge conditioning can be carried out through chemical or physical conditioning. The following section will introduce the chemical conditioning process, which is the most widely used method and relies on conditioning agents.
Conditioning agents are chemicals or substances that are added to sludge to modify its physical and chemical properties. The goal of using conditioning agents is to improve the dewatering characteristics, reduce odours, and enhance the overall handling and processing of sludge. These agents can affect the sludge’s structure, water content, and other properties, making it more suitable for subsequent treatment.
Organic polymers are used for sludge conditioning but without influencing the concentration of dry solids. They are highly efficient, and only a small amount of the polymer is required. Because of their high molecular weight, the polymers can also be easily separated in the clarifier. However, the downside of using organic polymers is that they are expensive and can drive up the cost of the entire process.
Inorganic chemicals such as lime, ash, and ferric chloride have been traditionally used for conditioning sludge because they help coagulate the solids and release the absorbed water. However, adding lime, ash, and ferric chloride can increase the dry solids produced by 20-30%.
Different conditioning agents serve different purposes, and their selection is based on the specific characteristics of the sludge and the treatment objectives. Lime conditioning is a widely used method and is the one that will be discussed in further detail.
Lime Conditioning
Lime can be applied to faecal sludge or septage at different stages of the treatment process, depending on the objectives and challenges.
Lime has both conditioning as well as stabilizing effects. As mentioned in the previous topic, due to its stabilizing benefits, it helps to inactivate pathogens and reduce odour in the sludge. As a conditioner, however, it modifies the physical and chemical properties that facilitate the separation of water from sludge.
Applying lime before the solid–liquid separation and dewatering stage offers enhanced dewatering. Due to the coagulant nature of lime, its addition to the sludge at this particular stage increases the overall sludge volume thus making it more manageable.
Alternatively, applying lime after the dewatering stage does not considerably increase the overall volume. However, this method requires specialised mechanical equipment to ensure adequate mixing. These devices need regular maintenance and repair, which depend on the availability of spare and replacement parts.
Regardless of the mixing stage, the long-term sustainability of lime-based treatment methods relies on the availability and affordability of hydrated lime. This highlights the need to supply this essential ingredient consistently to maintain the effectiveness of lime stabilisation and pathogen reduction.
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