Sludge Disposal: Lime Dewatering

Stabilization of wastewater sludge using lime is a common method applied to eliminate growth of pathogens and to get rid of undesirable odor prior to dewatering and disposal. There are basically two types of procedures involved in the stabilization process, whereby one is through addition of lime just before dewatering and another is after dewatering, which you would call it as "lime post-treatment" steps. In terms of lime usage, there are also two different types which is either using hydrated lime, Ca(OH)2 or quicklime, CaO or what you would call as dry lime. Usually in treatment process, quicklime is often selected because the exothermic reaction when mixed with water can easily disinfect and rid the sludge of pathogens.

Comparing between pretreatment and post-treatment process during addition of lime, the latter always has more advantages and is often the preferred method for use in wastewater plant. First of all, it allows use of quicklime and because the lime is dry, addition of it does not reintroduce water back to the sludge once it has undergone drying process. Second, problems related to scaling and other maintenance issues can be avoided. However, the negative point is that, post-treatment stabilization requires sufficient mixing to avoid formation of putrescible materials. As such presence of a good mixer is a must to ensure maximum efficiency coupled with a reliable dry lime feed system. A well-mixed sludge will create a soft texture that allows easy handling especially on the disposal part.

On the design of the sludge stabilization process, several points have to be taken into consideration as well especially on local regulatory control. With an expected quantity of produced sludge, steps must be taken to have a good lime dosing system with a mixer, which are able to treat the sludge and significantly reduced pathogen count prior to disposal. In most wastewater system, sludge stabilization often involved other processes as well and working in tandem together with the heat treatment system, anaerobic digester and also aerobic digester.

Recommended Engineering Books