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Sludge treatment unit operation design
CHAPTER
Sludge treatment unit operation design: chemical processes
23
CHAPTER OUTLINE Introduction ............................................................................................................275 Stabilization ...........................................................................................................275 Conditioning ...........................................................................................................276
INTRODUCTION As with the other areas of water process design, there are three kinds of sludges to consider: sewage sludges (high organics), waterworks sludges (low organics), and industrial effluent treatment sludges, which might be similar to one of the two preceding types, or entirely different (e.g., they might be very oily). Chemical treatment of sewage sludges may aim to stabilize or deodorize them to prevent odor nuisance, or pathogenicity. While waterworks sludges may have sufficient organic content for septicity to be a possibility, with the potential for taste or odor problems when water is recycled from sludge dewatering, this is not usually a problem in practice.
STABILIZATION Sewage sludges can be stabilized to prevent them “going off,” resulting in odor nuisance. This often also has the benefit of reducing pathogen count. Lime and heat treatment are the most common techniques. Lime or some lime-containing waste material may be added to sludge, with the aim of maintaining a pH of 12 or above for 2 h, with residual pH above 11 for a few days. This may be undertaken before or after dewatering. It is not a permanent solution, and does not kill all organisms present. The lime addition may have the effect of improving dewaterability by some processes if carried out prior to dewatering. Carrying out quicklime addition after dewatering has the secondary benefit of giving a useful temperature increase, which destroys parasite eggs. An Applied Guide to Water and Effluent Treatment Plant Design. DOI: https://doi.org/10.1016/B978-0-12-811309-7.00023-0 © 2018 Elsevier Inc. All rights reserved.
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276
CHAPTER 23 Sludge treatment unit operation design
CONDITIONING The main purpose of sludge conditioning is to improve dewaterability of sludges. Chemicals used to this end include inorganic coagulants, organic polymers, filter aids, and lime. Chemical dose requirements increase approximately as solids density of sludges decrease. Septicity of sludges also increases their chemical dosing requirements. For all three types of sludge, it is usual to add the chemical agents in a zone of very high shear, followed by sufficient residence time within a low shear area to allow floc to build prior to dewatering equipment. The most commonly used sludge conditioning chemicals are organic polymers, specifically polyacrylamides. Concerns over the carcinogenicity of acrylamide means that low-monomer grades of polyacrylamide are used in drinking water sludge treatment, and calculations are performed to ensure that levels of monomer recycled to process are always below maximum safe limits.
Sludge treatment unit operation design: chemical processes
23
CHAPTER OUTLINE Introduction ............................................................................................................275 Stabilization ...........................................................................................................275 Conditioning ...........................................................................................................276
INTRODUCTION As with the other areas of water process design, there are three kinds of sludges to consider: sewage sludges (high organics), waterworks sludges (low organics), and industrial effluent treatment sludges, which might be similar to one of the two preceding types, or entirely different (e.g., they might be very oily). Chemical treatment of sewage sludges may aim to stabilize or deodorize them to prevent odor nuisance, or pathogenicity. While waterworks sludges may have sufficient organic content for septicity to be a possibility, with the potential for taste or odor problems when water is recycled from sludge dewatering, this is not usually a problem in practice.
STABILIZATION Sewage sludges can be stabilized to prevent them “going off,” resulting in odor nuisance. This often also has the benefit of reducing pathogen count. Lime and heat treatment are the most common techniques. Lime or some lime-containing waste material may be added to sludge, with the aim of maintaining a pH of 12 or above for 2 h, with residual pH above 11 for a few days. This may be undertaken before or after dewatering. It is not a permanent solution, and does not kill all organisms present. The lime addition may have the effect of improving dewaterability by some processes if carried out prior to dewatering. Carrying out quicklime addition after dewatering has the secondary benefit of giving a useful temperature increase, which destroys parasite eggs. An Applied Guide to Water and Effluent Treatment Plant Design. DOI: https://doi.org/10.1016/B978-0-12-811309-7.00023-0 © 2018 Elsevier Inc. All rights reserved.
275
276
CHAPTER 23 Sludge treatment unit operation design
CONDITIONING The main purpose of sludge conditioning is to improve dewaterability of sludges. Chemicals used to this end include inorganic coagulants, organic polymers, filter aids, and lime. Chemical dose requirements increase approximately as solids density of sludges decrease. Septicity of sludges also increases their chemical dosing requirements. For all three types of sludge, it is usual to add the chemical agents in a zone of very high shear, followed by sufficient residence time within a low shear area to allow floc to build prior to dewatering equipment. The most commonly used sludge conditioning chemicals are organic polymers, specifically polyacrylamides. Concerns over the carcinogenicity of acrylamide means that low-monomer grades of polyacrylamide are used in drinking water sludge treatment, and calculations are performed to ensure that levels of monomer recycled to process are always below maximum safe limits.