102
4 Nitrogen and Phosphorous Recovery from Municipal Wastewater …
4.2.1.2 Stabilization
Sludge stabilization usually refers to reducing the sludge odors and putrescence as
well as the presence of pathogens. The main approaches for stabilization are chemical
methods, digestion (aerobic and anaerobic) and heat treatments.
– Anaerobic sludge digestion: Anaerobic digestion is the most common method for
sludge stabilization resulting in the reduction of Volatile Solids (VS) and production of biogas at mesophilic (35–40 °C) or thermophilic (55–60 °C) conditions.
Most of the large treatment plants use a two-stage anaerobic digestion system
consisting of a heated and mixed primary digester and an unheated and unmixed
secondary digester. In the primary digester, sludge is thickened to a dry solid
content of about 5% for several days. Hydrolysis and acidogenesis take place
forming volatile fatty acids (VFAs) along with NH 3 , CO 2 , H 2 and other byproducts by acidogenic (fermentative) bacteria. Then the sludge flows into the
secondary digester where the dissolved organics are converted into methane and
carbon dioxide by methanogenic bacteria [3, 4].
– Aerobic sludge digestion: It is performed in an open tank in the presence of
oxygen. Methane gas is not produced in this process. Aerobic systems are easier
to operate compared to anaerobic systems, however, they cost more due to the
power requirements for aeration [4].
– Alkaline stabilization (chemical method): In this process, alkaline compounds are
added to the liquid primary and/or secondary sludges to produce a homogeneous
mixture with a minimum pH of 12 after two hours of vigorous mixing, which
increases the mass of sludge solids, followed by supplemental dewatering [3].
– Thermal treatments: Thermal sludge treatment is referred to as the evaporation of
water from sludge by heat. Sludge is usually dewatered prior to drying to reduce
the energy costs for moisture evaporation. Thermal treatments result in increased
pathogen destruction, easier storage of the dried sludge and less transportation
costs [3].
4.2.1.3 Conditioning
The effectiveness of the sludge thickening and dewatering processes highly depends
on the conditioning of the sludge. Sludge conditioning can be performed using chemical or physical methods. Chemical methods involve the use of organic or inorganic
flocculants to support the formation of a porous and free draining cake structure.
The most used chemicals for conditioning are polymers, for thickening, and ferric
salt in conjunction with lime, for dewatering. The physical methods involve using
high levels of heat (175–204 °C) and pressure (1700–2800 kPa) for the length of 15–
40 min. In this process, hydrolysis of water matrix destroys the gelatinous components of the organic solids, hence resulting in improved solid–liquid separation.
However, the physical methods can lead to the production of extremely corrosive
liquids and the production of odorous gases [3].
4 Nitrogen and Phosphorous Recovery from Municipal Wastewater …
4.2.1.2 Stabilization
Sludge stabilization usually refers to reducing the sludge odors and putrescence as
well as the presence of pathogens. The main approaches for stabilization are chemical
methods, digestion (aerobic and anaerobic) and heat treatments.
– Anaerobic sludge digestion: Anaerobic digestion is the most common method for
sludge stabilization resulting in the reduction of Volatile Solids (VS) and production of biogas at mesophilic (35–40 °C) or thermophilic (55–60 °C) conditions.
Most of the large treatment plants use a two-stage anaerobic digestion system
consisting of a heated and mixed primary digester and an unheated and unmixed
secondary digester. In the primary digester, sludge is thickened to a dry solid
content of about 5% for several days. Hydrolysis and acidogenesis take place
forming volatile fatty acids (VFAs) along with NH 3 , CO 2 , H 2 and other byproducts by acidogenic (fermentative) bacteria. Then the sludge flows into the
secondary digester where the dissolved organics are converted into methane and
carbon dioxide by methanogenic bacteria [3, 4].
– Aerobic sludge digestion: It is performed in an open tank in the presence of
oxygen. Methane gas is not produced in this process. Aerobic systems are easier
to operate compared to anaerobic systems, however, they cost more due to the
power requirements for aeration [4].
– Alkaline stabilization (chemical method): In this process, alkaline compounds are
added to the liquid primary and/or secondary sludges to produce a homogeneous
mixture with a minimum pH of 12 after two hours of vigorous mixing, which
increases the mass of sludge solids, followed by supplemental dewatering [3].
– Thermal treatments: Thermal sludge treatment is referred to as the evaporation of
water from sludge by heat. Sludge is usually dewatered prior to drying to reduce
the energy costs for moisture evaporation. Thermal treatments result in increased
pathogen destruction, easier storage of the dried sludge and less transportation
costs [3].
4.2.1.3 Conditioning
The effectiveness of the sludge thickening and dewatering processes highly depends
on the conditioning of the sludge. Sludge conditioning can be performed using chemical or physical methods. Chemical methods involve the use of organic or inorganic
flocculants to support the formation of a porous and free draining cake structure.
The most used chemicals for conditioning are polymers, for thickening, and ferric
salt in conjunction with lime, for dewatering. The physical methods involve using
high levels of heat (175–204 °C) and pressure (1700–2800 kPa) for the length of 15–
40 min. In this process, hydrolysis of water matrix destroys the gelatinous components of the organic solids, hence resulting in improved solid–liquid separation.
However, the physical methods can lead to the production of extremely corrosive
liquids and the production of odorous gases [3].