4.2 Wastewater Sludge
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4.2.1.4 Dewatering
Dewatering is the last step of sludge treatment before disposal and is referred to as
the reduction of the floc-bond and capillary water content of the sludge. Although
the dewatered sludge still contains a large amount of water (up to 70%), it no longer
behaves like a liquid and can be treated as a solid material. The final sludge disposal
method has a great influence on selecting a suitable dewatering technique. Dewatering
could be performed using drying beds, belt filter press, centrifuge, filter press and
vacuum filter [3, 4].
4.2.1.5 Disposal
The sludge that has gone through treatments or dewatering is referred to as the
treated sludge, ready for disposal or utilization. Traditionally, treated sludge could
be disposed of/utilized using three main approaches: land applications as a soil conditioner, landfill disposal and incineration [9, 11, 12]. When the sludge is suitable for
land application, it is referred to as biosolids. Depending on the degree of stabilization, biosoilds could be used as a soil conditioner in agriculture, horticultural or
soil reclamation purposes. However, there are certain requirements for the pathogen
levels, presence of inorganic or organic chemicals and concentrations of metals in
the biosolids as they might contaminate soil if they are used as a soil conditioner.
In most wastewater treatment plants, the final destination of the treated sludge is
municipal sanitary landfills. Sludge should meet certain solid concentration requirements before it can be disposed of in landfills. Another option of sludge disposal is
incineration in which the moisture of the sludge is completely evaporated and the
organic solids are combusted into inert ash. Previously, dumping sludge in oceans
was an option for sludge disposal, while this method is no longer considered a viable
option since it is prohibited in many countries [3, 4].
Disposal of the sludge through the above-mentioned conventional methods is
becoming gradually more difficult as the sludge quantities are increasing fast, while
the disposal options are limited and more restricted. Thus, there has been a growing
interest in developing more environmentally friendly processes to reduce the volume
of the sludge for disposal and replacing the conventional sludge disposal methods
with others that can recover bioresources, such as nutrients (primarily nitrogen and
phosphorous), and bio-energy (organic carbon compounds) from sludge [9, 13]. The
energy recovery of the sludge generally includes the conversion of the sludge into
biogas, syngas and bio-oil that can be further converted into electricity, mechanical
energy and heat, as was discussed previously in Chap. 3. Resource recovery from
sludge involves the extraction of phosphorous, nitrogen, volatile acids, building materials (inorganics), etc. In the following sections, advanced technologies for nitrogen
and phosphorous removal from wastewater and sludge will be extensively introduced.
103
4.2.1.4 Dewatering
Dewatering is the last step of sludge treatment before disposal and is referred to as
the reduction of the floc-bond and capillary water content of the sludge. Although
the dewatered sludge still contains a large amount of water (up to 70%), it no longer
behaves like a liquid and can be treated as a solid material. The final sludge disposal
method has a great influence on selecting a suitable dewatering technique. Dewatering
could be performed using drying beds, belt filter press, centrifuge, filter press and
vacuum filter [3, 4].
4.2.1.5 Disposal
The sludge that has gone through treatments or dewatering is referred to as the
treated sludge, ready for disposal or utilization. Traditionally, treated sludge could
be disposed of/utilized using three main approaches: land applications as a soil conditioner, landfill disposal and incineration [9, 11, 12]. When the sludge is suitable for
land application, it is referred to as biosolids. Depending on the degree of stabilization, biosoilds could be used as a soil conditioner in agriculture, horticultural or
soil reclamation purposes. However, there are certain requirements for the pathogen
levels, presence of inorganic or organic chemicals and concentrations of metals in
the biosolids as they might contaminate soil if they are used as a soil conditioner.
In most wastewater treatment plants, the final destination of the treated sludge is
municipal sanitary landfills. Sludge should meet certain solid concentration requirements before it can be disposed of in landfills. Another option of sludge disposal is
incineration in which the moisture of the sludge is completely evaporated and the
organic solids are combusted into inert ash. Previously, dumping sludge in oceans
was an option for sludge disposal, while this method is no longer considered a viable
option since it is prohibited in many countries [3, 4].
Disposal of the sludge through the above-mentioned conventional methods is
becoming gradually more difficult as the sludge quantities are increasing fast, while
the disposal options are limited and more restricted. Thus, there has been a growing
interest in developing more environmentally friendly processes to reduce the volume
of the sludge for disposal and replacing the conventional sludge disposal methods
with others that can recover bioresources, such as nutrients (primarily nitrogen and
phosphorous), and bio-energy (organic carbon compounds) from sludge [9, 13]. The
energy recovery of the sludge generally includes the conversion of the sludge into
biogas, syngas and bio-oil that can be further converted into electricity, mechanical
energy and heat, as was discussed previously in Chap. 3. Resource recovery from
sludge involves the extraction of phosphorous, nitrogen, volatile acids, building materials (inorganics), etc. In the following sections, advanced technologies for nitrogen
and phosphorous removal from wastewater and sludge will be extensively introduced.