4.1 Wastewater Treatment
101
can be reused for agriculture. The removal of nutrients will be discussed in detail
later in this chapter.
4.2 Wastewater Sludge
As mentioned earlier, primary and secondary sludges are generated during the
primary and secondary treatments. The primary sludge contains mainly water
(between 97 and 99%) as well as highly putrescible organic matters [8]. The
secondary sludge or Waste Activated Sludge (WAS) is rich with dissolved organic
matter, and it consists of a complex activated sludge floc structure. Sewage sludge
could also come from septage or septic tanks from household on-site wastewater
treatment systems. The sludges are often combined together for further treatment
and disposal [4].
Sewage sludge is a mixture of organic substances or volatile matter including
proteins, carbohydrates and lipids as well as inorganics and water [9]. Primary sludge
has higher content of total solids (TS), around 2-8%, compared to the WAS (0.8–
1.2%). Both of the sludges contain 20–40% protein, 5–65% grease, 500–1500 mg
dm
-3 alkalinity, 200–2000 mg dm
-3 organic acids, and 18–29 MJ kg
-1 energy. In
addition, they have high contents of nitrogen and phosphorous (as high as 2–5%),
which could be recovered and used as fertilizers [10].
4.2.1 Sludge Treatment Methods
Sewage sludge treatment and disposal are the major factors in the design and operation of wastewater treatment plants [4]. Sludge management aims to reduce the
volume of the sludge and to stabilize the organic materials and can cost as high
as 60% of the total cost for a wastewater treatment plant [12]. Sludge treatment
includes a combination of thickening, stabilization, conditioning, dewatering and
disposal operations.
4.2.1.1 Thickening
As sludge contains a large amount of water, thickening is usually the first step in
sludge treatment in which the solids are concentrated and the volume of the sludge
is decreased substantially. The main approaches for thickening sludge include using
gravity, floatation or centrifugation. Gravity thickening is performed in tanks and it is
used for primary sludge and mixtures of primary and WAS. Dissolved air floatation
is a process in which air bubbles carry the solids to the surface and form a layer
of thickened sludge. Centrifugation is usually used for WAS and has three types:
solid-bowl decanter, disk-nozzle and basket [3, 4].
101
can be reused for agriculture. The removal of nutrients will be discussed in detail
later in this chapter.
4.2 Wastewater Sludge
As mentioned earlier, primary and secondary sludges are generated during the
primary and secondary treatments. The primary sludge contains mainly water
(between 97 and 99%) as well as highly putrescible organic matters [8]. The
secondary sludge or Waste Activated Sludge (WAS) is rich with dissolved organic
matter, and it consists of a complex activated sludge floc structure. Sewage sludge
could also come from septage or septic tanks from household on-site wastewater
treatment systems. The sludges are often combined together for further treatment
and disposal [4].
Sewage sludge is a mixture of organic substances or volatile matter including
proteins, carbohydrates and lipids as well as inorganics and water [9]. Primary sludge
has higher content of total solids (TS), around 2-8%, compared to the WAS (0.8–
1.2%). Both of the sludges contain 20–40% protein, 5–65% grease, 500–1500 mg
dm
-3 alkalinity, 200–2000 mg dm
-3 organic acids, and 18–29 MJ kg
-1 energy. In
addition, they have high contents of nitrogen and phosphorous (as high as 2–5%),
which could be recovered and used as fertilizers [10].
4.2.1 Sludge Treatment Methods
Sewage sludge treatment and disposal are the major factors in the design and operation of wastewater treatment plants [4]. Sludge management aims to reduce the
volume of the sludge and to stabilize the organic materials and can cost as high
as 60% of the total cost for a wastewater treatment plant [12]. Sludge treatment
includes a combination of thickening, stabilization, conditioning, dewatering and
disposal operations.
4.2.1.1 Thickening
As sludge contains a large amount of water, thickening is usually the first step in
sludge treatment in which the solids are concentrated and the volume of the sludge
is decreased substantially. The main approaches for thickening sludge include using
gravity, floatation or centrifugation. Gravity thickening is performed in tanks and it is
used for primary sludge and mixtures of primary and WAS. Dissolved air floatation
is a process in which air bubbles carry the solids to the surface and form a layer
of thickened sludge. Centrifugation is usually used for WAS and has three types:
solid-bowl decanter, disk-nozzle and basket [3, 4].