Sludge treatment

sludge treatment process

1. SLUDGE TREATMENT
Sludge is produced from both the primary and the secondary treatment of wastewater
in on-site (e.g. septic tank) and off-site (e.g. activated sludge) systems. This is inherently so
because a primary aim of wastewater treatment is removing solids from the wastewater. In
addition, soluble organic substances are converted to bacterial cells, and the latter is removed
from the wastewater.
The sludge treatment required because of sludge may be contaminated with heavy
metals and other pollutants, especially when industrial wastes are disposed into sewer. Besides,
sludge may be contain large numbers of pathogens. There are several of sludge treatment which
are stabilisation, thickening, dewatering, drying and incineration.


Figure 1 shows the process wastewater treatment

1. THICKENING

Sludge contains a high concentration of solids, but its water content is still high.
Combined primary and secondary sludge from an activated sludge treatment plant contains
about 2 % solids and hence 98 % water. One kg of dry sludge is associated with 49 L of
water. Thickening to 5 % solids means one kg of dry solids is associated with 19 L of water,
thus 30 L of water has to be removed.

2. Thickening is carried out in a sedimentation tank or in a sedimentation pond. The latter
is advantageous if land area is available, because the sludge can be allowed to settle over a
much longer period and a higher solids content of the thickened sludge is achieved. The
water removed from thickening needs treatment. It can be returned to the inlet of an off-site
wastewater treatment plant, or in the case of sludge from on-site units by an aerobic
treatment process such as lagooning.
2. STABILISATION
Sludge is stabilized to reduce their pathogen content, eliminate offensive odours, and
reduce or eliminate the potential for putrefaction. Technologies used for stabilization
include lime stabilization, heat treatment, aerobic digestion, anaerobic digestion and
composting.
2.1 Lime stabilization
 Lime is added to untreated sludge to raise the pH to 12 or higher.
 The high pH environment inhibits the survivals of micro-organisms, and thus
estimates the risk of sludge putrefaction and odour creation.
2.2 Heat treatment
 The process involves the treatment of sludge by heating in a pressure vessel to
temperature up to 260c at pressure up to 2760kN/mxm for approximately 30
seconds.
 The exposure of sludge to such conditions results in hydrolysis of proteinaceous
compounds, leading to cell distribution and the release of soluble organic
compounds and nitrogen.
2.3 Anaerobic digestion
 The process involves the anaerobic reduction of organic matter in the sludge by
biological activity.
 Anaerobic digestion consist of two stages that occur simultaneous in digesting
sludge :
 First, consist of hydrolysis of the higher molecular weight organic
compounds and conversion of organics acids by acid forming bacteria.
 Second, gasification of the organic acids to methane and carbon dioxide by
the acid splitting methane and carbon dioxide by the acid splitting methane
forming bacteria.

3. 2.4 Composting
 The objective of sludge composting is to biologically stabilize putrescible
organics, destroy pathogenic organisms, and reduce the volume of waste.
 The resulting end product is stable and may be used as a soil conditioner in
agricultural applications.


3. DEWATERING AND DRYING

Dewatering aims to reduce the water content further so that the solids content of the
sludge is about 20 % (equivalent to 1 kg dry sludge with 4 L of water). The sludge can then
be handled like a solid. Dewatering can be done mechanically using a filter press (employing
pressure or vacuum), or a centrifuge. It can also be done using drying beds. A drying bed
consists of a 30 cm bed of sand with an under-drainage. Sludge is applied on the sand bed
and is allowed to dry by evaporation and drainage of excess water over a period of several
weeks depending on climatic conditions. Bacterial decomposition of the sludge takes place
during the drying process while moisture content is sufficiently high. During the rainy
season the process may take a longer time to complete and sizing the area of the drying beds
should take this into account.