1. business and industrial
2. energy
3. coal

SUSTAINABLE SLUDGE HANDLING
Erik Levlin
Dep. of Land and Water Resources Engineering
Royal Institute of Technology (KTH)
S-100 44 Stockholm, Sweden
I.
PRINCIPLES OF SUSTAINABLE SLUDGE HANDLING
When the Brundtland Commission’s "Our Common Future" was published in 1987, the concept
of sustainable development rapidly became generally accepted. The definition of sustainability in
the Brundtland report is: "Development that meets the needs of the present without
compromising the ability of future generations to meet their own needs".
Harremoës (1996) gives the following interpretation of sustainability with a resource component
and a pollution component:
o Society should use its resources such that the society can continue its mode of operation
without exhausting its resources
o Society should protect the environment against irreversible damage, including protection of
unique species and habitats
Sludge may be regarded both as a resource which should be recycled in a proper way and a threat
to the environment. Different resources in the sludge include:
o Nutrients as phosphorus, nitrogen and potassium. Special consideration should be devoted to
phosphorus due to a possible scarcity in the future.
o Organic material for soil conditioning, energy production, adsorption materials or production
of organic compounds as organic acids.
o Inorganic material for reuse as precipitation chemicals, use in building materials and possible
recovery of some valuable metals.
Harmful substances in the sludge may be divided into pathogens, organics (including oil and
organic micropollutants) and toxic inorganics (heavy metals, asbestos, radioactive materials etc).
Sustainable sludge handling may therefore be defined as a method that meets requirements of
efficient recycling of resources without supply of harmful substances to humans or the
environment. The sludge handling should be performed in an energy and resource efficient way.
The different options that are available for handling chemicals (including sludge compounds)
with respect to ultimate fate are few, namely no use, reuse, convert, contain and disperse
(Harremoës, 1996). These options are illustrated for sludge handling (see Table 1).
1
Table 1. Illustration of different principal options for sludge handling
Option
Purpose
Application in sludge handling
No use
Stop use of an unwanted substance due to detrimental and
irreversible effects to the
environment
Effective control of industrial discharges, use of
environmentally friendly consumer products etc
to facilitate sludge use in agriculture and use of
sludge products
Reuse
Decrease of the amount reaching
the environment and of extraction
of mineral resources by reusing
the compound
Internal reuse of materials (as reuse of
precipitation chemicals) and external reuse (as
reuse of phosphorus as fertiliser)
Convert
Conversion of a substance from
an obnoxious form to a form,
acceptable for further transport by
air, or water or in solid form
Conversion of organics to methane gas (for
further use as energy source), solubilisation of
sludge components for product recovery,
conversion of sludge into compost etc
Contain
To contain the residues with as
low leaching ability as possible
Separate containment of toxic substances in the
sludge, inclusion or stabilisation of ashes from
sludge incineration etc
Disperse
Dispersion into environment
without negative impact
Effective dispersion of sludge in agricultural
use, effective dispersion of treated flue gases in
sludge incineration
The different options in sludge handling should be combined in an optimal way. Generally, an
effective control of industrial discharges and use of environmentally friendly consumer products
are prerequisites for an efficient sludge handling. Containment of pollutants should be restricted
to a minor part of the products used in society due to increasing shortage of land.
About 6.5 millions tonnes dry solid were disposed of or recycled in the 12 Member States of the
European Union in 1994 (European Commission, 1997). Trends in sludge production and final
sludge handling is illustrated in Figure 1. It is expected to 2005 a significant increase in sludge
production, a decrease in disposal and an increase in recycling and incineration.
Sludge production in Sweden is about 240,000 tonnes dry solids per year. Final sludge handling
was in 1988 in Sweden: Agriculture 35%, deposition on landfill 40%, land restauration 15%, and
green belts 10% (VAV, 1991). Ten years later (1998) the agricultural use had declined to 25%
and disposal on landfill had increased to 46 % (SEPA and SCB, 2000). At present, agricultural
use is low due to the recent recommendation (Autumn 1999) by LRF that agricultural spreading
should be suspended because of the presence of brominated flame retardants in sludges and their
possible negative effects on soils and organisms. Deposition on landfill of organic sludges will
be forbidden in 2005, and there is little experience on sludge incineration in Sweden. The
recommendation not to spread sludge on agricultural land, prohibition to dispose organic sludges
on landfills in the near future and the little experience on incineration have given a large pressure
on municipalities to change final disposal routes and different actors have very different opinions
on suitable routes. It is therefore natural that there exists at present an intensive sludge debate in
Sweden.
2
Figure 1. Sludge production and final sludge handling in the 12 EU Member States 1984-2005
(European Commission, 1997).
II.
PRACTICE IN SLUDGE HANDLING
II.1 Main methods
Sludge handling in a large centralised wastewater treatment plant consists of several possible
steps:
o
o
o
o
o
o
o
o
o
o
Preliminary treatment (screening, comminution)
Primary thickening (gravity, flotation, drainage belt, centrifuges)
Liquid sludge stabilisation (anaerobic digestion, aerobic digestion, lime addition)
Secondary thickening (gravity, flotation, drainage belt, centrifuges)
Conditioning (elutriation, chemical, thermal)
Dewatering (plate press, belt press, centrifuge, drying bed)
Final treatment (composting, drying, lime addition, incineration, wet oxidation, pyrolysis)
Storage (liquid sludge, dry sludge, compost, ash)
Transportation (road, pipeline, sea)
Final destination (landfill, agriculture/horticulture, forest, reclaimed land, land building, sea,
sale)
The main sludge handling technology in Sweden is for larger treatment plants: Primary
thickening (mainly by gravity), liquid sludge stabilisation (mainly by anaerobic digestion),
conditioning (mainly by use of polyelectrolytes), dewatering (mainly by centrifuges and belt
presses), transportation (mainly by road), and final destination (mainly agriculture, landfill, land
building, and land reclamation). The different methods for sludge volume reduction, sludge
stabilisation and final destination for sludges may be combined in different ways. Different
options are illustrated in Figure 2.
3
Sludge from mechanical, biological and chemical
Thickening (gravity, flotation)
Stabilization (anaerobic or aerobic
Dewatering (centrifugation, filter and belt presses)
Stabilization (composting, lime addition,
heat drying, incineration)
Sludge disposal (agriculture, landfill, land building)
Figure 2. Different options for sludge handling.
II.2 Restrictions of sludge handling
Sludge handling has two main purposes:
(1)
Stabilisation of the sludge by use of different methods as biological (anaerobic and aerobic
digestion and composting, chemical (mainly use of lime), and thermal (heat drying,
incineration and melting)
(2)
Volume reduction by use of thickening (gravity thickening, flotation and centrifugation),
dewatering (use of centrifugation, filters and presses), drying (natural and heat drying), and
incineration and melting
Stabilisation is used to obtain a sludge that does not change in time, i.e. a stable sludge, and that
does not cause odour problems. In addition stabilisation may reduce the number of pathogens in
the sludge, especially by use of chemical and thermal methods. Biological stabilisation removes
the biodegradable part of the sludge. Heat drying and lime addition inhibits further bacterial
growth. Lime containing sludge in contact with carbon dioxide in the air is neutralised and after
a certain time bacterial growth may start again. In incineration both biodegradable and nonbiodegradable organic materials are removed.
Main sludge handling in Sweden is for larger treatment plants: thickening (mainly by gravity
thickening), stabilisation (mainly by anaerobic digestion), conditioning of the sludge before
dewatering by polyelectrolytes, dewatering of the sludge (mainly by centrifuges or belt presses),
and transportation for final destination (mainly agriculture, landfill, land building and land
reclamation).
Sludge handling methods are restricted by several reasons such as regulations from authorities,
acceptance from different groups, technical problems, and costs. Some restrictions for the final
sludge destination are described in Table 2.
4
Table 2. Examples of restrictions for final sludge destination.
Final sludge
destination
Restrictions
Agricultural use
o Limiting values of metal concentrations in sludge use. These are in
Sweden as mg/kg DS: Pb 100, Cd 2, Cu 600, Cr 100, Hg 2.5, Ni 50,
and Zn 800
o Guidance values for indicator organic compounds. These are in
Sweden as mg/kg DS: Nonyl phenol 50, toluen 5, total PAH 3, and
total PCB 0.4.
o Maximum yearly amount of nutrients allowed to agricultural land
from sewage sludge (for Swedish conditions see SEPA, 1995)
o Limiting values of the yearly amount of metals that may be supplied
to agricultural land as g/ha and year (for Swedish conditions see
SEPA, 1995)
o Acceptance from food industry (food industry may not buy
agricultural products grown on land fertilised with sewage sludge)
o Public acceptance
Land disposal
o Availability of land disposal areas
o Limitation of maximum percentage of organic materials in the sludge
before disposal (which may necessitate the use of incineration)
o Fees of 250 SEK/metric ton for disposal of sludge on landfill
o Deposition of organic material will be prohibited by 2005
Land building and
reclamation
o Availability of land areas
o Environmental impact of application of sewage sludge
Incineration
o Permit to build an incineration plant
o Difficulties to find a suitable plant for co-incineration of sewage
sludge (with municipal solid wastes, biofuels, coal, cement, brick etc)
including permits
o Restrictions related to treatment of flue gases
o Restrictions related to further handling of produced ashes
Agricultural use of sludge is often regarded as the best alternative if the pollutants in the sludge
is below limiting and guidance values. However, lack of acceptans from food industry and the
public may make it difficult to use sludge for agriculture. Many attempts have been done to find
agreements for agricultural use of sludge. Thus, a national consultation group has been formed in
Sweden to stimulate the use of sludge in agriculture and to agree upon different actions and
voluntary precautionary measures to prevent the supply of unwanted chemicals and substances
into the sewer net. The group consists of representatives from the National Organization of
Agricultural Workers (LRF), the Swedish Water and Waste Works Association (VAV), and the
Swedish Environment Protection Agency (SEPA). Although this group has reached agreements,
the future of agricultural use of sludge is uncertain and under debate. A discussion is going on
concerning the availability of phosphate to plants of precipitated iron or aluminium phosphates.
Landfill of sludges will probably be restricted considerably in the future. In several countries (as
Germany) only sludges with a low percentage content of organics will be allowed for land
deposit. Land deposit of sludges can contribute to diffusive spread of materials as phosphorus
5
and metals due to leakage and emission of materials such as methane gas (a green house gas),
methylated metal compounds (such as methylated mercury) and odours. In order to reduce
landfill deposit of sludges a fee must be paid in Sweden of 250 SEK/metric ton of sludge. The
use of sludge for land building, restoration of land and use for covering of landfills may be
limited in the future due to lack of land and possible negative environmental effects.
Sludge incineration has gained much interest as a method for final handling of sewage sludges
(Jungvik, 1998). This technology is not yet practiced to a significant extent in Sweden. The
investment and operational costs are rather high and there may also be a problem to obtain a
permit to build an incineration plant. Therefore, attention has been directed towards coincineration in already existing incineration plants. Co-incineration may be applied in an
incineration plant for municipal solid wastes, for biofuels (wood, peat etc), coal or plants
producing building materials at high temperatures (cement, brick etc). Some experiments have
been done with co-incineration of sewage sludge together mainly with municipal solid wastes.
The experiments have in general been positive although some problems have been obtained
related to sintering of ashes and increased concentration of sulphur dioxide, nitrogen oxides and
volatile metals (mercury and cadmium) in the flue gases. The need for complementary
investments, if co-incineration is practised, in flue gas treatment or handling of ashes need to be
further studied. Possibilities to recover phosphorus from ashes have not yet been evaluated.
Studies have, however, started in this area.
II.3 Discussion of sludge handling practices
Main practises of sludge handling have been discussed above. In addition a lot of other methods
have been studied, although the methods have not been used significantly in practice. The sludge
handling methods in Table 2 may be modified in many ways to comply with requirements.
Different examples are given below:
o Agricultural use:
- Hygienisation of the sludge
- Removal of metals by treatment of sludge for instance by acids
- Improvement of sludge handling properties (composting, drying etc)
- Addition of alkaline materials (lime, cement dust etc)
- "Dilution" of sludge by addition of different materials (cocomposting with other materials, addition of weighting materials,
addition of special conditioning materials or rest products etc)
o Land filling
- Improvement of sludge handling properties
- Decrease of leaching from sludges and ashes
o Land building and
reclamation
- Improvement of sludge handling properties
- Decrease of leaching from sludges and ashes
o Incineration
- Pre-treatment of sludges before incineration
- Improvement in incineration technology
- Improvement of flue gas treatment
- Improvement of handling of ashes (including resources recovery
and leaching from ashes)
6
o Product recovery
- Improvement of product quality
- Decrease in use of chemicals and energy in product recovery
- Acceptance of products from industry and public
- Removal of toxic substanses as a sidestream
A wastewater treatment plant should have a flexibility in sludge handling and different options
should be considered. Sludge use for agriculture is often considered as the best alternative but
may have difficulties to be accomplished due to many restrictions. Landfills, use of sludge in
land building and land reclamation and incineration have limits related to resources recovery and
environmental impact. Product recovery from sludges is still in a development phase but has the
potential of both recover resources and destruct pollutants or remove them into a small
concentrated stream. The potential of products recovery from sludges will therefore be discussed
in more detail in the following.
III. SLUDGE VOLUME REDUCTION
III.1 Sludge volume reduction methods
Sludge from sedimentation and flotation units from water and wastewater treatment has a low
solids concentration and must be concentrated by different methods before disposal or use for
different purposes. Water in flocs exists as drainable water, capillary water and adsorbed and
internal water for instance in cells. Water may also be bound as crystallisation water in chemical
precipitates.
Thickening methods as gravity thickening, flotation and disc centrifuges can mainly remove
drainable water. Part of the water in capillaries can be removed by dewatering methods such as
belt and filter presses and bowl centrifuges. The applied pressure during dewatering limits how
much of the capillary water that can be removed. Water in fine capillaries and adsorbed water
can be removed by different drying methods. Most efficient is the use of heat drying. Finally,
internal water and crystallisation water are removed by methods as incineration and
sludge melting.
In order to facilitate dewatering of sludges the sludge is normally treated by conditioning
methods. Such methods include the addition of chemicals as polyelectrolytes and metal salts or
the use of heat or freezing.
Conditioning is a method to reduce the number of small particles and to build up larger flocs.
Metal salts and polyelectrolytes act as coagulation and/or flocculation agents. In heat treatment
the structure of the organic material such as proteins changes and a sludge with very good
dewatering properties may be obtained. During the heating process much materials in the sludge
goes in solution and a supernatant with a high concentration of organic materials and nutrients is
obtained. Freezing bursts cells and improves thereby the dewatering properties of the sludge.
In Sweden the dominant conditioning agent is polyelectrolytes. Freezing is used at a few places
with cold climate. During recent years a renewed interest has arisen on the use of heat
conditioning of sludges. This depends on the possibilities to diminish the sludge production,
improve the dewaterability compared with the use of polyelectrolytes and to recover different
7
substances released during the heat conditioning. Commersial processes with this technology are
KREPRO and Cambi.
The solids concentration of sludge is nearly doubled in thickening, which increases the solids
content by partial removal of the liquid. The concentration increase depends on the type of
sludge, with activated sludge increasing to about 3% solids and primary sludge to about 9%
solids. The most usual methods for thickening are gravity settling and flotation. Conventional
gravity thickening is simple and inexpensive. It is essentially a sedimentation process similar to
that which occurs in all settling tanks.
Gravity settling thickening is carried out in tanks which are usually equipped with a slow stirrer
to provide gentle agitation which enhances settling. Gravity thickening usually exhibits hindered
settling phenomenon. The degree to which waste sludge can be thickened depends on many
factors as the type of sludge being thickened and its volatile solids concentration. The initial
solids concentration has an effect on the degree of the concentration achieved. Hydraulic and
surface loading rates are also of importance. The supernatant from the thickener is returned to the
wastewater treatment process.
Flotation thickening is achieved by creating gas bubbles in the sludge. The bubbles attach to the
sludge particles, give them buoyancy and carry them up to the liquid surface. The accumulated
sludge is removed from the surface by scrapers and the clarified liquid is returned to the
wastewater treatment process. A dissolved air flotation unit is shown in Figure 3.
Surface sludge collector
Sludge
water
Influent sludge
Thickened
sludge
air
Bottem sludge collector
Water to Pressure tank
pressure tank
Figure 3.
Bottem
sludge
Flotation thickening of sludge.
Many methods of sludge disposal involve dewatering of the sludge. The objective of sludge
dewatering is to facilitate utilization, disposal or further processing of the sludge. The sludge
dewatering is a process of the removal of sufficient water from sludge to give it quasi-solid
characteristics and the reduction of volume and moisture content of sludge. Sludges may be
dewatered by use of belt press (see Figure 4), drying beds, lagoons, centrifuges, filter presses,
and horizontal belt filters.
8
Sludge
inlet
Dewatered
sludge
Figure 4. Belt press for sludge dewatering.
Dried sludge can be used as a fertilizer or a soil conditioner, but the costs of drying are high.
Incineration can be used as the ultimate sludge disposal method when the sludge has been
dewatered to give a solids content greater than about 30%. The heat of combustion of the sludge
solids is sufficient to evaporate the residual water content. The residual ash has to be disposed or
used in building material manufacturing. Sludge incineration, particularly for large cities, has the
advantage of removal organic solids and thereby reducing the weight and volume of solids.
9
III.2. Moisture distribution in sludge
Water in sludge can be classified as free water, interstitial water, surface water and bound water
(see Figure 5). Free water include void water that is not affected by capillary forces. This water
may partly be removed by thickening processes.
Free water
Interstitial water
Surface water
Bound water
Figure 5. A visualisation of moisture distribution in sludge (Tsang and Vesilind 1990).
Interstitial water is inside the flocs when sludge is in suspension and is present in the capillaries
when the cake is formed. This water may be partly removed by dewatering. Remaining capillary
bound water and surface water may be removed by drying. Bound water is chemically bound to
the solid particles and is not removed by drying but by the use of incineration.
III.3. Volume reduction in dewatering and thickening
During dewatering or thickening when the water is pressed out of the sludge, the volume
reduction will mainly follow the formula for volume reduction of suspensions with voids filled
with water. The specific gravity of sludge is a function of its volatile fraction and its fixed
fraction. The volatile fraction has a specific gravity near the value of water. The specific gravity
of the fixed fraction is often assumed to be 2.5.
The concentration and density of the excess sludge at Nowy Targ WWTP, Poland before and
after thickening and after dewatering are shown in table 3 as mean values for the years 1996,
1997 and 1998 (Hultman et al., 1999). Figure 6 shows sludge volume in l/kg dry solids versus
per cent of dry solids before thickening, after thickening and after dewatering.
Table 3. Concentration and density of the excess sludge at Nowy Targ before and after
thickening and after dewatering.
Parameter
Year
Before thickening
After thickening
After dewatering
Concentration, %
1996
1997
1998
0.58
0.75
0.63
3.89
2.67
2.47
15.77
14.00
14.00
10
1996
1002
1013
951
Density, kg/ m3
1997
1998
1003
1002
1010
1007
959
952
300
250
l/kg DS
200
Before
thickening
150
100
After thickening
50
0
0
1
2
3
4
5
6
DS, %
10
9
l/kg DS
8
After dewatering
7
6
5
4
10
12
14
16
18
20
22
24
DS, %
Figure 6. Volume in l/kg Dry Solids versus per cent of dry solids for sludge from Nowy Targ
before thickening, after thickening and after dewatering. The volume reduction at a sludge
density of 1 kg/ liter (100/% DS) is inserted as a dotted line (Hultman et al., 1999).
The present sludge production of about 3000 kg SS/day and a sludge concentration of about 13
% and a specific gravity of 0.95 requires a volume need of about 24.3 m3/day. A very efficient
dewatering with formation of a sludge cake or a combination of dewatering and natural drying
could reduce this volume need down to about 6 m3/day. With the voids filled with air the sludge
specific gravity is about 0.5.
Experiments at Nowy Targ have shown that a change from centrifugation to a filter press
increased the sludge solids concentration from about 15 % to 19 % if polyelectrolytes were used
as conditioning agents. The use of 0.17 kg FeCl3 and 0.83 kg Ca(OH)2 gave a after dewatering in
the filter press a value of 30 % dry solids.
11
With the assumption that added FeCl3 is precipitated as Fe(OH)3 and added Ca(OH)2 as CaCO3
the added amount of sludge dry solids is about 1.2 kg per kg supplied sludge before conditioning.
The volatile fraction of the sludge without addition of inorganic conditioning agents was about
80 %. After conditioning with inorganic agents the volatile fraction may be calculated to 34 %.
The VSS concentration after dewatering in the filter press may then be calculated to 15.2 % and
10.2 % with conditioning with polyelectrolytes and inorganic agents, respectively. The addition
of inorganic conditioning agents will therefore not cause a volume reduction compared with the
addition of polyelectrolytes.
Sludge from biological nutrient removal and other activated sludge process contain a high
proportion of intercellular water, which can not be removed with ordinary dewatering methods.
Processes which rupture intercellular water are required to remove intercellular water. Waste
activated sludge from biological nutrient removal can be dewatered to 12 to 15 % dry solids with
ordinary dewatering methods. Undigested primary sludge can be dewatered to 30 % and digested
primary and waste activated sludge can be dewatered to between 16 and 22 %.
III.4. Volume reduction in heat drying
The evaporation rate of different types of water is shown in Figure 7. After drying dry solids and
bound water remain. Dependent on the degree of heat drying some surface water may also
remain.
Rate of
surface
evapowater
ration dry
so- B
interstitial
lids
water
(B is bound water)
free
water
Weight of sample
Figure 7. Drying curve for identifying different types of water in sludge.
During drying the sludge changes characteristics. The sludge passes three zones:
o The wet zone, where the sludge is free-flowing and can be spread easily onto the heated tube
o The sticky zone, where the sludge is pastry
o The granular zone, where the sludge is crumbly in nature and mixes much more freely
During drying a certain amount of volatile substances are removed. If this amount is neglected
the total solids in the sludge is equal to the total sludge dry mass + the remaining water content.
12
III.5. Volume reduction in incineration
Ashes produced during incineration may either be handled wet or dry. Ash density is about 560
kg/m3 dry and 880 kg/m3 wet. Solmaz (1998) uses in a calculation example ash density of dry
and wet ash 800 kg/m3 and 1200 kg/m3, respectively. The water content in wet ash was about 6
% in the calculation example for polymer conditioned sludge.
Stabilization with lime will increase the inorganic content and thus decrease the volume
reduction achieved through incineration. A higher daily volume of ashes is produced if the
sludge is stabilized with lime. A typical dose of lime in stabilization is 0.45 kg CaO/kg dry
solids. For a sludge production of 3000 kg/day a lime dosage of 1350 kg is necessary and the
total sludge production will be 4350 kg/day of which 1950 kg is inorganic sludge. With a density
of ashes of 600 kg/m3 the volume produced of ashes is 2.25 m3/day.
Often sludge is burned at a temperature of about 800 to 850 ºC a temperature there the inorganic
material is in a solid phase as a porous ash with a density of about 600 to 800 kg/m3.
Solmaz (1998) has exemplified volume reduction of dewatered sludge conditioned either by
polyelectrolytes or lime and given approximate data for the volume needs per kg dry solids.
Table 4. Weight, volume and density for dewatered, dried and incinerated of two sludges, one
conditioned with polyelectrolytes and the other conditioned with lime (Solmaz, 1998).
Process
ConditioDry
ning agent Solids, %
Dewater.
PE
25
Dried
PE
90
Incin.
PE
100
Dewater.
lime
35
Dried
lime
95
Incin.
lime
100
Weight
Volume
1000
280
75
1000
370
175
1000
400
125
1000
525
290
Density
kg/l
1.0
0.7
0.6
1.0
0.7
0.6
Dry Solids Volyme,
l/kg DS
250
4.00
250
1.60
75
1.67
350
2.86
350
1.50
175
1.66
From table 4 it is seen that incineration is very advantageous as a method of sludge volume
reduction especially when organic materials are used for sludge conditioning. Additions of
inorganic chemicals, either for conditioning or for chemical precipitation, increases significantly
the sludge volume produced per day in incineration.
The sludge volume reduction depends on the reduction in volume in liter per kg dry solids
reached by dewatering and drying and and the reduction of dry solids solids reached through
incineration or digestion.
13
IV
BIOLOGICAL SLUDGE STABILISATION
Anaerobic digestion is a process in which organic material in the sludge is degraded without
contact with air (see Figure 8). The retention time in the digester is about 15 to 20 days. During
the process energy rich biogas is formed consisting of about 2/3 methane gas and 1/3 carbon
dioxide. The biogas may be used for energy production for heating purposes or production of
electricity. If the carbon dioxide is removed the remaining methane gas may be used as a fuel for
cars and busses.
In aerobic digestion the sludge is aerated for 15 to 20 days in order to remove biodegradable
organic material in the sludge. Problems related to aerobic digestion are the energy need for
aeration and cooling of the sludge during the winter period. Aerobic digestion has therefore
mainly come into use at small treatment plants.
Composting is also an aerobic stabilisation process. In this case a high sludge concentration must
be used, about 40 - 45 %. This may be obtained by addition of materials as wood chips or solid
wastes. Due to the high sludge concentration the heat produced during degradation of organic
material increases the temperature of the sludge up to 60 - 80 ºC, and thereby is the hygienisation
of the sludge improved.
CH4 + H2O
Fixed cover
Gas storage
Sludge
inlets
Sludge
outlets
Mixer
Sludge
heater
Figure 8.
V
Anaerobic digestion of sludge.
FINAL DESTINATION OF SLUDGE
Agricultural use of sludge is often regarded as the best alternative if the pollutants in sludge is
below limiting and guidance values. However, lack of acceptance from food industry and the
public may make it difficult to use sludge for agriculture. Many attempts have been done to find
agreements for agricultural use of sludge. Thus, a national consultation group has been formed in
Sweden to stimulate the use of sludge in agriculture and to agree upon different actions and
voluntary precautionary measures to prevent the supply of unwanted chemicals and substances
into the sewer net. Although this group has reached agreements, the future of agricultural use of
14
sludge is uncertain and under debate. A discussion is going on concerning the availability of
phosphate to plants of precipitated iron or aluminium phosphates.
Landfill of sludges will be restricted considerably in the future. In Sweden a tax of 250 SEK/ton
on all deposited solid waste was introduced year 2000 (SFS 1999:673). Deposition of incinerable
waste has been prohibited and in year 2005 there will be a ban on deposition of all organic
material on landfill (SFS 2001:512). Land deposit of sludges can contribute to diffusive spread
of materials as phosphorus and metals due to leakage and emission of materials such as methane
gas (a green house gas), methylated metal compounds (such as methylated mercury) and odours.
In order to reduce landfill deposit of sludges a fee must be paid in Sweden. The use of sludge for
land building, restoration of land and use for covering of landfills may be limited in the future
due to lack of land and possible negative environmental effects.
Sludge incineration has gained much interest as a method for final handling of sewage sludges.
Table 5 shows a comparison of sludge treatment methods.Incineration (ATV, 1997) and SCWO,
Super Critical Water Oxidation (Stendahl and Jäfverström, 2003, see figure 9), are methods that
eliminates all organic content. With SCWO can the potential energy of the organic material be
utilised. SCWO occurs in water of a supercritical phase at a temperature above 374 °C and a
pressure higher than 22 Mpa. Sludge incineration requires that the sludge has to be dried to 40 %
dry substance. The energy produced from the incineration is consumed by the drying and there is
therefore no net energy recovery from sludge incineration. With use of SCWO, energy can be
recovered also from sewage sludge and organic waste with too low dry substance for
incineration. Anaerobic digestion eliminates half of the organic content and half of the energy
can be utilised as methane gas. Co-incineration may be applied in an incineration plant for
municipal solid wastes, for biofuels (wood, peat etc), coal or plants producing building materials
at high temperatures (cement, brick etc). Some experiments have been done with co-incineration
of sewage sludge together mainly with municipal solid wastes. The experiments have in general
been positive although some problems have been obtained related to sintering of ashes and
increased concentration of sulphur dioxide, nitrogen oxides and volatile metals (mercury and
cadmium) in the flue gases. The need for complementary investments, if co-incineration is
practised, in flue gas treatment or handling of ashes need to be further studied. Recovery of
phosphorus from ashes involves many technical problems.
Table 5. Comparison of sludge treatment methods.
Energy gain
Composting
No
Rest product
Half of the organic content is oxidized
Anaerobic digestion
Half: 2.5 kWh biogas/kg DS Half of the organic content is oxidized
Incineration
No, the energy is used for
sludge drying
All organic content is oxidized
Super Critical Water
Oxidation
Almost all
5 kWh heat/kg DS
All organic content is oxidized
15
Steam
Steam Boiler
Gas/Liquid
Separator
Effluent
Cooler
Boiler Water
Pure Water,
Inorganics
Economiser
Boiler Water Pump
Waste
Water
Heater
Gas
400 °C
Reactor
250 Bar
CO 2, O2, N 2
Pressure
Release
°° °°°° °°
°°°°°° °° °°°°
°°° ° °°°° ° °
° °° ° °°
Oxygen
Vaporiser
Oxygen
600 °C
Tank Grinder High Pressure Pump
Oxygen Pump
Figure 9. SCWO, Super Critical Water Oxidation (Stendahl and Jäfverström, 2003).
VI
RECOVERY AND REUSE OF SLUDGE PRODUCTS
Another strategy for sludge handling that is to use the sludge as a source for different products.
At present development works are directed towards the use the digester gas as fuel for vehicles,
while the sludge should be used in agriculture. By use of improved sludge handling a better
product recovery and use may be obtained. Internal products may include solubilised organic
materials for improvements of biological phosphorus and nitrogen removal, production of
nitrification bacteria for seeding purposes from a separate nitrification step of supernatant from
dewatering of digested sludge, recovery of precipitation chemicals etc. These products have the
purpose to improve and to make the wastewater treatment more effective. External products may
include phosphorus compounds as magnesium ammonium phosphate and calcium phosphate and
improved use of sludge for energy production.
VI.1 Phosphorus Recovery
The nutrient in the sludge can be recovered and used as fertiliser in the agriculture. Of these
nutrients potassium, calcium, phosphate and nitrogen,phosphate is most important to recover.
Phosphate fertiliser is produced by mining of phosphate ores. More than 300 different phosphate
minerals are available, but only apatite (calciumphosphate, Ca3(PO4)2) is used for production of
fertiliser (Corbridge 1995). In 1995 the world phosphate rock production was 160 000 ton per
year (as P2O5), having tripled over the last 40 years. About 90% of this is used as fertiliser. At
this rate of consumption the known apatite reserves have been estimated to last for a period up to
1000 years. However, if the present increase in world population and the increasing need for
fertiliser for food production is taken into account, the supply of phosphate may well be crucial
within a century. Since nitrogen is the main part of the atmosphere the supply for nitrate is
unlimited. Recovery of nitrogen shall be done if the energy required for recovery is less than the
energy consumed by producing from nitrogen gas. In the wastewater treatment process
phosphorus is precipitated and transferred to the sludge phase. This process can be both chemical
with addition of iron or aluminium salts and biological. In the chemical process is phosphorus
precipitated as ferrous or aluminium phosphate. In the biological process is phosphorus at
16
aerobic conditions taken up by the micro-organisms. Also without salt addition some chemical
precipitation will occur with metal ions in the wastewater. The phosphate in the sludge will
therefore be present as both metal phosphate precipitations and incorporated in the biomass.
During anaerobic digestion the biomass will be converted to methane gas and the phosphate will
be released to the supernatant. The degree of phosphate release during digestion will thus depend
on the degree of biological phosphorus removal in the treatment process. The phosphate released
during digestion can be recovered from the supernatant by precipitation. Possible phosphate
precipitations are listed in table 6. The optimal precipitation struvite is achieved between pH 9.2
and 12. The experiments showed that the precipitation increased with increasing pH-level.
However, at the pH-levels for optimum precipitation, the concentrations in the solutions was 10
to 100 times higher than given by the equilibrium with the pK-values. Only at a low pH-level the
precipitation was in equilibrium with the pK-values with the difference increasing with
increasing pH-value.
Table 6 Possible phosphate precipitations (Mamais et al., 1994, *Stumm and Morgan, 1981).
Precipitation
MgNH4PO4
Name
MAP or struvite, magnesium
ammonium phosphate
pKs
12.6*
Precipitation rate Optimum at pH
Medium
7-8
Ca5(PO4)3OH
HAP or hydroxide apatite
57*
Very low
Mg3(PO4)2.8H2O
Bobierite
CaHPO4
Ca3(PO4)2
Calcium hydrogen phosphate
Tricalcium phosphate
9.5-11
Very low
6,6
24
The possibility of dissolving precipitated metal phosphate is also being studied. The phosphate
can be leached from the digested sludge or leached from the ash after incinerating the sludge.
Figure 11 shows the solubility of some metal phosphates (Stumm and Morgan, 1981). The
solubility increases with decreasing pH-value which indicate that phosphate can be recovered by
leaching with acid. Phosphate fertiliser is produced by leaching apatite with sulphuric acid
(McKetta and Cunningham 1990):
Ca3(PO4)2 (s) + 3 H2SO4 + 3x H2O → 2 H3PO4 + 3 CaSO4·xH2O (s)
17
mol/liter
1
0,1
0,01
0,001
0,0001
1E-05
1E-06
1E-07
1E-08
1E-09
1E-10
1E-11
FePO4
CaHPO4
Ca4H(PO4)3
AlPO4
1
2
3
4
5
6
7
8
9
Ca10(PO4)6(OH)2
Ca10(PO4)6(F)2
10 11 12
pH
Figure 11. The solubility of metal phosphates (Stumm and Morgan, 1981).
The precipitation of calcium sulphate makes the reaction to continue until all sulphuric acid is
consumed. For leaching aluminium or ferrous phosphate the pH-value must be lower. However,
aluminium or ferrous phosphate can also be dissolved at high pH-values. At a high pH-value
aluminium and iron are converted to metal hydroxide thus dissolving the phosphate (Stumm and
Morgan, 1981):
FePO4 (s) + 3 OH- → Fe(OH)3 (s) + PO43Another possibility to recover phosphate from ferrous phosphate is to use hydrogen sulphide.
VI.2 Sludge utilisation as building material
One other resource in sludge is the inorganic materials that can be used for production of
building materials. Any environmental hazardous contaminants are bound as mineral to the
material and utilisation of sludge reduces mining of raw material for production of building
material. Sludge from waste water treatment has been used for production of:
• Bricks (Alleman and Berman, 1984, Tay and Show, 1997).
• Lightweight aggregates in concrete (Tay and Show, 1997).
• Cementious material (Tay and Show, 1997).
Ash from sludge incineration can be used for production of building materials in three ways;. the
ash can be mixed with cement or concrete, a brick or some other object can be made of ash and
the ash can be melted and solidified as a ceramic material. The inorganic content in wastewater
sludge is more diversified. In incinerated and melted slag the main oxides are; SiO2 26-42%,
P2O5 12-27%, Fe2O3 5-25%, Al2O3 11-21% and CaO 8-14% (Ozaki et al., 1997). The inorganic
18
content is about half of the sludge weight. When sludge is used for brick manufacturing, the
organic content in the sludge is incinerated. The evaporation of the volatile part of the sludge
makes bricks produced of sludge and clay porous and of lower compression strength. This can be
avoided by incinerating the sludge and use of the inorganic ash for brick manufacture.
Bricks can be made from ash (Daido, 1998) by charging ash into a metallic mold with the shape
of the wanted brick and pressed into semi-manufactured bricks. The pressed bricks are than
sintered in a kiln type furnace at 1000 ºC during 10 - 12 hour. One problem with the bricks is the
higher water adsorption caused by capillary action of many fine pores. To decrease the water
adsorption the bricks were soaked into a silocone solution. Another problem is the variation of
the used ash. Since the components of ash are changeable mainly by the influence of rain, the
receiving of ash was stopped on rainy days. Further, the ash components were analyzed with
fluorescence X-ray method and an equation was introduced for calculating the suitable sintering
temperature.
A ceramic material can be produced by melting the ash and which thereafter is solidified
(Nomura, 1998, Endo et al., 1997). A high content of SiO2 and Al2O3 and a low CaO content
makes the melted ash to crystallise as mullite. However, crystallisation as anorthite will give the
glass ceramic better material properties, which can be achieved by using CaO as sludge
conditioning agent instead of polymers.
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