Optimization of the treatment and disposal of sewage sludge in the ATO of Como: options and scenarios assessment.

9 
 
Abstract 
Sludge produced by municipal wastewater treatment plants (WWTPs) amounts to only a few percent 
by volume of the processed wastewater, but its management accounts for up to 50% of total operating 
costs. Innovative solutions are being studied in order to minimize its production and to find new dis-
posal ways which are more cost-effective and less impacting on the environment. 
In the Province of Como, sludge management is one of the main issues. In this work, the situation is 
described, mainly referring to the biggest plants, both in quantitative and qualitative terms. Then actu-
al disposal routes are analyzed, which mainly concern agricultural reuse. New treatment and disposal 
solutions are assessed, mainly concentrating on thermal conversion processes: incineration and pyrol-
ysis/gasification. Sludge management scenarios are simulated, taking into account chains of technolo-
gies and analyzing the final economic and environmental results. Furthermore, varying some simula-
tion parameters, a sensitivity analysis is provided. 
Thermal conversion processes and electrodewatering seem to be cost-effective technologies which can 
be further investigated for a comprehensive project of diversification of sludge disposal routes in the 
ATO of Como. Finally, the problem of transparent and clear data gathering is mentioned, with the aim 
of developing a new WWTPs database, called SYST&MS. 
 
Keywords: sludge – disposal – incineration – pyrolysis – electrodewatering – database
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“Paris casts twenty-five millions of francs annually into the sea;  
an approximate amount given by the estimates of modern science. Science knows now that 
the most fertilizing and effective manures is the human manure. […]Do you know 
what these piles of ordure are, those carts of mud carried off at night from the 
streets, the frightful barrels of the nightman, and the fetid streams of subterranean 
mud which the pavement conceals from you? All this is flowering field, it is  
green grass, it is the mint and thyme and sage, it is game, it is cattle, it is the  
satisfied lowing of heavy kine, it is perfumed hay, it is gilded wheat, it is bread on 
your table, it is warm blood in your veins.” (Victor Hugo, Les Misérables, 1862) 
 
“Historically, it was common to see schematics that showed the water 
treatment scheme in detail […] and an arrow at the end that simply said ‘sludge 
to disposal’. ” (Neyens et al., 2004)
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1. Introduction 
The disposal and reuse of sewage sludge have assumed increasingly importance in recent years as part 
of the integrated water services because of the different impacts in terms of management, economic 
and environmental issues related to it. The interest has also increased because of the large-scale pro-
duction of rules and regulations on a European, national, and even regional level on the topic.  
In Lombardy, a production of around one million tonnes of sewage sludge was documented (IRER, 
2010). Just over half of the sludge, after appropriate treatment in specific platforms, is designed to ag-
ricultural reuse; the remaining half is sent, in roughly equal parts, to landfill or for incineration.  
Although recent surveys confirm a significant reduction in sludge quantities, due to the economic and 
industrial crisis and the consequent decrease of process residues that end up in wastewater treatment 
plants, production in Lombardy is going to increase, not only because of growing consumption and 
population, but also because the sewage and water treatment system will improve and will lead to 
connect all the users to the network. The tens of millions of Euros allocated, in Lombardy, for large in-
terventions in response to EC infringement procedure 2034/09, which imposes heavy penalties for 
purification and sewerage systems that will not be in accordance with limits within 2015, will lead to 
an improvement in the quality of rivers and the environment, but simultaneously to an increase in the 
quantity of sludge. 
With this in mind, it becomes even more pressing the necessity that authorities responsible for the 
protection of the environment intervene in the definition of a coherent planning framework, strategies 
and techniques to optimize the management of sewage sludge. Companies in the water sector, whose 
aim should be to identify technological systems to reduce the amount of sludge and minimize its eco-
nomic and environmental impacts, claim it. 
The same situation is found in the Como district, where the production of sludge amounted to approx-
imately 30,000 wet tonnes in 2013 and is going to increase because of the planned interventions to 
power up the WWTPs and the sewerage system. The main disposal route in the zone is agricultural re-
use, to which a certain number of issues are connected, mainly related to environmental (e. g. heavy 
metals leaching into soils used for landspreading of sludge) and management (e. g. availability of fields 
for sludge distribution) problems. 
The key word which must be bore in mind by the authority is disposal differentiation: in this way, new 
disposal routes are going to be studied in order not to convey all the sludge to agriculture. 
More and more attention is put on thermal treatments of sludge, such as incineration, gasification and 
pyrolysis, which are investigated in this work, together with new treatment options and the scenarios 
which can develop if these disposal routes are adopted.
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2. Conventional sludge management 
2.1 Sludge sources, quantities and management 
Both domestic and industrial wastewater treatment processes almost always involve the production of 
sludge, made of concentrated suspensions of the materials removed during the treatment. In fact, it is 
rare the case in which the purification can be obtained by gasification of the pollutants or by their sol-
ubilization in a form compatible with a proper disposal. In general, the treatment can be seen as a con-
centration in the sludge of substances initially in a dispersed, dissolved and suspended form and a re-
moval by processes culminating in a phase of solid-liquid separation (sedimentation or sometimes flo-
tation, filtration and centrifugation). As a result of the concentration step, the sludge contains a sub-
stantial portion of the pollutants originally present in the effluent; if not treated and disposed of cor-
rectly, it can then give rise to new phenomena of pollution. 
In urban sewage sludge, the volatile component is dominant, being formed by settled organic material, 
bioflocculated organic pollutants and excess biomass. Metal precipitates (hydroxides, phosphates, etc.) 
are present in substantial measure in the case where the cycles of purification include physical-
chemical or chemical steps, such as flocculation and precipitation of phosphorus. 
Sludges are characterized on the basis of the chemical (pH, alkalinity, organic substance, presence of 
nutrients and micro-pollutants), physical (humidity, specific weight, particle size, calorific value, rheo-
logical characteristics) and biological properties that influence the treatment and disposal. Moisture 
and specific gravity are related to the volume; the rheological properties to the hydraulic behavior; the 
physical properties to the applicability of the different systems of dewatering; chemical and biological 
characteristics to the need for stabilization and to identify ways for proper disposal. It is observed, 
however, that the characterization of sludge is not subject to standardization and that in many cases 
there is still uncertainty about the parameters to be considered as indicators and methods of defini-
tion. 
Treatment and disposal of sludge are one of the major aspects of the entire cycle of water purification, 
with heavy economic implications: the relative costs can approach, for certain plant solutions, 40-45% 
of the total costs of the construction and operation of water treatment plants (Bonomo, 2008). The re-
duction of the quantities to be disposed of and of their reactivity in the environment plays a crucial 
role in the schemes of purification. 
A typical municipal wastewater treatment plant produces three types of sludge: 
 
 Primary sludge, when the initial phase of sedimentation is expected. It is made up of suspended 
settled material fed with wastewater, separated by simple decantation without having under-
gone any process of transformation. Therefore it is always characterized by high putrescibility. 
 Secondary or biological sludge, made of biomass in excess (excess activated sludge, membranes 
films), including bacterial colonies and suspended, inert and volatile material, adsorbed or me-
chanically trapped. The level of putrescibility depends on the type of biological treatment ap-
plied. In some cases (extensive aeration treatment, trickling filters with low load) sludge may 
be stable enough to not require any additional treatment. 
 Tertiary sludge, produced by phases of filtration, flocculation or precipitation downstream of 
the biological treatment. In the presence of simple filtration, without the addition of inorganic 
reagents, the nature of the sludge is similar to that of biological sludge. Otherwise chemical 
precipitates are present in a variable percentage in function of the process applied, but often 
sufficient to produce a good degree of stability. 
 
In some cases different types of sludge may be mixed, within the same chain of water treatment. It can 
be foreseen, for example, the recycling of sewage sludge and tertiary ones (especially if produced by 
simple filtration) upstream of the primary sedimentation in order to improve the performance of the 
action exerted by the flocculation of excess biomass and get a better level of thickening. 
Other waste products in the water line (material retained by the grids and sieves, sand, fats and oils) 
have unique characteristics that require special disposal methods, distinct from those of the sludge. 
Hereafter they will not be taken into account.
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The values  reported in Table 1 are relative to the per capita production of sludge on a dry basis, with 
the corresponding humidity and the consequent production of wet sludge, for some biological pro-
cesses for the treatment of municipal wastewater. These values are intended to refer to the flow of 
sludge out of the water line, before the stages of thickening and dewatering. This is a guideline, de-
signed to provide orders of magnitude and to verify in accordance with local conditions. The quantities 
are reported on a dry basis in theoretical conditions of supply and resident population. 
Table 1: per capita production of dry and wet sludge after water line referred to urban wastewater 
Plant type per capita 
production  
humidity per capita 
volume 
[g inhabitant
-1
 
d
-1
] 
interval 
[%] 
typical 
[%] 
[l inhabitant
-1
 
d
-1
] 
Primary sludge 50-55 93-96 95 1.00 
Excess activated sludge 22-30 98.5-99 99 3.00 
Biological sludge from low load trickling filter  13-18 93-96 95 0.33 
Biological sludge from high lad trickling filter 20-25 94-97 95 0.47 
Excess activated sludge in absence of primary 
sedimentation 
55-60 98.5-99 98.8 4.50 
Excess activated sludge from extensive aeration 50-55 98.5-99 98.8 4.00 
Mixed primary and secondary sludge from acti-
vated sludge 
70-80 96-98 97 2.50 
Mixed primary and secondary sludge from low 
load trickling filter 
65-70 95-96 96 1.30 
Mixed primary and secondary sludge from high 
load trickling filter 
70-80 95-96 96.5 1.70 
(Source: Bonomo, 2008) 
2.2 Sludge characteristics 
It is useful to define the typical characteristics to understand the complex behavior and nature of 
sludge. For a complete discussion it is possible to refer to the books available in literature, such as 
Sanin, Clarkson, Vesilind, Sludge Engineering, 2011. 
Physical characteristics of sludge are: 
 
 specific weight, 
 solids concentration (total solids TS, suspended solids SS, dissolved solids DS), 
 settling (sludge volume index SVI), 
 floc/particle size and shape, 
 humidity and distribution of water (free, interstitial, vicinal and hydration water), 
 filterability and dewaterability (specific resistance to filtration SRF, capillary suction time 
CST), 
 rheology, 
 floc structure and porosity, 
 thermal conductivity. 
 
Chemical characteristics of sludge are: 
 
 pH, 
 alkalinity, 
 surface charge and hydrophobicity, 
 nutrient and fertilizer value, 
 heavy metal and toxic organics content.
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Biological characteristics of sludge are: 
 
 microbial community, 
 surface polymers (extracellular polymeric substances EPS). 
 
Typical characteristics of sludge originating from various treatment methods are shown in Table 2. 
Table 2: characteristics of sludge coming from different treatment processes 
 
(Source: Manara and Zabaniotou, 2012) 
2.3 Sludge treatment phases 
Sludge treatment may be composed by different stages; a typical process is summarized as follows 
(Metcalf & Eddy, 2003): 
 
 preliminary treatment (screening, comminuting), 
 primary thickening (gravity, flotation, drainage, belt, centrifuges), 
 liquid sludge stabilization (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, line addition, incineration, wet oxidation, pyrolysis, disin-
fection), 
 storage (liquid sludge, dry sludge, compost, ash), 
 transportation (road, pipeline, sea), 
 final destination (landfill, agriculture/horticulture, forest, reclaimed land, land building, other 
uses). 
 
Furthermore a brief explanation of the main processes is given. 
2.3.1 Thickening 
A decrease of moisture has a relevant reduction of the flow of sludge as a result, with obvious ad-
vantages in the subsequent treatments, thereby avoiding unnecessary hydraulic overloading in the 
stages of stabilization, conditioning and dewatering. The passage for example from 99% to 96% of
18 
 
moisture results in a reduction of four times in the amount of sludge. This result can be obtained 
through constructive and especially managerial adjustments in the operational units of both the water 
line and the sludge line: extension of the stay of sludge in the primary sedimentation tanks, which 
must then be built with hoppers of adequate capacity and depth; reduction of solid flow in secondary 
sedimentation tank downstream of the activated sludge tank; periodic shutdowns of the systems of 
mixing in the anaerobic digesters and in the tanks of aerobic stabilization, resulting in the discharge of 
water supernatants. One speaks in this case of simultaneous or contemporary thickening. With respect 
to such alternative it is common, at least in plants of medium and large size, the inclusion in the cycle 
of one or more specific steps which, in relation to the placement relative to the stabilization treatment, 
are divided into the stages of pre- and post-thickening. With pre-thickeners, the following advantages 
are achieved:  
 
 Greater ease and regularity of exercise of the primary sedimentation tanks, without the use of 
prolonged interruptions in the operations of sludge discharge with consequent risk of clogging 
in extraction circuits and production of odor due to putrefactive phenomena. 
 Containment of the area of secondary sedimentation (downstream of the activated sludge), in 
relation to less restrictive values for solids flux. 
 Limitation only to the sludge line of the movement of relatively concentrated suspensions on 
paths typically much shorter than otherwise needed and resulting in greater ease of adoption 
of appropriate measures for the proper functioning (cleaning devices in the pipes, volumetric 
pumps or similar). 
 Decrease of the heat transferred to the sludge if the heating is provided upstream of the stages 
of anaerobic digestion. 
 
The biological transformations that occur due to the stabilization processes improve the settleability 
of the sludge and then make the separation of additional quantities of water possible in post-
thickening. In many cases these operations are carried out, at least in part, within the same stabiliza-
tion reactor, with periodic shutdown of the mixing systems. The removal of the supernatant, as soon as 
the structure of the sludge changes, allows the reduction of the dimensions of the reactors, due to the 
lower volume of sludge maintained in it. The post-thickening allows the completion of the separation 
of the supernatants and the accumulation upstream of dewatering. The thickening is carried out by 
physical methods, normally by gravity, and, more rarely, by flotation, centrifugation or filtration. The 
use of flocculants allows an improvement of the reached moisture levels. Table 3 shows the perfor-
mances of different thickening systems.  
 
Table 3: typical thickeners performances 
 
Type of 
sludge 
Primary 
clarifier 
Dissolved air 
flotation 
Gravity 
thickener 
Belt thickener 
(with condi-
tioning) 
Centrifuge 
Total Solids Concentration after Thickening, Percent Solids 
RPS 5-7 - 8-10 9-12 9-12 
WAS - 3-5 1.5-2 5-7 5-7 
RPS+WAS 2 4-6 4-6 5-7 5-7 
RPS: Raw Primary Sludge; WAS: Waste Activated Sludge 
(Source: Sanin, Clarkson, Vesilind, 2011) 
2.3.2 Stabilization 
Many types of sludge have a high level of putrescibility and this is the case of the primary sludge from 
urban wastewater or biodegradable industrial products; also the sewage sludge from biological treat-
ment is usually putrescible to varying degrees, depending on the depurative process; sufficient stabil-
ity is achieved only in specific processes, such as extensive aeration and attached biomass with high 
surficial and volumetric loads. A stabilization phase, separated from the water line, is almost always
19 
 
necessary to ensure proper conditions for the final disposal, for odor containment and to improve the 
hygienic characteristics through pathogen reduction. 
The putrescibility of the sludge is connected to the presence of rapidly biodegradable material, subject 
to anaerobic transformations in the absence of oxygen. Stabilization can be obtained according to two 
different modes: 
 
 Transformation or destruction of putrescible organic matter; in the first case, aerobic or an-
aerobic biological processes are applied; in the second case the organic substance is complete-
ly eliminated by incineration processes. 
 Creation of environmental conditions that prevent the activity of the bacteria and thus the on-
set of anaerobic degradation phenomena; possible alternatives are of chemical nature, by alka-
lization, or of physical nature, by drying. 
2.3.3 Conditioning and dewatering 
The high moisture content of the sludge coming out from stabilization processes should be reduced to 
allow proper disposal in the environment. The characteristics of the sludge are generally not compati-
ble with the direct application of mechanical dewatering treatments, making necessary interventions, 
such as preliminary conditioning, almost always of chemical nature, in some cases of thermal nature, 
with the aim to: 
 
 Increase the speed of solid-liquid separation. 
 Increase the dry matter content of dewatered (or thickened) sludge. 
 Improve the quality of the separated liquid especially in terms of suspended solids. 
 
For the dewatering of fresh or stabilized sludge, conditioning is almost always necessary; it can be 
avoided only for some limited types of industrial sludge, especially in the case of inorganic nature; if 
dewatering is conducted through filtration processes, the purpose of conditioning is to improve the fil-
terability; in the case of centrifugation, to increase the size of the suspended particles, with advantages 
also for capture efficiency and quality of the centrifuged. 
The dewaterability of the sludge is conditioned by the presence of bound and interstitial water; other 
relevant factors are the organic content and its level of stabilization, the viscosity, the mechanical 
strength and especially the particle size distribution and the presence of extracellular polymers (EPS). 
The conditioning can be conducted by chemical (with the use of organic or inorganic additives) or 
physical means (thermally or with the use of sonic or electric energy). Chemical conditioning involves 
the addition of reagents to the sludge in order to achieve coagulation of colloidal or super-colloidal 
particles and their subsequent flocculation with reduction of the finely dispersed phase. Thermal con-
ditioning involves raising the temperature of the sludge up to value of 150 - 240 °C with contact times 
ranging between 30 and 90 minutes, depending on the type of sludge and the characteristics of the 
process. 
Sludge mechanical dewatering conducted by means of filtration and centrifugation aims at reducing 
the volume and weight of sludge, through partial separation of the liquid component, in order to make 
it compatible with the final disposal. The dry content achievable is such as to give the sludge the ap-
pearance of a soil (the sludge is defined shovelable i.e. able to be handled by mechanical means) and 
characteristics necessary for its final disposal in landfills or by thermal treatments and, if they fulfill 
the conditions, by agronomic use.  
Mechanical dewatering can be made through different systems; the most common are: belt presses, fil-
ter presses and centrifuges, whose advantage and critical points are shown in Table 4.