Sustainable sludge handling - EU Project Neptune

Neptune and Innowatech End User Conference 

Ghent January 27 th , 2010 

Sustainable sludge handling 

G. Mininni, C.M. Braguglia, A. Gianico 

IRSA (Water Research Institute) 

CNR (Italian National Research Council)

The problem 

In the coming years it is expected that in Europe, at least for large wastewater 

treatment plants (WWTPs), the only possible solution for sludge disposal will be 

transforming sludge into inert material by thermal processes. 

The present European landfill Directive 99/31 has restricted this disposal route, 

which was practically abandoned in those countries where the implementation 

of the above criteria in national legislations was very stringent (a limit of the 

organic carbon to 4-5 % in the wastes to be disposed off is fixed). 

Guidelines for agricultural utilisation will become progressively more stringent 

due to increasing health concerns about the widespread diffusion of pathogen 

and organic micropollutants in the environment (European Commission, 2000). 

Agricultural utilisation involving large amounts of sludge to be spread on land 

does not seem feasible for the following reasons: 

need of large extensions of fields and therefore long distances to be covered from 

WWTPs to the site of spreading; 

need of large storage volume required when sludge cannot be used (winter periods 

and when the fields are flooded); 

large WWTPs are often polluted by non controlled industrial discharges that might 

hinder agricultural utilisation of resulting sludge. 

Neptune project, contract no 036845, FP6-2005-Global-4, SUSTDEV-2005-3.II.3.2 2

The problem 

The volume of the sludge extracted from primary and secondary settling tanks is 

about 2% of the volume of treated wastewater (WW) but its treatment and 

disposal entails very high capital and operating costs, which can be accounted as 

high as 50% of the total costs of the WW treatment plant, i.e. 25-35 €/(person × 

year). 

Typical treatments for a large WW treatment plant include a first phase of 

concentration, generally carried out by gravity thickening, a biological aerobic or 

anaerobic stabilization, aimed to reduce biodegradable solids, odours and 

pathogens, and mechanical dewatering by centrifugation, belt-pressing or filterpressing. 

Often sludge processing is designed according to conventional systems, which 

might not be suitable for producing sludge with proper characteristics for the final 

outlet according to the legislative standards and avoiding any detrimental effects 

for the environment and any risk for the human health. 

According to the European Water Supply and Sanitation Technology Platform 

(WSSTP) in the large majority of cases, soil is, regardless of the technology or 

methodology selected, the final destination of vast quantities of treated sludge. 


European 

Union 

Landfill 

sites 

Current disposal options in Europe 

(% of sludge produced) 

Thermal treatments Composting Agricultural 

utilisation 

No agriculture 

18 23 7 45 7 

Austria 35 50 15 

Bulgaria 100 Some 

cases 

Czech 

Republic 

13

Per capita sludge production 

g/(person × d) 

Sludge production g/(person × d) 

Austria 55 

Brazil 33 

Canada 76 

Italy 38 

Finland 94 

Hungary 48 

Portugal 60 

Slovenia 20 

Turkey 60 

Medium value 54 


Towards a more sustainable 

sewage sludge management 

A more sustainable sewage sludge management could be attained through a 

separation of primary and secondary sludge before their treatment and disposal. It 

would thus be possible to maintain agricultural utilisation for the biological sludge 

(secondary) and to convert to inert material by incineration (on-site or off-site) only 

the primary sludge (Mininni et al. 2004). 

Characteristics of primary and secondary sludge are quite different in terms of 

quality (pollutants and nutrients) and in terms of suitability for thickening, digestion 

and dewatering. Secondary sludge is expected to be less polluted than primary 

sludge, and should be segregated and treated separately from primary sludge 

thus sustaining its agricultural utilisation. 

Sludge separation may also give flexibility to sludge management, decreasing 

dependency on conventional disposal options (as required in the European 

Directive 2008/98) as sludge of good quality (biosolids) can be recovered for 

agricultural utilisation while the remaining primary sludge can be treated by 

thermal treatments. The challenge in the coming years will be, in fact, assuring in 

sludge management the greatest flexibility, maximizing recovery of valuable 

products and energy sources and reducing disposal only to inert materials which 

cannot be recovered any more. 


10 

9 

8 

Assessment of primary and secondary 

sludge characteristics in the Neptune project 

(nutrients) 

Total Phosphorus 

Total Nitogen 

P=Primary sludge 

S=Secondary sludge 

Cobis 

Fregene 

P and N concentration (% of TS) 

7 

6 

5 

4 

3 

Roma Nord 

2 

1 

S 

P S P P S P S P S P S PS S 

0 

P S 

P 

S 

P 

S 

P 

S 

P 

S P 

P S S 

P 

S 

P 


Assessment of primary and secondary sludge 

characteristics in the Neptune project (metals) 

10000 

1000 

Cd primary Cd secondary Hg primary Hg secondary 

As primary As secondary Ni primary Ni secondary 

Cr primary Cr secondary Pb primary Pb secondary 

Cu primary Cu secondary Mn primary Mn secondary 

Zn primary Zn secondary 

Concentration (mg/kg dry solid) 

100 

10 

1 

0,1 

0,01 

Roma Nord Cobis Fregene 


Assessment of primary and secondary 

sludge characteristics in the Neptune project 

(organic micropollutants) 

40.000 

35.000 

Cobis 

PRIMARY 

SECONDARY 

30.000 

Concentration 

25.000 

20.000 

15.000 

Rome Nord 

Fregene 

Cobis 

10.000 

Rome Nord 

Fregene 

5.000 

Rome Nord 

Cobis 

Fregene 

0 

EOX 

(µg Cl/kg TS) 

MBAS 

(mg/kg TS) 

Total hydrocarbons 

C10-C40 (mg/kg TS) 


Secondary sludge disintegration 

One of the main goals in secondary sludge processing is improving the 

performance of the anaerobic digestion. 

Secondary sludge contains up to 70% of bacteria which can be resistant 

to anaerobic digestion. 

Acceleration of the process can be achieved by increasing the hydrolysis, 

which is the limiting step of the whole anaerobic process. 

Sludge disintegration treatments are able to disrupt biomass flocs and cell 

walls and to cause the release of the intracellular organic material. The 

subsequent increase in biodegradable material improves bacterial 

kinetics resulting in lower sludge quantities and, in the case of anaerobic 

digestion, increased biogas production. 

The most common methods for sludge disintegration are based on the 

use of ultrasounds (mechanical disintegration) at an energy input of 1-2 

kWh/kg dry solids or on thermal disintegration at temperature of 170°C 

and 8.5 bar (Cambi process) or at 150-180°C and 8-10 bar (Biothelys 

process). 


Process flow sheets with sludge 

triage 

Energy 

(heat) 

amount, 

composition, 

environmental 

impact 

Produced gas 

I 

E 

Primary sludge 

G 

Drying-Incineration 

with internal energy 

recovery 

Primary 

settling 

A 

Gravity 

thickening 

B 

Anaerobic 

digestion 

D 

Dewatering 

Chemicals, Energy 

F 

C 

Off gas 

L 

On-site drying 

Energy 

H 

Q 

Energy (transport) 

M 

Biological 

treatment 

WAS 

P 

Dynamic 

thickening 

Energy 

Effluent for 

further 

treatment 

amount, composition, disposal 

(heavy metals, icropollutants) 

Produced gas 

O 

Off-site 

coincineration 

R 

Energy (aeration) 

Chemicals (foam 

breakers?) 

Short aerobic 

thermophilic treatment 

with intermittent feed (5d; 

45°C) 

Ultrasound 

disintegration + 

anaerobic digestion 

Energy 

Thermal 

disintegration + 

anaerobic 

digestion 

Energy 

S 

Produced biogas to 

energy production 

U 

T 

Produced biogas to 

energy production 

W 

Chemicals 

Energy 

Dewatering 

V 

AA 

Chemicals 

Energy 

AB 

Dewatering 

X 

Chemicals 

Energy 

AC 

Dewatering 

Energy 

(transport) 

Y 

Z 

Energy 

(transport) 

Energy 

(transport) 

Sludge to agriculture 

nutrients 

(e.g. N, P) 

J 

amount, treatment, 

composition, disposal 

Solid 

residue 

K 

Fuel 

N 


Process flow sheets without 

sludge triage 

Primary 

settling 

Biological 

treatment 

Effluent for further treatment 

Primary 

sludge 

Sludge 

liquid 

C 

Sludge liquid 

A 

WAS 

Thickening 

B 

Mesophilic 

anaerobic 

digestion (37°C) 

G 

D 

F 

amount, composition, 

disposal 

Off gas 

H 

On-site thermal 

drying-incineration 

Chemicals, 

Energy 

No fuel is needed 

E 

P 

Mechanical 

dewatering 

amount, treatment, 

composition, disposal 

E 

Solid 

residue 

or 

P 

amount, composition, 

disposal 

Off gas 

Energy 

Off-site 

coincineration 

L 

O 

On-site drying 

N 

Fuel 

Energy (transport of 

dryed sludge; 90% DS) 

M 

Off gas 

Produced biogas to energy production 


Flow sheet for sludge incineration 


100 

Results with sludge triage 

COD SS VSS Total mass 

Total mass of primary+secondary sludge=16,27 (before treatment), 0,614 kg/m 3 WW (after treatment) 

Mass of dry solids=0,282 (before treatment), 0,152 kg/m 3 WW (after treatment) 

10 

12,3 

kg/m 3 WW 

3,97 

1 

Primary sludge 

0,363 

Secondary sludge 

0,159 

0,1 

0,0924 

0,123 

0,251 

0,0602 

0,01 

A Before thickening 

B After thickening 

D After digestion 

F After dewatering 

M After drying 

P Before thickening 

R After dynamic thickening 

S After short aerobic 

thermophilic digestion 

T After US dis. and 

digestion 

V After therm. dis. and 

digestion 

Y Sludge T after dewatering 

X Sludge S after dewatering 

Z Sludge V after dewatering 


Results without sludge triage 

100 

COD SS VSS Total mass 

16,24 

10 

kg/m 3 WW 

1 

0,281 

0,724 

0,161 

0,1 

0,01 

A Sludge before 

thickening 

B Thickened 


D Digested 


E Dewatered 


L Dried sludge 


70 

Net biogas production 

Biogas 

60 

H 2 O 

CH4 CO2 H2O 

50 

H 2 O 

CO 2 

CO 2 

CO 2 

CO 2 

L/m 3 WW 

40 

30 

H 2 O 

20 

23.8 

20.8 

H 2 O 

10 

CH 4 

CH 4 

CH 4 CH 4 

10.9 

13.5 

0 

Mixed 


Primary 



after ultrasound 

disintegration 


after thermal 

disintegration 


Off gas from thermal treatments 

9 

CO2 H2O N2 O2 

Off gas production (Nm 3 /m 3 WW) 

8 

7 

6 

5 

4 

3 

2 

1 

1.12 

7.54 

MS=Mixed sludge 

PS=Primary sludge 

1.35 

0.598 

3.63 

0.789 

0 

On-site 

incineration 

MS 

On-site 

drying 

MS 

Off-site 

incineration 

dryed MS 

On-site 

incineration 

PS 

On-site 

drying 

PS 

Off-site 

incineration 

dryed PS 


Sludge triage allows: 

 

 

 

Conclusions 

reducing the production of primary + secondary sludge with respect to conventional treatment, 

considering that secondary sludge can be appropriately treated by dynamic thickening, disintegration 

and digestion, thus reducing considerably both the water content and the biodegradable solids; 

reducing disposal problems only to primary sludge. To this purpose an integrated incineration 

process can be used thus minimizing the exhaust gas production to such low values (for a WWTP 

serving 500.000 PE 3.100 Nm 3 /h for primary sludge comparing to 5.800 Nm 3 /h for mixed sludge) 

that these type of plant can be considered like a pilot plant; 

giving more flexibility to sludge management considering that disposal of sludge is not accomplished 

by a unique solution. 

Sludge incineration should be preferentially performed with an on-site plant. The 

use of external incineration or co-incineration plants is not convenient whether the 

sludge has to be previously dried. In fact, the methane requirement for on-site 

thermal drying (34.4 and 71.7 L/m 3 WW, for primary and mixed sludge 

respectively) if no other waste heat is available, would be scarcely compensated 

by the biogas production from digestion only with flow sheet with sludge triage. 

With the conventional flow sheet energy requirement for sludge drying is much 

higher than the available energy with biogas. This option is not environmental 

friendly also considering the quite high amount of total gaseous effluent produced 

in drying and incineration (23.000-46.300 Nm 3 /h for primary and mixed sludge, 

respectively, for a plant serving 500.000 PE). 


Acknowledgment 

• This study was part of the EU Neptune project (Contract No 

036845, SUSTDEV-2005-3.II.3.2), which was financially 

supported by grants obtained from the EU Commission within 

the Energy, Global Change and Ecosystems Program of the 

Sixth Framework (FP6-2005-Global-4)

Sustainable sludge handling - EU Project Neptune

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