Energy Balance in a Novel Approach to Sludge Processing

Energy Balance in a Novel Approach to Sludge Processing

Giuseppe Mininni, Camilla Maria Braguglia, Andrea Gianico, Agata Gallipoli CNR - Istituto di Ricerca Sulle Acque

Via Salaria km 29,3 – Area Ricerca RM 1 00015 Monterotondo (Roma)

ABSTRACT In the coming years, large European wastewater treatment plants (WWTPs) will need to dispose of sludge by transforming it into inert material by thermal processes. In fact, the present European landfill directive 99/31 has restricted this disposal route which was practically abandoned by countries, where the implementation of the directive criteria in national legislations was very stringent, either imposing a limit of the organic carbon to 4 - 5 % in the wastes to be disposed (Germany) or including a leachate test with the very stringent limit of dissolved organic carbon of 80 mg/L (Italy). Moreover, guidelines for agricultural utilization, will become progressively more stringent due to increasing health concerns about the widespread diffusion of pathogenic and organic micropollutants in the environment (European Commission, 2000). Regardless, agricultural utilization 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 utilization of resulting sludge.

A more sustainable sewage sludge management system might be attained through a separation of primary and secondary sludge before their treatment and disposal. It would thus be possible to maintain agricultural utilization for biological sludge (secondary) and to convert to inert material by incinerating (on- or off-site) only the primary sludge (Mininni et al. 2004). In fact, 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. A sampling campaign carried out in Italy to assess the quality difference between primary and secondary sludge has highlighted that enrichment factors of organic micropollutants in primary sludge increase with respect to those in secondary sludge in the order of hydrocarbons, extractable organic halogens (EOX), anionic surfactants measured as methylene blue active substances (MBAS). Sludge separation may also give flexibility to sludge management, decreasing dependence on conventional disposal options (as required in the European Directive 2008/98), as sludge of good quality (biosolids) can be recovered for agricultural utilization while the remaining primary sludge can be treated by incineration. The challenge in the coming years will be

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Residuals and Biosolids 2010

Copyright ©2010 Water Environment Federation. All Rights Reserved.

assuring sludge management the greatest flexibility as well as maximizing the recovery of valuable products and energy sources while reducing disposal only to inert materials, which no longer contain useful compounds. One of the main goals in secondary sludge processing is improving the performance of anaerobic digestion. Secondary sludge contains up to 70 % of bacteria, so the above objective can only be achieved by accelerating the hydrolysis, which is the limiting step in the anaerobic process. Sludge disintegration treatments are able to disrupt biomass flocs and cell walls and to cause the release of the intracellular organic material. This treatment results in acceleration in the biological breakdown of particulate organic material into soluble, readily biodegradable fractions. 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 technologies already in use for sludge disintegration, such as ultrasounds, by means of shear forces, have shown that energy input can account for 1-2 kWh/kg TS (Khanal et al., 2007, Müller, 2001). In this context, IRSA has carried out anaerobic digestion tests with sonicated secondary sludge, in batch and semi-continuous mode, investigating the effect of pre-treatment on digestion performances and on particle surface charge, affecting dewaterability. It is worth noting that ultrasound pre-treatment (specific energy 5,000 kJ/kg TS, i.e., about 1.4 kWh/kg TS) considerably accelerates the reaction rate (doubling the k values), and biogas production increases by about 30 % with respect to the control reactor without sonication (Braguglia et al., 2008). A second way to increase performance of secondary sludge anaerobic digestion is by thermal pre-treatment. The Cambi process may give even better results due to the drastic conditions of this disintegration process (temperature of 160 – 170 °C, pressure of 6 - 9 bar, residence time of more than 20 minutes). The Cambi process combined with anaerobic digestion allows increased VS removal and biogas production of 40 – 50 % in comparison with conventional sludge anaerobic digestion without disintegration. Certainly, disintegration and digestion technologies of secondary sludge need to be optimised, taking into account both energy balance and sludge quality (reduction of volatile solids, dewaterability). In this paper, the energy balance of different treatment options will be presented. The conventional treatment routes with mixed (primary + secondary) sludge thickening, digestion, mechanical dewatering and on-site incineration or on-site drying and off-site co-incineration will be compared with advanced options where the primary and secondary sludge are kept separate. Primary sludge treatment is performed by the conventional route, whereas secondary sludge is thickened by dynamic thickening and then disintegrated by sonication or thermal treatment and then digested. This analysis points out that the advanced options are much more convenient than the conventional ones in terms of reduction of exhaust gas from the drying-incineration process and in terms of total energy requirements. KEYWORDS Energy, sewage sludge processing, sewage sludge characteristics, primary sludge, secondary sludge, anaerobic digestion, dewatering, thermal drying, incineration.

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https://www.researchgate.net/publication/11563425_Prospects_and_Problems_of_Sludge_Pre-Treatment_Processes?el=1_x_8&enrichId=rgreq-2b3d63ed-1de8-4195-8235-9685291d2909&enrichSource=Y292ZXJQYWdlOzI1NzU2MzE0NztBUzoxMDIwNzQ1NjYwNTM4OTRAMTQwMTM0Nzg4MTM2Mg==

https://www.researchgate.net/publication/248944608_Ultrasound_Applications_in_Wastewater_Sludge_Pretreatment_A_Review?el=1_x_8&enrichId=rgreq-2b3d63ed-1de8-4195-8235-9685291d2909&enrichSource=Y292ZXJQYWdlOzI1NzU2MzE0NztBUzoxMDIwNzQ1NjYwNTM4OTRAMTQwMTM0Nzg4MTM2Mg==

INTRODUCTION Management of sludge produced in the wastewater treatment plants (WWTPs) is a global issue with growing challenges that must address the concerns of all stakeholders, including facility administrators and operators, regulators, politicians, the scientific community, wastewater generators, taxpayers and the general public (Greater Moncton Sewerage Commission, 2008). The volume of the sludge extracted from primary and secondary settling tanks is about 2 % of the volume of treated wastewater (WW). In spite of this negligible volume, sludge treatment and disposal entails very high capital and operating costs, which can account for as high as 50 % of the total costs of the WW treatment plant, or 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 filter-pressing. Often, sludge processing is designed according to conventional systems, which might not be suitable for producing sludge with the proper characteristics for the final outlet according to the legislative standards and avoiding any detrimental effects on the environment and any risk for human health. The current situation in sludge disposal in European countries is quite diversified. Table 1 shows the current sludge disposal options in some European countries (Greater Moncton Sewerage Commission, 2008). The recent document by the Water Supply and Sanitation Technology Platform on “Sustainable Sludge Management in Urban Areas” (WSSTP, 2008) includes new data on the final sludge outlet in some European metropolitan areas (Table 2). In many countries and urban areas, agricultural utilization still plays an important role, while in some countries this option is banned. Composting seems to be quite popular as stabilized and sanitized sludge can be produced due to intense stabilization and process temperatures as high as 70 °C. The application of thermal treatments is also remarkable due to increased banning of landfill disposal and agricultural use. Therefore, it may be concluded that in the large majority of cases, soil is, regardless of the technology or methodology selected, the final destination of vast quantities of treated sludge (WSSTP, 2008). In countries where technology is less developed, direct agricultural application or landfilling are the typical ways for biosolids outlet. In countries where the stakeholders and policy makers have forbidden land application and where the landfill directive implementation into the national legislation is stringent, only high temperature destruction methods are available. In this paper, a new sludge management strategy is presented with specific regard to the energy balance of new sludge processing routes that are compared with conventional ones. This new strategy, sludge separation, is based on different outlets of primary and secondary sludge. In fact, primary sludge is generally more polluted than secondary sludge, especially with regard to hydrophobic organic micropollutants. However, secondary sludge is richer in nitrogen and phosphorus and is therefore more suitable for agricultural use than primary sludge, which should undergo an inertization process.

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Table 1 - Current disposal options in European countries (% of sludge produced)

Landfill sites

Thermal treatments Composting Agricultural utilisation

No agriculture

Other

European Union

18 23 7 45 7

Austria 35 50 15 Bulgaria 100 Some cases Czech Republic

13 <1 50 17 20

Finland 73 3 Flanders 88 12 Germany 3-6 20 (mono-inc.)+17 (other

thermal treatments) 32 25

(landscaping)

Hungary 60 0.8 39 Netherlands 58 (mono-inc. + 27 (drying) 15 Norway 7 65 12 16 Slovenia 30 47 (export to incineration) 15 7 1 U.K. 1.5 19.5 67 5.2 1.8

Table 2 - Final sludge destination (% of sludge produced) in some metropolitan urban areas of the European Union (WSSTP, 2008) Area Agriculture

directly Compost Re-

cultivation Landfill Thermal/

incineration Other

Prague 50 Barcelona 40 22.5 14 6.5 Genoa 18 33 7 42 Lisboa 90 0.5 9 0.5 Manchester 65 1 1 19 14 Oslo 60 5 31 4 Table 3 (Greater Moncton Sewerage Commission, 2008) shows the sludge production per capita in different countries. Table 3 - Per capita sludge production (g/person × d) in different countries 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

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SCHEMATIC OUTLINE Although rich in nutrients, organic substance and microelements, in some countries, sewage sludge is not accepted as agricultural fertilizer due to an awareness of the risks for contamination due to the possible presence of toxic compounds. Stabilization is a preliminary condition, both for sewage sludge agricultural use or landfill disposal. Anaerobic digestion has always been considered the principal process to stabilize sewage sludge, at least in big plants. Sludge separation could lead to more sustainable sludge management. Such a new concept is based on the following considerations: secondary sludge, which contains fewer pollutants than primary sludge, should be segregated and treated separately from primary sludge to increase chances of secondary sludge utilization in agriculture. Linear alkylbenzene sulphonates (LAS) concentration was reported to be 4,000 - 7,000 mg/kg dry solids for primary sludge and of 300 mg/kg for activated sludge due to sorption on particulate material of the raw sewage and aerobic degradation in biological treatment (De Henau et. al., 1989; Siegrist et al., 1989). Due to the low water solubility and lipophylic properties of polycyclic aromatic hydrocarbons (PAHs), these compounds are typically removed from sewage by adsorption onto the particulate material in the primary settling tank (Angelidaki and Ahring, 1999). If such a primary sludge is blended with secondary sludge, it might render the mixed sludge no longer acceptable for agricultural use. Secondary sludge, on the other hand, is much richer in nitrogen and phosphorus than primary sludge (5 – 8 % instead of 1.5 – 5 % of dry solids for nitrogen, and 1.6 - 6.0 % instead of 0.3 - 1.5 % for phosphorus), with consequently greater benefits for crops, which need smaller amounts of sludge to balance nutrient requirements. Sludge separation could also be advantageous from a technological point of view. Primary sludge, in fact, can be thickened, digested and mechanically dewatered much more easily alone than in a mixture with secondary sludge (Kopp and Dichtl, 2001). Moreover, primary sludge treatment can be more easily optimized for energy recovery and for final incineration. COLLECTION OF DATA OF PRIMARY AND SECONDARY SLUDGE QUALITY Three WWTPs in the Rome area were selected for a monitoring campaign, with the aim of assessing the pollution load and the nutrient contents of primary and secondary sludge. This monitoring campaign was specifically designed to verify whether the idea of sludge separation may be a solution to sustain sludge utilisation in the agriculture of biological (secondary) sludge. The three plants are Cobis Cesano, 90,000 Population Equivalent (PE); Fregene, 33,000 - 76,000 PE; and Roma-Nord, 780,000 PE. Wastewater Treatment Plant (WWTP) Description The municipal Roma-Nord WWTP is one of the four wastewater treatment plants serving the city of Rome. The plant is conventional, including screening, primary clarification and secondary treatment by activated sludge. This treatment plant is operated with very high sludge age (20 d). The plant is designed to treat an average WW flow rate of 4.1 m3/s. Both primary and secondary sludges are mixed together and treated in two anaerobic digesters with a total capacity of 18,000 m3.

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https://www.researchgate.net/publication/11563433_Influence_of_the_free_water_content_on_the_dewaterability_of_sewage_sludges?el=1_x_8&enrichId=rgreq-2b3d63ed-1de8-4195-8235-9685291d2909&enrichSource=Y292ZXJQYWdlOzI1NzU2MzE0NztBUzoxMDIwNzQ1NjYwNTM4OTRAMTQwMTM0Nzg4MTM2Mg==

The Cobis Cesano WWTP, which serves seven villages in a countryside area, is designed to treat an average WW flow rate of 0.25 m3/s. It is a conventional plant with primary clarification and secondary treatment by activated sludge. The sludge is aerobically treated and finally dewatered. The Fregene WWTP is located on the sea coast and is therefore subject to a large fluctuation of quantity and quality of treated WW, due to the seasonal resident changes. This plant is designed to operate with a winter organic load of 33,000 PE, treating 0.14 m3/s of WW, and with a summer organic load of 76,000 PE, treating 0.34 m3/s. The plant includes a primary settling and a secondary treatment by activated sludge. The sludge treatment includes a pre-thickening step followed by a mechanical dewatering step (without any biological stabilisation). Sampling and Results of Sludge Quality Sampling was performed on Roma Nord, Cobis and Fregene WWTPs, taking both primary and secondary sludge twice a day for 5 consecutive days to achieve a representative sample. In each sample, 10 L of primary sludge and 25 L of secondary sludge were collected. Figure 1 shows the nutrient concentrations of sludge samples from the three WWTPs. The average nutrient concentrations confirm that secondary sludge is richer in nitrogen (+60 %, +130 % and +170 %) and phosphorus (+85 %, +65 % and +190 %) than primary sludge.

Figure 1 - Nitrogen and phosphorus concentration in primary and secondary sludge

0

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8

9

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P an

d N

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cent

ratio

n (%

of T

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Total PhosphorusTotal Nitogen

P P P P P P P

S S S S S S S SP P P P P P

P

P

S S S S S S S

Roma Nord

Cobis

FregeneP=Primary sludgeS=Secondary sludge

SP

Figure 2 shows the metal content of primary and secondary sludges of Roma Nord, Cobis Cesano and Fregene WWTPs. No difference is found between the heavy metal content of primary and secondary sludge except for As whose concentration is higher in primary sludge

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than in secondary one eight cases out of nine. All values were well under (1 - 3 order of magnitude) the present European limits.

Figure 2 - Metals concentration of primary and secondary sludge (Roma Nord and Cobis)

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10000

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g/kg

dry

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id)

Cd primary Cd secondary Hg primary Hg secondaryAs primary As secondary Ni primary Ni secondaryCr primary Cr secondary Pb primary Pb secondaryCu primary Cu secondary Mn primary Mn secondaryZn primary Zn secondary

Roma Nord Cobis Fregene Analyses of organic micropollutants focused on extractable organic halogens (EOX), total hydrocarbons and methylene blue active substances (MBAS). Figure 3 shows the concentrations of these pollutants in primary and secondary sludge of the three investigated WWTPs. Total hydrocarbons are apolar compounds, which are classified as either aromatic or aliphatic, according to the presence or absence of benzene rings. Hydrocarbons originally derive from various petroleum fractions, and their toxicity increases according to the scale: paraffins, naphthalenes, olephines, and aromatic compounds. Generally, the smallest and most volatile molecules are the most toxic ones. It is more likely that WWTPs located in urban areas receive wastewaters contaminated by hydrocarbons and PAHs due to intense road traffic, petrol stations and micro industrial activities. Moreover, many private citizens change engine oils themselves but do not deliver the oil to authorised companies for their recovery. In many cases these lubricants are poured into the sewage, thus increasing the hydrocarbon loads to the WWTPs. These compounds could then be concentrated into sludge during primary settling. Some microorganisms are able to degrade organic compounds; for instance, in biological processes, such as sludge composting or anaerobic digestion, some PAHs and PCBs were reported to be destroyed by biodegradation (Lazzari et al., 2000; Bernal-Martinez et al., 2005).

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Figure 3 - EOX, MBAS and total hydrocarbon concentration of primary and secondary sludge

0

5.000

10.000

15.000

20.000

25.000

30.000

35.000

40.000

EOX(µg Cl/kg TS)

MBAS(mg/kg TS)

Total hydrocarbonsC10-C40 (mg/kg TS)

Con

cent

ratio

nPRIMARYSECONDARY

Rome Nord

Rome Nord

Rome Nord

Cobis

Cobis

Cobis

Fregene

Fregene

Fregene

Household detergents are the principal source of inorganic and organic chemical substances of domestic origin which are discharged into the environment, normally through wastewater drainage systems. The active matter of detergents is composed of natural and/or synthetic surfactants of different types. Linear alkylbenzene sulphonates (LAS) are the most frequently employed synthetic anionic surfactants (Prats et al., 1997) and typically they represent 70 - 80 % of MBAS in sludge. LAS are one of the most studied pollutants because they are included in the Working Document on Sludge of the European Commission (3rd draft) (European Commission, 2000). These results show that anionic surfactants, which are one of the most studied pollutants affecting sludge utilization in agriculture, are mostly adsorbed in primary sludge. Enrichment of organic micropollutants in primary sludge with respect to secondary sludge seems to increase in the order hydrocarbons < EOX < MBAS, confirming that secondary sludge is less polluted. Consequently, agricultural use of secondary sludge implies a lower pollutant load on land in comparison with mixed sludge, also considering that sludge application rate should be controlled by nutrient presence which is certainly higher in secondary sludge than in mixed sludge. COMPARISON OF SLUDGE PROCESSING SCHEMES The results of the sludge monitoring campaign pointed out that sludge separation might present many advantages over conventional sludge treatment, especially for big plants. In fact, possibilities of sludge recovery for agricultural use could be impaired by the bad quality of primary sludge when the two types of sludge (primary and secondary) are treated together.

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https://www.researchgate.net/publication/222206211_Removal_of_Anionic_and_Nonionic_Surfactants_in_a_Wastewater_Treatment_Plant_with_Anaerobic_Digestion_a_Comparative_Study?el=1_x_8&enrichId=rgreq-2b3d63ed-1de8-4195-8235-9685291d2909&enrichSource=Y292ZXJQYWdlOzI1NzU2MzE0NztBUzoxMDIwNzQ1NjYwNTM4OTRAMTQwMTM0Nzg4MTM2Mg==

Moreover, in the big WWTPs, the following further problems may obstacle use of mixed sludge in agriculture:

large volume of cake requires large extensions of fields and therefore long distances have to be covered from the WWTPs to the site of spreading;

a large storage volume is required when sludge cannot be used (winter periods and when the fields are flooded or covered by snow);

influent WW of large cities are often polluted by non-controlled industrial discharges that might impair agricultural utilization of resulting sludge.

Sludge separation might be the solution to sludge management for the big plants (> 250.000 inhabitants). In fact, primary sludge could be easily treated by a conventional scheme, including gravity thickening, anaerobic digestion, centrifugation and incineration, whereas secondary sludge needs an intensive digestion process to accomplish the requirements of good stabilisation (VS destruction > 40 %). This process can be achieved by sludge dynamic thickening and disintegration (either mechanically by ultrasound or thermally by the Cambi process) before digestion. This new sludge management scheme (Figure 4) was therefore compared to the conventional one (Figure 5) to highlight the main differences in terms of energy and environmental emissions. For the incineration of primary sludge (sludge separation) or mixed sludge (conventional sludge processing), an integrated process was considered (Figure 6). As an alternative to on-site incineration, on-site drying and off-site co-incineration (power plants or cement factories) or off-site incineration (in MSW incineration furnaces) were also considered. Figure 4 - Proposed flow sheet of sludge processing with sludge separation

Primary settling

Biological treatment

Primary sludge

Energy (aeration) Chemicals (foam

breakers?)Effluent for further treatment

WAS

Short aerobic thermophilic treatment

with intermittent feed (5d; 45°C)

Slu

dge

to a

gric

ultu

re

Energy (transport)

nutri

ents

(e

.g. N

, P)

Dewatering

Energy (transport)Drying-Incineration with internal energy recovery

Chemicals, Energy

Energy (heat)

Produced gas

Solid residue

amount, treatment, composition, disposal

amount, composition, disposal(heavy metals, icropollutants)

Dynamic thickening

Energy

Gravity thickening

Anaerobic digestion

amount, composition,

environmental impact

Ultrasound disintegration +

anaerobic digestion

Thermal disintegration +

anaerobic digestion

DewateringOn-site drying Off-site

coincineration

Produced gas

Energy

ChemicalsEnergy

Energy

Energy

A

B

C

D

FI

M

O

JN

P

R

Q

Dewatering

Dewatering

V

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E

H

Fuel

Off gas

K

L

G

ChemicalsEnergy

ChemicalsEnergy

Energy (transport)

Energy (transport)

Produced biogas to energy production


U

W

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Figure 5 - Conventional flow sheet of sludge processing Primarysettling

Biological treatment

Primary sludge

Energy (transport of dryed sludge; 90% DS)

Effluent for further treatment

WAS

Mesophilic anaerobic

digestion (37°C)


amount, composition, disposal

On-site thermal drying-incineration

Off gas

Sludge liquid

ThickeningOff-sitecoincineration

Mechanical dewatering

Off gas

Chemicals,Energy

On-site drying

amount, composition, disposal

or

Energy

Solid residue

amount, treatment, composition, disposal

Slu

dge

liqui

dNo fuel is needed

A

B

C

D

E

F

L

H OP P

G

Off gas

E

NFuel

M

Figure 6 - Flow sheet of an integrated process for sludge incineration

Assessment of performance of the processing units The mass and energy balance for the two flow sheets were fully investigated under the assumptions reported in Table 4.

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Table 4 – Assumptions for mass and energy balance of the flow sheet (Figure 4 and 5) Production Wastewater flow rate: 250 L/(per × d) COD concentration: 480 mg/L i.e. 120 g/(per × d) BOD5 concentration: 240 mg/L i.e. 60 g/(per × d) N concentration: 48 mg/L i.e. 12 g/(per × d) P concentration: 8 mg/L i.e. 2 g/(per × d) COD removal in primary sedimentation: 35 % SS removal in primary sedimentation: ~ 50 % SS concentration of primary sludge: 4 % VS concentration of primary sludge: 75 % of SS Production of secondary sludge according to Monod eq. with the following kinetic constants: Y 0.45, Kd 0.08 d-1, θc 6 d COD removal in secondary treatment: 90 % SS concentration in secondary sludge: 1 % SS in the effluent: 10 mg/L Elemental composition of primary sludge (% of VS): C 54.41 %, H 4.79 %, O 36.27 %, N 3.53 %, S 1 %; (% of SS) K 0.33 %, P 0.7 % Elemental composition of secondary sludge (% of VS): C 54.55 %, H 6.36 %, O 29.09 %, N 10 %; (% of SS) K 0.5 %, P 3.06 % Density of dry solids: 1,500 kg/m3 Density of water: 1,000 kg/m3 Thickening

inputinputoutput SSSSfractionweightSS )301.3()( −= , but not higher than 7%; SSinput (weight fraction) SS concentration of supernatant: 3,000 mg/L (from primary sludge thickening), 1,500 mg/L from secondary sludge thickening, 2,000 mg/L (from mixed sludge thickening)

)(045.2045.0)( fractionweightSSfractionweightSS inputoutput += (dynamic thickening of secondary sludge) Anaerobic digestion

θθd

d

kkfractionweightremovalsolidVolatile+

=1

)(

where kd = 0.06 d-1 for primary sludge, 0.04 for secondary sludge, 0.05 for mixed sludge, 0.048 for secondary sludge after ultrasound disintegration, and 0.064 for secondary sludge after thermal disintegration by CAMBI process. Mass balance according to the following formula by Buswell & Müller (1952):

Molar fraction of water in biogas: 5.504 % H2S concentration in biogas produced from primary and mixed sludge digestion: 600 ppm (in volume) P solubilization: 70 % Decrease of temperature of digester for heat losses: 1 °F/24 h Digestion time: 15 d Sludge temperature: 20 °C Digester temperature: 37 °C Short aerobic thermophilic stabilization

34285

4824483

224724 HCOdNHCOdcbaCHcdbaOHdcba

dNcObHaC +⎟⎠⎞

⎜⎝⎛ −+−+⎟

⎠⎞

⎜⎝⎛ −−+→+−−+

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nTCin

our

KVSVS

)1(1

)20(20 θγ −

°+=

where K20°C=0.025 d-1, γ=1.03, θ=5 d, n=number of reactors in series=1, T=45 °C Mass balance according to the following formula: CaHbOcNd + α O2→a CO2 + β H2O + d NO3

- + d H+

where 4

524 dcba +−+=α , 2

db −=β

Dewatering by centrifuge inout SSSS 00205.0087.0 += where SSout and SSin (weight fraction)

SS concentration of centrate: 3,000 mg/L Polyelectrolyte dosage: 5 g/kg SS Polyelectrolyte adsorbed to sludge particles: 90 % of input poly Polyelectrolyte solution concentration: 3 g/L On-site indirect drying + incineration (integrated process) Heat loss of fluidised bed furnace (FBF): 5 % of total input enthalpy Heat loss of boiler: 7 % of input heat Heat loss of dryer: 7 % of input heat Heat loss of condenser: 5 % of input heat Excess of air for methane combustion: 20 % Operating temperature of FBF: 850 °C Minimum oxygen concentration in exhaust gas from FBF: 6 % by volume SS concentration of dried sludge: 90 % Minimum solid concentration to the dryer to avoid clogging: 60 % Output temperature of exhaust gas from dryer: 100 °C Output temperature of exhaust gas from condenser: 50 °C Water temperature to condenser: 10 °C Input fresh air to indirect dryer: 1.5 kg/kg water to be evaporated Conversion of organic N to NO: 5 % of input N in VS Enthalpy from steam condensation in the indirect dryer: 474 kcal/kg On-site direct drying Concentration of water in the exhaust gas from drying (kg/kg)=0.1 of dry gas (about 14 % by volume) Figures 7 and 8 show the progressive reduction of major components during sludge processing for the two flow sheets shown in Figures 4 and 5, respectively. For the flow sheet with sludge separation, the total mass of primary sludge decreases from 3.97 (fresh primary sludge) to 0.363 kg/m3 WW (dewatered sludge) and to 0.103 kg/m3 WW (dried sludge), or 91 % and 97 % reduction, respectively. Correspondingly, COD and VS are reduced to 52 % with an SS of 42 %. The total mass of secondary sludge decreases from 12.3 to 1.54 kg/m3 WW (dynamic thickened sludge) and to 0.251 - 0.258 kg/m3 WW after thermal or mechanical disintegration, anaerobic digestion and dewatering and to 0.30 kg/m3 WW after short aerobic thermophilic treatment and dewatering, or up to 98%. It is interesting to note that total mass of raw secondary sludge is more than 3 times higher than that of raw primary sludge, whereas the total dry solid of secondary sludge is only 75 – 80 % of that of primary sludge.

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The total mass of mixed sludge for the conventional flow sheet is reduced from 16.24 (fresh mixed sludge) to 0.724 kg/m3 WW (dewatered sludge) and to 0.1795 kg/m3 WW (dried sludge), between 96 % and 99 % reduction. Correspondingly, COD and VS are reduced to 52 % with an SS of 43 %. Figure 7 - Reduction of components for the flow sheet with sludge separation

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e V

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erin

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kg/m

3 WW

COD SS VSS Total mass

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Figure 8 - Reduction of major components for the conventional sludge processing

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L Dried sludge

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The total output of dewatered sludge (primary + secondary) is therefore in the range 0.614 - 0.663 kg/m3 WW, while the corresponding value for mixed sludge is 0.724 kg/m3 WW (up to +18 and +9 %, respectively). Figure 9 shows the net biogas production, i.e., the total minus the requirement for sludge heating and for balancing the digester heat losses, from different types of sludge according to the flow sheets of Figure 5 (primary and secondary sludge) and Figure 6 (mixed sludge). It appears that the total methane production with sludge separation can be estimated at 31.7-34.3 Nm3/m3 WW, or approximately 7.92-8.57 Nm3/(person × d), whereas from mixed sludge, the net methane production is 23.8 Nm3/m3 WW, or approximately 5.96 Nm3/(person × d). This result indicates that with sludge separation, it is possible to gain 33 - 44 % more energy than with conventional process schemes. Figure 9 – Biogas production from primary, secondary and mixed sludge

Biogas

0

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70

L/m

3 WW

CH4 CO2 H2O

CH4CH4

CH4 CH4

CO2

CO2

CO2CO2

H2O

H2O

H2O

H2O

Mixed sludge

Primarysludge

Secondary sludgeafter ultrasounddisintegration

Secondary sludgeafter thermaldisintegration

23.820.8

10.913.5

Figure 10 shows the production of off-gas in thermal treatments of dewatered mixed and primary sludge. From on-site incineration by the integrated process shown in Figure 6, the total production of off-gas is estimated to be 1.12 Nm3/m3 WW (mixed sludge) and 0.598 Nm3/m3 WW (primary sludge), i.e., 5,800 and 3,100 Nm3/h respectively for a plant serving 500,000 inhabitants. An incineration plant of municipal solid waste (MSW) serving the same population would produce instead at least 120,000 Nm3/h of exhaust gas. This result indicates that the incineration of sewage sludge is much less environmentally problematic, considering the negligible exhaust gas production. Moreover, the production of mutagenic compounds like PCDD/F during sludge incineration is not expected, as the chlorine presence in sewage sludge is generally negligible.

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Figure 10 - Production of off gas from thermal treatments

0

1

2

3

4

5

6

7

8

9

Off

gas

prod

uctio

n (N

m3 /m

3 WW

)CO2 H2O N2 O2

1.12

7.54

1.35

0.598

3.63

0.789

On-siteincineration

MS

On-sitedrying

MS

Off-siteincineration

dryed MS

On-siteincineration

PS

On-sitedrying

PS

Off-siteincineration

dryed PS

PS=Primary sludgeMS=Mixed sludge

It is quite surprising that thermal drying by a direct contact dryer produces much more gaseous effluent (7.54 and 3.63 Nm3/m3 WW for mixed and primary sludge, respectively) than on-site incineration. This high flue gas production with drying is due to the great dilution required by fresh air in order to keep the temperature of the inlet gas to the dryer lower than 300 °C to avoid problems of combustion in the dryer and possible explosions due to the presence of fine particulates. Moreover, the humidity of exit gas from the dryer should be kept quite low (generally lower than 0.1 kg H2O/kg of dry gas) to guarantee that a sufficient gradient of water vapor is present in each section of the dryer to perform the operation with smaller equipment. The subsequent incineration of dried sludge in an off-site plant would produce an additional exhaust gas of 1.35 (mixed sludge) and 0.789 Nm3/m3 WW (primary sludge). The total volume of exhaust gas from on-site drying and off-site incineration of dried sludge is therefore 8.9 (mixed sludge) and 4.42 Nm3/m3 WW (primary sludge), equivalent to 46,300 and 23,000 Nm3/h for 500,000 people, respectively (7 - 8 times the volume of exhaust gas from on-site drying and incineration). On-site drying and off-site incineration of dried sludge is clearly not a convenient option, especially considering the methane consumption of 71.7 (mixed sludge) and 34.4 L/m3 WW (primary sludge) that is required for thermal drying. This energy requirement is quite high and cannot be balanced by the methane production in anaerobic digestion with the conventional flow sheet of Figure 5, where the net methane production is only ⅓ of that required for drying. With the flow sheet of Figure 4 with sludge separation, the balance between methane production and consumption (sludge drying) can be nearly achieved. CONCLUSIONS Sludge separation may be a good solution for sludge management for large WWTPs, where many units for sludge thickening, digestion and dewatering are available, thus making

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possible separate processing of primary and secondary sludge. The rationale of this approach is that biological sludge (secondary sludge) is less polluted than primary sludge, at least with reference to hydrocarbons, EOX and MBAS, and is more concentrated with nitrogen and phosphorus. Its agricultural use is therefore much more suitable than that of mixed sludge, as the poor quality of primary sludge can be detrimental. Moreover, the massive use of sludge produced in large plants is certainly a weak point of sludge management due to the large volume requirement for sludge storage during periods when agricultural use is not permitted by atmospheric and relevant field conditions. The big distances to be covered for sludge transportation from the WWTPs to the fields are another weak point.

This new strategy of sludge separation represents the same policy used for urban solid waste management (separate sorting of materials having different properties and compositions to increase opportunities for their recycling). Sludge separation allows:

to reduce 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 considerably reducing both the water content and biodegradable solids;

to limit disposal only to primary sludge. To this purpose, an integrated incineration process can be used, minimizing the exhaust gas production to such low values (for a WWTP serving 500,000 PE 3,100 Nm3/h for primary sludge, compared to 5,800 Nm3/h for mixed sludge) that these types of plants can be considered pilot plants;

to maximize biogas production from a separate digestion of primary and secondary sludge;

to increase flexibility of sludge management considering that sludge outlet is not accomplished by a unique solution.

Moreover, this study highlighted that sludge incineration should be preferentially performed with an on-site plant. The use of external incineration or co-incineration plants is not convenient, if the sludge must be previously dried. In fact, the methane requirement for on-site thermal drying (34.4 and 71.7 L/m3 WW, for primary and mixed sludge respectively), if no other waste heat is available, would hardly be compensated by the biogas production from digestion only with sludge separation. Without sludge separation (conventional flow sheet) energy requirement for sludge drying is much higher than the available energy with biogas. This option is therefore not environmental friendly, especially considering the quite high amount of total gaseous effluent produced in drying and incineration (23,000 - 46,300 Nm3/h for primary and mixed sludge, respectively, for a plant serving 500,000 PE). ACKNOWLEDGMENTS This research was carried out under the Neptune project funded by the European Commission under the Framework Program FP6 (Contract no. 036845). The monitoring campaign of the Italian WWTPs was carried out with the kind assistance and cooperation of ACEA S.p.A., the Roman Municipal Agency for Electricity and Environment.

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Energy Balance in a Novel Approach to Sludge Processing

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