Tsagarakis, Horan, Mara, Angelakis

Management of Biosolids from Municipal Wastewater Treatment Plants in Greece

by

Konstantinos P. Tsagarakis (1), Dr Nigel J. Horan (1), Prof. Duncan D. Mara (1) and

Dr Andreas N. Angelakis (2)

(1) University of Leeds, (2) Nat. Agric. Research Foundation, Inst. of Iraklio, GR (NAGREF)

University of Leeds, School of Civil Engineering, LS2 9JT, UK

Tel: (44) 113 2332319. Fax: (44) 113 2332243. E-mail cenkt@leeds.ac.uk, kandila@her.forthnet.gr

ABSTRACT

This paper aims to provide an overview of sludge (biosolids) management from all the municipal

wastewater treatment plants (MWTP) in Greece. Attention is given to three topics: treatment, disposal,

and quantities produced. For treatment, seven process streams are identified according to the units

employed (thickening, aerobic-anaerobic digestion, air drying, conditioning and mechanical dewatering).

Most of the MWTP employ thickening and dewatering, while digestion is employed mainly in large

conventional activated sludge plants. Only in a few plants is sludge treatment non-existent or inadequate.

The vast majority of MWTP in Greece dispose to landfills (80%). Other practices for sludge disposal

include within the curtilage (10%), reuse for agriculture (6%) and forestry (4%). Landfilling may have

severe health impacts on groundwater, as only a few have a sanitary design. Statistical data give a

production of 43 g DS per cap. d-1. Using this value, total sludge quantities per disposal site are also

presented on historical, present and future bases.

KEY WORDS

Sludge treatment; sludge disposal; sludge management; biosolids.

INTRODUCTION

The need for a common European policy in wastewater and sludge treatment has been examined under

the EEC 271/91 Directive on urban wastewater treatment and the EEC 278/86 Directive on sewage

sludge in agriculture. Apart from that, there are no other national or EU directives for sludge treatment

and disposal applied in Greece. The first one, under article 12, describes the obligations of member states

for sludge treatment. This states that that sludge arising from wastewater treatment shall be reused

whenever appropriate, and disposal routes shall minimise adverse effects on the environment.

Monitoring and evaluation of the solid products of the MWTP is essential. This need will be addressed in

this paper, which employs a selective presentation of sludge issues resulting from a survey of all Greek

MWTP. This survey aimed to reveal the wastewater treatment status in Greece, including sludge

Tsagarakis, Horan, Mara, Angelakis

treatment and disposal aspects. The main findings of the liquid line treatment have already been presented

in Tsagarakis et al.(1). 241 MWTP have been examined, 209 of which are activated sludge, 5 attached

growth, 24 natural systems, whilst 3 provide primary treatment. The natural systems and MWTP that

have failed are excluded from this study, which deals with 205 of them. A complete state of the art has

been processed through GIS(2), where all MWTP have been located on a map where, sludge treatment and

disposal sites have been inserted among others.

The quantities of sludge produced in Greece accounts for 0.6% of the European total(3). No matter how

small this portion may seem, the sensitive Mediterranean environment stresses the need for effective

management, to avoid irreversible pollution effects.

MAIN UNITS EMPLOYED FOR SLUDGE TREATMENT

Typically thickening, digestion, conditioning and dewatering are employed to treat a surplus sludge with

an average moisture content of 99.1%. Incineration and pasteurisation (disinfection) have not yet been

applied. The major unit options, thickening, digestion, conditioning and dewatering, and composting are

described below as they have been applied for Greece.

THICKENING

Gravity thickening with no chemical addition is mostly employed. Only in two of cases were gravity belt

thickeners reported. Gravity thickening would typically give a moisture content average of 96.4%. Thus,

it results in a 4 increase in solids concentration.

DIGESTION

Aerobic and anaerobic digestion is employed for the stabilisation of thickened and raw sludge. Aerobic

digestion is usually found in medium size installations, from 10,000 to 60,000 population equivalent

(p.e.). It is used for conventional and extended aeration activated sludge systems. Mesophilic anaerobic

digestion is used for medium and large conventional systems (>60,000 p.e.), although a 10,000 p.e.

installation has been found.

CONDITIONING

Chemical conditioning is employed for the improvement of the dewatering characteristics of thickened

and/or digested sludge. Polymer addition, ferric chloride and lime are used. It is only employed prior to

mechanical dewatering.

DEWATERING

Drying beds or mechanical dewatering with belt filter presses are employed. Centrifuges or other

Tsagarakis, Horan, Mara, Angelakis

mechanical devices are not used. Air drying is commonly applied to small installations. They have been

employed though for MWTP up to 67,000 p.e. Climatic conditions make air-drying an effective sludge

treatment as sunshine, high temperatures, dry weather, in most places during the summer period

contribute to the feasibility of drying beds. During this period, the sludge may meet the microbial

requirements for reuse purposes. On the other hand, there should be concern for heavy metal removal. As

it applies mainly to small installations, such quantities may be low enough for agricultural reuse. In 18

cases where belt filter presses were employed, there are drying beds for backup use in case of fault or

repair. Both methods give about the same moisture content average of 79.5%.

COMPOSTING

Composting is in its infancy. The first composting unit has just started operation at Kalamata plant, in

1997. The total capacity is 80 tonnes d-1 and treats municipal wastes with 10-12 percent wastewater

sludge (4).

SLUDGE TREATMENT CASES

Sludge treatment depends on the liquid line process. Based on the basic units of sludge treatment

described, seven cases are available. All cases are described below and depicted in Figure 1. The number

of MWTP that fall in each category, the total p.e. and today’s p.e. (t.p.e.), which reflects today’s capacity,

are presented in Table 1.

Table 1 Sludge Treatment Cases in Relation to the Number of MWTP, p.e. and t.p.e.

MWTP p.e. t.p.e. Sludge treatment case

(no) (%) (no) (%) (no) (%) 1 3 1.6 16,400 0.2 7,500 0.1 2 10 4.9 26,500 0.3 5,280 0.1 3 13 6.3 78,105 1.0 41,300 0.7 4 87 42.4 893,498 11.0 425,200 7.5 5 71 34.6 1,560,560 19.2 383,920 6.8 6 6 2.9 207,000 2.6 172,500 3.1 7 15 7.3 5,335,500 65.7 4,607,000 81.7

Total 205 100.0 8,117,563 100.0 5,642,700 100.0

CASE 1

There is no sludge treatment. Three installations operate under this situation, two very small and one

under expansion. The sludge in this case is wasted untreated. This option should not be considered when

planning a new installation and the MWTP without sludge treatment should be upgraded.

Tsagarakis, Horan, Mara, Angelakis

CASE 2

Sludge receives thickening only, before it is wasted. Ten small installations fall in this category. This is

also an inadequate treatment situation and upgrading of these installations should be considered.

Incorporation of dewatering units is necessary.

CASE 3

Sludge is only dewatered by exposing it for several weeks in drying beds. Handling problems may occur.

An incorporation of thickening units will be proved cost effective, as it will reduce the drying area

considerably.

CASE 4

Thickening and dewatering in drying beds take place. This is the most common option in small

installations. This is employed by 87 of the 205 MWTP examined. The size of these plants is mainly

small, they are located near agricultural areas and the source of wastewater is mostly domestic. Emphasis

should be placed on their performance and sludge quality to examine their individual applicability for

agricultural reuse of the produced sludge.

CASE 5

Thickening, conditioning and dewatering in belt filter presses take place. It is another very popular case

employed by 71 of the MWTP examined. Reuse possibilities should be examined as in case 4.

CASE 6

Thickening, aerobic digestion, conditioning and dewatering in belt filter presses take place. This case is

employed by the conventional activated sludge process and some extended aeration systems with primary

sedimentation. It is a higher energy requiring process than the previous but gives a stabilised sludge. Six

installations operate under this scenario.

CASE 7

Thickening, anaerobic digestion, conditioning and dewatering in belt filter presses take place. This case is

employed from 15 large installations. They operate well, and the sludge is highly stabilised. Energy

recovery is possible from the biogas produced. Only one installation has started producing electrical

energy from it but almost all the other installations falling in this category are in the process of doing so.

Because these installations are treating waste from large metropolitan areas, there is the danger of illegal

discharges of industrial type wastewater into the system. This parameter should be taken into account in

the examination of reuse possibilities.

Tsagarakis, Horan, Mara, Angelakis

QUANTITATIVE ASPECTS

It is essential to know the amount of DS production per p.e. to adequately design the sludge treatment

units. A report from WRc(5) (1994) gives a daily 40 g DS per caput production for Greece. For other

countries, it gives 35 to Portugal, 42 to Spain, 58 to Belgium, 62 to UK, 82 to France, and maximum for

Germany of 111 g per cap. d-1. Hamoda(6) calculates a statistical average from 26 installations in Ontario

and Kuwait of 95 g per cap. d-1 and relates sludge production with common operating parameters. EEA

and ISWA(7) propose an average dry weight per caput production resulting from primary and secondary

treatment of about 90 grams for European countries. Many parameters may influence the dry weight per

caput production. The type of sewage system (combined, separate), qualitative and quantitative

characteristics of the influent, processes employed, and efficiency of treatment are some of them.

Seasonal variations may exist within the same installation. Variations are expected to different countries

as well. Statistical data serving towns of about 5,000-800,000 p.e. gave a weighted average of 43 grams

for Greece with a range of 23-81. These installations operate well and receive normal pollutants. For

primary treatment this is calculated to be about 27 grams.

It is estimated that 55,426 tonnes of dry sludge were produced in 1997. Prediction data reveals that this

will increase to 79,211 tonnes in the year 2000 and 95,156 tonnes in 2005. Further estimation will be

avoided but given the fact that centralised wastewater treatment cannot be provided for more than 86% of

the total permanent population(2), the maximum sludge production is unlikely to exceed 160,000 tonnes

of DS per year, in the close future.

Table 2 Historical and Prediction Data of Quantities and Disposal of Sludge Produced from

MWTP in Greece

Year 2004 2002 2000 1998 1996 Dry weight (tonnes) 92,500 86,875 79,211 68,325 52,137

Within the curtilage 1.3 1.3 1.2 1.2 1.3 Agriculture 1.3 1.3 1.3 1.3 0.2 Composting 2.8 0.9 1.0 1.0 - Landfill 94.2 96.1 96.1 96.2 98.1

Perc

enta

ge p

er

disp

osal

site

(%)

Forestry and amenity 0.4 0.4 0.4 0.4 0.4

SLUDGE DISPOSAL SITES AND ASSESSMENT

The place where sludge is going to be disposed is of major importance. The recorded disposal sites for

sludge are landfill, within the curtilage, agricultural reuse, forestry and amenity applications. Sea disposal

has never been applied. The number of MWTP that fall in each category, the total p.e. and t.p.e. are

presented in Table 3 (1997). Sludge disposal sites, as used in Greece, are described below.

Tsagarakis, Horan, Mara, Angelakis

Table 3 Distribution of sludge in relation to the disposal site and number of sites, p.e. and t.p.e.

MWTP p.e. t.p.e. Disposal site

(no) (%) (no) (%) (no) (%) Landfill 163 79.5 7,832,532 96.4 5,481,680 97.1 Within the curtilage 22 10.7 127,486 1.6 68,100 1.2 Agriculture reuse 12 5.9 110,445 1.4 72,600 1.3 Forestry and amenity reuse 8 3.9 47,100 0.6 20,320 0.4 Total 205 100.0 8,117,563 100.0 5,642,700 100.0

LANDFILL

Sludge disposal to landfill is a common practice in Greece. From 205 MWTP, 163 dispose or are

designed to dispose to landfill. From these, only the three major installations (Psyttalia, Metamorphosi

and Sindos) that serve the two major cities with 4,200,000 p.e. and 4,020,000 t.p.e. which is 54 and 73%

of the total respectively. So, if the quantitative parameter is a priority, it is obvious that very careful

management policies should be applied at these MWTP. However, this does not imply that the remaining

sites should be neglected or underestimated. 97% landfilling is the maximum percentage for Europe that

has a weighted average of 27%(8).

WITHIN THE CURTILAGE

In 22 cases of mostly small installations, disposal is to a place within the plant, overflowing the drying

beds or just beside in an open area. This can also be into deep lined beds, lagoons, or on the nearby area

planned for expansion. This practice is employed because sludge quantities are low and authorities want

to avoid transportation costs. This is expressed as disposal within the curtilage. It is a cheap way of

sludge disposal, but it endangers the soil and causes ground water pollution from leakage when the site is

not lined. As an alternative to this practice, Kim and Smith(9) discuss sludge dewatering in reed beds. In

addition, they found that reed beds can be easily modified from sand-drying beds by planting reeds in the

sand layer and by adding 1-1.5 m freeboard above the sand drying bed side wall to provide long term

sludge storage.

AGRICULTURAL REUSE

Although this is not that popular, interest in land application of sludge has increased in recent years as its

beneficial impact on plants has been more appreciated. Sludge acts as a fertiliser due to its high nitrogen

and phosphorus contents. In addition, there can be beneficial effects due to its organic matter. On the

other hand, it contains great quantities of heavy metals and microbes. Heavy metals are of major concern

as they may enter the animal and human chain. Therefore, agricultural reuse of sludge is controversial. In

12 cases disposal was allowed for agricultural purposes, but it was not officially monitored or approved

of.

A few recent studies indicate the beneficial use of sludge in agriculture. All deal with MWTP in case

Tsagarakis, Horan, Mara, Angelakis

seven. A study(10) has involved an annual monitoring of the composition of sewage sludge produced by

the wastewater treatment plant of the city of Volos. The influence of sewage sludge addition to cotton

and soil properties was also monitored and was studied with a field experiment. The results showed that

sewage sludge quality was good in comparison to the limits imposed by the EEC. Sewage sludge

application to the soil significantly increased cotton yield improving some soil properties. The influence

on soil heavy metal concentration was low. Another study(11) investigated the suitability of sewage sludge

produced in the sewage treatment plant of the city of Larisa for agricultural usage. With a greenhouse

experiment, it was shown that sewage sludge application to the soil at a rate up to 120 tonnes per ha y-1

increased wheat and corn yields without damaging the chemical properties of the soil. The suitability of

reuse of the MWTP of Thessaloniki is examined another study(12). It is concluded that at an application of

10 tonnes per ha y-1 the P additions from sludge was in excess of crop requirements, whereas

supplemental N and K fertilisation would be needed to optimise crop yields. There were low heavy metal

concentrations that could meet the EEC regulations. However, lower application rates than 10 tonnes per

ha y-1 should be applied due to the relatively high Hg content. A recent study(13) has shown that sludge

produced from the largest MWTP of the country (Psyttalia) is not suitable for agricultural reuse due to its

high Zn content.

FORESTRY AND AMENITY APPLICATION

In eight cases disposal occurred in forested land or in recreational places such as parks and gardens; these

are referred as forestry and amenity. In this category, there is no high concern of transfer of heavy metals

to the man, and thus sludge reuse is more feasible. However, the danger of groundwater contamination is

still present.

COST OF SLUDGE TREATMENT AND DISPOSAL

The cost of sludge treatment is difficult to separate from that of the liquid line. Two basic reasons favour

that. Firstly, tendering does not usually regard the processes’ construction separately. Secondly, the

municipality usually owns the tip used for the solid waste disposal and permits the disposal of the sludge

as well. Thus, the only disposal cost is transportation. Sludge treatment cost is mostly referred to per p.e.

or m³ treated, although a few large installations have more detailed records, i.e. per kg BOD5 removed, or

per m³ sludge treated. A recent study by Stentiford and Lasaridi(14) reveals that landfill for organic waste

costs £3/ton compared to £15-40 in UK and £70-130 in Germany. Thus, it is estimated that the low price

of landfill makes any alternatives uneconomic. Priorities in Greece are thus likely to be the provision of

more engineered landfills.

CONCLUSIONS

Sludge treatment in Greece is undertaken under seven basic categories. Fifteen MWTP produce more

Tsagarakis, Horan, Mara, Angelakis

than 80% of the total sludge. These are anaerobically stabilised sludges, that studies have indicated that

could be suitable for agricultural reuse. However, it is produced in major urban areas where demand is

low and additional transportation costs will be required. The majority of the other MWTP employ

thickening and dewatering. Although quantities are small, agricultural land is readily available. Emphasis

should be placed on their performance and sludge quality to examine the individual applicability for

agricultural reuse of the sludge they produce.

Landfilling is the most popular disposal site for Greece. As there is no legislation to encourage reuse or

further treatment, it is going to remain the most popular option. Agricultural and forested land surrounds

the vast majority of installations, and thus sewage sludge reuse can be beneficial because of its nutrient

content. Although case studies have indicated that reuse is feasible, the health risks from microbial and

heavy metal concentrations should always be monitored.

A 43 g dry solids per caput production is proposed after statistical data consideration. According to this

sludge quantity in 1997 was 55,426 tonnes and in years 2000, and 2005 it is predicted to be 79,211 and

95,156 tonnes respectively.

ACKNOWLEDGEMENTS

The survey of all MWTP of Greece was financed from the EU-93 AVI 876 Project.

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Tsagarakis, Horan, Mara, Angelakis

case 1 Sludge disposalRaw sludge

case 2 Thickening

Sludgedisposal

Rawsludge

case 3 Dewatering

(air drying)

Sludgedisposal

Rawsludge

case 4 Thickening Dewatering

(air drying)Sludge

disposalRaw

sludge

case 5

Thickening Dewatering(mechanical)

Sludgedisposal

Rawsludge

ConditioningCH

case 6

ThickeningDigestion(aerobic)

Dewatering(mechanical)

Sludgedisposal

Rawsludge

Conditioning

CH

case 7

Thickening Thickening(optional)

Digestion(anaerobic) Dewatering

(mechanical)Sludge

disposalRaw

sludge

Biogas ConditioningCH

Figure 1 Sludge Treatment Line for Cases 1-7