Evolution of organic matter from sewage sludge and garden trimming during composting

transcript

Bioresource Technology 91 (2004) 163–169

Evolution of organic matter from sewage sludge andgarden trimming during composting

Marco Grigatti *, Claudio Ciavatta, Carlo Gessa

Department of Agro-Environmental Sciences and Technologies, University of Bologna, Viale Fanin no. 40, 40127 Bologna, Italy

Received 12 March 2003; accepted 4 June 2003

Abstract

To use compost appropriately in agriculture it is extremely important to estimate the stabilization level of the organic matter. In

this work, two different piles of compost were studied by means of (i) humification parameters (degree of humification––DH,

humification rate––HR, humification index––HI) prior to and after enzymatic hydrolysis of the extracted organic carbon, (ii) water-

soluble organic carbon (WSOC) and (iii) water-soluble nitrogen. A significant relationship between composting time, WSOC and

humification parameters after enzymatic hydrolysis (DHenz; HRenz; HIenz) was found.

Keywords: Compost; Enzymatic hydrolysis; Humification parameters; Water-soluble organic carbon; Water-soluble nitrogen

1. Introduction

The considerable amount of organic matter (OM)and the appreciable concentration of mineral nutrients

in sewage sludge and garden trimming provide an ex-

cellent source of plant nutrition when applied to soil

after an appropriate composting period (Terman and

Mays, 1973; De Haan, 1981; Chen et al., 1996; Zach

et al., 1999). The agronomical quality of compost pro-

duced from these biosolids is limited mainly by their

chemical composition as well as by the stability andmaturity of the OM (Iglesias-Jimenez and Perez-Garcia,

1992; Govi et al., 1993; Chefetz et al., 1996; Bernal et al.,

1998; Marchiol et al., 1999). Organic components of

compost undergo several important transformations

during the composting process, (Tiquia and Tam, 2000;

S�aanchez-Monedero et al., 2001) producing metabolites

which exhibit inhibiting or stimulating effects on plant

growth (Wong et al., 2001). The OM from sewage sludgeand garden plant trimming must be well stabilized be-

fore its addition to soil, otherwise it could have a toxic

effect on plants (Atiyeh et al., 2001). Many different

methods have been proposed to substantiate OM qual-

ity during the composting process. Nicolardot et al.

* Corresponding author. Tel.: +39-51-209-6217; fax: +39-51-209-

E-mail address: marco.grigatti@tin.it (M. Grigatti).

doi:10.1016/S0960-8524(03)00170-6

(1982) proposed monitoring the respiration intensity

during composting, while Nappi (1989) attempted to

correlate the degree of stabilization with nitrogen min-eralization. A number of bioassay methods on cress

(Lepidium sativum L.) seedling growth (Zucconi and de

Bertoldi, 1987), cress seed germination and physiologi-

cal parameters of sunflower (Helianthus annuus L.)

plants (Baca et al., 1990) have been proposed to evaluate

compost maturity; however none of these methods give

an absolute parameter. Compost is a very heterogeneous

biomass and the different chemical methods exploited todetermine maturity level are only suitable for certain

families of materials. Genevini et al. (1992) reported a

poor correlation between the degree of humification

(DH), compost protein and hemicellulose rich materials

due to the formation of humic-like molecules (Ciavatta

et al., 1990a). The relationship between composting time

and maturation level can be increased by treating an

organic extract with a sequence of unspecific hydrolyticenzymes (Ciavatta et al., 1990a). This enzymatic treat-

ment, together with the amount of water-soluble organic

carbon (WSOC) and water-soluble nitrogen (WSN),

provide a more thorough understanding of the compo-

sting process and compost stability.

The aim of this work was to follow the evolution of the

OM from sewage sludge and yard trimming during the

composting of two piles of compost, using humificationparameters, enzymatic hydrolysis, WSOC and WSN.

164 M. Grigatti et al. / Bioresource Technology 91 (2004) 163–169

2. Methods

2.1. Sampling and preparation of samples

Two different compost samples (‘‘A’’ and ‘‘B’’) were

taken from a composting plant over a period of 160 days

of composting in a static pile. The blend of raw mate-

rials in trial ‘‘A’’ at the beginning of the compostingprocess was made up of 70% (v/v) plant trimming

(mowed and pruned) and 30% (v/v) sewage sludge of

either urban or agro-industrial origin. The raw materials

in trial ‘‘B’’ were made up of 60% (v/v) plant trimming

and 40% (v/v) urban and agro-industrial sewage sludge

mixture. Samples were obtained following the procedure

suggested by I.P.L.A. (1998), dried in an air-forced oven

at 40 �C until they reached a constant weight and thensieved to 8 mm. The fraction <8 mm (/) was crushed

using a Tecator Cyclotec, 1093 (PBI), until all the ma-

terial passed through a 0.25 mm sieve.

2.2. Determination of ash content, pH and electrical

conductivity

The ash content was determined as follows: 5 g of dry

sample was placed in an oven at 650 �C until a constant

weight was achieved (about 6 h) (I.P.L.A., 1998).

A suspension of 5 g of sample and 50 ml of distilled

water was stirred for 30 min at 25 �C and, after filtra-tion, the pH was measured using a Crison pH-meter

(Micro TT 2022, Spain).

The electrical conductivity of the same solution was

then measured using specific equipment (Halosis SAT

type, Italy).

2.3. Determination of nutrients and heavy metals

The nutrient and heavy metal content was determined

in accordance with Italian and E.C. official chemical

methods for fertilisers (M.A.F., 1986).

2.4. Extraction and fractionation of the organic carbon

Two grams of compost sample was prepared as re-ported in Section 2.1 and extracted with 100 ml of

NaOH 0.1 M+Na4P2O7 0.1 M solution, at 65 �C using

a Dubnoff shaker bath at 110 rpm for 24 h under N2

atmosphere. The suspension was centrifuged at 5000 · gfor 30 min and then filtered through a 0.8 lm filter and

fractionated into humic and humic-like materials and

non-humified (NH) organic carbon (M.A.F., 1986;

Ciavatta et al., 1990b). The total organic carbon (TOC)in the solid and liquid samples was measured using the

methods suggested by Ciavatta et al. (1989, 1991), re-

spectively.

2.5. Enzymatic hydrolysis

Enzymatic hydrolysis of the liquid samples was car-

ried out as follows: 50 mg of extracted organic C (TEC)

was placed into a 100 ml Erlenmeyer flask. The volume

was brought to around 20 ml with de-ionized H2O and

then the solution was acidified drop by drop to pH 8

with 85% H3PO4. Next, selected enzymes (Merck, Ger-many) were sequentially added to the suspension. The

enzymes used were (i) 5 ml of a solution containing 150

lgml�1 of lipase (porcine, 130 FIP-units mg�1), (ii) 10

ml of a solution of 15 lgml�1 of lysozyme (from hen�segg white, 15,000 units mg�1), and (iii) 10 ml of a so-

lution of 40 lgml�1 of pronase (from Streptomyces

griseus, 95 PUK mg�1). The enzyme solutions were ad-

ded to the organic extract sequentially (lipase, followedby lysozyme, and finally pronase). Each enzymatic

treatment with a single enzyme was carried out in a

thermostatic shaker bath at 37 �C and 80 oscillations per

min for 90 min. Following the three incubations, the

suspension was filtered through a 0.45 lm Millipore

filter, and then fractionated following the procedure

suggested by Ciavatta et al. (1990a).

2.6. Water-soluble organic carbon and nitrogen

WSOC and WSN were extracted according to Chen

et al. (1998). The determination of TOC in solution was

carried out by dichromate oxidation (Ciavatta et al.,

1991), while the total nitrogen in solution was deter-

mined by acid digestion followed by automatic titration

(Kjeldhal) (M.A.F., 1986).

2.7. Statistical analysis

Statistical analysis was performed using a Sigma Plot

version 2000 software programme.

3. Results and discussion

3.1. Compost characteristics

The main chemical characteristics of the composting

trial samples at each specific stabilization step are re-ported in Table 1. The different moisture contents were

due to temperature changes throughout the year period.

Trial ‘‘A’’ was conducted throughout the spring–sum-

mer season and trial ‘‘B’’ in wintertime. As can be seen

in Fig. 1, it is possible to appreciate that the lower TOC

content of sample ‘‘B’’ compared to sample ‘‘A’’ is de-

pendent upon the different composition of the initial

blends of raw materials. The TOC and total Kjeldhalnitrogen (TKN) content decreased during the compost-

ing process due to the mineralization of the OM by

microorganisms.

Table 1

Moisture, pH, electrical conductivity and ash content in compost samples during the composting process

Sample Days of

composting

Moisture (%) pH (dil. 1:5) ECw (dSm�1) Ash (%)

A�, B� A� B� A� B� A� B� A� B�

1 0 67 68 7.41 7.17 2.5 3.5 37 42

2 15 38 62 8.67 7.45 3.2 4.6 39 51

3 30 17 45 7.84 8.23 4.2 3.6 40 57

4 40 16 22 7.43 8.42 4.5 2.1 48 61

5 70 12 49 7.51 8.47 3.3 1.6 47 66

6 100 12 53 7.70 8.38 2.4 1.7 50 65

7 130 11 50 7.70 8.12 2.0 1.7 48 65

8 160 11 48 7.70 7.79 2.0 1.9 49 65

A� ¼Compost A.

B� ¼Compost B.

All analyses were carried out in triplicate and relative standard deviations were less than 5%.

0 15 30 40 70 100 130 160Days of composting - Trial A

TOCTECHA+FANH

0 15 30 40 70 100 130 160

Days of composting - Trial B

TOC TECHA+FANH

Fig. 1. TOC, extracted (TEC), humified (HA+FA) and NH carbon in

samples of compost in trials A and B.

Table 2

Total content of various nutrients in compost samples at different stages of

Sample TKN (%) P2O5 (%) K2O (%)

A� B� A� B� A� B�

1 2.22 1.87 1.17 1.32 1.22 0.97

4 1.56 1.10 1.08 1.22 0.77 0.94

8 1.55 1.01 1.39 1.30 0.83 0.90

A� ¼Compost A.

B� ¼Compost B.

All analyses were carried out in triplicate and relative standard deviations w

M. Grigatti et al. / Bioresource Technology 91 (2004) 163–169 165

The data for TKN, phosphorus, potassium, magne-

sium, manganese and sodium (Table 2), was within the

normal range for these nutrients (Korboulewsky et al.,

2002). Total DTPA, extractable heavy metals and othertrace elements present in the different trial samples at the

end of the process are shown in Table 3.

3.2. Extractable and humified organic carbon

The extractable (TEC), humified (HA+FA) and NH

C content is detailed in Fig. 1. As reported by Ciavatta

et al. (1990b, 1993) the different trials confirmed that

TOC and TEC decreased predominantly during the first

phase of composting due to the intense mineralization

process. The NH fraction is lower than the humified

fraction in every sample analyzed. The humified Ccontent was overestimated particularly at the beginning

of the composting process, because a part of the NH

material interferes with the fractionation of humic sub-

stances (Ciavatta et al., 1990a).

The DH, the humification ratio (HR) and the index

of humification (HI) of the samples illustrated the evo-

lution of the organic fraction (Figs. 2–4). The high DH

and HR values (Figs. 2 and 3) at the beginning of theprocess are due to the interfering substances (mainly

proteins and lipids), while more authentic humification

parameter values can be obtained after an enzymatic

hydrolysis of the extracts (Ciavatta et al., 1990a).

maturation

MgO (%) Na (%) Mn (mgkg�1)

A� B� A� B� A� B�

1.20 2.00 0.12 0.14 358 259

1.81 2.70 0.12 0.13 247 292

1.96 2.56 0.18 0.10 261 330

ere less than 5%.

Table 3

Total and DTPA extractable heavy metals and some trace elements in samples of compost at the end of the composting period

Sample B Co Fe Cu Zn

Total DTPA Total DTPA Total DTPA Total DTPA Total DTPA

A� 9 n.d. 28 7.0 30,640 1432 146 13 212 93

B� 66 n.d. 33 7.2 19,872 800 164 28 312 98

Cd Cr Hg Ni Pb

A� 3.2 <0.5 35 <1 <0.5 n.d. 46 8 55 12

B� 1.3 <0.5 52 <1 <0.5 n.d. 57 5 41 11

A� ¼Compost A.

B� ¼Compost B.

All analyses were carried out in triplicate and relative standard deviations were less than 5%.

y = -0.0009x2 + 0.1732x + 65.089

R2 = 0.35; P = ns

y = -0.0023x 2+ 0.5144x + 44.469

R2 = 0.90; P<0.005

0 40 80 120 160

Days of composting - Trial A

y = -0.0009x2 + 0.1234x + 68.191

R2 = 0.30; P = ns

y = -0.0006x2 + 0.2289x + 40.809

R2 = 0.96; P<0.0005

0 40 80 120 160

DHDHenz

Fig. 2. Relationship between DH, degree of humification after enzy-

matic hydrolysis (DHenz) and days of composting in samples of com-

post of trials A and B, ns¼not significant.

y = 1E-04x2 - 0.0128x + 31.817

R2 = 0.01; P = ns

y = -0.0006x2 + 0.162x + 21.686

R2 = 0.83; P<0.05

0 40 80 120 160

HRHRenz

y = 0.0004x2 - 0.0737x + 34.4

R2 = 0.16; P = ns

y = 0.0001x2 + 0.0423x + 21.033

R2 = 0.86; P<0.01

0 40 80 120 160

HRHRenz

Fig. 3. Relationship between HR, humification ratio after enzymatic

hydrolysis (HRenz) and days of composting in samples of compost of

trials A and B, ns¼not significant.

3.3. Humification parameters after enzymatic hydrolysis

Continuously lower DH values were obtained after

the enzymatic hydrolysis of the TEC of compost sam-

ples when compared to the untreated samples (Fig. 2).

In trial ‘‘A’’, prior to enzymatic hydrolysis, the DH

showed an irregular trend with a negligible relationship

between composting time and DH (R2 ¼ 0:35; P ¼ ns).After enzymatic hydrolysis, the DHenz values decreased

and showed a more regular increasing trend during

composting. The relationship between time and DHenz

was statistically significant (R2 ¼ 0:90; P < 0:005) (Fig.2). In trial ‘‘B’’, the DH trend, both before and after

enzymatic hydrolysis, was very similar to that observed

in trial ‘‘A’’ (Fig. 2). The relationship between time and

DH was statistically significant after the enzymatic

treatment (R2 ¼ 0:96; P < 0:0005).The enzymatic treatment was highly effective on the

HRenz (Fig. 3). The relationship between HR and time,

before and after the enzymatic treatment, increases from

R2 ¼ 0:01, P ¼ ns to R2 ¼ 0:83, P < 0:05 in trial ‘‘A’’ and

from R2 ¼ 0:16, P ¼ ns to R2 ¼ 0:86, P < 0:01 in trial‘‘B’’. It must be emphasized that the HRenz showed an

asymptotic trend after about 3 months of composting.

Significant differences in the relationship between HI

calculated before and after the enzymatic treatment were

observed (Fig. 4). The statistical parameters showed a

good relationship between time and HIenz after the en-

y = 9E-05x2 - 0.0183x + 1.2699

R2 = 0.82; P<0.05

y = 2E-05x2 - 0.0041x + 0.553R2 = 0.39; P = ns0

0 40 80 120 160

HIHIenz

y = 3E-05x2 - 0.0104x + 1.4221

R2 = 0.95; P<0.001

y = 9E-06x2 - 0.0014x + 0.4982

R2 = 0.23; P = ns0

0 40 80 120 160

HIHIenz

Fig. 4. Relationship between HI, humification index after enzymatic

hydrolysis (HIenz) and days of composting in samples of compost of

trials A and B, ns¼ not significant.

y = 0.0741x2 - 16.33x + 1421.3

R2 = 0.85; P<0.01

y = 0.0694x2 - 17.218x + 1277.6

R2 = 0.92; P<0.0050

0 40 80 120 160

Days of composting

WSOC A

WSOC B

Fig. 5. Relationship between WSOC in samples of compost and

composting time of trials A and B.

y = -30.635x + 2778.3

R2 = 0.95; P<0.0001

y = -49,694x + 3190,5

R2 = 0,85; P<0.0050

35 45 55 65 75

DHenz (%)

Trial ATrial B

Fig. 6. Relationship between DHenz and WSOC in samples of compost

during composting process of trials A and B.

y = -73.44x + 2921.6

R2 = 0.81; P<0.005

zymatic treatment: R2 ¼ 0:82, P < 0:05 (Fig. 4, trial

‘‘A’’) and R2 ¼ 0:95, P < 0:01 (Fig. 4, trial ‘‘B’’).

The presence of the humic-like substances was sig-

nificantly higher at the beginning of the composting

process. These substances were used by the micro-

organisms both as an energy source and for synthesis of

humic substances (Ciavatta et al., 1990a; Adani et al.,

1995, 1997).

y = -76.076x + 2437.1

R2 = 0.92; P<0.00050

10 15 20 25 30 35

HRenz (%)

Trial A

Trial B

Fig. 7. Relationship between HRenz and WSOC in samples of compost

during composting process of trials A and B.

3.4. Total water-soluble organic carbon and nitrogen

The concentration of WSOC in the water extract

progressively decreased with maturation time in both

cases (Fig. 5). These results are in agreement with those

previously reported by Chen et al. (1998). At the be-

ginning of composting, the WSOC content was 1630

mg l�1 in trial ‘‘A’’ and 1400 mg l�1 in trial ‘‘B’’ and after

160 days it decreased respectively to 38% and 16% of the

initial values. A good correlation was found betweencomposting time and WSOC: R2 ¼ 0:85, P < 0:01 (Fig.

5, trial ‘‘A’’) and R2 ¼ 0:92, P < 0:005 (Fig. 5, trial ‘‘B’’).The WSOC was then correlated to humification

parameters calculated after the enzymatic treatment

(DHenz; HRenz; HIenz, Figs. 6–8). A significant corre-

lation was found between each parameter vs. WSOC in

both trial ‘‘A’’ and ‘‘B’’. In particular, the results of the

DHenz vs. WSOC showed a correlation of R2 ¼ 0:95,P < 0:0001 and R2 ¼ 0:85, P < 0:005 in trial ‘‘A’’ and

‘‘B’’ respectively (Fig. 6). Plotting HRenz vs. WSOC the

following relationships were observed: R2 ¼ 0:81,P < 0:005 in trial ‘‘A’’ and R2 ¼ 0:92, P < 0:0005 in trial

‘‘B’’ (Fig. 7). Finally, plotting HIenz vs. WSOC the re-

sulting relationships were R2 ¼ 0:97, P < 0:0005 in trial

‘‘A’’ and R2 ¼ 0:88, P < 0:001 in trial ‘‘B’’ (Fig. 8).

At the same time the WSN decreased from 280 to 180

mg l�1 and from 200 to 98 mg l�1 in trial ‘‘A’’ and ‘‘B’’respectively. TheWSN content showed a good correlation

y = 892.4x + 275.53

R2 = 0.97; P<0.0005

y = 1279.7x - 621.46

R2 = 0.88; P<0,001

0 0.6 1.2 1.8

Trial ATrial B

Fig. 8. Relationship between HIenz and WSOC in samples of compost

during composting process trials A and B.

with maturation time, but no significant correlation with

the humification parameters. These results confirm that

the microbial biomass subtracts nutrients, including sol-

uble N, for their metabolism during composting.

4. Conclusion

Composting is a biological, aerobic process in which

microorganisms utilize OM for their metabolism. The

parameters usually used to determine the maturity of

compost (DH, HR, HI) are often overestimated due tothe presence of humic-like substances such as proteins,

lipids and intermediate microorganism metabolites. A

significant relationship between composting time and

WSOC was found. Enzymatic treatment of the whole

extract produced very interesting results. The DH, HR

and HI calculated after enzymatic hydrolysis (DHenz;

HRenz; HIenz) showed a more realistic trend because this

treatment removes the humic-like substances. Further-more, the statistically significant relationship found

rbetween humification parameters after enzymatic hy-

drolysis (DHenz; HRenz; HIenz) and WSOC demonstrates

that is possible to monitor the composting maturation

process more easily and rapidly avoiding longer and

more expensive analytical procedures.

Acknowledgements

This research was supported with funds provided by

(i) ‘‘Ispettorato Centrale Repressioni Frodi’’ of the

Italian Ministry of Agricultural and Forestry Policies

(MiPAF) and (ii) Italian Ministry of ‘‘Universit�aa e della

Ricerca’’ (MIUR), 60% funds.

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