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

Evolution of organic matter from sewage sludge and

garden trimming during composting

Marco Grigatti *,Claudio Ciavatta,Carlo Gessa

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

Received 12March 2003;accepted 4June 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 di?erent piles of compost were studied by means of (i)humi?cation parameters (degree of humi?cation––DH,humi?cation rate––HR,humi?cation 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 signi?cant relationship between composting time,WSOC and humi?cation parameters after enzymatic hydrolysis (DH enz ;HR enz ;HI enz )was found.ó2003Elsevier Ltd.All rights reserved.

Keywords:Compost;Enzymatic hydrolysis;Humi?cation 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 and maturity 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 a nchez-Monedero et al.,2001)producing metabolites which exhibit inhibiting or stimulating e?ects on plant growth (Wong et al.,2001).The OM from sewage sludge and garden plant trimming must be well stabilized be-fore its addition to soil,otherwise it could have a toxic e?ect on plants (Atiyeh et al.,2001).Many di?erent methods have been proposed to substantiate OM qual-ity during the composting process.Nicolardot et al.

(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 sun?ower (Helianthus annuus L.)plants (Baca et al.,1990)have been proposed to evaluate compost maturity;however none of these methods give an absolute https://www.360docs.net/doc/485133506.html,post is a very heterogeneous biomass and the di?erent chemical methods exploited to determine maturity level are only suitable for certain families of materials.Genevini et al.(1992)reported a poor correlation between the degree of humi?cation (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 unspeci?c hydrolytic enzymes (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 humi?cation parameters,enzymatic hydrolysis,WSOC and WSN.

*

Corresponding author.Tel.:+39-51-209-6217;fax:+39-51-209-6203.

E-mail address:marco.grigatti@tin.it (M.Grigatti).0960-8524/$-see front matter ó2003Elsevier Ltd.All rights reserved.

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

Bioresource Technology 91(2004)

163–169

2.Methods

2.1.Sampling and preparation of samples

Two di?erent compost samples(‘‘A’’and‘‘B’’)were taken from a composting plant over a period of160days of composting in a static pile.The blend of raw mate-rials in trial‘‘A’’at the beginning of the composting process was made up of70%(v/v)plant trimming (mowed and pruned)and30%(v/v)sewage sludge of either urban or agro-industrial origin.The raw materials in trial‘‘B’’were made up of60%(v/v)plant trimming and40%(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 at40°C until they reached a constant weight and then sieved to8mm.The fraction<8mm(/)was crushed using a Tecator Cyclotec,1093(PBI),until all the ma-terial passed through a0.25mm sieve.

2.2.Determination of ash content,pH and electrical conductivity

The ash content was determined as follows:5g of dry sample was placed in an oven at650°C until a constant weight was achieved(about6h)(I.P.L.A.,1998).

A suspension of5g of sample and50ml of distilled water was stirred for30min at25°C and,after?ltra-tion,the pH was measured using a Crison pH-meter (Micro TT2022,Spain).

The electrical conductivity of the same solution was then measured using speci?c 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.o?cial 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 2.1and extracted with100ml of NaOH0.1M+Na4P2O70.1M solution,at65°C using a Dubno?shaker bath at110rpm for24h under N2 atmosphere.The suspension was centrifuged at5000·g for30min and then?ltered through a0.8l m?lter and fractionated into humic and humic-like materials and non-humi?ed(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:50mg of extracted organic C(TEC) was placed into a100ml Erlenmeyer?ask.The volume was brought to around20ml with de-ionized H2O and then the solution was acidi?ed drop by drop to pH8 with85%H3PO4.Next,selected enzymes(Merck,Ger-many)were sequentially added to the suspension.The enzymes used were(i)5ml of a solution containing150 l g mlà1of lipase(porcine,130FIP-units mgà1),(ii)10 ml of a solution of15l g mlà1of lysozyme(from hen?s egg white,15,000units mgà1),and(iii)10ml of a so-lution of40l g mlà1of pronase(from Streptomyces griseus,95PUK mgà1).The enzyme solutions were ad-ded to the organic extract sequentially(lipase,followed by lysozyme,and?nally pronase).Each enzymatic treatment with a single enzyme was carried out in a thermostatic shaker bath at37°C and80oscillations per min for90min.Following the three incubations,the suspension was?ltered through a0.45l m Millipore ?lter,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 version2000software programme.

3.Results and discussion

https://www.360docs.net/doc/485133506.html,post characteristics

The main chemical characteristics of the composting trial samples at each speci?c stabilization step are re-ported in Table1.The di?erent 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 di?erent composition of the initial blends of raw materials.The TOC and total Kjeldhal nitrogen(TKN)content decreased during the compost-ing process due to the mineralization of the OM by microorganisms.

164M.Grigatti et al./Bioresource Technology91(2004)163–169

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 other trace elements present in the di?erent trial samples at the end of the process are shown in Table 3.3.2.Extractable and humi?ed organic carbon

The extractable (TEC),humi?ed (HA +FA)and NH C content is detailed in Fig.1.As reported by Ciavatta et al.(1990b,1993)the di?erent trials con?rmed that TOC and TEC decreased predominantly during the ?rst phase of composting due to the intense mineralization process.The NH fraction is lower than the humi?ed fraction in every sample analyzed.The humi?ed C content 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 humi?cation ratio (HR)and the index of humi?cation (HI)of the samples illustrated the evo-lution of the organic fraction (Figs.2–4).The high DH and HR values (Figs.2and 3)at the beginning of the process are due to the interfering substances (mainly proteins and lipids),while more authentic humi?cation parameter values can be obtained after an enzymatic hydrolysis of the extracts (Ciavatta et al.,1990a).

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)EC w (dS m à1)Ash (%)A ?,B ?A ?B ?A ?B ?A ?

B ?

A ?

B ?1067687.417.17 2.5 3.5374221538628.677.45 3.2 4.6395133017457.848.23 4.2 3.6405744016227.438.42 4.5 2.1486157012497.518.47 3.3 1.64766610012537.708.38 2.41.75065713011507.708.12 2.0 1.748658160

11

48

7.70

7.79

2.0 1.9

49

65

A ??Compost A.

B ??Compost B.

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

Table 2

Total content of various nutrients in compost samples at di?erent stages of maturation Sample

TKN (%)P 2O 5(%)K 2O (%)MgO (%)Na (%)Mn (mg kg à1)A ?

B ?A ?B ?A ?B ?

A ?

B ?A ?B ?

A ?

B ?

1 2.2

2 1.87

1.17 1.32 1.220.97 1.20

2.000.12

0.1435825941.56 1.10 1.08 1.220.770.941.81 2.700.120.132472928 1.55

1.01

1.39

1.30

0.83

0.90 1.96

2.56

0.18

0.10261330

A ??Compost A.

B ??Compost B.

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

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

165

3.3.Humi?cation 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?0:35;P?ns). After enzymatic hydrolysis,the DH enz values decreased and showed a more regular increasing trend during composting.The relationship between time and DH enz was statistically signi?cant(R2?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 signi?cant after the enzymatic treatment(R2?0:96;P<0:0005).

The enzymatic treatment was highly e?ective on the HR enz(Fig.3).The relationship between HR and time, before and after the enzymatic treatment,increases from R2?0:01,P?ns to R2?0:83,P<0:05in trial‘‘A’’and from R2?0:16,P?ns to R2?0:86,P<0:01in trial ‘‘B’’.It must be emphasized that the HR enz showed an asymptotic trend after about3months of composting.

Signi?cant di?erences 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 HI enz after the en-

Table3

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?9n.d.287.030,64014321461321293

B?66n.d.337.219,8728001642831298 Cd Cr Hg Ni Pb

A? 3.2<0.535<1<0.5n.d.4685512

B? 1.3<0.552<1<0.5n.d.5754111

A??Compost A.

B??Compost B.

All analyses were carried out in triplicate and relative standard deviations were less than5%.

166M.Grigatti et al./Bioresource Technology91(2004)163–169

zymatic treatment:R2?0:82,P<0:05(Fig.4,trial ‘‘A’’)and R2?0:95,P<0:01(Fig.4,trial‘‘B’’).

The presence of the humic-like substances was sig-ni?cantly 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).

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 was1630 mg là1in trial‘‘A’’and1400mg là1in trial‘‘B’’and after 160days it decreased respectively to38%and16%of the initial values.A good correlation was found between

composting time and WSOC:R2?0:85,P<0:01(Fig. 5,trial‘‘A’’)and R2?0:92,P<0:005(Fig.5,trial‘‘B’’).

The WSOC was then correlated to humi?cation parameters calculated after the enzymatic treatment (DH enz;HR enz;HI enz,Figs.6–8).A signi?cant corre-lation was found between each parameter vs.WSOC in both trial‘‘A’’and‘‘B’’.In particular,the results of the DH enz vs.WSOC showed a correlation of R2?0:95, P<0:0001and R2?0:85,P<0:005in trial‘‘A’’and ‘‘B’’respectively(Fig.6).Plotting HR enz vs.WSOC the following relationships were observed:R2?0:81, P<0:005in trial‘‘A’’and R2?0:92,P<0:0005in trial ‘‘B’’(Fig.7).Finally,plotting HI enz vs.WSOC the re-sulting relationships were R2?0:97,P<0:0005in trial ‘‘A’’and R2?0:88,P<0:001in trial‘‘B’’(Fig.8).

At the same time the WSN decreased from280to180 mg là1and from200to98mg là1in trial‘‘A’’and‘‘B’’respectively.The WSN content showed a good correlation

M.Grigatti et al./Bioresource Technology91(2004)163–169167

with maturation time,but no signi?cant correlation with the humi?cation parameters.These results con?rm 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 to the presence of humic-like substances such as proteins, lipids and intermediate microorganism metabolites.A signi?cant 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(DH enz; HR enz;HI enz)showed a more realistic trend because this treatment removes the humic-like substances.Further-more,the statistically signi?cant relationship found rbetween humi?cation parameters after enzymatic hy-drolysis(DH enz;HR enz;HI enz)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 a e della Ricerca’’(MIUR),60%funds.

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