Soil Sci Plant Nutr., 36 (2), 243-250, 1990 243

Color Changes of Organic Wastes during Composting and Maturation Processes

Carlos Garc~a, Teresa Hern~.ndez, and Francisco Costa

Centro de Edafologia y Biologfa Aplicada de/ Segura, Apartado 195, 30080-MURCIA, Spain

Received December 15, 1988

Color changes of different mixtures of sewage sludge or city refuse during the compost ing and ma tu ra t ion processes were inves t iga ted by applying the method of measurement fo r the color of the mater ia ls based on the CIE 1931 S tandard Colorimetric System. Optical density changes of the alkaline ex t rac t s f rom the samples dur ing these processes were also determined. The stimulus value Y and Munsell no ta t ion value decreased rapidly dur ing the first 65 days of piling and then reached a cons tant value. While o rgan ic wastes were ro t t i n g and matur ing , the stimulus value Y and the WSC/N ra t io equally decreased. A positive cor re la t ion between the stimulus value Y and WSC/N ra t io with a cor re la t ion coefficient of 0,631 (p-0 .001) was observed. Corre la t ion coefficients between the stimulus value Y and C/N ra t io were no t significant. Optical density of the alkaline ex t rac t s f rom the different samples increased during the piling period and remained near ly cons tan t during ma tu ra t i on process.

I t was concluded t h a t the cons tant values of absorbance /ExC indicate a good matur i ty degree of the composts.

Key Words: color changes, degree of matur i ty , optical density, stimulus value IT.

Although the addition of organic residues into the soil increases its organic matter content, it may have a negative influence on the yield of the crops. Apart from toxicity problems originating from the accumulation of heavy metals in the soil (Heckman et al. 1987) or the increase of soil salinity when organic residues are added at high rates (Juste 1980), one of the principal causes of adverse effects arising from the disposal of compost on soil is derived mainly from their low degree of maturity and from the presence of non- stabilized organic constituents. When composts are applied to the soil before complete maturity they have a negative effect on the crops because soil microorganisms and plants compete for the nitrogen. Also extremely anaerobical conditions are often created and plant growth is retarded.

Several works have been carried out to determine whether the degree of maturity of the compost is sufficient for the compost to be harmless to the crops. The aim of the composting and maturation processes is to eliminate the phytotoxic substances of the raw materials which are harmful to the germination and to the growth of the plants (Kimber 1973).

All kinds of physical and chemical parameters have been described in the literature as indexes of the degree of maturity of the compost. In this report, the changes in the degree of lightness of different organic residues were investigated because these residues become darker

244 C. GARC[A, T. HERN,~NDEZ, and F. COSTA

during the composting and maturation processes (Sugahara et al. 1979, 1984). The correla- tions between the stimulus value Y (the degree of lightness) and other parameters which are considered to be suitable as indexes of the degree of maturity were also investigated. Optical density change of the alkaline extracts from the samples during this process was investigated and the correlation between the absorbance/extractable C (ExC) ratio and water-soluble C / N ratio were also determined.

MATERIALS AND METHODS

M a t e r i a l s . Seven mixtures from four organic residues: city refuse (CR), aerobic sewage sludge (AS), peat residue (PR), and grape debris (GD) were used in this study. The mixtures composted were: AS-GD 3/1 (A); AS-GD 1/1 (B); AS-PR 1/1 (C); AS-CR l/1 (D); CR-GD I/1 (E); CR-PR 1/1 (F); and CR (G). The ratios indicate the proport ion of each material in the mixture on the basis of their contents of organic carbon.

Composts were produced in a pilot composting plant. Approximately 2 m a of each mixture was moistened, piled in an outdoor facility (De Bertoldi et al. 1985) and thereafter turned and sampled every 13 days during 3 months, in controlling the temperature, humidity and aeration. Then the composts were allowed to mature during 4 months.

D e t e r m i n a t i o n of s t imulus va lue Y and c h r o m a t i c i t y c o o r d i n a t e s (x, y). Color change during the composting process was investigated by applying the method of measure- ment of the color of materials based on a 2 degree visual field X Y Z system (the CIE 1931 Standard colorimetric system, Smith and Guild 1931) using a Hunterlab D 25 A-2 color analyzer. In this system, the color is marked, as a rule, in accordance with the stimulus value Y and chromaticity coordinates (x, y). The latter are calculated fl-om the tristimulus values X, Y, Z according to the following equations:

x=X/ (X+ Y+Z), ),= r l ( X + Y+Z) .

Specification of colors according to their three attributes, value, hue, and chroma was obtained by Munsell notation. Munsell notation value was obtained fi'om the stimulus value Y, and Munsell notation hue and chroma were obtained from chromaticity coordinates (x, y).

D e t e r m i n a t i o n of c a r b o n and n i t r o g e n con ten t s . Total carbon (TC) was deter- mined by a Carmhograph 12 carbon analyzer. Extractable carbon (ExC) with 0.1 M sodium pyrophosphate at pH 9.8 (sample to solvent ratio of 1/10 w/v) and extractable carbon with water (WSC) (sample to solvent ratio of 1/10 w/v) were measured on a Beckman model 915 B carbon analyzer.

Total nitrogen (TN) was determined by the Kjeldahl method. Optical densities of the pyrophosphate extracts were measured at 450 nm on a Spectronic 1001 spectrophotometer (Morel 1982).

RESULTS AND DISCUSSION

The contents of the different forms of carbon and total nitrogen of the samples at five dif- ferent steps of the compost ing process and after the maturation period are shown in Table I.

It can be observed that the amounts of the different carbon fractions decreased in all the samples during the compost ing process as well as during the maturat ion period with the

Table 1.

Color Change during Organic Wastes Composting

Carbon and nitrogen contents of the composts at five different steps of the composting process and after maturation (% dry wt,),

245

Piling period (days)

Compost samples

A B C D E F G

ExC 1 8.15 6.05 6.97 7.88 5.47 5.76 7.45 13 7,14 5.63 6.80 6,89 5.08 6.41 6.23 39 4.97 4.15 6.08 5.43 3.85 5,80 6,47 65 4,04 4,20 5.29 3.93 2.73 4.81 3.61 91 3.16 3.56 5.63 2.97 2,83 3.62 2.78

210 3.58 4.20 5.81 2.91 3.83 4.84 2.99

TC 1 37.00 39.30 22.20 27. I 0 37.30 18.40 22.30 13 34.50 37.50 20.70 22.10 30,80 18.30 20.80 39 34.30 35.00 19.40 21.40 25.00 15.30 21.30 65 30.80 32,70 18.20 14.60 31.40 13.10 12.60 91 30.30 33.50 18.57 15.90 27.50 15.50 13.90

210 29.90 26.00 18.70 12.90 15.30 12,90 12.90

WSC I 4.56 3.51 3.12 4.12 2.25 1.27 2.76 13 2.87 2.54 2,10 2.50 1.72 1.32 2,16 39 1.98 1,59 1,40 2.24 1.26 0,96 2,21 65 1.04 1.30 0,79 1,26 1.09 0.51 0.66 91 0.95 1.19 0,74 0.84 0.83 0.41 0.72

210 0.34 0,47 0,30 0.24 0.61 0.24 0.33

TN I 4.32 3.29 2. I 0 2.25 I. 10 I. 14 1.45 13 3.42 2.47 1,89 2.05 1.27 1,01 1.31 39 2.80 2.26 1.57 1,86 1.03 1.03 1.01 65 3.60 2.78 1.66 1.55 1.49 0.96 1.12 91 2,09 2.59 1,28 1.34 1.30 0.98 1.01

210 2.32 2.48 1,31 1.19 1.29 0.95 1.05

C/N I 8,53 12,00 10.60 12.10 33.90 16,10 15.40 13 10.08 15,20 10.90 10.80 24,20 18.10 15.90 39 12.30 15.50 12.40 11.50 24.30 14,80 21.10 65 8.55 11.76 10.90 9.42 21.00 13,60 11.25 91 14.50 13.00 14.50 11.90 21.00 15.80 13.70

210 12.90 10.50 14.20 10.90 11,80 13.60 12.30

WSC/N I 1.05 1.07 1,48 1.83 2.04 1,11 1,90 13 0.84 1.03 I. 1 I 1.21 1.35 1.30 1.64 39 0.70 0,70 0.81 1.20 1.22 0,93 2.18 65 0.29 0.46 0.47 0.81 0.73 0.53 0.59 91 0.45 0.46 0.57 0,62 0.63 0.41 0.71

210 0.15 0.20 0.23 0.20 0.47 0.25 0.31

e x c e p t i o n o f the a m o u n t o f ex t r ac t ab l e c a r b o n wh ich inc reased l ight ly . T h i s p h e n o m e n o n

suggests tha t h u m i f i c a t i o n p r o d u c t s are resyn thes ized d u r i n g this pe r iod . It s h o u l d be no t ed

tha t the f r ac t ion o f w a t e r - s o l u b l e c a rbon , wh ich is the mos t eas i ly a t t acked by the m i c r o -

o rgan i sms , s h o w e d the h ighes t decreases in all the samples . T h e in tens i ty o f this decrease was

d i f fe ren t d e p e n d i n g on the na tu re o f the o rgan i c c a r b o n c o m p o n e n t s o f the d i f fe ren t

ma te r i a l s s tud ied . T h e r e f o r e the va lues o f this f r ac t ion m a y be used as i n d e x of the degree

o f m a t u r i t y o f the c o m p o s t . In a p r ev ious s tudy ( G a r c f a et al, 1987) we d e m o n s t r a t e d tha t

the W S C / N ra t io was a g o o d index o f the degree o f m a t u r i t y whereas the C / N ra t io , w ide ly

246 C. GARCfA, T. HERNANDEZ, and F. COSTA

Table 2. Tristimulus values, X, Y, Z, chromaticity coordinates (x, 3'), and MunseI1 notation values (v) of compost sampIes.

Piling Compost period X(%) Y(%) Z(7";) x y v

(days)

A 1 7.3 6.9 6.2 0.357 0.338 3.08 13 6.1 6.0 5.5 0.346 0.340 2.87 39 6.7 6,5 6.0 0,348 0.338 2.99 65 6.2 6.0 5.6 0.348 0,337 2.87 91 6.0 5.8 5,4 0.348 0.337 2.82

210 5.9 5.8 5.3 0.347 0.341 2.82

B 1 8.4 7.9 6,9 0.362 0.340 3.28 13 7.2 7.0 6.8 0.343 0.333 3.10 39 7.0 6.7 6.2 0,352 0.336 3,04 65 6.5 6.2 5.7 0,353 0.337 2.92 91 6.2 5.9 5.5 0.352 0.335 2.85

210 5.7 5.5 5.2 0.347 0.335 2.74

C 1 5,8 5.7 5.4 0.343 0.337 2.80 13 6.1 6.0 5.7 0.343 0.337 2.87 39 5.9 5.8 5.7 0.339 0.333 2.82 65 5,8 5.6 5,6 0.341 0.329 2.77 91 5.6 5.5 5.4 0.339 0.333 2.74

210 5.2 5.1 5.1 0.337 0.331 2.64

D 1 8.7 8.4 7.8 0.349 0.337 3.39 13 8.9 8.6 7.7 0.353 0.341 3.43 39 8.3 8. I 7.4 0.348 0.340 3.33 65 8.8 8.4 7.6 0.354 0.338 3.39 91 8.0 7.7 7,0 0.352 0.339 3.25

210 7,8 7.6 6,9 0.350 0.340 3.23

E 1 7.8 7,3 6.5 0.361 0.338 3.16 13 8,4 8,0 7.4 0.353 0.336 3,31 39 7.6 7, 1 6.9 0.352 0.328 3,12 65 6.6 6.3 5.8 0.353 0.337 2.94 91 6.6 6.3 5,7 0.355 0.399 2.94

210 6.1 5.8 5.6 0.348 0.331 2.82

F 1 8.1 7.8 7.7 0.343 0.330 3.27 13 7.4 7.0 6.9 0.347 0.328 3.10 39 7.9 7.7 7.5 0.342 0.333 3.25 65 6.7 6.5 6.3 0.343 0.333 2.99 91 6.6 6.4 5.8 0.351 0.340 2.97

210 6,7 6.5 6.3 0.344 0.333 2.99

G I 10.7 9,7 8.8 0.375 0.340 3.63 13 10.7 10.4 9.2 0.353 0.343 3.75 39 9.2 8,8 8.3 0,350 0.335 3.46 65 8.4 7.7 7.8 0.351 0.322 3.25 91 8,2 7.5 7.4 0.354 0.324 3.20

210 8.2 7.9 7.1 0.353 0.340 3.29

used as index , was no t usefu l fo r m a t e r i a l s such as s ludges , w i th l o w e r C / N ra t io va lues at

the b e g i n n i n g t h a n af ter t he c o m p o s t i n g or m a t u r a t i o n p ro ce s s ( T a b l e 1).

F r o m the a b o v e resul ts , it is sugges t ed tha t for t he m a t e r i a l s u n d e r s tudy , a c e r t a i n

Color Change during Organic Wastes Composting 247

Fig. 1. Munsell notation hue and chroma of the sludge composts on the (x,y)-chromaticity diagram at 4/value. x, sludge-grape debris (3/1) compost; a, sludge-grape debris (1/1) compost; A, sludge- peat residue compost; m sludge-city refuse (I/l) compost.

Fig. 2. Munsell notation hue and chroma of the city refuse composts on the (x,y)-chromaticity diagram at 4/value. �9 city refuse-grape debris (I / I ) compost; A, city refuse-peat residue compost; | city refuse compost,

period of maturation (4 months), after composting (91 days), is necessary for organic matter of the compost to become stable. In fact, we confirmed by germination studies (Garcfa et al. 1988) that the maturat ion period is necessary to avoid phytotoxicity in composts.

Table 2 shows the tristimulus values X, Y, Z chromaticity coordinates (x, y) and the Munsell notation va lue (v ) for all the samples studied. The stimulus value Y and Munsell notation value (v) decreased during the composting process due to the darkening process of the samples, which could include many complicated reactions such as aminocarbonyl reaction and /o r Maillard reaction. The stimulus value Y, which generally indicates the degree of lightness or darkness of materials, decreased rapidly during the first 65 days of piling and then remained nearly constant until the end of the composting and maturation processes. The group of the city refuse composts exhibited higher stimulus value Y than the sludge composts; grape debris and mainly peat residue decreased the stimulus value Y of the mixtures, indicating that the nature of the organic materials employed effects the stimulus value Y.

Figures 1 and 2 show the relation between Munsell notation hue and chroma for the sludge composts and city refuse composts, respectively, on the (x, y)-chromatici ty diagram at Munsell notation value 4. Although both groups of composts, sludge and city refuse composts, were differently situated in the (x, y)-chromatici ty diagram, the differences observed were slight. The darker the material, the lower the Munsell notation chroma values (AS-PR compost samples were the closest to the center of the diagram whereas the CR compost samples were the most distant). Munsell notation hue values were in the YR (yellow red) zone and AS compost samples showed the highest chroma values.

Table 3 lists the regression equations and correlation coefficients between the stimulus value Y of each compost and t h e i r W S C / N ratio, which we considered as a good index of the degree of maturity of the compost. It should be noted that the composts with peat residue exhibited the lowest correlation coefficients due to the darkening of the mixture produced by

248

Table 3.

C. GARC[A, T. HERN,~NDEZ. and F. COSTA

Regression equations and correlation coefficients between the stimulus value (Y) and WSC/N ratio (X).

Composts R. equations C. coefficients

A 3.'= 1.01 x+5.58 r=0.786" B y=2.35 x+4.99 r =0.947'** C y=0.11 x+5.57 ,"= 0.160 D I,'=0.80 x +7.43 r =0.886"* E ,v= 1.08 x+5.63 r =0.783" F y= l . l l x+6.14 r=0.740" G 3,'= 1.32 x+6.96 r=0.802"

Significant at *p=0.1, **0.05, and ***0.01, respectively.

Y('I~ 1011 1 9

Y:l.Z+/~ WS C/N* 5,74

r = 0,631 ( p < 0.001 )

A Q 0 XO ~176 ~

a A B

0,4 0B 1,2 1,6 2

W 5 C / N

Fig. 3. Relationship between water- soluble carbon/nitrogen ratio and the stim- ulus value of the compost, x, sludge-grape debris (3/1) compost; a sludge-grape debris (l/I) compost; zx sludge-peat resi- due compost; O, city refuse-grape debris (I/1) compost: A city refuse-peat residue compost: | city refuse compost: [], sludge-city refuse (1/1) compost.

this residue at tile beginning of the composting process and due to its resistance to degrada- tion. In general, sludge composts gave higher correlation coefficients than city refuse composts. The covariance analysis of the seven composts showed that the variable WSC/N was significant at p=0.01 and there were no significant differences between the variances and the test for the homogeneity of the slope (F=0.293, which is not significant). Therefore the seven slopes were equal and a general equation could be presented. Figure 3 shows the regression line using all the date. The regression equation is: Y = 1.44 W S C / N + 5 . 7 4 and is significant at p=0.001 (r=0.631). It can be observed that the points corresponding to the groups of sludge composts were situated below this line while those of the city refuse group were above the line. The values corresponding to the samples at the end of the piling period or to the mature compost samples were grouped in the initial zone of the straight line.

Correlation coefficients between the stimulus value Y and C/N ratio were not signifi- cant, mainly for the sludge mixtures, which indicated that this parameter (C/N ratio) is not a good index of the degree of maturity for materials with a low initial C/N ratio.

Figure 4 shows tile change of the absorbance/ExC ratio values of the alkaline extracts from the samples during the composting and maturation processes. This ratio increased

during the piling period in all the composts. The alkaline extracts from the AS-PR compost showed tile lowest increase, in the same

way as the solid AS-PR samples exhibited the lowest changes of the stimulus value Y. It should be noted that although ExC decreased during the composting process, the

Color Change during Organic Wastes Composting 249

4- Q

L) 3"

1, <I

0 39 91 210 0 39 91 210

D a y s

Fig. 4. Changes in the absorbance/extract- able carbon ratio of the alkaline extracts flom the sludge (a) and city refuse (b) composts during composting and maturation. • sludge-grape debris (3/I) compost; Q, sludge- grape debris (I/I) compost; /,., sludge-peat residue compost; O, city refuse-grape debris (I/I) compost; ,,, city refuse-peat residue con> post; o, city refuse compost; ", sludge-city refuse ([/1) compost.

Table 4. Regression equations and correlation coefficients between the values of absorbance/ExC ratio and WSC/N ratio.

Composts R. equations C. coefficients A y = - 1.47 x+1.88 r=-0.993"*** B 3" = -- 1.19 x + 1.82 r = -- 0.863* C .v= -0.28 x+ 1.97 r=--0.682 D 3,=--1.27 x+2.46 r=--0.941"" E y=--1.05 x+2.62 r=-0.960"** F y = - - 1.95 x+4.16 r=-0.857" G y=-0 .73 x+ 1.98 r=-0.962"**

Significant at *p=0.1, **0.05, ***0.01, and ****0.001, respectively.

optical density increased. This finding suggests that the samples were enriched in chromo- phore groups (likely conjugated double bonds) which may have been formed during the composting process as a result of the release of side chains from lignin-like molecules. On the contrary, during the maturation process there was an increase of the ExC content of the samples while the optical density remained nearly constant, indicating that the formation of humic acid-like polymers by condensation mainly occurs during this period of time.

Table 4 lists the regression equations and correlation coefficients for the values of absorbance/ExC ratio of each compost and their WSC/N ratio. It should be noted that, in general, the correlations of this parameter with the WSC/N ratio had higher level of significance than those of the stimulus value Y (see Table 3), although the AS-PR compost did not show again a significant correlation, There were not significant differences between the variances and the test for the homogeneity of the slope and a general regression equation using all the data could be presented: Y = --0.93WSC/N +2.29 and is significant at p=0 .00I ( r = -0.626).

The absorbance/ExC ratios were significantly correlated with the time with a level of significance (p=0.01; r=0.513) higher than that of the stimulus values Y (p~0.1; r = 0.2599). Therefore, it is considered that the absorbance/ExC ratio is a more useful index of the evolution during the cornposting and maturation processes.

Based on the study of the color of the solid samples, it appears that the stimulus value Y decreased with time during composting and that this parameter was significantly correlat-

ed with the WSC/N ratio (considered as a good index of the degree of maturity of the compost). However, this parameter may not be a good index of the maturity degree because constant values of Y are reached too soon (after 65 days of piling) and the changes are influenced by the nature of the organic material. Therefore, in agreement with Sugahara et al. 1984), it is concluded that the constant value of Y indicates the threshold of maturity

250 C. GARC[A, T. HERN,~NDEZ, and F. COSTA

only, and no complete matura t ion . On the other hand, the optical density of the a lka l ine

extracts reflects better the evo lu t ion of the organic materials dur ing compost ing. Also the

constant values of a b s o r b a n c e / E x C ratio can be a good index of complete ma tu ra t ion since

these values were only reached at the end of the matura t ion process.

REFERENCES

De Bertoldi, M., Vallini, F., Pera, A., and Zucconi, F. 1985: Technological aspects ofcomposting including modelling and microbiology. In Composting of Agricultural and Other Wastes, Ed. J.K.P. Gasser, p. 27- 40, Elsevier Applied Science Publishers, London

Garcra, C., Costa, F., and Herng.ndez, T. 1987: Evoluci6n de parametros quimicos durante el proceso de compostaje. 1#7 Proceeding 7 ~ Congreso Naeional de Quhaaica (in Spanish)

Garda, C., Hern~,ndez, T., and Costa, F. 1988: Efecto de los procesos de compostaje y maduraci6n de lodos y residuos s61idos urbanos sobre la germinaci6n. 1#7 Proceeding II Congreso Nacional de la Ciencia delSuelo (in Spanish)

Heekman, J.R., Angel, J.S., and Chaney, R.L. 1987: Residual effects of sewage sludge on soybean. I Accumulation of heavy metals. J. Envh'on. Qual., 16, 113-117

Juste, C. 1980: Advantages et inconvenients de l'utilisation des composts &ordures mdnageres comme emmendant organique des sols en support de culture. International meeting about compost, Madrid (in French )

Kimber, R.W.L 1973: Pkytotoxicity from plant residues Ill. The relative effect of toxins and nitrogen immobilization on the germination and growth of wheat. Plant Soil, 38, 543-555

Morel, J.L. 1982: L'evaluation de la maturitfi des composts urbains par une mfthode colorim6trique. Compost Inf., 10, 4-8 (in French)

Smith, T. and Guild, J. 1931: The C.I.E. colorimetric standards and their use. Trans. Opt. Sot., 33, 73-134 Sugahara, K., Harada, Y., and Inoko, A. 1979: Color change of city refuse during composting process. Soil

Sci. Plant Nutr., 25, 197-208 Sugahara, K., Koga, S., and lnoko, A. 1984: Color change of straw during composting. Soil SeL Plant Nutr.,

30, 163-173