The Science of the Total Environment, 114 (1992) 149-160 Elsevier Science Publishers B.V., Amsterdam

149

Time progress of the quality of a solid waste leachate

G. G e n o n , F. Marchese , P. Do t t a , O. Sivera, M. O n o f r i o

Politecnico di Torino, Dipartimento di Scienza dei Materiali ed Ingegneria Chimica, Corso Duca degli Abruzzi 24, Torino, Italy

(Received March 8th, 1990; accepted January 4th, 1991)

ABSTRACT

Leaching by percolation waters of toxic pollutants from solid wastes in a landfill occurs in a substantially different way to that suggested by elution tests in batch agitated reactors. Ef- fective release, in fact, is influenced by two important aspects that distinguish it from the reference standard: gradual removal of the pollutant and hence a possible change in the matrix; the chemical and physical characteristics of the percolation liquid itself, as determined by the acid or basic nature of the wastes. The composition of the percolate from an industrial waste landfill was evaluated to determine the validity of these general considerations. It was found interesting to correlate the composition of the wastes with the transfer of metals and organic substances to the percolating liquids, this being derived from both the flow of rain- water and the expulsion of interstitial liquid present in the wastes. The discrepancy between the real washout and its simulation by a laboratory test was also confirmed. From a practical standpoint, some conclusions are drawn with regard to disposal of the percolate and its treatability, these being primarily important in the case of organic substances (COD and mineral oils).

Key words." toxic pollutants; solid wastes; leachate

INTRODUCTION

W h e n assessing the e n v i r o n m e n t a l i m p a c t o f a landfil l for industr ia l wastes [1], its poss ib le po l lu t ion o f surface or u n d e r g r o u n d wate rs is the m o s t s ignif icant aspec t to which a t t en t ion m u s t be directed. C o n t a m i n a t i o n m a y

be the o u t c o m e o f acc iden ta l inf i l t ra t ion, due to p o o r t ightness o f the sys tem and loss o f percola te , o r sur face ou t f lowing into bodies o f wa t e r o f un t r ea t ed percola te . I m p o r t a n c e is thus a t t a ched to the tests for eva lua t ing such impac t

laid d o w n in the legis lat ion on the pe rmi t t ed d u m p i n g o f c o n t a m i n a t e d

0048-9697/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved

150 G. GENON ET AL

wastes [2]. Of these, the standard elution test [3], while undoubtedly signifi- cant as a yardstick for comparison, does not provide a sufficient simulation of the actual quality of the percolate that may be generated as a result of the contact between water and wastes [4]. With a view to obtaining a more realistic picture, therefore, an attempt has been made in previous papers [5,6] to simulate the real percolation taking place in a landfill and certain predic- tive assessments have been drawn from the outcome of these tests.

None the less, it is certain that the most interesting and significant result in definition of an impact on the environment can be obtained by determin- ing the actual quality of the percolate derived from a landfill. This quality is governed by a complex set of hydraulic, physicochemical and, on occa- sions, biological mechanisms [7] and is thus difficult to predict. This paper therefore, reports an investigation of the percolate quality of a large in- dustrial wastes landfill and its correlation with both the quality and quantity of the materials deposited. The elution pattern over time is also considered in a search for indications concerning possible changes in percolate quality.

FEATURES OF LANDFILL AND THE MATERIALS DEPOSITED

The landfill concerned lies in a heavily industrialised area not far from the centre of Turin (Italy). Some 100 000 cubic metres are currently being ex- ploited out of a total capacity of over 500 000 cubic metres [8].

The site is a 15-m deep disused quarry originally exploited for road con- struction aggregates. The high permeability of its gravel floor and its closeness (about 2 m) to the watertable have made it necessary to line the landfill with a triple impermeabilisation barrier (double HDPE sheet and a layer of clay). Monitor shafts are connected to the drainage layers between the HDPE sheets and above the clay layers. Bleed shafts have also been pro- vided for safety drainage in the event of an accident. Pipework installed above the top HDPE sheet is used to carry off the percolate for analysis and treatment. The backfill is exposed to the weather. Rainwater can thus reach the inside of the dumped material and will continue to do so until it is finally sealed with a waterproof layer when this section is completed. The mean Turin rainfall is about 800 mm per year, mostly in spring and autumn.

The landfill's characteristics as described above go well beyond the re- quirements of the current Italian legislation [9]. Even so, it has a limited materials acceptability with respect to the national standards laid down for a second-category, type C dump. The reception limits imposed by the Italian regulations are set out in Table 1.

In 6 months, the landfill has received about 17 600 tonnes of materials

TIME PROGRESS OF THE QUALITY OF A SOLID WASTE LEACHATE 151

(at least 50% dry matter by weight), mainly composed of the following com- ponents:

Asbestos from the removal of insulating material Slag containing asbestos Paint residues Combustion ashes and residues Dust from the treatment of smoke and fumes Toners and lampblack Sludge from galvanic processes Pharmaceutical industry sludges Sludge from physocochemical treatment processes Stabilised organic sludges Soil contaminated with solvents Other materials

6% 12.5% 13.3%

3% 6.2% 2.2% 5.6% 5.8% 4.6%

18.8% 6.6%

15.4%

Systematic analyses have been carried out to check all incoming materials: metal content by atomic absorption spectrophotometry, gravimetric deter- mination of water content and volatile substances, gas chromatography or colorimetry of specific components, such as phenols, chlorinated solvents, aromatic substances. Particular attention has been devoted to defining the correct procedure for pretreatment of the matrix so as to solubilise metals or selectively extract organic compounds prior to these analyses. Repeated or calibration tests showed that the repeatability and exactness of the analytical data were good with regard to the specific parameter determined. Account must be taken, however, of the fact that the analytical finding refers to a single sample, as significant as possible with respect to the dumped material, though not necessarily identical for each of its fractions. Repeated sampling and analysis were undertaken to get over this problem.

Determinations of the concentration of pollutants were also performed in order to assess the release of substances, primarily metals; these determina- tions were carried out in a stirred reactor with standard atomic absorption spectrophotometry. The percolate was regularly carried out and analysed by using the analytical procedures specific for waste waters [10]; particular at- tention was paid to the sample preservation and to the analysis time.

The periodic quantity of percolate that is collected was measured by volumetric meters connected to the pipe-line for transfer to storage.

152 G. GENON ET AL

T A B L E 1

A c c e p t a n c e l imi ts i m p o s e d for the d u m p ( the p a r a m e t e r s a re f ixed by I ta l i an law)

S u b s t a n c e s C onc . in the was t e Conc . E P A test

(mg/kg) (mg/l)

Ar sen i c - - < 10

M e r c u r y - - < 0.1

C a d m i u m - - < 0.4

T a l l i u m - - < 0.4

Ber i l l ium - - < 0.4

C h r o m i u m VI - - < 4

Lead - - - - A n t i m o n y - - - -

P h e n o l s < 50 000 a - -

C y a n i d e s < 5000 - -

I s o c y a n a t e s < 50 000 a - - O r g a n o a l o g e n a t e d < 0 . 0 1 / 5 0 0 000 b - -

C h l o r i n a t e d so lven t s < 1000/500 000 b < l 0

O r g a n i c so lven t s < 1000/500 000 b - -

Biocides - - - -

Mine ra l oils - - - -

P h a r m a c e u t i c a l c o m p . - - - - Pe rox ides etc. < 5000/500 000 b - -

E the r s < 1000/500 000 b - -

U n d . chem. subs t . - - - -

A s b e s t o s - - - -

Se l en ium - - < 0.6

Te l lu ry - - < 0.4

Polycicl ic a r o m . < 5000 - -

M e t a l - c a r b o n i l e s < 1000 - -

C o p p e r - - - -

Ac id -ba se s - - - -

P C B < 5000 - -

aFor s o m e toxic c o m p o u n d s lower l imits .

b D e p e n d i n g on the toxic i ty o f c o m p o u n d .

R E S U L T S

Results of the first 6 months of operation

T h e c u r v e i n F i g . 1 s h o w s t h a t w h i l e t h e a m o u n t o f w a s t e s d e p o s i t e d h a s

i n c r e a s e d i n a r e g u l a r f a s h i o n , t h e v o l u m e h a s b e e n n o m o r e t h a n s l i g h t l y i n -

f l u e n c e d , b u t s e e m s t o b e m o r e c l o s e l y l i n k e d t o t h e p r e c i p i t a t i o n f i g u r e s , a s

c a n b e s e e n i n F i g . 2 , w h i c h d e m o n s t r a t e s t h a t t h e r e w a s a r a p i d i n c r e a s e i n

TIME PROGRESS OF THE QUALITY OF A SOLID WASTE LEACHATE

percolate (103m 3) waste (lo6kg)

~ - ~ 15 i -.,f / ~

, ~ 10

~ ~ 5

0

90 120 150 180 time(d)

1.05

0.7

0.35

0

0 30 60

Fig. 1. First year of operation of the dump: dumped material and percolate collected.

153

percolate volume during a period of heavy rain (> 300 mm in 40 days), followed by a period of virtually no rainfall, during which percolation was confined to the small quantity of free interstitial liquid present in the wastes.

Despite the fact that this marked difference in liquid input naturally led to a substantial difference in flowtrough rate, the basifying power of the sludge did not change to any great extent. The pH value was alkaline and virtually constant.

The picture with respect to the release of pollutants is illustrated in Fig. 3, which shows the removal of heavy metals in function of the percolate collected. These metals are present in the form of ions leached from the hydroxides forming the matrix. The first point to note is that the pattern is substantially regular. There is no evidence of progressive inertization due to deposition of insoluble compounds on the surface of the washed-out waste, nor of true chemical leaching due to lessening of the basifying power of the

percolate (103m 3)

1.5

1.2 -

0.9 - V

0.6

0.3 I3 /

o

- - J F - - - ~ . - - - - V - - V

! i

4 8 12

A 300 mm rain

B .= 5 mm rain pH

11.

12

9

6

3

0 16

waste(lO6kg)

Fig. 2. Volume and pH of percolate.

1 5 4 G. GENON ET AL

600

400

200

0

metal removed (g)

i ! i .A/

0 0.2 0.4 0.6 0.8 1 1.2 1.4

percolate ( 10 3 m 3 )

Fig. 3. Leaching of heavy metals from waste.

A copper

v lead

• chromium

wastes placed in the landfill. When the quality of the eluate is related to that of the wastes as a whole (Fig. 4), it can be seen, especially with respect to copper but also cadmium and other metals, that the picture is primarily marked by two areas corresponding to a difference in the extent of the washout. The heavier precipitation in the initial period resulted in faster release, followed by a percentage drop.

When the metals themselves are compared it is seen that chromium and lead are retained by the matrix to a greater extent than copper and cadmium. The corresponding release values, in fact, are of the order of 3 g/t for the first two and 5-6 g/t for the second.

One interesting subject of inquiry is the comparison between the release observed and that deducible from standard laboratory EPA tests: as a general significance it offers a reliable assessment of the reliability of these standard tests.

The actual and estimated lead and copper release values are compared in Fig. 5. The latter were derived from elution analyses in the laboratory, since

600

400

200

0

metal wash-out(g)

[ ] [ ]

o 40 80 120 160

o copper

• lead

v chromium

quantity of metal in the dump (103 ko)

Fig. 4. Specific leaching of different metals from waste.

TIME PROGRESS OF THE QUALITY OF A SOLID WASTE LEACHATE 155

actual release (g)

6 0 0 I

4 0 0 ' • k a

8 • copper

[]lead

2 0 0

0 0 5 0 0 1000 1500

release evaluated by EPA

2000

tests (kg)

Fig. 5. Comparison of actual and estimated (EPA tests) release.

release evaluated in this way can be regarded as cumulative and free from saturation phenomena. The marked difference between the values on the two axes of the plot, even in their order of magnitude, leaving aside the initial in- crease in release owing to rain, is readily apparent. As suggested elsewhere [11], in fact, the substantial alkalinity of an industrial waste results in en- vironmental conditions that are very different from those associated with the buffered acetic acid test. This test, indeed, is designed to simulate different landfill conditions (mixed urban and industrial wastes) [12], and its modification and integration is currently under discussion.

Turning now to organic substances; Fig. 6 shows that the elution of mineral oils is substantially different from that of metals. An initial phase with only slight release is followed by an accentuation stage that then ap- pears to progress to saturation. This pattern is the result of the flow dynamics. Owing to their low mobility and solubility, a rather slow drainage mechanism of organic substances with respect to the matrix must be

(%)

min. oils eluted COD eluted

rain. oils in the dump • I

i 16

1 2 , - - i . . . . . .

i

4 . . . . F

o ~ J 0 350

percolate

7 0 0

(m 3)

org. waste in the dump

(°/.4 - - 12

o 1050 14o0

Fig. 6. Effect of percolation on specific release of organic substances.

156 G. GENON ET AL

established. This mechanism is not influenced by the already identified dif- ference in the rainfall (and hence the flow rate through the wastes), but by an independent flow. The contact time of the percolation liquid thus appears to be devoid of importance.

It is important to note that the ratio between the amount eluted and the amount present is two orders of magnitude greater than that for metals (5 kg/t for oils, 2-4 kg/t for COD: as opposed to a few g/l). Leaching, in fact, is not impeded by pH conditions slowing down dissolution, as was the case for metals. There is solely a delay in dissolution derived from the limited con- tact time and specific surface area available. Elution is slow, but proceeds towards values that are high in percentage terms compared with the total amount available, whereas for metals thermodynamic conditions (alkaline pH) promoting the precipitation of insoluble compounds lower the percen- tage release. A further point to be noted with regard to the organic substances is that the landfill is devoid of biological activity. The monitor shafts, in fact, showed a virtual absence of biogas. The salt load and the presence of toxic metals prevent any metabolic activity. This of course, would be very low in any event, since the organic substances in the landfill are of a technological and synthetic rather than a natural origin and hence refractory to biodegradation. Redox activity is also absent. Surface leaching, therefore, is the only phenomenon observed.

Quality of the percolate

Table 2 illustrates the average composition of the percolate in the first six months of operation. This is derived from the mean of the progressive per- colate quality results used to calculate the previous point. Apart from inter- pretation of the values observed, comparison of these results with the tabular limits and the treatability criteria is of more immediate interest. Comparisons of this kind are of importance in determining the expenses and the disposal problems the percolate is likely to raise.

Even though the results vary over time, it can be seen that the basifying power of the material in the landfill leads to percolate concentrations that are not much higher than the discharge acceptability limits. This is sufficient- ly reassuring with regard to the treatability of the percolate and its potential impact. The same conclusion, indeed, can be drawn with respect to all land- fills composed of industrial wastes only, especially those of an inorganic kind. Closer attention, however, is required in the case of organic pollution by COD, mineral oils, phenols and aromatic solvents. Their percolate con- centrations are decidedly high and well above the permitted discharge values. Specific tests must thus be conducted to determine the best ways of treat- ment. In view of these problems, one can suggest that in principle, subject,

TIME PROGRESS OF THE QUALITY OF A SOLID WASTE LEACHATE

TABLE 2

Mean composition of the percolate

157

Parameters Mean value Max. value Min. value Tab. A (mg/l) (mg/1) (mg/1) (mg/1)

pH 7.83 (10) 8.22 7.20 5.5-9.5 COD 24 230 (10) 86 460 352 160 Phenols 9.2 (10) 60.0 0.1 0.5 Mineral oils 520 (10) 1200 98 5 Cu 0.63 (10) 1.12 0.35 0.1 As 0.020 (10) 0.070 0.002 0.5 Hg 0.0200 (9) 0.1100 < 0.0025 0.005 Se 0.0071 (5) 0.0300 0.0008 0.03 Cd 0.003 (10) 0.009 0.001 0.02 Pb 0.104 (10) 0.590 < 0.005 0.2 Cr 0.340 (9) 1.110 0.012 2 Zn 2.17 (2) 3.50 0.84 0.5 Ni 2.21 (2) 2.70 1.38 2 Fe 1.35 (3) 1.60 1.10 2 Chlorinated 0.16 (2) 0.26 0.06 1

(n) Number of percolate samples from which the mean value has been evaluated.

of course, to verification, the wastes should be rendered inert or solidified [13] before they are dumped. A comparative assessment should in any case be made of the cost of final disposal of the percolate as such and the combin- ed cost of inertization and the less expensive disposal of a better quality per- colate.

Evaluation of the percolate through simulation tests

Simulation tests in accordance with a proven procedure [5], were run in the laboratory in the search for values to be used in predicting subsequent change in percolate quality. To obtain comparable data, the organic and in- organic constituents of the wastes were subjected to accelerated column and agitated reactor washout tests; the contact time is so reduced, but the simula- tion as regards the materials is correct.

In view of the release process concerned, washout in 1/kg was chosen as the parameter most indicative of the percolate pattern. The simulated release of inorganic pollutants and COD is illustrated in Figs. 7a and 7b, respective- ly. It will be seen that appreciable release is eventually followed by a marked reduction. In both cases, even though an eluent liquid simulating rainwater

158 G. GENON ET AL

release (mg) 40 I

30 ~r v , ~ ,

10 1 "

o ; 5 ~ ~

pH

10.5 • copper

v lead

~pH1

o pH2 t 3.5

____<..~ ~.~ J -- v - --V 0 0 100 200 300

wash-out ratio ( I /kg)

Dust deriving from fumes treatment ( lead 26000 rng/bgl

Electonic waste treatment sludge (copper9OOO mg/kg)

a

release (mg) pH

120 ~ v CO0

90 [ ~ ~ T - _ ~ _ ~ - _ ~ 10.5 vpl'l 60 7"

30 3.5

0 0 0 100 2 0 0 3 0 0

wash-out rat io (l/kg)

B iolegical sludge ( 13'1, organic substance )

Fig. 7. (a) Metals release arid pH of solution in column experiments. (b) Organic substances release in column experiments.

[14] was used, substantial alkahnization of the percolate results from the release of basic substances, as occurs in practice. It is this that greatly slows down the release of metals for obvious thermodynamic reasons. Comparison of the results of a s tandard EPA test and gradual elution tests equally per- formed with slightly acid, though not buffered, water in a co!umn, or through repeated contact in an agitated reactor (Fig. 8) in fact, shows that a markedly higher release occurs in the EPA test, where the pH is buffered to acid values, than in the other tests where an alkaline pH is established.

The conclusion to be drawn from the simulation results is that the washout ratio can be used to predict release patterns. In addit ion to this parameter, however, it is important to determine the quality of the environment (i.e. its pH and ion species), since it is primarily the thermodynamic equilibria established at the interface between the wastes and liquid that govern the quality of the percolate. A standard test will only be significant for simula- tion purposes if it succeeds in reproducing this aspect.

TIME PROGRESS OF THE QUALITY OF A SOLID WASTE LEACHATE 159

release (%) 6O

40 0(I )

2O ,I,,(2) 0 ~!r~

0 40 80 120

wash-out ratio ( l l i g ) (I] [PA stlndord test (pH5)

(2) repeated batch (pH 8.6 8.3)

13) elut ion in a column (pH 8.6-11.3)

i -- v----~) v

160 200

Fig. 8. Effect of pH and type of contact on the copper release.

CONCLUSIONS

Direct assessment of the quality of the percolate from an industrial wastes landfill is a matter of considerable interest, since it provides information unlikely to be found in the literature and very difficult to derive by extrapola- tion from results obtained in the laboratory.

The specific, full-scale experimentation described in this paper has shown that: • The amount of percolate is relatively slight, provided the humidity of the

waste itself is low, and is solely influenced by periods of heavy rain. • There is a regular release of none the less very small quantities of heavy

metals, owing to the basifying power of the industrial wastes placed in the landfill. • Organic substances are released to a much greater extent in percentage

terms. As a result, the quality of the percolate is such that considerable atten- tion must be devoted to its treatment. • Exploitation periods of only a few months are not sufficient to provide

evidence of a quality modification of the microenvironment in the mass of wastes of a landfill and hence the characteristics of its percolate.

ACKNOWLEDGEMENTS

The Authors wish to thank the Barricalla Company (Turin) for allowing us to use analytical data relating to the initial period of exploitation of the dump referred to in this paper.

160 o. GENON ET AL

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