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CURSO INTERNACIONAL SOBRE DISEÑO Y

DISPOSICIÓN FINAL DE RESIDUOS SÓLIDOS

(RELLENOS SANITARIOS)

 

 

 

 

 

 

 

 

 

 

 

 

 

WATER BALANCE AND LEACHATE QUANTITY

 

 

 

 

 

 

 

 

 

 

Dr. Peter Lechner

 

 

 

 

International Solid Waste Association

(ISWA)

 

 

PALACIO DE MINERÍA, MÉXICO D.F. 14-19 de Marzo de 1994

WGSL Course Mexico City, March 14-16, 94

WATER BALANCE AND LEACHATE QUANTITY

O.Univ.Prof.Dipl.Ing.Dr. Peter LECHNER

IWGF - Section Waste Management

Vienna University of Agriculture, Forestry and Renewable Resources

1. Introduction

As precipitation infiltrates through the landfill leachate is produced.

Leachate results from the biological, chemical and physical processes taking place within the landfill, coupled with a leaching effect as water trickles through the landfill. The product of these processes is a more or less highly polluted leachate whose constituents are heavily dependent on the condition of the landfill. Harmful germs may also be contained in the leachate (Table 1).

Table 1: Leachate analysis values for parameters with differences between acetic and methanogenic phase of a domestic waste landfill (EHRIG, 1989).

Parameter

Average

Range

Acetic phase

PH

-

6,1

4,5-7,5

BOD5

mg/l

13.000

4.000-40.000

COD

mg/l

20.000

6.000-60.000

BOD5/COD

-

0,58

-

SO4

mg/l

500

70-1.750

Ca

mg/l

1.200

10-2.500

Mg

mg/l

470

50-1.150

Fe

mg/l

780

20-2.100

Mn

mg/l

25

0,3-65

Zn

mg/l

5

0,1-120

Sr

mg/l

7

0,5-15

Methanogenic phase

PH

-

8,0

7,5-9,0

BOD5

mg/l

180

20-550

COD

mg/l

3.000

500-4.500

BOD5/COD

-

0,06

-

SO4

mg/l

80

10-420

Ca

mg/l

60

20-600

Mg

mg/l

180

40-350

Fe

mg/l

15

3-280

Mn

mg/l

0,7

0,03-45

Zn

mg/l

0,6

0,03-4

Sr

mg/l

1

0,3-7

2. Generation of leachate from landfills in water-deficient areas

Water is a scarce commodity in arid and semi-arid areas and pollution of surface and underground water resources can be disastrous to communities and households depending on these sources for domestic supply.

As many states of Mexico are largely water deficient areas concern must arise if landfills have the potential to cause unacceptable water pollution. Such pollution is also most costly and difficult to clean up once it has occurred. If nothing is done to ameliorate the situation, the pollution may persist in the groundwater for a long time, even though the source of the pollution has been removed.

Landfills receiving more than 750 mm of precipitation per annum will produce leachate, while in arid regions where annual precipitation is less than 300 - 400 mm, virtually all precipitation is evapotranspired.

Water Balance of a Landfill

The main factors influencing the water balance of a landfill are (see figure 1):

· Precipitation

· Surface run-off

· Evaporation and evapotranspiration

· Retention by the cover

· Storage by the refuse

· (Water production by biochemical processes)

· (Water losses through natural gas venting)

· Water output by leachate

Water losses through natural venting gas out of the landfill will condensate to a high

degree in the surface area.

 

Figure 1: Main factors influencing the water balance of a landfill

Figure 2: Some meteorological stations with average annual rainfall in millimeters

(Atlas Climatológico de México, 1939)

Once precipitation has passed through the cover layer, it will become leachate. It is a phenomena, that under common landfill practice, precipitation, respectively leachate migrates on special interconnected zones through the landfill long before an overall field capacity is reached. The existing of a main wetting front in the landfill, anticipated in some water balance models, is therefore not exactly right. Only if the field capacity of the waste is reached- with the age of a landfill it might become homogeneous- the water content in the refuse will not become lower than this field capacity.

It must also be recognized that good engineering and management of a landfill can be used to maintain a perennial water deficit within the fill even though there may actually be an excess of precipitation over potential evaporation. This can be done by

· Maximizing run-off and ID

· Minimizing infiltration into the refuse.

A suitably sloping surface and the installation of a carefully designed impervious

cover layer can achieve this.

The infiltration rate is strongly influenced by the kind of cover material that is used. Materials with a high field capacity should be preferred, for example waste compost.

Compost is a material with a very high content of organic matter (15 to 30 % DS!), which enables a very high field capacity (80 to 120 % DS!). On the other hand the very permeable surface and a possible strong vegetation prevents a good surface runoff, but forces evaporation resp. evapotranspiration.

Obviously the smaller the precipitation and the larger the evapotranspiration and runoff, the less the potential for the generation of leachate. These terms are particularly favorable in water deficient areas.

Leachate production is high from low compacted landfills without a soil cover. In case of highly compacting, at the landfill surface often ponding of rainwater can be observed. Under humid climatic conditions the average difference between precipitation and evaporation- independent from different vegetation types- is positive. The following figure presents leachate data from different landfills in the northern part of Europe (Federal Republic of Gerrnany/EHRIG, 1989).

 

Figure 3: Precipitation (mm/year) and leachate flow (mm/year and % of precipitation) at different landfills and years in the middle of Europe (EHRIG, 1989)

In water deficit areas, evaporation exceeds precipitation (figure 4).

Studies made in South Africa (BALL & BLIGHT, 1989; BLIGHT, VORSTER & BALL, 1987) produced strong evidence that if climatic conditions are such that a perpetual water deficit exists at the site of a landfill, no or very little leachate will be formed and exit the base of the landfill. Hence, if there is an adequate separation between the lowest level of refuse and the highest level of the regional phreatic surface, no groundwater pollution may occur (figure 5).

Figure 4: Water balance in water deficit areas

 

 

 

 

Figure 5: Different climate-types in water deficit areas

 

Example 1- Witwatersrand/South Africa, Example 2- Cape Town/ South Africa,

A winter rainfall area a summer rainfall area

(BLIGHT et al ... 1989) (BLIGHT et al ... 1989)

Precipitation (Rainfall)

The most critical situation occurs in the case of low rainfall intensity over a long period of time; cloud bursts, e.g. rains of extraordinary intensity result in a quick saturation of the cover material, with the result of a high surface run-off, so there is little infiltration into the landfill.

Rainfall data should be preferable obtained from measuring on site or alternatively from the nearest meteorological station (see figure 6).

Surface Run-off

The important facts, which influence the surface run-off are:

· Topography of the landfill

· Type of soil cover material

· Morphology of the soil cover

· Vegetation

A simple method for estimating the surface run-off is based on the general formula:

R = c x P

R . . . run-off

P ... precipitation

c ... coefficient

Table 2: Run-off coefficients proposed by SALVATO et al. (1971) for different soil cover materials and different vegetation types

Soil cover

Slope

(%)

Soil texture

Sandy

loam

Loamy

clay

Clay

Grassed oil

0-5

0,10

0,30

0,40

 

5-10

0,16

0,36

0,55

 

10-30

0,22

0,42

0,60

Bare soil

0-5

0,30

0,50

0,60

 

5-10

0,40

0,60

0,70

 

10-30

0,52

0,72

0,82

Evaporation and Evapotranspiration

The vegetation growing on the final cover of the landfill needs water for building plant tissue and causes a water loss by transpiration. In addition, water is evaporated from the soil depending on soil texture and climatic conditions. A distinction should be made between the period of landfill operating- maybe with intermediate cover- and the finished landfill with a final cover and revegetation.

Leachate Circulation

In regions with low annual precipitation (< 750 mm) a leachate circulation system reduces the quantity of leachate by evaporation and accelerates the biochemical decomposition process in the landfill. This leads to a drop in the decomposible organic content of the leachate and accelerates the production of methane gas.

3. Free Leachate Flow

Steps must be taken to ensure that under no circumstances- even in the long term- it will be possible for a build-up of leachate to occur in a landfill. Leachate must be able to exit from the area of the landfill following the natural gradient (LECHNER et al., 1993).

If a pit is completely filled, in other words, if there is no free leachate flow, there is a build-up of leachate as soon as the pumping system fails, even if the leachate collection system is optimally constructed. In an extreme case, at the relevant depth of filling or height of build-up, all the free water in the mineral base liner begins to permeate the liner. Permeation is now governed by the relationship v = k x (i-io). In other words, there is laminar flow through the liner. The mineral barrier liner is thus no longer "technically impermeable".

v ....... filter velocity (m/sec)

k ....... hydraulic conductivity in the linear range (m/sec)

i ........ hydraulic gradient

io ....... start gradient for the linear relation

In case of a free leachate flow the hydraulic gradient results only from the controlled flooding of the drainage system according to the hydraulic requirements for the runoff of the leachate. The hydraulic gradient will not generally exceed a value of 1,5. The value of Ki=1,5 relevant for the actual percolation through the base liner is thus much lower than Ki=30, which is the value used for the determination of the coefficient of permeability in the laboratory. At low hydraulic gradients the effect of the binding forces results in a non-linear relation between the filter velocity (v) and the hydraulic gradient (i), in mineral materials of low permeability (ki=30 < 10-8 m/sec).

In other words, the resistance to the percolation of leachate is virtually infinitely large at a low hydraulic gradient. This exponential relationship is explained by the fact that the adsorption water only contributes to the flow at an increasing hydraulic gradient. Only then does cross-sectional area of flow- and with it permeability- increase. As long as this is not the case, a mineral barrier liner of low permeability can therefore be describes as technically impermeable.

This is the reason for the decisive importance attached to free leachate flow. The necessity for free leachate flow is in most European countries now generally accepted.

Above ground landfill mounds have a potential for erosion of the topsoil layer in the cover. The appearance of leachate in the drainage system of such a landfill, especially in water deficient areas will signal the need to investigate the cover and, if necessary, to repair it.

4. Literature

BALL J.M., BLIGHT G.E. (1989): Movement of leachate from a New Landfill. ln:Proceedings of the Second International Landfill Symposium Sardinia 1989.

BLIGHT G.E., VORSTER K., BALL J.M. (1987): The Design of Sanitary Landfills to Reduce Groundwater Pollution. In: Proceedings of the Intern. Conference on Mining and Industrial Waste Management. SA Inst. of Civil Engineering. Johannesburg, 1987.

EHRIG H.J. (1989): Leachate Quality. In: Sanitary Landfilling Process, Technology and Environmental Impact. Academic Press 1989.

LECHNER P. et al. (1993): Reactor Landfill, Experiences Gained at the Breitenau Research Landfill in Austria. In: Proceedings of the Sixteenth International Madison Waste Conference, University of Wisconsin-Madison, 1993.

SALVATO et al. (1971) in: CANZIANI R., COSSU R.: Landfill Hydrology and Leachate Production. In: Sanitary Landfilling- Process, Technology and Environmental Impact. Academic Press 1989.

 

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