A STUDY ON A WASTE STABILIZATION METHOD IN CLOSED SYSTEM DISPOSALFACILITIESYoshikazu Otsuka*, Kazuei Ishii**, Masataka Hanashima***, Tohru Furuichi****, Mitsuo Kawaguchi*****
Working of fundamental investigation for waste stabilization,
Research group for control, Research Committee for Closed System Disposal Facilities
Chateau Takanawa 401, 3-23-14 Takanawa, Minato-ku, Tokyo 108-0074*Vice-Chief, **Group Leader, ***Chairman, ****Vice-Chairman, ***** Vice-Chairman of research committee

ABSTRACTin Japan because of opposition from residents who feel

Recent studies on the waste stabilization in a landfill site

uneasy about environmental pollution caused by the disposal

have revealed the effectiveness of a washout gradually,of solid waste. In response to this difficulty, a new type of

which removes some kind of chemicals from waste.

landfill site, called a “Closed System Disposal Facility”

However, there is no concrete method for evaluating(CSDF), has been studied and put into practice in Japan.

quantitatively the effectiveness of a washout. This study

Because the CSDF can control emissions to the neighboring

applies an optimal control theory and tried to evaluate aenvironment and the quality of landfill waste, the CSDF is

stabilization phenomenon of waste quantitatively. In theacceptable to neighboring residents.

evaluation, the TOC concentrations in the leachate were

In this study, “waste stabilization control” is discussed

calculated using a washout model in the process of the waste

since it is the most important function of the CSDF.

(bottom ash) stabilization a closed system disposal facility.

Specifically, we investigated ideas and factors relating toParticularly, the effects of three operating variables in thewaste stabilization and proposed an analytical method towatering for the washout, 1) intensity, 2) period, and 3)

promote waste stabilization.

interval of watering, on the TOC concentration profiles were

Research to find a theoretical explanation of the elution

evaluated quantitatively. For the optimization of the

behavior of toxic substances and other substances fromsprinkling operation, a total cost in a leachate treatment until

waste or an effective method of eluting toxic substances andthe TOC concentration reaches the acceptable level was usedother substances from waste is still in its infancy. Moreover,as an evaluation function. The optimization theory proposed

attempts to control this elution behavior and evaluate

in this study is a promising design method of watering andquantitatively various elution methods are at an even moreleachate treatment facilities for waste stabilization.

basic stage. The waste landfills in question are structures thatare highly susceptible to rainfall and other natural

1

INTRODUCTION
Construction of landfills has recently become very difficult

phenomena (because waste landfill sites are located in the
open air) and one of the challenges facing this research is theextensive level of disturbance encountered in attempting to“abolishment standard”, which is stipulated by the Japanesecontrol this elution behavior. To address this challenge,

government. For example, if the quality of leachate meets an

Hanashima and others have conceived a new type of

effluent standard or a local standard, the operation of adisposal facility called a closed-system disposal facility

leachate treatment facility can be stopped.

(CSDF), which has recently spawned various R&D

In this study, an analytical method to promote waste

initiatives on a variety of topics. A CSDF is a disposal sitestabilization was proposed to meet the abolishment standard,

covered by a roof to prevent rain from coming into direct

especially the standard for quality of leachate. We focused oncontact with landfill waste. Many people are hopeful thatbottom ash, which in Japan is usually disposed of by

CSDFs will facilitate control of the effects of the ambient

landfilling, and propose water leaching as the method toenvironment on landfill waste and that CSDFs will also

promote waste stabilization.

enable control of landfill waste itself. This has led to thedevelopment of multiple types of CSDFs. As of 2006, some
44 CSDFs have been constructed in Japan.

3

In this paper, we discuss several research projects relating

LANDFILL WASTE AND STANDARD IN
JAPAN

to the function of controlling the stabilization of landfill

3.1. Quality of landfill waste

waste, a key function of CSDFs. Specifically, we have

In recent years, the composition of waste accepted byadopted the method of watering to wash out organic

landfills in Japan has been changing, as shown in Figure 1.

contaminants in the bottom ash that comprises landfill waste.

More than half (54% or 5,682,000 t) of the total mass of

We discuss the effects that watering has on stabilization. We

landfill waste in the 2000 fiscal year was incineration ash.

also report on watering methods to promote this stabilization

The ratio of incineration ash to the total amount of landfill

and present ideas to determine the optimum watering method

waste increased with time, because increasing proportions ofto achieve stabilization.

the combustibles have been incinerated prior to landfillingand because recycling of incombustibles has been promoted.

2

Therefore, this study focused on bottom ash as the subject of

WASTE STABILIZATION

research.

The phrase “waste stabilization” has various meanings.
Generally, waste stabilization is a process in which organic
matter in solid waste is degraded to carbon dioxide and water

Mass of waste sent to landfill（thousand tons）
16,00014,000immobilized. However, while one may say that the stabilizedstate is a state where all pollutants in landfill waste arecompletely degraded or immobilized, it is in practice
impossible to meet this requirement. In other words, there isa gap between actual and ideal situations in waste
stabilization.
This study discusses waste stabilization from a realistic
viewpoint, so we set the level of waste stabilization as the

最終処分量（千t）or where hazardous compounds are degraded or

13,093
12,00812,00010,00011,35010,86910,514

3,820

3,444

3,084

1,765

1,770

1,692

1,749

6,034

5,909

5,760

5,733

5,682

８年度
1996

９年度
1997

10年度
1998
平成年度

11年度
1999

12年度
2000

5,180

4,334

1,879

8,000
6,000
4,000
2,000
0

Incineration ash

焼却残渣

Com
bustiblesの処理残渣
焼却施設以外

Incombustibles
直接最終処分Figure 1 Changes in the Mass of waste sent to landfill

3.2. Standard for termination of operation andsubstances in landfill waste, it is necessary to confirm theeffects various actions have in altering the physical properties

maintenance of landfill sites
The standard that defines the quality of leachate forof waste. It is important to utilize the information obtained totermination of operation and maintenance of controlled

estimate what types of factors affect stabilization. As shown

landfill sites in Japan is “the quality of the leachate collected

in Figure 2, we have postulated correlations between factors

into the leachate collection pipe must meet the effluent

affecting waste (bottom ash) in the landfill layers andstandards for two years; (a) all compounds in effluent

multiple leachate constituents in three scenarios, (a), (b) andstandard every six months; and (b) BOD (60 mg/L), and(c). We describe the dominant factors influencing elution

COD (90 mg/L) and SS (60 mg/L) every three months.”

from bottom ash.

With regard to aspects other than leachate, there are

4.1 Factors influencing physical properties of landfill

standards that specify the temperature, gas emission rates andwaste

settling of the waste layers.

(1) Landfill waste

4

FACTORS INFLUENCING CHANGES TO

As shown in Figure 2(a), landfill waste exhibits variable

PHYSICAL PROPERTIES OF LANDFILL

elution behavior in response to watering due to the following

WASTE

factors. Specifically, factors (a1) through (a4) are involved.

To achieve control of the elution velocity of various(a1) “Various forms of pretreatment of landfill waste and

CSDF

Fan
Roof

Watering(c1) Quality of watering
Waste

(a)

(c)

(a1) Quality of p retreatment(a2) Kind of covering soil(a3) Strength of compaction(a4) Height of landfilling waste(c2) Watering schedule
Intensity of watering
Interval of watering
Period of watering time

Moisture content
Oxygen content
pH
Temperature(b)(b1) Quality of roof(b2) Kind of ventilation(b3) Strength of aeration(b4) Interval of sucking gas

Landfilled waste
Gas collection pipe
Leachate
pH,COD,BOD,TOC,Cl,Ca,TN,NH4,SS
Heavy metals

Figure 2 Factors affecting changes in properties of landfill wasteplacement of additives in landfill waste,” factors related towatering schedule.

intermediate treatment prior to transportation of waste to the

4.2 Main influence and operation factors used in score

landfill site; (a2) “Quantity and quality of earth covering,”

table

factors related to earth-covering operations to prevent

In this study, we tried to develop a score table to extract thedispersal of landfill waste; (a3) “Location and order ofmore important factors from all those possible factors that

disposing of landfill waste,” factors related to the compacting

have effects on waste stabilization. We constructed the tableof waste during landfill operations; (a4) “Height ofby having 10 people carry out a paired-comparison of alllandfilling waste,” factors related to concentrations of somefactors and recording the level of correlation, from 0 to 2, of

elution materials.

the answers. For example, the amount of water supplied and(2) Structure and specifications of landfill facility

the amount of pollutants (inorganic substances) washed out

As shown in Figure 2(b), landfill waste exhibits variable

by watering are strongly correlated, so the answer is 2. On

elution behavior in response to watering depending on

the other hand, the structure of the roof of a CSDF and thefactors related to the structure of the landfill site andamount of pollutants (inorganic substances) washed out by

equipment installed at the facility. Specifically, factors (b1)watering are not correlated, so the answer is 0. The landfill

through (b4) are involved. (b1) “Roofing material used at

waste is assumed to be bottom ash.

landfill site,” a factor related to roof structure, which

The following information was extracted from the table.

influences the temperature of the landfill waste; (b2)・ The more important operational factors were amount of“Frequency of ventilation and number of ventilation spots,” asupplied water, watering schedule (period of watering,factor related to the method of ventilation used at the site,

intensity of watering, and frequency of watering),

which influences water evaporation from the landfill layers;

quality of supplied water, air supply method, and(b3) “Presence of gas collection pipes and location andwhether there had been any pretreatment before

number of gas-drainage pipes installed,” a factor related tolandfilling.

the method of supplying air to the landfill layers, which

・ The more important causal factors, which are related toinfluences the atmosphere inside the landfill layers; (b4)changes caused by operational factors in the“Interval of sucking gas,” a factor related to the control

characteristics of landfill waste, were moisture content

method of the evolved gas from the waste which affects

and water-holding capacity of the landfill waste.

chemical degradation and microbial degradation of landfilled

・ The more important phenomena occurring in waste

waste.

layers were biological or chemical degradation, quality(3) Wateringof leachate (COD, BOD, TOC, etc.), and generation ofheat and gas.

As shown in Figure 2(c), landfill waste exhibits variable
elution behavior in response to watering depending on
factors related to the method of watering. Specifically, factors

5

EXPERIMENTS ON WASTE STABILIZATION(c1) and (c2) are involved. (c1) “pH, salt concentrations andIn Japan, various kinds of experiment have been

temperature of water used for watering,” a factor related to

conducted on waste stabilization. These experiments are

the quality of water used in watering; (c2) “Intensity, Interval

divided into three categories with regard to the experimentalof watering and Period of watering time,” a factor related toconditions, as shown below.

a. Aerobic or anaerobic conditions, which depend on the
differences in an oxygen supply system.
b. Watering conditions (period of watering, intensity ofwatering, and frequency of watering).
c. Infiltration characteristics of water into the waste because

phase (L1) surrounding a solid phase (S). It also makes thefollowing assumption: eluted substances move from the solid
phase (S) to L1 and the elution velocity is proportional to theconcentration gradient between the two phases. Moreover,when there is migration of water as a result of watering,of differences in degree of compaction.

again there is movement of eluted substances from L1 to L2

Most of the experiments have been conducted at a

at an elution velocity proportional to the concentrationlaboratory scale of tens of centimeters, with pilot experiments

gradient between the two phases when the L1 phase comesat a scale of tens of meters.

into contact with the mobile phase (L2).
We define the activity from the start of watering (supply of

6

PROPOSED WATERING METHOD USING A

water to the waste layer) to the termination of watering as the

NUMERICAL MODEL“watering period.” Between any given watering period andWhen the authors looked at methods of watering to elutethe next watering period, there is a period in which watering

substances in landfill waste, they first proposed a numericalis discontinued. In other words, watering is not continuous.

model to simulate elution behavior after watering the

At CSDFs, the fundamental approach to the management ofrelevant substances (hereafter referred to as a “washoutwatering is this type of intermittent watering. The

model”). They then simulated quantitatively elution

non-watering interval is an important element in the effective

characteristics under various watering conditions using thistransfer of eluted substances to the mobile phase. The reason

model. Based on the results, they determined the wateringis that during the non-watering interval, eluted substances

method exhibiting the most effective elution behavior fromseep from the solid phase (S) to the immobile water phaseamong numerous watering patterns.

(L1) and accumulate in the immobile water phase. We think

6.1 The idea behind the washout model

this is because of the phenomenon of increased concentration

In the washout model, there are two phases in theof eluted substances in the L1 phase.

unsaturated flow within the waste layers: a mobile phase in

Based on the above, the washout model can be regarded

which liquid (water) is able to flow and an immobile phase in

as a modeling of the phenomenon of migration of eluted

which liquid (water) is unable to flow. The model is based on

substances during these intermittent periods, which is awhat is called a “two-phase model” in which there is
migration of dissolved substances caused by differences inmobile water

concentration between the two phases. In our research, Ishii

immobile water

expanded this two-phase model by making allowance for

L2

diffusion within the solid phase (particles of landfill waste).

L1

supplied water(or void space)

As a result, it was possible to incorporate migration ofdissolved substances from the solid phase to the immobile
water phase in the washout model. The basic idea of thewashout model is shown in Figure 3. As this figure shows,the model assumes that there is always an immobile-water

TOC

S

Particles ofbottom ash

klks

Figure 3. Idea of washout modelcritical element when considering watering operations andelements) should be included in the evaluation function toeffective elution of substances in a CSDF.

assess the optimal performance of multiple watering

6.2 Check of the model

methods? Looking at it the other way, different evaluation

In order to validate the accuracy of the washout model, we

criteria (evaluation elements) in the evaluation function could

conducted experiments using indoor column experimental

result in different optimum watering methods. Here weequipment1), 4) on elution of TOC found in actual bottom ash

focused solely on “cost” in our evaluation. Specifically, of theunder the watering conditions. We compared thevarious types of watering equipment that could be utilized toexperimental results obtained with the analytical results

stabilize landfill waste, we assumed that the optimum

calculated from the washout model. We confirmed that aswatering method would be the one with the minimum total

long as we can identify the appropriate parameters to input

cost, when relative costs are compared for the largest

into the washout model, it is possible to simulate actual TOC

water-treatment equipment, represented by an evaluation

elution behavior with this model.

function comprising the sum of costs from the construction

6.3 Determination of watering method

stage through the operation stage and the running stage. We

Based on the experimental results and results of otherexperiments, by changing various watering conditions suchpresent a specific example below.
7.1 Evaluation function

as watering intensity, period and frequency of watering,

As shown in equation (7), total cost of water treatment

different results are obtained in elution behavior for the same(CT) (¥) used as an evaluation function is the sum oftarget object. Based on the results of this research to date, weconstruction costs for water-treatment facilities (CI) (¥) plus

define these three watering conditions as manipulated

running costs for the water-treatment facilities (CM) (¥). As

variables and the results obtained by changing each of theseshown in equation (8), CI is expressed as a relational

manipulated variables (the final cumulative volume of water

expression consisting of volume of leachate treatedrequired and amount of time to completion of watering) as

Qday[m3/day] and coefficient of treatment capacity for thethe performance function. We view the watering pattern thattarget substances to be treated KI[¥, day/m3]. It varies, for

optimizes (maximizes or minimizes) the performanceexample, depending on the number of target substances to befunction as the optimum watering method for the CSDF.

treated in a situation involving high-level processing, such as
demineralization, when there is a large daily treatment
volume. As shown in equation (9), Qday is obtained from

7

OPTIMIZATIONOFSTABILIZATIONMETHOD

retardation factor R[-] taking into account watered areaA[cm2], watering intensity I[mm/h], watering period P[h],If the various effects arising from differences in watering

watering frequency F[1/day] and unsaturated flow ofmethods can be evaluated quantitatively, it should be possible

leachate in a vertical direction. As shown in equation (10), R

to propose an optimum watering method for effectively

is the time factor GV[sec] multiplied by seepage timeeluting substances contained in waste in order to stabilize

obtained by dividing seepage velocity V[cm/sec] by thewaste. When evaluating these effects, the choice ofdistance covered by seeping leachate H[m] from the wateredevaluation criteria, or evaluation function, to quantify thesurface to the bottom of the landfill.

effects is important. What evaluation criteria (evaluation

As shown in equation (11), CM is the sum of runningPeriod B (TB) Period C (TC)3,500

トータルコストと浸出水処理能力との関係（散水時間と強度を変化）

100[m3/day]50[m3/day]

3,000[¥/year]

I,F=3,1
I,F=5,1

2,500

I,F=10,1

Landfill
start
Treatme nt
start

CT [円]

10[m3/day]Time[year]

Landfill
completion
Treatme nt
completion

2,000

1,500

1,000

Watering
completion

Watering
start

500
0.0

Figure 4 Relationship between duration and cost of
running leachate-treatment facilities

period A, running costs for leachate-treatment facilities(CMB) (¥) in demineralization period B, and administrative
costs for leachate-treatment facilities (CMC) (¥) in the periodof preparation for abolishment C. CMA•CMB•CMC are
Qday and running-cost coefficients KA•KB•KC[¥•day/
year•m3] for each period A–C and fixed running costs
multipliedbywatering

periodTA•TB•TC[year], as shown in equations (12), (13), (14). The
B

relationship shown in Figure 4 is an example. Regarding
TA•TB•TC, TA is the landfill period while TB and TC are
B

5.0

10.0

15.0

20.0

25.0

30.0

35.0

TA+T
TA+TB+TC[年]
B+TC [year]

costs for leachate-treatment facilities (CMA) (¥) in landfill

CA•CB•CC[¥/year]

I,F=1,1CT [¥]< Qday･ KA,B,C+CA,B,C >cost of runningleachate-treatment facilities

Period A (T A)

B

obtained from functions fB•fC used to calculate respective

Figure 5 Relationship between CT and watering period

Qday =

R × A× I × P × F
1000

R = GV ×

V
H

(9)

(10)

CM = CMA + CMB + CMC(11)

CMA = (Qday × KA + CA) × TA

(12)

CMB = (Qday × KB + CB) × TB

(13)

CMC = (Qday × KC + CC ) × TC

(14)

B

equations (15), (16). TB and TC obtained from functions fB

TB = f B ( R, I , P, F )TC = f C ( R, I , P, F )and fC can vary substantially depending on the physical,

7.2 Challenges for optimizationwatering periods using R, I, P, F as variables, as shown in
B

B

(15)
(16)

chemical and biological characteristics of the target

Figure 5 shows an example of a calculation of total cost of

substances affected by watering as well as landfill conditions,water treatment CT using different watering methods. Theseand also by constraints on abolishment standards, such as

results show that simply shortening the watering period does

permissible concentration, etc. Consequently, functions fB

not lower leachate-treatment costs. The likely reason for thisand fC cannot be regarded as established (universallyis that construction costs CI are more dominant than runningapplicable) calculation formulas at this point of time.

costs CM within CT. The precision of this result depends on

B

CT = CI + CM

(7)

CI = Qday × KI

(8)

the precision of the watering period (TB•TC) depending on
differences in watering method (I, P, F) and the precision ofrunning costs CM. Improving the level of precision is an
issue to be addressed going forward. Another issue requiringfurther study is improvement of the evaluation function by
the selection of cost elements apart from water-treatment

ACKNOWLEDGMENTS
This paper summarizes the results of research on

costs and evaluation criteria apart from cost.

fundamental concepts of waste stabilization by the Research

7.6 Future studies

Group for Control, Research Committee for Closed System

In order to achieve practical application of a future

Disposal

Facilities.

Masahiko

Shioyama(Kubota

optimization model, elements that should be taken into

Corporation) provided advice on the content of this paper.

consideration (in addition to those mentioned above) are:Yasuyuki Yoshida (Environmental Technologic Consultant

・ Collection of data in other pilot-scale experiments

Co. Ltd.) advised on the quality of landfill waste in Japan,

・ Changing in characteristics of structure of bottom ash

Toshiki Tanaka (Kajima Eco-Solutions Co. Ltd.) advised
on influencing factors of physical properties changes,

layer
・ Elution phenomena of many components

Toshiyuki Kobayashi (Kobelco Corporation) advised on

・ Simplification of model to design watering equipment

extracting influence and operation factors for promoting

・ Arrangement and analysis of TB and TC of measured

waste stabilization, Akira Yajima (Hujita Corporation)Badvised on example of application of a model to a pilot-scaledata in existing landfill site.

experiment, and Atsushi Kaga (Nihon Suiko Sekkei Co.
8Ltd..) advised on factor of objective function. I express my

CONCLUSIONSThis paper presents the results of basic research on factors

appreciation to each of these persons.

influencing changes in properties of waste matter, thewashout model, watering methods and relevant evaluation

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Washout Model, Proceedings Sardinia 2003, Ninthmethods to achieve greater stabilization.

International Waste Management and Landfill Symposium.
2) Ricardo C. de Abreu, Enrique J. La Motta and Kenneth L.

This research has clarified the following.
1) The numerical-analysis model can predict quantitative

McManis (2004). Facultative Landfill Bioreactors (FLB):

TOC concentration in the pilot-scale experiments.

Results of a Pilot-Scale Study, Proceedings of GSP 142

2) The optimization theory may possibly be applied toWaste Containment and Remediation.

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Infiltration for Enhanced in situ Stabilization, Proceedings ofcollection of data in other pilot-scale experiments.

Sardinia 2003, pp. 400-401.

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4) Kazuei Ishii, Tohru Furuichi, and Noboru Tanikawa;

However, in actual situations physical, chemical and

Proposal of Watering Method for Waste Stabilization in

biological

Closed System Disposal Facilities by Using a Washout

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phenomenon is unsteady.

arerelatedandeach

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