About 60% of Generated Sludge is Reused in Japan

In sewerage systems, sewage sludge is inevitably generated through
wastewater transportation and treatment.  Appropriate treatment and
disposal of generated sludge, as well as water quality control of
treated wastewater, is essentially important for rational maintenance
and operation of sewerage systems.

The amount of generated sludge in Japan has increased year by year
in proportion to growing sewered population rate, and it reached 2.1
million Dry-solid ton throughout the year in 2002.  In future, the
amount will undoubtedly increase with further promotion of sewage
works and implementation of advanced wastewater treatment.

On the contrary, the available capacity of sanitary land fills for
sludge disposal is insufficient because of increasing amount of wastes
with the progress of social economy and difficulties to secure and
construct new dump yards.

Recently, the Basic Law for Establishing a Recycling-Based Society in
which cyclical use of resources and principals of proper disposal of
wastes are stated, has been established, and Waste Disposal Law has been
revised to strengthen the responsibility of polluter and prevent
insufficient treatment of waste.

To deal with the shortage of sanitary land fills area and follow the
above-mentioned laws, continuous efforts to establish and promote the
Recycling-Based Society are needed in the field of sewage works.

Annual change of generated and reused amount of sewage sludge was shown
in the following figure.  In 2002, about 39% of generated sludge was
disposed by sanitary landfill, and beneficial recycling rate of generated
sludge was about 60%. In recent years, the ratio of landfill has decreased,
and the recycling rate has steadily increased.


Especially, sludge utilization for construction works as cement ingredient

has been progressed.  Also, sludge utilization for construction works like

melted slag utilization as subbase course material, etc. has increased.

The amount of sludge utilization for agriculture and green area has been
almost constant.

Effective utilization measures of sewage sludge are shown in the following
Web Page.

Table Effective utilization measures of sewage sludge 

1) Sludge Utilization for Construction Work as Cement Ingredient

About 26% of generated sludge in 2002 was utilized as Cement Ingredient.

Inorganic matter of sewage sludge comprises silica, aluminum, calcium,
iron, etc. and the clay, which is used as cement ingredient, comprises
the same elements. Then, after generated sludge is dehydrated and
incinerated, incinerator ash is used as cement ingredient.  Also,
dewatered sludge, which contains organic matter and inorganic substance,
is utilized as fuel for cement burning and cement ingredient.

Cement

Ingredient		 Cement

Ingredient
Dehydrated cake Cement Incinerator Ash

2) Sludge Utilization for Construction Work

About 20% of generated sludge in 2002 was utilized for Construction Work.

In Japan, dewatered sludge is incinerated to reduce its volume, and

incinerator ash is used as soil stabilization additive, roadbed material, etc.

Also, incineration ash is mixed with other material, and tile, brick, clay

pipe, and so on, which contain incineration ash, are manufactured.  Recently,

new technology making brick only from incineration ash was developed.
Ash Contained Brick
Furthermore, dewatered sludge is melted for more reduction of volume than
incineration and stabilization of final product. The slag generated in the
melting process can be used as roadbed material, concrete aggregate, etc. and
also utilized as material for tile, permeable block, etc.
Example of Reuse of Melted slag
3) Sludge Utilization for Agriculture and Green Area
About 14% of generated sludge in 2002 was utilized for agriculture
and green area.  Sewage sludge consists of various organic matter and
inorganic matter and contains nitrogen and phosphorus. Then Sewage sludge
is very useful for agriculture and green area.  There are various
sludge forms for agriculture utilization such as dewatered sludge,
dried sludge, compost, and incineration ash.  However, in respect of fertilizer
effectiveness, convenience in handling, and sanitary affairs, compost is the most
desirable sludge form as a fertilizer.
In addition to those, carbonized sludge is also useful for soil conditioner or
soil for gardening.  Carbonized sludge is produced by carbonizing furnace, which
decompose sludge thermally under oxygen-depleted condition and consists of carbon
and inorganic components.  Then, it is also useful as a fuel like a charcoal and
has characteristics of absorption capacity for odors, colors, toxic organic
compounds, etc.
Powder compost Carbonizing furnace

4) Sludge Utilization for Energy Source

In the process of digestion of thickened sludge, methane gas is generated,

and it is used as energy source for electric power generation or warming of

digestion tank.  Sometimes, it is supplied to city gas.  The amount of digestion

gas, which is effectively reused in 2002 was 180 million m3.

Also, dried sludge or carbonized sludge is used as fuel, and exhaust heat from

incinerator or melting furnace is effectively reused as energy source for

area-wide air conditioning or electric power generation.