Bizkaiko Foru Aldundia - Diputación Foral de Bizkaia

Collection and Treatment of Lixiviates

The European Directive relating to the Dumping of Waste (April 29 1999) defines, among other things, the concepts of "treatment" and "leachate":

a) Treatment: These are physical, heat, chemical or biological processes, including separation, that change the characteristics of the waste in order to reduce their volume or hazardousness, facilitate handling or increase their value.

b) Leachate: This is any liquid that penetrates through the waste deposited and is emitted or contained in a Controlled tip. This is a highly contaminating agent that derives from the dumping of waste in controlled tips.
 
With regard to the treatment of leachates, the European Directive establishes that the appropriate measures will be taken with respect to the characteristics of the landfill and to the meteorological conditions, in order to:

- Control the rain water that penetrates the controlled tip basin.
- Prevent surface or underground water from penetrating into the waste dumped in the landfill.
- Collect contaminated water and leachates.
- Treat the contaminated water and leachates collected in the controlled tip so that they comply with the regulations required for their dumping.

Stripping

The Stripping Plant of the  Controlled Urban Waste tip Municipal in Igorre reduces the amount of ammonia in the leachate in order to reach concentrations of under 300 mg/litre, in accordance with the requirements of the regulations controlling the dumping of water into the general network of collectors of the Water Consortium of Bilbao-Bizkaia.

Once treated, the leachate will be recovered in the Bedia Sewage Treatment Plant, together with the urban and industrial waste water of the Middle and Lower Arratia Valley.

The first stage of this plant, opened on 26th April 1999, is fully compliant with the spirit and letter of the Directive. It was the first to be built in the whole of the Spanish state. This took four months and the investment involved amounted to 186,313 euros used in constructing homogenization raft and a desorption tower with the corresponding conditions required.

The second stage was carried out during the first half of 2004, with an investment close on 200,000 €. This included installing a decanter and a heater exchanger with boiler.

PROCESS:

This consists basically of a system to collect, store and regulate the leachate generated and the treatment system in itself.

Collection - storage - regulation system

This involves a system of concrete ditches that take the leachate to the pumping well which has a capacity of 200 m3. This contains two pumps with the following characteristics:

- Flow: 10 m3 /hour.

- Unitary power: 1.5 kW.

These pumps impel the leachate to a concrete pool with a capacity of 550 m3 in which two submerged stirring units operate. These are responsible for homogenising the leachate, preventing the sedimentation of the solids in suspension. The stirring units have the following characteristics:

- Unitary power: 2.5 kW.

- Operation: Timed

In this pool, the first removal of the ammonia is performed by means of natural stripping. To do this, it has an air injection unit, the major characteristics of which are as follows:

- Unitary power: 13.5 kW.

- Flow of suctioned air: 100 litres/second.

Another pumping system sent the homogenized lixiviate from the raft to the decanter where, by adding ferric chloride, the largest possible amount of solids in suspension was deposited into the bottom of said decanter to be evacuated into a container located nearby. In this manner, the density of the lixiviate is less, thus making it easier to carry out stripping.

Treatment system

The removal of ammonia is greatly dependent upon the temperature and the pH (hydrogen potential) of the leachate.

The increase in temperature is achieved by first sending the lixiviate from the decanter through a heat exchanger prior to reaching the desorption tower. A caustic soda dosage system raises the pH of the lixiviate. This dosage is administered automatically, maintaining a preset pH value in the lixiviate, which may be changed by the plant operator.

Next, the plant undertakes the ammonia removal process in order to attain the required concentrations of 300 mg/litre of ammonium nitrogen. This operation is carried out in a desorption tower which facilitates the release of the contaminant, which is drawn by the air introduced by a 8500 metres3/hour fan. This fan impels the air against the current upwards inside the tower.

The desoprtion tower is filled with pall rings of 2 inches in diameter. These are responsible for increasing the contact surface between the liquid and the air, favouring the release of the contaminant:

- Material: Reinforced polyester.
- Height of tower: 8 metres.
- Tower diameter: 1.3 metres
- Diameter of tower base: 2 metres.