13-jun-2006, STOWA

Background

The need to comply with Dutch environmental standards required partial or complete coverage of WWTP's in recent years. In the Netherlands biological filters are predominantly used to treat the ventilated process air from the primary treatment or sludge thickeners.

A study has been carried out to determine the most suitable construction and material for biofilters. Lava material is preferred as supporting material in biological filters, because of a stable and robust filter operation and associated low operating costs.

Description and working principle

Lava filters are biological filters using lava stones as support material on which micro-organisms are growing in a thin biofilm. Lava material provides a large surface area in combination with a relatively low density of the material itself. The latter characteristic limits the overall weight of the filter, while the large surface area increase filter removal efficiency.

Bacteria in the biofilm are breaking down the odour components in the air, such as hydrogen sulfide. For the biodegradation processes to occur the bacteria have to provided with sufficient oxygen supply as well as water and nutrients for cell growth.

The contaminated air enters the filter at the bottom of the filter and passes in an upward direction through the filter. Water is periodically sprayed on the surface of the biofilter and trickles down over the lava rock to the bottom, where it is collected. The spraying prevents a dry-out of the active bacteria in the biofilm and ensures a constant pH value in the filter. At the same time the spraying makes nutrients available to the bacteria.

The percolating water collected at the filter bottom contains odour components as well as sulphuric acid from the biological oxidation of hydrogen sulfide. Dependent on the process design the collected water is recirculated or subjected to further treatment.

Design guidelines / Technical data

Some indications on general design loading parameters were given by [1]:

Lava rock can be used in filters with a filter bed height of more than 1 metre. Typically filter height is in the range of 1 to 4 metres. Below design parameters of a typical lava filter at WWTP Bath (NL) are given:

The lava material is stable and not prone to settling processes if compared to other filling materials such as wood chips or peat. The size of the lava pebbles and stones varies between 8 to 64 mm, the specific density is about 1000 kg/m³. Usually a filter bed consists of several layers with different stone sizes. Parameters of a typical three layer structure of an filterbed with an overall height of 3,4 metres  are given below.

Measurements have shown that 90% of the odour components are removed in the first metre of a lava filter bed. In the figure below the influence of the filter bed height on the removal rate is shown.

Removal rate vs. filter bed height at WWTP Bath (The Netherlands) [1]

(hoogte denotes filter bed height)

Performance

General performance

The removal efficiencies of lava filters are reported to be in the same range or higher than those of filter with other support material, such as peat, woord chips, cocos fibres. Conversion capacities are dependent on the average loading rates (at certain temperatures).

Performance at specific installations

Some typical performance indicators of lava filters are given below:

¹GE denotes the term 'geureenheden' - odour units

Operational stability and maintenance

The operation and necessary maintenance of the filter are controlled by several parameters such as loss of pressure, pH value and bicarbonate concentration in the percolate as well as the required removal efficiencies.

Several remarks were made by [1]:

·       a stable operation is reported due to the inherent physical stability of the lava rock (settling does not occur)

·       the required maintenance is minimal compared to other, organic support materials such as wood chips

·       the life expectancy of the material is estimated at 15 years

Capital and operating cost

Some indications of the capital costs of a lavafilter are given by [1]. In the table below the costs are given dependent on the filter capacity, indicating that a centralised odour control can result in considerable savings. However these savings have to be set off against higher investment cost for additional pipework.

The operating costs are expected to be low compared to organic support materials because of a higher lifetime expectancy and reduced maintenance requirements.

In calculating the overall annual costs the longer lifetime expectancy has to be set off against higher transport costs, especially if large filters are to be built.

Reference installations

It is estimated that roughly 20 to 25% of the air treatment capacity on Dutch WWTP is based on lava filters. The number of reference installations has not been reported [1].

Suppliers / Patents

There are a few suppliers of lava support material for biofilters in the Netherlands, such as Bioway BV (www.bioway.net), Plasticon BV (www.plasticon.nl), KunststofWerktuigBouw BV (www.kwb.nl).

Literature references

[1]   Stowa report 2000-03: Biological air treatment systems on WWTP's. Design and dimensioning based on literature review and paractical experiences (in Dutch). 2000.