Lava filter

A constantly submerged lavafilter

Alava filteris abiological filterthat uses lavastone pebbles as support material on whichmicroorganismscan grow in a thinbiofilm.This community of microorganisms, known as theperiphyton,break down the odor components in the air, such ashydrogen sulfide.The biodegradation processes that occurs is provided by the bacteria themselves. In order for this to work, sufficient oxygen as well as water and nutrients (for cell growth) is to be supplied.

Method

Contaminated air enters the system at the bottom of the filter and passes in an upward direction through the filter. Water is supplied through the surface of the biofilter and trickles down over the lava rock to the bottom, where it is collected. Constant water provisioning at the surface prevents dry-out of the active bacteria in the biofilm and ensures a constant pH value in the filter. It also functions to make nutrients available to the bacteria.

A pile of lavastones in a lavafilter

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

Types of systems

At present: 2 types of systems are used;

1. constantlysubmergedlava filters (fortreatment ponds,combinedtreatment ponds/irrigationreservoirs,...)
2. not-submerged lava filters (for wastewater treatment; wastewater is simply sprayed on the pebbles with this system)

Constantly submerged lavafilters

These are constructed out of 2 layers of lava pebbles and a top layer of nutrient-free soil (only at the plants roots).On top,water-purifying plants(asIris pseudacorusandSparganium erectum)are placed. Usually, around 1/4 of the dimension of lavastone is required to purify the water and just likeslow sand filters,a series of herringbone drains are placed (with lava filters these are placed at the bottom layer).

The water-purifying plants used with constantly submerged, planted, lavafilters (e.g. treatment ponds, self-purifying irrigation reservoirs,...) include a wide variety of plants, depending on the localclimateand geographical location. Plants are usually chosen which areindigenousin that location for environmental reasons and optimum workings of the system. In addition to water-purifying (de-nutrifying) plants, plants that supplyoxygen,andshadeare also added inecologicwatercatchments,ponds,... This to allow a completeecosystemto form. Finally, in addition to plants, locally grownbacteriaand non-predatoryfish are also added to eliminate pests. The bacteria are usually grown locally by submergingstrawin water and allowing it to form bacteria (arriving from the surroundingatmosphere). The plants used (placed on an area 1/4 of the water mass) are divided in 4 separate water depth-zones; knowingly:

1. A water-depth zone from 0–20 cm;Iris pseudacorus,Sparganium erectum,... may be placed here (temperate climates)
2. A water-depth zone from 40–60 cm;Stratiotes aloides,Hydrocharis morsus-ranae,... may be placed here (temperate climates)
3. A water-depth zone from 60–120 cm;Nymphea alba,... may be placed here (temperate climates)
4. A submerged water-depth zone;Myriophyllum spicatum,... may be placed here (temperate climates)

Finally, three types of (non-predatory) fish (surface; bottom and ground-swimmers) are chosen. This of course to ensure that the fish may 'get along'. Examples of the three types of fish (for temperate climates) are:

• Surface swimming fish:Leuciscus leuciscus,Leuciscus idus,Scardinius erythrophthalmus
• Middle-swimmers:Rutilus rutilus
• Bottom-swimming fish:Tinca tinca

See also

• Constructed wetland
• Treatment pond
• Organisms used in water purification