Rôle de la fréquence des prélèvements de la biomasse produite sur les capacités épuratrices de Lemna minor L. Effect of the removal frequency of biomass production on the purification efficiency of Lemna Minor L.

Résumé

Les Lemnaceae en général, Lemna minor L. et L. gibba L. en particulier, font l'objet de recherches importantes au niveau international sur le rôle positif ou négatif que ces espèces peuvent jouer dans le domaine de l'épuration des eaux usées urbaines par les diverses techniques extensives.

Selon les travaux scientifiques réalisés en vraie grandeur comme en pilote, les auteurs insistent soit sur l'intérêt de ces espèces (productivité élevée, haute teneur en protéines, capacité de bioaccumulation de métaux lourds, prélèvements périodiques aisés, bonnes capacités épuratrices en N et P. ...) soit sur leurs inconvénients (effets néfastes sur l'épuration dans les lagunages à microphytes, anaérobiose de la nappe aquatique, prélèvements périodiques difficiles - ! -, capacités épuratrices médiocres, voire nulles, ...).

Dans ce contexte de résultats apparemment contradictoires, qui ne se limitent d'ailleurs pas aux seules Lemnaceae - loin s'en faut-, nous avons comparé, sous climat local, les rendements épuratoires de deux systèmes de bassins miniatures en série, rigoureusement identiques, alimentés par le même débit des mémos eaux usées urbaines durant toute la période de végétation : l'un des systèmes était peuplé de Lemna minor, l'autre n'en contenait pas.

Nos résultats font apparaître que l'efficacité épuratrice de Lemna minor dépend, entre autres, de la fréquence des prélèvements périodiques de la biomasse produite.

1. Lorsque les prélèvements sont peu fréquents (7 fois sur la période de végétation), le système à Lemna épure sensiblement mieux que le témoin non « planté » en ce qui concerne les matières en suspension et la charge organique. Il influence défavorablement, par contre, l'épuration tertiaire au niveau de l'azote et surtout du phosphore.

2. Lorsque les prélèvements sont fréquents (24 fois sur la période de végétation), le système à Lemna améliore considérablement la rétention des matières en suspension et celle de la charge organique. II augmente nettement l'épuration tertiaire au niveau de l'azote et reste sans effet pour le phosphore.

Mots-clés:

• Lemna,
• eaux usées,
• épuration,
• lagunage,
• macrophyte

Abstract

The affect of Lemnaceae, and in particular of Lemna miner L. and L. gibba L. on various extensive urban sewage treatment techniques is currently the subject of intense international research.

Researches on both full-scale and pilot plants stress either these species' advantages (high productivity, high protein content, heavy metal bioaccumulation, easy harvesting, high N and P purification efficiency, ...) or disadvantages (detrimental effect on microphyte ponding, anaerobiosis in the water layer, difficult harvesting, poor or non-existent purification capacity).Against this background of apparently conflicting results (and in this respect, Lemnaceae are far from being the only case), we have compared the performances of two systems under local climate, bath consisting of 4 strictly identical ponds in series (4 x 0.96 m²) supplied with an identical influx of the came urban sewage, one with a population of Lemna miner and the other without.

Each series of ponds was supplied with a hydraulic load of wastewater corresponding to the use of 6 square metres of pond per capita.

The two series of ponds were monitored in 1985 and 1986 during the periods of vegetative growth (from May to October inclusive).

In 1985, the duckweed biomass was harvested 7 times during the experimental period : each lime, 50 % of the plant cover was removed; this frequency amounts to roughly one harvest every fortnight in June, July, August and September.

In 1986, the duckweed biomass was harvested 24 times during the experimental period in the same manner as the previous year; this frequency amounts to 1 or 2 harvests every week in May, June, July, August and September.

The parameters chosen to characterize the pollutant load at the inflow and the outflow of each ponding system were measured twice monthly.

An amount of wastewater proportional to the volume in circulation was sampled automatically at each level. Our protocol required on sample of inflow every 30 minutes and one sample per litre of outflow. Consequently, average concentrations of the various parameters were assessed on the basis of a mixture of 14 x 48 inflow samples and a mixture of outflow samples equal in number to the volume in litres flowing through the system in two weeks.

Taking into account the automatically recorded hydraulic flow rates, theses average concentrations were then converted into absolute loads (flow rates) for each parameter and for every 14-day period. They are expressed in gram per 14 days.

Four parameters are discussed :

- suspended solids (S.S.)

- total chemical oxygen demand (total COD) on unfiltered samples, i. e. the total organic load

- total nitrogen on unfiltered samples (total N)

- total phosphorus on unfiltered samples (total P).

Our results show that the purification efficiency of Lemna minor does depend on the frequency of the biomass production removal.

1. When the removal is less frequent (7 times during the period of vegetative growth), the system with Lemna minor has purification rates that are significantly better than those of the unplanted control system as far as suspended solids and organic load are concerned. However, it has a negative impact on tertiary retention for nitrogen and especially phosphorus.

2. When the biomass production is frequently removed (24 times during the period of vegetative growth, the system with Lemna minor considerably improves the retention of suspended solids and of organic load. It also improves significantly tertiary purification as far as nitrogen is concerned, but bas no effect on phosphorus.

Keywords:

• Lemna,
• wastewater,
• pollution removal,
• stabilization pond,
• macrophyte