Effect of the removal frequency of biomass production on the purification efficiency of Lemna Minor L.
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.