Removal of municipal solid waste COD and NH 4–N by phyto-reduction: A laboratory–scale comparison of terrestrial and aquatic species at different organic loads

Abstract

Laboratory scale tests on phytodepuration of raw and pre-treated leachate from municipal sanitary waste were carried out with four vegetable aquatic and terrestrial species at different organic loads. We used the terrestrial species Stenotaphrum secundatum and the free-floating aquatic species Lemna minor, Eichhornia crassipes and Myriophyllum verticellatum to purify leachate from municipal solid waste. The organic load characterized by COD varied from 2–30 g m −2 day −1. Blanks using tap water served as controls. Duration of the experiments varied from 9–90 days. Maximum concentrations in the experiments were 1600 mg l −1 COD and 300 mg l −1 NH 4–N for S. secundatum. Best results in terms of COD, BOD, and ammonia removal were obtained for raw leachate with COD=2 g m −2 day −1 in free water surface (FWS) wetlands, and with 2 and 5 g m −2 day −1 in subsurface flow (SSF) wetlands. Results show that for pretreated leachate (labeled c) low in BOD and NH 4–N, the aquatic species showed low removal and stress even at the lowest load of COD=2 g m −2 day −1. We cannot say if this is due to the pretreatment itself or the chemical or microbial composition of this leachate. The Stenotaphrum system operated well with this load of leachate c. For untreated leachate (type a and b) the removal and plant growing conditions seemed good at COD=2 g m −2 day −1. For S. secundatum a load of COD=5 g m −2 day −1 operated well. All loads above COD=5 g m −2 day −1 caused low removal and stress, and the green parts of the plants disappeared. Oxygen was, however, consumed throughout the experimental period. For pretreated leachate (type c), the removal of COD were low (−24 to 17%) but good for NH 4–N (52–91%). This leachate also experienced high ammonia removal from the beginning of the experiments, probably due to existing consortia of nitrifying bacteria in it. Statistical analysis shows that the S. secundatum and L. minor systems maintained higher oxygen levels than the M. verticellatum and E. crassipes systems, when operated with tap water. For Lemna minor, this may be due to a better capacity for transporting oxygen into the water. With leachate all S. secundatum systems have higher oxygen levels than the aquatic systems, basically because the water content of the soil has been kept well below saturation. S. secundatum shows a significantly lower removal of COD than did the aquatic systems at a loading of COD=2 g m −2 day −1 of raw leachate. There is no significant difference between the systems in the removal of NH 4–N at a loading of COD=2 g m −2 day −1 of both types of leachate. E. crassipes has a lower removal of NH 4–N than M. verticellatum and S. secundatum at a loading of 5 g m −2 day −1 of COD of both types of leachate. In our experiments, it appears that the amount of free ammonia explains the toxicity of the leachate to the plants. This, however, does not exclude other possible toxic factors.