Hydroponic tertiary treatment

Three macrophytes are found to grow well in trenches filled with a secondary effluent of a wastewater treatment plant in the absence of any support medium: Canna, Cyperus and Paspalum. Daily concentration changes of nutrients in the wastewater as well as evapotranspiration of the macrophytes were studied batchwise. After the vegetation had covered the trenches fully, the plants removed daily, 1.2–2.6 g of total nitrogen, 0.24–0.33 g of phosphate-phosphorus and 0.3–0.9 g of sulphate-sulphur per sq m. While ammonium and phosphate concentrations always decreased in water, sulphate was found to increase due to evapotranspiration. The daily water losses due to the latter was up to 20 l•m−2 on sunny days. It was found that for extended nutrient removal under these conditions a hydroponic tertiary treatment plant could possibly produce no effluent at all. After harvesting, the aerial parts of the plant tissue contained comparable ash contents of 0.8–0.95% on wet base. Elementary analysis showed similar N and P contents of 23–26 g/kg and 2.3–3.8 g/kg respectively and differences in some inorganic elements of up to an order of magnitude between the three plants.