The nutrient assimilative capacity of maerl as a substrate in constructed wetland systems for waste treatment

Abstract

This study evaluated the performance of maerl (calcified seaweed) as a substrate for artificial wetland waste treatment systems. Pilot scale artificial wetlands were set up in the laboratory; three planted with Phragmites australis and three unplanted, and fed with a synthetic sewage solution. The effluent from the tanks was monitored over a period of 9 weeks for chemical oxygen demand, total nitrogen, total phosphorus, ammonium-N, total oxidised nitrogen, soluble reactive phosphorus, dissolved oxygen, pH and temperature. The data were analysed using repeated measures ANOVA to look for significant differences between treatments, and within treatments, over time. A batch incubation experiment was also carried out to ascertain the maximum adsorption capacity of maerl for phosphorus. Results obtained were compared with those in the literature for other substrates. Variability within and between treatments was high, but it was found that maerl effectively removed total phosphorus (98%). Nitrogen removal was less effective, with the tanks producing ammonium-N. The low nitrogen removal shown in the tanks was a factor of the short duration of the experiment; but ammonification did decrease in the planted tanks over time. Performance at removing nitrogen was normal when compared with figures in the literature, but phosphorus removal by maerl was considerably higher than gravel bed wetlands, and comparable with the very best figures given for artificial wetlands based on novel substrates such as shales and slags. This trial showed that maerl has great potential as a constructed wetland substrate, due to its high phosphorus-adsorbing capacity.