GROWTH AND MACRONUTRIENT REMOVAL OF WATER HYACINTH IN A SMALL SECONDARY SEWAGE-TREATMENT PLANT

Studies have been made of the growth characteristics of water hyacinth , Eichhornia crassipes (Mart.) Solms, and its ability to remove N, P a nd K, in a secondary settling pond of a small secondary sewage treatme nt plant serving both the academic and residential blocks of the Swire Marine Laboratory, University of Hong Kong. The treatment plant consi sts of, in series, a primary settling tank, a trickling filter compart ment and a secondary settling pond from which part of the treated wast ewater is recycled to the primary settling tank while the remaining ef fluent (1 to 2 m3 daily) mixes with and hence is diluted by the outflo wing seawater from the aquarium system of the Swire Marine Laboratory before discharge to the sea. Samples of wastewater have been taken reg ularly from the primary sedimentation pond, the outflow of the trickli ng filter, the secondary settling pond and the effluent of the treatme nt plant (before mixing with aquarium outflow) since January, 1992. Ph ysical, chemical and biological characteristics of the samples have be en determined and are typical of secondary effluents, with a mean pH o f about 7.5, total solids 1200 mg L-1, suspended solids 45 mg L-1, con ductivity 2000 muS cm-1, salinity 1 ppt, dissolved oxygen 2 mg L-1, BO D5 45 mg L-1, Kjeldahl-N 30 mg L-1, NH4-N 25 mg L-1, NO3-N 4 mg L-1, t otal P 10 mg L-1, K 35 mg L-1 and total coliforms of less than 10(5) c olonies 100 ml - 1. Water hyacinth plants have been stocked in the sec ondary settling pond as an integral part of the treatment plant so as to improve the quality of, as well as to retrieving and recycling nutr ient elements from, the wastewater. The plants are periodically harves ted to maintain an active growing crop. The growth rate, standing crop biomass, tissue nutrient composition, nutrient storage and accumulati on rate of two growth cycles, one from February 25 to March 18 (mean t emperature 17.6-degrees-C) and the other from 22 April to 12 May (24.8 -degrees-C) are reported. The water hyacinth assumed a relatively high standing crop biomass of 10 kg m-2 (5 to 6 t DM ha-1), and growth rat es of 48 and 225 g m-2 day-1, respectively, for the first and second g rowth period. Nutrient storage capacities were relatively high, at abo ut 20, 7.5 and 16.5 g m-2 for N, P and K, respectively. The nutrient c omposition was very high, reaching 5.42% for N, 1.97 for P, and 4.57 f or K. Both the stem and lamina accumulated high levels of N, while the petiole had the highest level of P and K. Apart from nutrient removal , the water hyacinth also helped to decrease the suspended solids, BOD 5 value and total coliforms of the wastewater. It is concluded that wa ter hyacinth improves the quality of wastewater in such small-scale se wage treatment plants and it is recommended that frequent harvests of water hyacinth would increase the treatment efficiency, especially dur ing the active growing season with high temperatures coupled with inte nse solar radiation.