USE OF WATER HYACINTH IN WASTEWATER TREATMENT AND BIOGAS PRODUCTION

Abstract

Although for a very long time water hyacinth has been considered a problem weed, this image is changing as a result of research work on different aspects of utilization of water hyacinth. From CIVILENGINEERING point of view, treatment of wastewater using water hyacinth and methane production from associated biomass are the two aspects of significance. Work on wastewater treatment and anaerobic digestion utilizing water hyacinth has been done mainly on pretreated wastewaters. Limits of poilutional parameters upto which plants can survive and actively participate in the treatment, quantitative assessment of contribution of water hyacinth in the overall treatment, the possibility of existence of poilutional stratification in water hyacinth pond and such other aspects have not been studied. Further, the effect of the presence of various concen trations of toxicants on methane production is also not known. Keeping these aspects in view, the following experimental work was planned. EXPERIMENTAL WORK Twenty nine sets of experiments were performed on wastewaters having initial COD ranging from 32.5 to 2550 mg/1, with and without water hyacinth in batch mode of operation in ponds of 0A5 m2 surface area. The purpose of these experiments was to study effect of water hyacinth on wastewater purification and to assess the limit of COD above which plants would not survive. As a next step, four ponds of 0.22 to 1.12 m depth were installed to be operated in continuous mode to study wastewater purification in continuous mode of operation1 and to see if stratification existed in the pond. Samples were analysed from various depths during the four months operation time. COD loading ranged from 66.9 to 267.6 kg/ha/day. Samples were analysed regularly for BOD,COD,solids,pH,nitrogen, phosphorus and alkalinity. in Anaerobic digestion was studied in the absence and presence of toxicants for batch and semicontinuous modes of operation by taking twenty five sets of experimental observations. These observations could be classified in four parts, each part having distinct aim of study. The aim of first part of experiments was to study the anaerobic digestion in batch and semicontinuous modes separately. Experiments of second part were conducted in six digesters to ascertain the reproducibility of the results in semicontinuous mode of operation. In the third one, experiments were carried out in fifteen digesters to see the effects of different concentrations of (i) nickel alone and (ii) combined doses of nickel and cadmium on gas production. Lastly effect of scale was studied in two digesters of different capacities. Throughout the experiments, total, volatile and fixed solids of feed and effluent, pH, volatile acids, alkalinity, total gas and its methane fraction were monitored regularly. RESULTS On the basis of the analysis of experimental data, the following conclusions have been drawn : Plants can survive and contribute towards treatment of wastewater only upto a soluble COD concentration of approximately 1000 mg/1 beyond which, the plants dry in wastewaters. In 15 to 18 days' time in batch mode of operation, percentage reduction in COD is about 96 per cent. Time variation of COD in the batch mode of wastewater treatment follows the relation, 1 - {o *«P -V *"Kt (i) where, _ 0.003 f K = purification constant equal to (0.24 +0.36 e iP) f = COD at any time t f = initial COD fo = COD after long Period of time t = time IV This information can be used to find the extent of treatment in a given time t or the detention time for required reduction in COD. Time variation of total solids and nitrogen is also found to follow the similar relationship as given by Eq. 1. During the treatment, total solids and kjeldahl nitrogen are also found to reduce by 66 and 7Z per cent respectively. In the ponds operated without water hyacinth, during the same time period COD reduction is about 79 per cent as compared to 96 per cent reduction in the presence of water hyacinth. This improvement of 17 per cent is attributed to the presence of water hyacinth. It may appear small but it is of particular signifi cance considering the recent strict effluent quality standards imposed by various governmental agencies. Variation of Kwith fp in the absence of water hyacinth follows the following relation, - .003 f K = 0.18, + 0.30 e P (2) Total solids and kjeldahl nitrogen reduction without the presence of hyacinth are also found to be lower, 55 and 43.5 per cent respectively. Arelationship for COD reduction, only due to water hyacinth, with time has been found, using the variation of COD with time for systems with and without water hyacinth. Variation of Kis then given by the following equation, - .003 f K = 0.05 + 0.1* e P (3) This gives a quantitative idea of direct and indirect influence of the presence of water hyacinth plants in wastewater ponds. In continuous mode of wastewater treatment, removal of COD, TKN, TP and TS is 5 to 6 per cent better in case of shallow pond as compared to deeper ponds. Lower pH values and depletion of DO to zero level are observed. However, in shallow pond alkalinity is reduced and in deep ponds alkalinity is increased. Slightly better treatment in shallow pond is attributed to the better interaction of waste water and roots of plants. Contrary to the facultative ponds, poilutional stratification is not found in any of the ponds operated under continuous mode. Obstruction by the water hyacinth mat to the entry of sunlight and transfer of heat from atmosphere to the pond or vice-versa and secondary circulation help in checking the stratification while incomplete coverage of the depth by plant roots encourages the stratification. Absence of stratification is attributed to the net result of all these processes. Based on the present work, design criteria for water hyacinth wastewater treatment system for the treatment of raw wastewaters have been developed. Organic loading rate upto 200 kg BODyWday and hydraulic residence time of 10 days are expected to give effluent BOD and suspended solids less than 30 mg/1. Preferable depth of the unit is found as 1.2 m. In batch mode, it takes about 100 days for complete digestion of water 3 hyacinth. Further, cumulative methane yield is found to be 0.22 m/kg of volatile solids added. The results of batch and semicontinuous modes of operation have 3 been analysed together. Methane yield ranges from 0.14 to 0.16 m /kg VS added 3 for the six digesters, run simultaneously at a loading of 2.35 kg VS/m /day and detention of 20 days. At this loading, analysis of effluent has indicated that acetogens and methanogens work well and no acid accumulations or pH disturbances are observed. Based on available data and the present studies, it is recommended that loadings of 1.6 to 3 kg VS/m3/day and detention of 12 to 20 days for anaerobic VI 3 digestion of water hyacinth is most appropriate. Methane yield of 0.15 to 0.20 m/kg VS added at 35° ± 2°C is expected under these conditions. Effect of various concentrations of nickel, and nickel and cadmium on the bioconversion of water hyacinth into biogas as a function of time has been studied. These studies indicate that metal ions essential for the growth of methanogens, e.g. nickel activate the digestion yielding more methane at lower concentrations. Such activation is found to occur upto nickel chloride concentration of 200 mg/kg wet weight of water hyacinth. For higher concentrations, the methane production decreases with increase in nickel ion concentration. If A and C represent activity and toxicant concentration parameter respectively, the variation of A with C can be represented by the following equations, Gasification activity, 1.45 0.45 (4) A = .05 C 1.5 C e e Methane production activity, A - LZ _ 9jL— (5) A - 0.05 C 3.0 C e e Here, C • c/cm c = Toxicant concentration c = Toxicant concentration at which activity is maximum m Presence of cadmium and nickel in combination is found to inhibit the gas production for the entire range of concentrations studied. On the other hand, methane production is found to be of the same order upto combined concentration of 200 mg/kg wet wt. Due to addition of metal ions, an increase in percentage of methane from approximately 70 to 85 per cent in the biogas is observed irrespective of the change in gasification activity.