Master of Science Environmental Water Resources Engineering and Management

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    item: Thesis-Abstract
    Modelling of growth cycle of water hyacinth : an application to Bolgoda Lake
    Fernando, PUD; Jayaweera, M
    Water hyacinth (Eichhornia crassipes (Mart). Sohns) is considered as a problematic aquatic weed in many lakes, irrigation canals, stagnant ponds, waterways and semi-wet areas in Sri Lanka. Bolgoda Lake is one of the freshwater bodies in Sri Lanka, which has been severely affected by excessive growth of water hyacinth thereby clogging the water ways and hence adversely affecting water quality. This study was conducted to determine the growth characteristic of water hyacinth under influence of natural, physical and chemical factors in Bolgoda Lake. The parameters considered in the study were as follows: biomass in dry weight, biomass production per day, phosphorus and nitrogen contents in plant tissues, phosphorus and nitrogen contents in the water body, pH, temperature, and salinity. The luxuriant growth of water hyacinth was observed during the study period, which occurred with the temperature ranging between 26-32 °C, pH from 6.67-7.76, salinity from 0-1.5 ppt and water nutrients from 4.6-17.4 mg Nil, 0.18-0.70 mg N03-NIl and 0.14-0.93 mg P/I and 0.02-0.16 mg P04-PIl respectively. Under such conditions, results revealed that hyacinth plants produced a biomass yield of 20 -1800 g dry weight/m' and the number of plants increased from 21 to 412 per m2 for the entire study period of 14 weeks with doubling time of around 13-15 days. The biomass production rate varied from 2.10-75.25 g dry weight/or' per day. Results of heavy metal uptake experiment suggest that rhizofiltration (metal absorption into roots) and phytoextraction (concentrate into the harvestable parts of roots or shoots) are the key mechanisms for removal of heavy metals from the aqueous phase. Phytoextraction was more responsible in translocating heavy metals to aboveground parts in initial few weeks and rhizofiltration became prominent at the later stages in which more metals are bound to below-ground parts. Once the heavy metal binding was complete, harvesting was suggested at the end of the 13th week during which more metals were adsorbed only to root zone. From this study it was shown that there exists a massive proliferation of water hyacinth stands in Bolgoda Lake with a great influence of nitrogen, phosphorus, pH and temperature. However, there has been a significant perishment of the existing stands of the vegetation from time to time due to the exposure to saline waters entering from the tidal action. A numerical model was developed to simulate the growth of water hyacinth in Bolgoda Lake, Sri Lanka. The model was first applied to experimental data from Sato and Kondo, (1981). Secondly, it was used to evaluate the management options to control the growth of water hyacinth in Bolgoda Lake. Model application showed how the model could be used to evaluates the management options to control the growth of water hyacinth and to reduce the available nutrients in the system. These options include harvest strategies (initial density and harvesting interval) and harvest rate. The maximum yield of 329 g / m2 dry weight was obtained when the rate of harvest was analogous to the initial density (at 100 g dry wt/rrr') in that the water hyacinths were harvested at a uniform rate every 20 days. The continuous harvesting is the major objective criteria to remove available nutrients in the water body and to control the excessive growth of water hyacinth in Bolgoda Lake.
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    item: Thesis-Abstract
    Sequencing batch reactor treatment of garment factory effluents in Sri Lanka : an empirical value for aeration factor
    Wijayatunga, WMC; Manatunge, J
    Abstract Most factories located outside the DOI industrial zones in Sri Lanka do not have proper sewage treatment systems and due to the high number of employees these factories are always encountered with the problem of handling large quantities of low strength sewage generated from lavatories, canteens and kitchens. In such factories sewage is usually handled using conventional septic tank - soakage pit systems. Such systems are commonly found infested with insects, and promote further breeding. They cause nuisance due to obnoxious odor, and is a major cause for severe ground and surface water pollution. It has been noted that these factories have critical problems of handling sewage during rainy season, especially when the factor)' is located in areas with high ground water table such as a marshy land or near a surface water body./ To solve the problem of handling septic tank effluents. Sequencing Batch Reactor (SBR) treatment plants have been provided in several factories due to their lower cost arid minimum use of land area. Most of these plants produce effluents of quality far better than what is stipulated by the Central Environmental Authority (CEA) effluent standards. Parameters such as BOD are sometimes being reduced even down to undetectable levels which are rather unnecessary. Moreover, it has been noted by the industry that there may be possibilities to cut down the monthly electrical cost by operating the treatment plants at optimal levels of treatment. On the other hand, treatment plant contractors have noted that the bid prices were relatively high due to over-treatment specified and set under conventional design criteria. Over-treatment and possibly higher than necessary, raw-BOD value for design purposes, seem to be the problem areas. Therefore, it is important to find a better value for aeration factor expressed in terms of oxygen dissolved in clean water (ODCW), for the use in calculations of aerobic sewage treatment plant designs, treating septic tank effluents. The lengthy calculations needed for designing aeration tanks and selection of number and type of aerators are therefore perceived to be correctly, easily and quickly done if a proper empirical value for aeration factor is available. ITic research was therefore carried out with the following objectives: 1.A reasonably accurate value for the oxygen requirement in oxidation of 1 kg of BOI) (Aeration factor) used in design calculations./ 2.To propose a reasonably correct value for septic tank effluent BOD for garment factories./ 3.To calculate the cost saving with a new value of aeration factor both operational and capital. It was found by the experiments carried out in this research, and samples taken simultaneously elsewhere that the average septic tank effluent BOD was around 250 mg/1 and ranged between 186 mg/1 to 284mg/l against the previously used value of 300 mg/1 which is 17% less./ The aeration factor used for carbonaceous oxygen in designing above SBR plants was 4kgOi/kg BOD and the value proposed in literature for various other countries range from 1- 2.5 kg02/kg BOD depending on temperature fluctuations. This research suggests 3.5 kg02/kg BOD, as an appropriate value for respiration factor in treating septic tank effluents from Sri Lankan garment factories. This is a reduction of 12.5% in oxygen requirement compared to previously used value. The effect of application of above two parameters in the form of electrical power and capital saving were approximately 27% and 25% respectively for the particular SBR design used for them research.