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Pistia stratiotes , an aquatic weed, was investigated as a substrate for biogas production in batch digestion. An inoculum was necessary to obtain biogas production from the weed. With Pistia only, production of carbon dioxide alone was high during the first five days of digestion but began to level off thereafter. With inoculated Pistia , a high rate of biogas production was sustained for nearly 10 days and the average methane content was 58–68%. The digesters charged with Pistia alone had significant concentrations of propionic, butyric, isobutyric, valeric, and isovaleric acids. These acids were not present in detectable concentrations, in the digesters running with inoculated Pistia , except during the first 4 days of the digestion when propionic acid was formed. When an inoculum was added to a ‘soured’ digester the performance of the latter improved dramatically.

Eight common aquatic weeds Salvinia molesta, Hydrilla verticillata, Nymphaea stellata, Azolla pinnata, Ceratopteris sp. Scirpus sp. Cyperus sp. and Utricularia reticulata were digested anaerobically to produce methane. The carbon to nitrogen (C/N) ratio, carbon to phosphorus (C/P) ratio, and the volatile solids (VS) content of the weeds varied widely. No trend between these factors and the methane yield was discernable; the possible reasons are discussed. The energy potential of the weeds per unit area of the weed crop was worked out. Natural stands of salvinia, such as the one employed in the present investigation, would yield energy (methane) of the order of 108 Kcal ha−1 year−1.

A method is presented for the spectrophotometric microdetermination of cobalt (II) in fresh waters, aquatic weeds, and animal tissues employing 7-nitroso-8-hydroxyquinoline-5-sulfonic acid. the method enables direct determination of cobalt (II) in presence of a large number of anions and cations commonly associated with cobalt, and is adequate for determining the metal in the concentration range of 0.001–10 ppm, which is commonly encountered in environmental samples.

In continuation of our earlier reports on the impact of heavy metals on certain annelids,1,2 amphibian larvae3 and arthropods,4 we present here studies on the impact of mercury (II), chromium (VI) and zinc (II) on copepod Cyclops sp. and larvae of mosquito Aedes aegypti relative to protozoans, cladocerans, molluscs and rotifers.5–8 The impact studies were done with the aid of static bioassays and computer aided analysis of mortality data. The Median Lethal Concentration (LC50) and Safe Concentration (SC) values were calculated and compared with the corresponding values reported previously for other arthropods and protozoans and with the minimum levels of these metals allowable in drinking waters, irrigation waters and effluent outfalls as per various national and international agencies (WHO,9 USEPA,10 USDI,11 NAS,12 FRG,13 USSR,13 Canada,14 and ISI15–17). The studies revealed that in many cases the prevailing standards are too liberal. The disparity between maximum allowable levels of zinc and its safe co...

In continuation of our studies on the toxicity of heavy metals1–3 we report the impact of mercury (II), zinc (II) and chromium (VI) on hatching of the pupae of mosquito Aedes aegypti. The studies were carried out in terms of abnormal responses in swimming and flying, incompleteness of the metamorphosis and mortality — all with respect to controls. The toxicity trend with respect to the mortality of pupae in partially or completely hatched conditions, as also with respect to reduction in the number of pupae reaching the mosquito stage was Hg > Zn = Cr. A comparison of lethal doze values of the metals obtained through present experiments with the levels allowable in irrigation waters, as per regulatory agencies based in U.S.A.,4 Canada5 and India,6 indicate that the permissible levels of zinc and chromium are higher by several orders of magnitude than the lowest lethal concentrations for pupae of A. aegypti. The studies thus point to a need for the revision of the existing standards.

Etude experimentale de la capacite de survie et de la croissance de Salvinia molesta dans des eaux contenant 1-10 et 100 ppm de zinc (II) et de la digestion anaerobie a 35 o C environ de cette plante aquatique

The paper starts with a report on the growing predominance of aquatic weeds of the genusSalviniain the waterbodies of the world. The various ways in whichSalvinia molesta, particularly, causes pollution and depletion water resources are then reviewed, as are the attempts made so far to control and eradicateSalviniathrough chemical, biological, and mechanical, means. It is shown not only that these attempts have failed to achieve their purpose but also that there are possibilities of environmental backlashes being caused by the introduction of chemicals or bioagents into the environment. The importance developing environmentally safe methods of utilizationSalviniahas been stressed, so that the cost of mechanical removal of the weed can be offset by the profits earned through its utilization.

The paper deals with the effect of traces of heavy metals copper (II), zinc (II) and nickel (II) on biogas production from aquatic weed Salvinia molesta (Mitchell). The experiments were carried out in all-glass air-tight digesters under anaerobic conditions at 37.5 ± 2.5 °C. Five controls were used with respect to which the stimulation in biogas was studied in duplicate catalysis experiments. The results indicate that all the three metals significantly catalyse the biogas production – the enhancement in the quantity of gas being in the range 13–50% with no adverse effect on the calorific value. The enhancement followed the trend Cu (II)> Zn (II)> Ni (II). The significance of the results and the possible mechanism of catalysis are discussed. It is also brought out that due to very low concentrations of the metals employed, the slurry ensuing the digestion can be safely disposed on land or utilised in irrigation.