Eutrophication is a serious environmental problem originates from excess nutrient load in water bodies. Health problems can occur where eutrophic conditions interfere with drinking water treatment (Bartram et al,1999).Luxuriant growth of micro and macrophytes is one of the issues associated with eutrophication that has many socio-economic implications in addition to unbalancing the ecosystem structure and function. Biogas from water hyacinth biomass by biomethanation is a possible solution to solve the problem in an environment friendly way. In the present study anaerobic digestion of water hyacinth was conducted in lab scale anaerobic bioreactor system.. Regular analysis of the operational parameters was done to closely monitor the performance of the system. Microbial community analysis of the ALBR and UASB were done by metagenomic approach.Quantitative analysis of the hydrolytic enzymes cellulase, xylanase and pectinase and amylase in ALBR and UASB were done and showed positive correlation with protozoan count.
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Key words: Water hyacinth, ALBR, UASB, Biomethanation, Protozoa
Water hyacinth (Eichornia crassipes) is an aquatic weed creating socio economic problems like hindrance to water transport, boating, fishing and almost all other water activities . Water hyacinth mats degrade water quality by blocking photosynthesis, which greatly reduces oxygen levels in the water. This creates a cascading effect by reducing other underwater life such as fish and other plants.I t also form a microhabitat for a variety of disease causing vectors like mosquitoes, snails etc. Recovering value added products like biogas from water hyacinth biomass by anaerobic digestion is a positive approach to address the issue in an environment friendly way.
MATERIALS AND METHODS
In the present study anaerobic digestion of water hyacinth was conducted in lab scale bioreactor unit. Water hyacinth was collected from the polluted water sources, Veli lake and Parvathy puthanar in Thiruvananthapuram Dist. The plants were transported and maintained in 4.5 m3 capacity water tank adjacent to Environmental technology lab in NIIST.
Plant biomass was subjected to different pre-treatment steps and digestion studies were done in batch experiments to get maximum biogas production. Different pretreatments like mechanical crushing, chopping, alkali hydrolysis and acid hydrolysis were tried. The volatile fatty acid generation, alkalinity production and biogas generated were monitored to assess the performance.
Based on the biogas yield in batch experiments, studies were extended to lab scale continuous bioreactor system. Two separate systems were tested for the digestion process. In one approach, a two stage system was tested (hydrolytic and biomethanation stages in separate reactors) and in the second approach a single stage reactor was tried. The performance of both the systems in terms of biogas yield was compared. The bioreactors were operated under different organic loading rates (OLR) to optimize the performance. Volatile fatty acid (VFA), Chemical oxygen demand (COD), MLSS, VSS etc. was constantly monitored to assess the performance of the system. The slurry characteristics after digestion were done for possible manure application. Quantitative analysis of the hydrolytic enzymes such as cellulase, xylanase, pectinase and amylase in the anaerobic reactor was analyzed and the enzyme activity was correlated with microbial communities in the reactor to understand the functional importance of different microorganisms in the treatment process. Molecular finger printing technique was adopted to follow the community structure and diversity in the treatment
RESULT AND CONCLUSION
Among different pre-treatment approaches tried, mechanical crushing of water hyacinth biomass resulted in faster digestion and higher biogas yield (Graph 1).
Between the two systems tried, the double stage system produced an average 0.409L biogas /g VS within 6 days, meanwhile, single stage system produced same quantity of biogas in 37days, from 2 kg wet weight of biomass. So in single stage system time duration for the complete digestion of the biomass is around 6 times higher than double stage system.
Among the different OLR (7, 11, 14 19 kg COD/m3.day ) tried, maximum methane production was obtained at an OLR of 11 kg COD/m3.day(Graph 2).
Without any external buffering agents or pH regulators, pH of the influent from ALBR and effluent from UASB was found to be between 6.2-7.2. Alkalinity of the reactor was maintained between 20 -30 meq/L. Volatile fatty acid (VFA) of the reactor was high during initial day and was reduced on subsequent days. Mixed liquor suspended solids (MLSS) and Volatile suspended solids (VSS) of the reactor sludge were 21g/l and 12.5g/l respectively.
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Activity of cellulase, xylanase, pectinase and amylase during the digestion process showed a similar pattern (Graph 3,4). From the second day, enzyme activity showed a similar patternand after ninth day it started to decrease.
The main protozoa present in the system were Menoidium, Cyclidium, Metopus, Colpoda, Rhyncomonas and Bodo(Graph 5). In anaerobic environments, protozoa are mainly responsible for the biological methane production owing to their physiological association with endosymbiotic methanogens (van Bruggen et al., 1983). A significant correlation was observed between protozoan count with enzyme activity (Graph 6,7)and biogas production(Graph 8) in the reactor, indicating the functional role played by protozoa in the digestion process. Ciliates of the genus Metopus have shown to be strict anaerobic organisms living in many anoxic habitats including marine sediments, municipal landfills, anoxic paddy fields and anaerobic reactors (Finlay and Fenchel, 1989).Metopus showed a positive correlation with gas production and enzyme activity(Graph 9). Denaturing Gradient Gel Electrophoresis for the archeal community analysis of ALBR and UASB showed a different community pattern
Characteristics of the slurry after digestion had a Total Kjeldahl Nitrogen of 8.70% and total phosphorous of 0.78% and presence of minerals indicating its probable use as manure.
Graph 1: Biogas yield from water hyacinth digestion after different pretreatment
Graph 2: Methane production from the reactor at different OLR
Graph 3: Showing the activity of cellulase, xylanase pectinase and amylase in the open ALBR.
Graph 4: Showing the activity of cellulase, xylanase, pectinase and amylase in the UASB
Graph 5: Diversity of the protozoa present the digester
Graph 6:showing correlation between celluase activity and protozoan count
Graph 7:showing correlation between pectinase activity and protozoan count
Graph 8:showing correlation between gas production activity and protozoan count
Graph 9: showing positive correlation between gas production and Metopus count