Biogranules containing photosynthetic bacteria for carbon dioxide reduction in palm oil mill effluent treatment

Abstract

Presently global warming is the most highlighted subjects in environmental issues which is related to greenhouse gases (GHG) emissions especially carbon dioxide (CO2). In Malaysia, one of the major sources of GHG is from industrial wastewater treatment such as ponding system to treat palm oil mill effluent (POME) where the accumulation of these gases will contribute to the greenhouse effect causing global warming. Since photosynthetic process offers the most effective and natural way of sequestering CO2, biogranules containing photosynthetic microorganisms were developed in a sequencing batch reactor (SBR) system using POME. A mixed sludge consists of sludge taken from a local sewage treatment oxidation pond, palm oil mill facultative pond treatment system and POME was used as seed sludge. Intermittent supply of light with intensity at 3600 lux was provided for 100 days with an organic loading rate (OLR) of 2.75 kg COD/m3/day, hydraulic retention time (HRT) of 4 hours and superficial air velocity of 2.07 cm/s. The developed biogranules had shown potential in retaining high accumulation of biomass concentration in the reactor (10.5 g/L), good settleability (43.5-102.9 m/h) and improvement in size from 0.5 to 2.0 mm as well as high physical strength at integrity coefficient (IC) of 2 %. The initial structure of sludge changed from dispersed loose shaped into denser, compact and more stable structure with sludge volume index (SVI) maintained between 10.30 to 14.80 mL/g SS leading to a good solid-liquid separation compared to conventional activated sludge. Also, the chemical oxygen demand (COD), nitrogen (N) and phosphorus (P) removal of 26 %, 21 % and 62 % were achieved during the development of the biogranules. The pigment analysis indicated the presence of the bacteriochlorophyll a implying the presence of purple photosynthetic bacteria. Molecular identification of the bacteria showed the presence of Enterobacter cloacae, Bacillus cereus, Lysinibacillus sp. which possess photosynthetic pigments. For CO2 reduction using the biogranules, approximately 18 to 21 % of CO2 removal was achieved due to possible formation of calcite were observed with FESEM-EDX. The biogranules had achieved a CO2 biofixation rate at approximately 0.234 g/L/day in a week while using the regression analysis; the maximum CO2 biofixation rate in a year was estimated at 1.733 g/L/day.