| Abstract | It is a well-established fact that septic tanks are a prevalent on-site treatment system to treat/collect toilet wastewater in most developing countries. Due to instabilities of organic loading rates (OLRs) and short hydraulic retention times (HRTs), septic tank effluents still contain high concentrations of organic matters and other pollutants. After operation for a certain period, there will be high accumulation of septic tank sludge which requires frequent desludging. To alleviate the above problems, a modified conventional septic tank with solar-heated water called as “Solar septic tanks” was considered to be an effective on-site sanitation technology. Because of the complex processes comprising of biological and physical reactions, the design and operation of solar septic tanks are still lacking. The ultimate goal of this research is to establish design criteria and operational conditions of solar-septic tanks. The experimental study, employing four laboratory–scale septic tanks (each with volume of 40 L) fed with diluted septage and operating at HRTs of 12, 24 and 48 h and temperatures of 40 and 30 oC. To evaluate technical feasibilities of increasing temperatures inside a septic tank, a pilot-scale solar septic tank with a size of 600 L and equipped with a locally made 2 m2 solar collector was operated for a 4-month period. The 40 oC laboratory-scale septic tank operating at HRTs of 12 h resulted in the lowest BOD5 and TCOD removal efficiencies of 67%, while those units operating at HRTs of 24 and 48 h resulted about 71 and 74 % for BOD5, and 76 and 78 % for TCOD, respectively. It was apparent that the HRT of 12 h did not provide sufficient residence time for solids sedimentation, resulting in the low TS and TVS removal efficiencies of 49 and 53 %, respectively. There were not significant differences in TS and TVS removal efficiencies at HRTs of 24 and 48 h, but higher than those of HRT of 12 h in the ranges of 68-70 % and 71-75%, respectively. At steady-state conditions, more methanogenic activities could be observed in the sludge layer of the septic tank operating at the temperature of 40 oC, resulting in less TVS or sludge accumulation and more methane (CH4) production than in the unit operating at 30 oC. Molecular analysis indicated more intense and diversity of methanobacteriales in the septic tank sludge operating at 40 oC than at 30 oC. The less TVS accumulation in the 40 oC septic tank would lengthen the period of septage removal, resulting in cost-saving in desluging and septage treatment.
The integrated kinetic model was developed to simulate performance of a solar septic tank and identify relevant parameters affecting changes occurring in biological pathways and treatment efficiencies of organic and solids removal, sludge reduction, and methane production. Validation of the model with the results obtained from the laboratory-scale septic tanks, actual septic tanks and literature was done with the correlation coefficient (R2) values of 0.85-0.90 which showed the applicability this model. Application of the integrated kinetic model to design and operation and cost-benefit analysis of increasing temperatures in septic tanks were discussed. However, it should be noted that the results from this study mainly obtained from the laboratory-scale septic tanks fed with the synthetically made of diluted septage and sludge reduction efficiency was observed within 4 month. Due to these limitations of this experiment, the recommendations for further study are further validation of the integrated kinetic model with the data of septic tanks operating at temperatures above 30-40 oC and investigation the sludge reduction efficiency in the septic tank in long term operation. |