Seasonal Variation on Extent of Stabilization of Faecal Sludge for Safe Disposal during Co-composting in Forced Aeration and Passively Aeration Process

Abstract

In many developing countries, a large proportion of municipal wastes especially organic solid wastes and Faecal Sludge are not properly disposed. Co-composting of Faecal Sludge (FS) and Organic Solid Waste (OSW) can contribute as efficient waste management tool and allows recycling of nutrients into agriculture thereby closing the nutrient circle. This study was conducted to investigate the effect of temperature variation and mass balance of forced and passive aeration on Co-composting process with different stages in wet and dry seasons. Degree and extent of degradation were determined through different composting process in first, second and third stage with different mix proportion of Faecal Sludge in both seasons. Degradation rates were analysed in composting for Faecal Sludge and solid waste mixture. For conducting experiment, suitable organic solid wastes were collected from a student’s hall and other sources. Organic solid waste was prepared according to waste proportion (vegetable wastes: food wastes: waste paper: sawdust) as 40:35:10:15. The Faecal sludge was collected from a septic tank. Always OSW and FS were mixed at four different ratios as 90:10, 85:15, 80:20 and 75:25 (OSW : FS). In wet season the number of reactors used in first, second and third stage were 32, 24 and 16 respectively with total 72 number reactors. Similarly for dry season a total of 72 reactors were used for the experiment. In another run 60 more reactors were used for degradation rate analysis. To investigate the effect of initial moisture content on volatile solids degradation and temperature parameters another were used. Therefore, total 218 reactors were used for whole research work. Forced aeration and passively aeration composting tests were done using series of reactors as described above according to a planned experimental program. Composting tests were performed in three stages of 15–30 days duration each. Maximum temperature of passively and forced aeration tests were 65 oC and 67 oC which raised within 5 days. In third stage the temperature was not increase much because in this stage the compost become stable and contain very less biodegradable volatile solids (BVS). In both seasons the peak temperature of all the passively aerated and forced aerated composting tests were almost same. It means that the Co-composting process can be applied smoothly over the year in Bangladesh to manage FS without any interruptian of seasonal effect. Results of both seasons showed that, the moisture content and volatile solids always decrease after composting in most of the composting process. For passively aerated process, the average percent reduction of volatile solids of first stage, second stage and third stage were 27.6%, 12.4% and 8.2% respectively for forced aerated process, the average percentges were 30.8%, 13.6% and 9.2% respectively. Percentage degradation of volatile solids in third stage was less because in this stage the compost become stable and contain very less BVS. In wet season, the absolutely total BVS degradation of the passively and forced aeration process were ranged in between 80 % to 92 %. In dry season the ranged were 73% to 91%. According to the mass balance, the volatile solids degradation in composting process using forced aeration was more than passively aeration process. The average maximum area under curve T-Ta of first stage and second stage was found 10764 oC.h and 9864 oC.h respectively. For every set of reactor for first and second stage, the total percentage of degradation was almost same. For detemination of optimum moisture content, the area under the temperature curve in oC.h and %VS reduction was highest in the initial moisture content range between 55% and 70%. The temperature dependence of the reaction rates of different satge passive and forced aeration composting tests clearly followed the Arrhenius equation. The actual reaction rates provided good correletion with reaction rates. Percentage degradation of volatile solids depending on temperature could be predicted well using a first order reaction model. Compost produced in first and second stage were stable with stability index of II to IV. All compost of third stage were more stable than first and second stage with stability index of IV to V according to LAGA (1985). The Temperature pattern, during composting, Degradation of biodegradable volatile solids and Stability index of produced compost were almost same in passive and forced aeration Co-composting process. Passive aeration composting system are economical in operations and maintenance and produce good quality compost. Therefore, the passively aeration Co-composting process can be applied smoothly in Bangladesh round the year to manage Faecal Sludge.