Alternative treatment for septic tank sludge: Co-digestion with municipal solid waste in bioreactor landfill simulators

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Abstract

Co-disposal of septic tank sludge had a positive effect on the municipal solid waste (MSW) stabilisation process in Bioreactor Landfill simulators. Co-disposal experiments were carried out using the Bioreactor Landfill approach aiming to solve the environmental problems caused by indiscriminate and inadequate disposal of MSW and especially of septic tank sludge. The simulator receiving septic tank sludge exhibited a 200 days shorter lag-phase as compared to the 350 days required by the control simulator to start the exponential biogas production. Additionally, the simulator with septic sludge apparently retained more moisture (>60% w/w), which enhanced the overall conversion of organic matter hence increasing the biogas production (0.60 m3 biogas kg−1 VSconverted) and removal efficiency of 60% for VS from the simulator. Alkaline pH values (pH > 8.5) did not inhibit the biogas production; moreover it contributed to reduce partially the negative effects of NH4+ (>2 g L−1) due to NH3 volatilisation thus reducing the nitrogen content of the residues. Associated risks and hazards with septage disposal were practically eliminated as total coliform and faecal coliform contents were reduced by 99% and 100%, respectively at the end of the experiment. These results indicate that co-disposal has two direct benefits, including the safe and environmentally sound disposal of septic tank sludge and an improvement of the overall performance of the Bioreactor Landfill by increasing moisture retention and supplying a more acclimatised bacterial population.

Introduction

Urban and rural areas without sewage coverage rely heavily on on-site sanitation systems such as latrines and septic tanks for temporary treatment and disposal of human excreta (Chaggu et al., 2002). The objective of the septic tank is to provide initial treatment to household wastewater by intercepting and separating solid faecal matter from the liquid part. However, these systems, which require to be ideally emptied every 2–3 years, are not handled properly and usually after 5–10 years are overloaded reducing their treatment efficiency (NDUDP, 2001). Available methods of septic tank sludge treatment and disposal like composting, anaerobic digestion, chemical treatment are often not feasible for developing countries due to high handling and operational costs. Therefore, these rich moisture separated solids are often employed in agriculture or aquaculture or discharged indiscriminately into lanes, drainage ditches, sewer networks, onto open urban spaces, and into fresh and marine water bodies, causing serious environmental and health impacts (Bradley, 1981, Chen, 1988, NDUDP, 2001).

On the other hand, MSW stabilisation processes in sanitary landfills are restrained due to the lack of sufficient moisture, keeping the waste intact for long periods of time (Allen, 2001). A more sustainable approach is to operate the landfill as a bioreactor in order to enhance the stabilisation process of waste by addition of liquids (Valencia et al., in press). However, the produced leachate is often not enough to cover the Bioreactor Landfill moisture requirements; therefore additional sources of moisture are constantly needed. Several investigations have been done with wastewater treatment plant (WWTP) sludge, rumen content from slaughterhouses, and dredging material (Reinhart and Townsend, 1998, Chan et al., 1999, Silva et al., 2004, Çinar et al., 2004). Co-digestion of WWTP sludge and the organic fraction of MSW (OFMSW) at mesophilic temperatures in a full-scale digestion plant improved the reactor performance as shown by the increased biogas production (Bolzonella et al., 2006). Studies carried out by Hartmann and Ahring (2005) on co-digestion of WWTP sludge, OFMSW and manure at thermophilic temperatures with liquid recirculation resulted in higher biogas production rates, slightly alkaline pH and gave no signs of inhibition by ammonia as was observed in the batch-type experiments. Neves et al. (2006) found that anaerobic co-digestion of coffee waste and sewage sludge enhanced the anaerobic digestion. However, coffee waste containing high levels of lignocellulose produced hydrolysis intermediates toxic for methane production. Despite legal and environmental constraints with the indiscriminate use of these materials, the result of these investigations revealed that co-disposal of these materials influences positively the waste stabilisation process. However, none of the previous experiments were done using septic tank sludge, which has different characteristics than regular sewage sludge (Harrison and Moffe, 2003). Research done by Leckie et al. (1979) and Leuschner (1989) reported that septic sludge (suggested as a poor inoculum source) influences negatively the process of MSW stabilisation by keeping pH at sub-optimal levels and hence inhibiting the development of methanogenic bacteria. However, in the previous co-disposal experiments no recirculation of liquids was implemented and therefore no pH control measures could be applied. Since septic tank sludge is already separated from the liquid part, it should be safely disposed following the solid waste management pathway; co-disposal with MSW could provide a solution to the problem of extra moisture requirement in Bioreactor Landfills, while it would also provide an efficient and cost-effective disposal of septic sludge (Metcalf and Eddy, 1991). Therefore, the objective of this study is to evaluate the effect of septic tank sludge addition to the MSW stabilisation process in Bioreactor Landfill simulators.

Section snippets

Bioreactor Landfill simulators setup and operation

The Bioreactor Landfills were simulated by using PVC pipes, 23 cm in diameter and 100 cm in height, with a working volume of approximately 0.031 m3, hermetically sealed with PVC lids and rubber rings attached at both ends. Reactors were equipped at the top with a unidirectional wet gas meter (Meterfabriek Schlumberger) to record the volume of biogas produced and a water inlet for recirculation purposes. At the bottom, a water outlet was installed to allow the produced leachate flow into a

Physical–chemical parameters

In both reactors the pH of the leachate (Fig. 1a) started at values around 5.2, and increased in the first week of operation to values around 7 and 6.5 for reactor with MSW and MSW + S, respectively. pH values of both reactors decreased slightly below 6 after 3 weeks of operation. Contrary to the pH values of the reactor with MSW, which remained constant at values slightly below 6 throughout the entire experiment, the pH of the reactor with MSW + S increased gradually from day 100 reaching values

Reactor performance

The initial increase of pH could be attributed to an increase of the buffer capacity of the leachate caused by the dissolution of salts, such as NH4+, contained in the waste into the leachate as observed by the increase of EC of the leachate (Fig. 1a). However, due to enhanced organic matter hydrolysis, accumulation of hydrolytic products (VFA) subsequently increased the acidity within both reactors hence reducing pH. The pH of the reactor treating only MSW was similar as in previous

Conclusions

Based on the findings of this study the following conclusions can be drawn.

  • Safe and relatively low cost disposal of septic tank sludge can be achieved by co-digestion with MSW in landfills; additionally it can help to solve the lack of moisture commonly encountered in Bioreactor Landfills, increasing the overall performance.

  • Contrary to the findings of Leuschner (1989), the addition of septic tank sludge proved to be beneficial to the stabilisation process of MSW in Bioreactor Landfills by

Acknowledgements

The authors acknowledge Mr. A. van der Berg (Biological Milk Factory) and ESSENT Milieu for supplying the septic tank sludge and MSW, respectively. The first author acknowledges his funding by the Mexican National Council of Science and Technology (CONACyT).

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