(663b) Fate of Nitrogen during Hydrothermal Treatment of Septage

Septic tanks are a popular method in rural areas to treat human waste. Waste enters the septic tank and settles so that solids collect in the bottom of the tank. Here organic matter is aerobically digested. Digestion only occurs partially, and is variable with each design of tank and the conditions of the tank (such as temperature and extent of cleaning). Septic tanks offer many advantages, such as the containment of the digestate and the containment of human waste. However, the resulting solid and liquid products must be pumped and disposed of, or further treated. One concern with the use of septic tank is the long term effects of untreated pollutants that are only present in the microgram/liter range, which have been shown to have serious impacts on aquatic ecology of surrounding water systems [1],[2]. Failure of septic tanks can also result in contamination of surrounding water sources with the various macronutrients present in the digestate and effluent of the tank.

Hydrothermal treatment has been investigated as a possible method for the treatment of municipal solid waste and sewage sludge [3]. Septage, or the combination of both septic sludge and effluent, can also be treated by hydrothermal means. Hydrothermal treat is an attractive option for additional septage treatment due to its high water content; there is no need for any drying or separation for solid treatment. More so, the high nitrogen content of septage can be recovered and reused as fertilizer. In this study septage was collected from local health department and hydrothermally treated. A full nitrogen balance was performed between soluble nitrogen species, as well as solid nitrogen content determined by CHNS. Experimental trials for this treatment used a batch reactor with a reaction time of 30 minutes. It was found that hydrothermal treatment at temperatures between 180-260 °C results in increased nitrogen in the process liquid compared to the untreated septage. In addition, treatment results in increased ammonia concentration in the liquid phase and a pH between 6 and 8. This generation of soluble nitrogen species is a result of the solid phase decomposing as it is heated.

References

[1]. Schwarzenbach, René P., et al. "The challenge of micropollutants in aquatic systems." Science 313.5790 (2006): 1072-1077.

[2]. Withers, Paul JA, et al. "Do septic tank systems pose a hidden threat to water quality?." Frontiers in Ecology and the Environment 12.2 (2014): 123-130.

[3]. Zhao, Peitao, et al. "Clean solid biofuel production from high moisture content waste biomass employing hydrothermal treatment." Applied Energy 131 (2014): 345-367.