Proceedings

Treatment of dewatering liquors using innovative expanded bed biofilm reactor technology

Dempsey, M., Advanced Bioprocess Development Ltd and Manchester Metropolitan University, UK

(free)

Abstract:
The expanded bed biofilm reactor (EBBR) is an innovative technology for wastewater treatment that provides up to 2,400 m2 of biofilm/m3 bioreactor and a biomass concentration of up to 42 kg/m3. These are the highest values of any current technology and provides over 100 times the pollutant-microbe contact time compared to e.g. activated sludge. EBBR processes are high-rate with a small footprint, thus reducing capex. For aerobic processes, our unique aeration system provides dissolved oxygen at low cost, thus reducing opex. Therefore, EBBR processes are low totex. At the heart of the EBBR are unique media particles of porous carbon, which has a specific surface area of 3,600 m2 /m3. This material is easily colonised by microbes, which are retained even with an upflow velocity of 36 m/h. During trials with sludge liquor, containing 300-1,200 mg NH3-N/L, nitrification rates of up to 3.6 kg NH3-N/m3 per day were achieved.

Introduction:
Since dumping of sewage sludge in European waters was banned in 1998, the main route for disposal has
involved a de-watering step, producing ammonia-rich wastewater known as sludge liquor or reject water. This liquid frequently contains high (g L-1) concentrations of chloride and sulphate, thereby making biological treatment difficult. However, destruction of ammonia via nitrification and denitrification represents a waste of a valuable resource that takes a considerable amount of energy to manufacture. Better to either recover the ammonia or convert it to nitrate, for re-use as fertilizer. Alternatively, return of nitrified wastewater to the secondary treatment process (e.g. activated sludge, IFAS or MBBR) can effectively recover the oxygen consumed for nitrification, because a range of heterotrophic bacteria can use nitrate as the terminal electron acceptor in place of dissolved oxygen. In this way, pre-denitrification can be accomplished in a secondary treatment anoxic zone. Here, an innovative process for nitrifying ammonia in sludge liquor (reject water from sludge de-watering operations) to produce nitrate is described.

When sludge liquor is fully nitrified, the nitrate can be used by heterotrophic bacteria to respire anaerobically, which prevents sulphate reduction and fermentation, thereby preventing wastewaters from becoming foul smelling. For example, it can be mixed with settled sewage (primary effluent) or return activated sludge (RAS), where low dissolved oxygen concentrations are found, which is a prerequisite for denitrification. In addition to consuming nitrate for respiration, the heterotrophic bacteria also mineralise organic matter. This strategy therefore provides solutions for both odour abatement and nitrogen removal.

In activated sludge processes, approximately 70% of the total energy consumption is used for the stirring and aeration systems (Hofken et al. 1992, cited in Bischof et al., 1996). These authors concluded that “it is quite clear that only cost-effective processes supported by energy-efficient equipment are the answer to economical wastewater treatment”. Most processes use co-current aeration (rising bubbles) because it is not possible to use counter-current systems, even though they are inherently more efficient. With expanded bed process technology, however, it is possible to use the more efficient, counter-current arrangement, as described by (Dempsey, 2011).

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