Erfahrungen zur Ammoniakentfernung mit Membrankontaktoren
Experiences with respect to ammonia removal with membrane contactors
In a conventional clarification plant, the water from sludge digestion (wash water containing sludge) is fed back into the waste water inlet. The ammonium load, that in this way can be recycled to the facility, can contribute 30% of the total ammonium loading. Because of this additional ammonia freight, a clarification plant requires larger activated sludge tanks and the need for oxygen increases, i.e., higher investment and energy costs arise in the treatment of the waste water. In a Fraunhofer IGB project, the filtrate of a biological sludge digestion developed at the IGB was studied. This wash water containing sludge has, because of the higher degree of decomposition in the sludge digestion, a higher ammonium content of 2 g/L than the wash water containing sludge in conventional clarification plants. Also, this muddy water is free of particles and microorganisms for the necessary microfiltration. These prerequisites permit the cost effective obtaining of the ammoni um from the sludge filtrate. To separate the ammonium from the muddy water, a hollow fiber membrane contactor is used. The filtrate is stripped with air. The ammonia laden air is washed with acids or freed of its ammonia load by cooling. Marketable products are thus obtained of ammonium salts such as fertilizer or ammonia solution for flue gas desulfurization. With this process the ammonium concentration in the muddy water can be reduced to over 99% without pH elevation. A higher pH value can considerably accelerate the ammonium removal by the shift of the ammonium/ammonia equilibrium. The addition of alkali has the disadvantage that the waste water becomes loaded with chemicals, in this case, dissolved salts. Technically worthwhile is the ammonium reduction by 90-95% that can be made with the "saved" energy through smaller ammonium loads in the clarification plant.