Description

The wastewater treatment plant (WWTP) of Altenrhein is located in the eastern part of Switzerland and has a regional responsibility for sewage sludge management. The technology of the WWTP consists of primary sedimentation, bio-filtration and activated sludge reactor in parallel, digestion, sludge dewatering and drying of self-produced and received sludge.


In this case study, the innovative membrane system for ammonia (NH3) stripping will be tested and evaluated in a large-scale unit, thus moving beyond state-of-the-art in physical nitrogen treatment of sludge liquid while recovering a valuable fertilizer.

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Objectives

  • Full-scale demonstration of recovery of nitrogen from wastewater by membrane stripping at WWTPs
  • First reference – feasibility of technology
  • Production of a marketable nitrogen fertilizer product
  • Operational and long-term experience
  • Optimization of the process
  • Gain more experience of the technology
  • Comparison to alternative technologies in costs and efficiency
  • Evaluation on energy balance of WWTP
  • Economic feasibility
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Challenges

In terms of costs and energy consumption anaerobic ammonium oxidation (anammox) is currently the optimal treatment of sludge liquids to reduce the internal N-load and to safe design capacity of WWTPs. Over the last years, experience in operating the anammox process showed some difficulties in process stability. Air, steam or membrane stripping are the only physico-chemical processes for the recovery of nitrogen as a valuable liquid fertilizer (ammonia sulfate) for agriculture. But free ammonia stripping requires large amounts of base.

Costs and energy consumption to produce the base are a significant part of the overall operational costs and energy consumption.

The novel processes, therefore, uses optimized pH and temperature conditions as well as CO2 pre-stripping, which reduces the amount of sodium hydroxide, the base ”in charge“, by up to 40%.

Ammonia can also be physically removed from sludge liquids by stripping ammonia from the water in gaseous from (NH3), with typical efficiencies of >90 %. In a two-step process, NH3 is stripped from the sludge liquid and subsequently removed from the gaseous phase by sulfuric acid. An innovative method is to use a cross-flow membrane that separates the sludge liquid from the sulfuric acid, resulting in a compact membrane treatment unit.

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