ACCELERATING IMPLEMENTATION OF EXTRACTIVE NUTRIENT RECOVERY AS AN INTEGRAL COMPONENT OF SUSTAINABLE NUTRIENT MANAGEMENT
FINDINGS OF THE RECENTLY COMPLETED WATER ENVIRONMENT RESEARCH FOUNDATION (WERF) STUDY ON EXTRACTIVE NUTRIENT RECOVERY
S Jeyanayagam, W Khunjar, A Pramanik, C Mehta
Publication Date (Web): 14 March 2016
Nitrogen (N) and phosphorus (P) are life-essential macronutrients. Production of synthetic fertilisers containing N and P is an energy-intensive process that uses non-renewable resources. Experts believe that economically extractable phosphorus reserves are being consumed faster than the geologic cycle can replenish them. In order to minimise the impact of nutrients on the environment, the current approach is to remove N and P prior to discharge to a water body. In this linear scenario, energy and resources are expended for fertiliser production as well as to remove N and P from wastewater. Achieving a circular economy demands a paradigm shift in how we manage nutrients and capture other resources and energy from wastewater.
This paper presents findings of the recently completed Water Environment Research Foundation (WERF) study on extractive nutrient recovery titled Towards a Renewable Future: Assessing Resource Recovery as a Viable Treatment Alternative (NTRY1R12). Extractive nutrient recovery entails harvesting N and P as a relatively clean nutrient product. Despite the obvious benefits of nutrient harvesting (recycle load management, chemical savings, reduced solids production, minimised struvite scaling, etc.) there remain technical, social and economic challenges towards a wider adoption of nutrient recovery.
Efficient extractive nutrient recovery involves a three-step framework comprising accumulation, release and extraction. Accumulation and release technologies are already commonplace at many nutrient removal facilities that utilise enhanced biological phosphorus removal and anaerobic sludge digestion. Achieving nutrient recovery at these plants involves adding the last extraction step.
The various technologies available to accomplish nutrient recovery may be characterised as embryonic, innovative or established. Currently six mature technologies are commercially available for full-scale implementation. These include Ostara, Multiform Harvest, NuReSys, Phospaq, Crystalactor and AirPrex.
A survey of 20 wastewater treatment plants provided the following: drivers for and barriers to the adoption of nutrient recovery, strategies for overcoming key barriers, lessons learnt designing and operating nutrient removal systems, and construction and operating costs.
As part of the WERF study, an Excel-based user-friendly tool called the Tool for Evaluating Resource Recovery (TERRY) was developed. TERRY allows users to perform high-level business case evaluation and examine payback scenarios by considering 13 factors. Fact sheets describing an array of struvite harvesting technologies are also available within the tool so that users can compare and contrast competing options.
During anaerobic digestion, the nutrients that are released tend to form insoluble inorganic compounds and are unavailable for subsequent recovery. University of Queensland, a project partner, conducted experimental evaluation of emerging concepts to maximise the availability of the released P and enhance the extraction efficiency through crystallisation.
The study investigated the two approaches to capture the released P: depressed pH during anaerobic digestion; and the use of chelating agents and ion exchange resins. Results confirmed that P solubility under acidic conditions (pH < 5.7) was higher than under neutral conditions, but methane production suffered. A shift in the microbial diversity was also observed. With respect to the use of additives, chelating agents were more effective than ion exchange resins. Another benefit of adding a chelating agent is the potential improvement of dewaterability and polymer cost savings.
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