AmmoniaRemoval from Sirofloc
Sewageusing Australian Natural Zeolite
ReportNo WSAA 113
Thereis ample literature to support the fact that the discharge of nutrients,ammonium and phosphorus, into rivers and streams strongly contributes toeutrophication of the water course. With this understanding of water pollutioncauses, the environment protection authorities have placed restrictions onnutrient discharges from both wastewater treatment facilities and other point-sourcecontributors such as abattoirs, fish-farms, tanneries, etc. A significantresearch effort has been directed towards removal of ammonium-nitrogen (NH4-N)using biological nitrification and denitrification techniques. Whilst thesetechniques have demonstrated effective removal of NH4-N down to verylow levels, the process is inherently slow and particularly sensitive toprocess disturbances (pH, temperature, NH4 concentration).
Pastresearch (notably USA, Hungary, Italy) into the use of the natural zeolite,clinoptilolite, as an ion-exchange medium for the removal of ammonium fromwastewaters indicated that its suitability was highly dependent on its sourceand pretreatment. In all cases, there was a definite selectivity of the zeolitefor ammonium ions, which could be exploited. However, the economics of theprocess appeared to be closely related to the process conditions adopted. Forexample, the use of zeolite on a once-through basis, whilst effective, wouldforce operating costs up to prohibitive levels. The points of entry of thezeolite into the process was also a moot point. The grain size of zeoliteemployed to maximise perceived benefits needed some fine-tuning for many of theresearchers.
Dueto the very recent commercial mining of Aquaclin, a proprietary grade ofAustralian zeolite mined by Zeolite Australia Limited since 1987, little in theway of characterisation of the material had been performed. It was thereforenecessary to start with fundamental equilibria and kinetic studies, using puresolutions of NH4Cl, toassess the ammonium adsorption capabilities of the zeolite. Following this, theeffects of competing cations, which would be present in SIROFLOC
Adownflow packed column (250 mm diameter) system was designed, fabricated andoperated for the pilot scale research. Pilot studies were initially performedusing ammonium chloride spiked tap water to obtain a base line from which theeffects of using sewage were compared. A range of NH4Clconcentrations and flowrates were trialed to ascertain the relationship betweenammonium removal and feed concentration or flowrate. The optimum loading ratesand regeneration conditions were established, to maximise the operatingammonium capacity of the zeolite whilst maintaining a practical hydraulic rate.Residence time distribution (RTD) studies were performed on the column, usingan inert tracer, to obtain the axial dispersion number (or deviation from plugflow) for the system. This would be used, along with equilibrium constantspreviously obtained, to characterise and predict column performance for anygiven set of process parameters.
Treatedsewage from the SIROFLOCŇ
Thezeolite was chemically regenerated, using caustic brine solution (equivalent tosea water at pH10), and optimum conditions were determined for this procedure.Full regeneration of the bed was impracticable, given time constraints, so apractical “industry” approach was adopted which terminated regeneration whenthe eluant ammonium concentration was equivalent to the incoming sewageconcentration. Repeated loading and regeneration cycles, under constant operatingconditions, revealed (over more than ten cycles) that the zeolite performancewas maintained under continuous operating conditions. It is assumed that therewould be some minor losses due to attrition, but this may not become apparentuntil hundreds of cycles have been performed.
Acost benefit analysis of the use of zeolite for ammonia removal has shown thatthe process economics hinge on the cost of salt used for regenerating thezeolite. If sea water can be used to regenerate the zeolite then the processwould appear to be extremely cost effective. Otherwise, some form of saltrecovery system needs to be developed. Ideally recovering the NH4+from the brine in a useable form, for example struvite fertiliser (MgNH4PO4.6H20).This is the focus of ongoing research at the CSIRO.
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