AmmoniaRemoval from SiroflocÒSTP treated

Sewageusing Australian Natural Zeolite

ReportNo WSAA 113

September 1996

SYNOPSIS

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).

TheSIROFLOCÒ sewage treatmentprocess (STP) is a high rate, compact process that has been demonstrated on thefull scale to remove suspended solids, oil and grease, heavy metals andphosphorus from domestic sewage. The removal of phosphorus is achieved bymanipulation of the solution chemistry, enabling precipitation and subsequentextraction. Such is also the case for other physico-chemical wastewatertreatment processes such as dissolved air flotation (DAF), vortex chemicallyassisted flotation, chemically assisted settling (CAS), membranemicro-filtration, etc. The ammonium remains soluble in these systems and passesthrough to the treated effluent stream. For ammonium to be removed from thiseffluent stream using biological techniques, the high-rate advantage of theprocess would be lost, and it would appear preferable to use biologicaltechniques for the whole treatment. A high-rate ammonium removal process,capable of complementing existing physico-chemical processes where N removal isrequired, is essential. To offer the capability to achieve this N removalallows flexibility and completeness to physico-chemical treatment.

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 SIROFLOCÒ STP effluent, were studiedusing pure solutions of NaCl, CaCl₂, Mg Cl₂ and KCl. Bothbinary and multicomponent systems were characterised, and data were applied toexisting models to predict ammonium adsorption under varying batch processconditions. The optimum chemical pretreatment technique was also determined.Effects of acidic solutions, using HCl, were determined both in terms ofaluminium dissolution (using ICP solution analysis) and zeolite crystal degradation(using XRD analysis).

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 SIROFLOCÒ STP pilot plantwas fed into the column at various flowrates to confirm optimum operatingconditions. The diurnal changes in sewage quality were seen to be “absorbed” bythe system, and effluent quality remained stable. Feed ammonium concentrationsranged from 20 to 60 mg NH₄-N/L, and effluent quality remained below2 mg NH₄-N/L prior to the onset of breakthrough. The operatingcapacity of the zeolite bed approached 5 mg NH₄-N/g. At the optimumflowrate, equivalent to 6 m/h filtration rate, sustained ammonium removal wasachieved for more than 20 hours (5800 litres).

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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