Ammonia Removal from Sirofloc® STP Treated Sewage using Australian Natural Zeolite
This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.
Report no. UWRAA 32
There is ample literature to support the fact that the discharge of nutrients, ammonium and phosphorus, into rivers and streams strongly contributes to eutrophication of the water course. With this understanding of water pollution causes, the environment protection authorities have placed restrictions on nutrient discharges from both wastewater treatment facilities and other point-source contributors such as abattoirs, fish-farms, tanneries, etc. A significant research effort has been directed towards removal of ammonium-nitrogen (NH4-N)using biological nitrification and denitrification techniques. Whilst these techniques have demonstrated effective removal of NH4-N down to very low levels, the process is inherently slow and particularly sensitive to process disturbances (pH, temperature, NH4 concentration).
The SIROFLOCÒ sewage treatment process (STP) is a high rate, compact process that has been demonstrated on the full scale to remove suspended solids, oil and grease, heavy metals and phosphorus from domestic sewage. The removal of phosphorus is achieved by manipulation of the solution chemistry, enabling precipitation and subsequent extraction. Such is also the case for other physico-chemical wastewater treatment processes such as dissolved air flotation (DAF), vortex chemically assisted flotation, chemically assisted settling (CAS), membrane micro-filtration, etc. The ammonium remains soluble in these systems and passes through to the treated effluent stream. For ammonium to be removed from this effluent stream using biological techniques, the high-rate advantage of the process would be lost, and it would appear preferable to use biological techniques for the whole treatment. A high-rate ammonium removal process, capable of complementing existing physico-chemical processes where N removal is required, is essential. To offer the capability to achieve this N removal allows flexibility and completeness to physico-chemical treatment.
Past research (notably USA, Hungary, Italy) into the use of the natural zeolite, clinoptilolite, as an ion-exchange medium for the removal of ammonium from wastewaters indicated that its suitability was highly dependent on its source and pre-treatment. In all cases, there was a definite selectivity of the zeolite for ammonium ions, which could be exploited. However, the economics of the process appeared to be closely related to the process conditions adopted. For example, the use of zeolite on a once-through basis, whilst effective, would force operating costs up to prohibitive levels. The points of entry of the zeolite into the process was also a moot point. The grain size of zeolite employed to maximise perceived benefits needed some fine-tuning for many of the researchers.
Due to the very recent commercial mining of Aquaclin, a proprietary grade of Australian zeolite mined by Zeolite Australia Limited since 1987, little in the way of characterisation of the material had been performed. It was therefore necessary to start with fundamental equilibria and kinetic studies, using pure solutions of NH4Cl, to assess the ammonium adsorption capabilities of the zeolite. Following this, the effects of competing cations, which would be present in SIROFLOCÒ STP effluent, were studied using pure solutions of NaCl, CaCl2, Mg Cl2 and KCl. Both binary and multicomponent systems were characterised, and data were applied to existing models to predict ammonium adsorption under varying batch process conditions. The optimum chemical pre-treatment technique was also determined. Effects of acidic solutions, using HCl, were determined both in terms of aluminium dissolution (using ICP solution analysis) and zeolite crystal degradation(using XRD analysis).
A downflow packed column (250 mm diameter) system was designed, fabricated and operated for the pilot scale research. Pilot studies were initially performed using ammonium chloride spiked tap water to obtain a base line from which the effects of using sewage were compared. A range of NH4Clconcentrations and flowrates were trialled to ascertain the relationship between ammonium removal and feed concentration or flowrate. The optimum loading rates and regeneration conditions were established, to maximise the operating ammonium capacity of the zeolite whilst maintaining a practical hydraulic rate. Residence time distribution (RTD) studies were performed on the column, using an inert tracer, to obtain the axial dispersion number (or deviation from plug flow) for the system. This would be used, along with equilibrium constants previously obtained, to characterise and predict column performance for any given set of process parameters.
Treated sewage from the SIROFLOCÒ STP pilot plant was fed into the column at various flowrates to confirm optimum operating conditions. The diurnal changes in sewage quality were seen to be “absorbed” by the system, and effluent quality remained stable. Feed ammonium concentrations ranged from 20 to 60 mg NH4-N/L, and effluent quality remained below2 mg NH4-N/L prior to the onset of breakthrough. The operating capacity of the zeolite bed approached 5 mg NH4-N/g. At the optimum flowrate, equivalent to 6 m/h filtration rate, sustained ammonium removal was achieved for more than 20 hours (5800 litres).
The zeolite was chemically regenerated, using caustic brine solution (equivalent to sea water at pH10), and optimum conditions were determined for this procedure. Full regeneration of the bed was impracticable, given time constraints, so a practical “industry” approach was adopted which terminated regeneration when the effluent ammonium concentration was equivalent to the incoming sewage concentration. Repeated loading and regeneration cycles, under constant operating conditions, revealed (over more than ten cycles) that the zeolite performance was maintained under continuous operating conditions. It is assumed that there would be some minor losses due to attrition, but this may not become apparent until hundreds of cycles have been performed.
A cost benefit analysis of the use of zeolite for ammonia removal has shown that the process economics hinge on the cost of salt used for regenerating the zeolite. If sea water can be used to regenerate the zeolite then the process would appear to be extremely cost effective. Otherwise, some form of salt recovery 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.