Enhancement of Nitrification in Wastewater Lagoons

This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.

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Report no. UWRAA 56

May 1993

Synopsis

Nitrifier washout appears to be the major factor limiting the extent of nitrification achieved in sewage treatment lagoons at the Werribee Treatment Complex near Melbourne. Keeping hydraulic retention times (HRT) in these lagoons above a minimum level is a key aspect of strategies to minimise nitrifier washout. However, the minimum required HRT varies, being dependent on prevailing nitrifier growth rates. These growth rates in turn depend on several factors, including lagoon water temperature, pH, dissolved oxygen content and ammonia concentration.

A parameter termed a Standardised Instantaneous Retention Time (SIRT) was introduced; this is calculated by multiplying the actual lagoon HRT at any time by terms accounting for the influence of factors affecting nitrifier grow thrates. An experimental program on a system of four pilot-scale ponds was undertaken to establish whether a critical SIRT value existed above which nitrifier washout would be unlikely to occur; if such a critical value existed it would make lagoon management much simpler. It was found that use of a modified SIRT, amended to incorporate a term involving pond algal concentrations, provided a useful guide to pond behaviour but could not be used on its own as a lagoon management tool.

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