Enhancementof Nitrification in Wastewater Lagoons

ReportNo WSAA 56

May 1993

SYNOPSIS

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

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

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