Vertical flow wetlands for tertiary wastewater treatment

Due to increasing focus on improving water quality within surface waters, it is anticipated that stringent ammonia discharge consents will be introduced to small wastewater treatment works in the coming years, with potential discharge consents of as low as 1mgNH₄-N/L. This is likely to require upgrading of secondary treatment works to include a polishing treatment stage. Vertical flow wetlands (VFWs) are aerobic treatment processes, making them the ideal solution for achieving nitrification to within the proposed discharge consent, however current uses are limited to treatment of raw water and primary effluents. The potential for VFWs under tertiary application has not previously been identified. This study aims at addressing this knowledge gap by determining performance capabilities and hydraulic behaviours at both full and pilot scale, and defining the optimal operational strategies in terms of hydraulic loading, dosing frequency and resting periods using pilot plant trials. Findings from the study have shown VFWs to achieve effluent ammonia concentrations of as low as 0.002mgNH₄-N/L from influent concentrations of up to 7.4mgNH₄-N/L, with almost complete nitrification observed in most cases. Additional onsite sampling provided a performance comparison against existing tertiary treatments, showing potential for the VFWs to outperform in terms of solids organics and nutrient removal. Pilot plant operational trials revealed application of prolonged resting periods and frequency of daily dosing to have no significant impact on either the hydraulic stability or treatment performance for VFWs in tertiary application. Pilot plant trials determined an initial stabilisation period of between 2 to 3 years is required during the start up of a VFW system, with hydraulic loading rates being increased gradually over time to avoid occurrence of clogging. An economic assessment determined the feasibility of tertiary VFWs to be comparable to existing conventional tertiary treatments.