Floating treatment wetlands supplemented with aeration and biofilm attachment surfaces for efficient domestic wastewater treatment

Abstract

Floating Treatment Wetlands (FTWs), comprising a floating matrix or framework vegetated with emergent wetland plants whose roots extend into the water below, have potential for treatment of wastewaters. Low oxygen concentrations beneath FTWs and wash-out of microbial communities can limit decomposition of organic matter and nitrification of ammonium-N in domestic wastewaters. This study aimed to test the utility of two different configurations of FTWs supplemented with mechanically aerated Biofilm Attachment Surfaces (BAS) to provide post-primary treatment of domestic wastewater. Two pilot-scale FTW treatment trains were developed at a domestic wastewater treatment plant in Hamilton, New Zealand using ten 1 m3 volume tanks, with 1 m2 square floating mats planted with Carex virgata. These were: (1) a single-pass (direct flow-through) system comprising an aerated FTW tank with subsurface BAS followed by four non-aerated FTW tanks in series, where (endogenous carbon inputs from wetland plants, along with organic carbon from the wastewater would promote denitrification; and (2) a recirculating system incorporating a pre-anoxic phase settling tank with FTW and BAS, followed by an aerated FTW tank with BAS from which 4/5 of the outflow was recirculated back to the initial anoxic stage where the (endogenous) organic matter in the incoming wastewater would provide the primary source of carbon to promote denitrification. The remaining 1/5 flowed into three non-aerated FTWs in series. Performance of each treatment stage was monitored monthly for one year from February 2015 to January 2016 for organic matter (BOD5: Biochemical Oxygen Demand; TSS: Total Suspended Solids), nutrients (nitrogen and phosphorus) and E. coli removal. The single-pass system had almost complete removal of both TSS and cBOD5 with >93%, maintaining the final effluent TSS and cBOD5 concentrations at below 5 and 4 mg L−1 respectively. The single-pass system had a lower NH4-N reduction (4.9 g m−2 d−1) compared with that in the recirculating system (6.9 g m−2 d−1), but achieved higher denitrification (1.7 g m−2 d−1) than that in the recirculating system (1.4 g m−2 d−1) as more organic carbon from the inflow wastewater was available in the single-pass system for denitrification. The recirculating FTW system achieved slightly higher removal of Total-P (TP: 44.9%) and Dissolved Reactive Phosphorus (DRP: 29.7%) than those in the single pass FTW system (TP: 36.7%; DRP: 24.3%), and both systems had a similar ~3-log E. coli reduction (from 106 to 103 MPN 100 ml−1) during the one-year experimental period. This one-year pilot-scale FTW study showed that FTWs supplemented with mechanically aerated BAS could be a practically option to retrofit existing waste stabilisation ponds to enhance organic matter, nitrogen and E. coli reduction. Furthermore, they may have some potential advantages over other types of intensified constructed wetlands, since they don’t need costly graded gravel (or other solid media), provide ready access to aeration diffusers for maintenance purposes, have low susceptibility to clogging, and can be more readily de-sludged if required.