Thompson, KeithTanner, Chris C.2026-08-202026-08-201994https://hdl.handle.net/10289/18559Constructed wetlands have the potential to provide simple, low-cost, low-maintenance, natural treatment systems for agricultural wastewaters. The aims of the present study were: 1. To evaluate the treatment performance of gravel-bed wetland systems supplied with dairy farm wastewaters and provide basic data for the development of suitable loading criteria. 2. To investigate the effect of wetland plants on treatment performance. 3. To investigate the effect of wastewater loading rates on plant productivity, nutrient uptake, and growth characteristics. Schoenoplectus validus (soft-stem bulrush) was chosen as the test species, since this is the plant most commonly used in wetland treatment systems in New Zealand, and is also in widespread use in the United States and Australia. The effect of loading rate and presence of plants on wastewater treatment was investigated over a 20 month period, in four pairs of gravel-bed wetlands operated with different loading rates of dairy shed wastewaters. Mean annual suspended solids (SS) removals of 75-85% were achieved in the wetlands, irrespective of loading rate or the presence of plants. In the planted wetlands, mean mass removal of carbonaceous biochemical oxygen demand (CBOD), during the first 12 months of monitoring, increased from 76 to 90%, total BOD (CBOD + nitrogenous BOD) from 50 to 80%, faecal coliforms from 95 to over 99%, total nitrogen (TN) from 48 to 75%, and total phosphorus (TP) from 37 to 74% as wetland retention time rose from 2 to 7 days (i.e. inversely related to loading rate). The unplanted wetlands generally showed similar performance to the planted wetlands at low loading rates, but poorer BOD, nitrogen and phosphorus removal at the highest loadings tested. Comparison of the performance of the horizontal-flow wetlands with that of up-flow wetland tanks, showed similar reduction of BOD and SS in relation to volumetric loading, but reduced N and P removal, particularly at high loading rates. Net annual plant production measured in the wetlands was high, rising from ~2.5 to 3.5 kg (dry weight) m⁻² with increasing wastewater loading, during the first growth season. Plant biomass and tissue nutrient levels declined down the wetland channels reflecting fertility gradients in the wetlands. Nutrient accumulation in living and dead plant biomass in mid-summer, after two years of wetland operation, although high compared to levels reported for natural communities, only represented 6 to 10% of the N removal and 6 to 13% of the P removal recorded for the wetlands. Above-ground litter inputs from plants were estimated to contribute significant quantities of readily and slowly available carbon substrates to the wetland surface, equivalent to between 60 and 125% of the available organic carbon loadings, and 80 to 190% of the CBOD loadings supplied to the wetlands in wastewaters. In further experiments, above and below-ground plant growth was compared in wetland microcosms batch-fed with a range of agricultural wastewater types and strengths, with and without nutrient additions, and in complete nutrient solutions. S. validus was capable of growth over a wide range of wastewater nutrient levels, but showed maximum growth at the highest levels tested. Reduced rooting depths were however noted in the higher strength organic wastewaters. Comparisons with root development in nutrient solutions and nutrient ammended wastewaters suggested that factors associated with organic compounds in the wastewaters (e.g. hypoxic stress caused by high BOD levels), rather than increasing nutrient levels, restricted root penetration. Similar reductions in the proportion of deep-growing roots were noted at high loadings in the larger-scale wetlands. These growth responses have important consequences for the design of subsurface-flow wetland systems, where bed depths are matched to plant rooting depth, to maximise contact between wastewaters and the root-zone.enAll items in Research Commons are provided for private study and research purposes and are protected by copyright with all rights reserved unless otherwise indicated.Thesis