Improving phosphorus removal in a surface flow wetland and land application system by geochemical augmentation with alum.

Constructed surface flow (SF) wetlands are commonly used for phosphorus (P) removal. Geochemistry of wetlands provides explicit mechanisms for permanent P-sequestration in sediments. This study had two goals: (1) Find P removal performance and rate at the highest alum doses that do not produce floc in an SF wetland; and (2) Determine potential improvements to P removal performance with low alum doses in a 140-ha land application system downstream from the wetland. The study started with a small fraction of a conventional, flocculation/sedimentation alum dose, then progressively increased the dose to observe initiation of floc formation and removal of P. For flows near 10 megaliters per day in an 0.8 ha SF wetland, doses started 189 L d-1 for two weeks, then increasing by 189 L d-1 every two weeks until the final two weeks at 946 L d-1. At an alum dosing rate of 189 L d-1 (alum concentration of 9.5 mg L-1), there was an order of magnitude improvement in P removal rates over literature values. Floc formation in the wetland was observed at 567 L d-1, but no significant improvement in P removal rates were observed until a conventional alum dose of 946 L d-1 was applied. Alum addition improved P removal performance in the land application system. In 2014, during which there was no alum dosing, the median effluent total P (TP) during the July-September dry season (groundwater dominated outflow) was 0.43 mg L-1. In 2015, (alum dosing August-October) median dry weather TP of 0.18 mg L-1 was significantly lower (p < 0.0001). Alum dosing in 2016 at 189 L d-1 produced a dry weather median of 0.28 mg L-1, which was significantly lower (p = 0.015) than in the 2014 median. Mean daily dry weather TP loads to the land application system were 44 kg d-1 in 2014, 45 kg d-1 in 2015, and 41 kg d-1 in 2016.

A pilot study of a subsurface-flow constructed wetland treating membrane concentrate produced from reclaimed water.

A pilot study was conducted for 7 months for the City of Oxnard, California, on the use of constructed wetlands to treat concentrate produced by microfiltration and reverse osmosis (RO) of reclaimed wastewater. The treatment performance of a transportable subsurface-flow wetland was investigated by monitoring various forms of nitrogen, orthophosphate, oxygen demand, organic carbon, and selenium. Significant mass removal of constituents was measured under two hydraulic residence times (HRTs) (2.5 and 5 days). Inflow and outflow concentrations of nitrate-N and ammonia-N were significantly different for both HRTs, whereas nitrite-N and total organic carbon (TOC) were significantly different during HRT2. Mass removal by the constructed wetland averaged 61% of nitrate-N, 32% of nitrite-N, 42% of ammonia-N, 43% of biochemical oxygen demand, 19% of orthophosphate as P, 18% of TOC and 61% of selenium. Mass removal exceeded concentration reductions through water volume loss through evapotranspiration. Calibrated first-order area-based removal rates were consistent with literature ranges, and were greater during HRT1 consistent with greater mass loads, higher hydraulic loading and shorter HRTs. The rate constants may provide a basis for sizing a full-scale wetland receiving a similar quality of water. The results indicated that engineered wetlands can be useful in the management of RO membrane concentrate for reclaimed water reuse.

Changes in fractal dimension during aggregation.

Experiments were performed to evaluate temporal changes in the fractal dimension of aggregates formed during flocculation of an initially monodisperse suspension of latex microspheres. Particle size distributions and aggregate geometrical information at different mixing times were obtained using a non-intrusive optical sampling and digital image analysis technique, under variable conditions of mixing speed, coagulant (alum) dose and particle concentration. Pixel resolution required to determine aggregate size and geometric measures including the fractal dimension is discussed and a quantitative measure of accuracy is developed. The two-dimensional fractal dimension was found to range from 1.94 to 1.48, corresponding to aggregates that are either relatively compact or loosely structured, respectively. Changes in fractal dimension are explained using a conceptual model, which describes changes in fractal dimension associated with aggregate growth and changes in aggregate structure. For aggregation of an initially monodisperse suspension, the fractal dimension was found to decrease over time in the initial stages of floc formation.