J.P.S. Sukias

Effect of water level fluctuation on nitrogen removal from constructed wetland mesocosms

Nitrogen removal processes were investigated at three frequencies of water level fluctuation, static, low and high (0, 2 and 6 d−1), in duplicate gravel-bed constructed wetland mesocosms (0.145 m3) with and without plants (Schoenoplectus tabernaemontani). Fluctuation was achieved by temporarily pumping wastewater into a separate tank (total drain time ∼35 min). Intensive sampling of the mesocosms, batch-fed weekly with ammonium-rich (∼100 g m−3 NH4-N) farm dairy wastewaters, showed rates of chemical oxygen demand (COD) and total Kjeldahl nitrogen (TKN) removal increased markedly with fluctuation frequency and in the presence of plants. Nearly complete removal of NH4-N was recorded over the 7 day batch period at the highest level of fluctuation, with minimal enhancement by plants. Redox potentials (Eh) at 100 mm depth rose from initial levels of around −100 to >350 mV and oxidised forms of N (NO2 and NO3) increased to ∼40 g m−3, suggesting conditions were conducive to microbial nitrification at this level of fluctuation. In the unplanted mesocosms with low or zero fluctuation, mean NH4-N removals were only 28 and 10%, respectively, and redox potentials in the media remained low for a substantial part of the batch periods (mid-batch Eh ∼+100 and −100 mV, respectively). In the presence of wetland plants, mean NH4-N removal in the mesocosms with low or zero fluctuation rose to 71 and 54%, respectively, and COD removal (>70%) and redox potential (mid-batch Eh>200 mV) were markedly higher than in the unplanted mesocosms. Negligible increases in oxidised N were recorded at these fluctuation frequencies, but total nitrogen levels declined at mean rates of 2.4 and 1.8 g m−2 d−1, respectively. NH4-N removal from the bulk water in the mesocosms was well described (R2=0.97–0.99) by a sorption-plant uptake-microbial model. First-order volumetric removal rate constants (kv) rose with increasing fluctuation frequency from 0.026 to 0.46 d−1 without plants and from 0.042 to 0.62 d−1 with plants. As fluctuation frequency increased, reversible sorption of NH4-N to the media, and associated biofilms and organic matter, became an increasingly important moderator of bulk water concentrations during the batch periods. TN mass balances for the full batch periods suggested that measured plant uptake estimates of between 0.52 and 1.07 g N m−2 d−1 (inversely related to fluctuation frequency) could fully account for the increased overall removal of TN recorded in the planted systems. By difference, microbial nitrification-denitrification losses were therefore estimated to be approximately doubled by low-level fluctuation from 0.7 to 1.4 g N m−2 d−1 (both with and without plants), rising to a maximum rate of 2.1 g N m−2 d−1 at high fluctuation, in the absence of competitive uptake by plants.

Advanced pond system for dairy‐farm effluent treatment

Abstract Two‐stage oxidation ponds have traditionally been used for the treatment of dairy‐farm wastewater in New Zealand, but are now considered unsuitable to discharge to waterways. The first full‐scale dairy‐farm advanced pond system (APS), a low‐cost and effective upgrade option for traditional ponds was evaluated over a 2‐year period. The system consisted of an anaerobic pond (AP) (the first pond of traditional oxidation pond systems), a high rate pond (HRP), a pair of algae settling ponds (ASP) and a maturation pond (MP) (which all replace the second pond of traditional systems). APS effluent quality was considerably higher than that of traditional ponds, with respective median effluent concentrations of biological oxygen demand: 43 versus 98 g m−3, total suspended solids: 87 versus 198 g m−3, ammoniacal nitrogen: 39 versus 106 gm−3, total phosphorus: 19 versus 27 g m−3, and Escherichia coli of 918 versus 70 000 MPN/100 ml. APS show great promise for upgrading traditional dairy‐farm oxidation ponds in New Zealand, particularly in areas where land irrigation is unsuitable.

Algal abundance, organic matter, and physico‐chemical characteristics of dairy farm facultative ponds: Implications for treatment performance

Abstract Six Waikato (New Zealand) dairy farm facultative ponds (DFPs), which met the larger sizes specified in recent dairy industry guidelines, were sampled monthly over an annual period. Median wastewater BOD5 was 65 g m‐3, suspended solids (SS) 206 g m‐3, ammoniacal N 37 g m‐3, total nitrogen 69 g m‐3, and faecal coliforms 24 000 (100 ml)‐1. This was 20–70% better than reported for DFPs built to previous guidelines, except for SS levels which were within reported ranges. However, performance was highly variable and only ½ of the DFPs studied consistently met an effluent standard of ≤ 100 g m‐3 BOD 5 and only one reached ≤ 150 g m‐3 SS. Removal of BOD 5 was much lower than recorded for SFPs in New Zealand with equivalent BOD 5 loading. Although the mean euphotic depth was only 0.11 m, algal biomass in DFPs was similar to that recorded for SFPs. Low phaeophytin concentrations and daytime oxygen exceeding 200% saturation in the shallow epilimnion on sunny days suggested a relatively healthy photosynthetic algal population was present in the DFPs. However, wastewater entering DFPs showed high median COD levels (1420 g m‐2). COD:BOD 5 ratios of c. 12.1 (compared with 1.5–1.8 for SFPs) and BOD 10 :BOD 5 ratios of c. 2 indicated the presence of a large pool of slowly degradable organic matter in the wastewater. This resulted in sustained exertion of BOD in the pond, explaining the “apparent” poor removal of BOD 5 by DFPs. Conductivity was found to be a useful single‐measure indicator of overall pond performance and management of sludge levels in the preceding anaerobic pond was identified as a key factor affecting DFP performance. Further improvements in dairy farm stabilisation pond performance are likely to be required on many farms to meet receiving water guidelines for the protection of water quality and aquatic life.

Removal of nitrate and phosphorus from hydroponic wastewater using a hybrid denitrification filter (HDF).

A laboratory-scale hybrid-denitrification filter (HDF) was designed by combining a plant material digester and a denitrification filter into a single unit for the removal of nitrate and phosphorus from glasshouse hydroponic wastewater. The carbon to nitrate (C:N) ratio for efficient operation of the HDF was calculated to be 1.93:1 and the COD/BOD(5) ratio was 1.2:1. When the HDF was continuously operated with the plant material replaced every 2 days and 100% internal recirculation of the effluent, a high level of nitrate removal (320-5 mg N/L, >95% removal) combined with a low effluent sBOD(5) concentration (<5mg/L) was consistently achieved. Moreover, phosphate concentrations in the effluent were maintained below 7.5 mg P/L (>81% reduction). This study demonstrates the potential to combine a digester and a denitrification filter in a single unit to efficiently remove nitrate and phosphate from hydroponic wastewater in a single unit.

Microbial Transport, Retention, and Inactivation in Streams: A Combined Experimental and Stochastic Modeling Approach

Long-term survival of pathogenic microorganisms in streams enables long-distance disease transmission. In order to manage water-borne diseases more effectively we need to better predict how microbes behave in freshwater systems, particularly how they are transported downstream in rivers. Microbes continuously immobilize and resuspend during downstream transport owing to a variety of processes including gravitational settling, attachment to in-stream structures such as submerged macrophytes, and hyporheic exchange and filtration within underlying sediments. We developed a stochastic model to describe these microbial transport and retention processes in rivers that also accounts for microbial inactivation. We used the model to assess the transport, retention, and inactivation of Escherichia coli in a small stream and the underlying streambed sediments as measured from multitracer injection experiments. The results demonstrate that the combination of laboratory experiments on sediment cores, stream reach-scale tracer experiments, and multiscale stochastic modeling improves assessment of microbial transport in streams. This study (1) demonstrates new observations of microbial dynamics in streams with improved data quality than prior studies, (2) advances a stochastic modeling framework to include microbial inactivation processes that we observed to be important in these streams, and (3) synthesizes new and existing data to evaluate seasonal dynamics.

Digestion of wastewater pond microalgae and potential inhibition by alum and ammoniacal-N

Algae are produced in considerable quantities in oxidation ponds, and may negatively affect receiving waters when discharged at high concentration. Thus in some instances they require removal prior to effluent discharge, which may be enhanced using flocculants such as alum. Harvested algal biomass could be anaerobically digested to methane for use as a renewable energy source, however, alum, has been reported to inhibit anaerobic digestion. Psychrophilic (20°C) anaerobic digestion experiments showed a 13% reduction in methane production with 200 g m(-3) alum in the flocculated algae, and a 40% reduction at an alum concentration of 1600 g m(-3). Elevated ammoniacal-N concentrations (785 g NH(4)(+)-N m(-3)) also inhibited algal digestion at 20°C when using an inoculum of anaerobic bacteria from a mesophylic municipal wastewater sludge digester. However, anaerobic digestion using a bacterial inoculum from a psychrophilic piggery anaerobic pond (in which typical ammoniacal-N levels range between 200 and 2000 g NH(4)(+)-N m(-3)) were unaffected by elevated digester ammoniacal-N levels and methane production actually increased slightly at higher ammoniacal-N concentrations. Thus, selecting an anaerobic bacterial inoculum that is already adapted to high ammoniacal-N levels and the digestion temperature, such as that form an anaerobic pond treating piggery wastewater, may avoid ammonia inhibition of algal digestion.