The effect of loess addition on the settling ability of activated sludge.

In this research, loess addition was investigated as a possible means of controlling the bulking sludge generated from a sequencing batch reactor (SBR) system treating a synthetic wastewater. The specific objective was to investigate whether loess changed the morphology of the sludge (i.e., influenced the relative abundance of filamentous species), as opposed to improving settling simply because the clay portion of the loess acted as a flocculating agent. To this end, two sets of batch tests were performed using 1 L reactors filled with bulking sludge from the SBR. The first set of batch tests investigated the effect of different loess concentration on the settling properties of the sludge; thus loess was added in concentrations of 0.0, 0.4, 2.0 and 5.0 g L(-1). The 5.0 g L(-1) loess concentration exhibited the most positive results on settling, bringing the modified sludge volume index (SVI) down into the target range of 150 mL g(-1). The second set of batch tests investigated filament length along with the modified SVI. It appeared that at the microbial level, 5.0 g L(-1)of loess caused no reduction in filament length, suggesting no reduction in the amount of filamentous microorganisms. This means that adding loess to a system after it has bulked has the potential to mask the bulking problem by improving settling, while not fixing the problem microbiologically.

Kinetic study of adsorption of arsenic onto New Zealand Ironsand (NZIS).

New Zealand Ironsand (NZIS), an iron-rich sand ubiquitous to the coast of the North Island of New Zealand was examined for the removal of arsenic (both As (III) and As (V)) by adsorption. Batch experiments were performed to evaluate the adsorption kinetics at three different pH conditions (3.0, 7.5 and 11.0). In addition, a column test was conducted to obtain the breakthrough curve and appraise the arsenic removal capacity of NZIS used as a filter media. The kinetic study showed that a very long contact time (>144 h) was needed to reach equilibrium and the nature of the adsorption was well described (R(2) value more than 0.96 at each pH condition) with a pseudo-second-order adsorption kinetic model for both As (III) and As (V). In column tests, a pore volume (PV) of 700 and 400 yielded a total arsenic level less than the WHO guideline value of 10 µg/L for As (III) and As (V), respectively.

Biodegradation of industrial-strength 2,4-dichlorophenoxyacetic acid wastewaters in the presence of glucose in aerobic and anaerobic sequencing batch reactors.

This research explored the biodegradability of 2,4-dichlorophenoxyacetic acid (2,4-D) in two laboratory-scale sequencing batch reactors (SBRs) that operated under aerobic and anaerobic conditions. The potential limit of 2,4-D degradation was investigated at a hydraulic retention time of 48 h, using glucose as a supplemental substrate and increasing feed concentrations of 2,4-D; namely 100 to 700 mg/L (i.e. industrial strength) for the aerobic system and 100 to 300 mg/L for the anaerobic SBR. The results revealed that 100 mg/L of 2,4-D was completely degraded following an acclimation period of 29 d (aerobic SBR) and 70 d (anaerobic SBR). The aerobic system achieved total 2,4-D removal at feed concentrations up to 600 mg/L which appeared to be a practical limit, since a further increase to 700 mg/L impaired glucose degradation while 2,4-D biodegradation was non-existent. In all cases, glucose was consumed before the onset of 2,4-D degradation. In the anaerobic SBR, 2,4-D degradation was limited to 120 mg/L.

Effects of hydraulic and solids retention times on productivity and settleability of microbial (microalgal-bacterial) biomass grown on primary treated wastewater as a biofuel feedstock.

High biomass productivity and efficient harvesting are currently recognized challenges in microbial biofuel applications. To produce naturally settleable biomass, combined growth of native microalgae and bacteria was facilitated in laboratory sequencing batch reactors (SBRs) using primary treated wastewater from the Christchurch Wastewater Treatment Plant (CWTP) in New Zealand. SBRs were operated under a simulated, local, summer climate (i.e., 925 µmol/m(2)/s of photosynthetically active radiation for 14.7 h per day at 21 °C mean water temperature) using 1.4- to 8-day hydraulic retention times (HRTs) to optimize growth. Solids retention times (SRTs) were varied from 4 to 40 days by discharging different ratios of supernatant and completely mixed culture. Biomass productivity up to 31 g/m(2)/day of solids was obtained, and it generally increased as retention times decreased. Biomass settleability was typically 70-95%, and the microbes aggregated into compact flocs as cultures aged up to four months. Due to a low lipid content of 10.5%, anaerobic digestion appeared to be the most appropriate biofuel conversion process with potential to generate 19,200 m(3)/ha/yr of methane based on settleable mixture productivity.

Two-phase anaerobic digestion of the organic fraction of municipal solid waste: estimation of methane production.

Energy generation from methane (CH(4)) is one of the primary targets of the anaerobic digestion process. Consequently, the focus of this study was to investigate the effect on CH(4) production of total solids (TS) loading (measured as % TS) and hydraulic residence time (HRT) during the treatment of the organic fraction of municipal solid waste (OFMSW). Laboratory-scale, two-phase anaerobic digestion systems were employed with each system consisting of an acidogenic reactor and a methanogenic reactor linked in series. The group A runs in the experiment explored the effect on digester performance of four variations in methanogenic HRT (15, 20, 25 and 30 days) at three different feed TS concentrations (8, 12 and 15%). The group B runs compared the actual methane yield (0.14 to 0.45 L g VSfeed−1)) to that predicted by the Chen-Hashimoto model. Results from the group A runs indicated that acidogenesis improved with an increase in % TS and a decrease in HRT; while, methanogenesis behaved inversely, achieving higher yields at the lower % TS and longer HRT values. In comparison with the group B runs, the Chen-Hashimoto model under-predicted (by an average of 16.5 ± 6.6%) the CH(4) yield obtained from the digestion of OFMSW.

Effect of mixture ratio, solids concentration and hydraulic retention time on the anaerobic digestion of the organic fraction of municipal solid waste.

This paper describes how the degradation of the organic fraction of municipal solid waste (OFMSW) is affected through codigestion with varying amounts of return activated sludge (RAS). Solid waste that had its inorganic fraction selectively removed was mixed with RAS in ratios of 100% OFMSW, 50% OFMSW/50% RAS, and 25% OFMSW/75% RAS. The total solids (TS) concentration was held at 8% and three anaerobic digester systems treating the mixtures were held (for the first run) at a total hydraulic retention time (HRT) of 28 days. Increasing amounts of RAS did not however improve the mixture's digestability, as indicated by little change and/or a drop in the main performance indices [including percentage volatile solids (VS) removal and specific gas production]. The optimum ratio in this research therefore appeared to be 100% OFMSW with an associated 85.1 ± 0.6% VS removal and 0.72 ± 0.01 L total gas g(- 1) VS. In the second run, the effect of increasing percentage of TS (8, 12% and 15%) at a system HRT of 28 days was observed to yield no improvement in the main performance indices (i.e. percentage VS removal and specific gas production). Finally, during the third run, variations in the total system HRT were investigated at an 8% TS, again using 100% OFMSW. Of the HRTs explored (23, 28 and 33 days), the longest HRT yielded the best performance overall, particularly in terms of specific gas production (0.77 ± 0.01 L total gas g(-1) VS).

The use of naturally generated volatile fatty acids for herbicide removal via denitrification.

This research focuses on the removal of 2, 4-D via denitrification, with a particular emphasis on the effect of adding naturally generated volatile fatty acids (VFAs) as a carbon source. These VFAs had been produced from an acid-phase anaerobic digester (mean VFA concentration of 3153 +/- 801 mg/L [as acetic acid]). The first step involved developing 2, 4-D degrading bacteria in a sequencing batch reactor (SBR) fed with both sewage and 2, 4-D (30-100 mg/L). Subsequent denitrification batch tests demonstrated that the specific denitrification rate increased from 0.0119 +/- 0.0039 to 0.0192 +/- 0.0079 g NO(3)-N/g volatile suspended solids (VSS) per day, when using 2, 4-D alone versus 2, 4-D plus natural VFAs from the digester as a carbon source. Similarly, the specific 2, 4-D consumption rate increased from 0.0016 +/- 0.0009 to 0.0055 +/- 0.0021 g 2,4-D/g VSS per day, when using 2, 4-D alone as compared to using 2, 4-D plus natural VFAs. Finally, a parallel increase in the percent 2, 4-D removal was observed, rising from 28.33 +/- 11.88 using 2, 4-D alone to 54.17 +/- 21.89 using 2, 4-D plus natural VFAs.

Effect of a starch-rich industrial wastewater on the acid-phase anaerobic digestion process.

This research investigated the effect of varying the starch-rich, industrial-wastewater component of mixtures with municipal wastewater fed to an anaerobic digester. A laboratory-scale, completely-mixed anaerobic digester was operated at an HRT of 30 h, an SRT of 10 d, and an ambient temperature of 21.5 +/- 1.5 degrees C. The industrial-to-municipal ratios tested were 1:3, 1:1, 3:1, and 100% industrial by volume. Steady-state, acidogenic conditions were achieved for all runs, except 100% industrial. The pH was observed to drop substantially as the industrial constituent of the feed increased. Net volatile fatty acids (VFA) production reached a plateau of approximately 800 mg/L at ratios of 1:1 and higher, while volatile suspended solids (VSS) reduction steadily increased as the industrial component rose. The specific VFA and soluble chemical oxygen demand (SCOD) production rates leveled off at approximately 0.070 mgVFA/ mgVSS.d and 0.124 mgSCOD/mgVSS.d, respectively, for all the mixtures investigated, except for 100% industrial. In this latter case, both rates dropped dramatically. Finally, acetic and propionic acid concentrations fell as the industrial proportion of the mixture increased. This was compensated by a rise in butyric acid production.