Colloidal Chemistry in Water Treatment: The Effect of Ca2+ on the Interaction between Humic Acid and Poly(diallyldimethylammonium chloride) (PDADMAC).

The complexation of humic acid (HA), as a major component of natural organic matter (NOM) in raw water, with polycations is a key step in the water treatment process. At sufficiently high addition of a polycation, it leads to neutralization of the formed complexes and precipitation. In this work, we studied the effect of the presence of Ca2+ ions on this process, with poly(diallyldimethylammonium chloride) (PDADMAC) as a polycation. This was done by determining the phase behavior and characterizing the structures in solution by light scattering and small-angle neutron scattering (SANS). We observe that with increasing Ca2+ concentration, the phase boundaries of the precipitation region shift to a lower PDADMAC concentration, which coincides well with a shift of the ζ-potential of the aggregates in solution. Light scattering shows the formation of aggregates of a 120-150 nm radius, and SANS shows that Ca2+ addition promotes a compaction in the size range of 10-50 nm within these aggregates. This agrees well with the observation of more densely packed precipitates by confocal microscopy in the presence of Ca2+. Following the precipitation kinetics by turbidimetry shows a marked speeding up of the process already in the presence of rather small Ca2+ concentrations of 1 mg/L. It can be stated that the presence of Ca2+ during the complexation process of HA with a polycation has a marked effect on phase behavior and precipitation kinetics of the formed aggregates. In general, the presence of Ca2+ facilitates the process largely already at rather low concentrations, and this appears to be linked to a compaction of the formed structures in the mesoscopic size range of about 10-50 nm. These findings should be of significant importance for tailoring the flocculation process in water treatment, which is a highly important process in delivering drinking water of sufficient quality to humans.

Forecasting and Optimizing Dual Media Filter Performance via Machine Learning.

Four different machine learning algorithms, including Decision Tree (DT), Random Forest (RF), Multivariable Linear Regression (MLR), Support Vector Regressions (SVR), and Gaussian Process Regressions (GPR), were applied to predict the performance of a multi-media filter operating as a function of raw water quality and plant operating variables. The models were trained using data collected over a seven year period covering water quality and operating variables, including true colour, turbidity, plant flow, and chemical dose for chlorine, KMnO4, FeCl3, and Cationic Polymer (PolyDADMAC). The machine learning algorithms have shown that the best prediction is at a 1-day time lag between input variables and unit filter run volume (UFRV). Furthermore, the RF algorithm with grid search using the input metrics mentioned above with a 1-day time lag has provided the highest reliability in predicting UFRV with a RMSE and R2 of 31.58 and 0.98, respectively. Similarly, RF with grid search has shown the shortest training time, prediction accuracy, and forecasting events using a ROC-AUC curve analysis (AUC over 0.8) in extreme wet weather events. Therefore, Random Forest with grid search and a 1-day time lag is an effective and robust machine learning algorithm that can predict the filter performance to aid water treatment operators in their decision makings by providing real-time warning of the potential turbidity breakthrough from the filters.

Characterizing concrete corrosion below sewer tidal levels at chemically dosed locations.

Unexplainable concrete softening below the water line has been observed by Sydney Water in their gravity sewer network, some of which is subjected to corrosion control methods using chemical ferrous chloride (FeCl2) dosing of the wastewater. We applied a combination of physical and chemical tools to determine the properties of the top 20 mm of concrete cores recovered from sewer pipes. These techniques consist of neutron tomographic imaging, scanning electron microscopy, hardness mapping, and pH profiling. Concrete cores were collected from roof (crown), tidal (wall) and below flow regions of gravity sewer pipes of Sydney Water's wastewater system from locations that received no treatment as well as locations dosed with FeCl2. All samples showed a degree of softening of the surface exposed to the sewerage with an associated depletion in calcium concentration and reduced pH in the same regions.

Effective Separation of CO2 Using Metal-Incorporated rGO Membranes.

Graphene-based materials, primarily graphene oxide (GO), have shown excellent separation and purification characteristics. Precise molecular sieving is potentially possible using graphene oxide-based membranes, if the porosity can be matched with the kinetic diameters of the gas molecules, which is possible via the tuning of graphene oxide interlayer spacing to take advantage of gas species interactions with graphene oxide channels. Here, highly effective separation of gases from their mixtures by using uniquely tailored porosity in mildly reduced graphene oxide (rGO) based membranes is reported. The gas permeation experiments, adsorption measurement, and density functional theory calculations show that this membrane preparation method allows tuning the selectivity for targeted molecules via the intercalation of specific transition metal ions. In particular, rGO membranes intercalated with Fe ions that offer ordered porosity, show excellent reproducible N2 /CO2 selectivity of ≈97 at 110 mbar, which is an unprecedented value for graphene-based membranes. By exploring the impact of Fe intercalated rGO membranes, it is revealed that the increasing transmembrane pressure leads to a transition of N2 diffusion mode from Maxwell-Stefan type to Knudsen type. This study will lead to new avenues for the applications of graphene for efficiently separating CO2 from N2 and other gases.

Application of a novel molecular technique to characterise the effect of settling on microbial community composition of activated sludge.

Activated sludge (AS) and return activated sludge (RAS) microbial communities from three full-scale municipal wastewater treatment plants (denoted plant A, B and C) were compared to assess the impact of sludge settling (i.e. gravity thickening in the clarifier) and profile microorganisms responsible for nutrient removal and reactor foaming. The results show that all three plants were dominated with microbes in the phyla of Proteobacteria, Bacteroidetes, Verrucomicrobia, Actinobacteria, Chloroflexi, Firmicutes, Nitrospirae, Spirochaetae, Acidobacteria and Saccharibacteria. AS and RAS shared above 80% similarity in the microbial community composition, indicating that sludge thickening does not significantly alter the microbial composition. Autotrophic and heterotrophic nitrifiers were present in the AS. However, the abundance of autotrophic nitrifiers was significantly lower than that of the heterotrophic nitrifiers. Thus, ammonium removal at these plants was achieved mostly by heterotrophic nitrification. Microbes that can cause foaming were at 3.2% abundance, and this result is well corroborated with occasional aerobic biological reactor foaming. By contrast, these microbes were not abundant (<2.1%) at plant A and C, where aerobic biological reactor foaming has not been reported.

Impacts of mixing on foaming, methane production, stratification and microbial community in full-scale anaerobic co-digestion process.

This study investigated the impact of mixing on key factors including foaming, substrate stratification, methane production and microbial community in three full scale anaerobic digesters. Digester foaming was observed at one plant that co-digested sewage sludge and food waste, and was operated without mixing. The lack of mixing led to uneven distribution of total chemical oxygen demand (tCOD) and volatile solid (VS) as well as methane production within the digester. 16S rRNA gene-based community analysis clearly differentiated the microbial community from the top and bottom. By contrast, foaming and substrate stratification were not observed at the other two plants with internal circulation mixing. The abundance of methanogens (Methanomicrobia) at the top was about four times higher than at the bottom, correlating to much higher methane production from the top verified by ex-situ biomethane assay, causing foaming. This result is consistent with foaming potential assessment of digestate samples from the digester.

Anaerobic digestion of soft drink beverage waste and sewage sludge.

Soft drink beverage waste (BW) was evaluated as a potential substrate for anaerobic co-digestion with sewage sludge to increase biogas production. Results from this study show that the increase in biogas production is proportional to the increase in organic loading rate (OLR) rate due to BW addition. The OLR increase of 86 and 171% corresponding to 10 and 20% BW by volume in the feed resulted in 89 and 191% increase in biogas production, respectively. Under a stable condition, anaerobic co-digestion with BW did not lead to any significant impact on digestate quality (in terms of COD removal and biosolids odour) and biogas composition. The results suggest that existing nutrients in sewage sludge can support an increase in OLR by about 2 kg COD/m3/d from a carbon rich substrate such as soft drink BW without inhibition or excessive impact on subsequent handling of the digestate.

Mechanism of water transport in graphene oxide laminates.

It is understood that nano-channels of graphene oxide membranes have a water flow mechanism which is similar to the water flow inside carbon nanotube pores. The water transport mechanisms recently proposed by various researchers suggest that membranes composed of graphene oxide laminates could be regarded as an assembly of many tiny carbon nanotubes stacked together with attached functional groups as spacers.

Effects of shearing on biogas production and microbial community structure during anaerobic digestion with recuperative thickening.

Recuperative thickening can intensify anaerobic digestion to produce more biogas and potentially reduce biosolids odour. This study elucidates the effects of sludge shearing during the thickening process on the microbial community structure and its effect on biogas production. Medium shearing resulted in approximately 15% increase in biogas production. By contrast, excessive or high shearing led to a marked decrease in biogas production, possibly due to sludge disintegration and cell lysis. Microbial analysis using 16S rRNA gene amplicon sequencing showed that medium shearing increased the evenness and diversity of the microbial community in the anaerobic digester, which is consistent with the observed improved biogas production. By contrast, microbial diversity decreased under either excessive shearing or high shearing condition. In good agreement with the observed decrease in biogas production, the abundance of Bacteroidales and Syntrophobaterales (which are responsible for hydrolysis and acetogenesis) decreased due to high shearing during recuperative thickening.

Effects of sludge retention time on oxic-settling-anoxic process performance: Biosolids reduction and dewatering properties.

In this study, the effect of sludge retention time (SRT) on oxic-settling-anoxic (OSA) process was determined using a sequencing batch reactor (SBR) attached to external aerobic/anoxic reactors. The SRT of the external reactors was varied from 10 to 40d. Increasing SRT from 10 to 20d enhanced volatile solids destruction in the external anoxic reactor as evidenced by the release of nutrients, however, increasing the SRT to 40d did not enhance volatile solids destruction further. Relatively short SRT (10-20d) favoured the conversion of destroyed solids into inert products. The application of an intermediate SRT (20d) of the external reactor showed the highest sludge reduction performance (>35%). Moreover, at the optimum SRT, OSA improved sludge dewaterability as demonstrated by lower capillary suction time and higher dewatered cake solids content.

Biosolids reduction by the oxic-settling-anoxic process: Impact of sludge interchange rate.

The impact of sludge interchange rate (SIR) on sludge reduction by oxic-settling-anoxic (OSA) process was investigated. The sludge yield of an OSA system (a sequencing batch reactor, SBR, integrated with external anoxic reactors) was compared to that of a control (an SBR attached to a single-pass aerobic digester). SIR (%) is the percentage by volume of sludge returned from the external reactor into the main bioreactor of the OSA, and was varied from 0% to 22%. OSA achieved greater sludge reduction when fed with unsettled sewage (sCOD=113mg/L) rather than settled sewage (sCOD=60mg/L). The SIR of 11% resulted in the highest OSA performance. At the optimum SIR, higher volatile solids destruction and nitrification/denitrification (i.e., conversion of destroyed volatile solids into inert forms) were observed in the external anoxic and intermittently aerated (i.e., aerobic/anoxic) reactors, respectively. Denitrification in the aerobic/anoxic reactor was inefficient without SIR. Effluent quality and sludge settleability of the main SBR were unaffected by SIR.

Impact of reduced water consumption on sulfide and methane production in rising main sewers.

Reduced water consumption (RWC), for water conservation purposes, is expected to change the wastewater composition and flow conditions in sewer networks and affect the in-sewer transformation processes. In this study, the impact of reduced water consumption on sulfide and methane production in rising main sewers was investigated. Two lab-scale rising main sewer systems fed with wastewater of different strength and flow rates were operated to mimic sewers under normal and RWC conditions (water consumption reduced by 40%). Sulfide concentration under the RWC condition increased by 0.7-8.0 mg-S/L, depending on the time of a day. Batch test results showed that the RWC did not change the sulfate-reducing activity of sewer biofilms, the increased sulfide production being mainly due to longer hydraulic retention time (HRT). pH in the RWC system was about 0.2 units lower than that in the normal system, indicating that more sulfide would be in molecular form under the RWC condition, which would result in increased sulfide emission to the atmosphere as confirmed by the model simulation. Model based analysis showed that the cost for chemical dosage for sulfide mitigation would increase significantly per unit volume of sewage, although the total cost would decrease due to a lower sewage flow. The dissolved methane concentration under the RWC condition was over two times higher than that under the normal flow condition and the total methane discharge was about 1.5 times higher, which would potentially result in higher greenhouse gas emissions. Batch tests showed that the methanogenic activity of sewer biofilms increased under the RWC condition, which along with the longer HRT, led to increased methane production.

Surface neutralization and H(2)S oxidation at early stages of sewer corrosion: influence of temperature, relative humidity and H(2)S concentration.

While the involvement of a range of environmental factors in sewer corrosion is known, a comprehensive understanding of the processes involved and the exact role of individual environmental factors in sewer corrosion is still lacking. The corrosion of concrete in sewer systems is reported to be initiated through chemical reactions (involving H(2)S and CO(2)) that lower the surface pH to a level then conducive for biological activity. However, the specific influence of environmental variables, such as H(2)S level, temperature, and relative humidity etc. remains unclear; although, they are expected to control these initial surface reactions of the concrete sewer pipe. We examined changes in the surface chemistry of concrete during the early stages of corrosion by exposing concrete coupons to thirty-six independent conditions in well-controlled laboratory chambers that simulated conditions typically found in various sewer environments across Australia. The conditions employed were combinations of six H(2)S levels, three gas-phase temperatures and two relative humidity levels. Our results indicate that the role of CO(2) on initial surface pH reduction is insignificant when compared to the influence of H(2)S. Within the first 12 months, a decrease in surface pH by 4.8 units was observed for coupons exposed to 30 °C and 50 ppm H(2)S, while significantly lower pH reductions of 3.5 and 1.8 units were detected for coupons exposed to 25 °C and 18 °C respectively, and 50 ppm H(2)S. Elemental sulphur was found to be the major oxidation product of H(2)S and elevated concentrations were detected at the higher levels of H(2)S, temperature and relative humidity. More significantly, the data obtained from the controlled chamber experiments correlated with those obtained from the field-exposed coupons. Hence, these findings can be extended to real sewer corrosion processes.

Fate of cyanobacteria and their metabolites during water treatment sludge management processes.

Cyanobacteria and their metabolites are an issue for water authorities; however, little is known as to the fate of coagulated cyanobacterial-laden sludge during waste management processes in water treatment plants (WTPs). This paper provides information on the cell integrity of Anabaena circinalis and Cylindrospermopsis raciborskii during: laboratory-scale coagulation/sedimentation processes; direct filtration and backwashing procedures; and cyanobacterial-laden sludge management practices. In addition, the metabolites produced by A. circinalis (geosmin and saxitoxins) and C. raciborskii (cylindrospermopsin) were investigated with respect to their release (and possible degradation) during each of the studied processes. Where sedimentation was used, coagulation effectively removed cyanobacteria (and intracellular metabolites) without any considerable exertion on coagulant demand. During direct filtration experiments, cyanobacteria released intracellular metabolites through a stagnation period, suggesting that more frequent backwashing of filters may be required to prevent floc build-up and metabolite release. Cyanobacteria appeared to be protected within the flocs, with minimal damage during backwashing of the filters. Within coagulant sludge, cyanobacteria released intracellular metabolites into the supernatant after 3d, even though cells remained viable up to 7d. This work has improved the understanding of cyanobacterial metabolite risks associated with management of backwash water and sludge and is likely to facilitate improvements at WTPs, including increased monitoring and the application of treatment strategies and operational practices, with respect to cyanobacterial-laden sludge and/or supernatant recycle management.

Fate of toxic cyanobacterial cells and disinfection by-products formation after chlorination.

Drinking water sources in many regions are subject to proliferation of toxic cyanobacteria (CB). Chlorination of source water containing toxic cyanobacterial cells for diverse treatment purposes might cause cell damage, toxin release and disinfection by-products (DBP) formation. There is limited information available on chlorination of different toxic CB cells and DBP formation potentials. This work: (1) determines the extent of lysis and toxins/taste and odor compound release in chlorinated natural water from CB cells (Anabaena circinalis, Microcystis aeruginosa, Cylindrospermopsis raciborskii, and Aphanizomenon issatsckenka) from laboratory cultures and natural blooms; (2) assesses the rates of oxidation of toxins by free chlorine under environmental conditions; (3) studies the DBP formation associated with the chlorination of CB cell suspensions. With chlorine exposure (CT) value of <4.0 mg min/L >60% cells lost viability causing toxin release. Cell membrane damage occurred faster than oxidation of released toxins. Kinetic analysis of the oxidation of toxins in natural water revealed significant differences in their susceptibility to chlorine, saxitoxins being the easiest to oxidize, followed by cylindrospermopsin and microcystin-LR. Furthermore, concentrations of trihalomethanes and haloacetic acids (<40 µg/L) and N-nitrosodimethylamine (<10 ng/L) as chlorination by-products were lower than the guideline values even at the highest CT value (220 mg min/L). However, the DBP concentrations in environmental bloom conditions with very high cell numbers were over the guideline values.

Application of powdered activated carbon for the adsorption of cylindrospermopsin and microcystin toxins from drinking water supplies.

Cylindrospermopsin (CYN) and microcystin are two potent toxins that can be produced by cyanobacteria in drinking water supplies. This study investigated the application of powdered activated carbon (PAC) for the removal of these toxins under conditions that could be experienced in a water treatment plant. Two different PACs were evaluated for their ability to remove CYN and four microcystin variants from various drinking water supplies. The removal of natural organic material by the PACs was also determined by measuring the levels of dissolved organic carbon and UV absorbance (at 254 nm). The PACs effectively removed CYN and the microcystins from each of the waters studied, with one of the PACs shown to be more effective, possibly due to its smaller particle diameter. No difference in removal of the toxins was observed using PAC contact times of 30, 45 and 60 min. Furthermore, the effect of water quality on the removal of the toxins was minimal. The microcystin variants were adsorbed in the order: MCRR > MCYR > MCLR > MCLA. CYN was found to be adsorbed similarly to MCRR.