The impact of filter bed depth and solids loading using a multimedia filter

Design and operation of tertiary wastewater filters are not always well understood because of the inherent complexities of the wastewater matrix. Here, comparison of single, dual, triple and quadruple media (anthracite, flint, alumina and magnetite) filters were made to understand how the depth, media type and solids concentration influenced performance. The filter was improved by making the media deeper; however, the top 0.1 m of the filter retained the most solids. Additional layers of filter materials from single, dual, triple to quadruple improved the filter performance for the same depth of filter bed

Impact of pre-treatment technologies on soil aquifer treatment

This study investigates the impact of pre-treatment options on the performances of soil columns simulating soil aquifer treatment (SAT). For this purpose a conventional activated sludge (CAS) process, a membrane bioreactor (MBR) and vertical flow reed beds were used as single units or in combination before SAT. The influent and effluent from each treatment train were monitored over three successive 6-month periods, corresponding to changes in the operational conditions of the MBR and CAS units from 6 days' sludge retention time (SRT) to 12 and 20 days. All the columns acted as efficient polishing steps for solids and bacteria. The column receiving effluent from the CAS system running at 6 days' SRT also presented high total nitrogen and total phosphorus removals, but this column was also associated with the lowest infiltration rates over that period. While the quality of the effluent from the column following the CAS process increased over 18 months of operation, the effluent quality of the columns receiving MBR effluent degraded. No correlations were found between variations in SRT of the MBR and CAS processes and the columns' performances. Overall, all columns, except the one receiving CAS effluent, underwent a reduction in infiltration rate over 18 months

Influence of microalgal N and P composition on wastewater nutrient remediation

Microalgae have demonstrated the ability to remediate wastewater nutrients efficiently, with methods to further enhance performance through species selection and biomass concentration. This work evaluates a freshwater species remediation characteristics through analysis of internal biomass N:P (nitrogen:phosphorus) and presents a relationship between composition and nutrient uptake ability to assist in species selection. Findings are then translated to an optimal biomass concentration, achieved through immobilisation enabling biomass intensification by modifying bead concentration, for wastewaters of differing nutrient concentrations at hydraulic retention times (HRT) from 3 h to 10 d. A HRT <20 h was found suitable for the remediation of secondary effluent by immobilised Scenedesmus obliquus and Chlorella vulgaris at bead concentrations as low as 3.2 and 4.4 bead·mL−1. Increasing bead concentrations were required for shorter HRTs with 3 h possible at influent concentrations <5 mgP L−1

Biofouling and scaling control of reverse osmosis membrane using one-step cleaning - potential of acidified nitrite solution as an agent

Biofouling is generally regarded as a major issue in reverse osmosis (RO) membrane filtration. Two-step chemical cleanings with alkaline and acidic agents are typically applied to restore the treatment capacity. In this study, the feasibility of one-step cleaning using free nitrous acid (FNA) was investigated as a novel low cost cleaning agent. The FNA cleaning solution was prepared by acidification of a sodium nitrite solution with hydrochloric acid. Seven fouled RO membranes collected from full-scale wastewater recycling and desalination plants were used to perform lab-scale cleaning trials. Membrane fouling characterisation revealed six of out of seven membranes were mainly bio-fouled, while one membrane was severely fouled by calcium carbonate. This study showed the feasibility of using FNA at pH 3.0 for biomass removal as well as for calcium carbonate scaling removal. The results from the lab-scale cleaning tests suggested that FNA can be used as a single cleaning agent for both biofouling and scaling removal. Cost analysis showed that FNA is a cost-effective solution for biofouling and scaling removal in RO filtration applications

The effect of high hydraulic loading rate on the removal efficiency of a quadruple media filter for tertiary wastewater treatment

It is well known that filtration removal efficiency falls with an increase in flow rate; however, there is limited supporting experimental data on how removal efficiency changes for filters with multiple layers of media and for wastewater filtration, a practice that is becoming more common. Furthermore, information is not available on the characteristics of particles that are removed at different flow rates. Here, a quadruple media filter was operated at hydraulic loading rates (HLRs) between 5 and 60 mh−1 with subsequent measurement of total suspended solids, turbidity and particle size distribution (PSD). Samples were collected from the filter influent, effluent and also from between media layers. Pressure changes across the filter layers were also measured. The solids removal efficiency of the filter varied inversely with the increase in filtration rate. However, the multiple media layers reduced the negative impact of increased HLR in comparison to a single media filter. High filtration rates were shown to transport solids, such that particle retention and headloss development was distributed across the entire depth of the multi-media filter. There was also a progressive decrease in the suspension particle size leaving each of the filter layers. The particle hydrodynamic force simulation was consistent with the changes in measured PSD through the filter layers

Recovery of ammonia from wastewater through chemical precipitation

Chemical precipitation is a consolidated technique applied in wastewater treatment to remove and recover phosphorous and ammonium that remain in the effluent after the anaerobic digestion treatment. The precipitate is magnesium ammonium phosphate hexahydrate (MgNH4PO4·6H2O), also known as struvite, and it is sold as a slow-release fertiliser. However, the value of struvite is quite low and has a limited market. Furthermore, it precipitates with heavy metals and other impurities that need to be removed to make the fertiliser commercially viable. This study looked at the thermal decomposition of struvite to recover added value products and recycle the magnesium for further precipitation. A kinetic study was carried out to understand the mechanism of decomposition and the formation of the different solid phases, which is fundamental for the design and optimisation of the technology. The thermogravimetric study confirmed that thermal decomposition is possible, but ammonia could not be completely released below 250 °C. The thermal analysis also led to the determination of the energy required for the decomposition, found to be 1.87 kJ g−1, which also includes the evaporation of water and ammonia. The kinetic study through the isoconversional method showed the presence of two major reactions, and the model-fitting approach identified the diffusion model as the best fit for the first reaction. The activation energy of the first reaction found with this method was 0.24 kJ g−1, comparable with the data obtained from the isoconversional method. The two-stage decomposition reactions were proposed, and the final calcination product was confirmed as magnesium pyrophosphate, which could be used in agriculture or dissolved in diluted mineral acids solution to separate the phosphate from the magnesium

A comparative evaluation of reverse osmosis membrane performance when combined with anaerobic or aerobic membrane bioreactors for indirect potable reuse applications

The filtration performance and fouling behaviour of reverse osmosis (RO) membranes was investigated for the post-treatment of aerobic (Ae) and anaerobic (An) MBR effluents treating municipal wastewater for potable reuse. Both MBR effluents followed by RO can produce a water quality sufficient for indirect potable water reuse, while fluorescence excitation-emission scan suggests RO can effectively remove disinfection by-products precursors, ensuring the safety for chlorine based reuse water distribution by rejecting the dissolved organic matters in MBR effluents. AnMBR effluent leads to more fouling when compared to the AeMBR effluent with an average membrane fouling resistance of 12.35 × 1013 m−1 and 8.97 × 1013 m−1. Elemental analysis and membrane surface imaging results demonstrate that the foulant deposition sequence is organic and colloidal at first, followed by inorganic substances, while TOC and Ca are the most deposited foulants from both effluents. The unremoved ammonia in the AnMBR effluent may partially go through in the RO permeate and exceed the threshold in Singapore's PUB NEWater standard, while experiencing a significantly higher deposition rate of 13.8 % than the nitrate (0.02 %) from the AeMBR effluent. The findings suggest that the combination of AnMBR with RO offers a more sustainable approach than with the AeMBR but nutrients removal, with the potential of recovery, is recommended before the RO membranes to limit the fouling propensity and achieve a permeate of sufficient quality

UV/TiO2 photocatalysis as post-treatment of anaerobic membrane bioreactor effluent for reuse

Advanced oxidation processes have been widely applied as a post-treatment solution to remove residual organic compounds in water reuse schemes. However, UV/TiO2 photocatalysis, which provides a sustainable option with no continuous chemical addition, has very rarely been studied to treat anaerobically treated effluents. In the current study, the removal of organics and nutrients from an anaerobic membrane bioreactor (AnMBR) effluent is evaluated during adsorption and photocatalysis processes under various conditions of TiO2 dose and UV intensity and compared to the effluent from an aerobic membrane bioreactor (AeMBR). The sequence for preferential adsorption on TiO2 was found to be phosphorus, inorganic carbon and then ammonia/organic carbon were found. The competing effect between the organics and nutrients, along with the low UV transmission efficiency caused by the need for high doses of TiO2, ultimately compromise the organic removal efficiency in the AnMBR permeate. TiO2 dosage was found to have a greater impact than UV intensity on improving the overall removal performance as nutrients are competing for the adsorption site but are not photodegraded. Under the same operational condition, the UV/TiO2 photocatalysis displayed a higher removal efficiency of organic matter and phosphorus in the AeMBR effluent due to a lower initial organics concentration and absence of ammonia as compared to the AnMBR effluent

Energy recovery from immobilised cells of Scenedesmus obliquus after wastewater treatment

Biomethane batch test of alginate beads and beads with algae at different stages of utilisation in the wastewater treatment plants showed that immobilised S. obliquus yield similar biogas and biomethane than freely suspended algae (between 60.51 ± 4.19 and 82.32 ± 2.17 mL g-1 VSadd) and that a pre-treatment stage was not necessary for the digestion process

Scale-up of reverse electrodialysis for energy generation from high concentration salinity gradients

Whilst reverse electrodialysis (RED) has been extensively characterised for saline gradient energy from seawater/river water (0.5 M/0.02 M), less is known about RED stack design for high concentration salinity gradients (4 M/0.02 M), important to closed loop applications (e.g. thermal-to-electrical, energy storage). This study therefore focuses on the scale-up of RED stacks for high concentration salinity gradients. Higher velocities were required to attain a maximum Open Circuit Voltage (OCV) for 4 M/0.02 M, which gives a measure of the electrochemical potential of the cell. The experimental OCV was also much below the theoretical OCV, due to the greater boundary layer resistance observed, which is distinct from 0.5 M/0.02 M. However, negative net power density (net produced electrical power divided by total membrane area) was demonstrated with 0.5 M/0.02 M for larger stacks using shorter residence times (three stack sizes tested: 10 × 10cm, 10 × 20cm and 10 × 40cm). In contrast, the highest net power density was observed at the shortest residence time for the 4 M/0.02 M concentration gradient, as the increased ionic flux compensated for the pressure drop. Whilst comparable net power densities were determined for the 10 × 10cm and 10 × 40cm stacks using the 4 M/0.02 M concentration gradient, the osmotic and ionic transport mechanisms are distinct. Increasing cell pair number improved maximum current density. This subsequently increased power density, due to the reduction in boundary layer resistance, and may therefore be used to improve thermodynamic efficiency and power density from RED for high concentrations. Although comparable power densities may be achieved for small and large stacks, large stacks maybe preferred for high concentration salinity gradients due to the comparative benefit in thermodynamic efficiency in single pass. The greater current achieved by large stacks may also be complemented by an increase in cell pair number and current density optimisation to increase power density and reduce exergy losses