Influence of intermittent wetting and drying conditions on heavy metal removal by stormwater biofilters

Biofiltration is a technology to treat urban stormwater runoff, which conveys pollutants, including heavy metals. However, the variability of metals removal performance in biofiltration systems is as yet unknown. A laboratory study has been conducted with vegetated biofilter mesocosms, partly fitted with a submerged zone at the bottom of the filter combined with a carbon source. The biofilters were dosed with stormwater according to three different dry/wet schemes, to investigate the effect of intermittent wetting and drying conditions on metal removal.Provided that the biofilters received regular stormwater input, metal removal exceeded 95%. The highest metal accumulation occurs in the top layer of the filter media. However, after antecedent drying before a storm event exceeding 3–4 weeks the filters performed significantly worse, although metal removal still remained relatively high. Introducing a submerged zone into the filter improved the performance significantly after extended dry periods. In particular, copper removal in filters equipped with a submerged zone was increased by around 12% (α = 0.05) both during wet and dry periods and for lead the negative effect of drying could completely be eliminated, with consistently low outflow concentrations even after long drying periods.     

Nutrient and sediment removal by stormwater biofilters: a large-scale design optimisation study

A large-scale column study was conducted in Melbourne, Australia, to test the performance of stormwater biofilters for the removal of sediment, nitrogen and phosphorus. The aim of the study was to provide guidance on the optimal design for reliable treatment performance. A variety of factors were tested, using 125 large columns: plant species, filter media, filter depth, filter area and pollutant inflow concentration. The results demonstrate that vegetation selection is critical to performance for nitrogen removal (e.g. Carex appressa and Melaleuca ericifolia performed significantly better than other tested species). Whilst phosphorus removal was consistently very high (typically around 85%), biofilter soil media with added organic matter reduced the phosphorus treatment effectiveness. Biofilters built according to observed 'optimal specifications' can reliably remove both nutrients (up to 70% for nitrogen and 85% for phosphorus) and suspended solids (consistently over 95%). The optimally designed biofilter is at least 2% of its catchment area and possesses a sandy loam filter media, planted with C. appressa or M. ericifolia. Further trials will be required to test a wider range of vegetation, and to examine performance over the longer term. Future work will also examine biofilter effectiveness for treatment of heavy metals and pathogens.     

Capturing phosphates with iron enhanced sand filtration

Most treatment practices for urban runoff capture pollutants such as phosphorus by either settling or filtration while dissolved phosphorus, typically as phosphates, is untreated. Dissolved phosphorus, however, represents an average 45% of total phosphorus in stormwater runoff and can be more than 95%. In this study, a new stormwater treatment technology to capture phosphate, called the Minnesota Filter, is introduced. The filter comprises iron filings mixed with sand and is tested for phosphate removal from synthetic stormwater. Results indicate that sand mixed with 5% iron filings captures an average of 88% phosphate for at least 200 m of treated depth, which is significantly greater than a sand filter without iron filings. Neither incorporation of iron filings into a sand filter nor capture of phosphates onto iron filings in column experiments had a significant effect on the hydraulic conductivity of the filter at mixtures of 5% or less iron by weight. Field applications with up to 10.7% iron were operated over 1 year without detrimental effects upon hydraulic conductivity. A model is applied and fit to column studies to predict the field performance of iron-enhanced sand filters. The model predictions are verified through the predicted performance of the filters in removing phosphates in field applications. Practical applications of the technology, both existing and proposed, are presented so stormwater managers can begin implementation.     

Algae removal by fine sand/silt filtration

A laboratory scale study was undertaken to determine the potential of a method of filtering algae from water using fine sand/silt as the filter media. Five median sand sizes (0.064–0.335 mm) and four bed depths (3.175–12.700 mm) were examined in constant head experiments with the algae Scenedesmus quadricauda. A total of 46 experiments were conducted with continuous measurements of filtration rate, head loss and effluent quality. All media with median sand sizes at or below 0.200 mm gave consistently high algae removal rates. The average removal was 97.27% (based on fluorescence) with a low average initial head loss across the filter media of 7.3 cm (median grain size diameter of 0.200 mm with the bed depth of 3.175 mm). Initial filtration rates obtained in the experimental apparatus were as high as 226 m³ m⁻²–day⁻¹ (3.84 gpm ft⁻²), comparable to conventional sand units. Run times were short due to surface clogging of the media. No chemical addition was required to obtain high removal levels.     

Variation among plant species in pollutant removal from stormwater in biofiltration systems

Biofiltration systems use vegetation to improve efficiency of pollutant removal from stormwater, but little is known of how plants vary in their capacity to improve biofilter effectiveness. We used a pot trial of 20 Australian species to investigate how species vary in the removal of pollutants from semisynthetic storm water passing through a soil filter medium. Effluent levels of total suspended solids (TSS), Al, Cr, Cu, Pb and Zn were similarly low for vegetated and non-vegetated soils, with reduction to <1-12% of levels in the stormwater input. N and P effluent concentrations were generally lower from vegetated than non-vegetated soils, but total N increased on average in effluent of both vegetated and non-vegetated soils relative to stormwater input. Effluent concentrations varied 2-4-fold among species for TSS, total N and P, total dissolved N (TDN), organic nitrogen and Cu, to more than 20-fold for NOx, NH4+, Mn, Pb and Fe. Species also varied markedly in pollutant removal per root mass (a means of standardising for plant size), with 18-50-fold variation among species in effluent concentrations of total P and N, TDN and organic N, to >150-fold variation in NOx and NH4+. Hence, choice of plant species may have marked effects on biofilter effectiveness.     

Hydraulic performance of grass swales for managing highway runoff

The hydraulic performance of grass swales as a highway stormwater control measure was evaluated in a field-scale study adjacent to a Maryland highway. Two common swale design alternatives, pretreatment grass filter strips and vegetated check dams, were compared during 52 storm events over 4.5 years. Swale performance is described via three regimes, dependent on the relative size of the rainfall event. Overall, half of the events were small enough that the entire flow was stored, infiltrated, and evapotranspirated by the swales, resulting in no net swale discharge. Swales significantly reduced total volume and flow magnitudes generally during events with rainfall less than 3 cm. While the majority of improvement can be attributed to the swales, inclusion of check dams increases swale effectiveness. Pretreatment grass filter strips produced mixed effects. The swales demonstrated essentially no volumetric reduction during large storm events, functioning instead as conveyance, and smoothing fluctuations in flow.     

Impact of water boundary layer diffusion on the nitrification rate of submerged biofilter elements from a recirculating aquaculture system

Total ammonia nitrogen (TAN) removal by microbial nitrification is an essential process in recirculating aquaculture systems (RAS). In order to protect the aquatic environment and fish health, it is important to be able to predict the nitrification rates in RAS's. The aim of this study was to determine the impact of hydraulic film diffusion on the nitrification rate in a submerged biofilter. Using an experimental batch reactor setup with recirculation, active nitrifying biofilter units from a RAS were exposed to a range of hydraulic flow velocities. Corresponding nitrification rates were measured following ammonium chloride, NH₄Cl, spikes and the impact of hydraulic film diffusion was quantified.The nitrification performance of the tested biofilter could be significantly increased by increasing the hydraulic flow velocity in the filter. Area based first order nitrification rate constants ranged from 0.065 m d⁻¹ to 0.192 m d⁻¹ for flow velocities between 2.5 m h⁻¹ and 40 m h⁻¹ (18 °C). This study documents that hydraulic film diffusion may have a significant impact on the nitrification rate in fixed film biofilters with geometry and hydraulic flows corresponding to our experimental RAS biofilters. The results may thus have practical implications in relation to the design, operational strategy of RAS biofilters and how to optimize TAN removal in fixed film biofilter systems.     

On‐site wastewater treatment using reactive filter media

The aim of this study is to reduce the phosphate concentration in treated wastewater using a small amount of the reactive filter media, Filtralite P. Biologically treated wastewater was passed through a filter with 215 g of Filtralite P. In the laboratory, the phosphate removal efficiency was 51% at a flow speed of 0.87 m/h. Under real conditions, in an experimental stand filled with 0.5 m³ of Filtralite P, the phosphate removal efficiency was 85% and the removal efficiency of total suspended solids was 57% after a 5‐month period. The residual phosphate concentration in the filtrate from the experimental stand was 1 mg/L of PO₄‐P after the 5‐month period. The experimental filtration plant was buried in the ground, and it did not freeze and worked well under winter conditions. The results of this study can be useful in the design and development of tertiary wastewater treatment plants in view of their sustainability potential.     

滤料粒径对地下水中铁、锰、氨氮、浊度去除效果的影响

滤料粒径是生物滤池设计的一个重要参数。采用滤料粒径分别为0. 8～1. 0、3～4、8～10 mm的3根成熟生物滤柱处理地下水,考察滤料粒径对铁、锰、氨氮、浊度去除效果的影响。结果表明,1^#、2^#、3^#滤柱出水的总铁平均浓度分别为0. 020、0. 037、0. 078 mg/L,锰平均浓度分别为0. 003 0、0. 005 1、0. 006 7 mg/L,氨氮平均浓度分别为0. 022、0. 030、0. 050 mg/L,浊度均值分别为0. 28、0. 69、1. 32 NTU,除3^#滤柱出水浊度不达标外,其余指标均满足国家标准。随着滤料粒径的增大,铁、锰、氨氮的沿程浓度明显升高,去除区域向下延伸,浊度主要在0～0. 4 m滤层被去除。     

Comparison of drinking water treatment process streams for optimal bacteriological water quality

Four pilot-scale treatment process streams (Stream 1 - Conventional treatment (coagulation/flocculation/dual media filtration); Stream 2 - Magnetic ion exchange (MIEX)/Conventional treatment; Stream 3 - MIEX/Conventional treatment/granular activated carbon (GAC) filtration; Stream 4 - Microfiltration/nanofiltration) were commissioned to compare their effectiveness in producing high quality potable water prior to disinfection. Despite receiving highly variable source water quality throughout the investigation, each stream consistently reduced colour and turbidity to below Australian Drinking Water Guideline levels, with the exception of Stream 1 which was difficult to manage due to the reactive nature of coagulation control. Of particular interest was the bacteriological quality of the treated waters where flow cytometry was shown to be the superior monitoring tool in comparison to the traditional heterotrophic plate count method. Based on removal of total and active bacteria, the treatment process streams were ranked in the order: Stream 4 (average log removal of 2.7) > Stream 2 (average log removal of 2.3) > Stream 3 (average log removal of 1.5) > Stream 1 (average log removal of 1.0). The lower removals in Stream 3 were attributed to bacteria detaching from the GAC filter. Bacterial community analysis revealed that the treatments affected the bacteria present, with the communities in streams incorporating conventional treatment clustering with each other, while the community composition of Stream 4 was very different to those of Streams 1, 2 and 3. MIEX treatment was shown to enhance removal of bacteria due to more efficient flocculation which was validated through the novel application of the photometric dispersion analyser.     

Occurrence and removal of priority pollutants by lamella clarification and biofiltration

This study investigates the occurrence of all priority substances (n = 41) listed in the Water Framework Directive and additional substances (n = 47) in raw sewage, as well as the removal performance of lamella clarification and biofiltration techniques. Once the efficiency of both types of techniques has been assessed for typical wastewater parameters, the differences in each technique's ability to remove pollutants becomes obvious; nevertheless, pollutant removal in quantitative terms still depends on the physico-chemical properties of the compounds used and operating conditions within the selected facility. For lamella clarification, the removal of organic chemicals was found to be primarily correlated with their sorption potential and, hence, strongly dependent upon log K_ow of the compound under study. Compounds with a strong hydrophobic character (log K_ow > 4.5) are removed to a significant extent (approx. 85%), while hydrophilic compounds (log K_ow < 3.5) are poorly removed (<20%). For biofiltration, the removal of chemicals appears to be compound-dependent, although this outcome involves several mechanisms, namely: i) physical filtration of total suspended solids, ii) volatilisation, iii) sorption, and iv) biotransformation of substances. Even if the complex processes within a biofilter system do not yield an accurate prediction of pollutant removal, two groups of chemicals can still be clearly identified: i) hydrophobic or volatile compounds, for which moderate to high removal rates are observed (from 50% to over 80%); and ii) hydrophilic, non-volatile and refractory compounds for which a low removal rate would be expected (<20%).     

Quantification of uncertainty in modelled partitioning and removal of heavy metals (Cu,Zn) in a stormwater retention pond and a biofilter

Strategies for reduction of micropollutant (MP) discharges from stormwater drainage systems require accurate estimation of the potential MP removal in stormwater treatment systems. However, the high uncertainty commonly affecting stormwater runoff quality modelling also influences stormwater treatment models. This study identified the major sources of uncertainty when estimating the removal of copper and zinc in a retention pond and a biofilter by using a conceptual dynamic model which estimates MP partitioning between the dissolved and particulate phases as well as environmental fate based on substance-inherent properties. The two systems differ in their main removal processes (settling and filtration/sorption, respectively) and in the time resolution of the available measurements (composite samples and pollutographs). The most sensitive model factors, identified by using Global Sensitivity Analysis (GSA), were related to the physical characteristics of the simulated systems (flow and water losses) and to the fate processes related to Total Suspended Solids (TSS). The model prediction bounds were estimated by using the Generalized Likelihood Uncertainty Estimation (GLUE) technique. Composite samples and pollutographs produced similar prediction bounds for the pond and the biofilter, suggesting a limited influence of the temporal resolution of samples on the model prediction bounds. GLUE highlighted model structural uncertainty when modelling the biofilter, due to disregard of plant-driven evapotranspiration, underestimation of sorption and neglect of oversaturation with respect to minerals/salts. The results of this study however illustrate the potential for the application of conceptual dynamic fate models base on substance-inherent properties, in combination with available datasets and statistical methods, to estimate the MP removal in different stormwater treatment systems and compare with environmental quality standards targeting the dissolved MP fraction.     

Bacterial, viral and turbidity removal by intermittent slow sand filtration for household use in developing countries: experimental investigation and modeling

A two-factor three-block experimental design was developed to permit rigorous evaluation and modeling of the main effects and interactions of sand size (d₁₀ of 0.17 and 0.52 mm) and hydraulic head (10, 20, and 30 cm) on removal of fecal coliform (FC) bacteria, MS2 bacteriophage virus, and turbidity, under two batch operating modes (‘long’ and ‘short’) in intermittent slow sand filters (ISSFs). Long operation involved an overnight pause time between feeding of two successive 20 L batches (16 h average batch residence time (RT)). Short operation involved no pause between two 20 L batch feeds (5 h average batch RT). Conditions tested were representative of those encountered in developing country field settings. Over a ten week period, the 18 experimental filters were fed river water augmented with wastewater (influent turbidity of 5.4-58.6 NTU) and maintained with the wet harrowing method. Linear mixed modeling allowed systematic estimates of the independent marginal effects of each independent variable on each performance outcome of interest while controlling for the effects of variations in a batch’s actual residence time, days since maintenance, and influent turbidity. This is the first study in which simultaneous measurement of bacteria, viruses and turbidity removal at the batch level over an extended duration has been undertaken with a large number of replicate units to permit rigorous modeling of ISSF performance variability within and across a range of likely filter design configurations and operating conditions.On average, the experimental filters removed 1.40 log fecal coliform CFU (SD 0.40 log, N = 249), 0.54 log MS2 PFU (SD 0.42 log, N = 245) and 89.0 percent turbidity (SD 6.9 percent, N = 263). Effluent turbidity averaged 1.24 NTU (SD 0.53 NTU, N = 263) and always remained below 3 NTU. Under the best performing design configuration and operating mode (fine sand, 10 cm head, long operation, initial HLR of 0.01-0.03 m/h), mean 1.82 log removal of bacteria (98.5%) and mean 0.94 log removal of MS2 viruses (88.5%) were achieved.Results point to new recommendations regarding filter design, manufacture, and operation for implementing ISSFs in local settings in developing countries. Sand size emerged as a critical design factor on performance. A single layer of river sand used in this investigation demonstrated removals comparable to those reported for 2 layers of crushed sand. Pause time and increased residence time each emerged as highly beneficial for improving removal performance on all four outcomes. A relatively large and significant negative effect of influent turbidity on MS2 viral removal in the ISSF was measured in parallel with a much smaller weaker positive effect of influent turbidity on FC bacterial removal. Disturbance of the schmutzdecke by wet harrowing showed no effect on virus removal and a modest reductive effect on the bacterial and turbidity removal as measured 7 days or more after the disturbance. For existing coarse sand ISSFs, this research indicates that a reduction in batch feed volume, effectively reducing the operating head and increasing the pore:batch volume ratio, could improve their removal performance by increasing batch residence time.     

Effect of organic load on phosphorus and bacteria removal from wastewater using alkaline filter materials

The organic matter released from septic tanks can disturb the subsequent step in on-site wastewater treatment such as the innovative filters for phosphorus removal. This study investigated the effect of organic load on phosphorus (P) and bacteria removal by reactive filter materials under real-life treatment conditions. Two long-term column experiments were conducted at very short hydraulic residence times (average ∼5.5 h), using wastewater with high (mean ∼120 mg L⁻¹) and low (mean ∼20 mg L⁻¹) BOD₇ values. Two alkaline filter materials, the calcium-silicate material Polonite and blast furnace slag (BFS), were tested for the removal capacity of total P, total organic carbon (TOC) and Enterococci. Both experiments showed that Polonite removed P significantly (p < 0.01) better than BFS. An increase in P removal efficiency of 29.3% was observed for the Polonite filter at the lower concentration of BOD₇ (p < 0.05). Polonite was also better than BFS with regard to removal of TOC, but there were no significant differences between the two filter materials with regard to removal of Enterococci. The reduction in Enterococci was greater in the experiment using wastewater with high BOD₇, an effect attributable to the higher concentration of bacteria in that wastewater. Overall, the results demonstrate the importance of extensive pre-treatment of wastewater to achieve good phosphorus removal in reactive bed filters and prolonged filter life.     

城市污水曝气过滤式化学除磷试验研究

磷污染与水体富营养化密切相关，研究开发经济、有效的除磷方法是解决当前水体富营养化问题的迫切需要。为此开展了曝气过滤式化学除磷的试验研究，考察了富铁填料层厚度、滤速、气量等对除磷效果的影响。结果表明，曝气过程和曝气量对除磷效果有重要影响，通过适量曝气既可以提高除磷率，又可以控制出水铁含量；当滤速≤3m／h时，滤层厚度〉0．4m便可使出水总磷达到排放标准；气水联合冲洗是对多孔富铁填料进行再生的有效方法。     

沸石-无烟煤生物滤池联合处理微污染水试验研究

考察了沸石-无烟煤双层滤料生物滤池处理微污染水的运行效能.结果表明,沸石-无烟煤生物滤池可以有效提高出水水质,对CODMn、NH3-N及浊度的去除率分别达到39.5%,93.6%和91.3%,而且工作区间主要在滤层上部40 cm内;反冲洗对滤料表面附着的微生物膜影响很小,生物膜在反冲洗后1.5 h内能恢复到反冲洗前的水平.     

Uncertainties of stormwater characteristics and removal rates of stormwater treatment facilities: Implications for stormwater handling

Stormwater runoff is a major contributor to the pollution of receiving waters. This study focuses at characterising stormwater in order to be able to determine the impact of stormwater on receiving waters and to be able to select the most appropriate stormwater handling strategy. The stormwater characterisation is based on determining site mean concentrations (SMCs) and their uncertainties as well as the treatability of stormwater by monitoring specific pollutants concentration levels (TSS, COD, BOD, TKN, TP, Pb, Cu, Zn, E.coli) at three full scale stormwater treatment facilities in Arnhem, the Netherlands. This has resulted in 106 storm events being monitored at the lamella settler, 59 at the high rate sand filter and 132 at the soil filter during the 2 year monitoring period.The stormwater characteristics in Arnhem in terms of SMCs for main pollutants TSS and COD and settling velocities differ from international data. This implies that decisions for stormwater handling made on international literature data will very likely be wrong due to assuming too high concentrations of pollutants and misjudgement of the treatability of stormwater. The removal rates monitored at the full scale treatment facilities are within the expected range, with the soil filter and the sand filter having higher removal rates than the lamella settler. The full scale pilots revealed the importance of incorporating gross solids removal in the design of stormwater treatment facilities, as the gross solids determine operation and maintenance requirements.     

Characterisation of adsorbents prepared by pyrolysis of sludge and sludge/disposal filter cake mix

Copper and zinc removal from water (pH = 5.0) using adsorbents produced from slow and fast pyrolysis of industrial sludge and industrial sludge mixed with a disposal filter cake (FC), post treated with HCl, is investigated in comparison with a commercial adsorbent F400. The results show that a pseudo-second order kinetics model is followed. The Langmuir-Freundlich isotherm model is found to fit the data best. The capacity for heavy metal removal of studied adsorbents is generally better than that of commercial F400. The dominant heavy metal removal mechanism is cation exchange. Higher heavy metal removal capacity is associated with fast pyrolysis adsorbents and sludge/FC derived adsorbents, due to enhanced cation exchange. Improvement of Zn²⁺ removal via 1 N HCl post-treatment is only effective when exchangeable cations of the adsorbent are substituted with H⁺ ions, which boost the cation exchange capacity. Increase of temperature also enhances metal removal capacity. Fast pyrolysis sludge-based adsorbents can be reused after several adsorption-desorption cycles.     

Effective disinfection of airborne microbial contamination in hospital wards using a zero‐valent nano‐silver/TiO2‐chitosan composite

A novel antimicrobial composite of zero‐valent silver nanoparticles (AgNPs), titania (TiO₂), and chitosan (CS) was prepared via photochemical deposition of AgNPs on a CS‐TiO₂ matrix (AgNPs@CS‐TiO₂). Electron microscopy showed that the AgNPs were well dispersed on the CS‐TiO₂, with diameters of 6.69‐8.84 nm. X‐ray photoelectron spectra indicated that most of the AgNPs were reduced to metallic Ag. Fourier‐transform infrared spectroscopy indicated that some AgNPs formed a chelate with CS through coordination of Ag⁺ with the CS amide II groups. The zones of inhibition of AgNPs@CS‐TiO₂ for bacteria (Escherichia coli and Staphylococcus epidermidis) and fungi (Aspergillus niger and Penicillium spinulosum) were 6.72‐11.08 and 5.45‐5.77 mm, respectively, and the minimum (critical) concentrations of AgNPs required to inhibit the growth of bacteria and fungi were 7.57 and 16.51 µg‐Ag/mm², respectively. The removal efficiency of a AgNPs@TiO₂‐CS bed filter for bioaerosols (η) increased with the packing depth, and the optimal filter quality (qF) occurred for packing depths of 2‐4 cm (qF = 0.0285‐0.103 Pa^?1; η = 57.6%‐98.2%). When AgNPs@TiO₂‐CS bed filters were installed in the ventilation systems of hospital wards, up to 88% of bacteria and 97% of fungi were removed within 30 minutes. Consequently, AgNPs@TiO₂‐CS has promising potentials in bioaerosol purification.     

Biofilter treatment of aquaculture water for reuse applications

Biotreatment of aquaculture water for recirculation purposes is a sensible mean to support the further growth of aquaculture industry without excessive water demands that are environmentally unsustainable. This study evaluates the efficacy of biofilter treatment of an eel (Anguilla japonica) culture pond water using different filter media and flow scheme arrangements. The experimental results demonstrate that biofilter systems packed with suitable filter media are capable of improving the quality of effluents for recirculation applications. The characteristics of the filter media appear to be more critical than biofilter flow scheme arrangements in affecting the efficacy of the biofilter treatment. Filter media with surface and structural characteristics are conducive to the development of biofilms and the capture of organic suspended matter are desirable to ensure good and consistent biofilter performance. Under such circumstances the bacterial "consortia" in the biofilter are capable of utilizing the captured organic suspended matter as an alternative substrate to support their metabolic activities when the concentration of the primary substrate (i.e., BOD) is low. For the eel pond water, a biofilter packed with filter media having cross-link structures and a high bed porosity, followed by another biofilter packed with a type of filter media having rough surfaces, produced the best results under the conditions tested. Moreover, a preliminary cost-benefit analysis confirms its cost advantages.