High frequency UV-Vis sensors estimate error in riverine dissolved organic carbon load estimates from grab sampling

High frequency ultraviolet - visible (UV-VIS) sensors offer a way of improving dissolved organic carbon (DOC) load estimates in rivers as they can be calibrated to DOC concentration. This is an improvement on periodic grab sampling, or the use of pumped sampling systems which store samples in-field before collection. We hypothesised that the move to high frequency measurements would increase the load estimate based on grab sampling due to systemic under-sampling of high flows. To test our hypotheses, we calibrated two sensors in contrasting catchments (Exe and Bow Brook, UK) against weekly grab sampled DOC measurements and then created an hourly time series of DOC for the two sites. Taking this measurement as a 'true' value of DOC load, we simulated 1,000 grab sampling campaigns at weekly, fortnightly and monthly frequency to understand the likely distribution of load and error estimates. We also performed an analysis of daily grab samples collected using a pumped storage sampling system with weekly collection. Our results show that: a) grab sampling systemically underestimates DOC loads and gives positively skewed distributions of results, b) this under-estimation and positive skew decreases with increasing sampling frequency, c) commonly used estimates of error in the load value are also systemically lowered by the oversampling of low, stable flows due to their dependence on the variance in the flow-weighted mean concentration, and d) that pumped storage systems may lead to under-estimation of DOC and over estimation of specific ultra-violet absorbance (SUVA), a proxy for aromaticity, due to biodegradation during storage.

Sources of dissolved organic carbon (DOC) in a mixed land use catchment (Exe, UK)

Many catchment management schemes in the UK have focussed on peatland restoration to improve ecosystem services such as carbon sequestration, water quality and biodiversity. The effect of these schemes on dissolved organic carbon (DOC) flux is critical in understanding peatland carbon budgets as well as the implications for drinking water treatment. In many catchments, however, peatland areas are not the only source of DOC, meaning that their significance at the full catchment scale is unclear. In this paper we have evaluated the importance of different land uses as sources of DOC by combining three datasets obtained from the Exe catchment, UK. The first dataset comprises a weekly monitoring record at three sites for six years, the second, a monthly monitoring record of 25 sites in the same catchment for one year, and the third, an assessment of DOC export from litter and soil carbon stocks. Our results suggest that DOC concentration significantly increased from the peaty headwaters to the mixed land-use areas (ANOVA F = 12.52, p < 0.001, df = 2), leading to higher flux estimates at the downstream sites. We present evidence for three possible explanations: firstly, that poor sampling of high flows may lead to underestimation of DOC flux, second, that there are significant sources of DOC besides the peatland headwaters, and finally, that biological- and photo-degradation decreases the influence of upstream DOC sources. Our results provide evidence both for the targeting of catchment management in peatland areas as well as the need to consider DOC from agricultural and forested areas of the catchment.

The impact of climate change on the treatability of dissolved organic matter (DOM) in upland water supplies: a UK perspective

Climate change in the UK is expected to cause increases in temperatures, altered precipitation patterns and more frequent and extreme weather events. In this review we discuss climate effects on dissolved organic matter (DOM), how altered DOM and water physico-chemical properties will affect treatment processes and assess the utility of techniques used to remove DOM and monitor water quality. A critical analysis of the literature has been undertaken with a focus on catchment drivers of DOM character, removal of DOM via coagulation and the formation of disinfectant by-products (DBPs). We suggest that: (1) upland catchments recovering from acidification will continue to produce more DOM with a greater hydrophobic fraction as solubility controls decrease; (2) greater seasonality in DOM export is likely in future due to altered precipitation patterns; (3) changes in species diversity and water properties could encourage algal blooms; and (4) that land management and vegetative changes may have significant effects on DOM export and treatability but require further research. Increases in DBPs may occur where catchments have high influence from peatlands or where algal blooms become an issue. To increase resilience to variable DOM quantity and character we suggest that one or more of the following steps are undertaken at the treatment works: a) 'enhanced coagulation' optimised for DOM removal; b) switching from aluminium to ferric coagulants and/or incorporating coagulant aids; c) use of magnetic ion-exchange (MIEX) pre-coagulation; and d) activated carbon filtration post-coagulation. Fluorescence and UV absorbance techniques are highlighted as potential methods for low-cost, rapid on-line process optimisation to improve DOM removal and minimise DBPs.

Oxidation and coagulation of humic substances by potassium ferrate

Ferrate (FeO₄²⁻) is believed to have a dual role in water treatment, both as oxidant and coagulant. Few studies have considered the coagulation effect in detail, mainly because of the difficulty of separating the oxidation and coagulation effects. This paper summarises some preliminary results from laboratory-based experiments that are investigating the coagulation reaction dynamically via a PDA instrument, between ferrate and humic acid (HA) at different doses and pH values, and comparing the observations with the use of ferric chloride. The PDA output gives a comparative measure of the rate of floc growth and the magnitude of floc formation. The results of the tests show some significant differences in the pattern of behaviour between ferrate and ferric chloride. At pH 5 the chemical dose range (as Fe) corresponding to HA coagulation was much broader for ferrate than ferric chloride, and the optimal Fe dose was greater. Ferrate oxidation appears to increase the hydrophilic and electronegative nature of the HA leading to an extended region of charge neutralisation. A consequence of the ferrate oxidation is that the extent of HA removal was slightly lower ( approximately 5%) than with ferric chloride. At pH 7, in the sweep flocculation domain, ferrate achieved much greater floc formation than ferric chloride, but a substantially lower degree of HA removal.

Sewage sludge-based adsorbents: a review of their production, properties and use in water treatment applications

The imposition of more stringent legislation governing the disposal and utilisation of sewage sludge, coupled with the growth in its generation and the loss of traditionally accepted disposal routes, has prompted a drive for alternative uses for sewage sludge. One option that exhibits special promise, due to its potential to valorise the sludge, is the conversion of the sludge into adsorbents. This paper seeks to review the published research in this field: it covers the means of production, the characteristics and the potential applications of sewage sludge-based adsorbents (SBAs). The literature has indicated that chemical activation utilising alkali metal hydroxides is the most effective technique for producing high surface area SBAs. In addition, acid washing is highly effective at raising the BET surface area of SBAs, especially when coupled with physical activation. Due to their relatively low microporosity, the phenol uptake of SBAs produced by physical activation is low, but through a combination of their favourable surface chemistry and relatively high mesoporosity, the best of these adsorbents can attain high uptakes of organic dyes. The SBAs produced by carbonisation, through their high cation exchange capacity, generally exhibit a high metal cation capacity. For further research, the following investigations are recommended: the utilisation of alternative chemical activation reagents; the optimisation of the most effective chemical activation techniques; the combined utilisation of different activation and surface chemistry modification techniques to produce application-specific adsorbents.

A comparison of ultrasound-based advanced oxidation processes for the removal of X-ray contrast media

The degradation of specific iodinated X-ray contrast media (ICM) compounds (viz: diatrizoate, iomeprol, iopromide, and iopamidol) by ultrasound irradiation in aqueous solution, with and without the presence of hydrogen peroxide or ozone, has been studied. Experiments were carried out at a constant ultrasound frequency of 20 kHz, at two power intensity values of 17.6 and 200.1 W cm(-2), and at five power densities up to 0.235 W ml(-1). Zero-order kinetic rate constants for the ICM degradation by ultrasound alone were calculated under certain sonication conditions. Pyrolysis appeared to contribute approximately 30%, and radical attack 70%, of the overall ICM degradation performance. The effect of ultrasound intensity on compound degradation (at a given power density) was found to play a negligible role, whereas ultrasound power density was found to be a major factor controlling the overall oxidation process under these conditions. The compound degradation by ultrasound alone was relatively minor, but the addition of hydrogen peroxide in the sonication process gave some improvement with a doubling in the degradation performance at the greatest applied peroxide concentration. The combination of gaseous ozone and ultrasound was found to be very effective in degrading ICM compounds and an almost complete compound removal could be achieved.

Enhancement of ozone oxidation and its associated processes in the presence of surfactant: degradation of atrazine

In this study, the degradation of atrazine (ATZ) by ozone (O3) oxidation and its associated processes (i.e. UV, UV/O3) in the presence and absence of surfactant was investigated and compared. A non-ionic surfactant, Brij 35, was selected. It was found that the presence of a low concentration of surfactant could improve the removal of ATZ by increasing the dissolution of ozone and the indirect generation of hydroxyl radicals. The saturated ozone level and the reaction rate constants were increased with increasing the concentration of surfactant and then decreased at higher surfactant doses at pH level of 2.5. A similar trend was observed at pH level of 7.0 in the presence of bicarbonate ion, because it is capable of deactivating the hydroxyl radicals generating at higher pH level. However, when the radical reactions become dominant in the ozonation (at pH 7.0 without bicarbonate), the saturated ozone level was higher than that with bicarbonate and the kinetic rate constants were increased first and levelled off with increasing of the dose of surfactant. Through the examining of a proposed unit performance index, the low concentration of surfactant is surely beneficial to the ozonation process. Besides, the direct photolysis and photo-assisted ozonation were compared to the ozonation. A significant enhancement on the decay rate of ATZ was resulted exclusively by adding the surfactant. An enhancement index for quantifying the improvement of the various processes was developed.

Coagulation of humic acid by ferric chloride in saline (marine) water conditions

The coagulation of a model seawater-humic acid solution with a hydrolysis metal salt (FeCl3) has been studied by monitoring floc size, solution pH, and zeta potential. The kinetic features of the orthokinetic coagulation have been demonstrated in relation to coagulant dosages, solution pH and zeta potential. Humic acid removal and floc charge reduction increased with coagulant dosage. Adjusting the solution pH prior to coagulation had a substantial effect on the treatment performance. By pH adjustment to pH 6, the greatest humic acid removal (by coagulation and subsequent membrane filtration) and the largest floc size was achieved at a FeCl3 dosage of 200 micromol l(-1). It is believed that the coagulation is characterised by competition between OH- ions and humic acid for ferric ions in the co-precipitation process. In acidic pH, where the concentration of OH- ions is low, humic acid molecules may compete more favourably for bonding sites in the co-precipitation, which leads to a more compact precipitation and a higher overall humic acid removal.

Thermal regeneration of granular activated carbons using inert atmospheric conditions

Thermal regeneration is increasingly being used for the recovery of field-spent granular activated carbons (GAC) generated by the water treatment industry. Despite its commercial success, conventional methods using oxidising conditions (usually steam) are known to damage the porosity of the regenerated carbons, thus reducing their adsorption capacity and economic value. This paper presents a comparative investigation into the benefits of using inert conditions for the regeneration of field-spent GAC. For the purpose of this work, a sample of spent carbon was regenerated in nitrogen and in steam to different degrees of burn off. The resulting samples were analysed for their porosity and surface area characteristics using nitrogen gas adsorption, and for their aqueous adsorption capacities using phenol and methylene blue. Experimental results showed that steam was sightly more effective than nitrogen at regenerating the total micropore volume and BET surface area of the carbons. However, these benefits were largely counteracted by greater losses in the carbon yield and damage to the narrow microporosity. Carbons regenerated in nitrogen exhibited greater adsorption capacities for the adsorption of small molecular size compounds (phenol) from solution, while carbons regenerated in steam adsorbed larger molecular size compounds (methylene blue) more effectively. However, when product yields were taken into consideration, inert regeneration was found to produce significantly better results than steam regeneration. An optimum process temperature was determined to be 950 degrees C.

Biomass development in slow sand filters

Microbial biomass development in the sand and schmutzdecke layer was determined in two full-scale slow sand filters, operated with and without a light excluding cover. A standard chloroform fumigation-extraction technique was adapted to routinely measure microbial biomass concentrations in the sand beds. Sand was sampled to a depth of 10 cm and schmutzdecke was also collected at the same random positions on the uncovered filter. Interstitial microbial biomass in the uncovered sand bed increased with time and decreased with sampling depth. There was a small accumulation of sand biomass with time in the covered filter, but no relationship was apparent between biomass concentration and depth in this filter. Schmutzdecke did not develop on the covered filter and was spatially highly variable in the uncovered condition compared to the consistent patterns observed in interstitial biomass production. It is speculated that microbial biomass in the sand of uncovered filters is largely related to carbon inputs from photosynthetic activity in the schmutzdecke and involves mechanisms that spatially distribute carbon substrate from the schmutzdecke to the sand. However, total organic carbon and dissolved organic carbon removals were similar in both filters suggesting that relatively small biomass populations in covered filters are sufficient to remove residual labile carbon during advanced water treatment and little further advantage to water purification and organic carbon removal is gained by the increased production of biomass in uncovered slow sand filter beds.