N.J.D. Graham

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.

The regeneration of field-spent granular-activated carbons

The thermal regeneration of field-spent granular-activated carbons (GAC) is being increasingly adopted as a cost-effective alternative to disposal. The success of this practice requires the adjustment of process conditions to maximise the recovery of the original carbon characteristics while minimising carbon loss. This paper describes an investigation into the regeneration of several field-spent GAC representative of those typically generated by the drinking water treatment industry. The carbons were initially investigated for their ash contents and inorganic compositions in order to determine the accumulation of metallic species that affect the regeneration process. Regeneration was conducted in steam at 800 degrees C over reaction times between 0 and 60 min in order to achieve different degrees of carbon gasification. Weight losses were determined for each condition and the resulting carbons characterised for their apparent density, porosity, surface area and aqueous adsorption characteristics. Results showed that spent carbons recovered most of their adsorption characteristics when heated to 800 degrees C under inert conditions. Steam gasification in the range of 5-10 wt% burn-off had some positive effects on the characteristics of the spent carbons which were in most cases counteracted by a reduction in the carbon yield. Steam gasification in excess of 15 wt% burn-off caused a rapid increase in the carbon mesoporosity but a significant deterioration in the carbon microporosity, BET surface area and adsorption capacity for organic species of small molecular size.

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.

Observations of the comparative hydrolysis/precipitation behaviour of polyferric sulphate and ferric sulphate

The hydrolysis/precipitation behaviour of polyferric sulphate (PFS) and ferric sulphate (FS) under conditions typical for coagulation and flocculation in water treatment are investigated by studying the rate of floc size development, the zeta potential and chemical structure of precipitates and the iso-electric point of re-suspended precipitates. The results detailed in this study strongly suggest that, under the test conditions, the hydrolysis behaviour of PFS was significantly different from that of FS, with evidence of a relatively lower rate of floc size development and colloid charge reversal in the investigated Fe(III) dose range and a different chemical structure of the precipitates formed from PFS and FS.

Removal of non-specific dissolved organic matter from upland potable water supplies—I. Adsorption

The presence of a dissolved organic component in potable water supplies is not only aesthetically undesirable, frequently imparting colour, tastes and odours to treated waters, but may also be associated with a variety of quality problems potentially hazardous to health. The removal of organic compounds is therefore imperative, and may be effected by adsorption onto activated carbon or a range of other porous solids. This paper investigates the amenability of non-specific natural organic compounds for removal by adsorption onto activated carbon, activated alumina, activated bauxite and bone char adsorbents.

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 slightly 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°C.

Removal of non-specific dissolved organic matter from upland potable water supplies—II. Ozonation and adsorption

Ozone has traditionally been used in the U.K. for the removal of organic colour from upland raw waters concomitantly converting the composite organic compounds to lower molecular weight, more biodegradable organic acids. However, ozonation has substantially no effect on the dissolved organic compound concentrations and, although the organic removal efficiencies of GAC filtration processes are generally improved due to enhanced biological removals, ozonation typically results in a deterioration of the organics adsorption. Consequently, this paper investigates the effect of preozonation on the organics adsorption efficiencies of activated carbon, activated alumina, activated bauxite and bone char adsorbents.

Filter pore flocculation as a mechanism in rapid filtration

Abstract The process of particle capture in a clean rapid filter has been simulated by means of a computer model which incorporates the mechanism of filter pore particle flocculation. The results of filtration simulations have highlighted the importance of the particle-particle collision efficiency factor and predict that under typical filtration conditions orthokinetic flocculation of particles in filter pores can be significant. However, the consequent improvement in filter performance as expressed by the filter coefficient is small. Comparative laboratory filtration experiments using kaolin suspensions destabilized with a cationic polymer have suggested that filter pore flocculation effects are greater than predicted by computer simulation.

Aqueous ozonation of a quaternary ammonium surfactant

Abstract The reactivity of a mixture of saturated and unsaturated quaternary ammonium surfactants towards ozone was examined at a total substrate concentration of 500 μg l−1. Saturated components were unreactive; unsaturated components of the surfactant were reactive towards ozone. A number of reaction products or their methylated derivatives were identified by gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS) and fast atom bombardment-mass spectrometry (FAB-MS). The reaction products identified were consistent with a 1,3-dipolar cycloaddition of ozone to a carbon-carbon double bond.

Alternative disinfectant chemicals for trihalomethane control ‐ a review

Abstract Chlorination is the most commonly used and best characterised process for disinfection of drinking water. Concern over the possible health risks of chlorination by‐products, particularly trihalomethanes, has led to a re‐evaluation of existing treatment processes and the benefits of using alternative disinfectants to chlorine. The general characteristics of, and the advantages and disadvantages of four possible alternative disinfectants (ozone, chlorine dioxide, chloramines and potassium permanganate) in drinking water treatment are reviewed.