Nigel Graham

LABORATORY STUDY OF ELECTRO-COAGULATION–FLOTATION FOR WATER TREATMENT

An electro-coagulation-flotation process has been developed for water treatment. This involved an electrolytic reactor with aluminium electrodes and a separation/flotation tank. The water to be treated passed through the reactor and was subjected to coagulation/flotation, by Al(III) ions dissolved from the electrodes, the resulting flocs floating after being captured by hydrogen gas bubbles generated at cathode surfaces. Apparent current efficiencies for Al dissolution as aqueous Al(III) species at pH 6.5 and 7.8 were greater than unity. This was due to additional reactions occurring in parallel with Al dissolution: oxygen reduction at anodes and cathodes, and hydrogen evolution at cathodes, resulting in net (i.e. oxidation + reduction) currents at both anodes and cathodes. The specific electrical energy consumption of the reactor for drinking water treatment was as low as 20 kWh (kg Al)(-1) for current densities of 10-20A m(-2). The water treatment performance of the electrocoagulation process was found to be superior to that of conventional coagulation with aluminium sulphate for treating a model-coloured water, with 20% more dissolved organic carbon (DOC) being removed for the same Al(III) dose. However, for a lowland surface water sample, the two processes achieved a similar performance for DOC and UV-absorbance removal. In addition, an up-flow electrocoagulator configuration performed better than a horizontal flow configuration, with both bipolar and monopolar electrodes.

Occurrence and control of nitrogenous disinfection by-products in drinking water – A review

The presence of nitrogenous disinfection by-products (N-DBPs), including nitrosamines, cyanogen halides, haloacetonitriles, haloacetamides and halonitromethanes, in drinking water is of concern due to their high genotoxicity and cytotoxicity compared with regulated DBPs. Occurrence of N-DBPs is likely to increase if water sources become impacted by wastewater and algae. Moreover, a shift from chlorination to chloramination, an option for water providers wanting to reduce regulated DBPs such as trihalomethanes (THMs) and haloacetic acids (HAAs), can also increase certain N-DBPs. This paper provides a critical review of the occurrence and control of N-DBPs. Data collated from surveys undertaken in the United States and Scotland were used to calculate that the sum of analysed halonitromethanes represented 3-4% of the mass of THMs on a median basis; with Pearson product moment correlation coefficients of 0.78 and 0.83 between formation of dihaloacetonitriles and that of THMs and HAAs respectively. The impact of water treatment processes on N-DBP formation is complex and variable. While coagulation and filtration are of moderate efficacy for the removal of N-DBP precursors, such as amino acids and amines, biofiltration, if used prior to disinfection, is particularly successful at removing cyanogen halide precursors. Oxidation before final disinfection can increase halonitromethane formation and decrease N-nitrosodimethylamine, and chloramination is likely to increase cyanogen halides and NDMA relative to chlorination.

Degradation of atrazine by manganese-catalysed ozonation : Influence of humic substances

A bench-scale, laboratory study has been undertaken on the ozonation of atrazine in the presence of manganese using a conventional gas bubble-contacting column. Manganese was present in solution either as Mn(II) or as in situ formed Mn(IV). The experimental results indicate that the presence of humic substances has a substantial influence on the Mn-catalysed ozonation of atrazine. In the presence of a small amount of humic substances (e.g. 1 mg/l as DOC), the Mn-catalysed ozonation is obviously enhanced. However, with the further increase of humic substance concentration, a negative effect of humic substances on the catalytic oxidation of atrazine is observed for both the cases of Mn(II) and Mn(IV). It is proposed that Mn-catalysed ozonation of atrazine follows a radical-type mechanism. At low concentrations of humic substances, both the manganese species and the humic substances may initiate and promote the formation of hydroxyl radicals during ozonation which enhance the destruction of atrazine. However, at higher concentrations, humic substances may scavenge these highly reactive hydroxyl radicals, thus decreasing the oxidation efficiency of atrazine.

The dynamic effect of pipe-wall viscoelasticity in hydraulic transients. Part II—model development, calibration and verification

A state-of-the-art mathematical model has been developed to calculate hydraulic transients in pressurized polyethylene (PE) pipe systems. This hydraulic transient solver (HTS) incorporates additional terms to take into account unsteady friction and pipe-wall viscoelasticity. Numerical results obtained were compared with the classic waterhammer solution and with experimental data collected from a PE pipe-rig at Imperial College (London, UK). Unlike the classical model, the developed HTS is capable of accurately predicting transient pressure fluctuations in PE pipes, as well as circumferential strains in the pipe-wall. The major challenge was the distinction between frictional and mechanical dynamic effects. First, the HTS was calibrated and tested considering these two effects separately: if only unsteady friction was considered, a major disagreement between collected data and numerical results was observed; when only the viscoelastic effect was considered, despite the good agreement between data and numerical results, the calibrated creep function depended on the initial flow rate. In a second stage, the combination of these dynamic effects was analysed: creep was calibrated for laminar flow and used to test the solver for turbulent conditions, and a good agreement was observed. Finally, the HTS was tested using creep measured in a mechanical test, neglecting unsteady friction, and a good agreement was obtained.

A comparison of the role of two blue-green algae in THM and HAA formation

The contribution of two blue-green algae species, Anabaena flos-aquae and Microcystis aeruginosa, to the formation of trihalomethanes (THMs) and haloacetic acids (HAAs) was investigated. The experiments examined the formation potential of these disinfection by-products (DBPs) from both algae cells and extracellular organic matter (EOM) during four algal growth phases. Algal cells and EOM of Anabaena and Microcystis exhibited a high potential for DBP formation. Yields of total THMs (TTHM) and total HAAs (THAA) were closely related to the growth phase. Reactivity of EOM from Anabaena was slightly higher than corresponding cells, while the opposite result was found for Microcystis. Specific DBP yields (yield/unit C) of Anabaena were in the range of 2-11micromol/mmol C for TTHM and 2-17micromol/mmol C for THAA, while those of Microcystis were slightly higher. With regard to the distributions of individual THM and HAA compounds, differences were observed between the algae species and also between cells and EOM. The presence of bromide shifted the dominant compounds from HAAs to THMs.

The dynamic effect of pipe-wall viscoelasticity in hydraulic transients. Part I—experimental analysis and creep characterization

The mechanical behaviour of the pipe material determines the pressure response of a fluid system during the occurrence of transient events. in viscoelastic pipes, typically made of polyethylene (pe), maximum or minimum transient pressures are rapidly attenuated and the overall pressure wave is delayed in time. this is a result of the retarded deformation of the pipe-wall. this effect has been observed in transient data collected in a high-density pe pipe-rig, at imperial college (london, uk). several transient tests were carried out to collect pressure and circumferential strain data. the pipe material presented a typical viscoelastic mechanical behaviour with a sudden pressure drop immediately after the fast valve closure, a major dissipation and dispersion of the pressure wave, and transient mechanical hysteresis. the creep-function of the pipe material was experimentally determined by creep tests, and, its order-of-magnitude was estimated based on pressure-strain data collected from the pipe-rig. a good agreement between the creep functions was observed. creep tests are important for the characterization of the viscoelastic behaviour of pe as a material; however, when pe is integrated in a pipe system, mechanical tests only provide an estimate of the actual mechanical behaviour of the pipe system. this is because creep depends on not only the molecular structure of the material and temperature but also on pipe axial and circumferential constraints and the stress-time history of the pipe system.

Disposal of water treatment wastes containing arsenic - A review

Solid waste management in developing countries is often unsustainable, relying on uncontrolled disposal in waste dumps. Particular problems arise from the disposal of treatment residues generated by removing arsenic (As) from drinking water because As can be highly mobile and has the potential to leach back to ground and surface waters. This paper reviews the disposal of water treatment wastes containing As, with a particular emphasis on stabilisation/solidification (S/S) technologies which are currently used to treat industrial wastes containing As. These have been assessed for their appropriateness for treating As containing water treatment wastes. Portland cement/lime mixes are expected (at least in part) to be appropriate for wastes from sorptive filters, but may not be appropriate for precipitative sludges, because ferric flocs often used to sorb As can retard cement hydration. Brine resulting from the regeneration of activated alumina filters is likely to accelerate cement hydration. Portland cement can immobilize soluble arsenites and has been successfully used to stabilise As-rich sludges and it may also be suitable for treating sludges generated from precipitative removal units. Oxidation of As(III) to As(V) and the formation of calcium-arsenic compounds are important immobilisation mechanisms for As in cements. Geopolymers are alternative binder systems that are effective for treating wastes rich in alumina and metal hydroxides and may have potential for As wastes generated using activated alumina. The long-term stability of cemented, arsenic-bearing wastes is however uncertain, as like many cements, they are susceptible to carbonation effects which may result in the subsequent re-release of As.

Precursors of nitrogenous disinfection by-products in drinking water--a critical review and analysis.

In recent years research into the formation of nitrogenous disinfection by-products (N-DBPs) in drinking water - including N-nitrosodimethylamine (NDMA), the haloacetonitriles (HANs), haloacetamides (HAcAms), cyanogen halides (CNX) and halonitromethanes (HNMs) - has proliferated. This is partly due to their high reported toxicity of N-DBPs. In this review paper information about the formation yields of N-DBPs from model precursors, and about environmental precursor occurrence, has been employed to assess the amount of N-DBP formation that is attributable to known precursors. It was calculated that for HANs and HAcAms, the concentrations of known precursors - mainly free amino acids are insufficient to account for the observed concentrations of these N-DBP groups. However, at least in some waters, a significant proportion of CNX and NDMA formation can be explained by known precursors. Identified N-DBP precursors tend to be of low molecular weight and low electrostatic charge relative to bulk natural organic matter (NOM). This makes them recalcitrant to removal by water treatment processes, notably coagulation, as confirmed by a number of bench-scale studies. However, amino acids have been found to be easier to remove during water treatment than would be suggested by the known molecular properties of the individual free amino acids.

Adsorption of humic acid by powdered activated carbon in saline water conditions

Abstract The adsorption of humic acid (HA) by powdered activated carbon (PAC) in saline waters has been examined in the absence and presence of metal salt coagulants. The study showed that adsorption of HA by PAC can be significantly greater in saline water compared to freshwater and low conductivity water. An optimal adsorption was attained at saline concentrations corresponding to synthetic seawater diluted to 12.5–25% of its original concentration. In undiluted synthetic seawater the adsorption of HA from solution by PAC was comparable with that of local tap water in terms of initial adsorption rate and total removal. The enhanced adsorption is believed to be a combination of reduced electrostatic repulsion between the HA and PAC at high salt concentrations, and chemisorption due to chemical bonding between the functional groups. The effects of adding a metal salt coagulant, either aluminium sulphate or ferric chloride, on overall HA removal were found to depend strongly on the coagulant dose, solution pH and the sequence of addition of the PAC and metal salt coagulant. Addition of the PAC shortly before the coagulant was found to give the greatest removal of HA.

EVALUATION OF INORGANIC ADSORBENTS FOR THE REMOVAL OF PROBLEMATIC TEXTILE DYES AND PESTICIDES

This paper evaluates three inorganic adsorbents (activated bauxite, fullers earth and a synthetic clay), relative to activated carbon, for the removal of several representative contaminants of major concern and frequent occurrence in UK textile industry efflupnts; reactive dyes; penlachlorophenol and Propetamphos. The results indicate that, for the removal of reactive dyes, the synthetic clay was the most effective adsorbent over the pH range from pH 5.5 to pH 8.5 and temperature range from 20 to 4Q?C,~although comparable dye removals were exhibited by activated carbon under neutral and alkaline conditions. Under acidic conditions activated bauxite was as effective as activated carbon. Pullers Earth was largely ineffective. With regard to the removal of pesticides activated «arbon was highly, effective^whereas *he three inorganic adsorbents showed negligible removals. ©1997 IAWQ. Published by Elsevipr Science Ud. .. : . : f i