John E. Van Benschoten

Chemical aspects of coagulation using aluminum salts—I. Hydrolytic reactions of alum and polyaluminum chloride

Abstract This is the first of a two-part series of papers investigating the chemistry of Al coagulants. This paper examines hydrolysis reactions of alum and polyaluminum chloride (PAC1). Part II of the series addresses the coagulation of fulvic acid by these coagulants. Monomeric, polymeric and precipitated Al were identified based on a timed spectrophotometric analysis. At typical A1 doses used in water treatment, alum showed no evidence of polymer formation. PAC1 consists of preformed polymers which are stable upon dilution below pH 6 and over the time frames encountered in water treatment. Solubility studies showed that alum and PAC1 precipitate to form different solid phases. Alum precipitates are adequately described by amorphous A1(OH) 3 (s) solubility. The polymeric structure of PAC1 is retained upon precipitation yielding a solid phase with different light scattering characteristics, electrophoretic mobility and solubility than alum floc. A1 hydrolysis is interpreted as a coordination reaction between A1 and OH − . Effects of low temperature on alum are shown to be a result of changes in OH − caused, in part, by the temperature dependence of the ion product of water. Hydrolysis products in PAC1 are preformed and therefore less sensitive to in situ hydrolysis than alum. Results suggest that when using alum, some of the adverse effects of low temperatures may be mitigated by an increase in pH, thereby maintaining a constant concentration of the complexing ligand, OH − .

Changes in fractal dimension during aggregation

Experiments were performed to evaluate temporal changes in the fractal dimension of aggregates formed during flocculation of an initially monodisperse suspension of latex microspheres. Particle size distributions and aggregate geometrical information at different mixing times were obtained using a non-intrusive optical sampling and digital image analysis technique, under variable conditions of mixing speed, coagulant (alum) dose and particle concentration. Pixel resolution required to determine aggregate size and geometric measures including the fractal dimension is discussed and a quantitative measure of accuracy is developed. The two-dimensional fractal dimension was found to range from 1.94 to 1.48, corresponding to aggregates that are either relatively compact or loosely structured, respectively. Changes in fractal dimension are explained using a conceptual model, which describes changes in fractal dimension associated with aggregate growth and changes in aggregate structure. For aggregation of an initially monodisperse suspension, the fractal dimension was found to decrease over time in the initial stages of floc formation.

Chemical aspects of coagulation using aluminum salts—II. coagulation of fulvic acid using alum and polyaluminum chloride

Abstract This is the second of a two-part series investigating chemical aspects of coagulation using AI salts. Part I of the series examined the hydrolytic reactions of AI. In this paper, the coagulation of fulvic acid (FA) by alum and polyaluminum chloride (PACI) is examined. An Al speciation methodology was used to examine complexation reactions between Al and FA for water treatment conditions. From pH 5 to 7 and at typical coagulant doses, hydrolysis and complexation of Al is described by a simple model based on the reaction stoichiometry between AI, FA and OH−. Model results show that when alum is used as a coagulant, Al complexed with FA is hydrolyzed to a ligand number, n = 2.7. This is similar to the ligand number for the prehydrolyzed PACI and explains similarities in dose requirements for these coagulants. Effects of temperature on coagulation performance are shown to be largely chemical in nature. Chemical aspects of coagulation are discussed and the importance of complex formation in coagulation is examined.

Aluminum Coagulation of Natural Organic Matter

The concentration and nature of natural organic matter (NOM) influence the processes selected for water treatment plants. Water supplies containing NOM dominated by humic substances require careful design and operation of the chemical pre-treatment step of coagulation. Humic substances in untreated waters exert a greater demand for chemical coagulants compared to particulate matter, and their removal is important for health reasons because they are precursors to trihalomethanes and other disinfection by-products. Furthermore, coagulation affects the performance of downstream particle separation processes of clarification (sedimentation or flotation) and filtration.

Combined use of heat and oxidants for controlling adult zebra mussels

Abstract The results of an experimental study are presented that demonstrate the use of chlorine or ozone to control zebra mussels at temperatures from 30 to 36°C. Control studies were conducted with no oxidant present. Three acclimation temperature ranges were tested: 0–5, 10–15, and 20–25°C. Chlorine was tested at 0.1 and 0.5 mg/L; ozone at 0.5 mg/L. Mortality was described by a cumulative normal distribution, from which times to 95% mortality were estimated and used as a dependent variable for hypothesis testing. Study results showed that the addition of chlorine or ozone was more effective than heat alone at test temperatures above 30°C. Compared to heat alone, the combined use of heat and oxidants decreased the time to 95% mortality by more than 95% at 30°C. Above 30°C, the benefits of the combined treatment strategy decreased with increasing test temperature. At 36°C, the benefits of the combined treatment strategy over heat alone were minimal. Acclimation temperature was important only for heat alone and for mussels acclimated at 0–5°C. The addition of chlorine or ozone at elevated temperatures can reduce mortality times by as much as three orders of magnitude compared to oxidant addition at ambient temperatures. The results of the study should be of significance to power plants or industries where excess heat is available to raise water temperatures.

Response of zebra mussel veligers to chemical oxidants

Abstract Experimental studies using chlorine, hydrogen peroxide and ozone showed two responses by zebra mussel veligers. First, veligers readily perceived the presence of oxidants (> 0.1 mg/l for chlorine and ozone) and responded by retracting the velum and ceasing swimming and feeding activities. Under these conditions, veligers are prevented from attaching to solid surfaces. Second, continuous exposure to these oxidants caused death. Complete mortality by chlorine or ozone (0.5–1 mg/l) at 18–22°C required continuous exposure of about 18 and 5 h, respectively. Either chlorine or ozone should be effective in controlling zebra mussel veligers. Hydrogen peroxide was observed to be effective in preventing attachment but only at high doses relative to chlorine and ozone.

Sorbing Vertical Barriers

This chapter presents an overview of vertical barrier technologies that use sorbing materials to remove contaminants from groundwater. Two classes of system are considered: 1) low-permeability earthen barriers, in which sorbing additives are used to reduce the diffusive flux of organic contaminants, and 2) high-permeability treatment walls designed to remove contaminants under advection-dominated natural groundwater conditions. The focus of the discussion is on the performance assessment of strongly sorbing barrier materials using laboratory tests. Emphasis is placed on the design and analysis of column studies to characterize the barrier sorption capacity and the appropriate formulation of mathematical models to extrapolate long-term barrier performance. Two case studies are considered: the amendment of soil-bentonite slurry walls with an organic-rich additives, and the use of natural zeolite to remove strontium-90 from groundwater.