J. Haarhoff

Seawater pretreatment for reverse osmosis: chemistry, contaminants, and coagulation.

The paper addresses the effects of salinity and temperature on the chemistry of important parameters affecting coagulation pretreatment including the ion product of water, acid-base chemistry, dissolved metal speciation, and precipitation reactions for aluminum and iron coagulants. The ion product of seawater is greater than for freshwaters and affects chemical hydrolysis and metal-hydroxide solubility reactions. Inorganic carbon is the main cause of seawater alkalinity and buffer intensity but borate B(OH)(4)(1-) also contributes. Buffer intensity is an important parameter in assessing coagulation pH adjustment. Mineral particles are relatively unstable in seawater from electrical double layer compression, and when present these particles are easily coagulated. Algal-particle stability is affected by steric effects and algal motility. Dissolved natural organic matter from algae and humic substances causes fouling of RO membranes and pretreatment removal is essential. Aluminum coagulants are not recommended, and not used, because they are too soluble in seawater. Ferric coagulants are preferred and used. The equilibrium solubility of Fe with amorphous ferric hydroxide in seawater is low over a wide range of pH and temperature conditions. Ferric chloride dosing guidelines are presented for various raw seawater quality characteristics. The effect of pH on coagulant dose and the role of buffer intensity are addressed. A dual coagulation strategy is recommended for treating seawater with moderate to high concentrations of algae or seawater with humic matter. This involves a low and constant dose with high charge-density cationic polymers using Fe as the main coagulant where it is varied in response to raw water quality changes.

A Case for Strong Municipal Governance: The Water Supply of Pretoria 1855–1935

Abstract The city of Pretoria, South Africa, was found at an artesian water source, exceptional in quality and quantity – its only source from 1855 until 1935. Despite being supplied so abundantly, Pretoria suffered the same problems with water supply and sanitation that were evident elsewhere in South Africa. This was due to deficiencies in municipal government. When the town was founded, it was managed by a magistrate, appointed by and reporting to the central government. The main reason for the unhygienic environment was the lack of clear authority in the nineteenth century. After 1903 the situation improved rapidly when the municipality, under the direction of a competent city engineer, could focus on improving its infrastructure. The limit of the water source in Fountains Valley was reached only in the 1920s, which forced the municipality to limit the lavish consumption of water by the Pretoria residents. This led to the introduction of comprehensive metering in 1927, which only curtailed the consumption by 15 per cent – much less than hoped for. The wasteful water use practices of Pretoria were not easily eradicated and a new water scheme had to be hastily built to augment the citys water supply.

The Distillation of Seawater on Ships in the 17th and 18th Centuries

The distillation of seawater, from our perspective, is a trivial heat transfer application. Its historical development for seaboard use, however, was a slow, arduous process, over almost two centuries before these systems were commonly embraced by the seafaring community. This slow progress is shown to result from a combination of factors—an erroneous belief that some additives had to be added to seawater before it could be properly distilled; unfamiliarity with the toxic effects of the copper of which the distillation stills were constructed; intense rivalry between commercial competitors, which impeded the flow and sharing of technical information; and the time spent on securing and protecting intellectual property rights within and between countries. Moreover, the principal inventors and entrepreneurs often claimed old and known ideas as their own. The paper illuminates these developments with particular emphasis on England, the Netherlands, and—to some extent—France.