Found the following excerpt of an article here:
http://www.probrewer.com/resources/libr ... pplies.php
After reading through it, I'm convinced that the only solution is to purchase water. Two comments: 1) who knows how it will end up reacting in your wort, and 2) the algal blooms are sometimes associated with increased industrial waste.
2. Algae
A nationwide survey published in 1958 (7) indicated that algae were considered by waterworks officials to be the most frequent causes of odors and flavors in water supplies, with decaying vegetation second in importance. For this reason, it will prove desirable to briefly discuss these organisms and, in particular, their influence on water supplies. We know very little about the potent odoriferous compounds which some species are capable of releasing in water. It is known that some of these compounds are not effectively removed by activated carbon.
Algae are common and normal in habitants of surface waters and are encountered in every water supply that is exposed to sunlight. While a few of the algae are found in soil and on surfaces exposed to air the great majority of them are truly aquatic and grow submerged in the waters of ponds, lakes, reservoirs, streams, and oceans. Many species have come to be recognized as important in water sup plies in many ways, such as by their capacity for modifying pH, alkalinity, color, turbidity, and lately the radio activity of water (2). Some are undoubtedly the most troublesome of the various types of nuisance organisms, but others can actually be put to good use in improving a water sup ply. For example, if it were not for the presence of algae, many of our waters would remain polluted. Algae provide much of the necessary oxygen to water which permits aerobic bacterial decomposition of organic matter. They allow many trickling filters in treatment plants to remain aerobic.
However, their presence and the presence of their by-products in brewery water can result in undesirable odors and tastes, clogging of filters and other equipment, as well as providing nutrient for bacteria. Corrosive activity of water is often increased as a result of algal growth. This can have far-reaching effects on the pipes in distribution systems.
Slime accumulations in the unlighted portions of distribution systems may be due to bacteria, carry over of coagulant, or to other organisms, but rarely to algae. In lighted areas blue-green algae, as a whole, are the most notorious slime producers.
Deep pits can be formed on the metal walls of exposed tanks as a result of the depolarizing action of the oxygen produced by algae. Algae in contact with submerged cement blocks have caused the complete disintegration of the concrete.
Often a considerable proportion of the decaying vegetation in water is composed of dead algal cells. The decay or decomposition is brought about by fungi and bacteria, including the actinomycetes. The latter are unicellular, filamentous organisms which are frequently looked upon as a separate group occupying a position between the fungi and the bacteria, although they are classified among the bacteria. As a matter of interest, odors that are produced through the activities of the fungi and bacteria may be either from intermediate products formed during decomposition or from special substances that are synthesized within the cells of the micro organisms. The latter appears to be true in the case of actinomycetes.
Algae of importance to water sup plies may be classified into four general groups; the blue-green algae, the green algae, the diatoms, and the pigmented flagellates.
Each year, a seasonal cycle is evident in the algal population of lakes and reservoirs. Diatoms generally in crease in number in late winter, often with two or three growth periods occurring during the spring months. In early summer, the green algae begin to flourish, followed in the late summer or early autumn by increased growth of blue-greens. Then there will follow a ]ate autumn maximum of diatoms. Throughout most of the winter, the diatoms and certain other algae may remain in the water, but with little or no increase in numbers, until conditions, in the late winter, stimulate the organisms to begin the cycle all over again.
Seasonal changes in thermoclines in relatively deep bodies of water can bring large quantities of algae to the surface, these conditions should not be confused with growth patterns.
A few algae are well known for the production of specific, distinctive odors and tastes while a large number of others are associated with odors and tastes that vary in type according to local conditions.
There are four types of odors usually associated with waters containing algae.
1. There is the "aromatic" odor which is sometimes described more specifically as that resembling a particular flower or vegetable. Common examples of these are geranium, nasturtium, violet, muskmelon, and cu cumber. In some cases, it is described as an attractive spicy odor but, in others, it may be very objectionable, for example, a skunky or garlicky odor. Relatively small numbers of diatoms and pigmented flagellates in water can produce some of these de scribed odors.
2. Then there is the "fishy" odor which is often produced by the same algae that are responsible for the aromatic odors, except that they are in larger numbers in the water. More specific terms that are often used to J describe the fishy odors are clam shell, cod-liver oil, seaweed, Irish moss, rockweed, and salt marsh. The difference between these odor impressions is not too significant as far as the group of algae responsible for them is concerned.
3. Next we have a type of odor that is somewhat aromatic which is best described as "grassy". It is the most common odor produced by green algae and is more apparent when the organisms are present in large numbers. It may also be due to certain blue-green algae and occasionally diatoms and pigmented flagellates.
4. The final type of odor is that which is often described as "musty" or "earthy". The latter is often associated with actinomycetes and with a few algae. It can vary from mild to decidedly pungent. The common musty odor in water is in most cases caused by blue-green algae and a few other forms. Odors in this group have also been described by such terms as "potato bin" and "moldy". Some waters have been reported as having weedy, swampy, peaty, straw-like and woody odors. These are possibly modifications or combinations of the grassy and musty odors.
The "septic", "pig-pen" or "putrefactive" odor of some waters is frequently linked to the presence of large accumulations of blue-green algae and occasionally some species of green algae. As these descriptive terms suggest, it is produced as a result of the decomposition of masses of algae, particularly where lack of sufficient oxygen permits the formation of odoriferous intermediate products from the algal proteins.
Chlorophenolic, iodoform, or medicinal odors and tastes may be produced by the action of chlorine on the products of certain algae.