Chlorination of Water Part 9

Abnormal conditions such as freshets, and storms, sometimes cause complaints regarding tastes and odours. Adams[3] found that the complaints in Toronto usually accompanied a change in the direction of the wind, a sustained east wind being the one most productive of trouble. The exact cause for this could not be ascertained but it was usually found that there was an accompanying increase in the number of microscopical organisms (plankton) present in the raw water.

Freshets usually increase the bacterial contamination and necessitate an increased dosage which may cause complaints.

Complaints as to tastes and odours can be best avoided by ensuring regularity of dosage, perfect admixture, and storage of the treated water for a reasonable period. These factors are discussed in detail elsewhere.

_Colic._ Although claims have been made that the consumption of chlorinated water has produced "colic" no corroborative evidence has been adduced and the symptoms have probably been due to some other cause. Dilute solutions of chlorine have been used as intestinal antiseptics in the treatment of typhoid fever without producing irritation of the mucous lining and the usual dose for this treatment is one grain of chlorine. Before taking a _medicinal_ dose of chlorine 140 gallons of water containing 0.1 p.p.m. would have to be consumed, a quant.i.ty greater than is ordinarily drunk in a year.

Chlorine and hypochlorites are destructive and irritant to skin and it is possible that hot chlorinated water has, in some instances, a similar effect.

It is inconceivable that the addition of minute traces of bleach or chlorine to water should cause it to extract abnormal amounts of tannin from tea but it is possible that free chlorine, when present, acts upon the tea extractives and produces compounds having obnoxious tastes and odours. Tannin to the ordinary tea drinker represents the disagreeable portion of the tea and an obnoxious taste in tea brewed with chlorinated water would consequently be ascribed to the extraction of abnormal quant.i.ties of tannin.

Almost all waterworks departments using chlorination have received complaints to the effect that the water had killed fish and small birds.

There is usually no evidence that the loss was due to chlorinated water but it is generally impossible to convince the owners that the process of water treatment was not the cause. Many continuous physiological tests have been made of the effect of chlorinated water on small fish and have shown that the concentration used in water treatment is without effect. The author kept a tank of minnows in one of the pumping stations for months without loss although the tank was continuously supplied with water that had been treated but a few seconds previously. The bleach solution was discharged into the suction of the pumps and the water for the fish test was taken from the discharge header.

It has been found on many occasions that fish are extremely susceptible to chlorine and hypochlorites. This knowledge has been sometimes used for such nefarious purposes as fish poaching, a few pounds of bleach in a small stream being a simple and most effective method of killing all the fish which are then carried down stream into a convenient net.

Chlorinated sewage effluents have also been known to destroy the fish life of the stream into which they were discharged.

The opinion of fish culturists as to the action of chlorinated waters upon fish eggs in hatcheries is almost unanimously to the effect that it is a destructive one. Fish eggs are extremely sensitive to chlorine and hypochlorous acid and very few will survive in a water containing 0.1 p.p.m. of free chlorine. The Department of Fisheries of the Dominion of Canada has informed the author that free chlorine in the water had a marked adverse effect on the hatching of the eggs of Atlantic salmon, Great Lake trout, pickerel, and whitefish, but no effect was noticed when free chlorine was absent. The Department has, however, decided to remove all the hatcheries to localities where water that does not require chlorination can be obtained.

The effect of chlorinated water upon seeds, plants, and flowers has been investigated by the Dominion Department of Agriculture and Dr. Gussow (Dominion Botanist) and Dr. Shutt (Agricultural Chemist) who were in charge of the work, have reported that water treated with hypochlorite caused no apparent injury to carnations and hybrid roses. Six varieties of wheat seed, after soaking in freshly prepared hypochlorite solutions (0.05 to 10 parts per million of available chlorine) were all sown on the same day. Germination was found to be uniform throughout and no effect of the chlorine was observed either as regards the rate of germination or the development of the young plants. Experiments on barley and oats produced similar results. Radishes, turnips, cuc.u.mbers, and beans also showed no r.e.t.a.r.dation in development after treatment with chlorinated water.

These experiments were conducted with solutions of bleach in distilled water, but identical results were obtained in a later series when the treated city supply (Ottawa) was used.

The results proved conclusively that statements alleging damage to plants, flowers, and seeds by the hypochlorite treatment of water are unfounded and do not merit the slightest consideration.

_Corrosion of Pipes._ Chlorinated water, it has been alleged on many occasions, causes rapid corrosion of galvanised iron water services and especially of the water tubes of boilers, water heaters, etc. When bleach is used for water treatment, a slight increase in the hardness is produced but as this is mostly due to calcium chloride, there is no corresponding increase in the salts that form a protective coating. The presence of traces of calcium chloride and chloro-organic compounds might tend to increase the corrosive properties of a water but this increase is probably so small as to be negligible.

If pipe corrosion is considered by the carbonic acid hypothesis, the use of bleach should tend to reduce it because bleach contains an excess of base that combines with a portion of the free carbonic acid. The results of routine tests for free carbonic acid made on the raw and treated waters at Ottawa are as follows:

-------+----------------------+------------------------- | CARBONIC ACID. | | PARTS PER MILLION | Year. +----------+-----------+ Nature of Treatment.

|Raw Water.|Chlorinated| | | Water. | -------+----------+-----------+------------------------- 1915 | 1.44 | 1.41 | Bleach 1916 | 0.92 | 0.85 | Bleach 1917 | 0.84 | 0.81 | Bleach first four months | | | Chloramine during last | | | eight months -------+----------+-----------+-------------------------

These figures shown that the hypochlorite treatment produced a small but definite decrease in the carbonic acid content and should, _caeteris paribus_, tend to reduce and not increase corrosion.

If the corrosion of pipes is considered according to the electrolytic theory, a slight increase, due to an increased electrical conductivity, might be antic.i.p.ated. The effect of the addition of hypochlorite upon the electrical conductivity of distilled water and the Ottawa River water is shown in Diagram VI.

[Ill.u.s.tration: DIAGRAM VI

Effect of Calcium Hypochlorite on Electrical Conductivity]

With the concentrations of hypochlorite ordinarily used in water treatment it is inconceivable that the slight increase in the electrical conductivity has any practical significance at low temperatures. The conductivity increases rapidly, however, with increase of temperature and any increment due to chlorination might produce a slight appreciable effect at temperatures approaching the boiling-point of water.

Liquid chlorine does not increase the conductivity to the same extent as an equivalent quant.i.ty of hypochlorite but it increases the carbonic acid content in proportion to the dosage used.

The author investigated the action of hypochlorite on galvanised pipes in 1914 and was unable to detect any definite corrosion with normal concentrations of chlorine. The experiments were made with 2-inch pipes and an examination of the first consignment received showed that, although the galvanising on the outside was perfect, the inner coat was very inferior: in some parts there was an excess of zinc that broke away on sc.r.a.ping whilst in others the iron pipe was bare.

A committee of the Pittsburg Board of Trade, appointed to investigate complaints as to pipe corrosion, reported in 1917 that they were largely due to inferior qualities of pipes and not to the method of water purification employed (slow sand filtration and chlorination).

The effect of chlorination on the _plumbo-solvency_ of water was investigated in 1904 by Houston who found that chlorine, as chloros, in amounts between one and ten parts per million, did not appreciably increase the plumbo-solvent action of either unfiltered or filtered water. Similar results were obtained by the author with the Toronto supply: raw lake water, filtered water, and water treated with 0.25 and 0.50 p.p.m. of chlorine, all dissolved the same quant.i.ty of lead in twenty-four hours. The amount in each case was too small to be of any significance.

BIBLIOGRAPHY

[1] Letton. J. Amer. Waterworks a.s.soc., 1915, 2, 688.

[2] Kienle. J. Amer. Waterworks a.s.soc., 1915, 2, 690.

[3] Adams. J. Amer. Pub. Health a.s.soc., 1916, 6, 867.

CHAPTER VI

BLEACH TREATMENT

The treatment of water with bleach alone has been largely supplanted by the liquid chlorine process but the following details will be of use on meeting conditions for which liquid chlorine cannot be used and also for the preparation of the hypochlorite solution required in the chloramine process.

The essential features of a bleach installation are the solution or mixing tanks, storage tanks, piping system, discharge orifice or weir, and sludge drain.

Bleach is usually sent out by the manufacturers in sheet steel drums, 39 inches high and 29-1/2 inches in diameter, which contain about 14 cu.

ft. of bleach and weigh approximately 750 pounds gross and 690 pounds net. It can be most economically purchased in car lots and if the consumption warrants this procedure storage should be provided for about 70 drums or rather more than one car load. According to Hooker[1] bleach loses 1 per cent of available chlorine per month in hot seasons and 0.3 per cent in cold ones so that it is advisable to carry as little stock as possible during hot weather. Hot weather also causes a further loss by accelerating the action of the bleach on the drum which rapidly disintegrates and cannot be handled. Bleach can often be purchased more cheaply in hot weather but such a policy is a short sighted one unless it is required for immediate use.

The general design of a hypochlorite plant is largely determined by the capacity but in all cases an effort should be made to avoid complicated details which may appear advantageous in the drafting office but do not stand up in actual practice. Many metals rapidly develop a protective coating on immersion in bleach solution but if this is removed by friction, rapid erosion ensues; bearing metallic surfaces should be reduced to a minimum.

_Mixing Tanks._ All tanks, whether mixing or storage, should be constructed of concrete and painted with two coats of asphalt.

Experience has shown that wooden tanks are not suitable. The author has used pine, oak, and cypress tanks but all were rapidly leached by the hypochlorite and ultimately had to be lined with concrete.

There is a considerable variation in the concentration of bleach solution made in mixing tanks at various works. Some operators use about one gallon of water per pound of bleach and mix the two to a cream by wooden paddles, revolving on a central axis, for 1-2 hours; the paddles are then stopped and the cream run out into the storage tanks and diluted to the required strength by pa.s.sing water through the mixing tank. There are two objections to this method: (1) the addition of small quant.i.ties of water to bleach tends to gelatinisation which may protect lumps from the further action of water and (2) a stratification of the solution occurs in the storage tank unless agitation is used.

Gelatinisation causes loss of available chlorine and stratification causes irregular dosage unless corrected by agitation, which necessitates power. Other operators mix the bleach and water to the final concentration in the mixing tank and discharge the contents into the storage tank, the intermittent process being repeated until the storage tank is full. Gelatinisation is avoided by using a low original concentration and as all batches are of equal density no stratification is produced.

At Ottawa the bleach is crushed and, after weighing, dumped into a circular concrete tank provided with a hinged wooden lid. The stirring arrangement consists of a bronze shaft on which an aluminium impeller is fixed which revolves in an iron tube set slightly above the bottom of the tank (see Fig. 1). After the requisite amount of water has been added the motor connected to the bronze shaft is started and the mixture pumped for 15-20 minutes; without waiting for the sludge to settle the contents are discharged into the storage tank and the operation repeated until the tank is full. The piping between the mixing and storage tanks is of galvanised iron of generous dimension so as to compensate for incrustation. The pipes are straight and are provided with crosses at every change of direction to enable excessive incrustation to be removed. The valves should be made of hard rubber or special bronze; if bra.s.s valves are used they will probably require renewing every twelve months.

[Ill.u.s.tration: Fig. 1.--Mixing Tank for Bleach.]

The concentration of solution necessarily depends upon local conditions but it is usually advisable to keep it below 2.5 per cent of bleach, which is equivalent to 0.85 per cent of available chlorine.

_Storage Tanks._ These should be built of reinforced concrete and painted inside with asphalt, which should be periodically renewed to prevent the solution seeping through to the reinforcement. At least two tanks should be provided so that one may be filled and allowed to settle before being put in operation. The hypochlorite discharge pipe is usually 6-9 inches from the bottom to permit the collection of sludge, which is run off when it reaches the elevation of the hypochlorite discharge. The sludge drain, which opens into the bottom of the tank, is usually a 4- or 6-inch cast-iron pipe, with suitable gate valve, which discharges into a common drain made of clay pipe.

The storage tanks should be provided with either gla.s.s gauges or float indicators to enable the orifice discharge to be checked up at periodical intervals.

_Regulation of Dosage._ The discharge of the hypochlorite solution is usually regulated either by maintaining a constant head on an orifice of variable dimension or by varying the head on an orifice of fixed dimension. The weir principle may also be used but it is not so well adapted for hypochlorite as for other chemicals.

In the constant head method, the head is maintained by a bronze valve connected to a float made of gla.s.s or tinned copper. In many cases the orifice is a rectangular slot in a bra.s.s plate and is adjusted by means of a bra.s.s slide operated by a micrometer screw. Bra.s.s plates are not very suitable as they become corroded and so reduce the size of the orifice; if the incrustation is removed the orifice will discharge more than the calibration indicates. Needle valves are unsuitable for similar reasons.

An example of an orifice feed box of the constant head type is shown in Fig. 2. A vertically arranged hard-rubber pipe pa.s.ses though a hard rubber stuffing box in the bottom of the tank and has one or more orifices near its upper end. The area of the submerged portions of the orifices is controlled by the hand wheel which is connected with the threaded stem of the pipe. The stem has sixteen threads per inch, and one revolution of the wheel will submerge the orifices one-sixteenth of an inch. The extent to which the orifices are submerged is indicated on the dial fixed to the side of the tank.