History of the Huddersfield Water Supplies
By T. W. Woodhead

CHAPTER VIII - TREATMENT OF WATER

FILTRATION

One of the main concerns of the waterworks engineer is the quality of the water in his reservoirs and this is directly related to the nature of the watershed, the land which is drained by streams, and the storage system, hence a knowledge of the factors prevailing under natural conditions is essential. As we have seen, the main water supplies of Huddersfield are from the peat-clad moorland plateau to the west and south-west of the town, a region favourable both from its altitude and high rainfall, for gravitational supplies. This is supplemented by numerous springs issuing from the clough sides and this spring water, percolating through the beds of sandstone of the Millstone Grit series is usually very soft and free from impurities.

The surface water is subject to pollution in various ways.

The chief impurities are:-

a) Organic matter : peat and peaty acids, washed down by heavy rains and these discolour the water ; living organisms, especially the filamentous green algae, blue green algae, diatoms and desmids ; and among animals, sponges, polyzoa and mollusks which formerly caused much trouble by their ability of growing in the water pipes and causing serious obstruction. The most serious organisms are bacteria of which Bacillus coli is important as an indicator of pollution from animal excrements and a possible carrier of more serious infection.
b) Inorganic matter : suspended matter like sand and mud ; and salts in solution.
c) Soft water has a solvent action on lead. On the other hand it economises soap and is of great value in the local textile trade.

To further ensure against pollution, land at the head of the reservoirs has been secured, farms closed and cultivation checked. All authorities are agreed that farm animals should be kept clear of catchment areas. Farms in the neighbourhood of reservoirs are a common source of pollution.

The presence of fish in the reservoirs is encouraged by many authorities and the Waterworks Committee on 14th May, 1886, decided to "grant the application of the Huddersfield Angling Association for permission to fish in Blackmoorfoot and Longwood Reservoirs upon certain terms, reduced the price of season fishing tickets and resolved to discontinue the issuing of the six days’ season tickets."

The presence of fish in the reservoirs serves the useful purpose of tending to keep down green algae, which often produce clogging in the filters ; the number of fish, however, should be controlled. Fishing in the storage reservoirs is not now permitted as it may increase the risk of pollution and angling is confined to the Longwood Compensation Reservoir.

Notwithstanding the presence of fish in this reservoir, difficulties occurred here with algae in 1933, and treatment with copper sulphate proved only a partial success.

The West Riding of Yorkshire Rivers Board made a survey of the River Holme and in October, 1936, published the results in a "Report upon Chemical and Biological Survey of the River Holme" by J.H. Garner, F.M. Brown and M. Lovett (7). In this they point out that the upper reaches of rivers near their sources in catchment areas are unpolluted, and the organisms present are only those characteristic of pure water. When mill effluents enter the river the pure water organisms are killed. They point out that Bacillus coli showed rather marked variations, evidently due to seasonal and weather conditions. "Except on rare occasions the counts were low in the waters of the upper reaches, and , although they were higher further downstream, they were not sufficiently high to indicate any material pollution by domestic sewage. From this evidence it would appear safe to assume that the pollution effects recorded in the surveys were almost entirely due to trade effluents.” In the water below Brownhill Reservoir they found only 5 per cc. of B. coli, but at Mytholmbridge the number had increased to 2,100 per c.c.

There is a growing tendency for the public to demand freer access to moorlands and mountains from which our main supplies are obtained. In this event care will be necessary to protect our supplies from pollution.

The intention of the Access to Mountains Bill, now (1939) before Parliament, was to allow the public to roam over moorlands and mountains, and similar uncultivated areas, without interference by the owners of the property. There was little objection to this, but Waterworks Authorities were rather concerned about moorlands, which are used as catchments areas for water supply purposes. It would be dangerous to allow picnic parties to wander about as they like in such places, and the Association of Municipal Corporations proposed that there should be some limitation of the freedom of the public on water-gathering grounds.

Dangers from all these sources have been experienced in the past and it became increasingly necessary to remove the impurities and correct the defects.

Since the Great War much attention has been paid to these problems by bacteriologists, biologists, chemists and engineers, and in consequence the standard of quality of drinking water has ben greatly raised. There has been established the "Freshwater Biological Association of the British Empire" with a research station at Wray Castle, Lake Windermere, which is engaged in intensive research on the fundamental problems relating to freshwater and of the life-histories and environment of freshwater organisms and their control. The Association is receiving the practical support and encouragement from the Ministry of Agriculture and Fisheries, the British Waterworks Association, and the principal Waterworks Boards and Municipal Corporations, and is yielding valuable results.

The provision of impounding reservoirs serves to some extent to purify the water. During the period of storage much suspended matter is deposited, organic matter is oxidized and rendered harmless, and during sunlight many bacteria in the surface layers of water are destroyed by its ultra-violet rays, but this is not constant on its action. For these changes to occur the water must remain at least a month in storage, but the condition may be entirely changed if the surface water becomes cooled (or warmed) to a temperature of 39.2 deg F. The surface water, now at its greatest density, convection currents are set up which may become sufficiently active to cause a complete overturn of the reservoir resulting in much turbidity. Hence the need for filtration.

The object of filtration and treatment is to remove suspended matter, to remove organisms, especially bacteria, and to correct softness and acidity.

For the following account of local filtration methods at our reservoirs, I am indebted to Mr. J. P. Beveridge, Waterworks Manager.

It was not until July, 1900, that any of the water supplied to the consumers was filtered. Previous to that date the water was delivered straight to the consumers from the impounding reservoirs at Blackmoorfoot, Deerhill and Wessenden, and from the Wessenden Springs and at Longwood.

In 1897 there was quite a number of complaints as to the colour and turbidity of the water being supplied from the Deerhill Reservoir, and after enquiry, the Waterworks Committee instructed the Waterworks Manager "to proceed forthwith with the preparation of plans for filter beds at Deerhill and submit the same when completed and also to carry out the Borough Analyst’s recommendation as to treatment of the water."

The construction of these filters was commenced in January, 1899, and they were brought into service for the first time in July of the following years. They consisted of three beds each of which was 1,200 square yards in area giving a total filtering area of 3,000 square yards (See Figure 47 below).

Figure 47. Deerhill Filter Station and Service Tanks Formerly Used as Slow Sand Filters

It will be of interest to give a short description of the construction and working of these filters.

Each filter bed consisted of a tank-like structure with masonry walls and concrete bottom set into the ground and six feet deep. Across the bottom of the bed and at regular intervals was placed a number of fire clay drains which were connected to a main collecting drain which ran through the centre of the bed for collecting the filtered water from the filter. On the top of these collecting drains was placed a layer of rubble about nine inches deep followed by two layers of gravel each six inches deep. These layers of rubble and gravel were so graded that they formed a mat sufficiently compact to hold a layer of sand approximately two feet in thickness (See Figure 44 below).

The water was led on to the top of the filter to a depth of approximately three feet and percolated through sand at a rate of approximately 1.5 to 2.5 gallons per square foot per hour. In the old days it was thought that the sand was the filtering medium but this is not so as the sand in itself is too coarse and only acts as a coarse strainer. After the filter has been in action for about twenty-four hours, a slimy layer forms on the top of and around the grains of sand and true filtration commences. This slimy layer is composed of algae and other low forms of life which form a very close mesh on top of the sand which is so fine that the bacteria and suspended matter in the water are retained on top of the filters. After the filter has been in action for some considerable time this film gets clogged up and sets up too much resistance to the passage of the water so that the filter has to be cut out of service and cleaned. This is done by draining the water off the filter and taking a skimming of sand, approximately one inch thick, off the top. After this sand has been washed thoroughly it is put back and the filter once more brought into service.

In February, 1907, the Corporation considered the question of filtering the water from Butterley Reservoir.

Figure 45. Interior Deerhill Filter House. Pressure Filters

Since the introduction of the slow sand filter beds at Deerhill great progress had been made in the filtering and treating of water for domestic supply by what is known as pressure filters (See Figure 45 above). These filters consist of closed steel cylinders about eight feet in diameter having a series of collection pipes in the bottom of each cylinder attached to which are gun metal strainers. On the top of the strainers is placed a layer of specially graded sand about 4 ft 6 ins. In depth. The unfiltered water enters at the top of the cylinder and is forced through the sand under pressure and the filtered water passes out through the before-mentioned strainers and collecting pipes to the clear water storage tank or direct to the distribution mains. These pressure filters differ from the slow sand filter in the following respect:-

Instead of allowing nature to form a slimy skin on top of the sand as is the case with the slow sand filters, a certain quantity of alumina sulphate is added to the water (See Figure 46 below) which combines with the carbonates in the water and quickly forms on the surface of the sand a gelatinous film which is impervious to micro-organisms and suspended matter however well divided the latter may be. It also removes the colouring matter from the water which sometimes proved to be a source of trouble with the slow sand filters.

Figure 46. Apparatus for Adding Chemicals to Water

As the Huddersfield Water is very soft, the carbonates to combine with the alumina sulphate are supplied to the water in the form of lime or chalk.

The pressure filters work successfully at between forty and fifty times the rate of the slow sand filters and therefore take up much less room than the latter. Moreover, they are cheaper to construct for equal filtering capacities, and much easier to control.

As has been previously stated, the bulk of the water supplied to the town is derived from moorland surfaces and is therefore acid in reaction and liable to attack lead pipes unless treated. To overcome this action an alkali in the form of lime or soda ash is added to the water. This used to be added to the water as it entered the impounding reservoir but difficulty was experienced in controlling the dose necessary to counteract this solvent action of lead. With the advent of the pressure filter, this difficult has been overcome by adding the lime to the filtered water immediately after filtration. This is done by inserting what is known as a venturi tube in the filtered water pipe line. Advantage is taken of the difference in pressure created in the throat of the venturi tube. The difference in pressure already mentioned causes a continual flow of water from the main through the tank and back to the venturi throat. As the flow of water through the tank is very slow, the water becomes saturated with lime and contains approximately seventy grains per gallon. The flow of lime water from the cylinder is controlled by a valve to give a predetermined dose of lime to the water passing through the main. As the flow of water from the filters varied, so does the flow of lime water in the same ratio.

Tests of the lime water passing into the filtered water main are taken throughout the day, the results of which are logged in a "Daily Test Sheet." Whenever the cylinder shows signs of "running off" it is cut out of service and recharged with lime. This plant is in duplicate so that the change over from one cylinder to the other is a very simple operation.

After considering the advantages of pressure filters as compared with the slow sand filters, the Waterworks Committee decided to install a battery of six of the former type at Longwood to deal with the Butterley Reservoir water which is conveyed to that place through a twenty-four inch diameter main. It was not until November, 1910, however, that this plant was brought into service as the work was held up owing to the difficulty in acquiring a suitable site for the filter house. On April 10th, 1916, an additional four filters were installed at this filter station. The filtered water is passed to the Lower Reservoir at an average rate of one million gallons per day and when required, it receives a supply from the Upper Reservoir which is fed from the Longwood Springs, hence the Longwood Lower Reservoir is a reservoir of spring and filtered water. Other pressure filters have been erected, making a total of sixty-one, a complete list of which is given in the following table:-

At the Blackmoorfoot North Filter Station, a Booster Plat was installed in September, 1938, to boost water through the filters when the Reservoir is low.

Since October, 1918, the whole of the water supplied to the consumers with the exception of the water from the Wessenden springs and Longwood Springs has been filtered.

In 1931 complaints were received with regard to the colour of the water being supplied from Deerhill. It has already been mentioned that all the water from this reservoir was filtered through slow sand filters and also that one of their drawbacks is the lack of power of being able to take all the colour out of the water. Experiments were carried out with alumina sulphate in an endeavour to overcome this, but the filters clogged up so quickly that the expense of cleaning the filter beds was prohibitive. After fully considering the matter, the Waterworks Committee decided to install a battery of twelve pressure filters in place of the slow sand filters and to clear the sand and gravel out of the filter beds and use them as storage tanks for the filtered water. The pressure filters were brought into service in July, 1933.

A short description of the construction of the pressure filters has already been given and it is now proposed to give a slight ides of how they are controlled and how the chemicals, alumina sulphate and lime or chalk, are added to the raw water. The alumina, which is delivered at the filter house in oblong blocks each approximately fifty-six pounds in weight, is placed on trays within a circular tank full of water and allowed to dissolve. When the blocks have dissolved, the saturated solution is pumped into the raw water main some little distance before it reaches the filter beds. The pump is operated by a water turbine which is inserted in the main and by this means the delivery of the pump is kept in strict relation to the flow of water to be filtered. The chalk or lime is injected into the raw water main in precisely the same manner as the lime which is injected into the filtered water main to counteract. The plumbo-solvent action of the water and which has already been explained. The quantity of alumina sulphate in grains per gallon added to the water is regulated by altering the stroke of the pump and the quantity of chalk or lime by the opening or closing of a valve.

After the filters have been in action for twenty-four to forty-eight hours according to the turbidity of the water being filtered, the sand is washed. The operation is carried out in the following manner. The valve controlling the raw water inlet at the top of the filter shell is closed, the wash-out valve is opened, and a flow of filtered water is forced up through the sand, which carries away all the impurities which have been retained by the filter. To help this cleansing action, arms, which are fitted to a central spindle within the filter, are rotated by means of shafting fixed along the top of the shell and which is driven by a small oil engine. The arms church up the sand and thereby set free the impurities. This washing operation is continued until the effluent from the filter shows clear in the special sight glass which is fixed in the side of the filter.

At most of our filter plants the carbonates to combine with alumina sulphate are added to the raw water in the form of a chalk mixture. This has given trouble from time to time as under certain conditions, the chalk goes lumpy or cakes inside the chalk chamber with the consequence that the flow of water through the cylinder does not stake up a sufficient quantity of chalk. When this happens not only does the filtered water deteriorate in quality but there is a danger of alum passing through the filter beds. When the latter happens, we get an afterflock in the clear water tank which proves very troublesome as the alum combines with the lime which is added to overcome the plumo-solvent action of the water.

To overcome this difficulty, experiments were carried out at the Deerhill Filter Station with what is known as the "double coagulation" system of water treatment. This is carried out by injecting a small quantity of sodium aluminate into the unfiltered water about one minute before the alumina sulphate is injected. The sodium aluminate, being alkaline in reaction, builds up a light flock which greatly assists the alumina sulphate to form a more tenacious and closer film on top of the sand and at the same time there is less chance of the free alum passing through the bed. Unfortunately this system did not prove a success owing to the deficiency of carbonates in the water. After further experiments were carried out, an entirely new system was devised which is proving highly satisfactory and most reliable. Sodium aluminate and soda ash (carbonate of soda) are dissolved in a tank in the proportion of half-a-pound to three-and-a-half pounds respectively. This solution is brought into contact with the solution of alumina sulphate just prior to being injected into the raw water main and immediately forms a very dense flock. The flock is injected into, and is thoroughly mixed with the raw water when passing through the water turbines.

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