Chapter VI: Part 6

This process naturally involves the use of an electric current and an apparatus for producing the hypochlorite. Of the latter there are several on the market, and an illustration of the Digby Meridioniser, which is manufactured and supplied by Messrs. Adams Hydraulics, Ltd., is given in Fig. 153. The particular feature of this apparatus consists in the manner in which the re-combination of the anode and cathode products are secured. Instead of the re-combination taking place in the main body of the electrolyte, it can only take place in the special porous compartment enclosing the electrodes. The re-combination may take place in either the anode or the cathode compartment, the products of the one compartment being conveyed to the other compartment. Thus the caustic hydrate from the cathode cells flows by gravity into the anode compartment, the two compartments being connected by a glass pipe. Fresh water or water containing an excess of alkali is run into the cathode cell. This process gives hypochlorite solution of low saline content, the only salt present in the resultant liquor being that due to diffusion, depending upon the porosity of the closely covering compartment walls, or upon such a reaction as that covered by the Blount hypothesis. A cross-section of this apparatus is shown at Fig. 153, in which A is the positive lead; BB negative leads, C outflow, D inflow, EE cathodes, F anode.

Another machine is that supplied by Messrs. Oxychlorides (1907), Limited. This machine consists essentially of a graphite anode of circular cross-section (except as to about 4 in. at the top, which is left open for the free escape of gases evolved during the electrolysis), and within it a metallic cathode of smaller circular cross-section. The annular space between the anode and cathode is filled with a solution of common salt, or with sea-water, through which a current is passed from a low-potential dynamo. In either case the ultimate result is to obtain some of the chlorine of the salt in an active or available form, the only difference being that in the case of using strong salt solution, a concentrated form of available chlorine may be obtained, while with sea-water a weaker solution results. Where sea-water is readily obtainable it is naturally more economical to make use of it, and to employ the larger volume of less concentration; while where sea-water is unobtainable and salt is expensive, or where chlorides in the effluent are objected to, it is more advantageous to prepare concentrated solutions, and to dilute them when required for use. This machine is made in various sizes to suit varying conditions. It was used by the Royal Commission on Sewage Disposal in connection with their experiments at Guildford, referred to in their Report of 1908, pages 198 to 201.

Electrolytic hypochlorite of magnesium is being produced daily, by the Borough of Poplar, for use as a disinfectant. The highly effective qualities of this solution, as well as the low cost of production, makes it a very valuable disinfectant, and for the results obtained every credit is due to Dr. F. W. Alexander, the Medical Officer of Health, who initiated the scheme, and to whose unbounded energy and enthusiasm the success of the work is due. This solution is, of course, equally suitable for the sterilisation of sewage effluents. The cost of production of this solution, of an average strength of from 4·5 to 5 grammes of available chlorine per litre (·45 to ·5 per cent. solution), is estimated at under one penny per gallon. The apparatus in use at Poplar has been supplied by the Farringdon Engineering Co., and an illustration of the plant is shown in Fig. 154.

It consists of four cells, each containing ten elements, consisting of one positive and two negative plates. The positive plates consist of thin platinum wire, wound upon slate slabs, and the negative electrodes are of zinc. The four cells are placed one above the other, so that the liquid passes through from one to the other by gravitation. The feed-tank at the top contains a solution of sodium chloride and magnesium chloride, and is fitted with a glass gauge to indicate the amount of solution in the tank. From this tank the solution passes through a small ball-valve cistern, so as to maintain a constant rate of flow. The feed-tank is also provided with a plate, operated by a chain carried over to the outside of the tank, by means of which the liquid to be electrolysed can be stirred from time to time, so as to secure a uniform strength of solution. The solution passes through the four electrolysers in series, being subjected to the action of a regulated current of 15 to 17 amperes at 230 to 250 volts, being 5·7 to 6·2 volts per cell. After the electrolysed solution leaves the last cell it runs into a small tank, where a fixed amount of hydroxide of magnesium is mixed with it, and it is claimed that by this means the solution is rendered stable, a quality which should be of much value where the solution has to be stored for any length of time.

The question as to what is the most suitable sterilising agent to use under certain conditions, and in what proportion it should be added to the sewage effluent, is a matter for the chemist and biologist. The method of application is, however, the duty of the engineer. As in the case of other chemicals, there are two ways in which it may be applied. The solution may be added to the sewage effluent in equal doses of varying strength, or in varying doses of equal strength. There is a third method, involving the variation of the dose and the strength of the solution, but while this is not impossible it is probably not practical. The chief difficulty to be overcome is the variation of both the rate of flow and the strength of the sewage, and the most practical solution is to prepare the sterilising agent of a uniform strength, and vary the doses in direct proportion to the flow of the effluent, the minimum dose being sufficient for the maximum strength of the sewage. This method was adopted by the author in the case of a small scheme of sewage disposal, which he prepared for a place where the only outlet for the final effluent was a discharge over an area of chalk subsoil, from which the water supply of a large town is drawn. In this case, he designed a simple apparatus which does not involve any special appliances, and which would be quite satisfactory for ordinary practical purposes. A new apparatus for the purpose in question has, however, recently been introduced by Messrs. Nixon and Mannock. As will be seen from the illustration, Fig. 155, it is based upon the application of the Venturi principle, and involves the use of a Venturi tube, as previously described under the heading of “Measuring Apparatus.” In fact, the same Venturi tube can be utilised to serve both for measuring the flow of the effluent, and for applying the sterilising agent in direct proportion to the flow of the effluent.

The apparatus consists of a cylinder C, the top of which is connected by means of a pipe fitted with a three-way cock to the “Upstream” end of the tube A. A similar connection is made from the bottom of the cylinder to the “throat” B. A piston of the type used in the Kent Standard Water Meters, and provided with a counterbalance weight, works in the cylinder by means of the difference of the pressure on the two sides of the Venturi tube. The chemical solution (e.g. a 5 per cent. solution of chlorine) is supplied to the underside of the piston, and the pressure on the upper side of the piston being greater than the pressure on the underside, the chemical is forced down by the piston and injected through the injection tube and regulating valve into the effluent at the “throat” of the Venturi tube. As the flow of the effluent through the Venturi tube produces a difference of pressure which varies as the square root of the velocity, the rate of injection will also vary in the same proportion. The injection is thus in exact proportion to the flow, and any variation of the flow will automatically cause a corresponding variation in the rate of injection.

When the chemical re-agent is exhausted, the piston will be at the bottom of the cylinder, and the pointer at zero. In order to recharge the cylinder with the chemical, the three-way cocks must be reversed by means of the hand lever, thereby cutting off pipes A and B, and simultaneously connecting the top of the cylinder to the waste pipe, and the bottom to the supply from the chemical storage tank, which is fixed at such a height that the head will rapidly force the piston up and re-fill the cylinder with the chemical. The three-way valves are then reversed, and the apparatus is again in full working order. The apparatus shown is applicable to the treatment of 1000 gallons per hour, and will only need recharging once per day of 24 hours.

A feature of this apparatus is that it is self-starting, and should the flow cease, the injection will also automatically stop, the static head on both sides of the piston being equal. There is absolute immunity from danger or over-injection of the chemical by this system, and this is a valuable factor in the treatment of potable water. Where absolutely necessary, the same firm can supply a de-chlorinating apparatus. By means of the indicator, the works manager is constantly informed of the exact amount of chemical injected, and the scale readings can be compared with those of a Venturi Meter operated by the same Venturi tube. This apparatus can be supplied of a larger size, and provided with automatic recharging gear for larger installations.

Whatever method may be adopted for applying the sterilising agent, it is essential in all cases to have a storage tank to receive the mixture in order to provide time for the chemical to have full effect. So far as can be ascertained at present, a storage capacity equal to one hour’s flow of the liquid to be sterilised will be sufficient under ordinary circumstances, but provision should be made for thoroughly mixing the chemical with the effluent and for drawing off any deposit which may occur in the tank without interfering with the normal working of the plant.

Although the present volume is devoted entirely to the disposal of sewage, it may be stated here that in the matter of sterilisation, the suggestions that have been made apply with equal force to drinking-water supplies. Where the water contains a considerable amount of matter in suspension, it would be advisable to provide means for ample storage and settlement before passing it through the sterilising plant.

_Note._—An apparatus for the injection of chlorine
solutions for the purpose of sterilising sewage effluents
and drinking water has recently been brought out by the
Candy Filter Co., Ltd., and has for some months been in
practical operation, dealing with 200,000 gallons of river
water per day for an important municipal waterworks in
the country. In this case the installation includes a
de-chlorinating process, and it is stated that the results
of tests in actual work show that the sterilised water
contains neither _B. coli_ nor free chlorine.

INDEX

PAGE
“Acme” spray nozzle, 151
“Adams-Cutler” distributor, 135
“Aerat” distributor, 145
Apparatus for contact beds, 205
” for dosing tanks, 167
” for measuring flow, 232
” for percolating filters, 106
” for sludge removal, 40
” for sterilisation, 242
Auto-mechanical syphons, 168, 169, 180

Birmingham fixed spray, 155

Candy-Whittaker bacterial tank, 60
” ” distributors, 106
Capacity of contact beds, 222
” of detritus tanks, 23
” of effluent settling tanks, 182
” of percolating filters, 222
” of sedimentation tanks, 29
” of storm-water tanks, 227
“Capillary trough” distributor, 146
“Carlton” adjustable fixed spray, 152
“Carlton” distributor, 123
“Carlton rotor” distributor, 121
Chemical mixers, 72
Cleaning gear for spray holes, 119, 122, 125
“Coleman” dosing valve, 168
Columbus fixed spray, 155
Contact beds, 189
” ” apparatus for, 205
” ” capacity of, 222
” ” filling material for, 203
” ” general design of, 190
” ” methods of construction of, 196
” ” ” of distribution on, 199
” ” method of operation of, 191
” ” necessity for testing, 198
” ” sub-drainage of, 201
“Cresset” distributor, 110

Decanting valve, 49
Detritus tanks, 23
” ” capacity of, 23
” ” Dortmund type of, 25
” ” sludge scraper for, 27
Dibdin, W. J., 68, 189
Digby meridioniser, 243
Disc sprays, 155, 157
Distribution by flooding filters, 157, 186
” methods of, 104
Distributors, 106
” cleaning gear for spray holes of, 119, 122, 125
” fixed, 143
” power-driven, 129, 140
” revolving, 106
” travelling, 138
Dortmund tanks, 25, 51, 52
Dosing apparatus, 166
” tanks, 162
” ” disadvantages of, 160
” ” necessity for, 159
Drainage of filters and beds, 90, 201
Ducat filter, 144

Effluent settling tanks, 182
Elliott and Brown, 59

“Facile” distributor, 119
Feed channels, 163
“Fiddian” distributor, 127, 138
Fieldhouse tank, 65
Filtering material, 101
” ” grading of, 103, 204
Fine-grain filters, 185
Fixed distributors, 143
” sprays, 147
Floating arm, 48
Floor-tiles, 91

Gauge weir penstocks, 233
Grading of material, 103, 204
Gravity disc sprays, 155, 157
Grit chambers (_see also_ DETRITUS TANKS), 23

Haller and Machell, 145
“Hanley” distributor, 140
“Hartley” distributor, 130
“Hodgson” distributor, 127
Humus pits, 182
Hydro-extractor for sludge, 81
Hydrolytic tank, 52
Hypochlorite solutions, 242—247

Imhof tank, 57
Intermitting valves (_see_ DOSING APPARATUS)

“Kessel” tank, 63

Lowcock, S. R., 51
Low-draught syphon, 167

Massachusetts Institute of Technology, 155
Material for filters and beds, 101, 185, 203
Measuring apparatus, 232

Non-septic cylinder, 61

Percolating filters, 85
” ” aeration of, 86
” ” capacity of, 222
” ” distributors for, 106
” ” floor-tiles for, 91
” ” floors for, 88
” ” general design of, 85
” ” material for, 101
” ” methods of distribution on, 104
” ” ” of feeding, 159
” ” planning of, 98
” ” sprinklers for, 106
” ” sub-drainage of, 90
” ” types of construction of, 87
” ” walls for, 95
Ponding valve, 178
Power-driven distributors, 129, 140
Precipitation tanks, 47
Preliminary processes (_see_ TANKS, _also_ SLATE BEDS)

Recording apparatus, 234—239
Revolving distributors, 106
Royal Commission on Sewage Disposal, recommendations, 3, 16, 23,
29, 32, 182, 224, 226, 227
Ryder, E. E., 93

Salford fixed spray, 155
Sand-filters, 185
“Scott-Moncrieff” distributor, 130
Screens, 3
” bars for, 4
” for deep sewers, 7
” mechanical, 7
” ” rake, cleaning gear for, 12, 14
” rotary, 5
” simple, 3
“Separator” tank, 65
Sewage mixers, 73
Shone and Ault, 53
“Simplex” distributor, 125
Skegness tank, 59
Slate beds, 67
” slabs and blocks, 68, 92
Sludge disposal, 71
” draining beds, 83
” drying apparatus, 81
” presses, 75
” pressing plant, complete installation, 77
” removal, 25, 28, 35, 71
” ” apparatus for, 27
” ” elevators for, 40, 41, 42
” ” well for, 38
Spray jets, 147
Sprinklers, 106
Stand-by tanks for storm water, 229
Sterilisation of sewage effluents, 240
Sterilising solutions, 242
” ” apparatus for the manufacture of, 242—247
” ” ” for the injection of, 247—250
“Stoddart” distributor, 142
Storm-water overflow, diverting plate for, 17
” ” fixed weirs, 17
” ” floating weir, 21
” ” movable weirs, 21
” ” swinging syphon for, 21
” ” weirs, 16
” tanks, capacity of, 227
” treatment, 227
Strength of sewage, 226
Supply pipes, 163
“Sypho-jet” distributor, 110

TANKS, 29
” Candy-Whittaker, 60
” capacity of, 29
” circular, 25, 51—59, 60, 61, 66
” continuous-flow settlement without chemicals, 29, 32
” ” ” ” with chemicals, 29, 32, 47
” decanting valve for, 49
” detritus, 23
” Dortmund type, 25, 51, 52
” Fieldhouse, 65
” floating outlet for, 48
” hydrolytic, 52
” Imhof, 57
” inlets and outlets for, 40—47
” ”Kessel” type, 63
” non-septic, 61
” precipitation, 47
” quiescent settlement without chemicals, 29, 32
” ” ” with chemicals, 29, 32
” rate of flow through, 34
” rectangular, 33, 36, 37, 43—50
” roofs for, 39
” ”separator”, 65
” septic, 29, 32
” Skegness, 59
” storm-water, 229
Taylor, W. Gavin, 105
Travelling distributors, 138
Travis, W. O., 52
Trays and troughs, 143
“Triple tank” system, 145

“Venturi” meter, 237

Walls of filters, 95
Waterbury fixed sprays, 155
Watson, J. D., 93, 130
Weirs, fixed volume, 21
“Whirl” spray jet, 149
Willcox and Raikes, 49, 96, 135

MANUFACTURERS OF APPLIANCES

Adamsez, Ltd.
Adams Hydraulics, Ltd.
Adamson, D. and Co.
Albion Clay Co., Ltd.
Ames-Crosta Sanitary Engineering Co., Ltd.
Blakeborough, J. and Son, Ltd.
Burn Bros.
Carlton Engineering Co.
Enock, A. G. and Co., Ltd.
W. E. Farrer, Ltd.
Farringdon Engineering Co.
Glenfield and Kennedy, Ltd.
Goddard Massey and Warner.
Ham Baker and Co., Ltd.
Harriman, W. and Co., Ltd.
Hartley, Causton and Richmond.
George Jennings, Ltd.
Johnson, S. H. and Co., Ltd.
George Kent, Ltd.
Birch Killon and Co.
Manlove, Alliott and Co., Ltd.
Mansfield, H. R.
Mather and Platt, Ltd.
Naylor Bros.
Nixon and Mannock.
Oxychlorides (1907), Ltd.
Patent Automatic Sewage Distributors, Ltd.
Septic Tank Co.
Smith, John and Co.
Stiff, C. L.
Stoddart, F. W.
Stott, S. S. and Co.
Whitehead and Poole.
Wolstenholme, J. and Co.

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