hold half the daily flow, and a lateral flow contact bed to hold one-third of a day's flow, in conjunction with percolating filters, is a scheme which will utilize a fall of about 10 feet to the greatest advantage. When less than 10 feet is available, the capacity of the liquefying portion of the plant (septic tanks and lateral contact bed) should be increased, so as to lessen the work to be done by the filters.

V. When, however, there is extremely little fall, say only five feet, the maximum amount of purification that can be effected by the anaerobic bacteria should be aimed at. The septic tank itself requires no fall, a septic tank should therefore be constructed capable of holding at least a day's flow. The effluent from the septic tank should also be passed through a lateral contact bed capable of holding half a day's flow. The sewage should be admitted to the lateral bed through a strainer of fine material. The effluent from the lateral contact bed might then be distributed by perforated pipes over a contact bed, the top layers of which for this purpose should be made of material from one-fourth to halfinch in size, the rest of the bed being of material between one-quarter and three-quarter inch in size. The effluent rising in the effluent chamber should be utilized to work the contact beds in rotation by some such mechanism as Cameron's or Adams'.

It will be noticed that it is suggested that a lateral contact bed, such as that of Messrs. Adams shown in Figs. 15, 16 (p. 100), and a contact bed proper are sufficient. The suggestion is made that the work of a contact bed can be increased by distributing the sewage over it intermittently, so that it acts as a percolating filter and a contact bed. In this case it should be understood that a sufficient area of contact bed must be provided to work with two fillings a day, and that the sewage should be intermittently distributed by perforated pipes over the whole surface of the contact bed. By such a scheme a satisfactory effluent could be produced, although only four or five feet of fall is available at the purification works, and by this means the expense of pumping could be avoided. If, however, the sewage contains brewery waste or the water supply sulphates, the septic tank effluent will probably become too offensive for such a scheme, and recourse must be had to pumping and a smaller septic tank.

VI. Lastly, when it is necessary to pump the sewage, advantage should be taken of this fact to apply the sewage intermittently to the filters by starting and stopping the pumps or by working in shifts. To enable this to be done wherever the engineering aspect of the question permits, the sewage should be allowed to gravitate through the septic tank or anaerobic bed, both of which require a slow continuous movement of the sewage, and it should not be pumped, unless it is for engineering reasons, until after it has been screened and liquefied. The motive power used for pumping may also be employed for spraying the sewage and making the intermissions in its application. If a Shone's ejector is employed to lift the sewage, it may also be used to spray the sewage over the filter, as is done at Chesterfield. This plan, however, is not admissible close to dwellings, or where the sewage is foul.

In all cases, before the engineer prepares his plans of construction, the scientific principles upon which the sewage is to be purified should be settled, and if special difficulties are likely to arise owing to the nature of manufacturing wastes admitted to the sewers, these should be settled in consultation with a chemist and bacteriologist, who has had special experience in dealing with this question. It should be borne in mind that by consulting a chemist at the outset, before the engineer has completed his plans, considerable expenditure and much trouble afterwards may be saved.

Afterword. The foregoing pages have brought the subject of the purification of sewage up to date at the time of issue, so far as practical results are concerned. It is very probable that before long modern views of electricity will bear fruit in this field of labour, and that in the future the action of a biological filter will be intensified by supplying it with ozonized air, or air in which the molecules of oxygen have been rendered less stable by exposure to Rontgen rays, high potential discharges, or the influence of some radio-active substance. For the moment, interest is again centered in the mechanical aeration of percolating filters, the removal of the top layer of sand from which, and the introduction of mechanical distributors, have opened up great possibilities.

PARTICULARS OF SEWERAGE AND SEWAGE DISPOSAL SCHEMES REQUIRED BY LOCAL GOVERNMENT BOARD.

Are any of the works outside the district, and have the provisions of the Public Health Act, 1875, been complied with?

Population to be served by the system.

Estimated dry wet weather flow, per head. State on what data the estimates are based.

Nature of sewage. (a) Domestic? (b) Are w.c.'s in use?
Trade effluents. Give particulars.
Are there any breweries?

Give number.

Is the separate system employed?

Are the pipes to be cement jointed? What description of pipe? Are there manholes at changes of direction in either plane?

Give gradient and diameter of any sewer which gives a less velocity, when flowing one-third full, of 3 feet per second.

Will the manholes ventilate? or are ventilating pipes to be used? If so, give area and height.

Have trial holes been dug on the line of pipe and site of works? As to wayleaves.

Storm overflows, particulars of. What will be the dilution of the sewage when they come into action? Show them on the plan within a 14-inch circle in red.

Means of flushing. Give contents of such tanks proposed, and state whether self-acting.

Is coal or other mining carried on in the neighbourhood?
What is the water-supply per head?

supply and situation of sources ?

DISPOSAL AREA.

What is the nature of

Is the scheme one of gravitation or pumping? If the scheme is one of pumping, give load to be overcome, distinguishing friction of rising main and brake horse-power (in duplicate) to be provided.

Have provisional agreements been entered into for the acquisition of the land?

Have trial holes, 6 feet deep, been sunk on the land, and what do they disclose? Give sections. State level of subsoil water. What is the area available for irrigation-by gravitation, by pumping?

What is the total area?

Cubic capacity of precipitation or bacteria tanks. Do they work continuously?

Are chemicals to be used?

Is the sludge to be pressed? If so, what becomes of the expressed liquid?

Is broad irrigation or filtration through land to be employed? Population per acre.

Are artificial filters or contact beds to be used? Do they work continuously? If not, give a statement showing the periods of working and rest.

Show the cycle of working, thus-
Filling,

hours; standing,

hours; emptying, hours; resting, hours.

Give their number and dimensions, also proposed rate of filtration per square yard of surface area. Further, give cubic capacity for filtering material and content (for sewage) of the same, taking this at a third gross content of filter structure below surface of filtering material. Are they to be worked by night as well as day? Give area of roughing filter for three D.W.F., or of land set apart for this storm water, and show the position on plan.

Is subsoil drainage proposed? If so, give particulars.

Are there any waterworks, collecting areas, or pumping stations within five miles of disposing ground. Give particulars, with geological section, if within five miles.

LEVELS. N.B.-ALL TO BE REDUCED TO ORDNANCE DATUM.

What is the level of invert of outfall sewer at disposal works?
Of surface level in precipitation tank?

Of outlet from the same?

Of bottom of the same?
At outlet into land carrier?

At inlet to artificial filters ?

Outlet from the same?

Give the highest and lowest level of land to be used for disposal.
Level of invert of effluent drain.

Of normal water-level (summer months) in stream.

Of flood-level ditto.

Is the land subject to flooding? (a) To what extent? (b) Show limit on plan. (c) Give level of highest known flood.

A general key-plan 6-inch ordnance is required, and this must be mounted on linen to fold in sections about foolscap size.

USEFUL DATA.

1 inch over an acre = 101 tons, or 22,623 gallons.

100 gallons to a square yard is 484,000 gallons, or, approximately, half a million gallons per acre.

15.68 grains per gallon = 1 ton to a million gallons.

N.B.-As an approximate rule, it may be taken that the square of the diameter of a pipe or tank of circular section, measured in inches, gives ten times the number of gallons in a lineal yard.

*The volume of sewage distributed through each 6" of a revolving arm should be proportionate to the figures in the columns marked *.