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Natural & Artificial Sewage Treatment

Chapter 10: VII. SOME OBSERVATIONS ON THE DEPOSITION OF SUSPENDED MATTERS IN TANKS.
By Alfred Stowell Jones
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The work explains and contrasts land-based natural purification with engineered methods for treating town sewage, detailing soil and subsoil roles, biological aerobic processes, and evaporation and plant uptake on sewage farms. It examines artificial approaches—septic tanks, intermittent and continuous contact beds, chemical precipitation and sludge management—discussing microbiology, oxygen consumption, nitrate formation and removal, effluent quality, and operational practice. Case studies, experimental results and contemporary commission reports inform practical recommendations on site selection, leveling, cropping, automatic appliances and management, with attention to public health implications and the need to balance nature's capacities with engineered supplements where land or conditions limit natural treatment.

VII. SOME OBSERVATIONS ON THE DEPOSITION OF SUSPENDED MATTERS IN TANKS.

Definition of the term “deposition.”

The term “deposition” shall here be held to mean the precipitation of the suspended matters without chemicals or other artificial means, i.e. the unaided subsidence of these matters at such a rate of flow that septic action is not set up within the tanks.

The question that is of interest here, is: Which is the most favourable rate of flow of the sewage through the tank, so far as the deposition of the suspended matters is concerned? To some extent the answer to this question will depend on the special characteristics of the particular sewage under consideration, but for general purposes the following observations will not be without interest.

Although of very great importance, this question does not appear to have received very general consideration, as the available number of careful experiments is but small.

Tank velocity at Barking.

It appears that the calculated velocity in the channels of the precipitation tanks at Barking is about 4 feet per minute, and that with this velocity about 77 per cent. of the suspended matters were deposited in the year 1894.

Tank velocity at Manchester.

At the Manchester tanks it is stated that a velocity of 3 feet 4 inches per minute is employed.

Velocity frequently adopted.

A rate of velocity now frequently adopted in this country for new works is 6 inches per minute.

Frankfort experiments.

In the settling tanks at Frankfort on the Main there are deposited about 84 per cent. of the suspended matters, with velocities ranging from 9½ inches to 16½ inches per minute.

Cassel experiments.

With a velocity of 7 inches per minute, it is stated that at the Cassel sewage works 97 per cent. of the suspended matters are retained in the tanks.

Hanover experiments.

At Hanover a set of interesting observations has lately been made, on tanks 246 feet long, with a view to ascertaining the most advantageous rate of flow.

With a velocity of 9·44 inches per minute, 62·7 per cent. of the suspended organic matters were precipitated, with a velocity of 14·17 inches per minute 61·7 per cent. were deposited, and with a velocity of 35·43 inches per minute 57·3 per cent.; from which figures it will be clear that there is not much difference in the result on the suspended matters between these velocities.

Against these results must be placed the results obtained with septic tanks, where, as has frequently been stated, a velocity of 1 inch per minute and a sojourn of twenty-four hours in the tank may be expected to lead to a deposition of about 60 per cent. of the suspended matters.

Reduction of cost.

Where, therefore, a previous septic treatment of the sewage by anaerobes is not necessary, it is clear that the substitution of ordinary settling tanks for septic tanks will be accompanied by a very considerable reduction of cost.