CONTACT BEDS.
The almost universal adoption at the present time of biological methods of sewage purification by means of artificial filters, is due entirely to the original experimental work of Mr. W. J. Dibdin, at the Barking Outfall Works of the London County Council. These experiments were carried out with a contact bed, and during the subsequent ten years an enormous number of works were constructed upon this principle. At the present time, however, it is a somewhat rare occurrence to find contact beds proposed for sewage disposal schemes of any size. It has been stated that the principle upon which they are operated is unscientific, that they rapidly become clogged and useless, and that, in any case, they are not capable of dealing with sewage at the same rate as percolating filters, or of producing such a high degree of purification. With regard to the first point it would be futile to endeavour to explain what is and what is not scientific. This must be left to the scientists. That contact beds have in many cases become clogged and useless cannot be denied, but there is also very little doubt that this unsatisfactory result has been due to one or more of the following causes: (a) overwork, (b) improper methods of operation, (c) the use of unsuitable material for filling the beds, (d) insufficient sub-drainage. It was most unfortunate that for some years the general idea of a contact bed was that it consisted of a simple excavation in the ground, filled with coke or similar material, into which the sewage was discharged, held up for two hours, and then drawn off; a very simple but crude affair altogether. It is now known that contact beds, like other systems, can only deal with limited volumes of sewage, the actual amount depending upon the character of the sewage and other factors; that there is a proper method of operating the beds, and that it must be strictly adhered to if the best results are to be produced; that unsatisfactory material is worse than useless, and that very ample means of sub-drainage are absolutely essential to the continued efficiency of the beds. It is probable that if these essential factors had been properly understood and acted upon from the outset, there would have been very few failures to record.
It has been stated that contact beds are obsolete, but there are engineers who even now recommend this system, and consider it satisfactory under some, if not under all conditions. In the opinion of the author contact beds are not obsolete, and there are cases where the conditions preclude the adoption of any other method of purification. Under these circumstances, it is considered desirable to describe in the following pages the details of design and construction which have been found by experience to be necessary to ensure satisfactory results.
General Principles of Design.—The first point to be decided before commencing the design of a scheme of contact beds is whether single, double, or triple contact is necessary to produce the desired degree of purification, and this will depend upon the strength of the sewage and the destination of the final effluent. Single contact alone will not be sufficient, except in a very few cases where the sewage is weak (highly diluted), and even then it will necessitate the use of fine-grade material for filling the beds, and consequently a tank effluent of exceptional quality as regards the matters in suspension in order that the fine material may not be rapidly choked. Where a sufficient area of land of a suitable character can be procured at a convenient level for treating the effluent from the beds, single contact may be adopted with material of medium-grade, but even in this case special attention must be devoted to the preliminary process in tanks, so as to reduce the amount of solids in suspension in the tank effluent to the minimum. As a rule it will be found safer to adopt double contact, as the primary beds may then be filled with coarse grade material, which will be less liable to choke, and it will not be necessary to rely so much upon the land or any other final process that may have been provided. In special cases, and particularly where the sewage is strong, or an exceptionally high degree of purification is essential, triple contact should be adopted, but the tertiary beds may consist of a set of sand filters similar in construction to those described on [pages 185 to 188]. In some quarters the question of the grading of the material is considered of slight importance, and very little difference has been made in the size of the material for the primary and secondary beds, but in the author’s opinion it is absolutely essential that each series of beds should be filled with finer material than the preceding series, and the material in the final stage of treatment should be as fine as possible, so long as it does not contain any dust. In making these statements, it is assumed that the question of sub-drainage will be dealt with on the lines recommended later under that heading.
Another factor which has an important bearing upon the general design of a scheme of contact beds, is the method of operation which is to be adopted. It is generally assumed that all contact beds are worked in what is known as eight-hour cycles: viz. 1 hour filling, 2 hours standing full, 1 hour emptying, and 4 hours standing empty for rest and aeration. There has, however, been a tendency in the past to overlook the fact that the periods of standing full, and of emptying the beds, are the only sections of the cycle which are, as a rule, under absolute control. Unless special provision is made for the purpose, the time taken to fill each bed depends entirely upon the rate of flow of the sewage to the works, and the period of rest empty also depends upon the frequency with which the beds are filled, and thus indirectly upon the rate of flow of the sewage. For example, a set of four beds designed to receive each three fillings per day in wet weather, should not receive more than one filling per day in dry weather. Assuming that one-half the total flow comes down in six hours, it will be found that it takes six hours to fill two beds in the middle of the day, or three hours to fill one bed. During the remaining eighteen hours the other two beds are filled, one of them in say six hours and the other in twelve hours. The times taken to fill the four beds in this scheme would therefore be—No. 1, three hours; No. 2, three hours; No. 3, six hours; and No. 4, 12 hours. In each case the period of filling is thus much in excess of the one hour prescribed under the eight hours cycle. The obvious remedy is to subdivide the total area into a larger number of smaller beds, but if this is carried to its logical conclusion it will be seen that there must be 24 beds if the time taken to fill any one bed is not to exceed one hour. While it is very desirable to arrange this subdivision in order to secure the proper cycle of operations, the number of schemes where it is economically practicable are few, and recourse must be had to some other method of reaching the same end. This has already been recognised by most engineers, and provision is now usually made for a tank known under various names, such as dosing tank, collecting tank, equalising tank or holding-up tank, in which the tank effluent is stored until the volume accumulated is equal to the capacity of one contact bed, and the latter is then filled within the regulation time of one hour. If the necessity for making provision on these lines to ensure the proper working cycle had been recognised in the early days of contact beds, it would doubtless have prevented the troubles which have arisen in many places.
From the preceding observations, it will be seen that it is very necessary to come to a decision as to the method of operation to be adopted, before designing any scheme of contact beds. If the method of subdivision into a large number of small beds is preferred, the planning of the separate series, and the probable cost of the additional work involved, must be taken into consideration. On the other hand, if a smaller number of larger beds with a suitable dosing tank are preferred, the extra fall required for the latter must be provided for, even if it involves the reduction of the depth which would otherwise be available for the beds themselves.
