Contact Filters—Single and Double
The treatment of sewage in a single contact filter is classed as a preliminary process and when treated in double contact beds or those arranged in tandem as a final process. A contact filter is a basin filled with broken stone, coke, slag or coarse gravel, thoroughly underdrained. The size of stone or other material to be used depends upon the degree of purification desired, and the manner of operating the beds. The smaller the stone the more brilliant the effluent will be, but all reports agree that the cost of operation will be greater and that there will be a more rapid loss of filter capacity. Experience has taught the superiority of the coarser material because the interstices being so large the bed is not so liable to choke. Watson advises a fine medium bed only when a highly purified effluent is desired, when it would be difficult to get rid of humus from the filtrate, when a high cost of maintenance is not prohibitive and when a temporary stoppage of the whole plant would not be a serious matter. He believes it is not suitable for installations of any magnitude. Beds have been built with various depths, the range being between four and seven feet. Some have been built shallower and have given good results. The method of applying the sewage is important. Some tanks are overfed and others are underfed. Francis E. Daniels, Director of Water and Sewage Inspection of the New Jersey State Board of Health, describes a method which has been found to be successful in plants in this state. At these plants the effluent is applied on the top and at one corner of the contact beds. At the point of application a small area of contact material from 6 inches to one foot deep is removed from the top of the bed, and fine cinders are substituted. An embankment about a foot high is constructed of the same material around this area so that all of the tank effluent applied to the beds strains through the cinders. Mr. Daniels says that a great deal of the suspended matter is thus removed from the tank effluent which reduces clogging and increases the life of the beds. It is Mr. Daniels’ experience that the value of underfed beds is diminishing. If the effluent is very septic this method has the advantage of reducing odors, but as Mr. Daniels has pointed out, the practise of reducing the storage capacity of tanks is becoming prevalent.
In many plants the sewage is distributed by mechanical appliances, some being motor driven and others cable driven. Springfield, Missouri, which uses a motor drive, reports a saving in power, first cost, moving weight, and maintenance, over the cable drive. Another advantage is that the length of the filter can be increased at will. The total cost of the distribution per million gallons according to Springfield’s experience is $1.25 for cable drive and $1.61 for direct motor drive.
After the sewage has been distributed on the beds so that the interstices are filled, it is allowed to stand for a time. The bed is then drained and rested. While standing the sewage comes in contact with a jelly-like film which forms on the surface of the stone, and important changes occur. As with the septic tanks contact beds require a certain period in which to ripen. The time of contact and the period of rest vary in different plants. The rate of filtration varies according to the construction of the beds, the range is between 600,000 and 1,200,000 gallons per acre per day. The effluent from single contact beds is not stable but that from double contact beds is non-putrescible and low in suspended matter, although somewhat turbid. It can be discharged without offense into small streams. Single contact beds have seldom been used for final treatment of sewage and fewer filters of this kind are now being constructed even in conjunction with any preparatory treatment. The general opinion is that this process is on the wane. Watson says, “It may now be assumed that percolating filters are being constructed in England in preference to contact beds wherever the conditions are suitable.” In America they are not being adopted for large installation but they are still considered for small disposal works. In their fifth report the Royal Sewage Commission of England states that taking into account the gradual loss of capacity of contact beds, a cubic yard of material arranged in the form of a percolating filter will generally treat satisfactorily nearly twice as much tank liquor as a cubic yard of material in a contact bed. Comparing the efficiency of contact beds and percolating filters it is claimed that the latter are better adapted to variations of flow and that the effluent is usually much better aerated; and apart from the suspended solids are of a more uniform character. With percolating filters the likelihood of odors is greater than from contact beds and there may be a greater nuisance from flies.
In the report of the City of Leeds, England, the results of very valuable experiments are given. It says, “Double contact beds give good results with crude sewage and excellent results with partially settled sewage or with septic effluent. Single contact beds are insufficient for dealing with crude sewage but give fair results with settled sewage or with septic effluent. The real difficulty with contact beds is to maintain capacity.”
The principal advantages of this process according to reports are low operating head, and less nuisance from odor and flies, and the disadvantages are large areas required and cost of maintenance.