The control of a contact bed may be either by hand or automatic, the latter being the more common. Hand control requires the constant attention of an operator and results in irregularity of operation, whereas automatic control will require inspection not more than once a day and insures regularity of operation. A number of automatic devices have been invented which give more or less satisfaction. The air-locked automatic siphons, without moving parts, have proven satisfactory and are practically “fool-proof.” The operation of these devices is explained in Chapter XXI.
257. The Trickling Filter.—A trickling or sprinkling filter is a bed of coarse, rough, hard material over which sewage is sprayed or otherwise distributed and allowed to trickle slowly through the filter in contact with the atmosphere. A general view of a trickling filter in operation at Baltimore is shown in Fig. 167. The action of the trickling filter is due to oxidation by organisms attached to the material of the filter. The solid organic matter of the sewage deposited on the surface of the material, is worked over and oxidized by the aërobic bacteria, and is discharged in the effluent in a more highly nitrified condition. At times the discharge of suspended matter becomes so great that the filter is said to be unloading. The action differs from that in a contact bed in that there is no period of septic or anaërobic action and the filter never stands full of sewage.
The effluent from a trickling filter is dark, odorless, and is ordinarily non-putrescible. Analyses of typical effluents are given in Tables 86 and 87. The unloading of the filter may occur at any time, but is most likely to occur in the spring or in a warm period following a period of low temperatures. It causes higher suspended matter in the effluent than in the influent and may render the effluent putrescible. The action is marked by the discharge of solid matter which has sloughed off of the filter material and which increases the turbidity of the effluent. Where the diluting water is insufficient to care for the solids so carried in the effluent, they can be removed by a 2–hour period of sedimentation. The effluent may become septic during this time, however. The nitrogen in the effluent is almost entirely in the form of nitrates, and the percentage of saturation with dissolved oxygen is high. The effluent is more highly nitrified than that from a contact bed, and its relative stability is also higher, thus demanding a smaller volume of diluting water.
Fig. 167.—Sprinkling Filter in Operation in Winter at Baltimore.
The principal advantage of a trickling filter over other methods of treatment is its high rate which is from two to four times faster than a contact bed, and about seventy times faster than an intermittent sand filter. The greatest disadvantage is the head of 12 to 15 feet or more necessary for its operation. Sedimentation of the effluent is usually necessary to remove the settleable solids. During the period of secondary sedimentation the quality of the filter effluent may deteriorate in relative stability. In winter the formation of ice on the filter results in an effluent of inferior quality, but as the diluting water can care for such an effluent at this time the condition is not detrimental to the use of the trickling filter. In summer the filters sometimes give off offensive odors that can be noticed at a distance of half a mile, and flying insects may breed in the filter in sufficient quantities to become a nuisance if preventive steps are not taken. The dissemination of odors is especially marked when treating a stale or septic sewage. The treatment of a fresh sewage seldom results in the creation of offensive odors.
| TABLE 86 | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Analysis of Crude Sewage, Imhoff Tank, and Sprinkling Filter Effluents at Atlanta, Georgia | |||||||||||
| (Engineering Record, Vol. 72, p. 4) | |||||||||||
| Temperature Fahrenheit | Parts per Million | Per Cent Saturation Dissolved Oxygen | Relative Stability | ||||||||
| Nitrogen as | Oxygen Consumed | Suspended Matter | |||||||||
| Organic | Free Ammonia | Nitrites | Nitrates | Total | Volatile | Fixed | |||||
| Crude Sewage | |||||||||||
| 1913 | |||||||||||
| Maximum | 77 | 15.6 | 21.8 | 0.1 | 3.0 | 100.0 | 371 | 154 | 163 | 47 | |
| Minimum | 61 | 10.4 | 16.5 | 0.1 | 1.4 | 78.3 | 222 | 98 | 112 | 11 | |
| Average | 70 | 12.8 | 18.8 | 0.1 | 2.2 | 90.6 | 285 | 126 | 138 | 28 | |
| 1914 (7 months) | |||||||||||
| Maximum | 74 | 16.0 | 33.4 | 2.3 | 431 | 48 | |||||
| Minimum | 60 | 9.5 | 18.1 | 1.6 | 279 | 12 | |||||
| Average | 66 | 13.4 | 27.1 | 2.0 | 351 | 30 | |||||
| Imhoff Effluent | |||||||||||
| 1913 | |||||||||||
| Maximum | 78 | 13.2 | 21.9 | 0.2 | 3.1 | 68.0 | 90 | 50 | 41 | ||
| Minimum | 58 | 6.5 | 16.8 | 0.1 | 1.1 | 53.1 | 35 | 42 | 21 | ||
| Average | 68 | 9.0 | 20.0 | 0.2 | 2.1 | 60.1 | 68 | 46 | 33 | ||
| 1914 (7 months) | |||||||||||
| Maximum | 77 | 10.3 | 30.3 | 2.0 | 73 | 48 | |||||
| Minimum | 59 | 4.1 | 18.0 | 1.5 | 49 | 34 | |||||
| Average | 65 | 7.7 | 25.9 | 1.8 | 65 | 43 | |||||
| Sprinkling Filter Effluent | |||||||||||
| 1913 | |||||||||||
| Maximum | 79 | 5.6 | 14.2 | 0.8 | 11.3 | 32.1 | 60 | 31 | 28 | 76 | 99 |
| Minimum | 55 | 2.6 | 6.2 | 0.5 | 5.8 | 23.6 | 33 | 26 | 28 | 52 | 88 |
| Average | 66 | 3.8 | 9.9 | 0.7 | 8.2 | 28.2 | 49 | 28 | 28 | 64 | 89 |
| 1914 (7 months) | |||||||||||
| Maximum | 77 | 8.5 | 20.7 | 11.2 | 106 | 79 | 99 | ||||
| Minimum | 55 | 4.4 | 8.8 | 3.6 | 40 | 55 | 89 | ||||
| Average | 63 | 5.7 | 15.2 | 7.2 | 62 | 65 | 95 | ||||
| TABLE 87 | ||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Efficiency of Sprinkling Filter Chicago, Illinois | ||||||||||||||||||||||||||||
| Depth of Filter 9 feet. Size of stone 2 in. to 3 in. | ||||||||||||||||||||||||||||
| Month | Organic Nitrogen | Free Ammonia | Oxygen Consumed | Nitrites | Nitrates | Dissolved Oxygen | Per Cent Putrescible | Suspended Matter | ||||||||||||||||||||
| Total | Volatile | Fixed | ||||||||||||||||||||||||||
| Influent, Parts per Million | Effluent, Parts per Million | Per Cent Removed | Influent, Parts per Million | Effluent, Parts per Million | Per Cent Removed | Influent, Parts per Million | Effluent, Parts per Million | Per Cent Removed | Influent, Parts per Million | Effluent, Parts per Million | Per Cent Removed | Influent, Parts per Million | Effluent, Parts per Million | Per Cent Removed | Influent, Parts per Million | Effluent, Parts per Million | Per Cent Removed | Influent, Parts per Million | Effluent, Parts per Million | Per Cent Removed | Influent, Parts per Million | Effluent, Parts per Million | Per Cent Removed | Influent, Parts per Million | Effluent, Parts per Million | Per Cent Removed | ||
| 1910 | ||||||||||||||||||||||||||||
| October | 5.1 | 2.8 | 45 | 12.0 | 4.6 | 62 | 30 | 15 | 50 | .90 | 7.8 | 0.0 | 8.5 | ∞ | 0 | 75 | 40 | 47 | 54 | 25 | 54 | 21 | 15 | 29 | ||||
| November | 5.9 | 2.5 | 58 | 12.0 | 5.9 | 51 | 35 | 15 | 57 | .76 | 5.9 | 0.0 | 8.1 | ∞ | 5 | 61 | 16 | 74 | 52 | 15 | 71 | 9 | 1 | 89 | ||||
| December | 4.6 | 3.0 | 35 | 12.0 | 6.9 | 42 | 39 | 20 | 49 | .07 | .45 | 6.4 | .15 | 2.6 | 17 | 2.0 | 8.4 | 4.2 | 35 | 85 | 40 | 53 | 60 | 26 | 57 | 25 | 14 | 44 |
| 1911 | ||||||||||||||||||||||||||||
| January | 6.3 | 4.8 | 24 | 11.0 | 7.0 | 36 | 42 | 20 | 52 | .08 | .15 | 1.9 | .27 | 2.2 | 8.2 | 3.0 | 7.8 | 2.9 | 38 | 112 | 43 | 63 | 68 | 29 | 57 | 44 | 13 | 70 |
| February | 9.0 | 4.8 | 47 | 10.0 | 7.2 | 28 | 46 | 20 | 56 | .09 | .15 | 1.7 | .50 | 2.6 | 5.2 | 2.6 | 8.0 | 3.1 | 29 | 100 | 49 | 51 | 64 | 32 | 50 | 37 | 17 | 53 |
| March | 8.3 | 3.5 | 58 | 9.9 | 6.4 | 35 | 47 | 21 | 56 | .09 | .15 | 1.7 | .34 | 3.2 | 9.4 | 2.2 | 6.6 | 3.0 | 28 | 106 | 37 | 65 | 63 | 22 | 65 | 43 | 15 | 65 |
| April | 6.4 | 4.0 | 37 | 8.3 | 3.6 | 69 | 38 | 21 | 45 | .16 | .21 | 1.3 | .53 | 4.5 | 8.5 | 2.1 | 7.1 | 3.4 | 9 | 113 | 68 | 40 | 59 | 35 | 41 | 54 | 33 | 39 |
| May | 7.6 | 5.4 | 29 | 9.2 | 2.4 | 74 | 33 | 31 | 6 | .08 | .38 | 4.8 | .15 | 7.5 | 4.3 | 0.1 | 7.7 | 77 | 6 | 88 | 150 | 1.7 | 54 | 70 | 1.3 | 34 | 80 | 2.4 |
| June | 5.9 | 3.2 | 46 | 11.0 | 0.6 | 95 | 28 | 16 | 43 | .00 | .30 | ∞ | .16 | 8.3 | 5.2 | 0.0 | 7.6 | ∞ | 1 | 92 | 77 | 18 | 56 | 36 | 36 | 36 | 41 | 1.1 |
| July | 6.2 | 4.2 | 32 | 11.0 | 1.3 | 88 | 34 | 26 | 24 | .00 | .36 | ∞ | .09 | 7.7 | 8.0 | 0.0 | 6.5 | ∞ | 4 | 155 | 130 | 16 | 74 | 61 | 18 | 81 | 69 | 15 |
| Note.—Italic figures represent increases. | ||||||||||||||||||||||||||||
Raw sewage cannot be treated successfully on a trickling filter. Coarse solid particles should be screened and settled out, in order that the distributing devices or the filter may not become clogged. The effluent from an Imhoff tank has proven to be a satisfactory influent for a trickling filter. A septic tank effluent may be so stale as to be detrimental to the biologic action in the filter.
In the operation of a trickling filter the sewage is sprayed or otherwise distributed as evenly as possible in a fine spray or stream, over the top of the filtering material. The sewage then trickles slowly through the filter to the underdrains through which it passes to the final outlet. The distribution of the sewage on the bed is intermittent in order to allow air to enter the filter with the sewage. The cycle of operation should be completed in 5 to 15 minutes, with approximately equal periods of rest and distribution. Cycles of too great length will expose the filter to drying or freezing and will give poorer distribution throughout the filter. Cycles which are too short will operate successfully only with but slight variation in the rate of sewage flow. In some plants it has been found advantageous to allow the filters to rest for one day in 3 to 6 weeks or longer, dependent on the quality of the effluent.