INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1926
407
Some Observations on Sewage Tank Gases’ By A. M . Buswell and S. I. Strickhouser STATEWATERSURVEYDIVISION, U R B A N AILL. ,
I
N THE summer of 1924 the attention of this laboratory
was called to the condition of the Imhoff tanks a t Decatur, Ill. During this first summer of their operation some of the tanks foamed to such an extent as to overflow the gas vents and completely cover the sewage in the sedimentation chambers. About the end of September all foaming ceased and the tanks became normal once more. It was thought that the composition of the gases evolved might furnish some clue to the cause of such foaming. I n an endeavor to correlate the gas composition with foaming, gas samples were secured from the Imhoff tanks a t Decatur, Highland Park, and Urbana-Champaign; also from an experimental septic tank maintained cooperatively by the State Water Survey Division and the Department of Farm Mechanics in the University of Illinois. Collection of Samples
The apparatus used in collecting the samples consisted of a n open-top, glass bell jar having the neck closed by a rubber stopper carrying a glass stopcock. The jar was weighted with lead in order that it might be lowered into the tank. The collection of the sample of gas was effected by attaching to the stopcock, by means of rubber tubing, a glass tube 5 feet long, having a short piece of rubber tubing closed by a pinchcock a t the end; then, with both cocks open, the bell jar was lowered into the tank until the liquid rose several feet in the tube. This was done to drive all air out of the jar, leaving it completely filled with liquid. The pinchcock was then closed, the jar raised almost to the surface, and the stopcock closed. At this point the glass tube, filled with liquid, was removed. The jar was allowed to remain suspended in the tank until it was about half full of gas, which required from 20 to 40 minutes. A rubber tube closed a t one end by a pinchcock was then attached to the stopcock and, with the stopcock open, the jar was lowered about 7 feet in the tank to put the gas under pressure. By opening the pinchcock, the gas swept the air out of the rubber tubing, after which the tubing was connected to a collecting bulb filled with water. The pressure on the gas in the jar was sufficient to force it into the bulb, displacing the water. More permanent collectors were placed in two of the tanks of the Urbana-Champaign Sanitary District. Each of these consisted of heavy, galvanized funnels, 13.5 inches in diameter and about 14 inches high. The area covered by each funnel was 1 square foot. A disk of wire screen with 0.25-inch mesh was soldered into the funnel 1 inch from the wide end, in a n attempt to prevent the entry of gas-lifted sludge into the 1-inch pipe to which the funnel was attached. This pipe was originally made up of three &foot sections. The outlet of gas from the collecting device was controlled by a 1-inch gate valve followed by a 0.25-inch reducer, a 0.25-inch gate valve, and a 0.25-inch nipple. A rubber tubing connection was then made from this nipple to the gas-collecting bulb. Upon opening the valves, the hydrostatic pressure on the gas forced it into the collecting bulb, displacing the water with which the bulb was filled. The whole apparatus was suspended in a gas vent by means of two crossbeams wider than the vent and clamped firmly to the pipe. 1
Received November 6, 1925.
This device gave fairly satisfactory service for a short time. It was then found that a n impervious layer of sludge had accumulated on the wire screen and prevented the entry of gas into the funnel. Since the depth reached by the apparatus was about 17 feet, it is probable that the sludge level was very close to the funnel. By removing one of the &foot sections of pipe the diaculty of keeping the pipe clear was greatly reduced. Analysis
All the analyses were made with a n Illinois gas apparatus. With this apparatus, absorptions are made in a Morehead buret, hydrogen is burned in an electrically heated copper oxide tube a t 180” C., and methane is burned in oxygen over mercury in a slow combustion pipet. The accuracy of the apparatus has been verified by its inventors, Professor Parr and his associates, by comparison with standard methods of gas analysis. Discussion of Data
The analytical data obtained are given in the accompanying table. A study of the results indicates that there are marked differences in the gases evolved from foaming tanks as compared with nonfoaming tanks. The carbon dioxide value is considerably higher in the foaming tanks and the nitrogen value is considerably lower. The methane content is only slightly lower. Very similar analyses were obtained a t Plainfield, N. J., where considerable trouble is experienced with foaming tanks.l The differences in the averages would be even greater if the Decatur results on the nonfoaming tanks were not included. These tanks, although not actually foaming a t the time of sampling, were presumably in the same general condition as the other tanks. The results are included. however, for the sake of fairness and completeness. The most striking differences are those exhibited by the Highland Park tanks. These receive sewage from different parts of the town, but in a private communication, S. A. Greeley stated that in his opinion
