Sludge

“ To show you the activity of that sludge, we got the raw sewage, perhaps a million bacteria per cubic centimeter, in the first aerating tank, within half an hour; the sludge contains anywhere from fifteen to twenty million bacteria per cubic centimeter. In this sludge we give the bacteria the natural environments for their work. We give them food, lodging, and air, and that is just exactly what aerobic bacteria need, and the desire is to have intensified the aerobic bacteria, which we have at all times when the plant is being properly operated. ”

“ It produces a better effluent than any other known process of sewage treatment except land treatment or intermittent sand filtration; it can be built upon a comparatively small area; it produces no objectionable odors or flies; it produces a sludge of sufficient value to meet the cost of its reduction to a fertilizer and therefore relieves the city of the difficult, complicated and wasteful method of sludge disposal common to all other processes ”

“ Besco-ter-Meer Sludge Drying Centrifuge at Milwaukee, Wisconsin Courtesy, Barth Engineering and Sanitation Co.
503Experiments on the drying of sludge by acid flotation have not progressed sufficiently to allow the installation of a working unit. The method, which has been applied principally to activated sludge, consists in adding a small amount of sulphuric acid to the sludge as it leaves the storage tank. The sludge is coagulated by this action, the coagulated material rising to the surface as a scum containing about 86 per cent moisture. ”

“ The sludge which settles out of the liquor into the sedimentation tank is then pumped back and discharged into the raw sewage as it enters, or while passing into the beginning of the aerating tank. The surplus sludge which settles in the sedimentation tanks is treated otherwise, which I will describe later. The process is one of aeration and nitrification practically. The sewage does absorb a great deal of oxygen from the air which is [Pg 100] forced into it through these diffusing methods. ”

“ When town sewage is pumped through a long rising main, it can often be spread on the land in its really crude state, and if the soil is clay ploughed up to receive it the sludge is most beneficial to its texture. ”

“ Sludge Disposal and Value Authorities are generally agreed that the sludge problem is the center of the entire sewage problem, because it causes more trouble and is the most expensive part of the treatment. ”

“ Kinnicutt, Winslow, and Pratt in their book on Sewage Disposal state:
With an average depth of 10 inches per dose of sludge of 87 per cent water content, one square foot of covered (glass) bed should dry to a spadable condition one cubic yard of sludge per year.
500The sludge is run on the bed in small quantities at periods from two weeks to a month apart. In favorable weather Imhoff sludge will dry in two weeks or less to approximately 50 to 60 per cent moisture. It is then suitable for use as a filling material on waste land, for burning, or for further drying by heat. ”

“ A sludge pipe is shown in Fig. 162 and 163. Under such conditions, when the sludge valve is opened the sludge should flow freely. The hydraulic slope to insure proper sludge flow should not be less than 12 to 16 per cent. Where it is not possible to remove the sludge by gravity an air lift is the best method of raising it.
The volume of the transition or scum chamber should equal about one-half that of the digestion chamber. The surface area of the scum chamber exposed to the atmosphere should be 25 to 30 per cent of the horizontal projection of the top of the digestion chamber. ”

“ Where land is not available to dig in the sludge, it is necessary to make it portable for removal and disposal away from where it is produced. This is best effected by filter presses. Appliances are made for this purpose, by which the sludge is pressed to a consistency of about 50 per cent. of moisture. The cost of effecting this is about 0.007 pound per head per annum. It is found in practice that where the sludge is produced by straining the solids from sewage before passing it on to land for purification, it requires a little lime to enable the press to work well. ”

“ When a plant is new, or if shut down for such a length of time that the sludge loses its activation, it is necessary to activate some new sludge. This is done by blowing air continuously through sewage either on the fill and draw method with periodic decantations of the supernatant liquid, or by the continuous-flow process, but more preferably by the latter. ”

“ He does not believe that under proper treatment sludge is capable of clogging the pores of the land or of injuring vegetation. He advises the laying out of the filtration-beds in ridges and furrows, the sewage only to flow into the furrows and not to be allowed to flood the ridges on which plants and vegetables are growing. As soon as the deposit of sludge on the sides of the furrows is sufficient to prevent infiltration in any great degree, the sewage should be withheld from the areas so affected. ”

“ A sludge pipe leads away from the bottom of the hopper-shaped sludge compartment, and at intervals of from one to four weeks the valve on this sludge pipe is opened for a short time and a small portion of the accumulated sludge is allowed to be forced out onto a sludge-drying bed by the weight of the sewage in the tank. ”

“ Activated Sludge Process Sewage treatment by aeration in the presence of sludge is the latest development in sewage disposal, and the process is attracting a great deal of attention in America. Milwaukee has constructed a plant to treat two million gallons of sewage a day. ”

“ Where the sludge-disposal area is 2 feet or more below the level of the surface of the sewage in the tank, and the floor of the latter is provided with a suitable sump in which the sludge may accumulate, the method of withdrawing the sludge by utilising the pressure of the head of liquid in the tank, as described in connection with the Dortmund type of detritus tank, may be adopted with advantage. ”

“ Settling solids are indicative of the sludge forming properties of the sewage and of the probable degree of success of treatment by plain sedimentation. Volatile settling solids indicate the property of the formation of offensive putrefying sludge banks. There is no chemical test which will indicate the scum-forming properties of sewage. Fixed settling solids indicate the presence of inorganic matter, probably gritty material such as sand, clay, iron oxide, etc. ”

“ High numbers of bacteria in the effluents have often resulted from the discharge of sludge from the sedimentation-basins onto the filters, due to the interference of ice on the action of the floating outlet for the basins, and this, rather than the direct effect of cold, is believed to be the direct cause of the low winter efficiency. ”

“ In some towns it appears to be mixed with lime and used as manure upon the fields, and in others it is mixed with the sludge of the sewage farms, and is then ploughed or dug into the soil of the farm. ”

“ When dried in machines the liquid contains much organic matter and is objectionable. The simplest method is to discharge the sludge upon drying beds of porous material and underdrained. The time for drying depends upon sewage treatment. Imhoff tank sludge will dry in less than a week, septic tank sludge in two weeks or more, and sludge from plain sedimentation will require about two months in summer and almost five months in winter. Cleveland, in order to overcome weather conditions at its experimental plant, built a covered sludge bed, modeled after standard greenhouse construction. ”

“ The reduction of organic matter in solution is considerable, averaging 17 percent.
This sludge at the bottom is a black muddy looking silt-like deposit containing about 60 percent of water and 5 percent of organic matter. Its value as a fertilizer is small.
27Although the amount of sludge is relatively small the accumulation requires removal occasionally, three or four times a year.
It is estimated that for every 1,000,000 gallons sewage there is about 3 cubic feet of dry matter. By chemical precipitation this would amount to from 20 to 25 cubic feet per 1,000,000 gallons. ”

“ The filtering material used could be gradually prepared from the sludge obtained after electrical treatment, unless it could be more profitably sold as a manure, and I am not a believer in the value of sewage sludge in large quantities. ”

“ After a period of 10 to 30 minutes the pressure is released, the cells are opened, and the moist sludge cake is 501removed. The liquid pressed from the sludge is highly putrescible and should be returned to the influent of the treatment plant. The pressing of wet greasy sludges is facilitated by the addition of from 8 to 10 pounds of lime per cubic yard of sludge. The cake thus formed is more cohesive and easy to handle. The output of the press depends so much on the character of the sludge that a definite guarantee of capacity is seldom given by the manufacturer. ”

“ Insufficient information is available to generalize on the amount of land required or the time before the land can be used for further sludge burial, or for other purposes. Indications are that the sludge may remain moist and malodorous for years and that the land may be rendered permanently unfit for further sludge burial. ”

“ The darkened ill ventilated and modestly heated conditions in the tank are all conducive to the propagation of a micro-organism known as bacteria. The activity of these organisms causes a chemical decomposition of the organic matter in the sewage a part passing off as gas and a 25part passing off in the effluent as inorganic matter, while another portion is deposited in the bottom of the tank as sludge. ”

“ As the liquid is drawn off, air enters freely between all the layers, so that the deposited solids are then immediately brought into close contact with air, from which the aerobic bacteria and other organisms can draw the oxygen they need for their life functions. The result is that the ultimate residue of solids is of quite a different character from sludge of the ordinary type. It is of a granular nature, which rapidly dries on a properly constructed draining bed, and, when dry, resembles ordinary peaty mould. ”

“ Advantages and Disadvantages.—Some of the advantages of the process are: a clear, sparkling, and non-putrescible effluent is obtained; the degree of nitrification is controllable within certain limits; the character of the effluent can be varied to accord with the quantity and character of the diluting water 470available; more than 90 per cent of the bacteria can be removed; the cost of installation is relatively low; and the sludge has some commercial value. ”

“ This sewage sludge upon entering salt or brackish water precipitates much more rapidly than in fresh water or upon land, and becomes relatively insoluble, hence the accumulation in harbors, e.g., Boston and New Bedford harbors and the estuaries of the Merrimac, Taunton and other rivers. ”

“ As the solids settle a scum which forms on top of the tank, keeps out light and air and produces a condition favorable for the bacterial activity caused by minute organisms known as anaerobic bacteria. These germs thrive and functionate best in the absence of oxygen, and their chief function in sewage treatment is the conversion of the solid organic matter into a soluble form, somewhat less complex in chemical composition. The sludge is rotted and when full bacterial action has taken place is humified. ”

“ Then again, if the day be wet, the slug works by day as well as by night. It is the sun and drought that he shuns, and not the light. Therefore the only effectual way to destroy slugs is to sow lime, in dust, and not slaked. The slug is wet, he has hardly any skin, his slime is his covering; the smallest dust of hot lime kills him; and a few bushels to the acre are sufficient. You must sow the lime at dusk; for then the slugs are sure to be out. Slugs come after a crop that has long afforded a great deal of shelter from the sun; such as peas and vetches. ”

“ Infection may come from the swirling dust of the railway roadbed, from contact with transitory or chronic carriers of disease, from green truck grown in gardens fertilized with night soil or sewage, from food prepared or touched by unclean hands or visited by flies or vermin, from milk handled by sick or careless dairymen, from milk cans and utensils washed with contaminated water, or from cisterns, wells, springs, reservoirs, irrigation ditches, brooks, or lakes receiving the surface wash or the underground drainage from sewage-polluted soil. ”

“ Pollution by Surface Refuse and Sewage.—This occurs where a large number of people congregate, as in cities, towns, etc., and very seriously contaminates the ground by the surcharge of the surface soil with sewage matter, saturating the ground with it, polluting the ground water from which the drinking water is derived, and increasing the putrefactive changes taking place in the soil. Here the pathogenic bacteria abound, and, by multiplying, exert a very marked influence upon the health by the possible spread of infectious diseases. ”

“ Cultivation and drainage are now fully understood to be the basis by which a healthy condition of air is to be obtained, next to that, cleanliness and ventilation; if either be neglected a sickly, mouldy, and unwholesome contamination of atmosphere ensues; the odour of a bog is proverbially mouldy, and so is that of an ill-ventilated house or cellar; dryness, or the fresh pleasant scent of clean water, are the antagonists of these; the aromatic odours of vegetation are opponents of putrefaction, and consequently of the development of the lower forms of life. ”