Activated Sludge

ACTIVATED SLUDGE [English "activated sludge", Fowler, 1912; exact translation - activated sludge], a voluminous, cotton-like sediment (suspension) of a brownish hue, devoid of unpleasant odor, consisting of a mass of microorganisms capable of rapid oxidation and nitrification of wastewater, settling well despite its very small specific weight. A. s. as a special term entered the practice of wastewater treatment simultaneously with the method of artificial aeration of wastewater (see), being an essential condition for the aerobic biochemical process mineralizing organic substances of domestic and some industrial wastewater. In essence, A. s. consists of the same mineralizing organisms that are active factors in wastewater purification on irrigation fields, filtration fields, "biological filters", in purification ponds; in natural conditions (in soil, in open bodies of water) the same organisms are responsible for the phenomena of "self-purification". However, as the main factor in calculation, A. s. acquires exceptional significance only in structures for artificial aeration of wastewater - in aeration tanks - "aerotanks", where wastewater is mixed with a certain volume of A. s. and subjected to blowing with air, which ensures the supply of oxygen to the liquid, the carrier of which, the living catalyst of oxidation processes, is the bacterial flora of A. s. The obtaining of A. s. by the first observers (Ardern and Lockett, 1912) directly from the aerated liquid required several weeks, but once obtained, A. s. has the ability for rapid growth. The above-mentioned wide distribution of the microbial complex constituting A. s. makes it possible to significantly accelerate the obtaining of large quantities of A. s. necessary for the initial start-up of aerotanks (and for experiments on a small scale). For this purpose, the "sediment" carried away with the liquid purified in so-called "continuous oxidizers" is successfully used. 24 hours of aeration is sufficient to obtain a typical activated sludge from such material. For the same purpose, one can use sludge from a pond, a river backwater, and even sludge washed from soil. - The dose of activated sludge has a significant influence on the rate of wastewater purification. The sludge dose is usually determined by volume, by settling the liquid aerated with sludge in a measuring cylinder for half an hour. In aerotanks, the sludge dose ranges from 10 to 25% (on average 20-25%) and depends on many conditions. To maintain the normal operation of activated sludge, the liquid being purified must receive, depending on the concentration, from 7.5 to 45 volumes of air (Eddy), and sometimes more. The operation of A. s. proceeds successfully at temperatures from +5 to +28°, but sharp changes in temperature are harmful to A. s. The action of A. s. manifests itself first of all in the rapid oxidation of the carbon part of wastewater, which is expressed in a sharp decrease in the value of oxidizability (by 50%) during the first 5-30 minutes of aeration. This first phase has much in common with the coagulation of colloids (see) and may, in reality, be that the huge surface of the A. s. flocs adsorbs the colloidal part of wastewater and, so to speak, purely physically reduces oxidizability and "oxygen demand of the liquid", while at the same time clarifying the wastewater liquid. This property of A. s. makes it possible to limit the treatment of wastewater liquid to only this first phase of the process ("coagulation" with activated sludge). The liquid after separation from A. s. (settling) is discharged, depending on local conditions, either directly into a body of water (Essen), or is transferred to ordinary bio-oxidizers (Moscow, Birmingham), the productivity of which is significantly increased due to such preparation of the liquid. But usually during aeration with A. s., the second phase of the process also manifests itself - nitrification of ammonium nitrogen, as a result of which one can obtain a "completely" mineralized liquid, which is quite stable, remarkably transparent, without any odor, with a negligible bacterial content. - Characteristic features of A. s. The specific weight (1.005-1.02) of A. s. differs little from 1; the sludge is easily stirred up, but, forming into large flocs, settles quickly. The maximum vertical flow rate in settling tanks for separating A. s. from purified liquid is from 0.5 to 0.8 mm per sec. The residence time of A. s. in the settling tank is limited (2 hours) by the fact that in a liquid containing nitrates (purified), in the presence of A. s. with insufficient oxygen, denitrification easily occurs with the release of gaseous N and CO₂, which raise the A. s. from the bottom of the settling tank to the surface. In terms of water content (98-99%), A. s. upon removal from the settling tank is one of the most voluminous "sediments". But the mass of substance removed in the form of A. s. is also greater, since in this way colloids not captured by ordinary settling tanks are also carried away in the sediment. The amount of A. s. excessively formed during the normal operation of an aerotank and subject to removal is at least 1% by volume of the amount of liquid being treated, but since the sediment of the wastewater liquid is also included in the composition of A. s., this may change the percentage of A. s. being removed. The volume of A. s. being removed can be greatly reduced by prolonged settling (from 2-3 hours to a day), if this does not harm denitrification. Chemical composition of the dry residue of A. s. in %. Cities an o *> B s п Я ш Manchester (England). . Milwaukee (U.S.A.). Houston (U.S.A.). . Essen (Germany) . . . 84.41 15.6 64.7135.3 67.8 32.2 66.9 33.1 58.2.Ul.8 1:1 5.8 1:1 1.14 2.60 2.3 2.1 2.90 5.80 2.20 5.00 The relatively high nitrogen and phosphate content makes it quite advisable to use A. s. as a fertilizer, but for widespread application, A. s. requires drying. In most cases, ordinary drying areas are used for this, and drying takes place in the open air. The absence of odor at this is very advantageous and distinguishes A. s. from other sediments. At large purification stations in America (Milwaukee, Houston), they do not stop at mechanical (vacuum filters, centrifuges) and thermal methods of drying A. s. At the same time, chemical treatment of the sludge (change in pH), and in winter - heating, turns out to be necessary. A very interesting method of treating A. s. (Imhoff) - to subject it before drying to methane fermentation (see) in Emscher basins or in special sludge fermentation chambers, as a result of which twice as much illuminating gas is obtained from activated sludge as from ordinary sludge.