Patent GB-745295-A (GB)
Abstract: 745,295. Treating sewage. DORR-OLIVER, Inc., [formerly DORR CO.]. March 6, 1953 [April 1, 1952; Feb. 2, 1953], No. 6317/53. Class 111. In the treatment of sewage sludge containing, in a substantially non-digested state, anaerobically-digestible matter, a sludge bed from which to produce thickened sludge is established and maintained in a thickening zone, both the solids detention time in the bed and the liquid loading rate being related to the solids loading rate, and the bed depth being maintained between upper and lower limits of operability, so that, despite the normal fluctuation of the solids concentration of the sludge supplied to the thickening zone, thickened sludge is obtained without undue downward changes of concentration; the detention time, expressed as the ratio of the volume of the sludge bed to the average rate of sludge withdrawal (" sludge volume ratio ") is maintained sufficiently short to discourage septicity and gasifica. tion of the solids within the above limits of bed depth by periodically determining the bed depth and adjusting the sludge withdrawal rate to maintain the above sludge volume ratio and the desired solids concentration; the admission of liquid into the thickening zone is controlled so that the liquid loading rate is maintained high enough to discourage septicity but not so high as unduly to affect the containment of solids, the liquid zone overlying the sludge bed at the same time being maintained deep enough to avoid undue loss of solids but not so deep as to encourage septicity; the admission of sludge solids into the zone is controlled so that the rate of solids loading is kept high enough both for determining the bed depth and for attaining the desired sludge solids concentration but not so high as unduly to affect that concentration and the containment of solids. Sludge from a primary clarifier 10, Fig. 6, is fed by conduit 19 having a valve 20 to a junction 37 where it meets one or more of the following: aerobically treated sewage passing from an aerobic treatment station 11 by way of conduits 34 and 36 and valve 35; aerobically treated sewage from the effluent conduit 22 of the station 11 by way of conduits 40 and 34 and valve 41; secondary sludge from a secondary clarifier tank 12 by conduits 24, 28 and 32 and valves 25 and 28a; and effluent from the tank 12 by conduits 23, 29 and 32 and valve 30. The mixture passes along conduit 38 into a thickener 13, from the bottom of which sludge having a solids concentration of 9 to 14 (or up to 24) per cent is withdrawn, and from the top of which supernatant liquid flows, e.g. to either end of the main flow. Where secondary treatment is not provided at all the effluent from the primary clarifier flows to waste along conduits 21 and 21a and through valve 21b and the auxiliary liquid is provided from a water supply of some kind and flows along conduit 13a through valve 13b, either to the junction 37 or, as shown, directly into the thickener 13. Where the mixture would otherwise consist only of primary and secondary sludge the liquor requirements of the thickener 13 are met either partly or wholly from the water supply 13a. The density of the sludge withdrawn from the thickener 13 is a function both of the ratio by volume of auxiliary liquid to raw sewage liquor (" liquor ratio ") and of the ratio of the sludge bed volume to the sludge volume withdrawn in unit time (" sludge volume ratio "). This function may be represented as a three-dimensional surface (Fig. 4, not shown) or the surface may be projected in the form of three families of curves as shown in Fig. 5. One or more of these families may be used to select the operating conditions appropriate to the production of a sludge of given density, for example 10 per cent solids (Q, Q<SP>1</SP>, Q<SP>11</SP>). In particular the solids concentration, when expressed as a function of detention time (which is equivalent to sludge volume ratio) in the thickener 13, rises steeply to a value between about 6 and 14 per cent after a detention time of one eighth to one quarter of a day and thereafter rises much less steeply to a value between 10 and 20 per cent at the end of one-anda-half days (Fig. 2). The thickened sludge is preferably digested in one (as shown at 46, Fig. 6) or more stages. In a modification the primary clarification effluent is subjected to biologic aerobic treatment and then to secondary clarification treatment, secondary sludge is returned to the initial clarification zone to yield therefrom a mixture of raw and aerobically treated sludge, and this mixture is subjected to the thickening operation (with auxiliary liquid) as before. In another modification raw primary sludge is mixed with a quantity of aerobicallytreated sewage and clarified in a secondary clarifying station, the controlled thickening operation being allowed to take place in this station, and the liquor ratio requirements being satisfied by the secondary liquor present in and passing through this station. For the purpose of determining the " sludge volume ratio " a test body G, Fig. 7, carried by suspension means E is lowered into the thickener tank 13. As the test body leaves the supernatant liquor and enters the bed of sludge B an apparent loss of weight is observed and the distance D between this point and the point of contact of the body with the bottom of the tank is equal to the depth of the sludge.