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Abstract The ,Purpose of the present investi.gation was to ~ the performance of a modified activated sludge process to be used in the treatment of wastes deficient in phosphorus and nitrogen~ The waste subject of the present study represents the wash effluent from an onion drying factory. Available nutrients in an organic waste should always satisfy the proportional build up of bacteria to the optimal requirements· of a given activated sludge~ ~ereforef an investigation on the partial removal. of organic matter fro~ solution by means of a low synthesis- ; high oxidation process has been adopted. Contact stabilisation as one of the modifications .of·activated sludge treatment was selected for experimentation. In this process, the waste stream flows into a contact basin witp. a retention period of 30 - 60 minutes; during which the org~cs are aQsorbed on the sludge floc. The activated s1udge is then separated from the mixed liquor in a sedimentation tank and passed into a stabi- lization basin to allow the adsorbed organics to be .., metabolized by available micro-organisms. Aeration is persued in the stabilization basin for 2 to 8 hours depen- dant on the strength of a given waste batch~ In order to test the engineering feasibility of this mode of operation studies have been conducted using a continuous-flow bench-scale system. The so formed sludge was recycled within a volumetric ratio of one part of sludge to one part of onion waste. The detention time in the contact column has been basically derived from the total volume or rate of flow in’ relation to the available size of aerated column. Adaptation of sludge has been performed along the·continuous run of tb?t system. The data obtained may be summarized as fqllows : 1 - When the retention period in the reaeration chamber was selected at 2 hours, an average BOD removal of 65 % was reached. However, it may be noted that during such a period, both tho percentages of BOD and COD removals tend to fluctuate. This, BOD removal ranges vary between 56 % and 87 % while the permenganate value ranged from 70 to , 2 - When the retention period in the re-aeration zone was raised from 2 to 4 hours, the ratios increased from 65 % to 80 % for BOD removals and from 74.4 % to 85.4 % for removal of COD. 3 - With a reaeration time of 6 hours and a contact period of 30 minutes, COD and BOD removals were raised to the order of 94 % and 93 % respectively. 4 - When the BOD load was changed from 5.35 lrg/m3• d to 2.72 kg/m3• d by increasing the detention period in the contact chamber from 30 to 60 minutes and the subsequent decrease of the hydraulic load from 48 m3/m3• d to 24 m3/m3• d, BOD removal up to 94.2 % was obtained. Corresponding pennanganate value removal 95.4 %. It is important to note that during this period of operation the syst em oper-at-edin a relatively stable manner with respect to effluent COD and BOD removals. Furthermore, studies on a laboratory scale were carried out to define the properties and characteristics of excess sludge, that determines its filterablity. An evaluation of -che effect of aerobic digesion and chemi-’ ca.L conditioning prior to filtration was considered. Experiments were carried out on thickened sludge from the stabilization zone without any further treatment. |