The study was carried out, from June 89 to December 1992, on 3 experimental concrete beds of 20 mê ach, constructed close to an extended aeration plant . Beds 1 and 2 were planted with reeds, bed 3 was considered as an unplanted control bed. During the first phase (June 89-May 90), most of the reeds in bed 2 died in spite of the influent sludge dose of 70 g. of SS.m"2.d~l. During the second phase (917 days) the 3 beds were aerated from the bottom and fed with sludge, directly extracted from the oxidation ditch (Dry Matter content 0.3 %). The influent sludge load varied between 120 and 160 g of SS.m^.d" 1 after the second growth season (with peak values of up to 215 g of SS.m'^.d"^ in the summer of 1991). Bed 3 didn't clog, and the major contribution of the reeds to maintaining a high and regular liquid conductivity in the sludge has been proven, allowing easier and higher dosing of the planted beds. Monitoring of the percolation flow emphasised a high mineralisation process in the rhizosphere. The accumulated sludge (DP Matter content =11%) can be dug out together with the reeds using a mechanical digger and sprea fields with a solid manure spreader. A regrowth of reeds occurs directly from rhizomes remaining m the drainage layer and in a few centimetres of sludge at the bottom of the bed.
Because the aeration system in an activated sludge plant typically represents a large part of the total energy requirements, designers and operators need accurate oxygen transfer information to make the aeration system as energy efficient as possible. This paper presents clean water tests performed at 38 wastewater treatment plants. The Specific Aeration Efficiency results (SAE, kgO2/kWh) are reported for: (1) large open channels (volume higher than 1000 m3), (2) small open channels, (3) total floor coverage cylindrical tanks, and (4) cylindrical tanks with a grid arrangement. Some practical guidelines can be drawn, some of them being: (1) high SAE can be achieved at small aeration tanks (< 1000 m3), applying cylindrical tanks with a total floor coverage arrangement of diffusers, volumetric blowers, and moderate air flow rates per diffuser area; (2) the high investment cost of this configuration can be justified with respect to a grid layout characterized by spiral liquid circulation which affects the oxygen transfer; (3) small open channels can meet sufficient SAE values but fail to meet in this range of tank volumes those of total floor coverage cylindrical tanks.
In France, many small sewage treatment plants are of the activated sludge/extended aeration type and, generally, only receive a part of their nominal organic load. They are mostly equipped with surface aerators whose Standard wire Aeration Efficiency is in the range 1.3-1.9 kg 02 kwh−1. Consequently the sludge age and the supply of oxygen are sufficient to obtain a large elimination of nitrogen from domestic waste water even when the sludge temperature is low.
However the concern is with the optimizing of the nitrogen treatment by the nitrification of ammonia in the aeration basin whilst avoiding any parasitic denitrification in the clarifier.
Amongst other investigations, 4 small domestic wastewater treatment plants were studied over several months. A number of adjustment modifications were imposed on them in order to optimize the elimination of nitrogen : The final concentrations to be reached were : N-NH4− lower than 5 mg.l−1 and N-NO3− lower than 3 mg.l−1.
The daily operating time of the aerators depends on the received load, the sludge concentration and the oxygenation capacity. When the works are under-loaded, the non-operational periods of the aerators should be as long as 1 1/2 and 2 hours in order to achieve the nitrates reduction. When the load is higher, the time required to attain anoxia after the shutdown of the aerators is shorter and the length of the non-operational periods can be reduced. These field experiments have allowed an evaluation of the tolerances around an optimum adjustment of the aeration operation : a reduction of 5 to 10 % of the daily aeration time may bring about an increase in the residual concentration of Kjeldahl nitrogen which reaches 10 rag.1−1. Conversely, sludge losses may take place in the clarifiers if the daily aeration time is increased by 5 to 10 % in the case of completely mixed basins equipped with slow vertical shaft aerators, and more than 15 % in the case of oxidation ditches (better denitrification probably due to the continuation of the current after stopping the rotors). The sludge concentration should be kept within fairly tight limits (± 0.5 g.1−1). It is however essential to rectify the aeration adjustments during the seasons : a rise in the sludge temperature of 10°C generates an increase in the total oxygen demand by 3 to 5 %.
The particular case of plants with anoxic zone ahead is finally discussed.
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