High rate aeration

High rate aeration Backmix activated sludge high strength domestic waste water, 75-90% removal. 

High rate aeration activated sludge, Backmix with recycle mean cell residence time = 5-10 d food/microorganism ratio = 4.5-17 mg BOD5/S kg MLVSS volumetric loading = 1.6-16 kg BODs/m MLSS = 4-10 g/L residence time = 0.5-2 h recycle ratio = 1-5. Air requirements = 25-100 m /kg of input BOD5. 

There is no standard aeration-tank shape or size. Aeration tanks can be round, square, or rectangular. Shallow aeration tanks are more difficult to mix than deeper tanks. Yet aeration-tank depths have ranged from 0.6 m (2 ft) to 18 m (60 ft). The oxidation-ditch systems tend to be shallow, while some high-rate diffused-aeration systems have used veiy deep tanks to provide more efficient oxygen transfer. 

Much of the information available on resistance of nickel-iron alloys to corrosion by mineral acids is summarised by Marsh. In general, corrosion rates decrease sharply as the nickel content is increased from 0 to 30-40%, with little further improvement above this level. The value of the nickel addition is most pronounced in conditions where hydrogen evolution is the major cathodic reaction, i.e. under conditions of low aeration and agitation. Results reported by Hatfield show that the rates of attack of Fe-25Ni alloy in sulphuric and hydrochloric acid solutions, although much lower than those of mild steel, are still appreciable (Tables 3.35 and 3.36). In solutions of nitric acid, nickel-iron alloys show very high rates of corrosion. 

While well-aerated near-neutral waters are normally much more corrosive than poorly-aerated waters, waters with near zero oxygen contents may cause high rates of corrosion if active sulphate-reducing bacteria, which can act as very efficient depolarising agents, are present. A corrosion rate of 1 5 mm/y has been observed on cast iron exposed to such a water. 

The primary distinguishing feature of the improved activated sludge treatment system is that high-rate DAF is the secondary clarifier for separation of suspended solids from the activated sludge aeration basin effluent, as opposed to secondary sedimentation alone in a conventional activated sludge system. 

Under the high-rate conditions, the fluid remains approximately 2 hours in the aeration basin, and the BOD of feed is reduced between 50 and 70%. When maximum conversion is desired, over 95% of the BOD can be removed. This is termed an extended activated sludge process, and the average residence time is at least 18 hours. The conventional conditions are a compromise, with 90% of the BOD being removed and the fluid being detained around 5 hours.34 Table 16-3 gives the design information for these systems. 

Twelve treatments were investigated (Table I) in which the residence times were nominally 1, 2, 3 and 4 days. At each residence time, the aeration rate was controlled at low, medium or high rates (runs 1L-4L, 1M-4M and 1H-4H respectively). The lowest two aeration rates controlled the mixed liquor redox potential in the range 0 to +100 mV Eh or +100 to +200mV Eh. Dissolved oxygen (DO) was not normally detectable at either redox value, but conditions were aerobic (11) and treatment at these redox values had been found effective on a laboratory scale (7). The high aeration rate maintained DO between 0.5 and 2 mg/1 so as not to inhibit... 

Completely-mixed aerated system (Figure 11.4) High rate activated sludge... 

Basic understanding of the effective kinematic viscosity of the emulsion is potentially important from the viewpoint of planning and operating industrial fluidized beds. There are mechanisms which lead to the formation of very small bubbles these are omitted here since the performance of larger bubbles usually dominates the rate processes under high rates of aeration. 

High Rate This process is characterized by a shorter hydraulic retention time of the mixed liquor in the aeration tank and a higher loading rate than the conventional process. Consequently, the effluent leaving the process is not as high a quality as in the conventional process, i.e., it has a higher BOD and suspended solids. It often precedes a second-stage nitriflcation process. 

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