(Conventional or Extended Aeration) Process
The conventional or fully aerobic activated sludge process was derived directly as the continuous alternative to the original batch processes which saw the introduction of this technology in England in 1914. Usually operated as nitrifying plug-flow processes treating municipal or industrial wastes, these systems were developed to satisfy BOD5 and solids in the effluent i.e. the 20:20 rule.
Suffering from the potential problems of the effects of toxicity and the effects associated with nitrification, which included loss of alkalinity, pH instability and high aeration requirements, some plants were operated at low sludge age to either avoid nitrification or to only partially nitrify.
These plants also demonstrated a significant attenuation of oxygen requirement along the plant which brought about a number of modifications to the process, including the so-called “complete mix” systems.
Since nutrient gradients in the aeration tank favor non-filamentous species, plug-flow characteristics in the aeration tank have been found preferable to minimize filament problems.
For stable operation and the production of a well nitrified effluent, sufficient influent alkalinity is required due to the consumption of alkalinity by the nitrification process. The nitrification of 1 mg of ammonia-N will consume 7.14 mg of alkalinity as CaCO3. Influent alkalinity should be high enough to accommodate this utilization and leave a residual of 60-80 mg/L alkalinity.
Fully aerobic plants are successfully operated over a wide range of sludge ages or loading rates (F/M ratios) depending upon the circumstances, and also applied to industrial wastes as well as municipal wastewater treatment. The earliest plants of this type were introduced around 1916.
The fully aerobic process is the simplest and most robust activated sludge system, and is used where removal of organic material is the main requirement and nitrification is not necessarily required.
Such systems can operate efficiently over a wide range of sludge ages – from 2-5 days (high rates, with sludge age below the minimum sludge age for nitrification ), to over 25 days (extended aeration with nitrification).
Where nitrification does occur, system performance can be affected (e.g. leading to denitrification in the clarifier). In tropical climates or temperatures above 25 degrees C, nitrification may need to be totally inhibited. One means to do this is to limit the aeration (maintain the DO well below 2 mg/L).
In some so-called fully aerobic plug flow plants, restriction of DO to repress nitrification along with the entry of partially fermented influent from anaerobic mains can cause the development of a partially PAO dominated biomass – as the requirement of anaerobic/aerobic cycling is met at least part of the time (and that is enough). The impact upon such a biomass is obvious if one cares to look (microscopically) or test (P in the biomass or effluent).
If phosphorus removal is regarded as the norm rather than the exception, the mechanism for sequestering of SCFA and ultimate removal of P is always available – if conditions allow it to occur (selection pressure). In most systems these conditions have been actively repressed (for example levels of DO in influent wastewater, introduction of complete mix (CSTR) plants in place of plug flow plants etc.).