The activated sludge process is the most widely used process for the biological treatment of domestic and industrial waste waters. Wastewater treatment plants based on the activated sludge process are in widespread use in developed and developing countries. The activated sludge model number 1 (ASM #1) is an internationally accepted standard for activated sludge modeling. It describes nitrogen and chemical oxygen demand within suspended-growth treatment processes, including mechanisms for nitrification and denitrification.
We analyse the biological treatment of a wastewater when a cascade of four reactors is used. We assume that each reactor in the cascade has the same volume. Operating conditions are investigated in which the first reactor is not aerated whilst the last two reactors are aerated. The second reactor may either be aerated or not aerated. The process configuration includes one settling unit and one recycle unit. The settling unit is placed after the final reactor of the cascade. Part of its exit stream is wasted and the remainder is fed into the first reactor. The recycle unit is also placed after the final reactor of the cascade. The entirety of its exit stream is fed into the first reactor.
The performance of a wastewater treatment plant can be characterised by a number of process parameters.Here we consider the nitrogen concentration in the effluent stream leaving the treatment plant (TNe). When the reactor configuration includesa settling unit this is defined by TNe=SNO+SNH+SND, where the state variables on the right hand are the concentration of soluble nitrate and nitrite nitrogen (SNO),soluble ammonium nitrogen (SNH), and soluble biodegradable organic (SND) respectively.
A combination of direct numerical integration and continuation methods are used to investigate the steady-state behaviour of the system. The governing equations were solved using both matlab (ode15s) and maple(lsode[backfull]). For continuation XPPAUT was used. We take the hydraulic retention time (HRT) as the bifurcation parameter, primarily allowing it to vary over the range0
Our results are summarised as follows.
1. When the second reactor is aerated the value of the recycle ratio that minimise the nitrogen concentration depends upon the value of the hydraulic retention time. There is a significant range of values for the hydraulic retention time over which the optimal performance is achieved by employing a ‘moderate’ recycle ratio.
2. When the second reactor is not aerated increasing the value of the recycle ratio always improves performance. Thus the recycle unit should be operated at the maximum attainable value of the recycle ratio.
3. If the maximum attainable value of the recycle ratio is ‘low’ then the second reactor should be aerated.If this value is ‘high’ then the second reactor should not be aerated.