## Stirrers

• the distribution of recycled external wastewater near the reactor bottom,

• the recycling of compressed biogas (not presented in Fig. 8.5).

In some reactors, two or three of these mixing processes are applied. Nevertheless, the case of nearly ideal mixing is rarely obtainable. It is absolutely necessary to install a system for degasifying the mixed liquor, i.e. the treated wastewater, suspended bacterial flocs and gas bubbles, before sedimentation. Several methods are available. In most degasifying systems, the water flows through tanks kept under a partial vacuum. In addition, the mass transfer rate and the gas/liquid interfacial surface are increased by the use of rotating stirrers or the formation of falling drops or trickling films. Temperatures fall during the degasification, resulting in lower CH4 and CO2 production rates and higher saturation concentrations of both gases. This significantly reduces the amount of gas bubbles which form during sedimentation.

Fig. 8.5 Contact process.

An anaerobic wastewater treatment process can be calculated easily if the following conditions are met:

• The process is controlled by acetate degradation.

• The reactor is completely mixed.

• The surplus bacteria are removed with the overflow.

• Bacterial decay is neglected.

We want to estimate the acetate concentration Se depending on the mean hydraulic retention time tR. Looking at Fig. 8.5, we write the bacterial balance for the reactor:

For the bacterial concentration at mixing point M, we obtain:

1+nR

with:

The concentration of bacteria in the recycled sludge XR must be calculated from the balance of a settler:

with Q0Xe as the overflow mass flow rate and QRXR as the mass flow rate of recycled sludge.

It is useful to define p, the separation coefficient of the settler: (Q0 + QR) X-Q0Xe

and considering Eq. (6.39):

(1+nR)X then, respectively:

Xe = X (1 - P) (1 + nR) (8.45) which is introduced into Eq. (8.43): 1+nR

introducing this into the balance of the reactor (Eq. 8.40) and considering Qm _ Q0 + Qr, we can write:

Using Monod kinetics: S

The acetate concentration in the effluent is represented by: Ks(1-p) (1+nR)

and the critical mean retention time follows for Se _ S0:

Pmax S0

For p = 0, nR = 0, S0P KS, we obtain the simple solution valid for a chemostat:

pmax

In large-scale contact reactors, the mixing energy is not sufficient to realize locally constant conditions (concentrations, temperature, pH), which means that considerable discrepancies exist between theory and practice. Nevertheless, the influence of tR, p and nR can be discussed fundamentally and can often be compared successfully with operational data.

Two-stage anaerobic contact processes can be advantageous, with a first stage predominately for the formation of lower fatty acids at a lower pH 5.5-6.5 and a second stage predominately for the methanization process at a higher pH 6.5-7.5. Because of the higher yield coefficient of acidogenic bacteria (Table 8.2; YX/S = 0.23 g MLSS (g COD)-1) compared with that of methanization (YX/S = 0.04 g MLSS (g COD)-1) and the higher reaction rate in the first stage, sludge concentrations are frequently high enough and must not be increased by sedimentation and sludge recycle.

In all systems described below, the bacteria are retained directly inside the reactor.

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