在生物膜中存在着如下几个重要的过程。
1. 基质的扩散过程
2.生物膜中基质利用方程
3.生物膜中的活性生物组分变化
对于平衡状态下的生物膜应当同时满足以上三个方程。
解以上方程的前提是合理的选取下列参数。
K
L
Y
b'
S
上图可以得到Steady-state的五个重要的趋势。
1. Steady-state状态下的生物膜损失速率biofilm loss rate等于生物膜的污泥龄SRT。 对于以某一个biofilm loss rate运行的系统来说，存在最低的基质浓度Sbmin， 可以通过下式计算
对于CSTR来说，这个操作的基质浓度是维持生物膜SRT的最低基质浓度。因此这里的与用不同的notation。如果,。With a CSTR, the effluent S value is the same at a given regardless of the influent substrate concentration. In contrast with a biofilm reactor, increasing the influent substrate concentration causes the effluent value for S to increase over The difference comes about because, in a CSTR, all microorganisms are exposed to the same S, while the concentration decreases with depth into a biofilm. As with any steady-state completely mixed biological process, having S lower than this critical level gives a negative growth rate and no possibility for sustaining steady-state biomass. Thus, remains a very key factor for the design and operation of both systems.
2. J and increase very sharply (on the logarithmic scale) as S increases slightly above . This rapid escalation occurs because J and increases together. A small increase to S allows greater J, which allows greater , however, greater also increases J. Thus, the positive feedback among S, J, and , allows a very rapid increase in J for S slightly above .
3. At some value of S > , the slope of J versus S declines from near infinity and approaches 1.0. This occurs at approximately S = 0.07 mg/ cm3 for the example in Figure 7.3.
4. For S large enough, the flux becomes equal to that of a deep biofilm. The concentration signifies the spot, which is S approximately equal to 0.11 mg/cm3 in Figure 7.3. For all the steady-state biofilm is deep, or St approaches zero before the attachment surface. The practical significance of having a deep biofilm is that J no longer depends on because additional biomass does not increase the reaction rate (for the same S) when all of the added thickness has s1 = 0. On the other hand, J and Xf, increase for larger S, because the substrate concentrations inside the biofilm rise in response to a higher S.
5. For very large S, the slope of J versus S declines gradually and eventually reaches the limiting case of one-half-order kinetics, or • Half-order kinetics is a well-known special case for deep biofilms; it is discussed later in this chapter. The practical impact of a reaction order less than one is that increases in S give less than proportional increases in J, and marginal increases in substrate removal go down when the system is operated well into the deep region.