Solubilization Of Particulate And High Molecular Weight Organic Matter

The conversion of particulate and high molecular weight organic matter into forms small enough for bacteria to take up and degrade is an important step in biochemical operations for wastewater treatment because such materials are commonly present in wastewaters and also arise from lysis reactions as discussed previously. In spite of that, relatively few studies have focused on those reactions. Perhaps this is because many types of particulate materials are attacked by distinctly different mechanisms, even though they are collectively referred to as hydrolysis.

The stoichiometry of hydrolysis is thought to be very simple, with organic material simply changing form. Consequently, most investigators have assumed that COD is conserved, i.e., that no energy is consumed. This is indicated in Figure 3.6. Because no energy is consumed, no electrons are removed and no terminal electron acceptor is used. Thus, the stoichiometric equation is simply:

Particulate substrate COD -» soluble substrate COD (3.76)

This means that the rate of formation of soluble substrate COD is equal to the rate of loss of particulate substrate COD.

In the face of complex situations in which reactions are ill defined, it is common for engineers to choose the simplest possible reaction rate expression, and that is what a number of investigators have done, assuming that hydrolysis is first order with respect to the concentration of particulate substrate, XS.M ' ' This approach, however, ignores the effect that the biomass concentration will have on the rate.

One group" performed an extensive literature survey before adopting a kinetic-expression for the hydrolysis of particulate organic matter patterned after that of Dold et al., " which is based on the work of Stenstrom:"

In this expression k„ is the hydrolysis coefficient (hr ') and is a half-saturation coefficient (mg particulate substrate COD/mg active biomass COD). An important characteristic of this expression is that even though the rate is first order with respect to the heterotrophic biomass concentration, it is controlled by the ratio of particulate substrate concentration to heterotrophic biomass concentration, rather than by the particulate substrate concentration alone. This is necessary because the reaction is thought to be surface mediated, depending on the presence of extracellular enzymes whose quantity will be proportional to the biomass concentration.""

Data on the values of k„ and Kx are very limited. Based primarily on the recommendations of Dold and Marais,^ one group 4 adopted a value of 0.092 hr ' for kh and a value of 0.15 for Kx.

The rate of hydrolysis will also be influenced by the electron acceptor concentration,even though no electron acceptor is used in the reaction. Under aerobic-conditions, an interactive, dual nutrient limitation expression has been adopted in a manner similar to that in Eq. 3.46. Under anoxic conditions, an expression similar to that in Eq. 3.48 has been found to be appropriate, with nitrate stimulating anoxic hydrolysis and oxygen inhibiting it. In both expressions, the effect of the particulate substrate should be given by Eq. 3.77. Under anaerobic conditions of short duration, hydrolysis is assumed to stop. While this would not be true for long term anaerobic conditions, it is consistent with observations in biochemical operations that cycle bacteria between aerobic and anoxic conditions."5 It would probably be satisfactory to use Eq. 3.77 alone to describe hydrolysis under fully anaerobic conditions, but with a k,, value much smaller than that used for aerobic conditions.

As seen in Eq. 3.61, biomass decay results in the formation of particulate biodegradable organic nitrogen. In addition, organic nitrogen will be associated with the particulate organic matter in the wastewater. All of this material will be converted into soluble, biodegradable organic nitrogen, SNS, (i.e., the nitrogen associated with amino acids and other soluble, nitrogen containing organic substrates) as the particulate substrate is hydrolyzed. The rate of generation of SNS (rSNS) is numerically equivalent to the rate of loss of particulate organic nitrogen (rXNS), which is proportional to the hydrolysis rate of particulate organic matter:25

where Xvs is the concentration of particulate, biodegradable organic nitrogen.

0 0

Post a comment