Thermophilic aerobic post treatment of anaerobically pretreated paper process water

Thermophilic waste- or process water treatment increases in importance as industries shift from end-of-pipe treatment towards integrated process water treatment. The need for process water treatment becomes evident as the levels of pollutants in industrial water circuits need to be controlled whereas the intake of fresh water generally diminishes. In the paper and board industry, high process water temperatures prevail and thus water treatment needs to take place under thermophilic conditions. In many cases, an anaerobic pretreatment method can be used but aerobic post treatment is required for polishing of the anaerobic effluent. This thesis describes research in which the aerobic post treatment of anaerobic effluent of a board mill was investigated under thermophilic conditions.

As a boundary condition for aerobic conversions, sufficient oxygen needs to be transferred from the gas phase to the liquid in which the bioconversion takes place. It was shown that although the oxygen saturation concentration decreases with a rise in temperature, this effect is fully compensated by the increased oxygen diffusion rate with the same temperature increase. The overall oxygen transfer rate thus remains constant in the temperature range of 20-55 °C.

Post treatment of anaerobic effluent in activated sludge reactors revealed several fundamental differences between mesophilic and thermophilic treatment. Firstly, batch and continuous experiments showed a lesser removal of complex soluble COD under thermophilic conditions when compared to mesophilic reference experiments. This could not be attributed to a higher production of soluble microbial products (SMP) at elevated temperatures. It is therefore expected that thermophilic biomass is unable to oxidize the same variety of complex soluble components as the mesophilic biomass is capable of.

Secondly, thermophilic effluents are often cloudy while effluents of mesophilic activated sludge reactors are generally clear. This is caused by smaller cohesion forces within thermophilic activated sludge flocs resulting in a higher sensivity towards shear forces and smaller floc sizes. Furthermore, fewer colloidal particles from the influent are adsorbed on the thermophilic sludge flocs and are washed out with the effluent. However, a clear thermophilic effluent can be obtained provided the influent contains little colloidal material.

The underlying causes for the weaker cohesion forces within the flocs are still unclear. The absence of protozoa at 55 °C was shown to be of minor importance regarding the effluent turbidity and could not account for this effect. Binding of hydrophobic pollutants on a hydrophobic surface was hardly affected by temperature and could not explain the observed effects either. Based on calculations using the DLVO theory it was shown that bacterial exo-polymers are of crucial importance in the flocculation process. These interactions are highly temperature dependant and are therefore expected to be the underlying cause for the differences in flocculation behavior.

Besides differences in removal efficiencies and flocculation behavior, also the kinetics of mesophilic and thermophilic activated sludge treatment differ. The maximum growth (and thus conversion rate) of biomass cultivated at 55 °C was a factor 1.4 higher than for a similar type of biomass cultivated at 30 °C. Decay rates are doubled with the same temperature increase whereas the gross biomass yields were similar. As a result, higher substrate conversion rates can be obtained under thermophilic conditions provided that a high concentration of thermophilic biomass is cultivated in the reactor by application of a high organic loading rate.

These kinetic advantages are however of little use when polishing the effluent of an anaerobic bioreactor. Under thermophilic conditions biomass growth will be limited since the organic loading rate is restricted by the need to retain and convert particulates from the anaerobic effluent and by the absence of readily biodegradable COD. Furthermore, biomass decay rates have doubled under thermophilic conditions. The combination of these factors diminishes the amount of active biomass in the thermophilic reactor and can not be compensated fully by the intrinsic higher conversion rates. Overall conversion rates in a thermophilic bioreactor can thus be lower as compared to a mesophilic reference system, depending on the applied loading rates.

Nevertheless, for application in the board industry these disadvantages can be dealt with as the water quality demands are relatively low. Additional treatment methods are however required in case higher water quality demands prevail.